101352703 colorants and auxiliaries vol 2

iii

Colorants and auxiliariesORGANIC CHEMISTRY AND APPLICATION PROPERTIES

Second Edition

Volume 2 – Auxiliaries

2002

Society of Dyers and Colourists

Edited by John Shore

Formerly of BTTG/Shirley and ICI Dyes (now DyStar), Manchester, UK

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iv

ISBN 0 901956 78 3

Copyright © 2002 Society of Dyers and Colourists. All rights reserved. No part of this publicationmay be reproduced, stored in a retrieval system or transmitted in any form or by any means withoutthe prior permission of the copyright owners.

Published by the Society of Dyers and Colourists, PO Box 244, Perkin House, 82 Grattan Road,Bradford, West Yorkshire BD1 2JB, England, on behalf of the Dyers’ Company PublicationsTrust.

This book was produced under the auspices of the Dyers’ Company Publications Trust. The Trustwas instituted by the Worshipful Company of Dyers of the City of London in 1971 to encouragethe publication of textbooks and other aids to learning in the science and technology of colour andcoloration and related fields. The Society of Dyers and Colourists acts as trustee to the fund.

Typeset by the Society of Dyers and Colourists and printed by Hobbs The Printers, Hampshire, UK.

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v

Contributors

John ShoreFormerly of BTTG/Shirley and ICI Dyes (now DyStar), Manchester, UK

Terence M BaldwinsonFormerly Dye & Information Service Manager, Yorkshire Chemicals plc, Leeds, UK

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Contents

Preface ix

CHAPTER 8 Functions and properties of dyeing and printing auxiliaries 471

8.1 The need for auxiliaries 4718.2 The general types and characteristics of auxiliaries 474

References 476

CHAPTER 9 The chemistry and properties of surfactants 477

9.1 Introduction 4779.2 Hydrophiles 4779.3 Hydrophobes 4779.4 Anionic surfactants 4799.5 Cationic surfactants 4859.6 Nonionic surfactants 4869.7 Amphoteric surfactants 4899.8 The general properties of surfactants 490

References 496

CHAPTER 10 Classification of dyeing and printing auxiliaries by function 497

10.1 Electrolytes and pH control 49710.2 Sequestering agents 50510.3 Macromolecular complexing agents 51910.4 Enzymes 53910.5 Preparation of substrates 55310.6 Dispersing and solubilising agents 63610.7 Levelling and retarding agents 64210.8 Thickening agents, migration inhibitors and hydrotropic

agents used in printing and continuous dyeing 64510.9 Treatments to alter dyeing properties or enhance fastness 66410.10 Agents for fibre lubrication, softening, antistatic effects,

soil release, soil repellency and bactericidal activity 70510.11 Foaming and defoaming agents 744

References 750

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CHAPTER 11 Fluorescent brightening agents 760

11.1 Introduction 76011.2 Mode of action of a fluorescent brightener 76111.3 Evaluation of FBAs: measurement of whiteness 76511.4 General factors influencing FBA performance 76811.5 Chemistry and applications of FBAs 77011.6 Brighteners for cellulosic substrates 77011.7 Brighteners for cellulose acetate and triacetate fibres 78111.8 Brighteners for nylon 78411.9 Brighteners for wool 78811.10 Brighteners for polyester fibres 79011.11 Brighteners for acrylic fibres 79911.12 Brighteners in detergent formulations 80311.13 Analysis of FBAs 809

References 811

CHAPTER 12 Auxiliaries associated with main dye classes 813

12.1 Introduction 81312.2 Acid dyes 81312.3 Azoic components 82012.4 Basic dyes 82412.5 Direct dyes 83212.6 Disperse dyes 83712.7 Reactive dyes 85612.8 Sulphur dyes 88212.9 Vat dyes 893

References 913

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ix

Preface to Volume 2

This Second Edition of a textbook first published in 1990 forms part of a series on colourand coloration technology initiated by the Textbooks Committee of the Society of Dyers andColourists under the aegis of the Dyers’ Company Publications Trust ManagementCommittee, which administers the trust fund generously provided by the WorshipfulCompany of Dyers.

The initial objective of this series of books has been to establish a coherent body ofexplanatory information on the principles and application technology of relevance forstudents preparing to take the Associateship examinations of the Society. This particularbook has been directed specifically to the subject areas covered by Section A of Paper B: theorganic chemistry and application of dyes and pigments and of the auxiliaries used withthem in textile coloration processes. However, many qualified chemists and colouristsinterested in the properties of colorants and their auxiliaries have found the First Editionuseful as a work of reference. For several reasons it has been convenient to divide thematerial into two separate volumes: 1. Colorants, 2. Auxiliaries. Although fluorescentbrighteners share some features in common with colorants, they have been treated asauxiliary products in this book.

This second volume of the book collects together a remarkable quantity and variety offactual information linking the application properties of auxiliary products in textilecoloration and related processes to as much as is known of the chemical structure of theseagents. The environmental impact of auxiliary products has become of major importanceand developments during the 1990s have necessitated substantial modification andexpansion of the text of this volume. The opportunity has also been taken to highlight novelchemical types of auxiliaries that are under evaluation to overcome or avoid many of thedrawbacks shown by traditional products. Thus the two volumes of this Second Edition arenow approximately equal in size, whereas in the 1990 edition Volume 2 was only about halfas big as its sibling.

Virtually all of this development and improvement of Volume 2, especially in the muchexpanded Chapters 10 and 12, is thanks to the thorough and painstaking work of TerryBaldwinson, who has carefully sifted through an extensive yet scattered range of primarysources. Our grateful thanks are due to John Holmes and Catherine Whitehouse for theirpatient copy editing and to the publications staff of the Society, especially Carol Davies, whohave prepared all the material in this new edition for publication.

JOHN SHORE

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Chapters in Volume 1

Chapter 1 Classification and general properties of colorants

Chapter 2 Organic and inorganic pigments; solvent dyes

Chapter 3 Dye structure and application properties

Chapter 4 Chemistry of azo colorants

Chapter 5 Chemistry and properties of metal-complex and mordant dyes

Chapter 6 Chemistry of anthraquinonoid, polycyclic and miscellaneous colorants

Chapter 7 Chemistry of reactive dyes

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471

CHAPTER 8

Functions and properties of dyeing and printingauxiliaries

Terence M Baldwinson

8.1 THE NEED FOR AUXILIARIES

There is hardly a dyeing or printing process of commercial importance that can beadequately operated by the use of dyes and water alone. Practically every colorant–substratesystem requires the use of additional products, known as auxiliaries, to ensure its reliablefunctioning and control. This was the case even centuries ago, when the use of natural vatand mordant dyes depended entirely on the proper, albeit rule of thumb, use of additives.These controlled pH, reduction, oxidation and mordanting to enable the dyes to be appliedto the natural fibres of those days. Many of the auxiliaries, like the dyes and fibres, were ofnatural origin. Dung and urine [1] were among the agents used, and soap was clearly thefirst surfactant to be employed. Indeed, from the standpoint of today, one can only wonderat the degree of purely empirical expertise so successfully developed and applied by theancient dyers and printers.

Our current level of understanding is clearly a phenomenal advance on the ancient arts,yet our need for auxiliaries remains. For example, even before dyeing or printing thesubstrate must be cleaned and wetted. Products are needed to convert non-substantive vatand sulphur dyes to substantive forms, to help stabilise the conditions that bring about thesubstantivity, and then to reconvert the dyes to their insoluble forms in the substrate(sections 1.6.1 and 1.6.2). Mordant dyes still require the appropriate chelating agents, aswell as other agents to create and maintain the optimum chelating conditions (section 5.8).Printers still need thickening agents to facilitate the localised application of dyes. Inevitably,however, the present-day plethora of dyes, fibres and coloration processes has createdadditional reasons for the use of auxiliaries, whilst the concurrent evolution of the chemicalindustry has satisfied these needs as they arose. Moreover, a vastly more comprehensiveunderstanding of the physico–chemical processes involved has enabled auxiliaries to beprecisely engineered for specific purposes.

Hand in hand with this theoretical knowledge, practical evaluation has becomeincreasingly sophisticated. Nevertheless, it is often difficult to differentiate betweenauxiliaries promoted purely for commercial reasons and those that serve a definite technicalneed. Dye manufacturers are acutely aware of the positive part played by auxiliaries inhelping to sell dyes, and dyers today are under constant pressure to use more of them. Someadditives offer cost savings by improving reproducibility and minimising reprocessing;nevertheless it is all too tempting to incorporate too many products without criticallyevaluating their efficacy, thus inevitably and unnecessarily increasing processing costs.Consequently it is more important than ever that the dyer or printer understands the

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472 FUNCTIONS AND PROPERTIES OF DYEING AND PRINTING AUXILIARIES

functions of auxiliary products and is equipped to evaluate their use realistically and tomonitor it continually.

As has been implied already, functional demands for auxiliaries continue to grow, witheach dye–fibre system and dyeing or printing process having particular needs. The primaryfunctions of auxiliaries are:(a) to prepare or improve the substrate in readiness for coloration by

– scouring, bleaching and desizing– wetting– enhancing the whiteness by a fluorescent brightening effect

(b) to modify the sorption characteristics of colorants by– acceleration– retardation– creating a blocking or resist effect– providing sites for sorption– unifying otherwise divergent rates of sorption– improving or resisting the migration of dyes

(c) to stabilise the application medium by– improving dye solubility– stabilising a dispersion or solution– thickening a print paste or pad liquor– inhibiting or promoting foaming– forming an emulsion– scavenging or minimising the effects of impurities– preventing or promoting oxidation or reduction

(d) to protect or modify the substrate by– creating or resisting dyeability– lubricating the substrate– protecting against the effects of temperature and other processing conditions

(e) to improve the fastness of dyeings, as in– the aftertreatment of direct or reactive dyes– the aftertreatment of acid dyes on nylon– the chroming of mordant dyes on wool or nylon– giving protection against atmospheric influences, as in UV absorbers or inhibitors of

gas-fume fading– back-scouring or reduction clearing

(f) to enhance the properties of laundering formulations (fluorescent brightening agents).

Some auxiliaries fulfil more than one of the above functions. For example, an auxiliary toimprove dye solubility may also accelerate (or retard) a coloration process, or an emulsifyingagent may also act as a thickening agent; pH-control agents may both stabilise a system andalso affect the rate of dye sorption.

Thus the range of auxiliaries available is very large indeed, covering a multiplicity of usesfor all stages of textile processing. However, a factor which has assumed great importanceregarding the use of auxiliaries in recent years is that of their effects on the environment. Inview of the extensive portfolio of products and processes, it is not surprising that good

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environmental management is complex. Undesirable effects from the use of auxiliaries maybecome evident during handling, through effluent discharge to surface waters, throughdischarge to the atmosphere (e.g. via stenter gases), through consumer contact with thefinished product (e.g. skin sensitivity) or during the eventual disposal of solid wastes (e.g.incineration or landfill). All these factors need careful consideration in the selection ofauxiliaries at all stages of processing. Compliance with good environmental practice may bevoluntary (preferably) or enforced by legislation, some countries having introduced quiteextensive and stringent requirements [2,3].

Many factors need to be considered: acute toxicity to mammals, toxicity to aquaticorganisms (fish, daphniae, algae) and waste water bacteria, biodegradability (aerobic oranaerobic), abiotic degradability (hydrolysis, photolysis, oxidation), ground mobility,bioaccumulation, carcinogenicity, mutagenicity and teratogenicity.

The textile wet processing industry produces particularly heavy discharges of effluent;hence the responsibility placed on it for environmentally good behaviour is indeed anonerous one, both technically and financially [4]. The preparation processes of desizing,scouring and bleaching, together with their associated wash-off processes, inevitably producea heavy biological oxygen demand in the effluent [5]. Hence there has been, and continuesto be, much research effort to improve the environmental performance of these areas.

There are two general approaches to good environmental practice. The first, termed ‘end-of-pipe’ solutions, requires all unacceptable matter to be removed from the effluent, or atleast to be reduced to acceptable levels. This is relatively difficult and expensive, requiringthe appropriate treatment facilities. The second attempts to minimise the need for end-of-pipe treatments by reducing the hazardous nature of the effluent in the first place. This canbe achieved, for example, by recycling and reusing useful constituents, especially reducingwater volumes through the use of low liquor ratios, and reducing the toxicity of the effluentby selecting ‘green’ chemicals and processing methods. In some cases, the volume of effluentcan be reduced by combining some processes, e.g. desizing, scouring and bleaching. Thepossibilities inherent in the second approach have stimulated much research work amongstmanufacturers and suppliers of auxiliaries, in terms of finding ‘greener’ products and moreacceptable processes for their use. These efforts will continue for the foreseeable future.

It is important to remember that auxiliaries nowadays are most frequently supplied asmore or less complex mixtures. It is essential from an environmental point of view toconsider the influence of the subsidiary components in the branded product as well as of themain substances. For example, even a small amount of solvent added to improve the stabilityof an auxiliary may pose problems of flammability or toxic vapours, necessitating carefulstorage and labelling. Constant monitoring of products and processes is necessary, sinceenvironmental requirements are continually changing as more information, matched byincreasing awareness of hazards, becomes available. It has been suggested that the mainconcerns to date have been in response to controlling listed substances rather than intackling fundamental environmental problems [5].

Wragg [6] has provided the following basic environmental check-list for textile wetprocessing:(1) Is the product free from species and/or by-products that are on the various control lists?

Lists of controlled chemicals are being continually extended, together with controls ontheir use. Usage of heavy metals, solvents and AOX-containing products will graduallydecrease.

THE NEED FOR AUXILIARIES

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(2) Is the manufacturing process suitable for safety and environmental controls? Processingcycles are being placed under greater control in terms of machine operation. Auxiliaryusage will be more specific and accurate to avoid over-consumption and to minimisewaste.

(3) Has the product or its components been assessed at any time for acute toxicity,carcinogenicity or irritant properties?

(4) Are the materials easily dealt with in waste treatment systems?(5) Has the product or its components been assessed at any time for toxicity to aquatic

species?

The overall result of environmental awareness has been to increase the interplay ofdevolution and evolution. Devolution has seen increasing restrictions, sometimes amountingto a complete ban, on the use of certain substances (e.g. alkylphenolethoxylates, which wereonce widely used) and a corresponding evolution of new products which, at least for thepresent, are environmentally acceptable. This interplay between devolution and evolution islikely to continue indefinitely. Environmental factors as they affect specific types of auxiliarywill be dealt with under the relevant sections of this volume.

8.2 THE GENERAL TYPES AND CHARACTERISTICS OF AUXILIARIES

An auxiliary has been defined [7] as ‘a chemical or formulated chemical product whichenables a processing operation in preparation, dyeing, printing or finishing to be carried outmore effectively, or which is essential if a given effect is to be obtained’. It is much harder todevise a classification system for auxiliaries than it is for dyes. This is undoubtedly one of themain reasons why there has been no incentive to produce an auxiliaries index comparablewith the Colour Index. It is difficult enough to put together a comprehensive yet manageablelist of general application types; it becomes even more difficult to classify them chemically,especially as many of them are more or less complex mixtures, are of imprecisely knownstructure or are the subject of a good deal of trade confidentiality. Although the Society ofDyers and Colourists has shown reluctance to be involved in this area, a very useful biennialtrade publication [8] has made considerable progress in the ordered listing of currentlyavailable commercial products. This first appeared in 1967. The seventeenth edition (2000)lists currently available products by trade name, application and suppliers. Unfortunatelythis publication does not include a listing by chemical type. Although the first section doesgive whatever chemical detail the manufacturers are prepared to divulge, these tend to bebland, broadly based descriptions such as ‘nonionic aqueous emulsion of modified wax’ or‘quaternary ammonium compound, cationic’. In spite of these shortcomings it remains anindispensable guide to the vast range of products on the market today.

The broadest classification of auxiliaries is achieved simply by dividing them into non-surfactants and surfactants, as detailed below.

Non-surfactants include simple electrolytes, acids and bases, both inorganic and organic.Examples include sodium chloride, sodium acetate, sulphuric acid, acetic acid and sodiumcarbonate, together with complex salts (such as sodium dichromate, copper(II) sulphate,sodium ethylenediaminetetra-acetate, sodium hexametaphosphate), oxidising agents(hydrogen peroxide, sodium chlorite) and reducing agents (sodium dithionite, sodiumsulphide). Anionic polyelectrolytes such as sodium alginate or carboxymethylcellulose, used

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mainly as thickening agents and migration inhibitors, also fall within the class of non-surfactants; so too do sorption accelerants such as o-phenylphenol, butanol andmethylnaphthalene, although they normally require an emulsifier to stabilise them inaqueous media. Fluorescent brightening agents (FBAs) form another large class of non-surfactant auxiliaries (see Chapter 11).

Surfactants are, in general, substantially organic in nature and structurally more complexthan most non-surfactants. It is difficult to define surfactants in a manner sufficientlyprecise to satisfy everyone. However, for the purposes of this book an adequate definition ofa surfactant is given by the Society’s Terms and Definitions Committee [7]: ‘an agent,soluble or dispersible in a liquid, which reduces the surface tension of the liquid’. Incoloration processes this reduction in surface tension usually takes place at a liquid/liquid orliquid/solid interface, although liquid/gas interfaces are also occasionally important. Ingeneral, a dramatic lowering of surface tension can be brought about by a relatively smallamount of surfactant; as little as 0.2 g/l of a soap such as sodium oleate will more than halvethe surface tension of water. This physical effect in solution is attributed to the molecularorientation potential of a relatively small hydrophilic moiety (a hydrophile) having strongpolar forces, juxtaposed with a relatively large (usually linear) hydrophobic moiety (ahydrophobe) having relatively weak electrostatic forces (Figure 8.1). In aqueous solution ordispersion the polar hydrophile tends to be oriented into the body of the aqueous phase,whilst the hydrophobe, by nature subjected to forces of repulsion by the aqueous phase, isoriented towards (or at) the interfacial boundary, which may be that between the solutionand air or between the solution and a fibrous (or other) substrate.

The surfactants used as textile auxiliaries can be divided into four major groups,depending on the type and distribution of the polar forces, an arrangement broadlyresembling the ionic classification of dyes. The general scheme is shown in Table 8.1.

Figure 8.1 Schematic diagram of surfactant

Table 8.1 General classification of surfactants

Degree of ionic charge on the

HydrophileClass of surfactant Hydrophobe (associated ion)

Anionic Weakly negative Strongly positiveCationic Weakly positive Strongly negativeNonionic Uncharged UnchargedAmphoteric These possess balanced negative and positive charges,

one or other of which dominates in solution dependingon the pH

THE GENERAL TYPES AND CHARACTERISTICS OF AUXILIARIES

Stronglypolarhydrophile Weakly polar hydrophobe

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REFERENCES1. H T Pratt, Text. Chem. Colorist, 19 (1987) 23.2. S Helman, Melliand Textilber., 72 (1991) 567.3. W Baumann, U Engler,. W Keller and W Schefer, Textilverediung, 27 (1992) 392.4. M Lomas, J.S.D.C., 109 (1993) 10.5. J Park and J Shore, J.S.D.C., 100 (1984) 383.6. P Wragg, J.S.D.C., 110 (1994) 1377. Colour terms and definitions (Bradford: SDC, 1988).8. Index to textile auxiliaries, 17th Edn (Bradford: World Textile Publications, 2000).

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477

477

CHAPTER 9

The chemistry and properties of surfactants


9.1 INTRODUCTION

A surfactant was defined in Chapter 8 as: ‘an agent, soluble or dispersible in a liquid, whichreduces the surface tension of the liquid’ [1]. It is helpful to visualise surfactant molecules asbeing composed of opposing solubility tendencies. Thus, those effective in aqueous mediatypically contain an oil-soluble hydrocarbon-based chain (the hydrophobe) and a smallerwater-solubilising moiety which may or may not confer ionic character (the hydrophile).The limitations of space do not permit a comprehensive detailed treatment of the chemistryof surfactants. The emphasis is therefore on a broad-brush discussion of the principal typesof surfactant encountered in textile preparation and coloration processes. Comprehensiveaccounts of the chemistry and properties of surfactants are available [2–13]. A useful andlucid account of the chemistry and technology of surfactant manufacturing processes isgiven by Davidsohn and Milwidsky [14].

9.2 HYDROPHILES

The basic purpose of the hydrophile is to confer solubility (aqueous solubility is always to beunderstood unless otherwise stated). The simple moieties most often employed are asfollows:(a) in anionic surfactants: sodium, potassium or ammonium cations, associated with

negatively charged groups on the hydrophobe such as carboxylate, sulphonate, sulphateor phosphate

(b) in cationic surfactants: chloride, bromide or methosulphate ions, juxtaposed with, forexample, positively charged quaternary nitrogen atoms

(c) in nonionic surfactants: ethylene oxide or propylene oxide moieties.

More complex hydrophilic moieties are sometimes encountered, however, such as mono-,di- and tri-ethanolamine and the corresponding isopropanolamines in anionic surfactants.Morpholine, once employed, is now obsolete owing to its toxicity.

9.3 HYDROPHOBES

There is a much wider choice of hydrophobes. Most are based on substantially linear long-chain alkanes, either saturated or unsaturated. These were originally obtained from naturallyoccurring fats and oils such as castor, fish, olive, sperm, coconut and tallow oils, but thesesources were later superseded by petroleum products which at that time were cheaper. Morerecently, not only has the price of crude oil escalated, but there has also been a growing

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478 THE CHEMISTRY AND PROPERTIES OF SURFACTANTS

awareness of the finite and diminishing nature of this resource. In 1995, some 75% fossil-sourced raw materials were used in the production of synthetic anionic surfactants (90% iflignosulphonates are excluded) [8], but it is foreseen that more biological materials will beused in the future. It is evident that for some time there has been a systematic and largeexpansion of vegetable oil production, especially in South East Asia [8]. Typical vegetableoils include: tallow, coconut, palm kernel, palm, soybean, linseed, cotton, rape andsunflower. Most cationic surfactants are still obtained from petrochemical olefins, alcohols,paraffins and aromatics, although some are derived from fatty acids [9].

The most common hydrophobes used as the basis for surfactants are those containingeight to eighteen carbon atoms, such as those listed as carboxylates in Table 9.1. Somehydrophobes are aromatic (benzene or naphthalene) moieties, often containing lower alkylsubstituents; dodecylbenzene (9.1) is a common example. Alkyl-substituted toluenes,xylenes and phenols, and mono- and di-alkylated naphthalenes (9.2 and 9.3), are also used.

Table 9.1 Examples of hydrophobes

No. ofcarbon Chemicalatoms name Trivial name and formula

8 Octanoate CaprylateCH3(CH2)6COO

10 Decanoate CaprateCH3(CH2)8COO

12 Dodecanoate LaurateCH3(CH2)10COO

12 9-Dodecenoate LauroleateCH3CH2CH=CH(CH2)7COO

14 Tetradecanoate MyristateCH3(CH2)12COO

14 9-Tetradecenoate MyristoleateCH3(CH2)3CH=CH(CH2)7COO

15 Pentadecanoate IsocetateCH3(CH2)13COO

16 Hexadecanoate PalmitateCH3(CH2)14COO

16 9-Hexadecenoate PalmitoleateCH3(CH2)5CH=CH(CH2)7COO

17 Heptadecanoate MargarateCH3(CH2)15COO

18 Octadecanoate StearateCH3(CH2)16COO

18 9-Octadecenoate OleateCH3(CH2)7CH=CH(CH2)7COO

18 9,12-Octadecadienoate LinoleateCH3(CH2)4(CH=CHCH2)2(CH2)6COO

18 9,12,15-Octadecatrienoate LinolenateCH3CH2(CH=CHCH2)3(CH2)6COO

18 12-Hydroxy-9-octadecenoate RicinoleateCH3(CH2)5CH(OH)CH2CH=CH(CH2)7COO

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The hydrophobes are usually, though not always, used in the form of acids, alcohols,esters or amines. Commercial products rarely contain a single pure hydrophobe, however;most are mixtures containing a range of hydrophobes, since the raw materials from whichthey are made are generally themselves mixtures of homologues. For example, a batch ofcoconut oil, a rich source of the lauric hydrophobe, may have the approximate compositionshown in Table 9.2, although the proportions of the individual components may vary by1–3% between batches. As is the general rule in naturally occurring fats and waxes, onlyeven-numbered carbon compounds are present; odd-numbered ones have to be made bysynthesis. Clearly, a surfactant produced from such a mixture will contain a very large, andvariable, number of homologues and isomers. Hence two products with the same nominalconstitution, but from different manufacturers, often differ in details of composition andproperties. This is one fundamental reason why a chemical classification of auxiliaries,analogous to that for dyes in the Colour Index, would be extremely difficult to devise.

C12H25

9.1

C8H17

9.2

C4H9

H9C4

9.3

Table 9.2 Approximate hydrophobe composition ofcoconut oil

Trivial No. of Amountname carbon atoms (%)

Caproate 6 0.5Caprylate 8 7.0Caprate 10 6.5Laurate 12 49.5Myristate 14 17.0Palmitate 16 8.5Stearate 18 2.5Oleate 18 6.5Linoleate 18 2.0

Any hydrophobe can yield each of the main (i.e. anionic, cationic, nonionic oramphoteric) types of surfactant in much the same way as the same chromogenic system canbe used in anionic, basic or disperse dyes. This will be demonstrated in the followingsections, dealing with each class of surfactant, using the cetyl-containing (C16H33)hydrophobe.

9.4 ANIONIC SURFACTANTS

Until recently this class accounted for by far the largest number of surfactants used inpreparation and coloration processes. This dominance is now challenged by the muchincreased use of nonionic types. The essential feature of the class is a long-chain

ANIONIC SURFACTANTS

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hydrophobe linked through an anionic grouping – usually carboxylate, sulphate (sulphuricester) or sulphonate, but occasionally phosphate, carboxymethyl or other group – to arelatively small cation, generally sodium, although ammonium, potassium and other cationsare also used.

Carboxylates (9.4, where R is the long-chain hydrophobe and X the cation) represent theoldest type of surfactants, since they could be obtained from naturally occurring fats and oilslong before the advent of the petrochemical industry; sodium heptadecanoate (9.5), forexample, incorporates the cetyl group as hydrophobe. Sodium stearate, sodium palmitateand sodium oleate are the simplest carboxylates generally used as surfactants. Alkylarylcompounds (9.6) are also known.

R COO X

9.4

_ +

NaC16H33COO

9.5

_+

COOH25C12 Na

9.6

_+

Many carboxylates are used in the form of soaps, obtained by alkaline saponification oftriglyceride fats and waxes of general formula 9.7. The three carboxylic ester groups(RCOO) may carry the same or different hydrophobes, generally containing eight to 22carbon atoms, the most common being laurate, palmitate and stearate among the saturatedtypes, and oleate and linoleate among the unsaturated ones. At ambient temperatures theunsaturated fats tend to be liquids and the saturated ones solids.

R1 COO CH

CH2

CH2 OOC

OOC

R2

R3

9.7

Particularly important as wetting agents are the disodium alkenylsuccinates (9.8), inwhich the saturated R group may contain from three to fourteen carbon atoms. Thesurfactant properties of these carboxylates, as with other types of surfactant, are dependenton the number of carbon atoms in the hydrophobe. Significant surfactant properties begin toappear in the C8 compounds, although the C8–C12 carboxylates are wetting agents ratherthan detergents. Better detergency and emulsifying properties become evident with C12–C18alkyl groups. Solubility decreases with increasing length of the alkyl group; the solubility ofsoaps, for example, reaches its useful limit with the C22 compounds. The major disadvantageof the carboxylates is that they tend to be precipitated by acids and hard water, since the freeacids and the calcium and magnesium salts of the carboxylates are insoluble. Thisdisadvantage provided the main technical reason for finding alternative products thatshowed tolerance to a wider range of processing conditions.

Modified carboxylates, in which the carboxylate moiety forms part of a carboxymethoxygroup, are also available. These are made by reaction of selected nonionic surfactants withchloroacetic acid. The result is a useful hybrid range, lacking the sensitivity of simple

CHHC

CH

R

CH2 COO

COO Na

Na

9.8

_

_+

+

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carboxylates to calcium and magnesium whilst retaining excellent detergency; thesecompounds are more stable to electrolytes than are the conventional nonionics and moresuitable for use at high temperatures as they are not susceptible to cloud point problems(section 9.8.2).

Sulphates or sulphuric esters of the long-chain fatty acids were the first alternative to thecarboxylates. They are essentially the half esters of sulphuric acid (9.9); the esterincorporating the cetyl hydrophobe (9.10) belongs to the important class of fatty alcoholsulphates. Such sulphates, using C8–C18 hydrophobes, are common.

R OSO3 Na

9.9

_ + C16H33OSO3 Na

9.10

_+

Just as there are mono-, di- and tri-carboxylate surfactants, the sulphates can also beprepared from products bearing mono-, di- and tri-hydrophobes. Indeed, the first sulphatesto be used were analogous to soaps in that they were the sulphation products of triglycerides.Although their chemistry can be represented in simple terms, it is worth re-stating that mostcommercial products are highly complex mixtures. For example, a sulphated triglyceride maycontain the following:– the sulphated glyceride proper– sulphated free fatty acid– unsulphated glyceride– unsulphated fatty acid– inorganic salts– traces of glycerol.

The range of hydrophobes present may also be unexpectedly broad, since the raw materialsoften consist of mixtures of symmetrical and/or mixed glycerides. As little as 60% of an oilmay be sulphatable; sulphation is never carried to theoretical completion and is often farbelow 100%. With these provisos in mind, the chemistry of the sulphated oils can beconsidered.

Many oils are used as starting materials: olive, castor, tallow, neatsfoot, cotton seed, rapeseed and corn oils are examples. Sulphated olive oil was the first sulphated oil to beproduced and was used as a mordant in dyeing as long ago as 1834. Sulphation usuallyoccurs at the double bonds of any unsaturated fatty acids in the glyceride (Scheme 9.1). Onthe other hand, in the preparation of the best-known of these products, Turkey Red Oil orsulphated (often wrongly termed ‘sulphonated’) castor oil, sulphation of the maincomponent, the glyceride of ricinoleic acid (12-hydroxy-9-octadecenoic acid), takes placepreferentially at the hydroxy group rather than at the double bond (Scheme 9.2). Suchproducts possess useful wetting, emulsifying and dye-levelling properties.

CH CH2 CH

OSO3 H

+ H2SO4 _ +CH

Scheme 9.1

ANIONIC SURFACTANTS

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At the present time, however, the long-chain alcohol sulphates already mentioned, suchas structure 9.10, and particularly the sulphated ethers are of greater importance. Thestability of the sulphates to mildly acidic conditions and to hard water is much better thanthat of the carboxylates and is sufficient for most purposes. Under more stringent acidicconditions, however, hydrolysis may take place.

Another type of sulphated product, an ester sulphate, can be prepared by esterifying afatty acid such as ricinoleic or oleic acid with a short-chain (C3–C5) alcohol and thensulphating. Such products are particularly useful foaming, wetting and emulsifying agents; anexample is sulphated butyl ricinoleate (9.11).

CH CH2

OH

CH CH CH2

OSO3

CH CH

H

+ H2SO4 _ +CH

Scheme 9.2

CH3(CH2)5CHCH2CH CH(CH2)7COO(CH2)3CH3

OSO3 H

9.11

+

More recent developments amongst anionic surfactants are the sulphated polyethers oralcohol poly(oxyethylene) sulphates (9.12, 9.13), prepared by ethoxylating the fatty alcoholto give a polyether containing a terminal hydroxy group that is then sulphated. Aromatichydrophobes may also be used to produce, for example, alkylphenol poly(oxyethylene)sulphates. In a general sense, a poly(oxyethylene) sulphate can be viewed as a partly anionicand partly nonionic surfactant, although the degree of ethloxylation of these products isgenerally much lower than that of the purely nonionic surfactants. Hence they aresometimes referred to as ‘lightly ethoxylated alcohol sulphates’; again, their actualcomposition may be a good deal more complex than indicated by their nominal structuralformulae. There has been increasing use of these derivatives in domestic detergents.

R (OCH2CH2)x OSO3 Na

9.12

_ + C16H33 (OCH2CH2)x OSO3 Na

9.13

_ +

Sulphonated anionic surfactants have the general structure 9.14, which should becompared with that of the sulphates (9.9). As well as the simple alkyl derivatives such asstructure 9.15, aromatic and particularly alkylated aromatic (alkylaryl) types are technicallyand commercially important. Indeed, sodium dodecylbenzenesulphonate (9.16) has long beenof great importance in domestic washing powders. Although it is no longer the only surfactantused in domestic washing powders, its economy, efficacy and environmental properties aresuch that it is likely to remain the dominant anionic surfactant in heavy-duty concentratedpowder detergents for some time [15]. Environmental studies have shown that 90% of suchlinear alkylbenzenesulphonates are removed by conventional sewage treatment, the remainderbeing virtually completely biodegradable in topsoil with half-life values varying from 3 to 25days [16,17].

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Nowadays these compounds are usually blended with other surfactants, includingnonionic types (section 9.6). In 1990 a typical low- or non-phosphate domestic detergentcontained 7% linear alkylbenzenesulphonate and 6% nonionic fatty alcohol ethoxylate [16].There is increasing use of the long-chain fatty alcohol poly(oxyethylene) sulphatespreviously described (e.g. 9.12) as a partial or complete replacement for linearalkylbenzenesulphonates [15] since they are made from renewable feedstocks such as tallowand palm oil [16].

Naphthalene and other aromatic hydrophobes are also used to produce sulphonates, suchas structure 9.17. Of greater importance, however, are the more complex condensationproducts that form the basis of many excellent dispersing, resist and aftertreating (syntan)agents. Typical examples are the condensation products of naphthalenesulphonates withformaldehyde, and the lignosulphonates derived from pulping processes; these are describedin more detail in section 10.6.1.

RSO3 Na

9.14

_ + C16H33SO3 Na

9.15

_ +

SO3H25C12 Na

9.16

_+

C3H7

SO3 Na

9.17

_ +

Sulphosuccinates are of particular interest not only for their technical properties but alsobecause structurally they combine the two hydrophile functions described earlier – thesulphonate and carboxylate moieties – in a single molecule (9.18). The sulphosuccinatediesters, however, are probably of greater commercial importance in textile processing thanare the monoesters. The most important example is sodium dioctylsulphosuccinate (9.19),but the dinonyl, dimethylamyl and di-isobutyl analogues are also used commercially. Asusual, a wide choice of hydrophobes is available and includes alcohols, lightly ethoxylatedalcohols, alkanolamides and combinations of these.

COO

SO3

H2C

CH

COOR

Na

Na

9.18

_

_+

+ COOC8H17

SO3

H2C

CH

COOC8H17

Na

9.19

_ +

Phosphate esters (9.20) represent a different class of hydrophile-characterised anionicsurfactants; mono- or di-esters can be formed depending on whether one or two alkyl groupsare present. Most phosphate esters are based on alcohols and especially their ethoxylates,

ANIONIC SURFACTANTS

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including aliphatic and alkylaryl types. Whereas the sulphates tend to be based on lightlyethoxylated alcohols, the phosphate esters are also made from more highly ethoxylatedproducts. Commercial products are complex mixtures (9.21) of monoester, diester, freephosphoric acid and free nonionic surfactant [18].

R O P OH

OH

O

R O P O

OH

O

R

9.20

R(OCH2CH2)xO P OH

OH

O

R(OCH2CH2)xO P O(CH2CH2O)xR

OH

O

HO P OH

OH

O

9.21

R(OCH2CH2)xOH

Monoester surfactant Diester surfactant

Free phosphoricacid

Free nonionicsurfactant

Phosphate esters are particularly useful for their alkali stability and wettability. It has beenshown [18] that a certain amount of ethoxylation of the hydrophobe is required to obtainalkali resistance, and that as the relative molecular mass (Mr) of the hydrophobe increases,the proportion of ethylene oxide required also increases. The greater the degree ofethoxylation, the greater the degree of alkali resistance, few materials showing good alkaliresistance with less than six moles of ethylene oxide per mole of hydrophobe. The samestudy [18] showed that wetting power decreased with increasing Mr of the hydrophobe.Incorporating 2–3 moles of ethylene oxide per mole of hydrophobe seems to give optimumwetting. It appears that as one attempts to increase the rate of wetting, alkali tolerancedecreases.

Other anionic surfactant types include the alkylisethionates (9.22), N-acylsarcosides(9.23), N-acyltaurides (9.24) and perfluorinated carboxylates, sulphonates (e.g. 9.25),sulphates and phosphates [13].

R O CH2CH2SO3 Na

9.22

_ +R C

O

N CH2COO

CH3

Na

9.23

_+

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9.5 CATIONIC SURFACTANTS

By far the most important types of cationic surfactant used in textile processing are thequaternary ammonium salts (9.26), in which R is usually a long-chain hydrophobe and R1,R2, R3 are lower alkyl groups. The most common anions in these and other cationicsurfactants are chloride and bromide: thus cetyltrimethylammonium chloride (9.27) istypical of this class of cationic surfactants. In fact, however, all four alkyl groups on thenitrogen atom can be varied to alter the balance of properties of the products. In thealkyldimethylmethallylammonium chlorides (9.28), an unsaturated aliphatic group is used.Aromatic components are also used, as in the important alkyldimethylbenzylammoniumchlorides (9.29), and both the aromatic nucleus and the alkyl groups in such products maycontain substituents (9.30 and 9.31). As important as the quaternary ammonium surfactantsare the pyridinium salts (9.32; R is a long-chain alkyl group), such as cetylpyridiniumchloride (9.33).

R1 C

O

N CH2CH2SO3

R2

Na

9.24

_ +O

SO3

CC

CF3

CF3

CF3C

CF2

CF2

F3C

F3C

Na

9.25

_+

R N R2

R3

R1

X

9.26

+

_H33C16 N CH3

CH3

CH3

Cl

9.27

_

+

R N CH2

CH3

CH3

C CH3

CH2

Cl

9.28

+

_

R N CH2

CH3

CH3

Cl

9.29

_

+

R N CH2

CH3

CH3

Cl

Cl

9.30

_

+

R N CH2CH2OH

CH3

CH3

Cl

9.31

+

_RN

X

9.32

+ _C16H33N

Cl

9.33

+ _

CATIONIC SURFACTANTS

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Imidazoles can be quaternised to yield cationic surfactants (such as structures 9.34 and9.35). Long-chain alkyl primary, secondary and tertiary amines can also be used as cationicsurfactants, but their use in textile processing is limited as a result of their insolubility inother than acidic aqueous media. The range of products available as cationic surfactants istruly enormous, including, for example, such complex products as alkylated mono- and di-guanidines and polyamines (9.36) containing more than one basic nitrogen atom.

R1C

N

N

CH3

H2C

CH2CH2

H2C

NHCO R2

Cl

9.34

_+

N

C

N

CH2CH3

CH2

RCl

9.35

_+

R (NHCH2CH2)xNH2

9.36

Many of these cationic products, including the quaternary amines and imidazoles, can beethoxylated (9.37, 9.38), forming cationic analogues of the ethoxysulphates andethoxyphosphates in the anionic series. They are essentially cationic/nonionic hybridsurfactants, variously described in manufacturers’ promotional literature as ‘modifiedcationic’, ‘weakly cationic’ or even ‘modified nonionic’. Their value lies in the fact that thecationic nature can be controlled by varying not only the alkyl substituents but also thedegree of ethoxylation. In addition the ethoxylate moiety confers useful emulsifyingproperties. Fluoro-containing cationic surfactants (9.39) can also be obtained [13].

R N (CH2CH2O)xH

(CH2CH2O)xH

CH3

Cl

9.37

+

_

RC

N

N

CH3

H2C

(CH2CH2O)xH

H2CCl

9.38

_+

(CH2)3 N CH3

CH3

CH3

CONHC7F15

I9.39

_+

9.6 NONIONIC SURFACTANTS

Nearly all nonionic surfactants contain the same type of hydrophobes as do anionic andcationic surfactants, with solubilisation and surfactant properties arising from the addition ofethylene oxide to give a product having the general formula 9.40. Usually, depending on the

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hydrophobe, aqueous solubility and detergent properties begin to be evident when x = 6, butin theory the degree of ethoxylation can be continued almost indefinitely. Optimalsurfactant properties are generally found when x = 10–15, although higher homologues (forexample, x = 50) are known. The ‘lightly ethoxylated’ sulphates mentioned earlier usuallycontain only 2–4 oxyethylene units per molecule. Thus a typical nonionic surfactant can berepresented by structure 9.41.

R (OCH2CH2)xOH

9.40

H33C16 (OCH2CH2)12OH

9.41

Although there are other types of nonionic surfactant, the great majority are adducts ofethylene oxide with hydrophobes derived from three sources:– fatty alcohols and alkylphenols– fatty acids– fatty amines and amides.

For many years the most common of these have been adducts with p-nonyl- andp-octylphenol, and to a lesser extent 2,4-dinonylphenol, p-dodecylphenol and1-alkylnaphthols. Since the hydrophobes used may be variable products conforming to anaverage nominal structure, and since the quoted degree of ethoxylation can also only beregarded as an average value, products having the same name (such as, for example,p-nonylphenol dodecaoxyethylene) may in fact differ in detailed composition and propertieswhen obtained from different manufacturers. These provisos should be borne in mind whenconsidering the examples below, even though there is a trend in some cases towards themanufacture of narrower fractions.

An example of an alcohol-based nonionic (9.41) has already been given. An alkylphenoladduct (9.42) is essentially similar; both alcohols and phenols give rise to the relativelystrong and stable ether link, a valuable property of this type of product. Analogues based onalkylthiols (9.43) may also be used.

H19C9 (OCH2CH2)xOH

9.42

H17C8 S (CH2CH2O)xH

9.43

Polyfunctional alcohols of varying complexity, such as polyethylene glycols (9.44) andpolypropylene glycols of varying chain length, also provide useful nonionic agents. Apolypropylene glycol molecule has a hydroxy group at each end to which ethylene oxide canbe added, forming random segments of poly(oxyethylene) and poly(oxypropylene). Thisresults in block copolymers, which can be engineered by control of starting materials andprocessing conditions to give products specifically suited to a wide variety of purposes byvirtue of wide variations in segment length and degree of polymerisation.

Whereas the alcohol and phenol derivatives are characterised by ether linkages, adductsof ethylene oxide with fatty acids give rise to both monoesters (9.45) and diesters. These are

R

(OCH2CH2)xOH

(OCH2CH2)xOH

9.44

NONIONIC SURFACTANTS

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less stable than the ethers in strongly acidic or alkaline media, however, hydrolysing to theoriginal fatty acid and polyethylene glycol.

Adducts of ethylene oxide with fatty amines can yield mono- (9.46) or di-substituted(9.47) products, as can the adducts with fatty amides (9.48, 9.49). In practice the productsformed are far from being as simple or as symmetrical as represented by these formulae since,amongst other things, the ethylene oxide addition takes place randomly.

A recent introduction in the area of nonionic surfactants is the alkylglycoside series.These are long-chain acetals of saccharides (9.50). Commercial products currently have anaverage alkyl chain length of 10–12 carbon atoms. These are manufactured from non-petroleum sources, being synthesised from glucose and fatty alcohols. Such acetals areregarded as eco-friendly, being said to be completely biodegradable [19] and having low skinirritancy. These surfactants possess wetting, foaming and detergency properties similar tothose of the corresponding alcohol ethoxylates but with higher solubility in water and insolutions of electrolytes. They are soluble and stable in sodium hydroxide solutions and showno inverse solubility characteristics.

H33C16CO(OCH2CH2)xOH

9.45

H23C11NH(CH2CH2O)xH

9.46

N

(CH2CH2O)xH

(CH2CH2O)xH

9.47

H23C11 H23C11 C NH(CH2CH2O)xH

O9.48

C N

(CH2CH2O)xH

(CH2CH2O)xH

H23C11

O

9.49

HO CH

CH

CH O

CH

CH O CnH2n+1

OH OH

CH2OH

9.50

The nonionic types so far discussed form the great majority used in textile processing. Ofcourse, a great many more can be synthesised, as the possible range of permutations andcombinations is truly enormous. Given appropriate conditions, ethylene oxide will react withalmost any proton-donating compound, but the choice in practice is restricted by economicfactors. Not all nonionic surfactants are ethoxylates, however. Analogous propylene oxideadducts are known; rather more different products include sucrose and sorbitan esters,alkanolamides and fatty amine oxides. The fatty acid esters of compounds such as sucroseand sorbitol exhibit surfactant properties. Some, such as the sorbitan fatty esters, areinsoluble in water but being oil-soluble they can be used as emulsifiers in oil-based systems,or they can be ethoxylated to render them water-soluble. Mention has already been made offatty amide poly(oxyethylene) adducts formed by condensation of a fatty acid with analkanolamine which is then ethoxylated; some complex alkanolamides have in themselves(i.e. without ethoxylation) some surfactant properties, however. They are made by the

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reaction of a fatty acid (such as lauric acid or a coconut fatty acid) with a secondaryalkanolamine (such as diethanolamine) to yield an amide, which then reacts further withdiethanolamine to give the water-soluble alkanolamide surfactant. Typical fatty amine oxides(9.51 and 9.52) are derived, for example, from the peroxide oxidation of tertiary aminescontaining at least one fatty-chain group.

H25C12 N

CH3

CH3

O

9.51

NO

C12H25

O

9.52

9.7 AMPHOTERIC SURFACTANTS

As mentioned in Table 8.1, amphoteric surfactants contain both an anionic and a cationicgroup. In acidic media they tend to behave as cationic agents and in alkaline media asanionic agents. Somewhere between these extremes lies what is known as the isoelectricpoint (not necessarily, or even commonly, at pH 7), at which the anionic and cationicproperties are counterbalanced. At this point the molecule is said to be zwitterionic and itssurfactant properties and solubility tend to be at their lowest. These products have acquireda degree of importance as auxiliaries in certain ways [20–25], particularly as levelling agentsin the application of reactive dyes to wool.

The simplest type is represented by the higher alkylaminoacids, such as compound 9.53;disubstituted amines can also be synthesised (9.54).

Ethoxylated products can also feature as amphoteric surfactants; an example is compound9.55, an alkylamine poly(oxyethylene) sulphate. Of particular interest in textile processing arethe trisubstituted alkylamino acids known as betaines; N-alkylbetaines (9.56; R = C8–C16alkyl) and acylaminoalkylbetaines (9.57; R = C10–C16 alkyl) are typical [30].

Sulphate and sulphonate analogues of the carboxylates, such as the sulphobetaine 9.58,can also be used as amphoteric agents.

H33C16NH2CH2COO

9.53

+ _

H33C16HN

CH2COO

CH2COOH

9.54

+_

HN

(CH2CH2O)xCH2CH2OSO3

(CH2CH2O)xH

H33C16

9.55

+

_

R N CH2COO

CH3

CH3

9.56

+ _

CONHCH2CH2CH2 N CH2COO

CH3

CH3

R

9.57

+ _

R N CH2CH2CH2SO3

CH3

CH3

9.58

+ _

AMPHOTERIC SURFACTANTS

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9.8 THE GENERAL PROPERTIES OF SURFACTANTS

9.8.1 Effects on the environment

The widespread use of these products focused attention on their environmental propertieslong ago, owing to the persistent foam-creating tendency of many surfactants whendischarged. However, the surfactants industry has a very good track record of responding toenvironmental problems, stretching back as long ago as the 1960s; that is, quite some timebefore the present environmental bandwagon began to roll. Karsa has provided a pointedreminder of this: ‘The most significant development in the West in the 1960s was thegrowing environmental awareness and concern for biodegradable components to overcomeproblems at sewage treatment plants and foam in watercourses. The result was an under-publicised and often forgotten fact that industry on both sides of the Atlantic voluntarilychanged from branched-chain alkylbenzenesulphonates’. Since then, ‘detergents have beenbased on biodegradable components, contrary to the impression given with some of theinformation supplied with today’s ‘green’ detergents, which would have one believebiodegradability is something new and exclusive to these products. This was among the firstmajor environmental moves by any industry and was ten years ahead of any UK or EEClegislation’ [16].

Major works dealing with environmental aspects of surfactants are available [26–30].The excellent biodegradability of the linear alkylarylsulphonates has already been

mentioned (section 9.4). The alcohol sulphates have low toxicity and alcoholpoly(oxyethylene) sulphates are even less toxic. Alkane sulphonates have high COD, BODand an MBAS degradation rate of 90%. Polyether carboxylates have excellentenvironmental properties and are non-toxic to the extent that they are used in cosmeticsand household detergents. Sodium-α-olefin sulphonates show rapid biodegradation due totheir linear structures. The α-sulphomonocarboxylic esters show good to excellentenvironmental properties and are also used in cosmetics and household detergents.Sulphosuccinates generally show 90% biodegradation after seven days and have a longhistory of safe use, being ranked as relatively non-toxic. Phosphorus-containing anionics arevery mild to the skin and are used in cosmetics, shampoos and lotions.

The toxicology of perfluorinated surfactants varies greatly; most are harmless, whilst someare amongst the most toxic non-proteins known, the structural differences between the twooften being relatively slight. Hence caution is needed in their use, even though they are sostrongly surface-active that they can be used in much smaller quantities than othersurfactants.

Cationic alkylammonium surfactants have shown 94% biodegradability [27].Amongst the nonionics, the use of linear primary alcohol ethoxylates has grown rapidly

since the 1970s, due in very large measure to their high degree of biodegradability undermost test procedures, both rapid primary and ultimate degradation [28]. Biodegradation ofsuch products is retarded by branching of the alkyl chain, this being cumulative. It is alsoretarded in secondary alcohol structures, by the addition of about 3 equivalents of propyleneoxide to the ethoxylate moiety and by an ethoxylate chain of more than 20 units. However,as Talmage points out [28], products containing these features are not present in the simplealcohol ethoxylates most commonly used in detergent formulations.

In contrast to the above trends, during the 1980s and 1990s there has been considerableenvironmental concern over the alleged effects of the nonionic nonylphenol ethoxylates.

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The concern seemed to be centred around possible bioaccumulation and the properties ofnonylphenol itself, potentially one of the major metabolites (products of biodegradation)released during the environmental breakdown of nonylphenol ethoxylates. The incompletebiodegradation was attributed to branching in the nonyl group and the presence of thearomatic phenyl ring. In the 1980s, particularly in Europe, there were calls for restrictionsand bans on the use of nonylphenol ethoxylates. This concern, not surprisingly, led to muchcareful and detailed research from which has evolved a clarified and much less alarmistpicture.

This research has been excellently reported by Naylor [31], who pointed out thelimitations of laboratory methods (the extent of biodegradation of nonylphenol ethoxylateshas been variously reported from 0% to 100%!) and the critical importance of determiningbiodegradability in conventional waste water plants using improved and streamlinedanalytical methods. This work, on American rivers and treatment plants, showed thatnonylphenol ethoxylates exhibited high treatability under conditions of extremely highloadings on waste water treatment plants. It was confirmed that nonylphenol is indeed themetabolite of highest toxicity but it is not a significant metabolite except under anaerobicconditions. It was shown that nonylphenol ethoxylates are extensively biodegraded (92.5–99.8% removal rates) in secondary treatment. Re-aeration studies have shown thatnonylphenol and nonylphenol ethoxylates contained in sewage sludge degrade when thesludge is applied to soil.

Thus there is a strong basis for the conclusion that nonylphenol ethoxylates are highlybiodegradable, do not accumulate in water, sediment or aquatic organisms and do not pose acredible threat to the environment. Hence, in 1995, Naylor [31] was able to say that, inAmerica, nonylphenol ethoxylates were by far the most important alkylphenol ethoxylates,accounting for 80% of the total volume and commonly found in formulations for fibre sizing,spinning, weaving, scouring and dyeing, as well as for water-based paints, inks, adhesives andmany institutional and household cleaning products.

Finally, in considering the environmental properties of surface-active auxiliariesgenerally, it should be borne in mind that they are more or less complex mixtures and hencethe presence of other components, such as solvents, electrolytes or sequestrants, needs to beconsidered in addition to the surfactants present.

THE GENERAL PROPERTIES OF SURFACTANTS

9.8.2 Application properties

Anionic and cationic products generally tend to interact with each other, usuallydiminishing the surface-active properties of both and often resulting in precipitation of thecomplex formed. Amphoteric compounds can also be incompatible with anionics in acidsolution but are generally compatible with cationics and nonionics. Interaction betweenanionic and cationic agents can sometimes be prevented by addition of a nonionic. In somecases, if an ethoxylated sulphate or phosphate is used as the anionic component a cationiccompound produces no obvious precipitation, since the oxyethylene chain acts as dispersantfor any complex that may be formed.

The main disadvantages of the carboxylates are their tendency to react with calcium andmagnesium ions in hard water to give insoluble precipitates and their insolubility in acidicmedia, although they generally have good wetting and detergent properties. Theacylsarcosides are less affected by calcium and magnesium ions, however, whilst the

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carboxymethyl surfactants are unaffected. The sulphates were specifically developed toovercome the drawbacks of the carboxylates and, like the phosphates, are stable towardscalcium and magnesium ions. As well as being outstanding detergents, the sulphonates arealso unaffected by strongly acidic or alkaline conditions, and the higher-alkyl members haveuseful lubricating properties. On the other hand, the sulphates can be hydrolysed by acidand sulphated monoglycerides can also be hydrolysed by alkali. Their wetting properties tendto be inferior to those of the sulphonates but they are particularly valuable as emulsifyingagents, especially in combination with nonionics. The sulphosuccinates have a highpropensity to foaming and their solubility is not generally good, but the monoesters havegood detergency properties and the diesters are particularly rapid wetting agents.

As a group, the phosphates have good stability to acid and alkali for most purposes, havelow foaming and good detergency properties and are biodegradable. They tend to be betterwetting agents than the sulphates and their solubility in organic solvents makes them usefulin, for example, dry cleaning. The perfluoroalkyl anionic surfactants are very expensive, butare powerful surfactants at very low concentrations and are stable in chemically hostileenvironments; they also exhibit surface activity in organic solvents.

Cationic agents generally are less useful than anionics as detergents but they have usefulproperties as softeners, germicides and emulsifiers.

Nonionic agents are generally compatible with both anionic and cationic types. They arealso stable to calcium and magnesium ions. With the exception of the fatty acid esters, whichare readily hydrolysed by acid and alkali, they are stable and effective over a wide range of pHvalues. A particular characteristic of nonionic surfactants is their inverse solubility: as thetemperature rises the solubility decreases, until a point is reached at which the surfactantattains its limiting solubility and therefore begins to precipitate out, causing cloudiness of thesolution. The temperature at which this occurs, known as the cloud point, depends on thenumber of oxyethylene units in the nonionic molecule in relation to the length of thehydrophobe. Thus, for any given hydrophobe, the cloud point increases with the increasingdegree of ethoxylation; for example, dodecanol heptaoxyethylene C12H25(OCH2CH2)7OH hasa cloud point of 59 °C, while that of the undecaoxyethylene homologue is 100 °C. Conversely,for a fixed number of oxyethylene units, the cloud point decreases with increasing size of thehydrophobe. The cloud points of nonionic agents are also generally lowered by the presence ofelectrolytes, the effect varying with the electrolyte and its concentration. It is important tobear this in mind when choosing nonionic agents for use in electrolyte-containing processes.This inverse solubility arises from the solubilisation of the nonionic molecules by hydrogenbonding of water with the ether oxygen atoms (9.59). As the temperature rises, the energywithin these bonds becomes insufficient to maintain their cohesion and dehydration takesplace, with a consequent decrease in solubility. A knowledge of the cloud point of a surfactantis useful, not only because of solubility effects but also because the surface activity tends to beoptimal just below the cloud point.

O

CH2

CH2

O

CH2

CH2

O

H

H

H

O H O H

O H9.59

n

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The tendency of nonionics to produce foam varies. Some, such as the block copolymers,are even used as defoamers. Their wetting, detergency and emulsifying properties also varywidely, depending to a large extent on the balance between the hydrophobic and hydrophilic(oxyethylene) portions.

The amphoteric agents exhibit excellent compatibility with inorganic electrolytes andwith acids and alkalis. Such is their stability in strongly acidic solution that they are evenused in cleaning compositions based on hydrofluoric acid [14].


9.8.3 The theory of surface activity

The physico-chemical theory of surface activity is a vast field and no more than broadprinciples can be touched on here; major reference sources exist for those who require moredetail of the relationship between chemical structure and the various surfactant propertiessuch as wetting, detergency and emulsification-solubilisation [32–36].

Surface activity is generally related to the balance between the hydrophobic andhydrophilic portions of the molecule. For example, among the anionic surfactants C8–C12alkyl hydrophobes tend to be predominantly wetting agents, whilst the C12–C18 homologuesexhibit better detergency and emulsifying properties. The alkylsuccinates andsulphosuccinates are particularly powerful wetting agents. Clearly, as the hydrophobiccharacter of the surfactant is increased, aqueous solubility decreases and oil solubilityincreases. Thus the balance between the hydrophobic and hydrophilic moieties of asurfactant is a critical factor in determining its major characteristics. This is referred to asthe hydrophile–lipophile balance, or HLB (the term ‘lipophile’, of course, being analogous to‘hydrophobe’). Whilst the HLB value is of general use in expressing the characteristics of asurfactant, it is of particular value in describing the formation of emulsions. For somegeneral purposes the HLB can be used qualitatively (referring, for instance, to low, mediumor high HLB), but for more precise work it is preferable to use a quantifying scale. Such ascale, put forward in the 1940s [37], covers a range of values from zero (the lipophilic orhydrophobic extreme) to a hydrophilic extreme of 20 or higher, with a value of 10approximately representing the point at which the hydrophilic and hydrophobic portions arein balance. This scale is especially useful in describing the properties of the nonionicethoxylates. For example, a low HLB value (4–6) signifies a predominance of hydrophobicgroups, indicating that the surfactant is lipophilic and should be suited for preparing water-in-oil emulsions. A value in the 7–9 range indicates good wetting properties. As the valueshifts towards increased hydrophilicity other properties predominate, values of 8–18 beingtypical for surfactants that will give oil-in-water emulsions, and values of 13–15 forsurfactants that show useful detergency. The HLB values required for solubilising propertiesare generally in the range 10–18.

The HLB of a relatively pure poly(oxyethylene) adduct can be calculated from theoreticaldata [37]. For these agents the HLB is an indication of percentage by mass of thehydrophilic portion, divided by five to give a conveniently small number. For example, if thehydrophilic portion of a purely hypothetical nonionic agent accounted for 100% of themolecule (such a product cannot, of course, exist), its HLB is 20. Similarly, a more plausibleproduct in which 85% of the molecule is accounted for by the hydrophilic portion has anHLB of 85/5 = 17. The ICI Americas Inc. method of calculating the theoretical HLB of asorbitan monolaurate nonionic having 20 oxyethylene units per molecule is given in

chpt9(2).pmd 15/11/02, 15:43493


Equation 9.1 (total relative molecular mass = 1226, of which 1044 is contributed by thehydrophilic portion) [37].

� � � �1044 1

HLB 100 17.01226 5

(9.1)

As explained earlier, however, the actual constitution of a surfactant rarely conforms to itsnominal structure. Consequently the theoretical method of calculation is of limited utility,practical methods being more reliable. The HLB value may be determined directly byanalysis or by comparison with a range of surfactants of known HLB values. An analyticalmethod for the sorbitan monolaurate described above uses Equation 9.2 [37].

� � � �� 45.5

HLB 20 1 20 1 16.7276

SA (9.2)

where S is the saponification number of the ester and A is the acid number of the recoveredacid. The saponification value of a product is the mass in milligrams of potassium hydroxiderequired to saponify one gram of the product; it can be found by saponification of theproduct with an excess of potassium hydroxide, followed by back-titration of the remainingalkali with hydrochloric acid. The acid value of an acid is the number of milligrams ofpotassium hydroxide required to neutralise a standard quantity, and can again be found bytitration. The comparative methods should always be used for the nonionic surfactants thatare not based on ethylene oxide, and also for ionic surfactants since the hydrophilicinfluence of the ionic group exceeds that indicated by the mass percentage basis (this canlead to apparent HLB values higher than 20).

Once the HLB values of a range of surfactants are known it is an easy matter to calculatethe HLB value of a mixture as follows:

Individual HLB Fractional HLB45% of surfactant A 16.7 0.45 × 16.7 = 7.5235% of surfactant B 4.0 0.35 × 4.0 = 1.4020% of surfactant C 9.6 0.20 × 9.6 = 1.92

Total HLB = 10.84

When preparing an emulsion, emulsification tends to be most efficient when the HLB of theagent matches that of the oil phase. Often a mixture of surfactants makes a more efficientemulsifying agent than a single product having the same HLB value as the mixture;similarly, if the oil phase to be emulsified is itself a mixture, its components will eachcontribute to the effective HLB value. It is this effective HLB that is the main criterion indesigning a suitable emulsifying system. The effective HLB value can be found by carryingout preliminary emulsification tests with agents of known HLB values. A useful procedure[37] uses two such emulsifying agents of widely differing HLB values mixed in variousproportions so as to give a range of intermediate HLB values. The HLB value of the mixturethat gives the best emulsion of the oil phase under test then corresponds to the effective

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495

HLB value of the oil phase. Further tests can then be carried out with different chemicaltypes of agents around this effective HLB value in order to find the optimum emulsifyingsystem.

9.8.4 Micelle formation

All surfactants in solution tend to form more or less ordered agglomerates of molecules,known as micelles. Pure water has a surface tension of about 72 × 10–3 N/m. As surfactantis added gradually to it, the surface tension falls quite rapidly (Figure 9.1) until, at a certainconcentration of surfactant, it begins to level off more or less sharply. At the point at whichthis levelling out takes place, the critical micelle concentration (CMC in Figure 9.1), thesurfactant molecules begin to orient themselves in clusters within the body of the solution,these clusters being more or less lamellar or spherical (Figure 9.2).

Concentration of surfactant/g l–1CMC

Sur

face

tens

ion/

N m

–1

Figure 9.1 Surface tension of water against surfactant concentration

Lamellar Spherical

Figure 9.2 Micelle formation

In water the surfactant molecules orient themselves with their hydrophobes at the centreof the cluster. The CMC is typically quite low, perhaps 0.5–0.2 g/l. At concentrations lowerthan this the molecules orient themselves only at the interfaces of the solution, and it is thiseffect which brings about the lowering of surface tension. Once the CMC is reached the


chpt9(2).pmd 15/11/02, 15:43495


interfaces become saturated and as the concentration increases micellar clusters ofmolecules begin to form in the bulk of the solution; there is little further reduction insurface tension beyond the CMC, nor are there changes in the other surfactant propertiessuch as wetting and foaming. In general, the CMC decreases with increasing size of thehydrophobe, and the CMCs of nonionic agents tend to be lower than those of ionic types,since with the nonionics micelles can form more easily in the absence of polar charges. Thisability to form micelles is vital to the efficacy of surfactants as emulsifying, dispersing andsolubilising agents.

REFERENCES 1. Colour terms and definitions (Bradford: SDC, 1988). 2. Kirk-Othmer encyclopedia of chemical technology, 3rd Edn., Vol. 22 (New York: Wiley, 1983). 3. M R Porter and M Porter, Handbook of surfactants, 2nd Edn. (Glasgow: Blackie Academic and Professional,

1994). 4. D R Karsa, J M Goody and P J Donnelly, Surfactants applications directory (Glasgow: Blackie Academic and

Professional, 1991). 5. M R Porter and M Porter, Recent developments in the technology of surfactants (Glasgow: Blackie Academic and

Professional, 1991). 6. K Y Lai, Liquid detergents (New York: Marcel Dekker, 1996). 7. M J Rosen, Surfactants and interfacial phenomena, 2nd Edn. (New York: Wiley, 1989). 8. H W Stache, Anionic surfactants: organic chemistry (New York: Marcel Dekker, 1995). 9. J M Richmond, Cationic surfactants: organic chemistry (New York: Marcel Dekker, 1990).10. V M Nace, Nonionic surfactants: polyoxyalkylene block copolymers (New York: Marcel Dekker, 1996).11. E G Lomax, Amphoteric surfactants, 2nd Edn. (New York: Marcel Dekker, 1996).12. I Piirma, Polymeric surfactants (New York: Marcel Dekker, 1992).13. E Kissa, Fluorinated surfactants: synthesis, properties, applications (New York: Marcel Dekker, 1993).14. A S Davidsohn and B Milwidsky, Synthetic detergents, 7th Edn. (Harlow: Longman, 1987).15. G Bevan, Rev. Prog. Coloration, 27 (1997) 1.16. D R Karsa, Rev. Prog. Coloration, 20 (1990) 70.17. Summary of Aachen conference, Alkylbenzenesulphonates in the environment, J. Amer. Oil Chem. Soc., 66

(1989) 748; full proceedings in Tenside Surf. Det., (Apr/May 1989).18. A J O’Lenick and J K Parkinson, Text. Chem. Colorist, 27 (Nov 1995) 17.19. R H Mehta and A R Mehta, Colourage, 43/45 (1996) 49.20. A Riva and J Cegarra, J.S.D.C., 103 (1987) 32.21. H Egli, Textilveredlung, 8 (1973) 495.22. W Mosimann, Text. Chem. Colorist, 1 (1969) 182.23. J Cegarra, Proc. IFATCC, Barcelona (1975).24. J Cegarra, A Riva and L Aizpurua, J.S.D.C., 94 (1978) 394.25. J Cegarra and A Riva, Melliand Textilber., 64 (1983) 221.26. D R Karsa and M R Porter, Biodegradability of surfactants (Glasgow: Blackie Academic and Professional, 1995).27. M J Schwuger, Detergents in the environment (New York: Marcel Dekker, 1996).28. S S Talmage, Environmental and human safety of major surfactants (London: Lewis Publications, 1994).29. C Gloxhuber and K Kunstler, Anionic surfactants: biochemistry, toxicology, dermatology, 2nd Edn. (New York:

Marcel Dekker, 1995).30. P Schöberl, K J Bock and L Huber, Tenside Surf. Det., 25 (1988) 86.31. C G Naylor, Text. Chem. Colorist, 27 (Apr l995) 29.32. K Tsujii, Surface activity: principles, phenomena and applications (San Diego: Academic Press, 1998).33. J C Berg, Wettability (New York: Marcel Dekker, 1993).34. A K Chattopadhyay and K L Mittal, Surfactants in solution (New York: Marcel Dekker, 1996).35. S D Christian and J F Scamehorn, Solubilisation in surfactant aggregates (New York: Marcel Dekker, 1995).36. A W Neumann and J K Spelt, Applied surface thermodynamics (New York: Marcel Dekker, 1996).37. The HLB system – a time-saving guide to emulsifier selection, Publication 103–3 10M (Wilmington: ICI Americas,

1984).

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497

CHAPTER 10

Classification of dyeing and printing auxiliariesby function


10.1 ELECTROLYTES AND pH CONTROL

The simplest auxiliaries of all are the neutral electrolytes such as sodium chloride andsodium sulphate. These are used in large quantities for dyeing cellulosic materials withdirect or reactive dyes and wool with anionic dyes. The major effect of electrolytes on dyesof this type is to increase the degree of aggregation of the dye anions in solution by thecommon-ion effect, the degree of aggregation varying markedly with dye structure (section3.1.2). The electrolyte suppresses ionisation of the dye in solution, thereby effectivelyreducing its solubility in the dyebath and modifying the equilibrium in favour of movementof dye anions from the solution into the fibre. The objective, of course, is to use theoptimum amount of salt to give the required rate and degree of exhaustion of the dyebath;too little electrolyte is ineffective whilst too much may aggregate the dye to an extent thatmay inhibit its diffusion into the fibre, thus giving a tendency to surface coloration only, oreven bringing about precipitation. The aggregating effect of electrolytes varies, sodiumchloride having a stronger effect than sodium sulphate, but it is generally decreased byraising the temperature.

This effect, which we may term the ‘salting-on’ effect, is the result of interactionsbetween electrolyte and dye. However, there may also be interactions between electrolyteand fibre, giving rise to a positive levelling action as electrolyte anions compete with dyeanions for the cationic sites in the fibre. Ionic surfactants (Table 8.1) can of course beregarded as electrolytes, although by hydrophobic interactions they tend to form micelles inconcentrated solution and hence may be referred to as colloidal electrolytes. In somerespects their levelling action is analogous to that of simple inorganic electrolytes – that is,ionic hydrophobes compete with dye ions of similar charge for sites of opposite charge in thefibre.

Electrolytes are used to promote the exhaustion of direct or reactive dyes on cellulosicfibres; they may also be similarly used with vat or sulphur dyes in their leuco forms. In thecase of anionic dyes on wool or nylon, however, their role is different as they are used tofacilitate levelling rather than exhaustion. In these cases, addition of electrolyte decreasesdye uptake due to the competitive absorption of inorganic anions by the fibre and a decreasein ionic attraction between dye and fibre. In most discussions of the effect of electrolyte ondye sorption, attention is given only to the ionic aspects of interaction. In most cases, thisdoes not create a problem and so most adsorption isotherms of water-soluble dyes areinterpreted on the basis of Langmuir or Donnan ionic interactions only. There are, however,some observed cases of apparently anomalous behaviour of dyes with respect to electrolytesthat cannot be explained by ionic interactions alone.

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498 CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

The fact is, ionic interaction between dyes, fibres and electrolytes is only part of the story.As Yang [1] has pointed out, hydrophobic interactions also need to be taken intoconsideration. Whilst this has been accepted for many years in relation to dye–fibreinteractions, the extension of the concept to interactions involving neutral electrolytes isnovel.

Yang quotes as one of several examples the fact that sodium chloride has a stronger effectthan sodium sulphate on decreasing the uptake of CI Acid Red 1 by nylon, which cannot beexplained on the basis of ionic interaction alone. It can, however, be explained in terms ofthe effect of the electrolytes on hydrophobic interaction, the same explanation also beingapplied to other examples. A lyotropic series is used to explain the effectiveness ofhydrophobic interactions, which always coexist with ionic interactions. A semi-quantitativerepresentation of the lyotropic series is shown in Figure 10.1. In such a series, neutral ionshave little influence on hydrophobic interactions. Kosmotropes increase hydrophobicinteraction and therefore tend to increase dye adsorption, whilst chaotropes decrease bothhydrophobic interaction and dye adsorption. Thus, in the example quoted above of CI AcidRed 1 on nylon, sodium chloride has a stronger effect on decreasing dye adsorption than themore kosmotropic sodium sulphate. On this basis, Yang has introduced a modified Donnanmodel that quantitatively predicts the various effects of electrolytes on either decreasing orincreasing dye adsorption.

Figure 10.1 Lyotropic series: effectiveness of hydrophobic interactions [1]

Rather more complex compounds that are currently being researched are the bolaformelectrolytes [2–4]. Bolaform electrolytes are organic compounds possessing two cationic ortwo anionic groups linked by a flexible hydrocarbon chain; the terminal groups may bealiphatic or aromatic (e.g. as in 10.1). Their interaction with sulphonated monoazo dyes inthe presence of poly(vinylpyrrolidone) as substrate has been studied in detail. However, itremains to be seen what commercial developments take place with these interesting

R N (CH2)n

CH3

CH3

N R

CH3

CH3

X X

10.1

++

__n = 3–12R = n-propyl, n-butyl or benzylX = halide, e.g. bromide

anions SO42– > CH3COO– > Cl– > Br– NO–

3 SCN–

cations Li+ > Na+ > K+ > Rb+ Cs+

kosmotropes chaotropes

(waterstructure-makers)

(waterstructure-breakers)

NEUTRAL

H2PO–4

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499

compounds. It seems likely that they would be used in complex formation rather than in themore traditional roles associated with electrolytes in textile processing.

In whatever role electrolytes are used, their effects on the environment need to beconsidered, particularly when discharged to effluent. High salt loading is undesirable inwaste water and sodium sulphate in particular causes corrosion of concrete pipes. It thusmakes sense to choose electrolytes carefully and to use the minimum amounts consistentwith obtaining the desired effects. Automatic dosing is helpful in this respect. The use ofshorter liquor ratios has been promoted on the grounds of economy (less water to heat andless liquor to treat subsequently). However, it should not be overlooked that when dyeing ina short liquor more rinsing baths are required to give the same residual concentration as at alonger liquor ratio. Weible [5] has demonstrated this effect for the washing-off of reactivedyes from fabric having a retention capacity of 4 l/kg using 60 g/l of electrolyte. At a liquorratio of 20:1, some 12 g/l and 2.4 g/l of electrolyte are found in the first and second rinsesrespectively. These rise to 30 g/l and 15 g/l when a liquor ratio of 8:1 is used. Equation 10.1was used by Weible to calculate these concentrations.

�scR

C (g/l)FV

(10.1)

Cs = concentration in rinsing bath (g/l)c = concentration in treatment bath (g/l)FV = liquor ratio (l/kg)R = retention capacity of goods (l/kg)

More detailed information on attempts to reduce the impact of electrolytes on theenvironment is given under the individual dye classes discussed in Chapter 12.

The great majority of coloration processes demand some control over the treatment pH,which varies from strongly alkaline in the case of vat, sulphur or reactive dyes, to stronglyacidic for levelling acid dyes. The concept of pH is a familiar one; its theoretical derivationcan be found in all standard physical chemistry textbooks and has been particularly wellexplained in relation to coloration processes [6,7] both in theory and in practice. We areconcerned here essentially with the chemistry of the products used to control pH and theirmode of action. It has been stated [7] that: ‘Unfortunately, pH control appears simple andeasy to carry out. Add acid and the pH decreases; add base (alkali) and the pH increases.However, pH is the most difficult control feature in any industry’.

The control of pH in textile coloration processes is ensured by three fundamentallydifferent techniques:(a) the maintenance of a relatively high degree of acidity or alkalinity(b) the control of pH within fairly narrow tolerances mainly in the near-neutral region(c) the gradual shifting of the pH as a dyeing proceeds.

Approach (a) is normally the easiest to control, and is used in the application of levellingacid and 1:1 metal-complex dyes to wool or nylon, and of the reactive, sulphur or vat dyes tocellulosic fibres. The agents traditionally used are the stronger acids and alkalis such assulphuric, hydrochloric and formic acids, sodium carbonate and sodium hydroxide. In

ELECTROLYTES AND pH CONTROL

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certain operations, particularly fixation in steam (as in printing), steam-volatile acids arereplaced with non-volatile products such as citric acid. Use of approach (a) can lead to themisconception already mentioned, that pH is easy to control, particularly as the dye–substrate systems involved are not normally sensitive to minor pH shifts. Nevertheless,strong acids and alkalis can react to produce quite drastic changes in pH; this can occur, forexample, when alkali is carried over from wool scouring into initially acidic dyebaths.

Wool and nylon absorb acid from dyebaths, thereby inducing a change in the dyebath pH,but wool will absorb significantly more acid than nylon [8] – a factor to be borne in mindwhen comparing results on these two fibres, especially in those systems using ‘half-milling’acid dyes for which the controlling agent is generally the weaker acetic acid; such systemsrepresent a compromise between approaches (a) and (b) and are moderately sensitive tochange in pH. In this area, the organic formic and acetic acids are of interest. Formic, ofcourse, is a stronger acid than acetic. Hence, acetic acid has been traditionally the preferredchoice for the adjustment of slightly acidic media, down to about pH 4, whereas formic wasthe choice below this level. It has been demonstrated, however, that for general purposesformic acid is preferred to acetic acid, particularly on economical and environmentalgrounds [9]. Formic acid has an extremely low BOD, being biodegraded to carbon dioxideand water. In any case, being a much stronger acid, smaller amounts are needed, thus givingless load for disposal. For example, in order to obtain pH values of 4.5, 4.0 and 3.3, theamount of formic acid 85% needed is, respectively, about 62%, 50% and 12% of that ofacetic acid 80%. Other advantages of formic acid are that it has a more powerfulneutralising effect than acetic acid and it is less corrosive than mineral acids.

Approach (b) needs greater awareness of the factors that not only determine pH but alsohelp to stabilise it against interference. Most of the dye–fibre systems requiring approach (b)are operated in the near-neutral region (pH 4–9) and are much more sensitive to minorchanges in pH. In addition, the pH of the water supply may vary, or drift during heating.Even the pH of pure water changes on heating, from 7.47 at 0 °C to 7.00 at 24 °C and 6.13at 100 °C, but that of the process water used in dyehouses and printworks can change muchmore drastically, most commonly showing an increase. Changes in pH on heating maycounteract the intended response of process liquors, especially in the central pH rangeassociated with approach (b); even more critical can be the effect of any acids or alkaliscarried over from previous processes.

The dye–fibre systems of obvious interest for approach (b) are milling acid and 1:2 metal-complex dyes on wool or nylon, basic dyes on acrylic fibres and disperse dyes on variousfibres. With wool and nylon there is often some overlap with approach (c) (section 12.2).

Where control is not too critical, simple electrolytes of weak bases with strong acids(such as ammonium sulphate) or strong bases with weak acids (such as sodium acetate) areoften used to produce slightly acidic or slightly alkaline media respectively. Ammoniumacetate is also commonly used, producing a less acidic effect than ammonium sulphate.Occasionally acetic acid and sodium carbonate are used, necessitating careful control andmonitoring. These simple expedients are not suitable for systems requiring more sensitivecontrol, however, and use of single electrolytes such as ammonium sulphate or sodiumacetate more properly belong to control systems based on approach (c). More precise controlis achieved by the use of buffering systems. By the use of electrolyte pairs, these systems setthe initial pH and exert a protective action that tends to resist changes arising fromcontaminants entering by way of the substrate or the water supply.

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Buffering systems are generally based on combinations of:– a weak acid together with the salt of this acid formed from a strong base, or– a weak base together with the salt of this base formed from a strong acid.

The most commonly used example of the first type is acetic acid/sodium acetate, whichfunctions well over the pH range 3.8–5.8. Acetic acid with ammonium acetate is also usedalthough it is less effective, especially in those boiling dyebaths from which ammonia canescape into the atmosphere, thus allowing the pH to fall. Such acetate buffers have theadvantage of low cost. Somewhat more expensive are the phosphate buffers, of which themost commonly used is a mixture of sodium dihydrogen orthophosphate (NaH2PO4) withdisodium hydrogen orthophosphate (Na2HPO4). Here, as with most polybasic acid systems,the distinction between the acid and its salt seems blurred at first sight. In fact, sodiumdihydrogen phosphate is the ‘acting acid’ and disodium hydrogen phosphate is its salt. Thetribasic orthophosphoric acid and its three salts can be used to produce a series of buffers,each active within a particular pH range:– orthophosphoric acid and the monosodium salt, main buffering region pH 2.5–3.5– the mono- and di-sodium salts, main buffering region pH 6–8– the di- and tri-sodium salts, main buffering region pH 10.5–11.

This can be seen from the titration curve for phosphoric acid [6] shown in Figure 10.2. Inpractice the mono- and di-sodium salt system is used most extensively, since this covers thepH range over which precise control is most often needed. These phosphate buffers aremore resistant than the acetate systems to temperature-induced changes.

2

4

6

8

10

12

Alkali added

pH

Mainbuffer region

Na3PO4

Na2HPO4

NaH2PO4

H3PO4

pH 6.2–8.2

Figure 10.2 Orthophosphate buffer system

The most common buffering system containing a weak base together with its salt formedwith a strong acid is ammonia with ammonium sulphate. Some useful buffers are obtainedfrom combinations of unrelated acids or bases with salts. The following combinations findoccasional use in textile coloration processes, but the acetates and orthophosphates are mostfrequently used:


chpt10(2).pmd 15/11/02, 15:44501


– pyrophosphoric acid (H4P2O7) and its salts (pH 3–9)– orthoboric acid (H3BO3), sodium tetraborate (borax Na2B4O7) and sodium hydroxide

(pH 8.1–10.1)– citric acid and sodium hydroxide (pH 2.1–6.4)– sodium carbonate and sodium bicarbonate (pH 9.3–11.3).

The pyrophosphate buffer is of particular technical interest as it can be used over therelatively wide range of pH 3–9. Unlike the orthophosphate titration curve, that for thetetrabasic pyrophosphate system is almost straight [6]. This linearity (Figure 10.3) meansthat effective buffering action is available across the whole pH range simply by using variouspairs of ionised components and varying their proportions; even so, however, it does notseem to be widely used.

3

5

7

9

pH

Strong acid added

Na4P2O7

Figure 10.3 Pyrophosphate buffer system

The mechanism of buffering can be described by reference to the acetic acid/sodiumacetate system. In aqueous solution sodium acetate can be considered to be practicallycompletely ionised (Scheme 10.1), the equilibrium being wholly to the right-hand side.Since acetic acid is a weak acid it is only slightly ionised, and the equilibrium represented byScheme 10.2 lies mainly to the left-hand side. This low degree of ionisation is even furthersuppressed in the presence of sodium acetate as a result of the common ion (in this caseacetate) effect operating through the law of mass action. The undissociated acetic acid is, ineffect, a ‘bank’ of hydrogen and acetate ions that can be brought into play as a neutralisingmechanism when either acidic or alkaline chemicals enter the system (either by deliberateaddition or adventitiously). If a small amount of an acidic solute is added to the mixture, theadded hydrogen ions combine with acetate ions to form undissociated acetic acid, which hasonly a minimal effect on the pH of the system. If an alkaline solute is added to the buffer,the added hydroxide ions react with the bank of hydrogen ions to form undissociated waterand so again the ionic balance and hence the pH remain essentially the same. Themechanisms of other buffering systems are similar: buffering action is increased by addingmore of the components, keeping their proportions constant.

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503

Approach (c) for pH control involves a deliberate shift of pH during the processing cycle,in a consistent direction rather than randomly. Systems of this type are particularly useful fornon-migrating anionic dyes on wool or nylon and have long been known in this connection.More recently, similar systems have been adopted for reactive dyes on cellulosic fibres. Thesimplest and most widely used of these systems consist of the salts of strong acids with weakbases or of strong bases with weak acids, examples being ammonium sulphate and sodiumacetate respectively. Ammonium sulphate, for instance, dissociates in aqueous media toyield the dominant strong-acid species of sulphuric acid, so lowering the pH (Scheme 10.3)at a rate that increases with temperature, especially when the ammonia formed can bereleased from an open dyebath. Ammonium acetate functions in the same way but does notyield as great a pH shift. Similarly, a solution of sodium acetate tends to produce thedominant strong-alkali species of sodium hydroxide (Scheme 10.4), thus increasing the pH.

CH3COONa Na + CH3COO

Scheme 10.1

CH3COOH H + CH3COOScheme 10.2

(NH4)2SO4 + 2H2O H2SO4 + 2NH4OH

2H + SO42–

NH4OH NH3 + H2O

Scheme 10.3

CH3COONa + HOH NaOH + CH3COOH

Na + OHScheme 10.4

Acetic acid (b.p. 118 °C) is not boiled off from open dyebaths as readily as ammonia butis rapidly flashed off in steam or dry heat processes, thus developing the maximum degree ofalkalinity under these conditions. The sodium salts of less volatile acids, such as sodiumcitrate, can be used to develop a lower degree of alkalinity.

If the process demands a gradual shift from about pH 9 to a slightly acidic pH, ammoniumsulphate together with ammonia can be used. This gives a safer, more uniform developmentof acidity than can be achieved by making additions of acid to an alkaline bath, although thedegree of acidity developed will clearly depend on the ease with which ammonia can escapefrom the system. In enclosed or partially enclosed machines this system does not function soefficiently [10–12].


chpt10(2).pmd 15/11/02, 15:44503


Another method of obtaining a pH shift in the direction of acidity is to use an organicester that hydrolyses to the alcohol and acid under the conditions of processing. Ethyllactate (Scheme 10.5) and diethyl tartrate (10.2) have been recommended for applyingmilling and chrome dyes to wool [13]. 2-Hydroxyethyl chloroacetate (10.3) andγ-butyrolactone (10.4), which hydrolyses during processing to give 4-hydroxybutyric acid(Scheme 10.6), have also been recommended. Such hydrolysable esters may be used alone,beginning at a near-neutral pH, but more likely in conjunction with an alkali to give ahigher starting pH. Thus γ-butyrolactone and sodium tetraborate (borax), giving a pH shiftfrom about 8 to 5.6, have been recommended for the dyeing of wool [14], as has2-hydroxyethyl chloroacetate with sodium hydroxide for the dyeing of nylon [15]. Suchhydrolysable esters are sometimes sold under proprietary trade names. The disadvantages ofhydrolysable esters have been their higher cost, a limited pH range and, where the dyebathis to be reused, the need for increasing quantities of ester to overcome the buffering effectcaused by the accumulation of salts [7,16]. Interest in these systems has declined due toenvironmental pressures on the one hand and the increased availability and sophistication ofautomatic dosing and monitoring systems on the other.

CH

HO

H3C

C

OCH2CH3

O

CH

HO

H3C

C

OH

OH2O

+ HOCH2CH3

Scheme 10.5

C

O CH2

CH2

CH2O

HO

HOCH2

CH2

CH2

C

O

H2O

10.4Scheme 10.6

HC

HC

HO COCH2CH3

O

HO COCH2CH3

O

10.2

O

CClH2C OCH2CH2OH

10.3

The advantages of automatic metering and monitoring devices were well described byMosimann [17] and have recently been re-emphasised [9]. Such devices are clearly of greatvalue in environmental terms since they are crucial in ensuring that the minimum quantityof agent is used, thus reducing the effluent load to a minimum. The use of strong acids andbases for control of pH-shift systems is obviously fraught with difficulties where theoperation is carried out manually. If a sophisticated automatic monitoring and dosing systemis used, however, the use of such compounds has certain very worthwhile advantages:

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505

(a) The adjusting chemicals are the cheapest available;(b) The entire range of pH values can be controlled by using just two chemicals;(c) Since a buffering system is not built up in the bath, the pH can be shifted in any

direction to any degree, easily and with the minimum addition of chemicals;(d) As a result of (c), any variation in the intrinsic pH of the substrate or water supply can

be easily neutralised;(e) Exhausted dyebaths can often be reused as there is no build-up of buffering agent; and(f) There are no environmental problems.

It should be noted that automatic dosing and monitoring of dyes and chemicals can be usedgenerally and is not restricted to pH control.

SEQUESTERING AGENTS

10.2 SEQUESTERING AGENTS

The tendency of soaps and other carboxylates to form insoluble complexes with calcium andmagnesium ions in hard water is mentioned in sections 9.4 and 9.8.2. Apart from decreasingthe efficiency of the anionic surfactant, deposition of such insoluble complexes on thetextile substrate can cause problems in subsequent processing and particularly in coloration.Even trace amounts of certain transition-metal or alkaline-earth elements may causeprocessing difficulties. The formation of ‘iron spots’, particularly in bleaching, is well known:multivalent transition-metal cations catalyse the decomposition of hydrogen peroxide(although divalent calcium and magnesium ions have a stabilising effect) and localisedstaining or tendering of the fibre may occur. In coloration trace-metal ions can react withcertain dyes, giving rise to precipitation, discoloration, unlevel dyeing and reduced fastness.

The processing water is the most obvious source of such extraneous metal ions, but otherpotential sources should not be overlooked. For example, trace metals may be dissolved fromthe surfaces of machinery and fittings. The substrate may already contain such metals, asmay also any chemicals or dyes used. Hence these problems cannot always be avoided simplyby ensuring the supply of suitable water – indeed, the overzealous treatment of water canactually lead to the presence of troublesome aluminium ions that were not originallypresent! Such problems can be solved using chemicals that react preferentially with themetal ions, effectively preventing them from interfering with the mainstream reaction orprocess. Such chemicals are aptly known as sequestering agents. Other terms frequentlyused in the literature include the derivative ‘sequestrants’ and ‘complexing agents’, althoughcomplexing does cover a wider field than just metal–ion chelation with which we areconcerned here.

Sequestering agents work by a mechanism of complex formation, often in the form ofchelation. A chelating agent contains substituents suitably located to form one or morechelate rings by electron donation to the metal ion (section 5.2), the resulting complexremaining soluble and innocuous under the conditions of processing. The most usefuldonating atoms are nitrogen, as found in amines or substituted amines, and oxygen in theform of carboxyl, phosphate or ionised hydroxy groups. As in the formation of dye–metalchelates (such as chrome mordant and metal-complex dyes), at least two electron-donatingatoms in the sequestering agent structure must be arranged so that a stable ring can beformed with the metal ion, the highest stability resulting from five- and six-membered rings.

chpt10(2).pmd 15/11/02, 15:44505


A great many chemicals exhibit sequestering capability but not all are of commercialvalue in textile processing. Earlier literature [18,19] mentioned three main types:– aminopolycarboxylates– phosphates, mainly inorganic– hydroxycarboxylates.

However, environmental awareness, in addition to commercial and technical exploitation,has resulted in considerable activity in this area, leading to a greatly expanded range ofproducts in recent years, as well as some conflicting statements with regard to theirenvironmental properties. The scheme of classification adopted here is as follows:– aminopolycarboxylates and their analogues, e.g. hydroxyaminocarboxylates– phosphates and phosphonates– hydroxycarboxylates– polyacrylic acids and derivatives.

10.2.1 Aminopolycarboxylates and their analogues

These are powerful chelating agents, having good environmental properties [20]. Importantmembers include:– ethylenediaminetetra-acetic acid EDTA (10.5)– diethylenetriaminepenta-acetic acid DTPA (10.6)– nitrilotriacetic acid NTA (10.7)

These products are sold as free acids or sodium salts. Analogues of these amino-polycarboxylic acids include the hydroxyaminocarboxylic acids. These structures are derived

N CH2CH2 N

O

CO CH2

O

COH2C

O

COH2C

O

CO CH2Na

Na

Na

Na

10.5

+

+_

_

_

_

+

+

EDTA

CH2

NCH2CH2 CH2CH2

C

O

O

N

O

CO CH2

O

CO CH2

N

O

COH2C

O

COH2C

Na

Na

Na

Na

Na

10.6

_

_

+

+

_

_ +

+

_+

DTPA

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507

by replacing one or more carboxymethyl groups of the aminopolycarboxylate by ahydroxyethyl group. Examples include:N-(hydroxyethyl)ethylenediaminetriacetic acid HEDTA (10.8)

in which one of the carboxymethyl groups of EDTA has been replaced by a hydroxyethylgroup

N,N-bis(hydroxyethyl)glycine DEG (10.9)in which two of the carboxymethyl groups of NTA have been replaced by hydroxyethylgroups.

N

O

CO CH2

O

CO CH2

CH2 C

O

O

Na

Na Na

10.7

+

+_

_

_ +

NTA

CH2N

CH2

CH2

CH2

CH2

HO

HO

C

O

O

Na

10.9

_ +

DEG

These compounds are not persistent in the environment, NTA degrading slightly morequickly than EDTA, DTPA or HEDTA [20].

These aminopolycarboxylates act as sequestering agents by forming complexes in whicheach metal ion is chelated into one or more five-membered rings. It is often assumed thatone molecule of sequestering agent interacts with one metal ion and for many practicalpurposes this is a valid assumption. The nature of the complexes actually formed, however,may depend on other factors such as the pH of the medium. It is difficult to represent suchstructures in detail, particularly as water of solvation is usually involved. It is convenient toadopt a simplified representation, omitting the water of solvation, as for the EDTA–calciumcomplex shown in structure 10.10, in which the arrows represent coordination bonds andthe calcium ion is held by three five-membered rings. At pH values below 11 the structuretends to be more like that shown in 10.11, which also resembles the complex formed withNTA (10.12).

C

H2C

OOC

CH2

O O

CH2CH2

N N

Ca2+

H2C CH2C

O

O

C

O

O

Na Na10.10

+_

_ _

+_

SEQUESTERING AGENTS

N

CH2

HO CH2

O

CO CH2

CH2CH2 N

O

COH2C

O

COH2C

Na Na

Na

10.8

+ _

_

_

+

+

HEDTA

chpt10(2).pmd 15/11/02, 15:44507


A more elaborate representation of an EDTA-metal complex (10.13), which gives someindication of the three-dimensional aspects of the structure, shows a complex of five five-membered rings [18]. A similar representation of a DTPA-metal complex shows a system ofeight five-membered rings.

N CH2CH2

CH2

CH2C

O

O

C

O

O

Ca2+ N

H2C

H2CC

O

O

C

O

O

Ca2+

10.11

_

_

_

_

N CH2

CH2

CH2C

O

O

C

O

O

Ca2+ C

O

O

Na

10.12

_

_

_+

O N

NO

M CH2

CH2

O

O

C

CH2C CH2

CH2

C

CH2C

O

O

O

O

10.13

10.2.2 Phosphates and phosphonates

Various polyphosphates are effective sequestering agents under appropriate conditions. Thebest known of these is sodium hexametaphosphate (10.14), the cyclic hexamer of sodiumorthophosphate. Further examples are the cyclic trimer sodium trimetaphosphate (10.15), aswell as the dimeric pyrophosphate (10.16), the trimeric tripolyphosphate (10.17) and otherlinear polyphosphates (10.18). All of these polyanions function by withdrawing thetroublesome metal cation into an innocuous and water-soluble complex anion by a processof ion exchange as shown in Scheme 10.7 for sodium hexametaphosphate. Hence thesecompounds are sometimes referred to as ion-exchange agents.

The disadvantage of the polyphosphates is that at the temperatures (100 °C or higher)used in many textile processes they can be hydrolysed into simpler phosphates that cannotretain the metal atom in the sequestered form. For example, dicalcium disodiumhexametaphosphate hydrolyses on prolonged boiling to yield the insoluble calciumorthophosphate. This is one of the main reasons why polyphosphate sequestrants are usedmuch less extensively than the more versatile and stable aminopolycarboxylates.

chpt10(2).pmd 15/11/02, 15:44508

509

A structural compromise between these two types of compound can also give productswith sequestering properties, although they are phosphonates rather than phosphates, sincethey contain C–P rather than C–O–P linkages. Examples of these aminopolyphosphonatesare:– ethylenediaminetetramethylphosphonic acid EDTMP (10.19)– diethylenetriaminepentamethylphosphonic acid DETMP (10.20)– nitrilotrimethylphosphonic acid ATMP (10.21)– hydroxyethylethylenediaminetrimethylphosphonic acid HEDTMP (10.22)– hexamethylenediaminetetramethylphosphonic acid HMDTMP (10.23)

Two sequestrants of the phosphonate class unrelated to aminopolycarboxylic acids are:1-hydroxyethane-l,1-diphosphonic acid HEDP (10.24)2-phosphonobutane-1,2,4-tricarboxylic acid PBTC (10.25)

PO

P

OP

O O

P PO O

P

O

OO

O

O O

OO

O O

O O

PO

P

OP

O O

P PO O

P

O

OO

O

O O

OO

O O

O O

Na

Na

Na

Na

Na

Na

Na

Ca2+ Ca2+

Na

10.14

+ 2 Ca2+ + 4 Na

_+

+_

+_

_+

+_

_

_+

_

_

_

_

_+

+

Sodium hexametaphosphate

Scheme 10.7

O

PO

P

OP

O O

O

O

O

O

Na

Na

Na

10.15

+ _

_+

+_

Sodium trimetaphosphate

O P O P O

O O

O O

Na

Na Na

Na

10.16

+ _ _+

_ _+ +

Sodium pyrophosphate

O P O P O

O O

O O

PO

O

O

Na Na

Na NaNa

10.17

++ __

+ +_ __ +

Sodium tripolyphosphate

O P O P O

O O

O O

PO

O

O

Na Na

NaNa Na

n

10.18

+ _

+_ _ + _

_ +

+

Sodium polyphosphate

SEQUESTERING AGENTS

chpt10(2).pmd 15/11/02, 15:44509


H2C P OH

OH

O

H2C P OH

O

OH

HO P CH2

HO

O

HO P CH2

O

HONCH2CH2N

10.19

EDTMP

CH2

NCH2CH2

PHO

O

HOHO P CH2

HO

O

HO P CH2

O

HOCH2CH2N

H2C P OH

OH

O

H2C P OH

O

OHN

10.20

DETMP HO P CH2

HO

O

HO P CH2

O

HON CH2 P OH

O

OH

10.21

ATMP

H2C P OH

OH

O

H2C P OH

O

OH

HO P CH2

HO

O

CH2

NCH2CH2N

CH2

HO

10.22

HEDTMP

H2C P OH

OH

O

H2C P OH

O

OH

HO P CH2

HO

O

HO P CH2

O

HON CH2CH2CH2CH2CH2CH2 N

10.23

HMDTMP

HO P C

O

HO

P

OH

CH3

OH

OH

O

10.24

HEDP

C CH2CH2

HO

O

C

CH2

C

P OH

O

OH

OHO

10.25

PBTC

C

OH

O

chpt10(2).pmd 15/11/02, 15:44510

511

The consumption of these phosphonates in textile processing is small in relation to that ofthe aminopolycarboxylates; they are mainly used in detergent formulations [21,22] assodium, potassium, ammonium or alkanolamine salts.

Environmentally, phosphates generally have been a sensitive issue, not least because theycan cause eutrophication of watercourses, and the situation is still not resolved completely.No aerobic or anaerobic bacterium has been found to date that will biodegradeaminopolyphosphonates under the treatment conditions used today, yet these products arenot biologically persistent. They are partially eliminated photolytically, partially absorbed insediment or eliminated by precipitation [23,24]. They show 50–80% elimination in theZahn–Wellens test. They show low aquatic toxicity and are non-toxic to humans, animalsand plants. Detailed ecological properties are listed by Schöberl and Huber [25]. Held [26]has investigated the ecological behaviour of sequestering agents based on phosphonic acidsin detail, concluding that although they contain phosphorus they do show ecologicaladvantages compared with other types and thus their use is justified.

10.2.3 Hydroxycarboxylates

The hydroxycarboxylic acids provide a range of sequestering agents of which the best knownare citric (10.26), tartaric (10.27) and gluconic (10.28) acids. The toxic oxalic acid (10.29)is now rarely used. However, these acids are much less important as sequestering agents fortextile processes than either the aminopolycarboxylates or the polyphosphates.Hydroxycarboxylates are easily biodegraded but do have a high COD. It has been pointedout [20] that glucoheptanoic acid (2,3,4,5,6,7-hexahydroxyheptanoic acid; 10.30) is alsoused in the USA, on the grounds that this compound is less prone to browning at hightemperatures although it possesses no other advantages, having less binding power as well asbeing more expensive than other hydroxycarboxylic acids.

C

HO

O

C

OH

C

C

H

H OH

O

10.26Citric acidHO

C

H

H

CC C

HO

O

C

H

OH H

OH

C

OH

O

10.27Tartaric acid

HO C C C C C C

OH

OH

H

H

OH

OH

H

H

OH

OH

H

10.28Gluconic acid

C C

HO

O

OH

O

10.29

Oxalic acid

HO C C C C C C C

OH

OH

H

H

OH

OH

H

H

OH

OH

H

H

OH

10.30

SEQUESTERING AGENTS

chpt10(2).pmd 15/11/02, 15:44511


10.2.4 Polyacrylic acids and their derivatives

Recent research has led to some more complex sequestering agents, particularly thepolymeric carboxylic acids referred to as polycarboxylates. These are, in effect,polyelectrolytes and as such have close similarities to the products described later asdispersing and solubilising agents (section 10.6), thickening agents or migration inhibitors(section 10.8). Common monomers used in the production of these compounds, either ashomopolymers or as copolymers with each other, include acrylamide (10.31) and variousunsaturated acids (10.32–10.34). The common and essential feature of these monomers isthe carbon–carbon double bond.

H2C C

CONH2

H

10.31

Acrylamide

H2C C

COOH

H

10.32

Acrylic acid

H2C C

COOH

CH3

10.33

Methacrylic acid

C C

COOH

H

HOOC

H

10.34Maleic acid

Polymers which have been suggested for use as sequestering agents [23,27] include:– poly(butadiene-1,2-dicarboxylic acid) EMA (10.35)

which is an ethylene–maleic acid copolymer– poly(α-hydroxyacrylic acid) PHAS (10.36)– poly(3-hydroxymethylhexatriene-1,3,5-tricarboxylic acid) (10.37)

which is a copolymer of acrylic acid with hydroxymethacrylic acid– poly(3-formylhexatriene-1,3,5-tricarboxylic acid) (10.38)

which is a copolymer of acrylic acid with 2-formylacrylic acid.

CH2 CH2 CH CH

HOOC COOHn

10.35

EMA

CH2 C

OH

COOHn

10.36

PHAS

CH2 CH CH2 C CH2 CH

COOHCOOH

CH2OH

COOH n

10.37

CH2 CH CH2 C CH2 CH

COOHCOOH

CHO

COOH n

10.38

chpt10(2).pmd 15/11/02, 15:44512

513

These agents are generally described as ion-exchange reagents rather than complex-formingchemicals. They tend to operate by sequestering the metal by an ion-exchange mechanismand as a result of their polyelectrolytic character they keep the complex dispersed.Oligomers with a molecular mass of 1200–8000 (i.e. relatively low) are said to give optimumsequestering power. Polymers of high molecular mass (i.e. Mr = 106–107) are useful asflocculating agents or migration inhibitors.

Although these acrylic oligomers and polymers show little decomposition in effluenttreatment, they pose no significant threat to the environment since they can be removedeasily by adsorption on activated sludge or by precipitation as an insoluble calcium complex[23]. Exhaustive tests have not revealed any adverse environmental influence. Their aquatictoxicity is negligible and toxicity to warm-blooded mammals is slight. Mutagenic,carcinogenic or teratogenic effects have not been found so far.

A further development [27] is the formation of so-called sugar–acrylate copolymers inwhich acrylic acid is copolymerised with glucose or other saccharides. Unlike othersequestering agents these polymers are said to be readily biodegradable, this being the mainreason for their development.

An unusual type of sequestering agent is triethanolamine (10.39). This compound ischeap and exclusively useful for complexing iron(III) in strongly alkaline solutions, e.g. up to18% sodium hydroxide. It does in fact remain active as a complexing agent even in morestrongly alkaline solutions although solubility can be a problem.

HOCH2CH2 N

CH2CH2OH

CH2CH2OH

10.39

Triethanolamine

10.2.5 The action of sequestering agents

Sequestering agents are often used rather indiscriminately, in amounts far in excess of thestoichiometric quantities required by the particular set of conditions. Instructions oftensimply state ‘add 0.5–1.0 g/l of a suitable sequestering agent such as EDTA’. Whilst this isconvenient for most purposes, it is worth bearing in mind that the action of sequesteringagents is governed by physico-chemical factors that, among other things, determine ahierarchy of efficacy. When the type and concentration of trace-metal ions to besequestered is known, a more discriminatory approach can be adopted regarding the choiceof agent. In some cases, including the treatment of water, this more precise specification oftype and quantity can be important.

Little more need be said here about the simple ion-exchange reactions such as thatbetween sodium hexametaphosphate and calcium ions (Scheme 10.7). It is useful, however,to consider in more detail those reactions involving chelation (Scheme 10.8). This is areversible reaction, the equilibrium being dependent on the process pH and theconcentrations of the reacting species (Equation 10.2). While chelated complexes are lessstable at higher temperatures, this effect can be ignored in practice. The factors involvedhave been discussed in some considerable detail by Engbers and Dierkes [20,23].

SEQUESTERING AGENTS

chpt10(2).pmd 15/11/02, 15:44513


Scheme 10.8

�s[CC]

K[SA][MI] (10.2)

The stability of the complex is generally expressed in terms of its stability constant, which isthe logarithm of the equilibrium constant (Ks in Equation 10.2) of the reaction in Scheme10.8. A high stability constant indicates a powerful sequestering effect. For example,amongst the aminopolycarboxylates the stability constant for a given metal ion generallyincreases in the order: NTA < HEDTA < EDTA < DTPA. Metals can also be listed inorder of increasing stability constant: Mg2+ < Ca2+ < Mn2+ < Al3+ < Zn2+ < Co3+ <Pb2+ < Cu2+ < Ni3+ < Fe3+. The stability constants at 25 °C for six metals with fourdifferent aminopolycarboxylates are shown in Figure 10.4.

30

20

10

Mg(II) Ca(II) Fe(II) Zn(II) Cu(II) Fe(III)

NTA HEDTA EDTA DTPA

log

Ks

Figure 10.4 Stability constants of aminopolycarboxylate chelates [20]

Thus for the series of sequestering agents and metal ions mentioned, the magnesium–NTA complex has the lowest stability and iron(III)–DTPA the highest. This scale of valueseffectively constitutes a displacement series. This means, in general, that in any systemcontaining more than one metal it is the metal forming the most stable complex (that is, thecomplex having the highest stability constant) that chelates preferentially. When these ionshave been completely chelated, any remaining sequestering agent then begins to sequesterthe metal that forms the complex having the next highest stability constant. Similarly ifiron(III) enters a system in which, for example, calcium is already chelated, the iron willdisplace the calcium since the iron complex has the higher stability constant. The calciumwill only remain chelated if sufficient sequestering agent is present to sequester both ironand calcium.

Adding protons or hydroxide ions to the system will influence the position of thechelation equilibrium. The stability constant of a complex is thus influenced by the pH of

Sequestering agent (SA) + Metal ion (MI) Chelated complex (CC)

chpt10(2).pmd 15/11/02, 15:44514

515

the system and pH is an important consideration in the choice of sequestering agents. Figure10.5 provides a very good illustration of the high sensitivity of the conditional stabilityconstant (Kc) to the pH of the system, using the same four aminopolycarboxylates in thepresence of iron(II) and iron(III). The pH-related stability constant is known as theconditional stability constant, log Kc. The effects of pH on the conditional stabilityconstants of the same four aminopolycarboxylates with other metal ions are shown inFigures 10.6 to 10.9.

1 3 5 7 9 11 13pH

5

10

15

20

log

Kc

DTPA-Fe(III)

ETDA-Fe(III)

HEDTA-Fe(III)

NTA-Fe(III)

DTPA-Fe(II)

EDTA-Fe(II)

HEDTA-Fe(II)

NTA-Fe(II)

Figure 10.5 Effect of pH on the conditional stability constants at 25 °C of Fe(III) and Fe(II) chelates ofaminopolycarboxylic acids [20]

1 3 5 7 9 11 13pH

5

10

15

20

log

Kc

DTPA-Fe(III)

DTPA-Cu(II)

DTPA-Fe(II)

DTPA-Zn(II)

DTPA-Ca(II)

DTPA-Mg(II)

Figure 10.6 Effect of pH on the conditional stability constants at 25 °C of metal chelates of DTPA [20]

SEQUESTERING AGENTS

chpt10(2).pmd 15/11/02, 15:44515


1 3 5 7 9 11 13pH

5

10

15

20

log

Kc

EDTA-Fe(III)

EDTA-Cu(II)

EDTA-Fe(II)

EDTA-Zn(II)

EDTA-Ca(II)

EDTA-Mg(II)

Figure 10.7 Effect of pH on the conditional stability constants at 25 °C of metal chelates of EDTA [20]

1 3 5 7 9 11 13pH

5

10

15

20

log

Kc

HEDTA-Fe(III)

HEDTA-Cu(II)

HEDTA-Fe(II)

HEDTA-Zn(II)

HEDTA-Ca(II)

HEDTA-Mg(II)

Figure 10.8 Effect of pH on the conditional stability constants at 25 °C of metal chelates of HEDTA[20]

1 3 5 7 9 11 13pH

5

10

15

20

log

Kc

NTA-Fe(III)

NTA-Cu(II)

NTA-Fe(II)

NTA-Zn(II)

NTA-Ca(II)

NTA-Mg(II)

Figure 10.9 Effect of pH on the conditional stability constants at 25 °C of metal chelates of NTA [20]

chpt10(2).pmd 15/11/02, 15:44516

517

It should not be overlooked that stability constants can be affected significantly by thepresence of electrolytes. For example, the stability constant of the Ca–EDTA complexchanges from 12 in distilled water to 10.5 in 0.1N KCl and to 8.5 in 1.0N KCl. Theinorganic polyphosphates tend to be most efficient under slightly acidic conditions whilstthe aminopolycarboxylates generally work best under neutral or alkaline conditions,although they still show some usefulness, and are used, at pH values of around 4.5.Generalisations like these can be misleading, however, since efficiency varies from one metalion to another at different pH values for each sequestering agent. The phosphates are goodsequestrants for magnesium and calcium but are considerably less effective for trivalentcations, which can be successfully sequestered with NTA, EDTA and DTPA up to about pH9. At higher pH values iron(III) tends to be precipitated from these complexes. It wasmainly for this reason that the hydroxyaminocarboxylates were developed, this basicallybeing their main use. For example, HEDTA will sequester iron(III) ions at pH 9 and DEGworks well at pH 12. Although effective with most metal ions, DEG will not sequestercalcium or magnesium, and HEDTA is also not as efficacious with these hard water ions asare the aminopolycarboxylates. At pH values above 12 iron(III) can be sequestered withtriethanolamine (10.39), either alone or together with EDTA.

Most divalent and trivalent ions, with the exception of the alkaline-earth metals, areeffectively chelated by the hydroxycarboxylates citric and tartaric acid, and citric acid willalso sequester iron in the presence of ammonia. Another hydroxycarboxylate, gluconic acid,is especially useful in caustic soda solution and as a general-purpose sequestering agent.

Clearly, the efficiency of sequestering action must be optimised for a specific set ofconditions. Thought needs to be given especially to the pH of the system and to whetherbroad-spectrum or specific sequestering is required. The extent of knowledge of the trace-metal ions present will determine whether a precise addition or an arbitrary excess of agentis appropriate. Finally, in some circumstances problems can arise from the use of certainsequestering agents that can remove the coordinated metal from a dye chromogen withsubsequent changes in shade or fastness properties. Metal-complex acid dyes and mordanteddyes are obviously vulnerable, but also many direct and reactive dyes contain coordinatedcopper atoms.

SEQUESTERING AGENTS

10.2.6 The uses of sequestering agents

Notwithstanding the comments made above in relation to the need to adopt a more or lesssophisticated approach to the selection and use of sequestering agents to target knowncontamination by trace metals, an attempt will now be made to provide generalrecommendations [28]. This approach will also indicate the wide range of uses for theseagents in textile wet processing.

Sized warp yarns sometimes contain metal salts or complexes (e.g. copper or zinccompounds) as fungicides or bactericides and these can interfere with enzyme action insubsequent desizing. Phosphonates such as ATMP, DETMP, EDTMP and PBTC are suitablefor use here, sequestering the heavy metals at pH 6.8–7.0, the effectiveness of each agentbeing dependent on the type of enzyme and the metal ions present. The use of thephosphonates HEDP or PBTC helps to reduce cellulose damage to a minimum in oxidativedesizing with persulphate.

In the kier boiling of cotton, the action of sodium hydroxide can be intensified by the use

chpt10(2).pmd 15/11/02, 15:44517


of sequestering agents. The phosphonates EDTMP, HEDP and PBTC have beenrecommended. In practice, however, the tendency is to use synergistic mixtures, such as:– phosphonate and gluconate– phosphonate and triethanolamine– aminopolycarboxylate and gluconate or triethanolamine.

Polycarboxylates may also be added to increase dispersing power and so reduce thepossibility of incrustations. Fine-tuning will again depend on the process details, themachine type and the degree of scouring necessary.

Phosphonates are useful additions in acidification treatments after alkaline processing toassist in the removal of metal compounds that have limited solubility in alkali.

Under the strongly alkaline conditions of mercerising, addition of either gluconate ortriethanolamine with a little HEDP is useful. The presence of a polycarboxylate helps toprevent precipitation on machine components.

Certain transition-metal salts catalyse redox reactions, leading to uneven treatment andperhaps damage to the substrate. Sequestering agents are therefore employed to complexthese metal ions and so to inhibit their catalytic activity. In reductive bleaching withdithionite, PBTC acts as a stabiliser at pH 5.5–6.5 and EDTA also gives good results. Athigher pH values, aminopolyphosphonates (e.g. EDTMP) and aminopolycarboxylates areuseful. Only triethanolamine is effective at pH 13. For oxidative bleaching with hypochloriteor chlorite, the amine oxides of ATMP and PBTC may be used. Aminopolycarboxylates areless suitable, whilst the amine oxide of NTA is unsuitable.

In the stabilisation of peroxide with silicate, the use of a polycarboxylate, perhaps incombination with a polyphosphonate such as DETMP (or PBTC in a lesser amount) helpsamongst other things in preventing fibre damage and incrustations on fabric or machine.Conversely, aminopolyphosphonates such as EDTMP or DETMP may themselves besuitable as stabilisers in the absence of silicate. Combinations of sequestering agents mayalso be used to obtain a synergistic effect, the following mixtures having been suggested foruse with silicates:– DETMP, HEDP and either gluconate or triethanolamine– DETMP, DTPA and gluconate.

Polycarboxylates may also be added to help prevent incrustations. It should be borne inmind, however, that magnesium is an essential component in most cases of stabilisation inperoxide systems, so any mixture of sequestrants should have minimum binding effect onthis metal ion.

In the pretreatment and dyeing of synthetic fibres, the aminopolyphosphonates can assistin the removal of oligomers.

Some dyes contain a coordinated transition metal as an essential part of theirchromogenic structure and this must be left undisturbed by any sequestrant used to complexextraneous metal ions in the system. Hence a balance of properties is needed, phosphatesand hydroxycarboxylates being useful. It is claimed that polycarboxylates can be molecularlyengineered to give the required balance of properties.

Wool is exceptionally prone to absorb metal ions, particularly in the weathered tips of thefibres, leading to shade differences on subsequent dyeing, especially if chelatable dyes areused. Hence sequestering agents can be essential additions to scouring, rinsing and dyeing

chpt10(2).pmd 15/11/02, 15:44518

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baths in order to remove this absorbed metal. However, it is difficult to say whichsequestrant or mixture of sequestrants gives the best results. Careful laboratory tests need tobe carried out beforehand, taking into account the chemical structure of the dyes to be used,the type of metal ions involved, the pH value of the dyebath and the type and concentrationof electrolyte present.

In many dyeing and printing operations, livelier, more intense colours can be obtainedwith better reproducibility and perhaps better fastness to rubbing by careful choice ofsequestering agent for use in the coloration process. The addition of triethanolamine withvat dyes can be beneficial in helping to prevent unnecessary loss of reducing agent or over-oxidation. Triethanolamine together with EDTMP or HEDP can be used where theconcentration of alkali is less than 10 g/l NaOH (or 22 ml/l caustic soda liquor 38°Bé).Bronzing of sulphur dyeings can often be prevented using triethanolamine withpolyphosphate, polyphosphonate or polycarboxylate. Azoic combinations can be verysensitive to metal contamination. EDTA, or perhaps combinations of polyphosphate,polyphosphonate or polycarboxylate, can help in the solubilisation of naphtholates and inthe stabilisation of their colloidal solutions, whilst EDTA, for example, can assist inprotecting solutions of diazonium salts from metal-induced catalytic decomposition.

Polyphosphates, or in lower amounts the very effective polyphosphonates, are helpful inapplications of fluorescent brightening agents. Sequestering agents can be useful additions inthe afterwashing of dyeings and prints, for example polyphosphates, polyphosphonates orpolycarboxylates such as, amongst others, a polyacrylate of molecular mass 3000–4000 or anacrylic–maleic acid copolymer. In the soaping of vat or azoic dyeings, recrystallisation isaccelerated and rubbing fastness improved by the use of sequestering agents, examples beingmixtures of polyphosphonates and polycarboxylates, such as HEDP and an acrylic–maleicacid copolymer.

The sugar–acrylate polymers [27] are recommended for applications similar to thosementioned above for polycarboxylate polymers and copolymers.

MACROMOLECULAR COMPLEXING AGENTS

10.3 MACROMOLECULAR COMPLEXING AGENTS

Macromolecular complexing agents have featured a good deal in recent research. Althoughthey do not yet appear to have attained any significant commercial use, they possessinteresting properties, not least their environmental advantages, that offer potential forfuture exploitation.

The term macromolecule includes all large molecules, including textile fibres andpolymers. The polyelectrolytes used as dispersing agents (section 10.6) or as thickeningagents and migration inhibitors (section 10.8) are examples of linear macromolecules. Cyclicmacromolecules are also known. An important feature of such macromolecules oftenresponsible for their functioning as auxiliaries is their ability to form complexes, particularlywith dyes or fibrous polymer segments. In the case of linear macromolecules, the complexesare generally formed by multipoint attachment with the smaller entity situated alongside themacromolecule. Cyclic macromolecules, on the other hand, may exhibit the interestingproperty of complexing another compound within its centre, the macromolecule completelysurrounding the complexed entity. Thus such agents have some functional similarity withsequestering and chelating agents. However, whereas sequestering and chelating agents aregenerally used to complex simple metal ions, the macrocyclic complexing agents are usually

chpt10(2).pmd 15/11/02, 15:44519


engineered to complex with bigger molecules such as dyes, polymer segments or surfactantsand do not act as simple metal-sequestering agents. It is these properties that havestimulated much research into the possible uses of macrocyclic complexing agents asauxiliaries in coloration processes or as agents for helping to clean up textile wastes. It isparticularly interesting that certain macrocyclic agents can be obtained from naturalreplenishable sources. Four types of macrocyclic complexing agents are considered here:– cyclodextrins– cucurbituril– crown ethers– liposomes such as phosphatidylcholine derivatives.

The discussion, however, is relatively brief in view of the fact that little commercialdevelopment seems to have taken place so far.

10.3.1 Cyclodextrins

Cyclodextrins are obtained by enzymatic depolymerisation and extraction from starch. Theycomprise rings of D-glucose units and α-, β-, or γ-cyclodextrin can be obtained [29],depending on whether 6, 7 or 8 glucose units are present in the ring (10.40). The dimensionsof the outer and inner surfaces increase as the number of glucose units increases (Figure10.10). The important characteristic feature of these cylindrical structures that influences theirproperties lies in the different nature of their surfaces, the outer being essentially hydrophilicwhilst the inner is essentially hydrophobic [29–31]. Thus the outer surface confers aqueoussolubility whilst the inner surface is capable of attracting suitably configured hydrophobicmoieties into the cavity, thus forming a complex, sometimes referred to as an inclusioncomplex. Obviously, the size of the hydrophobic moiety to be complexed must be such that itcan fit into the cavity of the cyclodextrin used (i.e. α, β or γ).

In addition, the versatility of cyclodextrin macromolecules as a group is enhanced by thefact that derivatives of cyclodextrins can be prepared [32]. This is achieved, for example, by

O

O

O

O

OH

HO

O

O O

O

O

O

O

O

OH

OH

HO

HO

HO OHHO

HOHO

HOOHHO

OH

OHOH

OH 10.40a

α-Cyclodextrin(6 units)

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521

1530 16901370

780

500 650 850

780

-cyclodextrin -cyclodextrin -cyclodextrin

Figure 10.10 Dimensions of cyclodextrins [29]

OHO

OO O

HOHO

O O

O

O

OH

OH

HO

HO

OHHO

HOOH

HO

OH

O

O

O

O

O

O

OH

HO OHOH

OH

HO

HO

OH

10.40b

β-Cyclodextrin(7 units)

O

O

O

O

O

O

OO

O

O

O

O

O

O

OO

OH

OH

OH

OH

HO

HO

HO

HO

HO OH

HO

HO

OHHO

OH

OH

HOHO

HOHO

OHOH

OHOH10.40c

γ-Cyclodextrin(8 units)


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substituting to various degrees the hydrogen atoms of the hydroxy groups (Table 10.1).Introduction of a hydroxypropyltrimethylammonium chloride grouping gives a cationicderivative, whilst the monochlorotriazinyl substituent gives rise to a derivative that isanalogous to a reactive dye. This cyclodextrin derivative, therefore, is capable of reactingwith fibrous macromolecules that contain nucleophilic groups.

Table 10.1 Typical derivatives of β-cyclodextrin [32]

R Group

β-cyclodextrin

2-hydroxypropyl

2,3-dihydroxypropyl (glyceryl)

2-hydroxyhexyl

n-butylglyceryl

2-ethylhexylglyceryl

phenylglyceryl

o-tolylglyceryl

sodium carboxymethyl

2-hydroxypropyltrimethylammonium

monochlorotriazinyl

H

CH2 CH CH3

OH

CH2 CH CH2OH

OH

CH2 CH CH2CH2CH2CH3

OH

CH2 CH CH2

OH

O CH2CH2CH2CH3

CH2 CH CH2

OH

O CH2CHCH2CH2CH2CH3

CH2CH3

CH2 CH CH2

OH

O

CH2 CH CH2

OH

O

H3C

CH2 COONa

CH2 CH CH2

OH

N CH3

CH3

CH3 Cl

NN

NCl

ONa

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Once complex formation has taken place, the physico-chemical nature of both thecyclodextrin and the complexed substance is changed. For example, complexing diminishesthe vapour pressure of volatile compounds and increases the stability of compounds that aresensitive to light or air [32]. The aqueous solubility of some sparingly soluble compoundscan be increased by complexing, this being attributable to the influence of the hydrophilicexterior of the cyclodextrin. Complexing with cyclodextrin can also increase the hydrolyticstability of some compounds, including dyes [29]. The functionality of cyclodextrincomplexes, like that of metal-sequestrant complexes, is governed to a large extent by thestability constant of the complex and this is markedly influenced by stereochemical factors.

Cyclodextrins have ecologically advantageous properties. Not only are they producedfrom natural and replenishable sources, they are biodegradable, non-toxic and possess noallergenic potential [32,33]. They are commercially available in bulk quantities at anoptional degree of purity and have been used for many years in pharmaceuticals.

The general uses of cyclodextrins (i.e. non-textile as well as textile) have been reviewed[30]. Research into possible textile applications has been ongoing since the 1950s and hascovered many aspects, from preparation of substrates through coloration processes tofinishing, as well as effluent treatment. A brief review of more recent textile-related researchis given here to demonstrate the wide range of potential applications of these interestingproducts.

Substrate preparation

It has been claimed that complexes of β-cyclodextrin with anionic surfactants, notablyhigher fatty alcohol ethoxylates, improve scouring efficiency on cotton and wool inlaboratory-scale processing [34]. Residual surfactants carried over from preparation canhave undesirable effects in subsequent processing. When cyclodextrins complex withsurfactants, their surface activity is reduced. Hence cyclodextrins are potentially useful forthe removal of residual amounts of surfactants from substrates [35]. The use of α- andβ-cyclodextrins has been studied in this context with one cationic, one anionic and four

OO O

OOORRO

OR ROROOR

OO

ORRO

O

O

ORRO

RO

OR

OR

OO

RO

OO

RORO

RO OR

OR RO

O

OR


chpt10(2).pmd 15/11/02, 15:44523


nonionic agents. α-Cyclodextrin tended to form weaker complexes than β-cyclodextrin.Methylated β-cyclodextrin was especially useful. The hollow interior of a β-cyclodextrinmolecule is capable of the optimum accommodation of a benzene ring and is thusparticularly good for complexing ethoxylated phenols.

Acid dyes

The formation of complexes of the fluorescent tracer dye ammonium 1-phenyl-aminonaphthalene-8-sulphonate (10.41) with cyclodextrins has been investigated withfavourable results, especially in environmental studies [33]. The ability of this dye tocomplex with cyclodextrins has been exploited mainly as an analytical tool in the study ofcyclodextrin applications, since its fluorescence is easily measured. The interaction of α-, β-and γ-cyclodextrins with azo acid dyes containing alkyl chains of different lengths has beenreported [36,37]. The formation and isolation of solid complexes between β-cyclodextrinand CI Acid Red 42, CI Acid Blue 40 or Erionyl Bordeaux 5BLF (Ciba) have been reported[29].

NH SO3NH4

10.41

Basic dyes

The structure and formation constants of α-, β- and γ-cyclodextrin complexes withazoniabetaine dyes have been studied, the formation constants decreasing in the order:β-CD > γ-CD > α-CD [31]. Complexes of methylene blue (CI Basic Blue 9) withcyclodextrins have been examined, β-cyclodextrin giving the highest stability constant. Thiswas indicative of almost ideal fitting of the dye molecule into the cyclodextrin cavity [38].Complexing with cyclodextrins increased the fluorescence intensity of the dye, this effectalso being highest with β-cyclodextrin.

Direct dyes

The application to cotton of CI Direct Orange 46, Red 81 and Blue 71 at 90 °C in thepresence of a cyclodextrin, sodium chloride and borax (to give pH 9) gave results that variedwith the dye and auxiliary combination present [39]. The formation and isolation of solidcomplexes between β-cyclodextrin and CI Direct Orange 40, Orange 46, Blue 86 or Green26 and between γ-cyclodextrin and Green 26 have been reported [29]. The complexes ofβ-cyclodextrin with Orange 46 or Green 26 were evaluated for the dyeing of cotton withthem in the presence of electrolyte at 90 °C. Greater amounts of the complexes were needed

chpt10(2).pmd 15/11/02, 15:44524

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to obtain depths matching those obtained conventionally using the uncomplexed dyes. It isnot clear whether complexing with the cyclodextrin interferes with the capability of the dyeto interact with the fibre in the usual way. The stability of these complexes does not appearto be high.

Disperse dyes

Detailed dyeing studies have been carried out using complexes formed between disperse dyes(Table 10.2) and cyclodextrins. One study [40,41] evaluated the complexing and subsequentdyeing properties of five related disperse dyes (10.42) on polyester. The results indicated thatdyes which have no ortho substituents in the diazo component formed 1:1-complexes withα-, β- and γ-cyclodextrins. Dyes having electron-withdrawing groups in both of these orthopositions formed 1:1-complexes with β-cyclodextrin and 2:2-complexes with γ-cyclodextrin.Dyes 3 to 5 have a substituted diazo grouping that is larger than the cavity of α-cyclodextrinand so they are unable to form complexes with this macrocyclic molecule. It was suggestedthat such complexing could be used as a retarding mechanism in dyeing with disperse dyes,although these studies were restricted to a maximum temperature of 90 °C.

Table 10.2 Dyes used in complexing with cyclodextrins [40,41]

Dye X Y R MSCS

1 H H CH2CH3 4.352 H H CH2CH2OH 4.353 Cl Cl CH3 7.294 Cl Cl CH2CH2OH 7.295 NO2 Br CH2CH2OH 8.21

MSCS maximum size of cross-section of the substituted ring in the diazocomponent

X

O2N

Y

NN N

CH2CH2OH

R

10.42

Buschmann et al. [29,42] have studied the formation and isolation of solid complexesbetween disperse dyes and cyclodextrins. Disperse dyes free from their usual diluents, suchas dispersing agents, were used. Complexes were formed as listed in Table 10.3. CI DisperseBlue 79 did not form complexes with either β- or γ-cyclodextrin. The complexes with CIDisperse Orange 11, Orange 29, Red 82, Violet 1, Blue 165, Resolin Red FRL or ResolinYellow 5GL were studied in the dyeing of polyester at 130 °C. An important aspect of thesedyeings was that they were carried out using only the dye–cyclodextrin complex in water,without the addition of dispersing or levelling agents. Good level dyeings of high exhaustionwere obtained, even though complexing with cyclodextrins increases the aqueous solubility


chpt10(2).pmd 15/11/02, 15:44525


of disperse dyes. Thus particularly important advantages are claimed for this process overthe traditional method of dyeing with disperse dyes: the fact that no dispersing or levellingagents are needed coupled with higher exhaustion leads to much less pollution of theeffluent.

Table 10.3 Formation of complexes between cyclodextrins anddisperse dyes [42]

With β-cyclodextrin: CI Disperse Yellow 3CI Disperse Yellow 42CI Disperse Orange 11CI Disperse Orange 29CI Disperse Violet 1CI Disperse Violet 31CI Disperse Blue 56CI Disperse Blue 165Resolin Red FRL (DyStar)Resolin Yellow 5GL (DyStar)

With γ-cyclodextrin: CI Disperse Orange 11CI Disperse Orange 29CI Disperse Red 82CI Disperse Violet 1CI Disperse Blue 56CI Disperse Blue 165Resolin Red FRL (DyStar)Resolin Yellow 5GL (DyStar)

Reactive dyes

The formation and isolation of solid complexes between cyclodextrins and reactive dyeshave been reported, but no dyeing results were presented [29]. Complexes were formedbetween β-cyclodextrin and CI Reactive Orange 16, Violet 5, Blue 38 or Blue 114 andbetween γ-cyclodextrin and CI Reactive Blue 38 or Blue 114.

The use of β-cyclodextrin and its fibre-reactive heptasubstituted monochlorotriazinederivative (10.43) has been studied in an effort to minimise the amount of theenvironmentally undesirable hygroscopic agent urea that is necessary when printing withreactive dyes [43]. Although this detailed research covered many variables as regards printpaste additives, it involved only one reactive dye. It was found that the 300 g/kg ureanormally used could be reduced to 75 g/kg when 40 g/kg of the reactive cyclodextrinderivative was used or to zero when 80 g/kg of the cyclodextrin was used, comparable resultsbeing obtained in all cases. Such cyclodextrins may function in several possible ways. Theexterior hydrophilic surface of the cyclodextrin may enhance dye solubility in the same wayas traditional hygroscopic agents and the hydrophobic cavity may assist this by complexingwith the dye. The reactive cyclodextrin derivative may first react with the fibre and thenattract further dye through its ability to absorb dye into its hydrophobic cavity.

Finishing

It has been demonstrated [32,44] that the various β-cyclodextrin derivatives shown in Table10.1 can be applied to the surface of appropriate fibres by dyeing methods traditionally used

chpt10(2).pmd 15/11/02, 15:44526

527

for those fibres. When the agent has become attached to the fibre surface, the cavity of thecyclodextrin derivative is still available for the accommodation of appropriate hydrophobicguest compounds. Such guest chemicals may be, for example, perfumed, bactericidal,waterproofing or stain-resist agents. Thus the concept offers a versatile potential for thedevelopment of special finishing effects.The mechanisms by which certain β-cyclodextrin derivatives can become successfullyattached to respective fibres are analogous to those operating between dyes and fibres. Themonochlorotriazine derivative can be applied to cellulosic fibres either:– by padding and fixing for 5 minutes in saturated steam at 100 °C or 3 minutes contact

heat at 150 °C– or by printing followed by drying for 2 minutes at 100 °C and steaming in saturated steam

for 8 minutes at 100 °C.

The protective action of the alginate thickening agent in the print paste is believed to play asignificant part in the success of this printing method. Application of the reactiveβ-cyclodextrin by an exhaust method, as used for hot-dyeing monochlorotriazine reactivedyes, was unsuccessful. This is probably because the cyclodextrin derivative, although amonochlorotriazine, does not possess the structural features, such as a planar molecule and aconjugated system of double bonds, that play an important role in the substantivity ofreactive dyes for cellulosic fibres. Nor was padding followed by a cold (15 h at 25 °C) or hot(4 h at 80 °C) batching treatment successful. Several derivatives with hydrophilic (2,3-dihydroxypropyl or 2-hydroxypropyl) or lipophilic (n-butylglyceryl, 2-ethylhexylglyceryl oro-tolylglyceryl) substituents showed evidence of fixation to polyester by exhaust applicationat 130 °C. The anionic sodium carboxymethyl derivative could be fixed to nylon and thecationic 2-hydroxypropyltrimethylammonium derivative became fixed to an acrylic fibre byexhaust application at atmospheric pressure and 95–98 °C.

ORO

OO O

RORO

O O

O

O

OH

OH

HO

HO

HO

RO

OR

O

O

O

O

O

O

OR

HO OHOH

OH

HO

HO

OR

N

N

N

Cl

ONa

10.43

R =

HO OH

OH


chpt10(2).pmd 15/11/02, 15:44527


Effluent treatment

Since cyclodextrins form complexes with various other substances, including many dyes andsurfactants, it is clear that they could be useful in effluent treatment. They are potentiallysuitable for the reduction or removal of polluting substances either by immobilisation or bysolubilisation and extraction and thus can accelerate detoxification [30].

Summary

Cyclodextrins, discovered as long ago as 1930, have been the subject of much research sincethe 1950s, covering the whole field from textile preparation to effluent treatment. Theircapability to form complexes with surfactants, dyes and segments of fibrous polymers is nowwell-established, yet little or no commercial exploitation appears to have grown out of theirresearch and environmental potential. Most of the work has been done with β-cyclodextrinand its derivatives, followed by γ-cyclodextrin. The cavity of α–cyclodextrin appears to betoo small for it to be widely used in textile applications. Thus there are limits to complex-forming capability depending on the molecular size and nature of the cyclodextrin inrelation to the molecular size, nature and configuration of potential guest compounds.

Much of this research has been carried out with single guest compounds. Evidently, thebehaviour of these systems is highly specific, leading to a need for considerable fine-tuningin extending the concept to more heterogeneous commercial conditions. For example, therehave been few investigations of cyclodextrins with typical trichromatic mixtures of dyes, inwhich the compatible behaviour of all the components may be difficult to resolve. This isimplicit in the work of Yun et al. [45], who studied the compatibility of β-cyclodextrin with27 water-soluble dyes. They demonstrated marked specificity in cyclodextrin–dyeinteractions and found that this was influenced by electrolytes and surfactants, as well as bypH. Complexing with β-cyclodextrin protected some dyes from the effects of salts and acids;this could be desirable or problematic, depending on requirements. It was also found thatβ-cyclodextrin improved the levelling of certain acid dyes [45].

Further problems may arise from the possible effects of residual cyclodextrins onsubsequent processes. For example, as described above, a cyclodextrin can be used toremove residual surfactants from a fabric after scouring, yet it is possible that any residualcyclodextrin could itself interfere with subsequent coloration or finishing processes to thesame extent as the surfactants that have been eliminated.

10.3.2 Cucurbituril

Cucurbituril, like cyclodextrin, is a macrocyclic complexing agent [46]. An advantage of thiscompound is its potentially low cost, being made from glyoxal, urea and formaldehyde. It is acyclic hexamer (10.44) containing six acetyleneurein residues linked by methylene groupsbetween the nitrogen atoms and is configured like a wristband (10.45), composed of six8-membered rings alternating with pairs of 5-membered rings. Also like cyclodextrin, thiscylindrical macromolecule has a hydrophobic cavity into which a dye molecule, or a partthereof, can be accommodated, the chemical structure of the dye having a distinct influenceon the stability of the complex [46]. Cucurbituril can also complex with substances otherthan dyes, including alkaline-earth and alkali metal ions [47].

In the case of a typical dyebath composition containing dye, electrolyte and surfactants,

chpt10(2).pmd 15/11/02, 15:44528

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and possibly alkaline-earth cations from hard water, there are many competing concurrentinteractions involving cucurbituril. For example, the following possibilities coexist [46]:– the dye forms a complex with the cucurbituril and simultaneously is associated in micellar

complexes with the surfactant– the surfactant forms a complex with cucurbituril– the electrolyte ions interact with cucurbituril.

Each factor may affect the others, depending on relative concentrations and pH.Cucurbituril requires quite strongly acidic conditions for solubilisation, hence its use intextile processing is likely to be very limited. It has mostly been investigated in connectionwith the removal of colour from textile effluents [46,48–53].

10.3.3 Crown ethers

Crown ethers and related structures are macrocyclic organic compounds generally composedof repeating ethylene (CH2CH2) units separated by hetero atoms such as oxygen, nitrogen,sulphur or phosphorus [54]. Other alkylene sub-units such as methylene (CH2) or propylene(CH2CH2CH2) may also be included but are less common. In contrast to the cyclodextrinsand cucurbituril, these macrocyclic complexing agents possess an electron-rich and highlypolar cavity and a hydrophobic exterior. Usually they are readily soluble in organic solvents.They have been known since the 1930s. Two typical structures are 10.46 and 10.47. Oxygenis the hetero atom most commonly incorporated into the ring, but nitrogen (azacrowns),sulphur (thiacrowns) or phosphorus (phosphacrowns) are also known. Organic moietiessterically equivalent to the ethylene unit (such as 1,2-benzo) can be incorporated, as canmost carbohydrates with vicinal dihydroxy groupings. Crown ethers are usually named asx-crown-y, from the number (x) of atoms composing the macrocyclic ring and the number(y) of hetero atoms contained within it. If one of the oxygen atoms in structure 10.46 issubstituted by nitrogen, it becomes monoaza-12-crown-4.

Not all crown ethers have been tested for ecological or toxicological properties. Some areirritants and some are known to be toxic, although those tested do not show high toxicity.Nevertheless, amongst those that have not been tested, some may be hazardous to health.

O

CN

C C

N

NC

N

O 6

10.44

CHH

H H

CHH

O

N N

N N

O

O

NN

O

N N

O

N N

O

NN

O

N

N

N

ON

N

N

O

cavity

10.45

Cucurbituril


chpt10(2).pmd 15/11/02, 15:44529


The functional characteristic of these compounds that is of interest from the viewpoint oftextile processing is their capability to accommodate alkaline-earth and alkali metal cations,as well as a variety of other species, within their cavities. Stability constants (Equation 10.3)are again used, both as a measure of ligand strength and as a hierarchical indicator ofcomplexing capability.

� cs

d

kK

k (10.3)

kc = rate constant for complex formationkd = rate constant for dissociation of the complexKs = stability constant

The stability constant is dependent, amongst other things, on the solvating medium. Forexample, for a simple crown ether kc is usually very large and kd also large, but in nonpolarsolvents kd is much smaller than kc, so that KS increases with decreasing polarity of thesolvating medium.

It is generally accepted that for complexing to occur the cavity of the crown ether mustcontain convergent binding sites (as, for example, the inwardly directed oxygen atoms in10.46 and 10.47), whilst the entity to be complexed must have divergent binding sites. Anexample is shown in 10.48, the formation of which is facilitated by the hydrogen atomsdiverging from the central nitrogen of the methylammonium cation. It is the three N–H–Ohydrogen bonds that stabilise the complex.

The potential of crown ethers for use as auxiliaries in textile coloration processes does notappear to have been evaluated recently, although their potential to complex with alkaline-

CH2H2C

OH2C

H2C

O CH2

CH2O

CH2

O

H2C

10.46

12-crown-4

O

O

H2C

H2CO

CH2

CH2

O

O

CH2

CH2

H2CO

H2C

10.47

Dibenzo-18-crown-6

H

N

H

H

O

O

O

H2C

H2C

CH2

H2C CH2

H2C

O

CH2

CH2

CH3

Cl

10.48

+

_

chpt10(2).pmd 15/11/02, 15:44530

531

earth and alkali metal ions has been demonstrated with styryl dyes containing an aza-15-crown-5 macroheterocyclic moiety (10.49) [55].

10.3.4 Liposomes

Liposomes, also known as lipid vesicles, are aqueous compartments enclosed by lipid bilayermembranes [56,57]. Figure 10.11 shows how lipid bilayers are arranged in the liposome andthe lipid structures in large unilamellar vesicles and multilamellar vesicles. Lipids consist oftwo components:– an elongated hydrophobic moiety– a hydrophilic end group.

10.49 D = Styryl dye chromogen

D N

H2C

H2C

OCH2

CH2

OCH2

H2C

H2C

OCH2

CH2

OCH2

polar

non polar

polarcavity

Liposome

MLV LUV

Figure 10.11 Liposome structures, including multilamellar vesicles (MLV) and large unilamellarvesicles (LUV) [57]

When these lipids are dispersed in water, they spontaneously form bilayer membranes (alsocalled lamellae) which are composed of two monolayer sheets of lipid molecules with theirhydrophobic surfaces facing one another and their hydrophilic surfaces contacting theaqueous medium. In the case of phospholipids such as phosphatidylcholine (10.50), thestructure consists of:


chpt10(2).pmd 15/11/02, 15:44531


– hydrophobic component: two hydrocarbon chains (R1 and R2)– hydrophilic component: glyceryl ester, phosphate and choline groups.

CH2

O

CR1 O

CH

CH2

OC

R2

O

O P O

O

O

CH2 CH2 N CH3

CH3

CH3

10.50

+

_

Phosphatidylcholine

These structures are effective encapsulating systems for either hydrophilic or hydrophobiccompounds. They can be obtained not only in uni- or multilamellar forms but also indifferent particle sizes with varying degrees of aggregation. They are particularly useful inbiological and pharmacological applications. Recent research in various areas of textile wetprocessing has revealed further potential in these sectors. However, the methods ofpreparing liposomes so far reported are not readily adaptable to commercial processingconditions, as can be seen from the following typical procedures used by de la Maza et al.[57–60]. Clearly, substantial development work is needed before these techniques becomecompatible with bulk-scale textile wet processing.

Large unilamellar vesicle liposomes

Reverse-phase evaporation in a nitrogen atmosphere was used to prepare lipids. A lipid filmpreviously formed was redissolved in diethyl ether and an aqueous phase containing thedyebath components added to the phospholipid solution. The resulting two-phase systemwas sonicated at 70 W and 5 °C for 3 minutes to obtain an emulsion. The solvent wasremoved at 20 °C by rotary evaporation under vacuum, the material forming a viscous geland then an aqueous solution. The vesicle suspension was extruded through a polycarbonatemembrane to obtain a uniform size distribution (400 nm).

Multilamellar vesicle liposomes

A lipid film was formed from a chloroform solution of egg phosphatidylcholine by rotaryevaporation in a nitrogen atmosphere and under vacuum. An aqueous phase containing thedyebath components was then added to the lipid film. The solution was swirled to transferthe lipid from the flask and to disperse lipid aggregates; glass beads being added to facilitatedispersion. The resulting milky suspension was centrifuged for 5 minutes and then extrudedthrough a polycarbonate membrane to obtain a uniform size distribution (400 or 800 nm).

Liposomes made from pure phosphatidylcholine or containing lipids that are found in thecell membrane complex of wool (e.g. cholesterol) have been used to encapsulate aqueouschlorine solutions in chlorination processes [61,62]. The results showed improvements in

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the uniformity and homogeneity of oxidative treatment, minimising degradation of the wooland facilitating subsequent treatments.

The application of acid dyes to wool using liposomes has also been researched. The dyesused were the milling acid dye CI Acid Blue 90 [57] and the neutral-dyeing 1:2 metal-complex dye CI Acid Yellow 129 [60]. Dyeing conditions were 90 °C and pH 5.5, usingvarious ratios of phosphatidylcholine:dye. In the work with CI Acid Blue 90, both uni- andmultilamellar vesicles were used. Dye exhaustion decreased with increasing concentration ofphospholipid (Figure 10.12) but the amount of bonded dye increased with increasing lipidconcentration (Table 10.4). The percentage of dye bonded to wool (Cb) was expressed byEquation 10.4:

�� a e

ba

100(C C )C

C (10.4)

where Ca = mg/g dye absorbed by wooland Ce = mg/g dye extracted from wool by ethanol and ammonia.

Dye

exh

aust

ion/

%

Time/min20 60 100

100

80

60

40

20

A

Dye

exh

aust

ion/

%

Time/min20 60 100

100

80

60

40

20

B

0.0 0.5 1.0 2.0 4.0Concentration (mmol/l)

Figure 10.12 Exhaustion of CI Acid Blue 90 by untreated wool in dyeing with LUV (A) and MLV (B)liposomes [57]

Table 10.4 Amounts of dye bonded to wool usingLUV and MLV liposomes at different lipid concen-trations with CI Acid Blue 90 [57]

Bonded dye (%)

Lipid concentration(mmol/1) LUV MLV

4.0 84 772.0 78 741.0 77 730.5 68 680 62 62


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The work with CI Acid Yellow 129 used only unilamellar vesicles. The liposomes againsuppressed exhaustion but increased dye–fibre bonding, leading to better fastness properties.It is claimed that liposomes can be used to control the rate of exhaustion.

The application of CI Disperse Violet 1 to wool with phosphatidylcholine [58] andphosphatidylcholine/cholesterol [59] liposomes has been investigated. Figures 10.13 and10.14 show that exhaustion decreases with increasing concentration of liposome, an effectwhich may be used to control exhaustion rate. It is claimed that liposomes enhance the dyedispersion efficiency, being superior to conventional dispersing agents. Dye–fibre bondingforces and levelling of the dye are also said to be improved.

Exploration of the use of liposomes in wool processing stems from the similarity thatexists between the bilayer structure of the cell membrane complex of wool and that of theliposomes. Merino wool contains about 1% by weight of lipids, these forming thehydrophobic barrier of the cell membrane complex. Cholesterol is one of the main lipid

20 60 80 100 120Time/min

40

20

60

80

100

Dye

exh

aust

ion/

%

40

Phosphatidylcholine lipid concn (mmol/l)

0.51.0

1.52.5

Figure 10.13 Exhaustion rates of CI Disperse Violet 1 on untreated wool in dyeing using liposomes atdifferent lipid concentrations and constant dye concentration [58]

20 40 60 80 100 120Time/min

20

40

60

100

80

Dye

exh

aust

ion/

%

Phosphatidylcholine/cholesterol lipid concn (mmol/l)

1.252.5

3.0

Figure 10.14 Exhaustion rates of CI Disperse Violet 1 on untreated wool during dyeing in thepresence of MLV liposomes at different lipid concentrations and constant dye concentration [59]

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components in wool; hence its use in combination with phosphatidylcholine (Figure 10.14).One of the ideas behind this research, which remains valid despite limited commercialprospects as yet, is to focus attention away from electrostatic forces of attraction towardshydrophobic interactions, which are now accorded greater importance. In dyeing, forexample, the idea is that the hydrophobic liposome will encapsulate dye molecules by meansof hydrophobic interaction, the liposome–dye complex then being absorbed into thehydrophobic centre of the cell membrane complex via further hydrophobic interaction. Thisis why claims are made for increased hydrophobic bonding of the dyes to the fibre.

Along similar lines, synthetic cationic (10.51) and anionic (10.52) double-chainsurfactant vesicles have been investigated for the dyeing of polyester with a monoazodisperse dye [63]. The results were moderately encouraging from technological, economicaland environmental viewpoints, although problems and inconsistencies were observed. It isoften difficult to explain results with disperse dyes on the basis of structural chemistry alone,since they can be influenced by variations in dispersion characteristics and instability duringdyeing.

N

CH3CH3(CH2)10CH2

CH3(CH2)10CH2 CH3

X+_

X = Br or Cl10.51

PO NaO

O O

CH3(CH2)14CH2

CH3(CH2)14CH2

10.52

_+

10.3.5 Chitin, chitosan and their derivatives

Chitin is the second most important natural polysaccharide produced by biosynthesis,exceeded only by cellulose, to which it is closely related in structure. It was first isolated byBraconnot in 1811 and thus its ‘original and spectacular’ properties have been recognised fora long time [64]. Chitin is found in crabs, lobsters and other crustaceans, spiders and otherarthropodic insects, and the cell walls of fungi. Like cellulose (10.53), chitin (10.54) is a 1,4-β-D-glucopyranose. Both have a linear sequence of pyranose rings linked by 1,4-glycosidicbonds, a non-reducing end group and a reducing end group in the cyclic hemiacetal form.The characteristic difference between them is that chitin has an acetylamino group in the2-position, compared with the 2-hydroxy group in cellulose. Chitin exists in threepolymorphic forms, depending on the directions of adjacent polymer chains, the alternatingα-chitin structure being the most common [64].

O

HO

OH

OH

CH2OH

O

OH

OH

CH2OH

O

O OH

OH

OH

CH2OH

O

n–2

10.53

Cellulose


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Chitosan (10.55) is a derivative of chitin made by alkaline hydrolysis resulting indeacetylation to give a primary amino group in the 2-position. Chitin is less hydrophilic thancellulose, whilst chitosan is more basic than either of the others. Structures 10.53–10.55 arerepresentative only. Variations occur, depending on the source of the chitin and itstreatment during and after harvesting. Chain length, average molecular mass and molecularmass distribution vary. There is also the question of impurities and the degree ofdeacetylation of chitosan, which is usually 75–95% [65]. Knittel and Schollmeyer [65] haveoutlined the nature, properties and uses of chitin and chitosan, including indications of theiruses in textile processes [65]. Roberts has provided an excellent textbook [64]. There arealso the proceedings of two symposia, although these do not deal with textile processingapplications [66,67].

O

HO

NHCOCH3

OH

CH2OH

O

NHCOCH3

OH

CH2OH

O

O OH

NHCOCH3

OH

CH2OH

O

n–2

10.54

Chitin

O

HO

NH2

OH

CH2OH

O

NH2

OH

CH2OH

O

O OH

NH2

OH

CH2OH

O

n–2

10.55Chitosan

Numerous substituted derivatives of chitin and chitosan are known [67]; some importantexamples are shown in Scheme 10.9. The possibility of forming either anionic (5,7,8,11) orcationic (9,12) derivatives should be noted. The O-carboxymethyl (5) and N-carboxymethyl(11) polymers are of particular interest as they have stronger complex-forming capabilitieswith metal ions than either unsubstituted chitosan or EDTA [65]. In practice, derivativesformed by substitution via the 2-amino group of chitosan are more common than thosesubstituted via the 6-hydroxy position of the glucopyranose grouping [65].

Chitosan features far more than chitin in research into applications. This is largely due totheir difference in solubility characteristics, chitosan being more amenable to practicalmanipulation. Chitin is in fact rather more intractable than cellulose, since it is insoluble inthose solvents, such as cuprammonium hydroxide, that are commonly used to dissolvecellulose. Chitin is soluble in hot concentrated solutions of certain inorganic salts capable of

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a high degree of hydration, the order of effectiveness being: lithium thiocyanate > calciumthiocyanate > calcium iodide > calcium bromide > calcium chloride. Chitin also dissolves,with some degradation, in concentrated hydrochloric acid, sulphuric acid (someO-sulphation taking place) or phosphoric acid, but not in nitric acid. Certain organiccarboxylic acids, such as formic, dichloroacetic or trichloroacetic, will also dissolve chitin.

Chitosan, on the other hand, interacts with inorganic acids to yield cationicpolyelectrolytes, their solubility depending on the nature of the anion. Thus it is soluble indilute hydrochloric, hydrobromic, hydroiodic, nitric or perchloric acid, but may beprecipitated from hydrochloric or hydrobromic solutions as the acid strength is increased.Chitosan forms water-soluble salts with most carboxylic acids. Hence it is chitosan, ratherthan chitin, that has come to the fore in a remarkably wide range of end-uses, includingsuch diverse fields as medicine, personal care, contact lenses, biotechnology, food,agriculture, effluent treatment, analysis, textile finishes and coatings. Although usage intextiles is relatively small as yet, its availability, environmental compatibility and remarkableversatility offer considerable potential.

Both chitin and chitosan are manufactured commercially on a large scale. Chitosan isavailable in powder, gel, solution, film, membrane, fibre and bead forms. Interest in all formsand levels of purity is high and continuing to expand [67]. Chitosan is produced from amplyreplenishable biological sources and is readily biodegradable, non-toxic and non-allergenic.It has bactericidal and fungicidal properties and actively promotes wound-healing.

The ability of chitosan to form complexes is of particular interest. Being slightly basic, itwill readily form complexes with anionic compounds. Initially it forms into micelles withsmall amounts of anionic surfactants, leading to precipitation of a complex as theconcentration of the anionic surfactant increases. Chitosan will complex with anionic

O

NHCOCH3

OH

CH2OH

O

O

NHCOCH3

OH

CH2OCH2COONa

O

O

NH2

OH

CH2OH

O

O

NHCH2COONa

OH

CH2OH

O

O

NHCOCH3

OSO3Na

CH2OSO3Na

O

O

NHCOCH3

OH

CH2OCH2CH2OH

O

O

NHCOCH3

OPO3H2

CH2OPO3H2

O

O

N

OH

CH2OH

O

O

NHCOCH3

ONa

CH2ONa

O

O

NHCOCH3

OCOR

CH2OCOR

O

O

+NH3

OH

CH2OH

O

O

NHCOR

OH

CH2OH

O

CH3

R CH3

OCOR

X

1 2

34

5

6 7 8

9 10

11

12

_

+_

R = alkyl

Typical derivatives of chitin (1) and chitosan (2) [67]: alkali-chitin (3), O-acylchitin (4), O-carboxymethylchitin (5), O-hydroxyethylchitin (6), chitin O-sulphate (7), chitin O-phosphate (8), chitosan salt (9), N-acylchitosan (10), N-carboxymethylchitosan (11), trialkylammonium salt (12)

Scheme 10.9


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polyelectrolytes leading to the formation of polycationic/polyanionic complexes of highmolecular mass. This capability to complex with anionic substances is further enhanced ifcationic derivatives of chitosan are used. The degree and strength of complexing dependson:– the nature and ionic strengths of the cationic and anionic species– the spacing of the charged ionic groups, as influenced by the relative molecular masses

and spatial configurations of the components.

This property is clearly of interest for the removal of anionic substances from effluentstreams, for example. However, since such complexing often results in an increase inviscosity as complexing proceeds, such systems can be used to produce gels or viscousliquids. Hence there is the possibility of using these complexes as print–paste or pad–liquoradditives to control migration. Weakly basic chitosan or its more strongly basic derivativeswill complex with anionic fibres and can therefore be used as finishes or pretreatments tomodify selected properties of the fibres. They are already used, for example, in hair sprays orfor complexing with and isolating proteins.

It is not surprising, therefore, that chitosan and its basic derivatives will complex withanionic dyes. Giles et al. [68,69] researched the use of chitosan for the removal of dyes fromeffluent as long ago as 1958. The binding capacity of chitosan for anionic dyes is pH-dependent, but it has been reported [65] that in effluent treatment as much as 10 g dye perkg chitosan can be complexed at pH values above about 6.5. Similarly, chitosan has beenused for the aftertreatment of direct dyeings on cotton to improve their fastness.

The complexing of chitosan and its basic derivatives with anionic substances is paralleledby compatibility with cationic and nonionic compounds. Similarly, the anionic derivatives ofchitosan show complex formation with cationic agents and are compatible with anionic andnonionic compounds. The capability of these chitosan derivatives to complex with certainmetal ions, notably those of the transition series, is also important, having possibilities forthe removal of metal salts from effluent. The hierarchy in terms of binding capacity is:Cr(III) < Cr(II) < Pb(II) < Mn(II) < Cd(II) < Ni(II) < Fe(II) < Co(II).

Chitosan will readily react with formaldehyde via its primary amino groups [65]. Thecapability of chitosan and its basic derivatives to complex with anionic fibres has alreadybeen mentioned. In this context, the bactericidal and fungicidal properties of these chitosancompounds are useful. The fact that fibre-reactive chitosan derivatives can be preparedfurther increases these possibilities. Chitosan compounds containing long-chain alkyl groupsexhibit fabric-softening properties and can be incorporated into finishing formulations forthis purpose. The fact that charged chitosan derivatives can interact with appropriate fibretypes gives scope for their use as levelling agents and to modify dye absorption in either apositive or negative sense, depending on circumstances and dyeing requirements. Forexample, they are claimed to reduce dye uptake variations between mature and dead cotton.

The use of chitosan derivatives in print pastes, to reduce the content of the environ-mentally unfavourable hygroscopic agent urea necessary when applying reactive dyes, has beenevaluated [43]. It was found that in recipes normally requiring 300 g/kg urea this could bereduced to 75 g/kg by adding either 20 g/kg chitosan or 4 g/kg N-hexylchitosan. Although thisdid not give a significant increase in dye yield, the replacement of most of the urea by abiodegradable chitosan polymer offered significant promise.

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10.3.6 Summary

In conclusion, it is noteworthy that cyclodextrins, liposomes and chitin derivatives are allreadily available from renewable biochemical sources and offer advantages ofbiodegradability and safety in use. However, it needs to be borne in mind that this fact alonedoes not necessarily mean that they are entirely environmentally innocuous in the long run.Demands on resources for the husbanding and processing of bioforms that may be necessaryin order to sustain demand for commercially viable qualities and quantities can exertdeleterious effects, not least because they may give by-products that present problems ofutilisation or disposal [70].

10.4 ENZYMES

10.4.1 Structure and properties of enzymes

Enzymes are proteins, i.e. sequences of amino acids linked by peptide bonds. The sequenceof amino acids within the polypeptide chain is characteristic of each enzyme. This leads to aspecific three-dimensional conformation for each enzyme in which the molecular chains arefolded in such a way that certain key amino acids are situated in specific strategic locations.This folded arrangement, together with the positioning of key amino acids, gives rise to theremarkable catalytic activity associated with enzymes.

Their molecular masses range from about 10 000 to more than 1 000 000. Each enzymecan catalyse an indefinite amount of chemical change without itself being consumed ordegraded by the reaction, although most enzymes lose their activity gradually under theconditions of use due to an inherent instability. Enzymes are produced by all living cells andare of two types:– exoenzymes: these are expelled by the manufacturing cell into the surrounding

environment, where they can break down organic compounds such as proteins, starchesand fats into more soluble components of lower molecular mass; and

– endoenzymes: these remain within the living cell and are transformed or broken down bythe action of coenzymes to produce relatively large amounts of energy and the cellcomponents needed for cell processes.

Clearly, it is the exoenzymes that are of interest in textile processing, an area which has seenconsiderable development in recent years. Originally used only in the preparatory processesof scouring and desizing, they are now also used to modify textile surfaces in finishing as wellas in effluent treatment.

Although enzymes are present in living systems they can exhibit powerful activity undercertain conditions. Their behaviour within the cells of a correctly functioning (i.e. healthy)living entity is controlled by a sophisticated biochemical system designed to maintainoptimum activity according to the needs of the living cells. In an external application suchas textile processing, such biochemical control is not in place. Hence care is needed in theuse of enzymes. Repeated inhalation of enzyme dust is associated with a comparatively highrisk of respiratory allergy in susceptible persons. Undue exposure can cause irritation ofmoist skin, eyes and mucous membranes. The manufacture and use of enzymes is usuallygovernment-controlled.

ENZYMES

chpt10(2).pmd 15/11/02, 15:44539


More than 3000 different enzymes have been extracted from animals, plants and micro-organisms. Traditionally, they have been used in impure form since purification is expensiveand pure enzymes may be difficult to store and use. There is usually an optimumtemperature and pH for maximum activity of an enzyme. Outside these optimum conditions,activity may simply be held in check or the enzyme may become ‘denatured’, i.e. altered insuch a way that activity is lost permanently, although some forms of denaturing arereversible. Many enzymes are also sensitive to transition-metal ions, the effect being specificto particular metal ions and enzymes. In some cases, certain metal ions are essential for thestability and/or activity of an enzyme. In other cases, metal ions may inhibit the activity ofan enzyme. Similarly, certain organic compounds can act as enzyme inhibitors or activators.

An enzyme consists of a polypeptide chain with a particular spatial configuration specificto that sequence of amino acids. The molecule twists and turns, forming structural featuresthat are catalytically active, these being known as active sites. There may be more than oneactive site per enzyme molecule. Sometimes an auxiliary catalyst, known as a coenzyme, isalso needed. Apparently, only the relevant active site of the enzyme comes into contact withthe substrate and is directly involved in the catalysed reaction. The active site consists ofonly a few amino acid residues. These are not necessarily adjacent to one another in thepeptide chain but may be brought into proximity by the characteristic folding of the enzymestructure. The active site may also include the coenzyme. The remainder of the enzymemolecule fulfils the essential function of holding the components of the active site in theirappropriate relative positions and orientation.

Thus the alkaline protease obtained from Bacillus licheniformis with a molecular mass ofabout 27 000 consists of 274 amino acid residues and has serine and histidine as active sites.Pancreatic trypsin with a molecular mass of about 24 000 contains 230 amino acid residuesand also has serine and histidine as active sites. Papain (molecular mass about 23 000 and211 amino acid residues) has cysteine and histidine as active sites.

The molecular folding of the backbone chain of the enzyme, as well as the distributionand content of amino acids [71] plays a decisive role in determining the characteristicspecificity of an enzyme with regard to its reactions. This folding is markedly affected bytemperature, for example. As the temperature rises, the chain gradually unfolds until a pointis reached at which the enzyme becomes ‘denatured’ and the catalytic activity is lost.

Most enzymes are highly specific, catalysing only one specific reaction. They may actupon only one isomer of a particular compound and are then described as stereospecific.Others are less specific, being able to catalyse several (usually related) reactions. Part of theactive site is involved in binding to the substrate and another part is responsible for makingor breaking chemical bonds. It appears that, for some enzymes, binding of the substrateproduces a change in conformation which brings the key functional group of the enzymeinto the required position for reaction to take place. Thus there must exist between theenzyme and the substrate a close stereochemical fit or complementarity, analogous to thesituation between a lock and its key. Regarding stereospecific behaviour, certain enzymesexhibit a remarkable ability to discriminate between asymmetric right-hand and left-handmolecular configurations.

Enzymes may be named trivially or more formally. Trivial naming tends to predominate inindustry and two trivial systems exist:(1) a suffix (-in or -ain) is added to a root indicating the source of the enzyme, e.g. papain

from papaya or pancreatin from pancreatic cells

chpt10(2).pmd 15/11/02, 15:44540

541

(2) the suffix -ase is added to a root indicative of the substrate or reaction involved, e.g.lactase acts on lactose, cellulase hydrolyses cellulose and glucose oxidase oxidisesglucose.

In formal nomenclature, a decimal numbering system is used. Only a brief description can begiven here; more complete accounts can be found elsewhere [72,73]. The system requiresfour numbers. The first number gives the class of enzyme according to the following scheme:(1) Oxidoreductases(2) Transferases(3) Hydrolases(4) Lyases(5) Isomerases(6) Ligases.

The second and third numbers give the subclass according to the type of reaction which ischaracteristic of the enzyme, e.g.1. Oxidoreductases

1.1 acting on the CH–OH group of donors1.1.3 with O2 as acceptor

The fourth number is the serial number of the enzyme in its subclass.

With regard to the specificity of enzymes, there are four main types:(1) Enzymes that catalyse the reaction of only one substrate are known as absolute

enzymes.(2) Stereospecific enzymes catalyse reactions with one type of optical isomer but may also

react with a series of related compounds of the same configuration. Many proteolyticenzymes hydrolyse only peptide bonds linking laevorotatory (L-) amino acids.

(3) Enzymes that react with a specific type of ester linkage are known as generalhydrolysing enzymes. Thus lipases hydrolyse a wide range of organic esters. Generally,phosphatases will break down phosphate esters into phosphoric acid and an alcohol.

(4) This group is characterised by enzyme attack at a certain specific point in a molecule.Examples are:(a) Some proteolytic enzymes act at a location where the adjacent amino acid (e.g.

phenylalanine) contains a benzene ring.(b) Some hydrolytic enzymes attack the interior bonds of a molecule. Thus α-amylase

attacks the mid-chain region of the starch molecule and of glucosidic fragmentsformed from starch.

(c) Other hydrolytic enzymes attack the end groups of saccharidic macromolecules.Thus β-amylase attacks the end groups in starch molecules, splitting off twoglucose units in the form of a maltose residue. Amyloglucosidase attacks the non-reducing ends of starch or its hydrolysis products to split off single glucose units.

As mentioned above, certain metal ions may be necessary for activity or stability. Thuscalcium is needed for bacterial α-amylase. Magnesium or cobalt is needed with glucoseisomerase. Calcium stabilises the starch-liquifying bacterial α-amylases but inactivates theglucose isomerase that may be used subsequently. Many enzymes contain an additional non-

ENZYMES

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protein component, referred to as a coenzyme or prosthetic group. This may be an organicmolecule, a vitamin derivative or a metal ion. In most cases the coenzyme participatesdirectly in the catalytic reaction.

The four main groups of enzyme activity mentioned above are covered by the six classesof enzymes already listed.

Oxidoreductases

Such enzymes catalyse reactions involving electron transfer. Oxidases use molecular oxygenas an electron acceptor (Scheme 10.10). Dehydrogenases remove hydrogen atoms from thesubstrate and transfer them to an acceptor other than oxygen.

Hydrolases

Enzymes in this group are capable of hydrolysing a substrate. Examples include:

Substrates Types of enzymestarch amylasesproteins proteinases and peptidasesnucleic acids nucleasesfats lipasesesters (e.g. phosphates) esterases (e.g. phosphatases)

CH

OCH

CH

CH CH

CH2OH

HO

OH OH

OH C

OCH

CH

CH CH

CH2OH

HO

OH OH

O+ O2

glucose

oxidase

10.56 10.57D-Glucose D-Gluconolactone

Scheme 10.10

Transferases

These catalysts bring about the transfer of a particular chemical group from one substance toanother. Examples of groups that can be transferred include alkyl, formyl, carboxyl,aldehyde, keto, acyl, glucosyl, nitrogen-, phosphorus- or sulphur-containing groups. Thus atransaminase transfers an amino group and a transmethylase transfers a methyl group.Scheme 10.11 shows the transfer of an amino group using a transaminase.

R

C

COOH

H NH2

R′

C O

COOH

R

C O

COOH

R′

C

COOH

H NH2+transaminase

+

α-Amino acid α-Keto acid

Scheme 10.11

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543

CH3

N

CH2

H3C CH3

CH2

O

C

CH3

O

CH3

N

CH2

H3C CH3

CH2

OH

H2O

cholinesterase+ CH3COOH

10.58

10.59

Acetylcholine

Choline

Scheme 10.12

Lyases

These enzymes catalyse the non-hydrolytic cleavage of bonds in a substrate to removespecific functional groups. Examples include decarboxylases, which remove carboxylic acidgroups as carbon dioxide, dehydrases, which remove water, and aldolases. Thedecarboxylation of pyruvic acid (10.60) to form acetaldehyde (10.61) takes place in thepresence of pyruvic decarboxylase (Scheme 10.13), which requires the presence of thiaminepyrophosphate and magnesium ions for activity.

Scheme 10.12 shows the action of cholinesterase in the hydrolysis of acetylcholine (10.58)to choline (10.59).

CH3

C

COOH

O

CH3

C

H

Opyruvic

decarboxylaseand coenzyme

+ CO2

10.60 10.61Pyruvic acid Acetaldehyde

Scheme 10.13

Isomerases

These catalysts facilitate the interconversion of isomeric compounds and include racemases,optimerases, cis-trans isomerases, intramolecular oxidoreductases and intramoleculartransferases. Scheme 10.14 shows the conversion of an aldehyde to a ketone by triosephosphate isomerase.

Ligases or synthetases

Such enzymes catalyse the condensation of specific compounds, accompanied by thebreakdown of a pyrophosphate bond in adenosine triphosphate (10.64). Adenosine is thecondensation product of a pentose (D-ribofuranose) and a purine (adenine). Scheme 10.15shows the action of glutamine synthetase on a mixture of L-glutamic acid (10.65) and

ENZYMES

chpt10(2).pmd 15/11/02, 15:44543


CHO

C

CH2

H OH

O

P

OH

OHO

CH2OH

C

CH2

O

O

P

OH

OHO

triose

phosphateisomerase

10.62 10.63

D-Glyceraldehyde phosphate Dihydroxyacetone phosphate

Scheme 10.14

OHC

HC CH

CH

O O P OH

O

OH

P

HO

O

HO

CH2

OPHO

O

HON

N

N

N

NH2

10.64

Adenosine triphosphate

COOH

CH2

CH2

C

COOH

H NH2

CONH2

CH2

CH2

C

COOH

H NH2

+ NH3 + ATP

glutamine

synthetase+ ADP + H3PO4

10.65

10.64

10.66L-Glutamic acid L-Glutamine

Scheme 10.15

ammonia. Adenosine triphosphate (ATP) is converted into the diphosphate (ADP) andphosphoric acid is formed.

More comprehensive accounts of enzyme chemistry, behaviour and technology areavailable [71,73–75]. In addition, general biochemistry textbooks contain more or lessdetailed accounts of these topics.

10.4.2 Enzyme applications in textile processing

Enzymes have traditionally been closely associated with the desizing of cellulosic fabrics. Inrecent years, however, the sphere of possible, if not actual, uses has widened considerably.

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545

Further developments can reasonably be expected, particularly as much of this vigorousresearch is motivated by environmental concerns. An overview of this activity is givenbelow.

Mercerisation, scouring and alkali boiling of cellulosic fibres

The traditional mercerising of cotton presents quite hostile conditions for enzymes. Hence itis not surprising that little use of enzymes has been reported, either in traditionalmercerising or as an alternative means of obtaining similar effects. Cegarra [76] hasconcluded that, because of the strongly alkaline nature of mercerising solutions and theresultant transformation of the cellulose structure, it seems rather unlikely that enzymes willprovide alternatives to alkali in the near future. Even so, it is pertinent to study the effectsthat mercerisation may have on any subsequent enzyme treatment. It has been shown [77]that enzymatic hydrolysis is accelerated on cellulose that has been mercerised withouttension compared with stretch mercerisation.

Possible uses of enzymes in scouring or alkali boiling offer somewhat more scope, althoughconditions can still be rather hostile as regards alkalinity and temperature. Nevertheless,there are some emergent signs. Various enzymes, such as cellulases, pectinases, lipases andproteases, have been compared [77], leading to the tentative conclusion that cellulases givethe best results for the removal of impurities, together with slightly inferior whiteness, asimilar loss in strength and less contaminated effluents, compared with the traditionalalkaline scour. Use of either a pectinase or a pectinase/cellulase mixture for the removal ofpectin from cotton has also been studied [78]. Effective removal using such enzymes wasfound at 40 °C and pH 4.5, giving a higher degree of whiteness than alkaline washing at theboil for the same degree of cellulose degradation. Cellulase can facilitate the alkalinescouring of viscose [79], enabling the concentration of alkali (36–60 g/l) traditionally usedto be reduced by 5–10 g/l, and giving, moreover, a more uniform and consistent swellingprocess than when alkali is used alone. Another study [80] also demonstrated possibilitiesfor using enzyme formulations in cotton scouring.

ENZYMES

Desizing of cellulosic fabrics

The enzymatic desizing of cellulosic fabrics is a long-established standard process.Amylolytic enzymes are used to convert any type of starch size into water-soluble productswithout affecting the cellulosic fibres. Using enzymes in their natural or modified state,products are available to allow desizing at 20–70 °C, 70–90 °C or 85–115 °C [81]. Cegarra[76] has intimated that, given the availability of such products, further studies are likely tobe concentrated on formulations allowing simultaneous desizing and scouring in an alkalinemedium, replacing the present two-stage process.

Nevertheless, research continues to explore improved desizing processes. Advantageshave been claimed for lipases [82] and traditional amylase desizing can be improved with thehelp of a thermostable lipase, giving both technical and environmental advantages [83].Cotton sized with poly (vinyl alcohol) (PVA) is generally desized in water at about 80 °C.However, a mixture of two different PVA-degrading enzymes gives equivalent desizing atonly 30–55 °C and pH 8.0. Extending the enzymatic treatment time to 4–6 hours (comparedwith one hour) resulted in minimum residual PVA [84]. Environmental benefits were also

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found, since the PVA content in the liquid waste after desizing for four hours was negligible.Advantages for oxidoreductases over amylolytic enzymes have been observed, since theybreak down lignin impurities and are effective over a wider range of temperature and of pH[85].

Bleaching of cellulosic fibres

The possibility of catalysing the action of hydrogen peroxide by enzymes is an interestingone, but the need to avoid fibre damage is critical and so such catalysis by a peroxidase isnot currently practical. However, the careful use of glucose oxidase in conjunction with anenzymatic desizing process is reported [86] to permit the novel and eco-friendly use ofstarch-containing effluent liquors for subsequent bleaching. This allows the use of hydrogenperoxide as the oxidising agent, together with gluconic acid (10.28) which has outstandingsequestering properties and good biodegradability. Ecological and economic advantages areclaimed, including minimising the effluent pollution load, reducing chemical consumptionand processing under mild conditions.

After bleaching it is important to ensure that the fibre does not contain residual hydrogenperoxide since this can interfere with subsequent processes, particularly coloration. Certainenzymes, particularly catalases, used to eliminate peroxide are bio-friendly and time-saving[87], thus having significant advantages over traditional methods [88]. Such a techniquecan be used in either batchwise or continuous washing-off after bleaching to give rapid andcomplete decomposition of any residual peroxide [89].

Dyeing of cellulosic fibres

Enzyme processing before, during or after dyeing is an active area of study. Enzymepretreatment may have beneficial or adverse effects on subsequent coloration. The action ofenzymes during coloration may improve the coloration process or provide a combinedprocess, such as desizing/coloration or coloration/biofinishing. With the emergence ofbiofinishing techniques, it is important to know how such enzyme treatments are affected byany prior coloration process. Most of the published work deals with enzyme pretreatments oraftertreatments.

In one study [90], enzyme pretreatment increased colour yield without affecting fastnessproperties. However, pretreatment of cellulosic fibres with cellulase lowered the subsequentfixation of homobifunctional triazine reactive dyes but did not impair the fixation of othertypes of reactive dyes [91]. Another study suggested that the enhanced brightness ofreactive dyeings was greater with triazine dyes than with vinylsulphone types when cottonwas pretreated or aftertreated with cellulase [92].

The enzyme biofinishing of cotton after dyeing was found to be inhibited by direct orreactive dyes but not by vat dyes [93]. In another investigation of reactive dyes in thiscontext, biofinishing was variously influenced by the type of dye–fibre bond, the type ofchromogen, the presence of metal ion, the number of reactive groups per molecule and evenby the dye application method [91]. Yet another study [94] showed that cellulasepretreatment boosts dye exhaustion and cellulase aftertreatment increases the apparentdepth of the dyeing. Interactions between cellulase enzyme pre- or post-treatments andreactive or direct dyes have been studied by Buschle-Diller et al. [95], with the objectives of

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elucidating the mechanism of enzymatic degradation and specifying optimum conditions fora combined dyeing/biofinishing process.

Denim washing

The practice of stonewashing of dyed cotton denim fabrics to give a ‘distressed’ or washed-down appearance was traditionally carried out with pumice stones. This was labour-intensive, time-consuming, caused abrasion of the fabric surface and created debris. Severalauthors have described how the stonewashed effect can be produced more advantageouslyusing cellulase enzymes to partially or wholly replace the stones [76,96–101]. The enzymesprovide a controllable means of surface attack of the fibres, thus bringing about the desireduneven appearance. Advantages claimed include savings in time and labour, much lessfabric abrasion and no debris. The enzymes used are neutral or acidic cellulase preparations,which may contain endo-, exo- or beta-gluconases. The finisher can exploit the differingcharacteristics of acidic and neutral cellulases by employing washing procedures that takeadvantage of each type of formulation [98].

Biofinishing of cellulosic fabrics

Biofinishing, or ‘biopolishing’ as it is more popularly known, is similar to denim washing inits use of cellulase enzymes, although the effects intended are quite different. The process isdesigned to eliminate, by dissolution, the cellulosic fibrils projecting from the surface of thefabric. This treatment results in [76]:– a cleaner, smoother surface– a softer, cooler feel– improved resistance to pilling– brighter, sometimes deeper colours.

The precise effects obtained are dependent on the fabric quality, the type of cellulaseenzyme and the application conditions, but no mechanical forces are involved in removal ofthe fibrils. The process has attracted considerable attention and is now one of the mainmethods of defibrillating lyocell fabrics [94,101–114]. Simultaneous treatment with cellulaseand protease enzymes has been applied to the biofinishing of wool/cotton blends [115].

Acidic cellulases at pH 4.5–5.5 and 45–55 °C or neutral cellulases at pH 6–8 and 50–60 °Care effective in biofinishing [106,107]. Heavier fabrics and lower enzyme concentrations needlonger treatment times but 30–60 minutes is a typical duration. The treatment is terminatedby inactivating the enzyme, either by raising the pH to 10 or by increasing the temperature to75 °C for 10–15 minutes. The process is usually monitored by assessing the weight loss of thefabric; a weight loss of 3–5% usually represents an adequately finished effect without excessiveloss of fabric strength [107]. Dissolution of the cellulose involves depolymerisation asillustrated in Scheme 10.16 [107].

ENZYMES

Wool processing

It is more difficult to control the enzymatic processing of wool. Hence there is a greaterdanger of fibre damage compared with cellulosic fibres. Since cellulose is a highly crystalline

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O

HO

OHOH

CH2OH

O

OHOH

CH2OH

O

OH

O

HO

OH

OHOH

CH2OH

O

HO

OHOH

CH2OH

OH

H2Ocellulaseenzyme

+

Scheme 10.16

material possessing only limited amorphous regions, it is relatively easy to restrict the actionof enzymes to the surface of the fibre and to the amorphous material, thus leaving thestrength of the fibre unchanged [116] In the case of wool, however, proteases and lipasescatalyse the degradation of different components of the fibre. Proteases, having diffused intothe interior of the fibre, hydrolyse parts of the endocuticle and proteins in the cellmembrane complex. This is difficult to control and can lead to serious damage of the fibre.SEM micrographs have shown the complete damage of wool fibres and released cortical cellscharacteristic of uncontrolled attack by protease enzymes [116].

Three types of enzyme may be selected for the treatment of protein fibres [76,99]:(1) Proteases, which can be classified as either peptidases or proteinases. These cleave

polypeptide chains eventually into their component amino acids. Peptidases can befurther classified as endopeptidases (which act on the main-chain amido groups alongthe polypeptide molecule) or as exopeptidases (which act only at terminal amino acidresidues).

(2) Lipases, which mainly hydrolyse fatty esters, especially triglyceride esters of fatty acids.(3) Lipoprotein lipases, which act on the lipoproteic bonds of lipoproteins (combinations of

proteins with fatty ester molecules), thus breaching the hydrophobic barriers formed bythese compounds.

The most widely used of these types are the proteases, but the others may be useful in somecircumstances. A characteristic feature is that individual enzymes are highly specific in theiraction, so that although one protease may yield the required effect, another may fail to do so.

Bleaching of wool

A serine protease that is stable to hydrogen peroxide and is active in an alkaline medium hasbeen found and marketed [76,117,118]. In fact this enzyme becomes more active with

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increasing concentration of peroxide. This enzyme increases the whiteness of wool directlyby decolorising the natural yellowish hue of the fibres. Hence, depending on the degree ofwhiteness required, this enzyme can be used either alone or in combination with hydrogenperoxide to effect the bleaching of wool. Serine protease can also be applied with bleachingagents that operate by a reductive mechanism.

Carbonising of wool

The traditional method of carbonising with sulphuric acid is environmentally undesirableand can easily lead to fibre damage. Hence it is not surprising that research has beendirected towards alternatives in which enzymes are used to remove the cellulosic impuritiesfrom wool. Cellulases and lignases are mainly used but others have been proposed [116]:(1) Removal of plant impurities by hydrolases, lyases or oxidoreductases.(2) Cellulolytic and pectinolytic enzymes used to reduce the amount of sulphuric acid

required.(3) Incubation of wool with cellulases facilitated subsequent removal of burr with no

chemical or physical damage to the wool.(4) Application of a mixture of cellulases, pectinases and lignases, again without damage to

the wool.

Dyeing of wool

The effects of enzyme treatments on the subsequent dyeability of wool have been evaluated.One investigation included both chlorinated and unchlorinated wool [119]. Wool wastreated with a protease at 50 °C and pH 7.5, followed by dyeing with CI Reactive Reds 28and 116. The enzyme-treated wool showed more rapid dyeing and higher absorption with noeffect on fastness. These effects were greater on the chlorinated wool than on theunchlorinated control. Alternatively, the enzyme-treated wool could be dyed at a lowertemperature. The effect of pretreatment with a neutral protease on dyeing with acid dyeshas also been examined [90,120], increased colour yields again being observed. It isessential, of course, to determine whether the economies of increased yield or lower dyeingtemperature exceed the additional cost of enzyme treatment, and whether the durability ofthe wool is adversely affected.

Shrink-resist finishing of wool

This is an area of considerable research activity, comparable with the enzymaticstonewashing and biopolishing of cotton. However, there has been less success in translatingthis research into commercial processes. Evidently, the technical use of enzymes for woolfabrics will not become widespread for another five to ten years [121]. Since certain enzymescan remove cuticular scales from wool fibres, it is not surprising that they are of interest forshrink-resist finishing, either alone or in combination with traditional chlorination or resin-application processes. Interest in this area is acute, because of the environmentaldisadvantages of chlorination procedures. These yield absorbable organohalogen (AOX) by-products, which accumulate in the effluent and ultimately may give rise to toxicity problemsin the food chain if taken up by aquatic organisms [116]. Hence there is considerable

ENZYMES

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commercial potential for an enzymatic descaling process that could wholly or even partiallyreplace chlorination. The critical factor is to achieve the optimal degree of descalingreproducibility, with minimal effect on fibre strength.

Ideally, the anti-felting effect should be achieved using ‘soft chemistry’ withoutapplication of a synthetic resin and the entire process should be environmentally innocuous,producing no harmful substances [116,122]. This ideal has yet to be attained. It wasoriginally thought that the large protease molecule would not be able to penetrate the fibrecuticle. If so, attack would be limited, as with chlorination, to the cuticular scales with onlyminor deterioration in mechanical properties attributable to damage in the interior [123]This proved to be too simplistic a viewpoint, however, as some proteases even attack thehighly swellable cell membrane complex preferentially, possibly penetrating this region bychannelling beneath the cuticular scales [123–125]. Moreover, microscopic examination hasindicated that enzymatic action on wool is not uniform, some fibres remaining practicallyintact whilst others are damaged considerably [126]. Most anti-felting investigations havebeen carried out with proteases but other types have also been examined, e.g. a proteindisulphide isomerase which rearranges the disulphide bonds of cystine residues [127] andtransglutaminase which introduces new crosslinks into the keratin structure [128].

Protease activity towards wool can be increased by addition of sodium sulphite or bisulphite,either with the enzyme treatment or as a pretreatment [122]. Pretreatment with oxidisingagents may also increase the effect of certain enzymes; hydrogen peroxide, dry chlorine,peracetic or performic acid, wet chlorination, potassium permanganate and peroxymono-sulphuric acid (H2SO5) have been used in this way [122]. Sulphite reduction increasesproteolytic activity by cleavage of cystine disulphide bonds in the cuticle to form thiosulphonicacid groups, a reaction known as sulphitolysis [122,129,130]. When preceded by oxidativetreatment, the action of sulphite yields electron-withdrawing sulphonic acid groups in thesulphur-rich cuticular layers, selectively activating the nucleophilic degradative reactioncatalysed by the protease and thus preferentially directing the enzyme action to the cuticle[122,129,131]. Not all proteases are activated by sulphite, however [126].

Although the present situation and the way ahead appear uncertain, it is clear thatenzyme treatment alone does not fulfil the technical requirements for shrink-resist finishing.Even with enzyme treatment, some degree of chlorination (with the attendant AOXproblems) and/or application of a resin will still be required. Two-stage or even three-stageprocesses have been proposed [116]:(1) (i) Treatment with permanganate; (ii) proteolytic enzyme treatment; This gave

complete descaling.(2) (i) Treatment with papain (protease), monoethanolamine hydrosulphite and urea; (ii)

treatment with dichloroisocyanuric acid; (iii) a second enzymatic treatment.(3) (i) A combined protease treatment; (ii) wet chlorination or oxidative treatment (using

sodium hypochlorite and potassium permanganate); (iii) application of a polymer.(4) (i) Enzyme treatment; (ii) treatment in saturated steam.(5) (i) Enzyme treatment; (ii) high-frequency radiation.(6) The Schoeller Superwash 2000 process [132]: (i) so-called ‘black box’ pretreatment;

(ii) enzyme treatment; (iii) application of a low-AOX polyamide resin.

Most enzyme treatments of wool are carried out at about 50 °C for 30–60 minutes. Theamount of enzyme required depends on the specific enzyme type and its commercial

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strength. Optimal pH also depends on the enzyme type. In a study of sixteen commercialproteases for which the optimal pH varied from 3 to 10.5 [122], it was found that onlypapain (optimal pH 6.5–7) and alkaline proteases conferred shrink-resistance on sulphite-treated wool and these tended to cause too much fibre damage. It is thus clearly apparentthat in this area of enzyme activity there is still scope for further development to meet thedesired targets.

ENZYMES

Biofinishing of woolEnzymes can be used to modify the surface of wool fibres in order to improve lustre, softness,smoothness or ‘warmth’ of the fabric. Since such processes involve attack on the cuticularscales of the fibre, there is clearly a resemblance to shrink-resist treatments and similarmethods are used [116]:(1) (i) Treatment with potassium permanganate, ammonium sulphate, acetic acid and

bisulphite; (ii) treatment with a proteolytic enzyme.(2) Descaling by application of a heat-resistant neutral protease to confer a cashmere-like

feel.(3) Combined use of dichloroisocyanurate and a proteolytic enzyme.(4) Complete removal of degraded or damaged portions of the wool (not merely the

cuticle) using: (i) protease treatment; (ii) formic acid rinse and application of asoftener.

(5) (i) Treatment with dichloroisocyanurate; (ii) neutralising and incubating with papain;(iii) steaming at 100 °C.

Only empirical tests have so far been carried out, however [76]. In a detailed but small-scalestudy, various options were examined for the sequence: (i) oxidative treatment; (ii) proteasetreatment; (iii) application of softener, including exhaust or pad application [133].

The following products were examined:

Oxidising agents

(a) dichloroisocyanuric acid,(b) potassium peroxymonosulphate,(c) magnesium monoperoxyphthalate hexahydrate,(d) sodium hypochlorite.

Enzymes

Papain and four protease formulations that varied from neutral to alkaline as regards optimalpH for activity.

Softeners

(a) a weakly cationic softener,(b) a cationic silicone micro-emulsion,(c) a cationic emulsion modified with a silicone elastomer.

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The results varied widely:– descaling: from none to full– fibre damage: from none to severe– strength loss: from -6% (i.e. a slight increase in strength) to +30%– Whiteness Index (original value -2): from -1.9 to +25.8.

Irrespective of the descaling effect, development of a ‘soft lustre’ depends on application of asoftener. These experiments were positive in demonstrating the possibility of descaling thefibres and in perceptibly improving lustre under mild conditions. In particular, it was shownthat papain is effective at the remarkably low concentration of 50 mg/l, showing a highdegree of specificity after chlorination.

Degumming and desizing of silk

The use of enzymes in silk degumming or desizing is well-established [76,99,134–136]. In astudy of eight enzymes under optimised conditions [137], weight losses of 24 ± 3% wereobserved in most cases but trypsin and pepsin gave extremely poor results. Increasing thetreatment time at the optimal concentration of enzyme gave no further significant weightloss. There was no significant strength loss in the case of degummed silk and lustre wasimproved. In earlier work, degumming with papain was as effective as alkaline degummingand superior to other methods [138]. Nevertheless, all the parameters must be carefullycontrolled to prevent attack on the silk itself [76].

Dyeing of silk

Enzymes have been used to facilitate the dyeing of silk with reactive dyes [139].

Application of enzymes in laundering

As surfactants are often used in textile processing, it is important to note that anionic orcationic surfactants can inhibit the action of enzymes, as has been reported in the case ofcellulases used for the treatment of cotton [140]. Dyes can also inhibit enzyme activity: forexample, CI Direct Red 28 has been shown to have a much greater inhibitory effect than CIAcid Orange 7 [141].

Many domestic and laundry washing formulations contain at least one enzyme. Alkalineproteases, with serine active sites and optimal activity at pH 9–10.5, are mostly used [73].Much attention is given to the degree of temperature toleration and to compatibility withother components of the commercial product. In some countries (e.g. the USA) it issufficient to have temperature tolerance up to 50–55 °C, whereas elsewhere (e.g. in Europe)toleration to 100 °C may be required for laundry detergent formulations. Papain, forexample, has broad activity and is thermally stable but is unsuitable, as is trypsin, on accountof incompatibility with perborate, many boosters and all bleaches. A protease derived fromBacillus licheniformis is much more suitable, this being compatible with surfactants, chelatingagents such as phosphates, EDTA and NTA, as well as with fluorescent brightening agentsand perfumes.

Rather more offbeat investigations have centred on micro-organisms belonging to the group

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Archaea or Archaebacteria, which live in sulphurous waters around undersea volcanic vents.An extraordinarily stable enzyme which functions even at 135 °C and survives at pH 3.2–12.7has been identified [142]. This enzyme has been termed STABLE (stalk-associatedarchaebacterial endoprotease). It is suggested that such exceptional stability may beattributable to unusually large Mr and tight folding of the protein chain. Suggested usesinclude washing powders and detergents, as well as industrial catalysts. It is even proposed thatsuch remarkable properties may have contributed to the early evolution of life on earth [142].

PREPARATION OF SUBSTRATES

10.5 PREPARATION OF SUBSTRATES

By one of those perversities of terminology often encountered in textile wet processing, theterm ‘auxiliaries’ generally includes all chemicals used in preparation and finishing processes,even though in these cases such chemicals often provide a primary rather than a secondary(auxiliary) function as in coloration processes. Thus the chemicals used in preparation arediscussed in this section. It cannot be overstressed that the success of any coloration processrelies on the state of the substrate presented for coloration; moreover, thorough preparationcan often do much to reduce the need for auxiliaries in subsequent processes. Methods ofpreparation for cellulosic and wool substrates are also discussed elsewhere [11,143]. In thischapter the emphasis is on the chemistry of the products used rather than on the technologyof processing.

10.5.1 Scouring

The purpose of scouring is to reduce to an acceptable level the amounts of fats, waxes, oilsand dirt present. Apart from the aesthetic benefits of a clean substrate, the major technicalreason for scouring is to improve the extent and uniformity of absorbency for subsequentprocesses, especially coloration. Usually the objective is the complete removal of allextraneous matter but on occasion only partial removal is the aim, since a certain residue ofoils, for example, will aid such processes as spinning, weaving or knitting. Scouring isparticularly important with natural fibres, which obviously contain much more extraneousmatter than do synthetic fibres.

In scouring, surfactants function as primary, rather than auxiliary, agents. The basicrequirements are for good wetting power and detergency, the latter property generallyincluding the ability to remove, emulsify and suspend the extraneous matter in the liquor.Not all effective detergents possess good wetting properties; hence a combination of surface-active agents to provide both wetting and detergency may be preferable. Detergency can besignificantly improved by the use of additional compounds usually referred to as ‘builders’,the chief of which is undoubtedly alkali in the form of sodium carbonate or hydroxide.Alkaline phosphates such as sodium orthophosphate, sodium pyrophosphate or sodiumtripolyphosphate may also be used in those countries where they do not contravene the localenvironmental regulations. Alkalis function mainly through saponification of the waxes, fatsand oils on the substrate, thus. rendering them water-soluble and more amenable to removaland suspension by detergents.

If the processing water or the substrate contains cations such as those of calcium,magnesium or iron, a sequestering agent should be added to the scouring liquor. Apart from

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their ability to sequester metal ions, many of these agents also possess useful detergent-enhancing powers. The aminopolycarboxylates are generally preferred, both for theirsequestering ability and for their stability in warm to hot alkaline liquors. Polyphosphates areoccasionally used at lower temperatures but are less efficient in alkaline media. Whenincorporated in the scouring medium certain organic solvents, such as pine oil,trichloroethylene, perchloroethylene, triethanolamine or glycols, can greatly aid the removalof greasy matter, particularly mineral oil which may be a component of any lubricating oilspresent on the substrate. However, the use of these additives must be in accordance withlocal environmental regulations.

Soaps are occasionally still used for scouring, although anionic or nonionic syntheticdetergents are almost always preferred. Among the anionics, fatty alkyl sulphates,sulphonates and phosphates are commonly used. Ethoxylated fatty alcohols are typicalnonionic scouring agents. Ethoxylated nonylphenol was once the most common nonionicproduct used. This came under quite severe environmental scrutiny but, as explained insection 9.8.1, there is now a good deal of evidence to suggest that nonylphenol derivativesare not so environmentally damaging as at first thought.

Proprietary scouring agents range from single-component surfactants to complex,specially formulated mixtures that contain some or all of the above mentioned types ofcomponent matched to give a balanced or compatible product. As mentioned in section9.8.3, better emulsifying properties are generally obtained with a carefully selected blend ofsurfactants rather than with a single product. It has been demonstrated [144] that in thecase of ethoxylated nonylphenol and ethoxylated fatty alcohol surfactants, the broader themolar mass distribution of the combined surfactants the better is the scouring efficiency, andthat it is inappropriate to aim at homogeneity of the nonionic formulation in scouring. Inselecting a suitable product, thought should be given to the ease with which it can be rinsedout of the substrate and to any effects that residual quantities may have on subsequentprocessing. Fabrics destined for printing, in particular, need the highest degree of uniformabsorbency and cleanliness [145], free from residual surfactants that may cause bleeding orhaloing of the printed design into the surrounding area.

Scouring is of crucial importance in wool processing: important, because the raw fibrecontains 20–60% of extraneous matter in the form of grease, suint, dirt, sand and vegetablematter; and critical, because the fibre is so easily damaged by hot alkaline treatments. Therehave been considerable changes in wool scouring practice, evolving mainly fromcorresponding changes in the types of lubricants used on the fibre, although environmentalfactors have also played a part. It has been pointed out that the pollution load from a woolscouring mill can be similar in magnitude to the average discharge from a small town [11].Hence there are heavy environmental pressures on wool scourers. This impact has providedthe impetus to develop systems which use as little water and energy as possible and reduceeffluent contamination through the recovery of some of the components.

Examples of such comprehensive systems are the WRONZ and Siroscour (CSIRO)techniques, which use minimal volumes of water whilst producing wool of optimal quality.The essential feature of the WRONZ treatment [146] is the passage of the greasy liquorfrom the first stage through a heavy solids separating tank and a centrifugal separator, fromwhich the partially degreased liquor is returned to the first stage. The Siroscour systemdepends on concentration destabilisation to increase centrifugal recovery of grease and dirtand to reduce water usage even further [146]. In concentration destabilisation wool grease,

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dirt and suint salts are allowed to build up to very high levels in the scour bowls so that theresulting unstable emulsion can be cracked easily by heating to 95 °C, allowing the greaseand dirt to be centrifugally separated. Separation occurs in three stages [147]:(1) superficial dirt (easy to remove)(2) grease(3) persistent dirt (difficult to remove).

Advantages claimed for this process [148] include: improved whiteness, decrease in ashcontent, improved grease recovery and quality, optimisation of water consumption, efficientdirt removal from effluent and a reduction in treatment costs. An excellent review of woolscouring (and of wool processing generally) up to 1984 is available [146].

Soap and alkali were traditionally used and very greasy wools were scoured with alkalialone, forming a soap in situ by saponification of the wool grease. Such methods are nowrarely used, having been supplanted by nonionic surfactants under neutral or alkalineconditions. Octa- and nona-ethoxylated nonylphenols are the preferred nonionics,providing environmental considerations permit their use, since they are unsurpassed fordetergency. Alternatively, ethoxylated straight-chain fatty alcohols are preferred on thegrounds of superior biodegradability. The advantages of nonionic detergents over soapsinclude greater efficiency under neutral conditions, stability in hard water, lower cost andmore efficient removal of grease (although this is one area where over-degreasing can be adisadvantage). Syndets are desorbed more easily in difficult rinsing situations (in yarncheeses, for example), although they are not as efficient as soap for the suspension of dirt. Inplace of the alkaline sodium carbonate, the tendency is to use neutral sodium sulphate as adetergent builder.

Apart from the impurities present in raw wools, typical formulations [146] for lubricatingwool fibres are:(1) water-miscible polyglycol lubricants; these are mainly used on carpet yarns and can be

readily removed using a neutral nonionic surfactant;(2) mineral wool oil (mineral oil with a nonionic lubricant); normally extracted using a

nonionic surfactant, although in some cases a little alkali may also be useful;(3) a combination of mineral oil, olein fatty acids and triglycerides; nonionic surfactant

with sodium carbonate can be used, but for more complete removal (as wheresubsequent shrink-resist processes are carried out) soap and sodium carbonate must beused;

(4) natural vegetable and/or animal oils; these are still used on woven worsteds scouredtraditionally with soap and sodium carbonate, although even here there is a gradualtrend towards more neutral systems.

Nowadays, proprietary mixtures of lubricating agents are formulated with ease of removal inscouring very much in mind. Consequently, scouring processes are generally mild, using anonionic surfactant at about 50–60 °C [11].

Aqueous scouring is expensive in terms of water use and effluent treatment and it cancause entanglement of delicate wool fibres. Solvent scouring offers an effective alternativebut it is essential that the solvent does not enter the environment. Earlier solvent-basedprocesses included the use of perchloroethylene in which 8–18% water had been emulsifiedwith a surfactant. Current processes are based on hexane (de Smet process), 1,1,1-


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trichloroethane (Toa/Asohi process) [11,149] or a commercial product called Triwool (ICI)in the Wooltech process [150–152].

The Wooltech process is claimed to be environmentally friendly and to produce wool ofsuperior quality. The Triwool solvent is non-flammable, does not deplete ozone from theupper atmosphere and is not a known carcinogen. The process specifically excludes water,being designed primarily to avoid fibre entanglement. Current use is for the scouring of wooltops, where it gives about 2–3% higher yields than aqueous scouring. The recovery of rawwool grease is claimed to be 99%. The process offers environmental advantages overconventional aqueous scouring and gives softer wool fibres of enhanced tensile strength andelasticity, which is beneficial for spinning and weaving.

In the laundering of garments and household textiles, as opposed to the scouring of in-process material, agents are often added to the liquor to prevent redeposition of soilextracted from the fabric during washing, typical products being the hydroxyethyl,hydroxybutylmethyl and carboxymethyl ethers of cellulose. It has been shown, using carbonblack soiling on shrink-resist treated wool, that the lowest redeposition was obtained withhydroxybutylmethylcellulose and the highest with the carboxymethyl ether [153].

The other major natural fibre, cotton, contains a significant proportion of extraneousmatter such as seeds, fats, waxes, colouring matter and dirt, as well as substances such assizes and lubricants applied during processing. Unlike wool, however, it has outstandingstability in alkali and withstands strongly alkaline treatments ranging from severe caustickier boiling to milder treatments with soap and soda [143,154]. It is difficult to detach theeffect of scouring from the complete sequence of desizing, scouring, mercerising andbleaching, since they all contribute to improved absorbency and cleanliness. Traditionalcaustic treatment in kiers is carried out at the boil or in some cases at up to 120 °C, using1–2% o.w.f. alkali. This treatment bursts the seed motes and saponifies fats and waxes,converting the fatty esters into sodium salts and glycerol. This in situ formation of soapsnaturally aids cleaning. Nevertheless, synthetic detergents are often added to aidpenetration through wetting and to increase detergency.

The surfactants selected must be highly stable in the strongly alkaline conditions, as wellas in hard water. Anionic surfactants of the fatty alkyl and alkylaryl sulphate types have beenpreferred, although the use of phosphate esters is increasing. A synergistic mixture isbeneficial, one component (C10–C13) to aid wetting, the other (C14–C16 ) as a detergent.The sulphosuccinates, often a first choice for wetting ability, cannot be used here as they arehydrolysed under such strongly alkaline conditions. A sequestering agent is usually added inorder to remove metal ions that would create problems in subsequent bleaching. Addition ofa mild reducing agent guards against alkaline oxidative tendering of the fibre throughoxycellulose formation and also promotes a degree of bleaching. The reducing agent will alsoreduce any iron(III) contamination to iron(II) ions, which are easier to remove by thesequestering agent. A commercial kier boiling additive may contain some or all of thesecomponents. Suppression of foam may also be a requirement. Semi-continuous andcontinuous scouring systems are more common nowadays [154]. The auxiliary needs inthese pad–steam processes are generally the same as those for batch scouring, except thatthe selection and balancing of components is much more critical in order to secure optimaltreatment during the short dwell times.

Contrary to the usual practice of scouring cotton and its blends under alkaline conditions,McCaffrey and Santokhi [155] have cogently argued the case for scouring knitgoods under

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acidic conditions. This starts from the assertion that the degree of impurity removalresulting from conventional alkaline scouring is probably not really necessary for cottonknitgoods. In the case of knitgoods, as opposed to woven fabrics, a milder scour is desirableto retain some of the natural fats and waxes, resulting in a fabric with a softer handle andimproved sewability [143]. It has been demonstrated [155] that acidic scouring (pH 4.5 withacetic acid) in the presence of a self-emulsifiable knitting lubricant can give results as goodas traditional processes on knitgoods, except that impurity removal is inferior butacceptable. In addition there are significant advantages, particularly from economical andenvironmental aspects. Acidic scouring uses approximately one-third less water than analkaline scour. Alkaline processes often require a separate acidic rinse to neutralise thealkali, whereas residual acid is easily rinsed out. Acidic scouring also offers an opportunity toreduce chemical consumption and hence costs. These savings minimise effluent treatmentand reduce the effects of effluent on the environment. Acidic scouring gave a reducedprocessing time and lower weight losses, both factors that contribute to improvedproductivity.

Compared with wool and cotton, the scouring procedures for synthetic fibres arerelatively simple since these fibres contain fewer impurities. Most of these have at least somedegree of water solubility; the most important are sizes and lubricants. The major sizes usedare poly(vinyl alcohol), carboxymethylcellulose and poly(acrylic acid), all of which arecompletely or partially water-soluble. Sometimes aliphatic polyesters are used.

Secondary acetate and triacetate fibres generally respond to a light scour with soap orsynthetic detergent, usually at 60–75 °C, this being sufficient to remove soil, oil, sightingcolour and any antistatic agent, although temperatures can range from 30 to 90 °C [156].Anionic synthetic detergents, such as the poly(oxyethylene) sulphates, are preferred beforedyeing with disperse dyes since low cloud-point nonionic scouring agents, if carried over intothe dyeing process, can interfere with the stability of the dye dispersion at highertemperatures. Addition of a sequestering agent is helpful in hard water. Care should betaken if alkali is added, especially on secondary acetate, since these ester fibres can behydrolysed to cellulose under hot alkaline conditions. Nevertheless, in the S-finishingprocess this alkali sensitivity is exploited to effect a carefully controlled surfacesaponification of the fibres to improve drape and antistatic properties [157]. Sodiumhydroxide is applied together with an anionic surfactant to aid wetting and uniformity oftreatment. S-finishing is more usually carried out on triacetate fibres, reducing the totalacetyl content from about 62 to 59%.

When scouring synthetic fibres that are to be dyed with disperse dyes, nonionic scouringagents are best avoided unless they are formulated to have a high cloud point and are knownnot to adversely affect the dispersion properties of the dyes. Conversely, when scouring acrylicfibres, anionic surfactants should be avoided [156] because they are liable to interfere with thesubsequent application of basic dyes. These fibres are usually scoured with an ethoxylatedalcohol, either alone or with a mild alkali such as sodium carbonate or a phosphate.

Polyamide and polyester fibres are generally scoured using an alkyl poly(oxyethylene)sulphate and sodium carbonate. Some polyester qualities are subjected to a causticisationtreatment with sodium hydroxide in the presence of a cationic surfactant to give a lighterfabric with a silkier handle [154,156]. This treatment involves etching (localisedsaponification) of the polyester surface and is broadly analogous to the S-finish used ontriacetate fibres. The process has attracted considerable interest in recent years but its


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popularity is likely to decline as the availability of polyester microfibres increases. If theweight loss during caustic reduction of polyester is restricted to 15–17% there is little effecton the breaking strength or the viscosity of the fibre [158], since hydrolysis is limited to thesurface of the fibre. If the weight loss is greater than 20%, however, deeper layers of the fibreare attacked and the fibre strength and viscosity are reduced. The crystalline regions of thefibre show considerable resistance to hydrolysis. These factors, however, may varyquantitatively depending on the structure and morphology of the particular polyester.

The cationic surfactant normally used with the alkali acts as a catalytic accelerant [159];quaternary ammonium compounds are most often used. The kinetics of the process havebeen studied [159], showing that careful control of all parameters is essential. Thetreatment results in an increase in surface polarity of the fibre. The substantivity for dyesand the rates of wetting-out and dyeing are increased. The increase in wettability arises fromthe formation of free carboxyl and hydroxy groups [160]. Although sodium hydroxide is thepreferred alkali, other alkalis have been investigated [161], the order of activity being: KOH> NaOH >> Na2CO3. Saponification also increased at lower liquor ratios.

Cationic accelerants vary in their efficacy [161]. Other types of accelerant have also beenevaluated. In one study [162], comparisons were made between tetra-ethylammoniumbromide, benzyltriethylammonium chloride, poly(diallyldimethylammonium chloride) andthe diethyldimethylammonium derivative of a benzenesulphonate polyglycol ester. It wasfound that the cationic polymers had a greater effect than the simple quaternary ammoniumcompounds of lower molecular mass. This effect was attributed to the capability of thepolymers to enter into hydrophobic interaction with the fibre surface. Ethylenediamine hasalso been found to accelerate the alkaline hydrolysis of polyester [163].

Alkaline hydrolysis in a solvent (dimethylformamide, dimethylsulphoxide or dimethyl-acetamide) containing sodium hydroxide has been investigated [164]. Fabric geometry [165]and the degree of heat setting of the polyester also influence the results. As the temperatureof heat setting was increased, the accelerating effect of dodecylbenzyldimethylammoniumchloride decreased [166]. Basic-dyeable polyester is particularly sensitive to alkalinehydrolysis [167]. In some cases, saponification has been used to produce special effects suchas a leather-like finish [168].

10.5.2 Desizing

Desizing is an essential part of the purification process for woven fabrics. Sizes perform anadhesive and lubricating function. After drying, the size forms a protective film on thesurface of the warp yarns, bonding the protruding fibrils to produce a smoother yarn withimproved tensile strength and abrasion resistance. The objective of sizing is to improveweaving efficiency by reducing the number of yarn breakages, reducing frictional wear ofloom parts and allowing increased running speeds.

Individual size polymers may be used alone or in combination with one another and theirperformance may be further improved by the addition of other components such as waxesand lubricants. However, whilst sizing offers many benefits in the subsequent weaving of theyarns, it is anathema as far as wet processing is concerned. A typical sized yarn may containas much as 34% of impurities, distributed as shown in Figure 10.15. These impurities caninterfere with wetting-out and with bleaching. They may also affect coloration processes.Depending on the type of size and the dyes used, dye uptake may be increased or resisted;

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hence uneven distribution of size may lead to unlevel coloration. There may also be adeleterious effect on fastness properties, since coloured size is likely to be attached to thefibre only superficially. Consequently, it is invariably essential to remove sizes and lubricantsthoroughly before further wet processing.

Approx 4% Up to 20% Up to 10%

fatsoilswaxesabraded metal

hemicellulosepectinsproteinsseed husksfruit capsulescoloursalts

size

Figure 10.15 Cotton warp yarn and its impurities [169]

This removal of size residues inevitably raises environmental questions. Unfortunately,the various size polymers and their associated additives respond to different methods ofremoval. It is therefore highly desirable to know which sizes and other components havebeen used in a given case so that appropriate methods of removal can be formulated. This isnot always easy, particularly in commission dyehouses or printworks where sizing has beencarried out elsewhere. Analytical procedures are available but these require appropriatefacilities and expertise. Once desizing has been carried out there arises the question of howto dispose of the effluent.

The range of size polymers available has expanded to the point of being extremelycomplex [169], both in terms of the main types of size and the numerous combinationalpossibilities that they represent. The present overview of salient aspects covers thefollowing: chemistry of size polymers, the use and properties of sizes, desizing methods,analysis of size polymers and environmental aspects. A summary of the equipment used forsizing is available [170].

Chemistry of size polymers

Recent reviews dealing with the chemistry of size polymers are available [169,171].Sometimes a distinction is made [172] between primary sizes, secondary sizes and binders.This distinction appears to be an arbitrary one depending mainly on the proportions presentin a mixture, although other factors may also be pertinent. There is a great deal of similarityin essential chemistry between typical size polymers, thickening agents used in printing andmigration inhibitors used in continuous dyeing. An overall summary [169] of the main typesof size polymers available is given in Table 10.5. Similar but less comprehensive lists aregiven elsewhere [171,172].


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It is important to recognise that the molecular characteristics (average molecular massand distribution, possible degree of substitution) of such polymers can be varied quitewidely, with attendant changes in properties. Thus polyacrylic acids and their salts may bedescribed as either sizes or binders. The distinction is related to the proportions used: to beeffective as a binder [171] such a polymer must constitute at least 10% of the dry weight of asize formulation. A product marketed specifically as a binder may have molecularcharacteristics that provide properties different from those of a similar product marketed as asize component. For example, an acrylic size may be engineered specifically for adhesion andfilm-forming properties whereas an acrylic binder may be designed to enhance film elasticitywhen added in smaller quantities (about 10%) to a size formulation. It is therefore importantto bear in mind that different polymers based on similar chemistry can be engineered toprovide suitability for specific uses.

The chemistry of starches, galactomannans, modified starches, modified celluloses andalginates is discussed in section 10.8.1 on natural thickeners. The main starches used are

Table 10.5 Chemical types of main size polymers [169]

Vegetable productsNative Starches Potato, maize, wheat, rice,

sago, tapiocaResinsVegetable gums Arabic, Locust bean, Senegal,

TragacanthMosses and algae Moss starch, sodium alginatePectin

Modified Starch derivatives Water-soluble starchesBritish gumStarch ethers Hydroxyethyl starch

Hydroxypropyl starchStarch esters Carboxymethyl starch

Phosphate starchStarch carbamate

Cellulose derivatives Cellulose esters Carboxymethyl celluloseMethyl cellulose

Animal productsGlueGelatinCasein

Synthetic productsPolyvinyl compounds Partially saponified

poly(vinyl acetate)Fully saponified poly(vinyl acetate)Copolymers with crotonic acidCopolymers with vinyl acetate

Acrylic acid copolymers with methacrylic acidwith acrylic acid esterswith acrylonitrile

Maleic acid copolymers with styrenewith ethyl vinyl etherwith butadiene

Poly(ethylene glycol) copolymers with isophthalic acid

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potato starch in Europe, maize starch in America, and rice, maize, tapioca or sago starchesin the Far East [169].

Animal glue is a complex colloidal mixture of proteins. The related gelatins are alsocomplex heterogeneous mixtures of proteins. They are strongly hydrophilic and rich in theamino acids glycine, proline, lysine, hydroxyproline and hydroxylysine. Casein is aphosphoprotein obtained from the milk of mammals.

Poly(vinyl alcohol) has the structure 10.67. Poly(vinyl acetate) is the fully esterifiedderivative of poly(vinyl alcohol), in which the –OH groups are replaced by –OCOCH3groups. As indicated in Table l0.5, commercial polyvinyl sizes are effectively copolymers ofpoly(vinyl acetate) and poly(vinyl alcohol) that vary in the degree of saponification of theester groups. These products may comprise 100% of either polymer, or combinations of thetwo monomers in any proportions. Crotonic acid (2-butenoic acid), widely used in thepreparation of resins, may also be a component. This compound exhibits cis–trans isomerism(Scheme 10.17). The solid trans form is produced readily by catalysed rearrangement of theliquid cis isomer.

CH2 CH CH2 CH CH2 CH CH2 CH

OHOH OH OH

10.67

CC

H

H3C

COOH

H

CC

H

H3C

H

COOH

Crotonic acid mp 72°C bp 180°C

Isocrotonic acid mp 14°C bp 169°C

Scheme 10.17

Polymers based on acrylic acid have gained considerable importance in recent years.Their essential chemistry is discussed in section 10.8.2 on synthetic thickeners. Copolymersof acrylic acid with acrylonitrile and methyl acrylate (10.68) contain a random distributionof cyano, ester and acidic sidechain groups [169].

CH2 CH CH2 CH CH2 CH

C CN

CH2

C

CH CH2

C

CH CH2

C

CH

CNO OCH3 O OH O OCH3 O OH

10.68

The polyester sizes used have a much lower average molecular mass than polyester fibres.These structures (10.69) contain sulphonic acid groups and may be water-soluble or water-dispersible types. The degree of sulphonation is low [171]. If these resins are subjected to ahigh pH, the sulphonate groups can be hydrolysed, giving an insoluble resin that is verydifficult to remove from the fibres.


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O C R1 C O

O X1 O

R2 O

X2

C R3 C

OOn

10.69

R1X1R2X2R3

(1)(2)(3)(4)(5)

=====

Aromatic ring—SO3

–, —OCH2CH2CH2SO3– or —CO2CH2CH2SO3

– substituent on the aromatic ring R1CH2CH2 (n = 2–10), CH2CH2CH2 (n = 1–2) or CH2—cycloalkyl—CH2—CH2OCH2CH2CH2SO3

– substituent on aliphatic group R2, in which case X1 = HCycloaliphatic or aliphatic hydrocarbon of 2–6 carbon atoms

Rather more specialised sizes are used in certain applications. For example, a reactivepoly(dimethylsiloxane) (section 10.10.2) is recommended for the sizing of some industrialtextile fabrics [173].

The application and properties of sizes

As already mentioned, the essential aim of sizing is to increase productivity in weaving. Thisis achieved through a reduction in yarn breakages that permits increased running speeds.Indeed, the high speeds of modern weaving processes could not have been realised withoutcorresponding improvements in sizing technology. The most important requirements of asize formulation can be summarised as follows [174]:– high adhesion and good film-forming properties on the yarn, together with good elasticity

of the applied film– low tendency to foam in the application liquor– freedom from skin formation in the application liquor– good storage stability– good compatibility of wash-off liquors containing different size components– appropriate compatibility with alkalis and bleaching agents if desizing is not carried out

separately from scouring and bleaching.

To these may be added economy and ease of removal from the substrate, as well as afavourable response to effluent treatment.

The adhesive strength of a size film is an important consideration as it has a bearing onstability during weaving [175]. Adhesive strength depends on such factors as type of sizepolymer, additives present (e.g. wetting agent), sizing liquor temperature, yarncharacteristics, viscosity index of the size formulation and degree of saponification ofpoly(vinyl acetate) sizes. Size dissolution rate is also an important factor; this needs to beknown if desizing is to be carried out effectively and efficiently [176].

Many factors will determine the choice of size type and formulation. A general scheme ofsize use relative to different fibre types is given in Table 10.6. It is useful to classify sizesaccording to those physico-chemical properties that largely determine the method ofdesizing, as shown in Table 10.7. Evidently, some degree of synergistic chemistry is involvedin determining the specific suitability of size polymers for certain fibres. For example, there isan obvious similarity of molecular structure between starch and cellulosic fibres, offering

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substantial scope for hydrogen bonding between hydroxy groups. The similarity in molecularstructure between polyester sizes and synthetic fibres facilitates hydrophobic bondingbetween hydrocarbon segments of the polymer chains.

Table 10.6 Sizing agents for different substrates [169]

Size: Natural Synthetic

Substrate Starch CMC Gums Glue AACo AECo PVA PE PVCo

Staple fibre yarnsCellulosic + + + + + +Polyester/cellulosic o o o + + +Nylon/cellulosic o o o + + +Wool o o + + +Polyester/wool o o + +Polyester, nylon o o + +

Filament yarnsViscose + + + + +Acetate + + +Triacetate + + +Nylon + +Polyester + + +

+ Aloneo Only in combination with synthetic size

CMC Carboxymethylcellulose Gums GalactomannansAACo Acrylic acid copolymers PVA Poly(vinyl acetate)AECo Acrylic ester copolymers PE PolyestersPVCo Polyvinyl copolymers

Table 10.7 Size properties and methods of removal [169] (slightly modified)

Physico-chemicalcharacteristics Type of size Method of removal

Chemically degradable starches enzymatic or oxidativemodified starches

Water-soluble acrylic acid copolymers rehydration and dissolutionpoly(vinyl acetate/alcohol)carboxymethylcellulosecertain modified starches

Water-resistant polyesters neutralisation and dispersioncertain acrylic acid copolymers

Economic factors play a major part in the selection of sizes. For this reason, starch sizesand their mixtures continue to be the most widely used, particularly on cellulosic substrates.Nevertheless, more costly size polymers may be economically justifiable if this can be offsetby higher productivity in weaving. High productivity generally demands high elasticity and


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strong adhesion, provided mostly, if not exclusively, by synthetic sizes. Water-jet weavingmachines require water-resistant sizes. Some sizes are adversely affected by hightemperatures (as in heat setting) or by treatment at inappropriate pH values and theseeffects can make their removal more difficult.

The choice and combinations of different size components must take account of manyfactors if optimum results are to be obtained. Much has been published regarding theoptimisation of size formulations in relation to desizing processes [177–183]. Cotton warpyarns sized with starch are normally woven at high humidity (80% and above) to keep yarnbreakages low, as the starch film is brittle at low humidity. It has been shown [183],however, that improved weavability at moderate relative humidity (e.g. 65%) can beobtained using: (a) starch/acrylamide or hydroxyethyl starch at not less than 15% add-on; or(b) poly(vinyl alcohol), which gave excellent results even at a low add-on of 5–6%.

Addition of acrylamide to starch improved the performance of cotton yarn more thanacrylamide alone, but addition of poly(vinyl alcohol) to starch lowered the performance ofthe yarn compared with poly(vinyl alcohol) alone. Overall, taking into account economicconsiderations, stringent pollution requirements and the needs of desizing, the single-component hydroxyethyl starch showed optimum acceptability for weaving performance atmoderate relative humidity.

The incorporation of waxes or lubricants (section 10.10.1) is an important considerationthat should not be overlooked. These components exert a significant influence onweavability and on the conditions and efficiency of desizing. The lubricant may be added asa component of the size formulation, or applied separately by kiss-roll after sizing. Lubricantsmay themselves be used as sizing agents, particularly on synthetic warp yarns. Indeed, it hasbeen argued [184] that such treatments give results comparable with traditional sizes asregards fibre strength and weaving performance. They offer advantages of savings in heatenergy during drying and in manpower and space, as no special sizing unit is required.

Desizing methods

As indicated earlier in Table 10.7, it is helpful to select desizing methods according towhether size polymers are chemically degradable, water-soluble or water-resistant.

Desizing by chemical decomposition is applicable to starch-based sizes. Since starch and itshydrophilic derivatives are soluble in water, it might be assumed that a simple alkaline rinsewith surfactant would be sufficient to effect removal from the fibre. As is also the case withsome other size polymers, however, once the starch solution has dried to a film on the fibresurface it is much more difficult to effect rehydration and dissolution. Thus controlledchemical degradation is required to disintegrate and solubilise the size film without damagingthe cellulosic fibre. Enzymatic, oxidative and hydrolytic degradation methods can be used.

The traditional approach is enzymatic desizing, in which an α-amylase or diastase enzymeis used to attack the 1:4-glycosidic links in the starch, breaking down the macromoleculesinto small soluble saccharides such as maltose and glucose. Scheme 10.18 is a simplifiedrepresentation based on hydrolysis of the amylose component of starch; similar reactionstake place with the amylopectin component (section 10.8.1). In addition to the enzyme asurfactant is required to ensure rapid and thorough wetting, including penetration of the sizefilm, and emulsification or solubilisation of lubricants. Nonionic surfactants are less likely todeactivate the enzyme than anionic agents. The enzyme liquor is generally applied by

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impregnation immediately after singeing. With modern equipment running at speeds up to170 m/min, the role of the surfactant is a critical one [169]. A carefully formulated mixtureof surfactants may give the best balance of properties for rapid and thorough wetting,together with the removal of lubricant waxes or oils. The concentration of enzyme neededdepends on the type and amount of size present and particularly on the potency of thecommercial brand used.

O

HO

OHOH

CH2OH

O

OHOH

CH2OH

O

OH

OHOH

CH2OH

O O

O

HO

OHOH

CH2OH

O

OHOH

CH2OH

OHO

O

HO OH

OHOH

CH2OH

n

Amylose component of starch

enzymaticdegradation

Maltose Glucose

Scheme 10.18

Temperature and pH are critical parameters that are also brand-related. Enzymes areavailable covering the range 20–120 °C [143,169] and from acidic to alkaline pH. Thoserequiring mildly alkaline conditions are usually preferred, since the degraded fragments fromthe branched amylopectin component of starch usually require such conditions forsolubilisation, rice and tapioca starches having rather higher amylopectin contents [143].The presence of salt or calcium ions can also influence the behaviour of the enzyme.Hickman [143] has summarised typical conditions (Table 10.8). The effect of electrolytes(NaCl, Na2SO4, CaCl2 and MgCl2) on the aqueous solubility of saccharidic size polymers,including potato starch, starch esters and galactomannans, has been studied in detail [185].It is important to know whether a fungicide, such as a halogenated phenol, has been addedto protect the size formulation against mildew, since such fungicides are toxic to enzymes.

There are some circumstances in which enzymatic desizing is inefficient:– if insufficient space is available for the batching of enzyme-impregnated fabric– if branched-chain starches such as tapioca are present they can be difficult to degrade,

depending on the degree of ageing– if fungicides have been used to protect the starch from mildew attack– if oxidative desizing can be adopted and combined with scouring or bleaching, thus

minimising energy requirements.


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Table 10.8 Optimum conditions for enzymatic desizing [143]

Effect of

Optimum temperatureEnzyme Optimum pH range (°C) NaCl Ca ions Time

Malt diastase 4.5–5.5 55–65 o + 12–24 hPancreatic amylase 6.5–7.5 40–55 + + 12–24 hBacterial amylase 6.5–7.5 65–75 + + 1–4 hBacterial amylase 7.0–8.0 100–120 + + 1–2 min

+ Improved desizingo No effect

The primary alternative to enzymatic treatment is oxidative desizing. This is extremelypopular worldwide, especially in the Far East [169], and its popularity is increasing [143],mainly for the economic reasons outlined above. The oxidants most often used are hydrogenperoxide and sodium or potassium persulphate. There may be a considerable risk of fibredamage by persulphate, however, as this oxidant tends to degrade both starch and cellulose.The mechanism of degradation includes the hydrolysis of amylose (see Scheme 10.18) andamylopectin, as well as the formation of aldehyde (10.70), carboxylic acid (10.71), keto(10.72) and diketo (10.73) groups. There may also be ring cleavage [169] to give diacidssuch as tartaric (10.74) and oxalic (10.75).

O

HO OH

OHOH

CH O

10.70

O

HO OH

OHOH

CHO O

10.71

O

HO OH

OHO

CHO O

10.72

O

HO OH

OO

CHO O

10.73

C OH

OH

HO

O

CHO O

10.74

CC

O O

HO OH

10.75

This oxidative reaction is generally carried out simultaneously with a caustic scour at100–130 °C, offering the economic advantages of a combined scouring and desizing process.If peroxide is selected as the oxidant, this also exerts a bleaching action. In addition tooxidant and sodium hydroxide, the auxiliaries required include a magnesium salt, sodiumsilicate or an organic stabiliser for the oxidant, a sequestering agent (e.g. DTPA;

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diethylenetriaminepenta-acetic acid) and a wetting agent. The role of the magnesium saltand oxidant stabiliser is discussed elsewhere (section 10.5.3). Recommended conditions forbatchwise oxidative desizing at high temperature are given in Table 10.9.

Table 10.9 Oxidative desizing at high temperature [143]

Concentrations (% owf)

Magnesium sulphate heptahydrate 0.005 0.005 0.005Sodium hydroxide (100%) 2–6 2–4 9–12Stabiliser 0–1 0–1 0–1DTPA (40% solution) 0.2 0.2 0.2Hydrogen peroxide (35%) 5–15 5–15 5–15 or sodium persulphate 0.2–0.5 0.2–0.5 0.2–0.5Wetting agent 0.2–0.5 0.2–0.5 0.2–0.5

Temperature (°C) 100 100 120–130Time (min) <15 15–60 1–2

In this table, the lower amounts of sodium hydroxide provide desizing only, whereas thehigher concentrations provide oxidative scouring. The concentrations of the variouscomponents are critical to ensure efficiency of action without fibre damage. Hickman [143]gives the following guidelines:(1) Rapid desizing treatments require more critical control of alkali and oxidant

concentrations.(2) Increased alkalinity for a given oxidant concentration tends to increase chemical

damage.(3) Increased oxidant concentration above the minimum required for desizing increases

chemical damage.(4) Persulphates promote desizing, rather than bleaching, and require more critical control

of concentration than does hydrogen peroxide.(5) Mixing of persulphates and hydrogen peroxide is not recommended in pad–steam

desizing.(6) To desize oxidatively by a batchwise process, the oxidant must be added when the

alkaline fabric reaches top temperature.

It is possible to degrade and solubilise starch size residues using cold peroxide and alkali butbatching times are long (typically 16–20 hours). This simultaneous desizing and bleachingprocess requires 30 g/l sodium hydroxide (100%) and 50 ml/l hydrogen peroxide (35%),together with wetting agent, detergent, stabiliser and sequestering agent. The desizing effectcan be improved by addition of persulphate without risk of significant fibre damage at thistemperature [169].

It has been demonstrated [186] that the inclusion of polyacrylamide in either enzymatic oroxidative desizing formulations results in increased pick-up of the liquor by the sized warpyarns. Desizing by hydrolytic degradation of starch during the traditional kier-boiling treatmentusing 3°Bé sodium hydroxide liquor at 110 °C is now rarely encountered as it is a slow andexpensive process [169].


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In Table 10.7 are listed the following size polymer types in the water-soluble category:acrylic acid copolymers, poly(vinyl acetate/ alcohol), carboxymethylcellulose and certainmodified starches. It is necessary to clarify what is meant by water-soluble in this context.All these polymers form colloidal solutions and the dissolving process is not instantaneous.When films formed by such polymers come into contact with water, there is an initial periodduring which they absorb water. The rate at which water diffuses into the polymer filmvaries widely, depending on the polymer structure and the heat treatments that the film hasreceived. This phase is known as rehydration and is characterised by the formation of a gel.Only when the polymer has absorbed sufficient water does it actually begin to disintegrateand dissolve to give a viscous colloidal solution. Higher temperatures and the presence ofsurfactants generally increase the rates of rehydration and dissolution. Figure 10.16illustrates relative dissolution rates in cold water of various water-soluble size polymersprepared as 60 × 15 mm films of 200 µm thickness [169]. The disintegration level representsthe completion of rehydration. Thus in any desizing process relying solely on aqueousdissolution, it is always important to allow adequate time for rehydration.

4 8 12 16 20Treatment time/min

Dissolution

DisintegrationA B C D

E

Film thickness 200 m Dimensions 60 × 15 mm

ABCDE

Size CB/CAStarch etherPartially saponified poly(vinyl acetate)Carboxymethylcellulose, salt freeFully saponified poly(vinyl acetate)

Figure 10.16 Dissolution rates of films of water-soluble sizing agents in water at 20 °C [169]

Not all modified starches are suitable for removal by aqueous dissolution alone. Suchmodifications of natural starches are carried out to reduce solution viscosity, to improveadhesion and ostensibly to enhance aqueous solubility. Commercial brands vary [169],however, from readily soluble types to those of limited solubility. Indeed, some may be asdifficult to dissolve as potato starch if they have been overdried. It is thus very important tobe sure of the properties of any modified starch present. If there are any doubts aboutaqueous dissolution, desizing should be carried out by enzymatic or oxidative treatment.Even if the size polymer is sufficiently soluble, it is important to ensure that the washing-offrange is adequate. Whilst the above comments relate to modified starches, other sizepolymers such as poly(vinyl acetate/alcohol) and acrylic acid copolymers vary from brand tobrand with regard to ease of dissolution.

Carboxymethylcellulose, an excellent film-former, is a highly effective size on cellulosicsubstrates but has poor adhesion to synthetic fibres. It is easily desorbed, hot water generallybeing sufficient, although surfactant and alkali are usually added to increase the efficiency of

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removal. A further advantage of carboxymethylcellulose is that heat setting does not affectthe ease of subsequent removal, nor is it sensitive to alkaline or acidic pH. Indeed, it hasbeen stated [171] that if anyone has a problem removing a carboxymethylcellulose size, it isunlikely that they could remove much of anything from a piece of fabric! Nevertheless,Angstmann and Bassing [169] caution that this size does require much water and an initialswelling time.

The poor adhesion of carboxymethylcellulose to synthetic fibres means that where suchfibres are present, it can only be effective in combination with a synthetic size polymer(Table 10.6). This needs to be taken into account when considering suitable desizingprocedures. If this cellulose derivative is to be used in conjunction with an electrolyte-sensitive acrylic acid copolymer, it is advisable to choose a salt-free carboxymethylcellulose.

Poly(vinyl acetate/alcohol) sizes are also described as water-soluble and are widely used,either alone or in combination with most of the other types, across the whole range of fibresand blends [169,171]. However, this category covers a wide range of commercial products,differing greatly in quality and ease of removal. Indeed, some are quite difficult to remove,thus necessitating careful selection [187]. Detailed studies of factors affecting the removal ofwater-soluble sizes, particularly poly(vinyl alcohol) types, have been published [188–190].

Apart from molecular mass, the behaviour of polymers of this type depends strongly onalcohol group content, usually expressed as the percentage degree of saponification of thepoly(vinyl acetate) from which it is derived. Poly(vinyl alcohol) forms a strong film thatshows good adhesion to cellulosic fibres but poor adhesion to polyester. These films canusually be removed quite readily with hot water and detergent. The presence of a mild alkalimay be acceptable [171] but is perhaps best avoided [169]. Poly(vinyl alcohol) is sensitive toalkali addition and is precipitated by strong alkali, making the residues very difficult toremove. The wash-off parameters should be optimised for efficiency of size removal, sincehigh temperatures and long liquor ratios are environmentally and economicallyunfavourable. The amount of size polymer applied and the presence of any additives need tobe carefully considered. Care should be taken in the heat setting of fabrics containingpoly(vinyl alcohol) size. Too high a temperature for too long a time induces crystallisation,making the size residues much more difficult to remove.

Poly(vinyl alcohol) can be modified with crotonic acid (Scheme 10.17) to give sizecopolymers that have higher solubility and lower sensitivity to alkali [169]. Although thesesizes are generally regarded as water-soluble, they are more readily removed by alkalineoxidative desizing methods using either persulphate or peroxide [169], the polymer beingdegraded into smaller segments as indicated in Scheme 10.19.

CH2

CH

CH2

CH

CH2

OH OH

CH2

C

CH2

C

CH2

O O

CH2

C

OH

O

C

C

CH2

O

HO

O

Na2S2O8

or H2O2

+

Scheme 10.19


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Many acrylic acid copolymers are water-soluble but unlike poly(vinyl alcohol) they arenot degraded by alkali. In fact they need alkali for effective desizing as they are more solubleat alkaline pH than in neutral solutions. They are sensitive to acidic media, which shouldnot be used. Solubilisation occurs by the formation of sodium carboxylate groups from theanionic polyacid. The polyelectrolyte formed in this way is readily soluble and shows a rapidrate of dissolution. However, the presence of electrolytes such as magnesium or calcium saltsfrom hard water can inhibit removal [191].

The desizing of water-soluble size polymers can be summarised as follows. Batchwise orcontinuous methods can be used; in both cases an adequate supply of hot water is neededduring the washing-off. Hot water and detergent are needed to remove poly(vinyl alcohol)or carboxymethylcellulose. The addition of alkali may be beneficial withcarboxymethylcellulose. Alkali is essential with modified starches and acrylic acidcopolymers. Poly(vinyl alcohol) can be degraded effectively by alkaline oxidation.

Water-resistant sizes include polyesters and certain acrylic acid copolymers. These aremainly used for the sizing of synthetic filament warp yarns, some formulations beingparticularly useful in water-jet weaving. The acrylic acid size copolymers described as water-soluble are generally partially neutralised during manufacture to give partial formation of thesodium salt; complete conversion to the sodium salt is carried out using alkali in desizing.Water-dispersible types, on the other hand, are normally partially neutralised with ammoniaduring manufacture and these are more suitable for water-jet weaving. When the sized yarnis dried the ammonia is evolved, leaving the less soluble acrylic acid form. This lowersolubility is a significant advantage in water-jet weaving. A yarn sized with a sodium acrylatecopolymer will have a typical pH of 6–7 when dry, compared with pH 4–5 for an ammoniumacrylate type [171]. This difference in pH needs to be accounted for when desizing, so thatmore alkali is needed for the desizing of ammonium acrylate copolymer sizes. Typicaldesizing conditions include a detergent with up to 5 g/l sodium hydroxide or sodiumcarbonate at 70–90 °C [171]. The pH should be carefully monitored in order to maintainalkalinity throughout. A sequestering agent may be needed because transition-metal ionscan insolubilise acrylic acid copolymers, one atom of trace metal reacting with two or threecarboxylic acid groups in the size polymer.

Polyester sizes differ from acrylic acid sizes in the nature of the solubilising groups present,namely sulphonic acid rather than carboxylic acid groups, solubility behaviour beingdependent on the degree of sulphonation with respect to the average molecular mass of thepolymer (10.76). In general, however, the degree of sulphonation is low (10.69) and thepolymer is rather sensitive to alkali. Too strongly alkaline conditions result in hydrolyticcleavage of the ester groups (Scheme 10.20) to give some fragments without solubilisinggroups and these are difficult to remove in desizing [169]. Consequently, although alkali anddetergent are needed for desizing, the amount of alkali is less than that needed for acrylicacid sizes and the alkalinity should not be allowed to rise above pH 9. Typical conditions are0.5–1.0 g/l sodium carbonate with detergent at 90–95 °C [169]. A sequestering agent mayalso prove useful. Batchwise treatment, or more usually a pad-wash range, can be used.

Polyester resins can be highly beneficial as additives to other size polymers, although a greatdeal of care and expertise is required in formulation [192]. Viscosity, for example, is animportant factor in the warp sizing process. The viscosity of some sizes, such as poly(vinylalcohol), is significantly affected by temperature fluctuations. The addition of a polyester resintends to minimise such changes in viscosity. Surface tension is another important parameter

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that is subject to variability and polyester resins can help to stabilise it within the optimumrange. Such resins have beneficial effects on rheological behaviour, film formation and filmcharacteristics, giving significant improvements in yarn abrasion resistance, good surfacelubrication and a softening of the surface fibres. Certain acrylic acid or vinyl copolymer resinsbehave in a similar way to polyester resins in enhancing the performance of conventional sizeformulations. The degree of expertise required in formulating such complex mixtures of sizepolymers for optimal performance has led Mayfield [192] to emphasise the advantages of usingproprietary products formulated by specialists. These products may contain up to thirtyingredients, including trace additives that can effectively influence the many componentspresent. These formulations can be very accurately combined and evaluated for chemical andphysico-chemical compatibility by specialist suppliers.

Analysis and monitoring of size polymers and desizing processes

The many technical factors involved in desizing and the need for economy andenvironmental accountability emphasise the importance of monitoring and analysis.However, only a brief outline can be given here. A review of analytical procedures andsimple laboratory methods for size determination is available [193]. Methods are given forsize determination directly on the fibre surface, for the extraction of components of lowmolecular mass and for their subsequent estimation in solution.

Size polymers on polyester can be determined by staining tests with CI Basic Red 22, CIReactive Red 12, iodine/potassium iodide solution, or a mixed indicator. The extraction ofsize components and their determination in solution using a variety of reagents to give acharacteristic coloration or a coloured precipitate has been described. Methods usingfluorescence spectroscopy with a fluorescent cationic dye (e.g. Pinacryptol Yellow or CIBasic Orange 14) were also described.

O C R1

O

C

SO3Na

O

O

R2 O

X2

C R3

O

C O

O

R1 O

SO3Na

C R2

O

C

X2

O

O

HO C R1

O

C

SO3Na

OH

OHO R2 OH

X2

HO C R3

O

C OH

O

HO C R2

O

C

X2

OH

OHO R1 OH

SO3Na

10.76

hydrolytic cleavage

In structure 10.76 (see also 10.69) either X1 or X2 contains a sulpho group, but not both.R3 is an unsulphonated aliphatic group

(= X1)

(=X1)

Scheme 10.20


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An extremely useful technique for measuring the amount of size applied is non-contacton-line determination of water absorption [194]. The moisture content of sized warps can bederived from microwave absorption by the water present.

The identification of anionic poly(acrylic acid) sizes can be carried out by staining with afluorescent cationic dye (CI Basic Orange 14) followed by spectroscopic measurement ofexcitation wavelength and fluorescence emission [195,196]. Such methods can also be used(with CI Basic Orange 14 or CI Basic Red 1) to detect and estimate carboxymethylcellulose,poly(vinyl alcohol) and starch derivatives [197].

A method is available, utilising on-line near-infrared reflectance spectroscopy, forcontrolling the uniform application of poly(vinyl alcohol) size [198].

The desizing of cotton can be monitored using a dyeing test supported by assessment ofwettability [199,200]. The dyeing test requires a 10 g/l solution of CI Direct Red 83. Thistest can usefully be supplemented by the TEGEWA drop test to determine wettability. Asimple method of estimating the efficiency of desizing with poly(vinyl alcohol) and starch-based sizes depends on the determination of TOC (total organic carbon) and COD(chemical oxygen demand) [201].

Environmental aspects

Since size add-on is customarily in the region of 10–20%, desizing clearly produces quite asizeable pollution load. Although many size polymers are biodegradable, they exhibit highbiological oxygen demand (BOD) and chemical oxygen demand (COD). These high valuesare compounded by the major contribution of size residues. High liquor ratios or copiousquantities of wash-off water facilitate the desizing process but these large volumes of wasteliquor pose additional difficulties quite apart from the initial cost, both financially andenvironmentally. In Table 10.10 the water consumption, BOD and pollution loadcontributed by desizing are compared with those from other wet processes on cotton [202].Typical BOD and COD values determined for the main classes of size polymers are given inTable 10.11 [203]. The COD of a typical cotton desizing effluent is composed of 42% fromsizing agents and fibre fragments, 40% from impurities in the cotton and 8% fromsurfactants [204].

Table 10.10 Water and effluent data arising from the wet processing ofcotton [202]

Water consumption BOD Pollution loadProcess (% of total) (% of total) (% of total)

Desizing 5 22 >50Scouring 1 54 10–25Bleaching 46 5 3Mercerising 2 <4Dyeing 8 5 10–20Printing 7 6 10–20Washing-off 30 1 5Finishing 1 7 15

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Clearly, one option to reduce the add-on is to use high-efficiency size formulations.However, there is a limit to what can be achieved by this approach. Even if the add-on isreduced to only 5%, the pollution load is still substantial. The two main options to facilitatedisposal are: (a) recovery of size polymers; and (b) biological effluent treatment. Recovery ofsize polymers, particularly from water-soluble synthetic sizes, is based on extraction washingusing the minimum quantity of water. Recovery rates in the region of 50% have been quotedfor poly(vinyl alcohol) and carboxymethylcellulose size formulations. It is necessary to applyone of three concentration techniques: precipitation, condensation or ultrafiltration [205].

Precipitation is only really possible with poly(vinyl alcohol) and is seldom applied totextile effluents, normally only to eliminate size residues from effluent liquors. Thecondensation technique exploits heating to drive off water. This is energy-intensive and istherefore in decline. Ultrafiltration is long-established commercially and is the preferredconcentrating technique. The principle of ultrafiltration, capable of separating particles inthe range 0.05–0.15 µm, has been compared with other systems of filtration, such as reverseosmosis, nanofiltration and microfiltration [206]. A sequence of treatments can be used,such as ultrafiltration to recover most of the size followed by biological treatment of theresidue. In some cases ultrafiltration renders an effluent acceptable for discharge to themunicipal water treatment system, but this depends on local regulations.

One major advantage of ultrafiltration is that it can facilitate the reuse of recoveredmaterials [207]. In the case of sizing, however, this clearly depends crucially on thecomposition of the recovered material. If a blend of sizing agents is present, it does notnecessarily follow that the recovered material will contain these components in the original(required) proportions for reuse. Nor is reuse a possibility where desizing has been carried outby a degradative process, such as enzymatic or oxidative desizing. Thus the scope for reuse isfavoured where single-component formulations of non-degraded size polymer can berecovered. Simple formulations, however, may not meet all the weaving requirements. Hencearguments for and against the relative merits of simple and complex formulations are common.

The basis of ultrafiltration is that a liquor is passed through a membrane many times untilthe required concentration of the permeate is attained. Fouling of the membrane can be aproblem and regular cleaning and disinfection of the membrane is recommended.Ultrafiltration of poly(vinyl alcohol) and starch sizes offers economic advantages over

Table 10.11 Specific COD and BOD values found for important sizing agents [203]

Specific COD Specific BODSizing agent value (mg O2/g) value (mg O2/g)

Starch 900–1000a 500–600Carboxymethylcellulose 800–1000a 50–90Poly(vinyl alcohol) ca. 1700a 30–80b

Polyacrylate 1350–1650a <50Galactomannan 1000–1150a 400Polyester dispersion 1600–1700a <50Protein size 1200a 700–800

a Taking account of moisture content of commercial productb With non-adapted inoculum


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discharge to the sewer [208,209]. A starch carbamate size effluent had its entry COD of19 159 mg O2/l reduced by ultrafiltration to 672 mg O2/l, a recovery yield of almost 97%.Starch ethers, such as hydroxypropyl and carboxymethyl derivatives, have given recoveryvalues of 92–97% [209]. Size polymers suitable for recovery by ultrafiltration must exhibitthe following characteristics: water solubility, thermal and mechanical stability,bioresistance, good washing-out properties and low to moderate viscosity [210]. Poly(vinylalcohol), carboxymethylcellulose and certain acrylic sizes meet these demands. The ultimatebenefits of ultrafiltration from economic and environmental aspects are that not only cansize be recovered for reuse but also the water that passes through the membrane can berecycled into the washing-off range [211].

Biological treatment of a desizing effluent usually involves a two-stage anaerobictreatment using cultivated methane bacteria, possibly followed by a final aerobic treatment[209]. Typically, a dwell time of 12–17 days will give about 80% degradation of a starch size.Such treatment usually gives an effluent acceptable for discharge to the municipal sewagesystem. Several factors other than the nature of the size polymer can affect the efficiency ofbiological treatment, notably the degree of adaptation of the biological inoculum and thetemperature at which digestion takes place. The method of evaluation can also influence theresults obtained. Figures 10.17 and 10.18 illustrate the effects of adaptation of the inoculumon the bioelimination of a modified poly(vinyl alcohol) size [212], as assessed by theSapromat [213] and Zahn–Wellens [214] test protocols respectively. The effect of digestiontemperature on the same polymer using optimally adapted inoculum is shown in Figure10.19. Thus seasonal temperature fluctuations need to be considered. It is possible toformulate mixtures of size polymers so that the effect of digestion temperature is minimised.Data showing the effects of temperature on the rates of biodegradation of hydroxypropylstarch and of a formulation containing poly(vinyl alcohol) and hydroxypropyl starch [215]reveal that the polymer mixture is much less sensitive to the temperature of digestion. Therate of influx and even of rainfall can also exert some influence on the rate of elimination.

Time/days

4 12 20 28

20

40

60

100

80

Bio

degr

adat

ion/

% adapted

non–adapted

Figure 10.17 Biodegradation rate curves for poly(vinyl alcohol) with adapted and non-adaptedinoculum according to the Sapromat test [212]

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The following scale has been proposed to assist discussions of biodegradability [216,217]:

Degree of biodegradationafter 14 days Description80–100% easily biodegradable60–80% moderately biodegradable 0–60% difficultly biodegradable

Many starches and modified starches are easily biodegradable, although some are less so. Inan evaluation of the biodegradability of ten starch-based sizes over seven days [218], four

Time/days2 4 6

20

40

60

100

80

DO

C–e

limin

atio

n/%

optimally adapted

non–adapted

Figure 10.18 DOC-elimination rate curves for poly(vinyl alcohol) with optimally adapted and non-adapted inoculum according to the Zahn-Wellens test [212]

Time/days6 12 18

20

40

60

100

80

DO

C–e

limin

atio

n/%

23 oC

15 oC10 oC 8 oC

Figure 10.19 DOC-elimination rate curves for poly(vinyl alcohol) with optimally adapted inoculum atvarious temperatures [212]


chpt10(2).pmd 15/11/02, 15:44575


were degraded to 95–100%, two to 80–95%, three to 60–80% and only one to less than 60%.Whilst carboxymethyl starches are easily biodegradable [218], carboxymethylcelluloses aregenerally difficult to degrade [211,217]. The biodegradation of carboxymethylcellulose canbe as low as 8–13% under aerobic conditions, although up to 54% can be eliminated underanaerobic conditions [211]. Galactomannan sizes are biodegradable to 80% within sevendays [216,217].

It was mentioned earlier that poly(vinyl alcohol) sizes show wide variability in aqueoussolubility. They are also extremely variable with regard to biodegradation characteristics,from 15% to 95%. It is not surprising, therefore, that they have been the subject of muchresearch [187,203,211,212,219] and not a little controversy. Reference 203 lists twelvefurther references concerned with the microbial degradation of poly(vinyl alcohol).Misunderstandings have undoubtedly arisen through a failure to appreciate the need for anadequate digestion temperature and effective acclimatisation of the bacterium. There can bea drastic fall in efficiency of degradation at temperatures lower than 10–12 °C, since thedesorption rate of the degrading micro-organisms at such temperatures exceeds the growthrate [203,219] and bioactivity is lower in any case. The fluctuations are influenced byvariations in the composition of commercial size formulations, including the presence oforganic chemicals such as methanol and acetic acid remaining from the polymerisationprocess, which may increase or decrease the biodegradability. In general, poly(vinyl alcohol)sizes can be degraded or eliminated by an optimally adapted inoculum in an appropriatesewage sludge phase, giving decomposition rates of 57–65% within seven days but with apotential for up to 100% elimination [211,212]. Trials have been carried out at a lowtemperature (8 °C) to demonstrate that even in winter poly(vinyl alcohol) can be eliminatedby more than 90%, admittedly at a slower rate [211,212].

Acrylic size polymers generally show poor biodegradability (typically 3–10%), althoughrecent research [211] has achieved up to 37% decomposition in special anaerobic cultures.However, these anionic polyelectrolytes can be readily eliminated from effluents byadsorption on sludge [220]. Acrylic size polymers may be precipitated in the third stage nowincluded in many water treatment plants for the removal of phosphates, using aconventional inorganic precipitant such as iron(III) chloride. Some size formulations mayrequire the addition of a specific precipitant of this kind, whilst others may be removed bybioelimination in the sludge. Polyester sizes may be disposed of by similar bioeliminationmethods [220]. Deliberate precipitation creates a need for subsequent disposal, but withacrylic size polymers this solid waste has high calorific value and is suitable for disposal byincineration. An advantage of acrylic size polymers arising from their ease of removal byprecipitation is that they can be readily recycled, so that they need not enter the sewagesystem. Ultrafiltration is the preferred method of recycling [220,221].

10.5.3 Bleaching

The three primary oxidants associated with textile bleaching are hydrogen peroxide (H2O2),sodium hypochlorite (NaOC1) and sodium chlorite (NaClO2). There are other oxidisingand reducing agents occasionally used (or proposed for use) in bleaching; these will be dealtwith later. For the present, general factors affecting the use of the above three oxidisingagents will be discussed. Many technical factors govern the selection of one bleaching agentover another:

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– generic type of fibre (cellulosic, wool, synthetic)– physical form of fibre (yarn, woven or knitted fabric)– product and process costs– stability, and therefore reliability– versatility of process (batchwise or continuous)– degree of whiteness obtained– extent of any fibre damage.

Environmental factors have now become important in bleaching and are having far-reachingeffects on the choice of bleaching agent. A particularly important parameter is theabsorbable organic halogen value, commonly referred to as AOX. This is generally expressedas the mass of organohalogen compounds absorbed per unit mass of activated charcoal,there being three main steps in the analytical procedure [222]:(1) Absorption of all organic halogen compounds by activated charcoal, taking care to

avoid misleading results by uptake of inorganic salts such as sodium chloride(2) Combustion of the activated charcoal and collection of the halogens thus released(3) Quantitative determination of the halogens.

This aspect came to the fore with the well-known and much publicised discovery of thepotent toxin and carcinogen 2,3,7,8-tetrachlorodibenzo-p-dioxin (10.77), commonly buterroneously referred to simply as ‘dioxin’, in effluent from chlorine-based bleaching processesfor wood pulp in papermaking. Although legislative requirements vary considerablyworldwide the trend has been to ban or severely restrict the discharge of AOX-containingliquors. Typically, an AOX value of 0.5 mg/l must not be exceeded in waste water releasedfrom a textile wet processing works. This compares with a typical limit for drinking water ofnot more than 0.01 mg/l. In some legislative criteria there are limits on discharge from eachprocessing line in a works, as well as on overall discharge. Typical values for three oxidisingagents in cotton bleaching processes where no attempt has been made to minimise the AOXvalue are listed in Table 10.12.

O

OCl

Cl

Cl

Cl

10.77

Table 10.12 AOX values for cotton bleached with variousoxidising agents [223]

Oxidising agent AOX (mg/l)

Sodium hypochlorite 27.00Sodium chlorite 2.40Hydrogen peroxide 0.92


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The seriousness of the AOX problem is highlighted by the fact that there is currently noeconomically acceptable solution for the specific removal of halogenated organic compoundsfrom effluents [224]. There are important technical reasons why, over the years, hydrogenperoxide has progressively gained ascendancy over sodium hypochlorite as a bleachingagent. In view of the AOX values in Table 10.12 it is clear that there will be additionalpressure, this time on ecological grounds, to continue the phasing out of hypochloritebleaching in favour of peroxide. This is in spite of the fact that there are some steps (to bementioned later) that can be taken to reduce the AOX values of effluent from hypochloritebleaching. The intention here is to concentrate discussion on the chemistry of the productsused. Hydrogen peroxide, because of its pre-eminence in bleaching technology, will be dealtwith first, followed by sodium hypochlorite, sodium chlorite and other bleaching agents. Theinitial discussion is concerned mainly with cellulosic fibres, with some reference to syntheticfibres; wool and silk are then dealt with separately. The processing details of bleaching andthe machinery used are dealt with elsewhere [11,143,225,226]. Excellent historical accountsare also available [227,228].

Peroxide bleaching

The advantages favouring the pre-eminence of hydrogen peroxide (over 90% of cottongoods are bleached with peroxide) include [143,225]:(1) Stability and consistency of supply

Most of the hydrogen peroxide solution supplied for textile bleaching is acidic (pH 4.5–5.0) because it shows maximum stability under these conditions. Additives are presentto increase its stability further at this pH.

(2) Environmentally friendlyAs indicated above, peroxide does not contribute to AOX values and potentiallydecomposes completely into water and oxygen during effluent treatment.

(3) Versatility of applicationPeroxide can be used over a wide range of application conditions in batchwise andcontinuous methods, the latter being predominant. The success of continuous peroxidebleaching is attributable to the relatively rapid rate at which bleaching takes place,although long-dwell processes are also established. Peroxide baths do not causesignificant corrosion of machinery. Peroxide gives a good white and can be used evenwithout prior scouring of the material. It is particularly suitable for combining withother processes, e.g. scouring, desizing, application of fluorescent brighteners, as well asthe bleaching of many blends.

The technical disadvantages of hydrogen peroxide are relatively minor compared with theprocess costs:(1) Fibre damage

This may occur by free-radical formation, especially in the presence of transition-metalions such as those of iron or copper. Similar mechanisms can result in thedecomposition of peroxide but there are means of controlling or avoiding this problem.

(2) Quality of whitenessHigher levels of whiteness and more attractive tones (i.e. neutral or bluish rather thanyellowish or reddish whites) are attainable with hypochlorite, although whites of thehighest quality are produced using the two bleaching agents sequentially.

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(3) Cost-effectivenessIf the costs for treating AOX-containing effluent are omitted, peroxide bleaching ismore costly than hypochlorite bleaching. Data has been presented [222] showing thatthe chemical costs of a continuous bleaching process with alkaline hydrogen peroxideare on average about six times higher than those from sodium hypochlorite. Inbatchwise processing, peroxide can be twice as expensive as hypochlorite [222].

As mentioned above, hydrogen peroxide is available commercially as a stabilised liquid ofpH 4.5–5.5. Solid peroxides have been proposed, with claims of bleached fabrics with greaterstrength and more stable whites [229], but it is difficult to foresee such solid peroxidesgaining a commercial foothold at the expense of liquid hydrogen peroxide. Hydrogenperoxide exerts little bleaching action at pH 4.5–5.5 and requires activation to produce ableaching effect. The principal means of activation is alkali addition. Given the requireddegree of alkalinity, temperature provides a further means of controlling the bleachingaction, such as an overnight dwell at ambient temperature or from 3 to 20 minutes at 100°C. Figure 10.20 shows the degree of whiteness attained at various pH [226], together withthe corresponding curve for the amount of peroxide remaining in the treatment bath. Thisclassic activation curve for hydrogen peroxide shows that bleaching is generally best carriedout at pH 10.5–11.0. The amount of alkali required to give this pH will vary with the type ofprocess, sodium hydroxide being the alkali most commonly added, although sodiumcarbonate or phosphate may be used.

Initial pH

9.5 10.0 10.5 11.0 11.5

75

80

85

95

90

20

40

60

100

80

CIE

whi

tene

ss

Res

idua

l H2O

2/%

Figure 10.20 Optimisation of pH in hydrogen peroxide bleaching [226]

Under alkaline conditions an additive is required for stabilisation of the peroxide, whichis necessary to avoid undesirably rapid decomposition with loss of bleaching efficiency and/or damage to the fibre. Traditionally, the most common stabilising agents have been thecolloidal sodium silicates. The formulae of sodium silicates are best represented in terms ofthe ratio of sodium oxide to silica, which is 1 in sodium metasilicate (Na2O:SiO2) and 2 inthe orthosilicate (2Na2O:SiO2). These silicates, however, are crystalline forms in which thisratio is 1 or greater. In the colloidal forms originally preferred for peroxide bleaching the


chpt10(2).pmd 15/11/02, 15:44579


ratio is less than 1. For example, in the so-called ‘alkaline glass’ form the ratio is 1:2, whilstin the so-called ‘water glass’ form it varies from 1:1.6 to 1:1.38. The colloidal silicates areefficient and economical stabilisers but care is needed to ensure efficient washing-off inorder to avoid silicaceous deposits on the fabric and equipment. A simple and rapid testprocedure has been developed to determine the relative suitability of stabilisers, with a viewto keeping the running surfaces of a continuous bleaching range free from deposits [230].Although such colloidal forms have been preferred, the crystalline meta- and orthosilicatescan also be used and may provide easier washing-off.

The required degree of alkalinity is generally obtained by the addition of sodiumhydroxide, sodium carbonate or a phosphate, the amount of alkali varying with the type andquantity of silicate used. Since the bleaching action yields acid, sufficient alkali is requiredfor neutralisation as well as absorption by the cellulose. The mechanism by which thesestabilisers act is complex, although the elements of buffering action and sequestering oftransition-metal ions, such as those of iron(III) and copper(II), undoubtedly contribute.Magnesium ions also play an essential part in the mechanism and must be added (as thesulphate, for example) if sufficient are not already present in the system. It is important torecognise that whilst transition-metal ions catalyse the decomposition of peroxide, thealkaline earth elements stabilise it. In the absence of calcium and magnesium even silicatescan act as bleach activators [19].

The problems associated with silicaceous deposits have led to the adoption of more costlyorganic stabilising agents that also aid in plant cleaning and reduce the incidence ofreprocessing. These organic stabilisers are often commercially blended products which mayor may not contain magnesium salts [143], the three main types being aminopolycarboxylatesequestering agents, protein degradation products and selected surfactants. The preferredsequestering agents, in terms of both sequestering ability and stability to oxidation, areDTPA (10.6), either as its sodium or its magnesium salt, and its hydroxy derivatives [18].Relatively simple methods of evaluating the efficiency of stabilisers have been used, butmore reliable results are obtained with statistical experimental methods involving a realisticsimulation of the bleaching process [231,232].

The mechanism of peroxide bleaching

The details of this mechanism have been the subject of much debate over the years.Dannacher and Schlenker [233] have reviewed the various hypotheses and appliedexperimental criteria to establish the most likely mechanism. The fact that the bleachingeffect varies widely with reaction conditions suggests that the actual bleaching agent is nothydrogen peroxide itself but another species liberated from the peroxide under the influenceof pH and temperature. This much is generally agreed; there is, however, much debateregarding the liberated species. Since molecular oxygen has no bleaching effect, nascent oratomic oxygen has been proposed as the active species on the basis that it is readily liberatedfrom the perhydroxide anion (HOO–), according to Scheme 10.21. Dannacher andSchlenker reject this, since calculations show that the formation of oxygen atoms isenergetically highly unfavourable and is not expected to occur under bleaching conditions.

Singlet oxygen has also been proposed and this can indeed be formed, particularly inmixtures of hydrogen peroxide and sodium hypochlorite. However, carefully designedexperiments showed that under these conditions sodium hypochlorite has neither a direct

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bleaching effect (because it is consumed in the reaction with peroxide) nor an indirectbleaching effect via singlet oxygen. Dannacher and Schlenker also examined the effect ofsinglet oxygen derived from the thermal decomposition of an endoperoxide (10.78). Theyconcluded that singlet oxygen has no bleaching effect when homogeneously dispersed in aliquor and that alkaline hydrogen peroxide solution contains no singlet oxygen.

The active species most often cited in recent times has been the perhydroxide anionmentioned previously (Scheme 10.21), even though it shows a decrease in redox potentialwith increasing pH. Hydrogen peroxide exists in aqueous solution in a dissociatedequilibrium with the perhydroxide anion (HOO–) and the peroxo dianion (–O-O–), as inScheme 10.22. In view of the predominance of this hypothesis, Dannacher and Schlenkersubjected it to a thorough investigation, studying the bleaching effect of hydrogen peroxideat 60 °C over the range of pH 2 to 13 and obtaining the results shown in Figure 10.21. If the

H

HOHOO HO [O]H2O2 +

_ __

+

Scheme 10.21

CHCH

CHC

CH2CH2COO

OO

10.78

_

H2O2

H

HO

H

HOHOO

+

_

+

__

O O_ _

Scheme 10.22

pH

2 4 6 8 10 12 14

20

40

60

100

80

Sta

in r

emov

al/%

ab

Experimental valuesH2O2

HOO–

Figure 10.21 Relative stain removal after 60 min bleaching at 60 °C [233] 6.5 × 10–3 mol/l hydrogenperoxide in buffer solution. Relative share (%) of the protolytic forms of hydrogen peroxide in the totalamount at 60 °C (–O–O– negligible in this range)


chpt10(2).pmd 15/11/02, 15:44581


perhydroxide anion were the effective agent one would expect the bleaching effect toincrease with concentration and thus with increase in pH beyond the equilibrium point ofpH 11. Figure 10.21, however, shows that at the point where the concentration ofperhydroxide anions begins to exceed that of undissociated hydrogen peroxide, thebleaching effect decreases considerably, indicating that the perhydroxide anion is clearly notthe effective bleaching agent [233].

As shown in Scheme 10.23, the perhydroxide anion can give rise to both perhydroxyl andhydroxyl free radicals. These free radicals have been proposed as the active bleachingagents. Dannacher and Schlenker carried out bleaching tests in the presence of scavengersfor the hydroxyl free radical, 4-nitroso-N,N-dimethylaniline (10.79) and potassiumhexacyanoferrate(II) (10.80) separately at a concentration of 10–3 mol/l [233]. Thebleaching effect was just as powerful in the presence of the scavengers as in their absence. Itwas concluded that the hydroxyl free radical has no bearing on the bleaching effect ofhydrogen peroxide.

H O O H O OH2O2 HO HO+ ++

Perhydroxideanion

Hydrogenperoxide

Perhydroxylradical

Hydroxylradical

Hydroxideanion

_

Scheme 10.23

N N

CH3

CH3

O

10.79

NC Fe CN

CNNC

CNNCK K

KK

10.80

+

+

+

+

Consideration of pKa values suggests that the perhydroxyl radical is dissociated almostquantitatively in the optimum pH range for peroxide bleaching, giving rise to the conjugatebase .O–O– known as the superoxide radical ion (Scheme 10.24). When Dannacher andSchlenker carried out tests using hydroquinone (10.81) as a scavenger for the superoxideradical ion, it was found that the bleaching effect decreased with increasing concentration ofscavenger (Figure 10.22). These results suggested that the effective species in hydrogenperoxide bleaching is the superoxide radical. The agreement between experimental andcalculated results for superoxide concentration at different pH values was taken as furthersupport for this mechanism [233].

H O O O OHO

H

_

+

_

Perhydroxylradical

Superoxideradical ion

Scheme 10.24

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583

However, these conclusions have been vigorously challenged by Spiro and Griffith [234].The bleaching tests of Dannacher and Schlenker were carried out on cotton stained withtea. Spiro and Griffith pointed out that tea stains are complex mixtures of colouredpolyphenolic compounds with unknown protonation constants. They claimed that kineticstudies of peroxide bleaching are more easily interpreted using individual colouredcompounds with known properties such as phenolphthalein (10.82), alizarin (10.83) orcrocetin (10.84), a yellow polymethine colorant isolated from saffron. Spectrophotometricstudies by Spiro and Griffith of the bleaching of these compounds with hydrogen peroxidedemonstrated that none of these reaction rates was affected by addition of a scavenger forhydroxyl and perhydroxyl radicals, which were therefore ruled out as effective species. Theydo concur with Dannacher and Schlenker, however, that singlet oxygen can also be ruledout on the basis of scavenging experiments. Offering a radically different interpretation,Spiro and Griffith claimed that all the kinetic data can be explained quantitatively on theassumption that the perhydroxide anion (at higher pH values) and hydrogen peroxide itself(at lower pH values) are the only active oxidising species. Further studies of the bleaching ofmalvidine chloride (10.85) at pH 1.5–4.0 showed that the only significant oxidant was thehydrogen peroxide molecule, which was thus not as inert as Dannacher and Schlenkerapparently believed. Spiro and Griffith concluded that in the peroxide bleaching of a widerange of colorants there is no need to invoke any oxidising species other than theperhydroxide anion and molecular hydrogen peroxide.

HO OH

10.81

Hydroquinone

Hydroquinone concentration/mmol l–1

0.5 1 2 3 4 5

20

0

40

60

100

80

Sta

in r

emov

al/%

With 6.5 × 10–3 mol/l hydrogen peroxideWithout hydrogen peroxide

Figure 10.22 Effect of increasing hydroquinone concentration on the relative stain removal after 60min at pH 10.5 [233]


chpt10(2).pmd 15/11/02, 15:44583


Thus the detailed mechanism of peroxide bleaching is not yet finally resolved. It shouldbe borne in mind, however, that the work of Dannacher and Schlenker was carried out at60 °C on tea-stained cotton, whereas Spiro and Griffith studied the decolorisation ofindividual colorants at 21–25 °C in the absence of a textile substrate.

Fibre tendering and process control

In an optimally controlled process free from transition-metal ions hydrogen peroxidebleaching is remarkably safe, there being no reported detrimental effects of bleaching ataround 100 °C or for more than several hours [143]. Under such conditions, most of theperoxide appears to be consumed in the oxidation of chain end units of the cellulosemacromolecule. The other major effect on the substrate is oxidation of secondary hydroxy toketo groups, accompanied by the formation of very few aldehyde or carboxyl groups [235].

Owing to the relative inactivity of keto groups in cellulose, the bleached effect is stableand is not susceptible to yellowing. This action of hydrogen peroxide is not entirely randomamongst all cellulosic hydroxy groups but is directed mainly towards the C3 hydroxy groupon randomly located glucose units in the amorphous regions of cellulose, as many as 8–17keto groups being formed per macromolecule. The formation of keto groups is thought tooccur as a result of transient generation of the free radical form of the C3 hydroxy group, asin Scheme 10.25. Free radicals can be formed from the C3 hydroxy groups following reaction

C

O

O

HO OH

10.82

Phenolphthalein

O

O

OH

OH

10.83

Alizarin

C C

O

HO

CH

CH3

CH CH C CH

CH3

CH CH CH C CH

CH3

CH CH C C

CH3

O

OH

10.84Crocetin

O

OH

HO

OH

Cl

10.85

+_

OH

OCH3

OCH3

chpt10(2).pmd 15/11/02, 15:44584

585

between hydrogen peroxide and perhydroxide anions (Scheme 10.23) although other factorsmay be involved, such as a localised concentration of radicals in the amorphous regions ofthe fibre. A supply of free radicals can be generated by heat treatment, without requiring thepresence of transition-metal ions as a catalyst. The number of oxygen atoms consumed perchain scission exceeds 100, compared with 26 for hypochlorite bleaching. The yield ofoxidised hydroxy groups is less than 10%, whereas it is about 40% for hypochloritetreatment. Under these circumstances, the decomposition of hydrogen peroxide and itsoxidative modification of cellulose can be adequately regulated by a well-formulatedstabiliser, so that an unacceptable degree of fibre damage is avoided.

R C H

R

OH

R C

R

OH

R C

R

OH

R C

O

R

H2O

H2OHO

HO+ +

+ +

Scheme 10.25

In the presence of certain transition-metal ions, however, notably Fe(II) or Fe(III), thedecomposition of hydrogen peroxide and its oxidative attack on cellulose are accelerated.This results in uncontrolled damage, even to the extent in some cases of forming holes in afabric. Although sequestering agents and other stabilisers, such as silicates, can decreasesuch severe degradation, in practice they cannot be relied upon to give consistent totalprotection in the presence of catalysing metals. In such cases, a pretreatment specifically toremove such metals before bleaching is recommended [143].

Various metals can be present in raw cotton, the amounts varying with source andtreatment (Table 10.13). There has been a trend in recent years towards increased metalcontamination of textiles, giving rise to a great deal of research in this area. Indeed, Reicher[236] cites 29 other references in the period 1987–91 alone. Rotor spinning has becomemore prevalent in recent years and this is known to give an increased incidence of localisedcritical concentrations of metal contamination. Despite the overall complexity of metalliccontamination in cotton fibres, iron is the main culprit with regard to catalytic damage inperoxide bleaching [236,237] and hence is the only one considered here.

Metal-ion catalysis of hydrogen peroxide decomposition can generate perhydroxyl andhydroxyl free radicals as in Scheme 10.26 [235]. The catalytic effects of Fe2+ and Fe3+ ionsare found to be similar [235]. It is not necessary for the active catalyst to be dissolved [237],as rust particles can be a prime cause of local damage. The degradative free-radical reactioncompetes with the bleaching reaction, as illustrated in Scheme 10.27 [237]. Two adverseconsequences arise from the presence of free radicals:– fibre damage– rapid decomposition of hydrogen peroxide, leading to a corresponding loss of bleaching

action.


chpt10(2).pmd 15/11/02, 15:44585


Table 10.13 Metal ion content of six different cotton samples obtained byatomic absorption studies [236]

Sample: 1 2 3 4 5 6Metal (mg/kg)

Al 23 21 32 47 25 22Ba 1.5 1.5 1.6 1.2 2.0 1.5Ca 580 600 840 830 580 630Cu 3 4 2 5 3 2Fe 34 100 110 790 36 94K 4500 4600 4500 5300 4700 4700Mg 490 510 520 530 520 550Mn 3.2 3.5 4.0 6.7 3.3 3.7Na 220 230 230 330 330 320Ni nd 1.2 nd 2.1 nd 1.0P 240 270 260 330 270 270Pb nd nd nd 7.6 nd ndS 340 360 360 430 380 380Sr 5.4 5.6 5.6 5.9 5.7 6.0Ti 0.3 0.25 0.7 1.4 0.8 0.7Zn 4.5 7.3 5.3 35 4.9 13

nd Not detected in practice (Ni < 1 mg/kg; Pb < 3 mg/kg)

H O O H

H O O H O O

Fe2+

Fe3+

Fe3+

Fe2+

HO HO++

+

+

+_

_

Scheme 10.26

H O O H

H O O

H O

Perhydroxide anion

Fe ion catalyst

Hydroxyl radical

Hydrogenperoxide

Alkaline stabiliser

Fe ion catalyst

Bleaching

Fibre damage

_

Scheme 10.27

This is an important but very difficult area for research, particularly regarding theformulation and evaluation of stabiliser systems. Non-reproducibility of results is a seriousproblem and in commercial practice a single batch can unexpectedly give rise to damagedespite using the same protective measures that had been successful with apparently similar

chpt10(2).pmd 15/11/02, 15:44586

587

batches. In research, it is difficult to obtain consistent metal-containing samples direct fromnature, whilst artificially contaminated specimens do not always give results directlycomparable with substrates contaminated naturally.

In both research and practice, critical localised concentrations of metal contaminationcan be difficult to detect. Potassium hexacyanoferrate(II) (10.80) gives an intense deep bluecoloration with iron(III), permitting extremely sensitive detection of tiny iron spots even byvisual inspection. It is recommended as a quality control measure on batches of cottondestined for bleaching [237]. However, in view of the random distribution of metal traces,even the most sensitive test cannot guarantee freedom from contamination throughout abatch of goods to be bleached.

Certain researchers have preferred soluble salts such as iron(III) nitrate [236] torepresent deliberate contamination, whilst others have used insoluble forms. However, eveniron(III) oxide in the form of rust is found to vary in catalytic activity depending on physicalform. Although uniform distribution of the contamination, at least below a relatively lowconcentration, has been claimed to be less troublesome than localised concentrations, thereis not even agreement on this. A further complication is that different studies have beencarried out in either the absence or the presence of a cellulosic substrate. With theseprovisos in mind, the catalytic behaviour of trace metals and the effects of some preventiveagents will be outlined.

The parameter most commonly monitored in this research is the extent of hydrogenperoxide decomposition. Measurement of tensile strength and/or the degree ofpolymerisation can be useful indicators of fibre damage. The effect of iron(III) ionconcentration in accelerating the rate of peroxide decomposition is shown in Figure 10.23,using a system comprising hydrogen peroxide, sodium silicate and magnesium sulphate at 95°C and pH 12. The effects of pH and Fe(III) concentration on decomposition are indicatedin Figure 10.24. Only slight changes in these variables can greatly influence the degree ofdecomposition.

Treatment time/min

5 10 15 20 25 30

20

40

60

100

80

Hyd

roge

n pe

roxi

de d

ecom

posi

tion/

%

5 mg/l1 mg/l0

Figure 10.23 Effect of Fe(III) ion concentration on rate of hydrogen peroxide decomposition inabsence of substrate [237]. Initial concentration 2.9 g/l H2O2, Sodium silicate 5 g/l, Magnesiumsulphate 0.2 g/l, 95 °C, pH 12


chpt10(2).pmd 15/11/02, 15:44587


The effect on the decomposition rate of replacing sodium silicate and magnesiumsulphate by a phosphonate stabiliser is shown in Figure 10.25. In this case, at aconcentration of 2 g/l of the specific phosphonate used proved more effective in retardingFe(III)-catalysed peroxide decomposition than the combination of 5 g/l sodium silicate and0.2 g/l magnesium sulphate represented by Figure 10.23. However, doubling theconcentration of silicate and magnesium sulphate also brought about a considerableimprovement in stability (compare Figures 10.23 and 10.26).

pH

109 11 12 13

20

40

60

100

80H

ydro

gen

pero

xide

dec

ompo

sitio

n/%

20 mg/l10 mg/l 5 mg/l 1 mg/l 0

Figure 10.24 Effects of pH and Fe(III) ion concentration on hydrogen peroxide decomposition inabsence of substrate [237]. Initial concentration 2.9 g/l H2O2, Sodium silicate 5 g/l, Magnesiumsulphate 0.2 g/l, 95 °C, 30 min

Treatment time/min

105 15 20 25 30

20

40

60

100

80

Hyd

roge

n pe

roxi

de d

ecom

posi

tion/

% 10 mg/l 5 mg/l 0

Figure 10.25 Effect of phosphate stabiliser on Fe(III)-catalysed decomposition of hydrogen peroxidein absence of substrate [237]. Initial concentration 2.9 g/l H2O2, Phosphonate stabiliser 2 g/l, 95 °C,pH 12

chpt10(2).pmd 15/11/02, 15:44588

589

The results shown in Figures 10.23 to 10.26 were obtained in the absence of a textilesubstrate. Those in Figure 10.27 were determined in the presence of cotton and involvedmeasurement of the degree of polymerisation of the cellulose [236]. These curves show theeffects of increasing the concentration (up to 40 g/l) of an unspecified commercialsequestering agent on both peroxide decomposition and the degree of polymerisation of thecellulose. The contaminant present on the cotton was iron(III) nitrate applied artificially. Theinitial concentration (3 g/l) of sequestering agent present was that recommended by themanufacturer. Under these conditions the loss of peroxide was 80%, accompanied by anunacceptable lowering of the degree of polymerisation to less than 1000. It was necessary toincrease the concentration of complexing agent by several times the recommended amount inorder to obtain acceptable protection. Such an increase may be impossible to justify, however.

Treatment time/min

105 15 20 25 30

20

40

60

100

80

Hyd

roge

n pe

roxi

de d

ecom

posi

tion/

% 5 mg/l1 mg/l0

Figure 10.26 Effect of magnesium silicate stabiliser on Fe(III)-catalysed decomposition of hydrogenperoxide in absence of substrate [237]. Initial concentration 2.9 g/l H2O2, Sodium silicate 10 g/l,Magnesium sulphate 0.4 g/l, 95 °C, pH 12

Sequestering agent/g/l

105 15 20 25 30 35 40

20 500

1000

1500

2000

2500

40

60

100

80

Hyd

roge

n pe

roxi

de d

ecom

posi

tion/

% Average degree of polym

erisation

A

BA

B

A Peroxide decompositionB Degree of polymerisation

Figure 10.27 Effect of sequestering agent concentration on the decomposition of hydrogen peroxideand degree of polymerisation of cotton cellulose [236]


chpt10(2).pmd 15/11/02, 15:44589


It is evident that careful optimisation of all the parameters involved is essential forefficient bleaching with minimal fibre damage. The need for close control of pH,minimisation of liquor ratio and coordination of the component concentrations withtreatment time and temperature is critical [226]. Titration of samples of the liquor taken atregular intervals has been the traditional method of analysis. An obvious disadvantage is theunavoidable delay after sampling before corrective action can be taken. Hence the trend hasbeen towards continuous monitoring. A method of continuous on-line monitoring andcontrol of a continuous bleaching range, using measurement of pH, conductivity andtemperature, has been described [238].

The use of biosensors and chemosensors to monitor peroxide in continuous ranges hasbeen examined [239]. Biosensors incorporate electrochemical/enzymatic features withoxygen detectors. Chemosensors depend on anodic or electrolytic oxidation and are morerobust under the conditions prevailing in the textile industry. This technique is limited toperoxide concentrations from 0.01 to 100 mg/l, so dilution of the test sample from anoperating level of 10–50 g/l is necessary. Chemosensors offer a rapid means of determiningthe concentration of peroxide. Being small, they can be fitted quite close to the point ofmeasurement with a direct link to the bleach liquor dispenser. In continuous flowmeasurement, where liquor is transported continuously through the measuring instrument,continual monitoring of the prevailing concentration is provided. By means of suitableelectronic control equipment connected to the sensor system, automatic logging ofmeasured values is possible and, if necessary, appropriate adjustment of the liquor can takeplace immediately [239].

An alternative electrochemical method is based on the detection of hydrogen peroxide atan electrode supplied with a reference voltage [240]. Many electrode materials, however,give proportional measurements only up to 0.02 mol/l but a special electrode material of theglassy carbon type provides linearity between current signal and peroxide concentration upto 50 g/l. The measuring cell has five components: the measuring, reference and backelectrodes determine the current signal, whilst a pH and temperature probe providescompensation for the pH- and temperature-dependence of the current signal. In spite of theadvantages offered by these systems most monitoring still depends on manual titration, withmanual intervention at the dispensing stage where necessary [240].

Typical peroxide bleaching bath formulations

The composition of bleaching formulations varies widely with the machinery available andthe condition of the substrate. Typical formulations and conditions have been given [143]for batchwise processing (Table 10.14) and for continuous processing (Table 10.15). Theneed to conserve energy in recent years has led to growing interest in cold pad–batchperoxide bleaching, in which padded fabric is batched without uneven drainage or surfacedrying for 15–24 hours at ambient temperature. Table 10.16 gives typical pad liquorformulations [143,225,241]. A good wetting agent is required to ensure rapid and thoroughwetting of grey fabrics at 25–35 °C, together with an efficient detergent to assist in removalof fats and waxes. A persulphate (up to 5 g/l) may also be added to assist desizing [143]. Theaddition of a peroxide activator based on 1,2,4-triazole has been suggested as a means ofaccelerating bleaching at about 30 °C [242].

chpt10(2).pmd 15/11/02, 15:44590

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Table 10.14 Recommended conditions for batchwise peroxide bleaching [143]

Kier Jig Winch or PackageAdditives (% owf) (% owf) jet (g/l) machine (g/l)

Magnesium sulphate heptahydrate 0.1 0.1Wetting agent 0.5–2 0.5–2Sodium silicate (79Tw) 2–3 3–5 7 2–7Organic stabiliser 1–1.5 1–2 0.5–2Caustic soda (100%) 0.6–1.4 0.25–0.8 5–15 5–15Hydrogen peroxide (35% solution) 3–5 2–5 5–15 5–15

Liquor ratio 4:1 3:1 15–20:1 8–10:1Temperature (°C) 95 95 95 90Time (min) 60–120 60–120 60–120 60–120

Table 10.15 Recommended conditions for continuous peroxide bleaching [143]

J-box conveyor(woven goods Roller-bed Pressure Jemco J-box

Additives (g/l) in rope form) steamer steamer machine (knitgoods)

Magnesium sulphate heptahydrate 0.1 0.1 0.1 0.1 0.1Wetting agent 2–5 2–5 2–5 0.5 5Sodium silicate (79Tw) or organic stabiliser 5–10 10–20 5–10 1.5 10Caustic soda (100%) 2–5 5–15 2–5 4 5Hydrogen peroxide (35% solution) 15–30 45–60 30–45 2 45

Liquor ratio 1:1 1:1 1:1 10:1 1:1Temperature (oC) 95–98 95–98 120–140 95–100 95–98Time (min) 60–120 10–30 1–2 40–60 60–90

The above recommendations are for caustic scoured or peroxide desized fabrics. For fabrics not so treated, theamounts of caustic and peroxide should be increased by 50% [143]

Table 10.16 Recommended conditions for cold pad-batch peroxidebleaching

References

Additives (g/l) 225 143,241

Magnesium sulphate heptahydrate 0.1DTPA (10.6) (40%) 0.1Wetting agent 2–5Organic stabiliser 0–15 6–10Sodium silicate (79Tw) 10–15 8–12Sodium hydroxide (100%) 10–20 8–15Hydrogen peroxide (35%) 40–60 40–50


chpt10(2).pmd 15/11/02, 15:44591


In some circumstances, hydrogen peroxide can be combined with an organic activator.One such process suggests the use of hydrogen peroxide and urea for the bleaching of linen[243]. Recommended additions are 7 g/l hydrogen peroxide, 8 g/l urea and 1 g/l nonionicwetting agent for 2.5 hours at pH 6, 95 °C and 50:1 liquor ratio. Under these conditionsurea accelerates the decomposition of peroxide, which is normally slow at pH 6. In thepresence of urea, peroxide decomposition is believed to proceed according to Scheme 10.28[243]. In this scheme, hydrogen peroxide and urea interact to form an unstable complexwhich then decomposes to yield hydroxyl radicals. These attack more of the peroxidemolecules to yield perhydroxyl radicals, hydroxyl radicals and molecular oxygen. It is alsolikely that urea interacts with perhydroxide anions from hydrogen peroxide according toScheme 10.29. This anionic complex of urea can be a further source of hydroxyl radicals.The influence of temperature on the whiteness index of linen subjected to this process isshown in Figure 10.28. The process may be applied to scoured or unscoured linen [243].

The success of peroxy bleach activators such as tetra-acetylethylenediamine (10.86) indetergent formulations for low-temperature laundering has encouraged trials of thisapproach in peroxide bleaching [244,245]. TAED is colourless, odourless, non-toxic, non-sensitising, non-mutagenic and stable on storage. During biological treatment it is degradedto carbon dioxide, water, ammonia and nitrate; ethylenediamine is not detected onbiodegradation [245]. TAED seems to be an ideal peroxide activator in the neutral toweakly alkaline range in textile bleaching, being entirely benign environmentally. Such an

OC

H2N

H2N

H O O HOH

CH2N O

H2N

O H

OHC

H2N O

H2N

O H

OC

H2N

H2N

H O O

H O O

H2O2 H2O

H2O2 HO H2O O2

OH

HO

+

2+

+ +

+ ++

Scheme 10.28

H O O H H O O

C O

H2N

H2N OC

H2N O

H2N

O HH O O

H +

+

+ _

_

_

Scheme 10.29

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593

approach permits bleaching at lower temperatures and near-neutral pH values. Advantagesof this process over hot alkaline peroxide bleaching include:– at neutral or weakly alkaline pH fibre damage is only slight and catalytic fibre damage is

almost unknown– low temperatures (60 °C or lower) permit energy savings– handle is improved, since cotton waxes remain on the fibre– the process can be used for coloured goods, since colour bleeding is minimal at low

temperatures– the process is suitable for regenerated cellulosic fibres, giving less swelling of cellulose– the process is less sensitive to hard water.

The disadvantages include:– TAED powder is difficult to dissolve in water– disintegration of cotton husks is not as good as in alkaline bleaching.

The formulations listed in Table 10.17 have been suggested [244].The mechanism of activation is believed to be as follows. In an alkaline medium,

hydrogen peroxide yields the perhydroxide anion (Scheme 10.22), which reacts with TAED(10.86) to form diacetylethylenediamine (10.87) and the peracetate anion (10.88) as inScheme 10.30 [244]. At pH 8–9, the peracetate anion is in equilibrium with free peraceticacid, as in Scheme 10.31 [244]. The peracetic acid reacts with the peracetate anion to formnascent oxygen which is the active bleaching agent, as in Scheme 10.32 [244]. Furtherpossible activators suggested by Kleber [244] include:PAG penta-acetylglucoseBOBS sodium p-benzoyloxybenzenesulphonateNOBS sodium n-nonanoyloxybenzenesulphonateTAGU tetra-acetylglycoluril

Temperature/oC

40 60 80 100 120

20

10

40

30

60

50

Whi

tene

ss in

dex

Scoured fabricUnscoured fabric

Figure 10.28 Effect of temperature on the whiteness index of linen bleached using urea-activatedhydrogen peroxide [243]. Treated with 7 g/l hydrogen peroxide and 8 g/l urea for 150 min at pH 6 and50:1 liquor ratio


chpt10(2).pmd 15/11/02, 15:44593


DADHT diacetyldioxohexahydrotriazinePAP phthaloylaminoperoxycaproic acid.

Certain activated peroxide systems are specifically designed for bleaching wool and thesewill be mentioned later.

The final step after peroxide bleaching is to ensure that the goods do not contain residualperoxide. Reducing agents have been used traditionally for this purpose. However, the possi-bility of using environmentally friendly catalase enzymes should not be overlooked [87–89].

Table 10.17 Recommended conditions for TAED-activated peroxidebleaching [244]

Pad-steamHydrogen peroxide (35%) 5.16 ml/lSodium carbonate 6.36 g/lTAED 3.42 g/lStabiliser 3 g/lWetting agent 2 g/lDefoamer 1–2 drops

Pad to 100% liquor pick-up and steam for 30 min

Cold pad–batchHydrogen peroxide (35%) 8.6 ml/lSodium bicarbonate (10% w/v) 84 ml/lTAED 5.7 g/lStabiliser 10 g/lWetting agent 4 g/lDefoamer 1–2 drops

Pad to 100% liquor pick-up and batch for 2 hours at ambient temperature

N CH2CH2 N

CCH3

CCH3CH3C

CH3C

O

O O

O

2H O O

NH CH2CH2

CH3C

O

NH

CCH3

O

2CH3C

O

O

O

+

+

10.86

_

_

Perhydroxideanion

DAEDPeracetateanion

TAED

10.8710.88

Scheme 10.30

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595

Hypochlorite bleaching

As indicated in Table 10.12, bleaching with sodium hypochlorite is the mostenvironmentally damaging of all bleaching processes with regard to AOX values.Consequently, despite the economical and technical benefits of this bleaching process, theuse of hypochlorite will continue to decline and may even be banned in some countries.

The advantages of this bleaching agent include:(1) economical attractiveness(2) lower risk of catalytic fibre damage, although some chemical damage can occur

depending on temperature and pH(3) powerful bleaching action.

The disadvantages, in addition to the environmentally sensitive aspects already mentioned,include:(1) hypochlorite bleaching does not compete effectively with the rapid peroxide bleaching

process(2) despite the intense whiteness that can be produced, hypochlorite bleached goods are

prone to subsequent yellowing on storage(3) the substrate must be scoured before hypochlorite bleaching(4) many dyes and fluorescent brighteners are destroyed or degraded by hypochlorite

bleaching(5) stock solutions of sodium hypochlorite are unstable and must always be chemically

analysed before use.

Sodium hypochlorite is commercially available as an alkaline solution, normally containingthe equivalent of 12–14% available chlorine. However, this is so unstable that analyticaltesting of its strength is always necessary before use. Calcium hypochlorite (bleachingpowder), stabilised by adding lime, has been used in the past but this product is no longerused in textile bleaching.

The mechanism of hypochlorite bleaching appears to be considerably less controversialthan peroxide bleaching. The pH-related active species in sodium hypochlorite are shown inFigure 10.29 and Scheme 10.33. The pH range 9–11 is the most suitable for hypochloritebleaching. The active bleaching species is the hypochlorite anion ClO–. In fact bleaching

OO

CH3C

O

OO

CH3C

O

H

+ H2O + HO_

_

Scheme 10.31

OO

CH3C

O

OO

CH3C

O

H HO

CH3C

O

O

CH3C

O

+ + + 2 [O]_ _

Scheme 10.32


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can be accelerated by lowering the pH but the liberation of hypochlorous acid HClOdramatically increases damage to the fibre. This reaches a maximum at pH 6–8 and suchprocesses have never been exploited commercially. Higher temperatures also increase therate of bleaching and especially the extent of fibre damage, so that hypochlorite bleaching isusually carried out at ambient temperature for several hours. The effect on cellulose ismainly one of depolymerisation as a result of chain scission. Degradation includes the non-specific formation of a minor proportion of aldehyde, keto and carboxyl groups, the last-named predominating at the usual pH of bleaching (Figure 10.30). The formation ofaldehyde groups, however, results in a tendency for the bleached fibre to yellow on storage.In the traditional process, the optimum pH of 10–11 is carefully controlled by addition ofalkali, usually sodium carbonate. Typical bleach liquor formulations are given in Table 10.18.

pH2 4 6 8 9 10 11 12

60

20

40

100

80

Com

posi

tion/

%

Cl2 HClO [ClO]–

HClO HClO

Figure 10.29 Effect of pH on the composition of sodium hypochlorite solutions [246]

Figure 10.30 Functional groups formed by oxidising cotton cellulose with sodium hypochlorite overthe pH range 5–10 [235]

pH6 7 8 9 10

0.6

0.2

0.4

1.0

0.8

Func

tiona

l gro

ups/

meq

uiv

per

100

g pe

r 10

equ

iv. O

2

CO

CHO

COOH

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597

NaClO

HClO

ClHClO

Na ClO

H ClO

Cl2 H2OH

pH > 10

pH 5–6

pH < 2+

+

+

++

+ _

+ _

+_

Scheme 10.33

Table 10.18 Recommended conditions for hypochlorite bleaching [143]

Package J-boxCistern Jig Winch machine conveyor

Available Cl2 (g/l) 2–4 2–4 1–2 1–2 2–5Sodium carbonate (g/l) 2–4 2–4 1–2 1–2 2–4Treatment time (h) 3–4 1–2 1–2 1–2 1–2

Despite its technical usefulness, hypochlorite bleaching faces severe environmentalpressures because it yields AOX values well in excess of permitted levels. The AOX valueobserved increases with the active chlorine content of the bleach liquor (Figure 10.31) andwith the time of treatment (Figure 10.32).

The state of the substrate, including the source of the cotton and its degree of purity, hasa major influence on the AOX value. It has been shown [247] that under similar bleachingconditions pure cotton cellulose gives an AOX value of 8–9 ppm, scoured or desized cottonsgive intermediate values of 13–60 ppm and raw cotton 70 ppm or higher. Although purecellulose already gives a measurable AOX value, much higher values arise from impurities incotton. The concentration of hypochlorite applied and the treatment time can scarcely be

Active chlorine/g/l

1 2 3 4 5 6 7 8

20

40

60

80

WhitenessAOX content/ppm

Figure 10.31 Influence of active chlorine concentration on AOX content and whiteness [247]


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varied widely in practice, since they have been adjusted already to minimum valuesdetermined by the degree of whiteness required and the economy of the process [222]. Thisimplies that attempts to reduce the AOX value must concentrate on removing impurities asmuch as possible during the pretreatments (scouring, desizing and washing-off) beforehypochlorite bleaching [222,247,248].

The purification of cotton can be improved considerably by pretreatment with hydrogenperoxide and this gives even lower AOX values after hypochlorite bleaching [222]. Thisdouble bleaching, however, considerably increases process and chemical costs and demandsadditional processing capacity. Würster and Conzelmann [222] have estimated that suchmeasures can lead to a reduction in pollution levels by about 70% at justifiable cost, offeringthe possibility of bleaching cotton with hypochlorite without exceeding permissible AOXloading of the effluent. Nevertheless, it seems inevitable that hypochlorite bleaching willcontinue to decline on environmental grounds.

After bleaching with hypochlorite it is always necessary to remove or inactivate anyresidual chlorine present. The importance of this is highlighted by the fact that theconcentration of undissociated hypochlorous acid reaches a maximum at pH 6, giving thegreatest risk of damage to the fibre. Hence it is essential to avoid lowering the pH to neutralduring washing off before an antichlor treatment has been given. The traditional antichlorshave been sodium bisulphite, or less often the sulphite or dithionite, but the current trend isto use hydrogen peroxide (Scheme 10.34) on environmental grounds [143].

Bleaching time/min

Rat

e of

incr

ease

/%

50 100 150 200

20

40

60

80

AOX contentWhitenessChlorine consumption

Figure 10.32 Rates of increase of whiteness, AOX content and chlorine consumption with time ofbleaching [247]

ClO HSO3 Cl HSO4

ClO H2O2 Cl H2O O2

+

+ ++

+__ __

_ _

Scheme 10.34

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Chlorite bleaching

Sodium chlorite is commercially available as an alkaline powder (80%) or as an alkalineliquid (26%) of specific gravity 1.25. As indicated in Table 10.12, chlorite gives considerablylower AOX values than hypochlorite, although not as low as peroxide. Bleaching withchlorite differs significantly from peroxide or hypochlorite bleaching, as it is generally carriedout under acidic conditions. The general advantages of chlorite bleaching include:(1) no need for special cleaning treatments before bleaching, although such preparation

does give rise to lower AOX values(2) the risk of chemical damage is low(3) minimal scouring action of the acidic bleach results in lower weight losses(4) softer handle and good sewability due to low degree of removal of fats and waxes(5) this oxidising bleach is the least sensitive to accelerated damage by metallic

contamination(6) washing off is easier than with alternative bleaching agents(7) useful for synthetic fibres and particularly important for acrylic fibres.

The disadvantages include:(1) toxic and unpleasant chlorine dioxide vapour can be liberated(2) acidic chlorite solutions are highly corrosive and thus demand highly specialised and

expensive equipment(3) no rapid chlorite bleaching process is available(4) residual fats and waxes can be advantageous (see above), but they can also be

disadvantageous by giving lower absorbency(5) many dyes and fluorescent brighteners are destroyed or degraded by chlorite bleaching.

As with hypochlorite, the mechanism of chlorite bleaching does not appear to becontroversial. The pH-related active species in sodium chlorite solution are shown in Figure10.33. Initially, sodium chlorite hydrolyses in solution, as indicated in Scheme 10.35.Chlorous acid, however, dissociates in water to a limited extent only. Acidic or acid-releasing agents, sometimes referred to as activators, are needed to lower the pH and raisethe concentration of chlorous acid to a level suitable for bleaching. However, asdemonstrated in Figure 10.33, the reactions in solution are complex. Evidently the chloriteanions formed undergo the various reactions shown in Scheme 10.36, producing chlorinedioxide vapour (ClO2), chlorate anions (ClO3

–), chloride anions (Cl–) and oxygen. Figure10.34 illustrates the amounts of chlorite anions, chlorine dioxide, chlorate anions (whichhave no bleaching action) and oxygen produced at various pH values from a sodium chloritesolution (1 g/l) after 1 hour at 95 °C in a stream of nitrogen [235]. This demonstrates veryclearly that decomposition according to Scheme 10.36 is very slow at pH 5 and almostceases above pH 6. Below pH 5 the decomposition products are mainly chlorine dioxide andchlorate anions. Oxygen accounts for less than 5% of the products formed bydecomposition.

The rate of chlorite bleaching increases as pH decreases, but only between pH 2 and 9 isthe rate proportional to the concentration of chlorous acid present in solution. At low pHvalues, evolution of the noxious and corrosive gas chlorine dioxide increases. In practice it isnecessary to keep the pH above 3 in order to minimise the formation of chlorine dioxide. Itis necessary to monitor the pH during chlorite bleaching because acid is liberated by the


chpt10(2).pmd 15/11/02, 15:44599


pH2 4 6 8 10

20

40

60

80

100

Com

posi

tion/

%

HClO2

ClO2

HClO2HClO3

[ClO2]–

Figure 10.33 Effect of pH on the composition of sodium chlorite solutions [249]

pH2 3 4 5 6 7

2

4

6

8

10

12

Con

cent

ratio

n/m

mol

l–1

ClO2

O2

[ClO3]–

[ClO2]–

Figure 10.34 Decomposition of sodium chlorite solution as a function of pH [235]

NaClO2 H2O NaOH HClO2+ +

Sodiumchlorite

Chlorousacid

Scheme 10.35

ClClO2

5 ClO2 + 2 H 4 ClO2 + Cl + 2 HO

3 ClO2 2 ClO3 + Cl

+ 2 [O]

_ + _ _

_ _ _

_ _

Scheme 10.36

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reactions, creating a need for careful adjustment or compensation. Bleaching is generallycarried out at about pH 4. In some processes, particularly continuous or short liquormethods, this pH shift is allowed for by setting the initial pH somewhat higher (pH 4.5–6.5).Sodium nitrate is sometimes added to inhibit corrosion of stainless steel vessels to someextent. Sodium dihydrogen phosphate is a useful pH buffer able to act as an activator forsodium chlorite to improve the bleaching performance. Table 10.19 gives some typicalprocessing conditions.

Table 10.19 Recommended conditions for chlorite bleaching [143]

Winch Package J-box ColdAdditives (g/l) Cistern Jig or jet machine conveyor bleach

Sodium chlorite (80%) 2–4 2–6 1–3 2–4 20 20–25Sodium dihydrogen phosphate 0.5–2 0.5–2 1 0.5–1Sodium nitrate 1–2 1–3 1–2

pH 3.8–4.2 3.8–4.2 3.8–4.2 3.8–4.2 6–6.5 6–6.5Temperature (°C) 80–85 80–85 80 80 80–85 20Time (h) 3–4 1–2 1–2 1–2 1–4 16–18

Aftertreatment with detergent (2–5 g/l) and sodium carbonate (2–5 g/l) often enhanceswhiteness and may improve fabric absorbency, particularly if the goods have not beenscoured before bleaching. Antichlor treatment is unnecessary for white goods but may berequired before coloration. A convenient antichlor treatment involves combining thedetergent aftertreatment with sodium perborate, percarbonate or thiosulphate [143].Traditional reductive antichlors such as sodium bisulphite are not recommended, since theirresidues can be just as troublesome as chlorite residues.

Under optimum bleaching conditions, chlorite does not degrade cellulose but degradationcan occur from excessive oxidant concentration, prolonged treatment time or non-optimumpH conditions, the main effect being depolymerisation. Formation of some aldehyde groupsis suspected, since the bleached goods can be susceptible to yellowing on storage.

The popularity of chlorite bleaching has always been restricted by the toxic and highlycorrosive nature of chlorine dioxide, which even attacks stainless steel. Hence equipmentcosts for chlorite bleaching are high. Environmental aspects, in particular AOX values andthe toxicity of chlorine dioxide, will increasingly mitigate against the process in future. Asalready noted (Table 10.12), chlorite bleaching reaches a significant AOX level, althoughthis is only about one-tenth of that produced by hypochlorite bleaching of the same sample.It should also be borne in mind that chlorite bleaching is recommended for some syntheticfibres as well as cellulosics. Kleber [224] considers that consent levels for AOX can easily bemet in practice and that chlorite bleaching will continue.

It is important to examine the influence of impurities or additives, such as sizes andlubricants, since these are often prime sources of higher AOX values rather than thesubstrate itself. It is equally important to assess the contribution of auxiliaries in thebleaching bath to the total AOX value. Kleber [224] has reported several studies of systemsrequiring compliance with an AOX consent of 3 ppm. Table 10.20 shows that: (a) synthetic


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Table 10.20 Effect of surfactant addition on AOX values afterchlorite bleaching of synthetic fibres [224]

AOX value (ppm) after chlorite bleaching

Fibre Without surfactant With surfactant

Polyester 0.6 2.8Polyamide 0.6 2.6Acrylic 1.2 2.8

fibres are negligible contributors to AOX values; and (b) the inappropriate choice of anauxiliary (in this case a surfactant) can have an adverse effect. Similarly, the fibremanufacturer’s yarn finish may have a positive or negative influence on the AOX value.Thus careful attention should be given to all yarn additives and to the auxiliaries used inbleaching.

The possible benefits of prescouring to remove such contaminants should also beconsidered. Alkaline pretreatments, including boiling off of cotton, have a profound effecton AOX values after chlorite bleaching (Tables 10.21 and 10.22). It can be beneficial, fromthe viewpoint of both AOX and whiteness, to follow a chlorite bleach with a peroxidetreatment. Linen yarns after an alkaline scour and chlorite bleach gave a whiteness value of63.9 with an unacceptably high AOX value of 8.0 ppm. These results were improved to 78.5and 1.2 ppm respectively after peroxide treatment [224].

Peracetic acid bleaching

In traditional peroxide bleaching, hydrogen peroxide is activated by alkali. Acids, bothinorganic and organic, can also be used to activate peroxide by the formation of a peracid.Peracids can be effective oxidative bleaching agents and, at least potentially, offer analternative to the environmentally sensitive chlorine bleaches. Although known for quite

Table 10.21 Effect of washing on AOX and whiteness values before andafter chlorite bleaching [224]

Whiteness (460 nm)

Residual AOXSubstrate fat (%) (ppm) Before bleach After bleach

Cotton: untreated 3.8 54.6 81.3washed 2.2 2.4 61.5 87.7

Viscose: untreated 6.2 68.7 81.1washed 0.1 2.8 72.0 84.1

Nylon: untreated 2.6 82.3 80.6washed 0.9 1.2 81.6 84.9

Acrylic: untreated 2.8 70.9 75.5washed 0.7 1.5 72.5 78.5

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some time, they have not achieved much use in practice, mainly because of their high costand difficulties in preparing and handling them. Recent environmental concerns, however,have revived interest in their potential. Comparison with chlorine bleaches is one aspectoften highlighted, but ultimately they will have to compete both technically andeconomically with peroxide bleaching. Peracetic acid is undoubtedly the peracid of greatestinterest for textile bleaching, although persulphate and others have also been evaluated.

Peracetic acid is produced in equilibrium with acetic acid by the reaction of aceticanhydride with hydrogen peroxide, as in Scheme 10.37. Alternatively, peracetic acid can beproduced by acid-catalysed oxidation of acetic acid with hydrogen peroxide, as in Scheme10.38. Characteristic of a peracid is the perhydroxide anion (H–O–O–), or in some instancesthe peroxo dianion (–O–O–). Originally, the bleacher had a choice of using eithercommercial peracetic acid (as a 38% solution) or preparing the peracetic acid in situ usingacetic anhydride as in Scheme 10.37. The 38% peracetic acid solution has a pungent odour,attacks human skin and is highly inflammable; it requires special transport and storagefacilities. Preparation of the peracid in situ is both hazardous and unpleasant, as it requiresthe handling and storage of acetic anhydride. One part of peroxide is reacted with six partsof anhydride for 4 hours at ambient temperature to give 80% yield [244,250,251], duringwhich the explosive diacetyl peroxide may also be formed as a by-product, as in Scheme10.39. Thus, although peracetic acid is environmentally friendly, hydrolysing to acetic acidand oxygen, its manufacture and use do pose quite severe hazards. Hence it is hardlysurprising that it has been used only rarely.

More recently, manufacturers have introduced safer versions containing only 12–15%active material [225]. Typical formulations of these so-called equilibrium mixtures are givenin Table 10.23, together with their flash points [251–253]. Although these lower-strength

Table 10.22 Effect of desize and boil-off of cotton on AOX and whiteness values beforeand after chlorite bleaching [224]

Whiteness (460 nm)

AOX ChloritePretreatment (ppm) Before bleach After bleach consumption

Bleach only 4.4 58.0 87.9 51.4Desize* and bleach 7.2 61.2 89.1 57.3Desize*, boil-off & bleach 2.8 62.7 89.9 55.0

* Starch sized and enzyme desized

O

CH3C

CH3C

O

O

OO

CH3C

O

H

CH3C

O

O H

+ H2O2 +

Aceticanhydride

Peraceticacid

Aceticacid

Scheme 10.37


chpt10(2).pmd 15/11/02, 15:44603


OO

CH3C

O

H

CH3C

O

O H

H+ H2O2 + H2O

+

Scheme 10.38

O

CH3C

CH3C

O

O

OO

CH3C

O

H

CH3C

O

O

O

CCH3

O

CH3C

O

O

H

+ +

Aceticanhydride

Peraceticacid

Diacetylperoxide

Aceticacid

Scheme 10.39

Table 10.23 Typical formulations of lower-strengthperacetic acid equilibrium mixtures

Peracetic acid 35–38% 15% 5%Hydrogen peroxide 7% 23% 27%Acetic acid 39% 16% 6%

Flash point (°C) 62 96 97

The remainder of these formulations consists of water, catalyst(e.g. sulphuric acid) and stabiliser

CH3C

O

O

O H

CH3C

O

O O H+

Peraceticacid

Acetylradical

Perhydroxylradical

Scheme 10.40

mixtures are less hazardous than the 38–40% strength, they still pose problems in use. Oralingestion, inhalation or contact with skin or mucous membranes leads to strong andsustained cauterising or burning and eczema that is difficult to heal [254]. They are quitestable when correctly stored but should not be stored in enclosed vessels or pipework, nor incontact with catalysing metals: ventilated containers of stainless steel or aluminium of atleast 99.5% purity are suitable [254]. These hazard constraints are likely to continue torestrict the use of peracetic acid in comparison with relatively hazard-free peroxide.

Bleaching is thought to take place through the perhydroxyl free radical (Scheme 10.40)[223]. Various attempts have been made over the last decade to elucidate the mechanismand demonstrate the potential of peracetic acid in bleaching. Temperature and pH arecritical parameters with regard to the rate and degree of bleaching on the one hand and the

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extent of fibre damage on the other. It is difficult, however, to be dogmatic about theseeffects since the published results (and conclusions!) show variability because of thedifferent conditions and substrates selected by the researchers.

Examination of the influence of pH is complicated by the fact that the bleaching reactionlowers the pH due to the liberation of acetic acid [254]. Hence the results are affected bywhether this effect is counteracted by the use of a buffer. It has been pointed out that thispH shift increases with the content of bleachable material present [254]. In theory, optimalbleaching should take place at pH 8.2, which corresponds to the pK value of peracetic acid[253]. The results in Table 10.24 have been obtained on knitted cotton bleached for 30minutes at 60 °C with 2.5 g/l peracetic acid, the original degree of polymerisation of thecotton cellulose being about 2600. An investigation on linen [254] revealed no bleachingunder acidic conditions. Whiteness reached a maximum at pH 7–8 but there was noincrease beyond pH 8 as the peracetic acid decomposed to give acetic acid solution.Likewise, pH 8 has been recommended for the bleaching of cotton/acrylic blends with 2–10g/l peracetic acid at 75 °C in the presence of a phosphate buffer [252]. Conversely, pH 3–6has been indicated for the bleaching of nylon carpets with 1–5 g/l peracetic acid for 15–45minutes at 60–70 °C [252]. Somewhat conflicting results [255] have been obtained on astarch-sized cotton fabric using 1% peracetic acid for 30 minutes at 75 °C and a long liquorratio (Figure 10.35).

Table 10.24 Effect of the pH of peracetic acid bleach-ing on the brightness and degree of polymerisation ofcotton cellulose [251,253]

Brightness Degree ofpH value polymerisation

3 37 26505 43 27207 62 26309 65 2430

pH6 8 10

70

75

80

Whi

tene

ss

Figure 10.35 Effect of pH on the whiteness of starch-sized cotton fabric in a one-stage pretreatmentwith peracetic acid at a long liquor ratio [255]


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As indicated in Table 10.25, markedly different effects can be obtained at alkaline pHvalues depending on whether sodium hydroxide or sodium carbonate is used to obtain therequired pH [256]. These results were obtained on desized and scoured cotton bleachedwith 1% peracetic acid for 1 hour at 50 °C and 50:1 liquor ratio. It is evident that fibredamage was much higher in the region of the pK value of peracetic acid when sodiumcarbonate was used compared with sodium hydroxide. Sodium carbonate resulted in morerapid decomposition of peracetic acid above pH 6, although this was not accompanied byincreased formation of the species active in bleaching, both alkalis giving virtually the samedegree of whiteness for a given pH. The optimum conditions in most cases are found at pH6–7.

Table 10.25 Effect of alkali used withperacetic acid at various pH values on thedegree of polymerisation of cotton cellulose[256]

Degree of polymerisation

pH Alkali: Na2CO3 NaOH

4 2750 27805 2800 25506 2850 25507 2680 27208 1850 27609 1350 2800

Table 10.26 Effect of temperature of peracetic acidbleaching on the brightness and degree ofpolymerisation of cotton cellulose [251,253]

Temperature Brightness Degree of(°C) value polymerisation

20 38 265040 44 260060 60 255080 66 2500

The effects of temperature are shown in Table 10.26 for the bleaching of knitted cottonfor 30 minutes at pH 6–7 with 2.5 g/l peracetic acid, the original degree of polymerisationbeing about 2600 [251,253]. Thus both whiteness and fibre damage increase progressivelywith bleaching temperature. In practice it is desirable to avoid excessive treatmenttemperatures, the preferred range being 50–80 °C [251,253]. At higher temperatures apungent vapour is evolved.

Under optimum conditions of pH and temperature, a treatment time of 20–60 minutes isgenerally adequate. The degree of whiteness increases with increasing concentration ofperacetic acid. The optimum concentration is dependent on the type of process and

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substrate, but relatively high concentrations of peracetic acid have little effect on the degreeof polymerisation. For example, increasing the oxidant concentration from 1.75 to 8.75 g/limproved whiteness by about 25%, whilst the degree of polymerisation was lowered by onlyabout 9% [253]. This is in marked contrast to bleaching with sodium hypochlorite. Testswith 100 mg/l iron showed that this transition metal has no significant influence onwhiteness or fibre damage [253] under otherwise optimum conditions. The addition of asurfactant can improve wettability and penetration. Indeed, this may exert more influenceon whiteness than raising the treatment temperature [256].

In the context of peracid bleaching it is worthwhile recalling the reaction outlined inScheme 10.30, in which peracetic acid is produced in situ by the action of the activatortetra-acetylethylenediamine (10.86) on hydrogen peroxide [244].

Peracetic acid bleaching is not widely practiced, so it is not possible to give typicalformulations and conditions. However, various process recommendations have been given[251–256]. It has been demonstrated that peracetic acid bleaching combined with ultrasonictreatment gave higher whiteness values and less fibre damage than conventional bleachingwith peroxide [257]. Low temperature bleaching with peracetic acid at 30 °C, catalysed byincorporation of 2,2′-bipyridyl (10.89) in an alkyl sulphate surfactant, has been proposed[258].

N

N

10.892,2′-Bipyridyl

Combined bleaching processes

Combinations of more than one bleaching process can be beneficial. The sequence of ahypochlorite bleach followed by a peroxide bleach is common: the second stage canconsiderably reduce the AOX value imparted by the preceding hypochlorite treatment. Ithas also been suggested that the sodium hypochlorite in this two-stage sequence caneffectively be replaced by peracetic acid, lowering the AOX values even further. A review ofcombined bleaching processes for weftknit cotton fabrics is available [225].

Monitoring bleaching processes

Accurate monitoring of bleaching processes is essential for efficiency, economy andprotection of the environment. The importance of checking raw materials, including thesubstrate, should be obvious. There is also a need to monitor the actual process liquors andto evaluate the results obtained on the substrate. A useful account of such procedures isavailable [143].

Bleaching from the viewpoint of fibre type

Most cotton is bleached with peroxide, by far the greater proportion by continuous methods.Synthetic fibres seldom require bleaching, but where it is necessary either peroxide orchlorite bleaches are recommended. Most regenerated cellulosic fabrics are only bleached


chpt10(2).pmd 15/11/02, 15:44607


for fluorescent whites and pastel shades. Any of the three oxidative bleaches may be used[143]. Peroxide bleaching requires 5–15 g/l hydrogen peroxide (35%) at 80–100 °C,depending on the type of machinery selected, together with sequestrant, caustic soda andstabiliser for the peroxide. Chlorite bleaching on the winch requires 0.5–1.5 g/l sodiumchlorite (80%) at pH 4, treatment conditions being 45–90 minutes at 90–95 °C. Peraceticacid (3 g/l) treatment for 60 minutes at 65 °C and pH 7 in the presence of sodiumhexametaphosphate is also suitable.

Wool bleaching

Wool poses special problems, although only approximately 1.5% of the world production ofwool is bleached directly. However, an unknown amount of so-called ‘top-up’ bleaching alsotakes place, usually by adding hydrogen peroxide to the last bowl of the scouring process.Reviews of wool bleaching, which includes oxidation, reduction and sequential oxidation–reduction processes, are available [11,259,260]. A particular problem with wool lies not somuch in its initial natural colour as in its tendency to show yellowing under the influence oflight, dry heat or wet alkaline treatments. It is particularly important to realise thatbleaching often exacerbates this problem. The complex chemical mechanisms involved inthe yellowing of wool have been reviewed in detail [259] and will not be discussed furtherhere.

Oxidative bleaching of wool is invariably carried out with hydrogen peroxide. The activespecies involved is likely to be the same as on cellulosic substrates but specific reactions withwool amino acid residues must be considered. The primary reaction is oxidation of cystinedisulphide bonds leading to the formation of cysteic acid residues (Scheme 10.41). Therupture of disulphide crosslinks, with attendant increase in urea-bisulphite and alkalisolubility values, adversely affects fibre properties. As the severity of bleaching conditionsincreases, the urea-bisulphite solubility remains little changed but the relationships betweenalkali solubility and cysteic acid (Figure 10.36) and between cystine and cysteic acid (Figure10.37) are almost linear [259,261,262]. Tyrosine, tryptophan and methionine residues areoxidised by hydrogen peroxide [259]. In order to retain commercially acceptable fibreproperties, the alkali solubility of bleached wool should not exceed 30% [259,263]. Thedegree of attack on the fibre depends on the nature of the wool itself. Chlorinated wool ismore sensitive than untreated wool but Hercosett-treated wool is less sensitive (see Figure10.38) [259,264].

As with cellulosic fibres, the bleaching of wool with peroxide requires carefuloptimisation of several parameters: peroxide concentration, pH, temperature, treatmenttime, choice and concentration of stabiliser and possibly choice and concentration ofactivator. A typical formulation for batchwise bleaching of wool is given in Table 10.27.Sodium pyrophosphate (10.16) or tripolyphosphate (10.17) are generally the stabilisers ofchoice but silicates can also be used. Bleaching for two hours at 50–60 °C and pH 8.5–9 iscommon, although in some cases careful optimisation can reduce the time to one hour [11].Alkaline conditions are appropriate for cellulosic bleaching but may cause quite severedamage with wool and thus careful control is necessary. Alternatively, acidic conditions (pH5 at 80 °C) can be chosen, using a suitable peroxide activator to produce a peracid.

Only 15–25% of the hydrogen peroxide present is consumed in a typical bleachingprocess. In order to minimise wastage, some bleachers reuse liquors several times,

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609

CH2 S S CH2 CH2 SO3H HO3S CH2

H2O2

HO+_

Cystine Cysteic acid

Scheme 10.41

Cysteic acid/%

1 2 3 4

20

40

60

80

Alk

ali s

olub

ility

/%

Figure 10.36 Relationship between alkali solubility and cysteic acid content of peroxide-bleachedwool [259,261]

Cysteic acid/%

1 2 3 4

8

9

10

11

12

Cys

tine/

%

Figure 10.37 Relationship between cystine and cysteic acid content of peroxide-bleached wool[259,261]

replenishing the bath with fresh peroxide after determining the deficit by titration. Thenumber of batches that can be bleached in the same replenished liquor is limited bydiscoloration of the bath by impurities desorbed from the wool [259]. There is a furthereconomic benefit in that the peroxide replenishment decreases with each successive batch(Figure 10.39), possibly because of the build-up in the bath of soluble proteins, which have astabilising effect.


chpt10(2).pmd 15/11/02, 15:44609


2

20

25

30

35

40

4 106 8

40 oC

Time/h

Alk

ali s

tabi

lity/

%

20

25

30

35

40

Alk

ali s

tabi

lity/

%

2 4 106 8

50 oC

Time/h

2

30

31

32

33

34

4 106 8

40 oC

Time/h

Whi

tene

ss in

dex

2

30

31

32

33

34

4 106 8

50 oC

Time/h

Whi

tene

ss in

dex

Chlorinated woolUntreated woolHercosett wool

Hercosett woolChlorinated woolUntreated wool

Figure 10.38 Effect of treatment time on the alkali solubility and Jaquemart whiteness index of woolbleached with 2-vol. hydrogen peroxide at 40 °C and 50 °C [259,264]

Table 10.27 Recommended conditions for the peroxidebleaching of wool [259]

Hydrogen peroxide (35%) 10-30 ml/lPhosphate stabiliser 2-4 g/lWetting agent 1 g/l

Treatment for 1–16 hours at 40–60 °C

Figure 10.39 Bleach bath regeneration in the peroxide bleaching of wool [259]

Batch number2 3 4 51

200

600

1000

1400

Hyd

roge

n pe

roxi

de 2

7%/m

l

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611

A typical liquor formulation for pad application in a continuous or semi-continuoussystem is given in Table 10.28. Batching for 24–48 hours is followed by backwashing,although the use of radio-frequency heating to give a batch temperature of 50–60 °C canreduce the treatment time to 2–4 hours.

Table 10.28 Recommended conditions for the padbleaching of wool [259]

Additives g/kg

Hydrogen peroxide (35%) 40Thickener 8Isopropanol 16Wetting agent 10Formic acid 2Antifoam 3Fluorescent brightener (optional) 5

Although various processes may have been subjected to optimisation in recent years as aresult of economic pressures, a survey in the 1980s revealed disparities between differentsectors of the industry as summarised in Table 10.29 for batchwise wool bleaching methods.

Table 10.29 Worldwide practice of batchwise wool bleaching[259]

Processing time: <3 hours >3 hours Totals

Loose stock 17.8 34.3 52.1Slubbing 5.1 10.5 15.6Yarn 11.6 10.2 21.8Piecegoods 4.7 5.8 10.5

100.0

As with the peroxide bleaching of cellulosics, the presence of transition metals in thebleaching of wool with peroxide has a catalytic effect that can damage the fibre. Thiscatalytic effect has been exploited in a process for the bleaching of heavily pigmented wools.The fibre is first mordanted with a ferrous salt in the presence of hypophosphorous acid(H3PO2), then rinsed to remove the iron from the wool keratin but not from the melaninpigment granules. Hydrogen peroxide is then added to destroy the pigment granules by afree-radical mechanism [11,259,265].

Reductive bleaching of wool is mostly carried out with stabilised sodium dithionite.Derivatives such as sodium formaldehyde-sulphoxylate (CI Reducing Agent 2) or zincformaldehyde-sulphoxylate (CI Reducing Agent 6) as well as thiourea dioxide (CI ReducingAgent 11) are also used. The chemistry of these reducing agents is discussed in Chapter 12.Apparently little is understood about the reactions between these reducing agents and thepigments responsible for the natural colour of wool. Sulphitolysis of the wool is known totake place, the reducing agent reacting with the disulphide bonds of cystine to give a Bunte


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salt (Scheme 10.42) [266]. Rupture of the disulphide bonds results in an increase in alkalisolubility of the fibre, to which the formation of sulphonic groups also contributes.

C

CH

O

NH

CH2 S S CH2 CH

C O

NH

C

CH

O

NH

CH2 S SO3 HS CH2 CH

C O

NH

HSO3+

_

Cystine Bunte salt Cysteine

_

Scheme 10.42

Typical conditions for bleaching with stabilised dithionite are given in Table 10.30[11,259]. A warm rinse is then given, with an addition of 1 ml/l hydrogen peroxide (35%) inthe final rinse to eliminate sulphurous odours. The sulphoxylates are used at pH 3.0 for upto 30 minutes at 90 °C, but these tend to produce a harsh handle as well as an unpleasantresidual odour. Thiourea dioxide (1–3 g/l) can be used at 80 °C and pH 7 for one hour [11].A sequestering agent is added to prevent metal-catalysed decomposition of the thioureadioxide. The active bleaching species is sulphinic acid (Scheme 10.43) [11]. Thioureadioxide has less effect on the physical properties of the fibre than other reductive bleachingagents [267]. Fluorescent brighteners can be applied together with a reductive bleach butthey may increase the subsequent tendency to photo-yellowing.

Table 10.30 Recommended conditions for thereductive bleaching of wool [11,259]

Stabilised sodium dithionite 2–5 g/lWetting agent 1%

pH 5.5–6.0Temperature 45–65 °CTime up to 1 hour

SC

H2N

H2N

O

O

SC

HN

H2N

OH

O

OC

H2N

H2N

HO S OHH2O

+

Thioureadioxide

Formamidine-sulphinic acid

Urea Sulphinicacid

Scheme 10.43

Reductive bleaches are generally less costly than oxidative bleaches but tend to give agreenish white compared with the reddish white tones from peroxide. Combining the two inan oxidation/reduction sequence gives a more neutral white, this being known as ‘a full

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bleach’. The sequence is generally carried out in separate baths. However, an interestingattempt has been made to integrate the two processes [268–271]. This approach is ofsignificant interest chemically. A combination of hydrogen peroxide and thiourea is used butthe sequence of reactions is complex. Initially the wool is given a conventional oxidativebleach with 8 g/l hydrogen peroxide (30%) and 2 g/l sodium pyrophosphate (10.16),beginning at pH 9.0 and ambient temperature, then raising at 1 °C/minute to 60 °C andmaintaining at this temperature for one hour. Titration shows that only about 10% of theperoxide is consumed and the redox potential is around +200 to +400 (i.e. oxidative). Theobjective in the second stage is to make use of the residual peroxide. The pH is lowered to8.2–8.7 and then to 5.0–5.5 with acetic acid, whilst still at 60 °C. Thiourea (1.68 g/l) is thenadded and ten minutes is allowed for thiourea to react with peroxide to give the reducingagent thiourea dioxide, as in Scheme 10.44. Adequate time at this pH for the reaction totake place is essential. Ammonia is then added to give pH 6.8–7.2. The redox potentialbecomes -600 to -700 (i.e. reductive) and reductive bleaching proceeds at 60 °C for 25minutes. The thiourea dioxide formed in Scheme 10.44 is hydrolysed to urea and the activereducing species, the sulphinate anion, during this phase, as in Scheme 10.45. Thesulphinate anions in turn react with wool and with the coloured pigments present on thewool, being thereby oxidised to sulphate anions (Scheme 10.46). Any residual reducingactivity can be quenched by adding a small amount of peroxide.

SC

H2N

H2N

SHC

HN

H2N

SC

HN

H2N

OH

O

SC

H2N

H2N

O

O

+ 2 H2O2

pH 52 H2O +

Thiourea Hydrogenperoxide

ThioureadioxideScheme 10.44

SC

H2N

H2N

O

O

OC

H2N

H2N

O S OH2O

+_ _

Thioureadioxide

Urea SulphinateanionScheme 10.45

O S O

O

S

O O

O

+ 2 [O]

_

_

__

Sulphinateanion

SulphateanionScheme 10.46


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The control of conditions, particularly of pH and the molar quantities of reactants, iscritical; otherwise, variant reaction routes are possible. Unfortunately, thiourea is asuspected carcinogen and this appears to be one reason why this two-stage redox processhas not been readily adopted, even though thiourea is easy.to handle and the bath can bemonitored colorimetrically to ensure complete conversion. In an attempt to overcome thisproblem, it has been found [272] that a cyclic analogue of thiourea, namely thiocyanuricacid or s-triazine-2,4,6-trithiol (10.90), can replace thiourea in this application.Thiocyanuric acid is commercially available, being used as a heavy metal precipitant inwaste water treatment and as a vulcanising agent. It is best used in the bleaching process asa 15% solution of the trisodium salt. The process is claimed to produce exceptionalwhiteness with minimum fibre damage [272].

SH

N

N

N

HS

HS10.90

Thiocyanuric acid

Semi-continuous and continuous reductive processes are best carried out with activatedhydroxymethanesulphinates, since dithionites (even when stabilised) are unsuitable due tobeing oxidised too quickly by air [273].

The environmental aspects of wool bleaching have recently been reviewed [273]. Thereis a tendency to replace phosphates by phosphate-free activators. Reductive bleachingagents are not environmentally friendly as they are oxygen-depleting, but pollution bydithionite is kept to acceptable levels if the residual concentration is less than 0.3 g/l.Thiourea dioxide offers the advantage of contributing only half the sulphur load fromdithionite, but it also introduces detrimental nitrogen. The carcinogenic potential ofthiourea has already been mentioned.

Bleaching in particular is an area where many formulated commercial products areoffered. These have the advantage of providing the bleacher with a single (optimised)product to simplify the preparation of bleach liquors, compared with having to store andprepare several separate components. Little information is available on the detailedconstitution of such mixture products, although hints are sometimes given by the suppliers.Some bleachers, however, may prefer the greater freedom to adjust liquors according torequirements by the judicious use of separate components. Special considerations areneeded where bleaching is carried out in association with other processes or as a means of‘brightening’ goods that have been already dyed.

10.5.4 Mercerising

Mercerising is an alkaline treatment often given to cotton yarns or fabrics, the objectivebeing to increase fibre lustre, strength and dyeability. These effects are brought about byalkaline swelling of the fibres with or without tension. Accounts of practical aspects ofmercerising treatments are available [143,235,274]. Processing options include: cold or hot,

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wet or dry, chain or chainless and batchwise or continuous. Consideration must be given tothe position of mercerising in the overall sequence of preparation, since it may be carriedout on grey, partially prepared or fully prepared goods. The maximum degree of swellingattainable decreases the later the stage at which mercerising is carried out. Accordingly,mercerising of grey-state goods gives the maximum potential for swelling but fibrepenetration and uniformity of treatment are most difficult to achieve. Grey mercerising alsohas the severe disadvantage of giving maximum fouling of the caustic soda liquor.

Mercerising is usually positioned either between desizing and boiling-off or betweenboiling-off and bleaching [274,275]. Care should be taken to avoid carry-over of ironimpurities into the bleaching stage. It is also important to ensure thorough and uniformrinsing after mercerising, as any localised residual alkali will lead to uneven bleaching [275].Whilst these are generally the two most favourable positions for mercerising, theirdisadvantage lies in interrupting the flow of the continuous preparation sequence, sincemercerising is the slowest pretreatment stage. This is where hot mercerising (60 °C) canbecome an advantage, because the quicker rate of swelling allows this step to be more easilyincorporated within the processing range as a whole [275].

Mercerising after bleaching gives the least fouling of the liquors but increases thepossibility of fibre yellowing. Moreover, fibre swelling and absorptivity are less evident,sometimes leading to problems in subsequent processing, particularly in continuousprocesses where rapid uptake and maximum absorption are required [275].

In chain mercerising, the weft threads of the fabric are kept under tension on a clipstenter. In a chainless merceriser the fabric dimensions are controlled by a series of rollers.The fabric is usually woven slightly wider to allow for some weft shrinkage. Yarn ismercerised in hanks between two movable rollers which create the required tension whilstknitted constructions may be mercerised in either slit or tubular forms. Chain and chainlessmercerising have been compared [276].

It is important to be aware of the machine configuration as this influences the targetconditions, including the concentration of caustic soda applied and its subsequent washing-off. The three stages that are important from the viewpoint of auxiliaries are: mercerisingzone, stabilising zone and washing zone. In the mercerising zone the goods are impregnatedwith caustic soda liquor and are subjected to the relevant means of controlling tension. Inthe stabilising zone the concentration of alkali is reduced to a level at which the fabric againbecomes dimensionally stable, losing the plasticity imparted by the concentrated alkali in themercerising stage. The counterflow washing principle is applied to reduce the alkaliconcentration. Only when dimensional stability has been restored are the goods ready forthe washing-off section in which residual alkali is removed and the fabric neutralised.

Traditionally, mercerising has been carried out cold (10–20 °C). This imparts themaximum degree of swelling but this is attained at the slowest rate. Hot mercerising hasbeen introduced more recently and this is carried out at 60–70 °C. The characteristics of thetwo processes are compared in Table 10.31 [274,277]. Table 10.32 presents a comparison ofthese processes from the viewpoint of results obtained in relation to the stage at whichmercerising is carried out, as well as analogous trends for modal fibres [275,277]. Thecharacteristic feature of hot mercerisation is that the essential chemical and physicalchanges do not take place at the higher initial temperature but in the subsequently cooledfabric as it passes through a traditional tensioning process [143]. Hot mercerising, however,has not yet achieved significant commercial success [143]. In cold mercerising, the


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treatment time needs to be longer and is usually 40–45 seconds, compared with 25–35seconds when mercerising at 60 °C [276]. The optimum effect is achieved when the fabrictakes on a glassy, transparent appearance just before the stabilising section. A combinationof hot and cold impregnation is possible [276].

Table 10.31 Comparison of hot mercerising with conventional cold mercerising of cotton [274,277]

Conventional mercerisation (10–20 °C) Hot mercerisation (70 °C)

strong fibre swelling less fibre swellingslower swelling rapid swelling slower relaxation rapid relaxation incomplete relaxation good relaxation higher residual shrinkage lower residual shrinkagesurface swelling complete swelling unevenness evennesstighter fibre packing looser fibre packing firmer handle softer handle slow NaOH diffusion rapid NaOH diffusionless lustre optimised lustre less strongly swollen fibres moderately swollen fibres in surface zone of yarn throughout yarn cross-section background less lustrous background equivalent lustre

Table 10.32 Comparison of cold and hot mercerising processes for bleached andunbleached cotton and for modal fibres [275,277]

Cold HotProperty Substrate mercerised mercerised

Degree of polymerisation grey cotton = =bleached cotton = =modal fibre … …

Breaking strength grey cotton = =bleached cotton = =modal fibre < >

Dyeability grey cotton ≤ ≥bleached cotton > <modal fibre > <

Lustre grey cotton < >bleached cotton < >modal fibre >/< >/<

Dimensional stability grey cotton < >bleached cotton < >modal fibre < >

Handle (flexibility) grey cotton ≥ ≤bleached cotton … …modal fibre > <

Crease recovery angle grey cotton < >bleached cotton … …modal fibre < >

= Similar to< Less than> Greater than the other method

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There is a so-called dry mercerisation process [275] in which the fabric is padded withcaustic soda liquor at 20–25 °C and then dried in a stenter at about 130 °C. An immersiontime in the pad trough of 7–10 seconds is sufficient but the goods need a total saturationtime in the alkaline liquor of 30–40 seconds, i.e. from the nip to entry into the drying zone.

The type of cotton and its condition determine to a large extent the degree anduniformity of mercerisation. Fibres with a relatively rounded cross-section, exceptionalfineness and consistency tend to give the highest degree of mercerisation [274]. Theseproperties also play a part in determining the mercerising conditions.

Ideally, the maximum possible degree of mercerisation would be obtained if the goodswere repeatedly mercerised (for example, twice at around 70 °C and then for a third time at10–15 °C) but such a procedure is economically impractical [274]. In practice it is essentialto aim for an optimum rather than the maximum degree of mercerisation, a compromisebetween what is desirable or ultimately possible and what is economically feasible onavailable machinery.

In the context of the above basic requirements of the process, the chemicals used aresodium hydroxide as the primary agent and a surfactant-based auxiliary to aid rapid andeven penetration as an important secondary requirement.

The viscosity of solutions of sodium hydroxide increases with concentration anddecreases with temperature as shown in Figure 10.40. It is the higher viscosity of coldconcentrated solutions which makes fibre penetration so difficult in cold mercerising.Nevertheless, despite the slower rate of penetration at low temperatures, the ultimate degreeof swelling is greater (Figure 10.41). Thus in this investigation, maximum swelling at 25 °Cwas obtained with 4–6 mol/l sodium hydroxide solution. These curves illustrate that thechange in swelling behaviour with temperature becomes more critical at low concentrationsof alkali. Traditionally, solutions of sodium hydroxide in the 6.25–6.5 mol/l range havegenerally been used in cold mercerising. In fact cellulose swells even in pure water but to alesser extent than in alkali, the swelling in water being reversible whereas alkali-inducedswelling is irreversible. Aqueous swelling affects only the readily accessible amorphousregions of cotton, whilst alkali also greatly affects the crystalline regions.

Temperature/oC

10 20 30 40 50 60 70

2

4

6

8

Dyn

amic

vis

cosi

ty/1

0–3

Pa

s

A

B

C

ABC

531 g/l NaOH/42 oBé297 g/l NaOH/30 oBé167 g/l NaOH/20 oBé

Figure 10.40 Change in viscosity of NaOH solutions with temperature [274]


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The mechanism is thought to be one of ionic hydration [143,279,280], arising fromreplacement of some of the water of hydration by cellulosic hydroxy groups as shown inScheme 10.47. When a hydrated ion pair is absorbed by the cellulose, three molecules ofwater are released and are replaced by three hydroxy groups. These are unlikely to belocated in the same anhydroglucose unit. The liberated water molecules now occupy agreater volume than when associated with the ion pair, thus causing swelling of the fibre[143]. It is important to realise that swelling involves only partial molecular disruption ofcellulose; complete disruption would produce dissolution or at least dispersion. Hydrogenbonds in the crystal structure are ruptured but the van der Waals forces remain intact, thusenabling the cellulosic matrix to behave as mobile sheets held in close contact by the vander Waals forces. This parallel arrangement of the cellulose chains is enhanced by thetension applied during mercerisation and the surface of individual fibres becomes smoother,giving an increase in lustre [276].

Sodium hydroxide concentration/mol l–1

2 4 6 8 10

1.2

1.4

1.6

1.8

2.0

Deg

ree

of s

wel

ling

0 oC

25 oC

100 oC

Figure 10.41 Effect of temperature on degree of swelling of cotton fibres by sodium hydroxide (molarsodium hydroxide solution contains 40 g/l NaOH) [278]

OHNa

OH

.nH2O

[cellulose](OH)3.Na .(n–3)H2O + 3 H2O

[cellulose](OH)3++

_

+ _

Scheme 10.47

The more highly crystalline the fibre, the more it will resist the mercerising treatment.Initial swelling always takes place in the amorphous regions, followed by penetration of thecrystalline material by alkali. As indicated earlier in Figure 10.41, optimum conditions forswelling are obtained with about 4–4.5 mol/l NaOH at 25 °C, this being the most favourableconcentration for penetration of the crystalline regions and thus for enhancing dye affinity. Afurther optimum is reached at a concentration of 6.5–7.5 mol/l NaOH, since it is at thisconcentration and 0 °C that the fibre cross-section changes from kidney-shaped to circular,

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giving enhanced lustre. This is why 6.25–6.5 mol/l NaOH has been the standardconcentration in traditional cold mercerising. Greif considers that a concentration of at least6.75 mol/l and preferably 8.75 mol/l NaOH is necessary for the highest degree of mercerisation[274]. In the so-called continuous addition mercerising process, a liquor concentration ofabout 13 mol/l sodium hydroxide is used at 70 °C, applied to squeeze-wet goods [274]. Thisfacilitates quicker penetration and gives a higher degree of mercerisation. The activeconcentration, however, falls to the usual level of about 6.75–8.75 mol/l after a dwell time ofabout 15–25 seconds, since the fabric already contains about 60% moisture. Nevertheless, it isclaimed that a better effect is obtained by this technique of ‘dilution from above’.

Strictly speaking, regenerated cellulosic fibres cannot be ‘mercerised’ although they canbe given a ‘causticisation’ treatment. There is a critical concentration of caustic soda thatcauses dissolution of regenerated cellulose: about 65 g/l. The much higher concentrationsused in conventional mercerising do not cause dissolution of cotton cellulose, of course.Regenerated cellulosic fibres can be given a causticisation process using 15–50 g/l sodiumhydroxide. There is some advantage in using higher temperatures as swelling is therebyreduced. Viscose staple fabrics for dresswear are often causticised to enhance dye receptivity,increase the brilliance of colour and obtain a softer handle [279].

Addition of a wetting agent to the mercerising liquor gives better penetration and moreeven treatment. However, the choice of wetting agent depends on the fabric to bemercerised and the position of mercerising in the preparation sequence as a whole. Theneed for this additive is greatest with grey yarn or piece goods. Goods that have been givenan alkaline boil-off or bleach already have much better wettability, so the need for a wettingagent is not so great. Indeed, such goods may still be saturated from a previous process and iffed directly into the mercerising liquor, there is no need for a wetting agent [280].

The properties of a wetting agent for mercerising can be summarised as follows [235,280]:– good solubility and stability in 10M (400 g/l) sodium hydroxide solution; the stability

should be maintained under the conditions of alkali recovery by centrifugal separation orvacuum evaporation

– high wetting ability and high efficiency at low concentrations in the strongly alkalinesolutions used; this is particularly critical in continuous processing and on grey goods

– low affinity for the fibre; together with high efficiency at low concentrations this aidssubsequent rinsing and recovery

– low foaming.

Suitable products include [235,280]:– alkylarylsulphonates– sulphated aliphatic alcohols, the most efficient being those of low molecular mass (i.e.

4–8 carbon atoms), such as sodium 2-ethylhexylsulphate (10.91); branched chains aremore efficient than linear ones [235]

– some short-chain alkylphosphonate esters, e.g. sodium methyloctylphosphonate.

CHCH2OSO3Na

CH3CH2

CH3CH2CH2CH2

10.91


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A product such as 10.91 may require blending with about 10% each of butanol andunsulphated 2-ethylhexanol to give an effective formulation in terms of solubility, stabilityand wetting power. It is useful for a wetting agent to exert some degree of detergency acrossthe whole range of caustic soda concentrations, including the lower concentrations forefficiency during washing-off [281].

Yarn shrinkage provides a good measure of the efficiency of a wetting agent undermercerising conditions [280]. Figure 10.42 illustrates results for an effective product, yarnshrinkage being complete in about 30 seconds.

Figure 10.42 Mercerising shrinkage rate using 5 g/l of a commercial wetting agent [280]

In the so-called stabilising zone, the concentration of the alkali is reduced, by acounterflow washing system for example, until the fabric regains dimensional stability. In thesucceeding washing-off zone, most of the residual alkali is removed and the fabric isneutralised, typically with acetic acid [274,276]. The stabilising zone generally gives a spentliquor containing 40–80 g/l sodium hydroxide [274,282,283]. Desorption is more rapid thebetter the preparation of the cotton and the remarkable claim has been made that it ispossible to remove more than 75% of the alkali in only 2 seconds [284]! Even after thoroughrinsing, however, there is always a small amount of residual bound alkali in the fibre [285].

The spent liquors may contain lint and residual size that can be removed by filtration.Weakly alkaline liquors represent a cost problem, however. Although limited amounts of lessdilute liquor may be recycled and used in boiling-off or scouring, the major proportion becomesa rather troublesome component of the effluent load. Neutralisation simply increases the saltcontent of the effluent. Recovery of the alkali by vacuum evaporation is the usual procedure[282,283].

Anhydrous liquid ammonia can also be used to enhance the absorption properties ofcotton [143].

10.5.5 Wool processing

Milling, a process peculiar to wool, is carried out to develop its felting propensity. Traditionalwool goods such as felt hats and blankets are milled under slightly acidic conditions, sulphuricacid being the main agent. Acid milling is particularly useful for dyed goods, which may not

Time/s

20 30 12010

5

10

15

20

Shr

inka

ge/%

B

C

D

DC

B

A

ABCD

Water only200 g/l NaOH280 g/l NaOH300 g/l NaOH

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have adequate fastness to neutral or alkaline treatments. Alkaline milling conditions are stillused for woven piece goods traditionally known as milling cloths, maximum milling takingplace using soap at pH 10. The higher-melting soaps, such as those based on tallow and palmoils, have been preferred to give the required gelatinous solution and lubricating properties.Greasy woollens are often milled in sodium carbonate solution, which saponifies the naturalgrease to a soap. Even with such woven pieces, however, the trend is towards milling in almostneutral conditions, for which milling aids based on nonionic and anionic surfactants are useful.Some wool yarns are milled simply by tumble drying of the wet yarns, whilst knitted garmentsare milled in rotary-type machines using a nonionic surfactant with sodium bicarbonate orpolyphosphate. Solvent-based systems in which a small amount of water is emulsified in thesolvent by an appropriate surfactant have also been used [146], this often forming part of asequence in which scouring, milling and shrink-resist finishing are all carried out in the samemachine.

Another process peculiar to wool is carbonising. This exploits differences between woolkeratin and cellulose in their response to strong acid, wool being substantially more stablewhereas cellulose is degraded. Hence strongly acidic conditions are required to removecellulosic impurities from wool. Dilute sulphuric acid (4–8%) is most commonly used [286].Other mineral acids (e.g. hydrochloric acid) or inorganic compounds that are strongly acidicin solution (e.g. aluminium chloride) may also be used. However, whilst aluminium chloridehas been used to carbonise wool blends containing other fibre types such as polyester orcellulose acetate that are sensitive to sulphuric acid, it is not used commercially to any greatextent [286]. Arylsulphonic acids and thionyl chloride (SOCl2) are also suitable. Thionylchloride is hydrolysed to hydrochloric acid and sulphur dioxide, so it has been used incarbonising either as a vapour or as a solution in perchloroethylene [286].

All carbonising processes involve the following steps [286]:(1) Immersion (or spraying) to impregnate the wool with the appropriate acid solution(2) Drying to concentrate the acid(3) Baking to dehydrate and carbonise the cellulosic impurities(4) Crushing and dedusting to remove the charred cellulosic debris(5) Neutralisation of the residual acid.

As already mentioned, sulphuric acid is by far the most common carbonising agent. Intraditional processes, it is applied at 4–5% concentration with a dwell time of 3–5 minutes.So-called rapid processes apply 7–8% sulphuric acid with very short dwell times, typically 5seconds. When used alone, there is a danger that localised droplets of highly concentratedsulphuric acid can be formed, with consequent damage to the wool. The critical conditionsfor this to occur are met when the acid concentration reaches 40–45% [286–288].

Auxiliaries are generally used to prevent this localised damage and to ensure efficientwetting and penetration. Many products have been suggested [286], surfactants being themost important. These must be stable to the hot acidic treatment, of course. Anionic sur-factants such as alkylbenzenesulphonates have been used, as well as nonionic types such asnonylphenol polyoxyethylenes of 6 to 9 ethylene oxide units per molecule. Typically, the con-centration of surfactant present is only 0.025–0.04%, depending on the composition of thesurfactant [286]. Mixtures containing a polyethoxylated nonionic and an alkylarylsulphonateanionic, however, can significantly increase the risk of wool damage compared with the use ofeither component separately [286,289].


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Since wool is attacked most rapidly by sulphuric acid of intermediate concentration, it isimportant that drying is carried out either at a relatively low temperature so that reaction ofthe acid with wool is slow, or very quickly so that the time of exposure of the wool to thecritical acid concentrations is brief [146]. Ideally, all the sulphuric acid in the wool isabsorbed chemically as bound acid that causes little hydrolytic damage. It is the free acidthat can concentrate locally and cause serious degradation. The acid picked up by thevegetable impurities, on the other hand, is free acid that has the desirable effect of beginningthe process of attacking the cellulose [286].

Large amounts of residual acid may cause damage to the wool, so that carefulneutralisation after baking is an essential and important stage of the process. Carbonisedfabrics allowed to accumulate without neutralisation at moderate humidity may sufferconsiderable damage, so it is essential that neutralisation should take place as soon aspossible after carbonising. Neutralisation with ammonia or a mixture of ammonia andammonium acetate is achieved more rapidly than with sodium carbonate or sodium acetate;the ammonia is best used cold [146,286,290].

The pH of carbonising effluent can be adjusted, of course, to meet dischargerequirements. However, this can lead to undesirably high levels of sulphate. The slime insewage pipes produced under anaerobic conditions in turn produces hydrogen sulphide fromthe sulphates present. This hydrogen sulphide gas is oxidised on the sewer walls, promotingthe growth of sulphuric acid-producing anaerobic bacteria with consequent damage toconcrete pipes [286]. It is possible to minimise sulphate levels by coagulation with acombination of aluminium salts and lime at pH 10 [286,291].

Mechanism of shrink-resist finishing

Shrink-resist processes for wool came into prominence with the growth in importance ofmachine-washable wool. These processes have a decisive bearing on the selection of dyes asregards fastness demands. Numerous approaches have been suggested, normally involvingoxidative modification of the epithelial scales, the application of a polymer to the fibresurface, or a combination of both. In the case of polymer deposition, two approaches arepossible. The polymer may be applied either as a surface film masking the epithelial scales orto link together neighbouring fibres in a process sometimes known as ‘spot-welding’. Thesedifferences are clearly illustrated in Figure 10.43. Special equipment is needed for interfacialcondensation polymerisation, however, and a further restriction is that it can only be applied

Scale masking by polymer deposition

Scale linking by interfacial condensation polymerisation

Figure 10.43 Mechanisms of shrink-proofing by polymer application [292]

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to fabric, whereas polymer deposition can be used at any stage of wool processing. For thesereasons, polymer deposition remains by far the most popular method commercially. Thedegradative modification or removal of the wool scales is sometimes referred to as‘subtractive’ shrink-resist finishing, whilst polymer deposition techniques are described as‘additive’ [293].

Subtractive shrink-resist treatments

The chemical basis of subtractive treatments is the oxidative breakdown of disulphide bondsin the cystine-rich epithelial scales of wool to form cysteic acid residues (Scheme 10.41).There is also some hydrolysis of amide groups in the main peptide chain. Acidified sodiumhypochlorite was originally the preferred oxidising agent. However, difficulties in achievingconsistent and uniform treatment without excessive fibre damage led to its replacement bysodium dichloroisocyanurate (10.92), also applied under acidic conditions. The use of this‘chlorine generator’ provides a more gradual and controlled release of chlorine than can beachieved with hypochlorite. Additives include sodium dioctylsulphosuccinate as wettingagent, as well as the acidifying medium such as acetic acid with a buffer such as sodiumformate/formic acid.

O

N

N

N

O Cl

Cl

O

Na

10.92

+_

Liposomes (section 10.3.4) have been suggested as auxiliary agents in wool chlorinationsince they give improvements in the consistency and homogeneity of the oxidativetreatment, minimising degradation of the wool and facilitating subsequent treatments[61,62].

Although chlorination with sodium dichloroisocyanurate is still by far the mostcommonly used method of shrink-resist finishing, there is considerable concern over theenvironmental influence of its AOX contribution. For this reason, its usefulness coulddecline in future and there has been considerable investigation of alternatives to thisattractively cost-effective treatment.

One possibility is to use an atmosphere of nitrogen containing 3% fluorine gas, for whicha commercial-scale plant is available [294]. This dry fluorination process provides aneffective subtractive treatment capable of replacing chlorination. Since it requires a halogengas, it might be thought to be just as likely as chlorination to infringe AOX regulations.However, organofluorides cannot be detected by the current AOX test method. In any case,owing to the strength and stability of the carbon-fluorine bond, it is unlikely thatcarcinogenic species are formed. Furthermore, as this is essentially a surface modification,the formation of fluoride ions is limited. It is considered that current effluent legislationcovering fluoride ions should not restrict commercial adoption of this process [294]. Thedesign of the machine eliminates the risk of loss of gaseous fluorine.

Corona discharge, bombardment of the wool fibre surface with electrons of sufficientenergy to break covalent bonds, has also been applied to the improvement of shrink


chpt10(2).pmd 15/11/02, 15:44623


resistance [294–296]. Collisions between electrons and molecules of oxygen and nitrogen inthe atmosphere results in the formation of ozone and oxides of nitrogen. Subsequentreaction between free-valence species on the substrate surface and the corona atmosphereleads to the formation of a polar surface. This can facilitate adhesion of polymer if thesubtractive stage is followed by an additive phase [294]. The plasma glow dischargetreatment of wool in non-polarised gases like air, oxygen or nitrogen, usually combined witha polymer treatment, also shows promise as a zero-AOX treatment for the shrinkproofing ofwool [297–299]. The plasma treatment does not damage the fibres, yet considerably reducesthe felting potential of the wool, particularly by enhancing the effectiveness of polymertreatment.

Enzymes have been proposed as a means of subtractive shrink-resist treatment. Their usehas been discussed already in section 10.4.2. There are difficulties, however, in thecommercially successful application of enzymes to wool at present.

Alternatives to sodium dichloroisocyanurate as oxidising agent include potassiumpermanganate and peracids such as peracetic acid, peroximonosulphuric acid (Caro’s acid)and peroximonophthalic acid, of which by far the most important in terms of currentinterest is peroximonosulphuric acid (10.93), usually known as permonosulphuric acid Thepure acid can be obtained in crystalline form, but its salts are unstable. In a detailed study ofthe effect of oxidising agents on wool, their relative reactivities generally corresponded withthe degree of shrink resistance but this was not related to redox potential [300], the order ofreactivity being: aqueous chlorine > dichloroisocyanurate = permonosulphate >permanganate and salt > peracetic acid > permanganate > persulphate = hydrogenperoxide. Permanganate does not produce adequate effects unless it is applied fromsaturated salt solution [301], a situation hardly likely to make it a preferred commercialchoice.

OS

OO

O H

O H

10.93

Permonosulphuric acid looks quite promising and appears to have the greatest potentialof all other oxidants as an alternative to chlorination. The high reactivity ofpermonosulphuric acid with wool makes it particularly suitable for continuous treatments,where only a short time is available for reaction. The reactivity can be controlled by pHadjustment (Figure 10.44), the most suitable range being pH 3–5. Lowering the pHincreases the rate of oxidation but also increases the likelihood of uneven treatment.Compared with chlorination this product shows the following advantages, although it is lesseffective in minimising felting of wool fibres [301]:– No AOX problems– No yellowing of the wool– Superior uniformity of treatment– Almost odourless– Less likely to attack dyes by oxidative fading.

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The difference with regard to yellowing of the wool is quite marked (Figure 10.45).Permonosulphuric acid is suitable for use at any processing stage and on all the usual dyeingequipment. The goods are first treated in an acidic liquor at ambient temperature untilvirtually all the active oxygen has been consumed. A reductive treatment with sodiumsulphite is then given in the same liquor at a slightly alkaline pH, followed by rinsing. Thereis some conflict regarding the parameters involved but careful investigation [301] has led tothe following recommendations:(1) After wetting out, the wool is treated in the same bath with 4–6% of a commercial

permonosulphuric acid formulation for 30–60 minutes at 25 °C and pH 4–5(2) To the same bath are then added 0.5% sodium carbonate and 5% sodium sulphite (pH

8). The temperature is raised to 35–50 °C and maintained for 20 minutes.

5 10 20 30 40 50 60Treatment time/min

1

2

3

4

5

Oxi

dant

/%

pH 8pH 6pH 4pH 2

Figure 10.44 Influence of pH value of treatment bath containing a commercial permonosulphateformulation on the rate of reaction with dyed wool at 25 °C and 20:1 liquor ratio [301]

1 2 3 4 5 6Oxidant concentration/%

25

26

27

28

29

30

Deg

ree

of y

ello

win

g (D

IN 6

167)

B

A

B

AAB

DichloroisocyanuratePermonosulphate

Figure 10.45 Variation of degree of yellowing of wool with concentration of commercialdichloroisocyanurate and permonosulphate formulations [301]


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Chlorination treatments, of course, are invariably followed by a reductive aftertreatment. Inthe case of permonosulphuric acid, this is even more important as the sulphite treatmentsignificantly enhances the shrink resistance (Figure 10.46).

10 6 7 8 92 3 4 5 10Sodium sulphite/%

10

20

40

30

Shr

inka

ge/%

Figure 10.46 Influence of sulphite concentration on the felting shrinkage (IWS TM31) of dyed wooltreated with 5% of a commercial permonosulphate formulation [301]; untreated wool shrinkage 60%

Permonosulphuric acid treatment confers only a modest shrink-resist effect which usuallyneeds to be improved by a subsequent additive treatment. It has been suggested [300] thatthe most likely mechanism for inhibiting felting by permonosulphuric acid treatment is theremoval of degraded protein from below the exocuticle, producing a modified surface with areduced differential friction. The direct formation from cystine residues of lowconcentrations of Bunte salts has been confirmed, as indicated in Scheme 10.42.

The use of peroximonophthalic acid (10.94) has been reported as a shrink-resist agent[302,303]. When comparing dichloroisocyanuric acid, permonosulphuric acid andpermonophthalic acid, it was observed [303] that dichloroisocyanuric acid reacts so rapidlythat it is difficult to control the evenness of chlorination. This treatment tends to causestiffening and yellowing of the wool. Consequently, it is used at lower concentrations thanwould be needed for full shrink-resist properties, the balance being achieved by subsequentapplication of a polymer. Permonosulphuric acid does not cause these problems but can onlybe used successfully on woven cloth, unless a polymer is applied subsequently.Permonophthalic acid does not exhibit the disadvantages of dichloroisocyanuric acid.Furthermore, it can be applied to wool in all its forms to give an adequate degree of shrinkresistance without the need for subsequent polymer treatment.

C

C

O

O

H

O

O

O H

10.94

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Since the handle of the wool is not impaired, permonophthalic acid can be applied at alevel that gives full protection from shrinkage, this being the reason why the cost of polymertreatment can be avoided. Currently however, there is a major disadvantage: althoughavailable for experimental work as its magnesium salt, this is expensive and not yet availablein commercial quantities. However, a process has been proposed [303] whereby it can bereadily and inexpensively prepared in about 80% yield at 25–30 °C by stirring togetherphthalic anhydride and hydrogen peroxide in water for about an hour, maintaining the pH atabout 5. This process is analogous to that for the preparation of peracetic acid (Scheme10.37), although phthalic anhydride is less hazardous and easier to handle than aceticanhydride. Nevertheless, the stability of the resulting permonophthalic acid is poor, itsoxidising power decreasing through decomposition by about 5% per hour. As withpermonosulphuric acid, a subsequent reduction stage using sodium sulphite is needed toeliminate residual oxidant.

Additive shrink-resist treatments

As mentioned previously, additive treatments involve the application of a polymer to thefibre. This is usually prepared before application and contains reactive groups. However, it isalso possible to form the polymer in situ within the fibres. The traditional approach is toapply the polymer after a subtractive oxidation treatment but environmental concern overAOX problems is increasing demand for additive treatments that can stand alone. There isno denying that the oxidative step can facilitate subsequent treatment with a polymer, sincethe scission of cystine disulphide bonds to yield cysteic acid residues provides useful reactivesites for crosslinking or anchoring the polymer.

The desirable properties of shrink-resist polymers are [301]:(1) The treatment must impart maximum shrink resistance.(2) High substantivity for wool. This is clearly linked with the foregoing requirement.

These two factors are particularly important when application takes place after anAOX-free oxidative stage, since such treatments generally impart lower initial shrinkresistance than chlorine-based subtractive treatments. Indeed, these two requirementsmay need to be fulfilled so effectively that the oxidative stage before polymer treatmentcan be omitted.

(3) Uniform application properties on wool in all forms.(4) The treatment should not lead to harshness or stiffness of the fabric, thus obviating the

need for a softener. Indeed, if the polymer itself provides a degree of softness, this is anadded bonus.

(5) Suitable for application by batchwise or continuous methods.(6) Should have no adverse effects on frictional characteristics of wool fibres or yarns.(7) Should have no deleterious effects on the colour or fastness of dyes.(8) Should not contribute to AOX values.

No shrink-resist polymer developed so far meets all the above requirements [301]. There isclearly some similarity with easy-care finishing of cotton. Although effective crosslinkingagents are readily available for application to cotton, the morphological complexity of thewool fibre is such that an equally effective polymer has yet to be identified for wooltreatment [304].


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Still the most widely used shrink-resist polymer treatment is that associated with theHercosett process. This treatment always follows on from an oxidative treatment withdichloroisocyanuric acid. Despite its popularity, it is doubly suspect environmentally becauseboth the oxidative first stage and the polymer itself contribute to AOX values. The polymerhas aqueous solubility and is a cationic polyamide-epichlorohydrin resin, theepichlorohydrin contributing to AOX values. Since the polymer is cationic it hassubstantivity for the anionic sites in wool, including those produced by the oxidativetreatment. The polymer contains azetidinyl cationic groups, which are reactive with avariety of nucleophiles leading to insolubilisation of the polymer [11]. Covalent bonding isalso possible through cysteine thiol groups.

Despite the popularity of the chlorination-Hercosett route, it is clear that AOX problemswill often enforce the adoption of alternatives, many of which have already been developed.Non-AOX polymers include polyethers, polyurethanes, polysiloxanes, polyquaternarycompounds and multifunctional epoxides.

Polyethers of various types are of particular importance and include the following types:– polyethers solubilised by reactive Bunte salts– polyethers solubilised by carbamoyl sulphonate groups– thiol-terminated polyethers– aziridine-terminated polyethers.

The first two types are of long-standing commercial availability. They are applied usingmagnesium chloride as catalyst and are crosslinked by addition of ammonia. An importantfactor is that magnesium chloride induces a cloud point at about 50 °C, leading to thephysical form essential for functioning of the polymer [11].

Polyurethanes have also been used for many years. They can be applied from a solventsuch as perchloroethylene, but such solvents are increasingly under environmental scrutiny.An aqueous polyurethane formulation is normally applied by padding, followed by baking at150 °C using sodium carbonate as catalyst.

Polysiloxanes as shrink-resist finishes have been developed from their traditional uses assofteners and water repellents; as such their chemistry is discussed in section 10.10.3. Thiswas a natural trend as many shrink-resist finishes tend to impart a harsh handle to wool.

Polyquaternary compounds are useful in that as well as conferring shrink resistance theymay also improve acid dye fastness to wet treatments.

A useful and detailed comparison between specific examples of a polyether, a cationicpolysiloxane and a polyquaternary compound is available [301]. This review includes detailsof practical application via various processing routes available for loose stock, tops, yarn,knitted garments and woven or knitted piece goods. As mentioned earlier no single polymerfulfils all requirements and combinations of different types are sometimes used. Someindication of this is given in Table 10.33.

Whilst many methods have been proposed for preventing shrinkage of wool fibres, nosatisfactory method operates without some damage to the hydrophobic nature of the fibresurface [305,306]. Nevertheless, it has been claimed that treatment of wool with amultifunctional epoxide, glycerol poly(glycidyl ether), in a saturated solution of sodiumchloride gives excellent shrink resistance without impairing the wool surface [305,306].Apparently this polymer is able to crosslink the cuticular cells and decrease the prominenceof their edges. There must be doubts, however, about the commercial feasibility of using the

chpt10(2).pmd 15/11/02, 15:44628

629

saturated brine solution necessary to bring about effective treatment. Such a medium wouldbe difficult to handle, costly and environmentally undesirable, even though it is AOX-free.

The dyeing of wool with bifunctional reactive dyes can enhance its resistance to felting.This observation has been exploited in a shrink-resist process using a specifically designed‘colourless reactive dye’ as the effective agent [307,308]. The product developed is thetrifunctional reactive compound, dipotassium 2-chloro-4,6-bis(4′-sulphatoethylsulphonyl-anilino)-s-triazine, also known as XLC (10.95). This reactant has substantivity for wool andimparts shrink resistance through crosslinking, being an example of a non-polymericadditive treatment. The crosslinking activity is centred mainly in the low-sulphurmicrofibrillar proteins through their high content of lysine and histidine residues. Aparticularly interesting approach is to apply this compound together with reactive dyes, inwhich case it reacts and crosslinks to an even greater extent than in the usually shortershrink-resist process [308].

Table 10.33 Comparison of typical shrink-resist polymers of the non-AOX types for application to wool[301]

Polymer type Advantages Restrictions

Polyether with Outstanding shrink-resist effect Restricted range of applications reactive groups on pre-oxidised wool Not applicable before dyeing

Durable soft handle Special dissolving requirementsSuitable for piece goods, garments and loose stock.Cost advantage over polysiloxane type

Polysiloxane with Most effective shrink-resist Costly but high-quality finish amino groups siliconeSemi-microemulsion Durable handle after finishing.

No crosslinking agent or catalyst requiredAll processing stages suitableLong liquor application after pre-oxidationPadding possible without pre-oxidation

Polyquaternary compound Shrink-resist effect after No improvement in handle pre-oxidation Shrink-resist effect weakerExceptional improvement in than with other types fastness Odour possible onAll processing stages suitable chlorinated woolImproves shrink-resist effect of polyether and polysiloxane types

N

N

N

Cl

HNNHCH2CH2SO2 SO2CH2CH2KO3SO OSO3K

10.95

XLC


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A variation of this technique utilises reactive surfactants based on Bunte salt acetateesters of dodecanol and dodecane-1,12-diol [309]. Results using mono-, bi- andtrifunctional versions have been reported (10.96–10.98 respectively). These are water-soluble and highly surface-active compounds. They can be applied to wool by padding incombination with 20 g/l sodium sulphite and a locust bean gum thickener to give 100%pick-up, followed by batching at 20 °C for 24 hours. Rinsing completes the process. Theresults in terms of shrink-proofing are shown in Figure 10.47. It is evident that reactantefficiency increased with functionality: 15%, 10% and 3% of the mono-, bi- andtrifunctional agents respectively were required to obtain zero shrinkage, from which it ispredicted that only 1–2% of a tetrafunctional agent would be needed. The monofunctional

1 3 5 8 12 1510

Applied agent concentration/% owf

10

20

30

0

50

40

Are

a fe

lting

shr

inka

ge/%

MonofunctionalBifunctionalTrifunctional

Figure 10.47 Effect on shrink resistance of functionality of agent applied by the pad–batch coldprocess [309]

CH2C

O

OC12H25

SSO3Na

10.96Monofunctional

OC

O

CH2NaO3SS

C12H24 O C

O

CH2 SSO3Na

10.97

Bifunctional

C

O

O

C12H24 O

C CH2

O

SSO3Na

C

O

O

C12H24 O

C CH

O

OHCH2NaO3SS

10.98Trifunctional

CH2

C

O

O

C12H24 O

C CH2

O

SSO3NaCH2

chpt10(2).pmd 15/11/02, 15:44630

631

agent in fact gave better results than expected, showing clear evidence in scanning electronmicroscopy of agent–fibre spot-welding. This is believed to occur via the formation of mixeddisulphides from the dodecanol derivative and wool protein Bunte salts from the reaction ofdisulphide-rich proteins in the wool with sulphite (Scheme 10.48) [309].

Mention has already been made of the effectiveness of corona or plasma treatment inincreasing the influence of subsequent or concurrent polymer treatment. As examples ofpolymers used in this way, mention can be made of reactive cationic polysiloxane [294] andpolymerisation on the fibre of tetrafluoroethylene or hexafluoropropylene [299]. Waterrepellency was also improved by the fluorinated polymers. Tetrafluoroethylene gave superiorshrink resistance; this polymer covered the scale edges of the wool, whereas this did notoccur with poly(hexafluoropropylene).

Shrink resistance can be achieved by graft polymerisation of vinyl monomers onto wool[292]. Suitable monomers include methyl, ethyl and butyl methacrylate, particularly themethyl ester. Polymerisation is initiated by a redox system, potassium bromate and cobalt(II)acetate, with a co-solvent such as 2-butoxyethoxyethanol to improve both efficiency andselectivity. Grafting of 100% monomer is possible in three hours or less at 50 °C. Ultimatelyup to 950% can be achieved; such high levels, however, are impractical. There are somerather severe obstacles to the commercial development of this process: methyl methacrylateis flammable and is a respiratory irritant, whilst use of the co-solvent is also undesirable. It issuggested [292], based on a limiting volume model, that the mechanism is characterised bythree stages: initial grafting and void-filling within the fibre, disruption of crystalline regionsand polymer growth on or through the fibre surface with formation of interfacial bonds (i.e.spot-welding) at high levels of grafting.

Whilst elimination (by oxidation) or masking (by polymer deposition on the cuticularscales) are the accepted mechanisms by which shrink resistance is achieved, there isevidence that other factors need to be considered, particularly as it is possible to obtain ashrink-resist effect without degradation or masking of the scales. A review is available [310]of the mechanism of chlorine-based shrink-resist processes.

Whilst chlorine-based processes are well understood from a mechanistic viewpoint, thereare differences between these and the permonosulphuric acid processes. Understanding ofthe mechanism of permonosulphuric acid treatment has improved in recent years but thereare still aspects requiring elucidation [300]. An important difference between these twotypes of oxidative treatment is that chlorine-based processes lead to scale modification or

S([wool] S)n [wool] S S [wool]

(NaO3SS)n [wool] SNa [wool] SSO3Na

SSO3Na

S S [wool] (SSO3Na)n

+ Na2SO3

+ Wool protein Bunte salts

Dodecanol derivative

Mixed disulphides

Scheme 10.48


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destruction with zero differential friction as the target. Permonosulphuric acid processes,even when followed by a sulphite treatment, leave the differential friction essentially thesame as untreated fibres, apart from some longitudinal striations at the base of the scales andsome detachment and rounding of scale edges [300].

Chlorine water acting on wool produces Allwörden bubbles or sacs by raising of theepicuticle [11]. This occurs through the formation of osmotically active oxidation products.Cleavage of peptide bonds and especially oxidation of the disulphide bonds to producesulphonic acid residues result in the formation of soluble peptides responsible for theincrease in osmotic pressure within the semi-permeable membrane. Permonosulphuric acidtreatment, with or without a sulphite aftertreatment, also forms Allwörden sacs, as dohydrogen peroxide, potassium permanganate and peracetic acid [300]. On immersion inwater, wool treated with permonosulphuric acid shows major decreases in magnitude offriction and differential friction compared with untreated fibres, especially following asulphite aftertreatment. This may be attributed to degraded protein at the fibre surfaceacting as a lubricant or to changes in swelling properties of the wool surface.

The sulphite aftertreatment is particularly important with permonosulphuric acidtreatment. Evidence for the underlying mechanism is available from analysis of sulphuroxidation products formed in the various processes (Table 10.34). It is evident from theseresults that the concentration of [RSSO3]– anionic groups necessary to change thehydration of the fibre surface is achieved by the reaction of bisulphite with cystine monoxideresidues to give the required cysteine-S-sulphonate groups [311].

Table 10.34 Relative amounts of sulphur oxidation products formed during shrink-resistprocessing of wool [11,311]

Treatment Oxidation product Frequency (cm–1) Quantity

Chlorine-Hercosett Cysteic acid (RSO–3) 1042 ***

Permonosulphate **Permonosulphate +bisulphite *

Chlorine-Hercosett Cystine monoxide (RSOSR) 1076 *Permonosulphate ***Permonosulphate +bisulphite *

Chlorine-Hercosett Cysteine-S-sulphonate (RSSO–3) 1024 **

Permonosulphate *Permonosulphate +bisulphite ***

In the case of polymer deposition, it has been pointed out [293] that the masking effectat the scale edges may be less important than mutual adhesion of fibres in the yarn, since thethickness of the polymer film (0.1 µm) is much less than the average height of scale edges(1 µm). This effect is more analogous to spot-welding.

The particular requirements of shrink-resist processes in relation to wool fabric printinghave been described [312]. In addition to dimensional stability, there is a need for ease of

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diffusion and absorption of dyes in printing. Chlorination is particularly effective because therapid reaction limits the oxidative attack to the fibre surface. Dye affinity and thehydrophilicity of the fibre are increased by the anionic sulphonic acid groups formed and thesurface barrier to diffusion is lowered. However, this is an AOX-loading process.Permonosulphuric acid, although AOX-free, is an inadequate preparation for printing evenwith a polymer aftertreatment. Hydrogen peroxide, activated by the recyclable catalystsodium tungstate, offers a suitable process [312] that is AOX-free. Oxidation remainsconcentrated at the fibre surface because of the high reactivity of peroxide, giving resultsanalogous to those from chlorination. A subsequent polymer treatment may also be given.The process involves padding to 75% pick-up in a solution equivalent to 8% by weight ofsodium tungstate and 3% by weight of hydrogen peroxide. The padded fabric is rinsed after areaction time of two minutes and the tungstate can be recovered from the rinsing water.

10.5.6 Combined processes

The economics, at least on paper, of combining two or more processes to gain major savingsin time and energy have long been sufficiently attractive to motivate research in thisdirection. For example, one-stage desize–scour, scour–bleach, desize–scour–bleach, scour–dye and even (paradoxically enough) bleach–dye operations have been, and are, operated.The major disadvantage of such combined processes stems from difficulties of operation,especially compounded where there is a high probability of incompatibility, as in enzymedesizing–bleaching or in bleaching–dyeing. In addition, any process that is combined withscouring and/or desizing may be subject to interference from the products desorbed ordecomposed by those processes. Under these circumstances the choice of chemical additionsbecomes critical. For example, in combined desizing–bleaching the enzyme must be stable tooxidation and both processes must be operable under the same conditions of time,temperature and pH. Similarly, other products added, such as surfactants for wetting anddetergency or metal ion sequestering agents, must fulfil their primary task and not interferewith other functions, while themselves being unaffected by the conditions.

Scour–dye operations in particular need careful planning backed by detailed knowledge,especially in regard to the effects of desorbed impurities on the stability and exhaustion ofthe dyes. In the combined scouring and dyeing of wool or nylon with acid dyes the detergentshould ideally function as a levelling agent. The crucial factor in scour–dye processesinvolving disperse dyes is stability of the dye dispersion in the presence of the detergent anddesorbed impurities. This becomes increasingly critical at higher temperatures (say 130 °C)and with difficult application conditions, such as tightly woven fabrics on beams. Hencesuch operations are more frequently used in jet dyeing, for example. Incompatiblesurfactants and greasy soil can have disastrous effects on dye dispersions, leading toagglomeration of dye particles and deposition of coloured oily stains on the substrate, withextensive breakdown of the dispersion in severe cases. Surfactants can have a highlyselective effect on the rate and extent of exhaustion of disperse dyes and must therefore beselected accordingly.

In spite of these difficulties successful combined processes are indeed operated, althoughthey require vigilant monitoring. The promised economies must not be squandered by lostprocessing time and costs of damage and reprocessing. It would be surprising if themanufacturers of dyes and chemicals were more enthusiastic than textile processors about


chpt10(2).pmd 15/11/02, 15:44633


such combinations, in view of their understandable caution about guaranteeing thebehaviour of individual products in circumstances for which they are not intended. Nowthat environmental and economic factors limit research into new products, the fusing ofnormally sequential processes into a single one will remain a worthwhile technical andeconomic goal.

Possibilities for combining the three main preparatory processes for cotton (desizing,scouring and bleaching) remain economically attractive. One process for achieving thisinvolves treatment of loomstate cotton with an aqueous solution containing 3 g/l sodiumchlorite, 1 g/l hydrogen peroxide, 2 g/l nonionic wetting agent and 10 g/l disodium hydrogenphosphate for 90 minutes at 95 °C, pH 10 and 20:1 liquor ratio [313]. In another processthe fabric is impregnated with 20 g/l sodium chlorite, 0.05 g/l potassium permanganate and2 g/l nonionic wetting agent, followed by treatment for 30–60 minutes at 90 °C and pH 10[314]. The concentrations of the oxidants, the pH and time of treatment were critical,however. In a modification of this process, impregnation with 20 or 30 g/l sodium chlorite,respectively 3 or 1 g/l potassium chromate and a nonionic wetting agent is followed bytreatment for 90 minutes at 90 °C and pH 6 [315]. All these processes are subject to theusual criteria regarding machinery that is resistant to corrosion by sodium chlorite, whilstthe discharge of effluent containing chromate has environmental implications in manycountries.

An AOX-free alternative [316] is impregnation with 4 g/l hydrogen peroxide, 8 g/l ureaand 2 g/l nonionic wetting agent, then treatment for 60 minutes at 95 °C, pH 8 and 20:1liquor ratio [316]. This results in a bleached fabric with excellent wettability and withoutserious fibre degradation. The urea interacts with hydrogen peroxide to form an unstablecomplex, which then decomposes to form hydroxyl and perhydroxyl radicals, according toScheme 10.28 [316]. Urea exhibits undesirable environmental characteristics in somerespects, however.

In all the above processes, the optimised quantities of the chemicals indicated will bespecific to the substrate quality evaluated. They would require further re-optimisation foreach substrate to take account of the type and concentration of size, the presence of otherimpurities and the degree of natural yellowness. In particular, the amount of oxidant willneed to be adjusted to give the optimum balance between oxidative desizing and the degreeof bleaching required.

A commercially established system for the combined desizing, scouring and bleaching ofcellulosic fibres and blends is the Raco-Yet (Kieinewefers) system [317,318]. The fabric isimpregnated successively with 23–26 ml/kg sodium silicate (38°Bé), 32–40 g/kg sodiumhydroxide (100%), 20–26 ml/kg Cottoclarin AS and 40–65 ml/kg hydrogen peroxide (50%)before steaming. Cottoclarin AS is designed specifically for this process, having excellentwetting properties, freedom from foaming, high dispersing, emulsifying, complexing anddetergency powers. The process is applicable to a variety of size polymers and their mixtures,including starch, hydroxypropyl starch, poly(acrylic acid), poly(vinyl alcohol),carboxymethylcellulose and polyesters. Desizing, boiling-off and bleaching can be achievedin 1–3 minutes. Savings in processing costs and exceptional flexibility of operation areclaimed, since recipe changes can be made in less than one minute. It is essential to carryout a thorough analysis of each substrate and to adjust the recipe and processing speedaccordingly. Table 10.35 gives an indication of the flexibility of the process together withtypical results [318].

chpt10(2).pmd 15/11/02, 15:44634

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Table 10.35 Examples of application of the Raco-Yet system to cellulosic fabrics and blends [318]

Polyester/100% Polyester/ Cotton/ viscose/

Fabric 100% Cotton cotton polyester linen(130% Cotton shirting 65:35 88:12 70:20:10pick-up)* 320 g/m2 190 g/m2 170 g/m2 290 g/m2 185 g/m2

Size type Starch None PVA Starch None

Sodium silicate (38 Be) ml/kg 26 26 23 26 26Sodium hydroxide (100%) g/kg 40 40 32 40 40Cottoclarin AS ml/kg 26 26 20 26 26Hydrogen peroxide (50%) ml/kg 65 65 40 65 65

Treatment time (min) 2 1.5 1.5 3 1.5Running speed (m/min) 60 80 80 40 80

Evaluation Grey Treated Grey Treated Grey Treated Grey Treated Grey Treated

Absorptivity (mm)After 15 seconds 0 15 0 18 0 15 0 32 0 18After 30 seconds 0 20 0 24 0 20 0 41 0 25After 60 seconds 0 25 0 31 0 25 0 52 0 33Whiteness:%REM 56.0 80.3 90.2 112.5 50.1 86.1 61.8 81.2 Berger 16.1 64.0 104.5 152.6 43.3 75.7 20.1 70.8Degree of polymerisation 2760 2280 3080 2670

* concentrations are per kg of fabric

It is possible to use an enzyme with hydrogen peroxide in a combined desize–bleach butgreat care is needed in selection of the enzyme and optimisation of the concentrations[319].

Mercerising (high concentrations of alkali; cold) is particularly difficult to combine withdesizing, scouring and peroxide bleaching (lower concentrations of alkali; hot).Nevertheless, a combined one-stage desize, scour, bleach and slack mercerise process hasbeen attempted [320]. This involves impregnation of the fabric in a 10–30% sodiumhydroxide liquor containing 20 g/l hydrogen peroxide and 50 g/l trichloroethylene for


chpt10(2).pmd 15/11/02, 15:44635


3 minutes, the results being dependent on impregnation temperature (20–100°c). The fabricis squeezed and (without predrying) is heated to 120 °C, the results being influenced by theduration of the heating step. It is claimed that, with a wet immersion treatment for3 minutes at 40 °C and subsequent treatment for 30 seconds at 120 °C, the physicalproperties of the treated cotton are similar to those from a conventional two-stage approach.The use of trichloroethylene, of course, is highly sensitive both environmentally and as ahealth hazard (toxic to the liver).

An electrochemical system combining scouring, mercerising and bleaching has beenproposed. It is a non-polluting method based on an electrochemical cell, the cathode ofwhich produces the base to mercerise and bleach, whilst the anode produces an acid toneutralise the base remaining after mercerisation [321].

A mercerising-type effect and bleaching of viscose fabrics can be achieved simultaneouslyusing liquid ammonia, the bleaching agent being a peroxidated urea derivative that causesless damage than conventional oxidants, together with improved dye absorbency [322].

Mercerising has been combined with vat dyeing in a continuous process [323]. Cottonfabric is padded with an aqueous suspension of a vat dye in a sodium chloride solutioncontaining caustic soda for mercerising. After drying, the dyeing is developed by padding inan alkaline solution of reducing agent, steaming and soaping.

Further combined processes involving dyeing include:(1) Dyeing cotton yarn with selected direct dyes and simultaneous bleaching with peroxide.

It is claimed that the peroxide also increases the colour yield [324](2) Combined dyeing and easy-care finishing of cotton using bis-nicotinotriazine reactive

dyes and DMDHEU in a pad–dry-HT steam process [325](3) Combined dyeing and finishing of polyester/cotton using a liquid ammonia medium

[326](4) Dyeing polyester/cotton with reactive and disperse dyes and imparting a crease-resist

finish [327].

10.6 DISPERSING AND SOLUBILISING AGENTS

10.6.1 Dispersing agents

Dispersing agents are substances that promote the more or less uniform and stablesuspension of relatively small particles in a given matrix. We are concerned here with themost common type of dispersion encountered in textile coloration, the solid-in-watersystems typified especially by disperse dye technology, as well as the insoluble forms of vatand sulphur dyes. Pigments are also extremely important examples of solid-in-liquiddispersions but form a specialised case fully dealt with in Chapter 2. Also excluded from thissection are other systems that depend mainly on a large increase in viscosity for theirsuspending action; these are more appropriately dealt with in section 10.8. An in-depthaccount is available [328], covering in particular the essentials of colloid science asapplicable to dispersions, the preparation of dispersions (solid-in-liquid and liquid-liquid) aswell as foams (section 10.11). An extensive account of the uses of dispersions is alsoavailable [329]; this includes pigments and the incorporation of colorants in polymer meltsbut is otherwise concerned with non-textile applications.

Any formulation of solid particles in a liquid medium is more or less unstable as a result of

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(a) gravitational settling effects and (b) attractive forces between particles tending to lead toparticles adhering, thus increasing the susceptibility of the system to gravitational effects.Two aspects need to be considered: the initial preparation of the dispersion, and itssubsequent stabilisation during storage and use; only rarely will one agent satisfy the needsof both. Individual dyes vary widely in their requirements and any given dye may requiredifferent treatments depending, for example, on its microcrystalline form and the applicationprocesses for which it is intended. Therefore specific types of dispersing agents, or mixturesof them, are frequently needed to obtain the optimum dispersing action. There are two maingroups of such agents:(1) Surfactants, mainly of the anionic and nonionic types(2) Water-soluble polyelectrolytes, most usually of the anionic type.

The chemistry of surfactants has been described already. They usually play a subsidiary rolein dispersions involved in textile coloration. The polyelectrolytes may be convenientlydivided into two categories:(1) Acrylic acid copolymers, sulphonated polyvinyl compounds, alginates and

carboxymethylcellulose. Some of these may require addition of other chemicals (e.g.alkali) in order to ensure aqueous dissolution. These polymers are less important asdispersing agents for disperse, vat and sulphur dyes than in areas such as pigmentapplications and as thickeners in textile printing or migration inhibitors in continuousdyeing (section 10.8).

(2) The condensation products of formaldehyde with arylsulphonates or lignosulphonates,these being the major types of polyelectrolyte of interest in the manufacture and use ofdisperse dyes [330,331].

As with surface-active agents, the detailed chemistry of these products is a good deal morecomplicated than is indicated by the nominal structures frequently quoted. Mostcommercial products are mixtures of which the nominal structure represents a basic typeonly. Indeed, the detailed chemistry of the more complex products is still only partiallyunderstood. These provisos should be borne in mind when considering the structures givenbelow.

The sulphonated aromatic condensation products form a large and varied group, sinceformaldehyde will condense with many aromatic compounds [330], including sulphonatedarylamines, phenols and aliphatic ketones; the range of commercially important products isrelatively limited, however. One of the oldest is the condensation product of naphthalene-2-sulphonic acid and formaldehyde (10.99), in which the degree of condensation is thought to

HOCH2 CH2 CH2 CH2 CH2OH

SO3Na SO3Na SO3Na SO3Na0–4

10.99

DISPERSING AND SOLUBILISING AGENTS

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correspond to between two and ten naphthalene units, although the quantitativedistribution of the condensates varies widely. Also of importance are the similarly structuredcondensation products of (a) phenols with formaldehyde and sodium sulphite (structures10.100 and 10.101, depending on molar ratios) and (b) p-cresol and 2-naphthol-6-sulphonicacid with formaldehyde and sodium bisulphite (10.102 and 10.103). The types representedby structures 10.100–10.103 are also widely used as syntans (section 10.9.4).

The lignosulphonates comprise a variable group of products derived from wood pulping.

HOCH2

OH

CH2

OH

CH2SO3Na

CH2 CH2

OH

CH2OH

CH2SO3Na

OH

n

10.100

HOCH2

OH

CH2OCH2

CH2SO3Na

OH

CH2OCH2

CH2SO3Na

OH

CH2OH

CH2SO3Nan

10.101

CH2

NaO3S

OH HO CH2SO3Na

CH2

HO CH2SO3Na

10.102

CH2

NaO3S

OH HO CH2SO3Na

CH2

HO

SO3Na10.103

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639

Their highly complex structures are only partially understood, although enough is known toenable representative structures to be proposed showing the major functional groups. Thereare two distinct processes whereby lignins are extracted. The first is an acidic digestionprocess in which the wood is pulped with sulphite or bisulphite. The second is an alkalineprocess, the so-called kraft process, in which the wood is treated [331] with sodium sulphidein autoclaves at pH 13 and 160–175 °C. The product is precipitated by careful addition ofacid, filtered off and washed free from inorganic ions, then sulphonated to increase itsaqueous solubility. Structure 10.104 has been proposed as being representative of kraft ligninprior to sulphonation [331]. An alternative partial representation of a lignosulphonatestructure [330] is that of structure 10.105, which shows sulphonation as having taken placemainly at the CH=CH link. The molecular configuration is such as to give sphericalparticles. The final nature of the product varies enormously depending, amongst otherthings, on:(a) purity, especially the content of electrolytes such as sodium sulphate(b) the number of hydroxy groups present(c) the degree of sulphonation(d) the relative molecular mass (2000–1 000 000) and its distribution.

This wide variability need not be a disadvantage provided it can be controlled to givereasonable consistency from batch to batch, since this enables products to be designed togive the optimum efficiency for the particular application concerned. The kraft ligninsprovide greater scope for modification, particularly of the degree of sulphonation andmolecular size, and are also much more amenable to production in low-electrolyte forms[331]. Further information on the modification and behaviour of lignosulphonate dispersingagents is given in section 12.6.1.

The basic similarity between these major types of dispersing agents and the surface-activeagents discussed earlier lies in their amphiphilic nature (the possession of a combination of

OCH3

CH2

OH

CH

HOCH2

HS

HOCH2CH2 O

HC CH

OCH3

OHCH

CH2

HC

HO

CH2

OCH3

OHHOOC

OCH3

CHCH

OH

CH3O

CHO

H2CCH

CH2

CH2

OC

OCH3HO

CH2OH

H2C

HC

O

O

OH

OCH3

HC

OCH3

CH

CH2O

CH

10.104


chpt10(2).pmd 15/11/02, 15:44639


hydrophobic and hydrophilic moieties). The polyelectrolytes, however, are of much largermolecular size than conventional surface-active agents.

When converting a conglomerate mass of relatively coarse particles into an aqueousdispersion (as in the manufacture of disperse dyes) there are two broad phases to beconsidered. In the first phase the dual aim is that of mechanically grinding the particlesdown to the required size and of obtaining as narrow a range of particle size as possibleduring the actual preparation of the dispersion. Maintaining these particles in a stabilisedsuspension constitutes the second phase.

Particle size alone is not the main criterion; its distribution is equally important. This isbecause all dispersions are metastable. As well as tending to settle as a result of gravitationalforces, there is also a thermodynamic tendency towards a reduction in the free energy of thesystem. This is manifest as a continuing increase in particle size leading ultimately to asevere deterioration in the dispersion quality (as already mentioned). Smaller particles tendto be attracted towards larger particles, with which they then form even larger particles. Theopportunity for particle growth is therefore much less when all the particles are of similarsize than when the range of sizes is large.

The actual comminution of the coarse particles is usually carried out mechanically – forexample, by grinding an aqueous slurry of the colorant in a rotatory mill containingrelatively large, hard and inert grinding media such as pebbles. The process is facilitated byefficient wetting of the particles and lowering of interfacial tension. It is therefore preferableto add dispersing agents that have some surface activity and good wetting properties.Microfissures are created as the particles are broken down. The surface-active properties ofthe dispersing agent enable it to penetrate these microfissures, hindering agglomeration andfacilitating further comminution. The amphiphilic agent becomes adsorbed and oriented onthe surfaces of the particles, providing a protective sheath of like repellent charges, theforces of which eventually exceed the forces of attraction between the particles and thusstabilise the dispersion. This protective sheath becomes of critical importance during thesecond phase by stabilising the suspension of particles, both in storage and in subsequent

OH

OCH3CH

O[lignin]

CH

O [lignin]

CH

H2C

SO3Na

CH

SO3Na

OH

CH3O

CH

HOCH2

CH

NaO3S

HOCH2

HO

CH3O

CH

HOCH2

OCH3

OH

10.105

O

CH

CH

HOCH2 O

CH

SO3Na

chpt10(2).pmd 15/11/02, 15:44640

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use. Hence different dispersing agents may well be required to provide optimum dispersingand stabilising power in the two phases.

The dual amphiphilic nature of the polyelectrolyte condensates and lignosulphonatesdescribed above, with their hydrophobic groups juxtaposed with many polarisable ionicgroups, renders them highly efficient dispersing agents. It is necessary, however, to make acareful choice from the various grades of dispersing agents available with respect to theirmolecular size and charge distribution. Optimum dispersing ability depends on matching thesteric, hydrophobic and ionic properties of the dispersing agents relative to thecharacteristics of the particles to be dispersed. The forces that may be operative inadsorption of surfactants onto disperse dye particles have been listed [330] as ion exchange,ion pairing, hydrogen bonding, van der Waals dispersion forces, polarisation of π–electronson aromatic systems and hydrophobic interaction. It follows that anionic and nonionicsurfactants, judiciously selected, may be used in conjunction with the polyelectrolytes to aidthe dispersing mechanism [330].


10.6.2 Solubilisation

It is necessary to differentiate between simple solutions and the process of solubilisation incolloidal solutions. A non-colloidal solution is a homogeneous single-phase system of a solutedissolved in a solvent, examples being an aqueous solution of sodium chloride or a solutionof methylnaphthalene in acetone. The term solubilisation refers to the homogeneous mixingof an otherwise insoluble agent, the solubilisate, into a liquid medium by addition of asolubilising agent, invariably a surfactant. This agent acts as an amphiphilic bridge betweensolubilisate and medium. For example, methylnaphthalene will not dissolve to anysignificant extent in water, but its solubilisation in water to give an apparently clear colloidalsolution can be brought about by the use of a surface-active agent such as nonylphenolpoly(oxyethylene) sulphate. Hence the distinction between solution and solubilisation innon-colloidal and colloidal situations generally.

Solubilisation can be viewed as one end of a reversible colloidal continuum that beginswith wetting and proceeds through dispersion or emulsification to solubilisation, each ofthese stages being characterised by the size and nature of the particles. In this sensesolubilisation is an extension of emulsification (or dispersion) in which the proportion ofsurfactant has been increased to the level where the discrete droplets (or particles) thatcharacterised the emulsion (or dispersion) have become completely absorbed into thesurfactant–medium phase. In some cases the surfactants used to produce an emulsion (ordispersion) may need modification (a change of hydrophile–lipophile balance) beforecomplete solubilisation can be brought about. Similarly, if a solubilised system is diluted byaddition of the liquid medium, a point will usually be reached at which the solubilisedsystem changes to an emulsion (or dispersion). It is indeed possible to have all the stages ofwetting, emulsification (or dispersion) and solubilisation present at the same time todifferent degrees.

It is beyond the scope of this section to discuss the complex physico-chemical parametersof solubilisation in detail. Useful relevant works of reference are available [332–335]. Itfollows, however, that since solubilisation is essentially an extension of emulsification (ordispersion), the factors discussed in section 9.8.3 in regard to emulsification are alsopertinent to solubilisation. Theory in this area is a useful guide but much still depends on

chpt10(2).pmd 15/11/02, 15:44641


empiricism. As in emulsification, each system to be solubilised will present its own specificrequirements with regard to the type and amount of surfactant(s) required. In some casessolubilisation is aided by adding a small amount of a water-miscible solvent such as analcohol or glycol, although the environmental and the health and safety aspects of suchadditions nowadays requires careful consideration. Conversely, the presence of electrolytescan have a deleterious effect. Temperature too can be important; a system that is anemulsion (or dispersion) at one temperature may become solubilised at a different (usuallyhigher) temperature.

There are two main approaches to solubilisation in textile wet processing. One is thedeliberate preparation of a solubilised product for use as an auxiliary agent, as in theproprietary carriers formulated for dyeing polyester with disperse dyes. The other is as aconcomitant of the process itself, as in the solubilisation of fats and oils during scouringprocesses and in the disperse dyeing process. In both situations more than one stage of thecolloidal continuum of wetting-dispersion/emulsification-solubilisation may be present atany one time.

10.7 LEVELLING AND RETARDING AGENTS

Level dyeing problems can be divided into two broad categories [336]:(1) Gross unlevelness throughout the material: this type of unlevelness is primarily related

to the dyeing equipment or process; the substrate is often uniform in properties, bothchemically and physically

(2) Localised unlevelness: this is primarily related to physical and/or chemical non-uniformity of the substrate; typical examples are barriness in nylon or polyester dyeingand skitteriness in wool dyeing.

There are also two fundamental mechanisms that can contribute to a level dyeing:(1) Control of rate of exhaustion of dye so that it is taken up evenly(2) Migration of dye after initially unlevel sorption on the fibre.

Either or both of these mechanisms may operate to a greater or lesser extent in a given dye–fibre system, although a general trend towards better fastness properties has dictated the useof dyes that show low, if any, propensity to migration, thus placing the emphasis for leveldyeing on the control of exhaustion rate. Physical factors such as temperature and frequencyof liquor/substrate contact (governed by rate of liquor circulation in a jet, beam or packagemachine) can be used to exert some degree of control over these mechanisms. Slower ratesof heating usually favour more even uptake of dye and higher temperatures tend to increasemigration or diffusion. In some cases level dyeing can be influenced by dyebath pH and/orthe presence of electrolytes. This section, however, is more concerned with the control oflevelness by means of chemical auxiliaries, generally known as levelling or retarding agents.

Since levelling agents are invariably surfactants, they may be anionic, cationic, nonionicor amphoteric in nature. Sometimes combinations of these are used. The chemical structureof commercial products is seldom revealed, however; hence only general principles can becovered here. The main mechanisms by which levelling agents operate [337–341] are asfollows:

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643

(a) nonionic agents usually form water-soluble complexes with the dye, some degree ofsolubilisation being involved

(b) ionic agents are primarily dye- or fibre-substantive; in the former case they tend toform complexes with the dye and there is competition between the levelling agent andthe fibre for the dye, while in the latter case the competition is between levelling agentand the dye for the fibre.

In complex formation the principle, as far as levelling action is concerned, is usually thesame irrespective of whether nonionic or ionic agents are used, although the mode ofcomplexing is different. The attractive forces between agent and dye create acounterbalancing mechanism against dye–fibre attractive forces, restraining the uptake ofdye by the fibre. As the temperature of the dyebath increases the complex gradually breaksdown, progressively releasing the dye for more gradual sorption by the fibre. Clearly, for aneffective levelling agent that functions by this mechanism the stability of the agent–dyecomplex, governed by forces of attraction between agent and dye, is crucial. If these forcesare so weak that a relatively unstable complex is formed, restraining or levelling action maybe inadequate. On the other hand, strong forces of attraction may result in a complex that istoo stable to break down as the temperature rises, so that the dye is effectively entrapped bythe agent in the solution phase and is not available for sorption by the fibre. The objectivetherefore is to formulate the levelling agent so that it forms a dye complex of optimum,rather than maximum, stability relative to the conditions of application. This is done byadjusting the hydrophilic–lipophilic balance of the surfactant. The problem lies in the factthat the dye–agent interaction is so specific that different members of a range of dyes mayeach require a different balance. Hence commercial levelling agents may contain more thanone surfactant.

A difficulty that arises with ionic levelling agents is that they may form an insolubleprecipitate with ionic dyes of opposite charge; this can be obviated in various ways. In thefirst instance attention should be paid to the concentration of the surfactant; where initialaddition of surfactant to the dyebath causes precipitation of the agent–dye complex, furtheradditions of surfactant often lead to its solubilisation. Alternatively, a further surfactant maybe added to solubilise the complex; a nonionic agent will not itself react with either the dyeor the original ionic surfactant to form a further insoluble complex, but its addition mayfurther complicate the relationship between the hydrophobic–hydrophilic balance of theionic agent and the dyes to be complexed. Due regard also needs to be paid to the cloudpoint of the nonionic agent under the conditions of use. This does not preclude the use of arelatively hydrophobic nonionic agent, since its cloud point may be effectively raised in thepresence of the ionic agent (subject to possible interference from any electrolytes or solventspresent in the dyeing system). Similarly, if there is any danger from the cloud point of anonionic surfactant used as the primary levelling agent (as with disperse dyes, for example),a suitable anionic surfactant may be added to effectively raise the cloud point, again payingdue attention to any effect the anionic agent may have on the complexing–liberatingperformance of the nonionic agent.

The third method of obviating precipitation of an ionic agent–ionic dye complex is tochoose what effectively amounts to a ‘modified’ ionic agent. Ethoxylated anionic andethoxylated cationic agents are particularly useful in this respect. The ethoxylation tends to

LEVELLING AND RETARDING AGENTS

chpt10(2).pmd 15/11/02, 15:44643


reduce the ionic character of the agent, thus giving rise to weaker but more controllableforces of attraction for dye ions, and the oxyethylene chain can further function as adispersing–solubilising moiety for the agent–dye complex. In a sense this is basically similarto using a mixture of ionic and nonionic agents as described above except that a single agentis used, thus facilitating the aim of obtaining the optimum complexing–liberating balance.

Dye–agent complexes of lower net charge are formed when the ionic agent is added tothe ionic dye solution. As the concentration of agent is increased a point is reached atwhich all the dye is complexed and its ionic charge has been neutralised. Beyond this point,as more agent is added, the agent–dye complex takes on the charge of the complexing agent(i.e. the opposite to that of the dye itself). This brings about a change in the partitioncoefficient of the complex between water and organic solvents [336], modifying theelectrical and solution properties of the dye and so altering its affinity for the fibre.

Fibre-substantive levelling agents are usually of the same ionic type as the dye, that isanionic agents are used with anionic dyes and cationic agents with cationic dyes, the aim beingto create a system in which levelling agent and dye both compete for the sorption sites in thefibre. Just as the complexing type of levelling agent has to be carefully chosen so as to obtainthe optimum complexing–liberating properties, so must the competing type of levelling agentbe chosen such that its ionic power gives the optimum level of competition relative to the dye–fibre system concerned. If the ionic power is too weak, it will not function as an effectivelevelling agent; if it is too strong, it may exert blocking effects, preventing sorption of the dye.Ideally the balance should be such that the smaller surfactant ions are adsorbed by the fibremore quickly than are the larger dye ions, but the agent–fibre interaction needs to be weakenough to permit subsequent displacement of the surfactant ions by the dye ions.

As the forces of dye–fibre interaction vary from one dye to another, the ionic power of thelevelling agent must be suitably adjusted through its hydrophilic–hydrophobic balance to givethe optimum properties. This can be done either by careful choice of a single surfactant or bythe use of mixtures, which has gained prominence in recent times. For example, the stronglyanionic character of a long-chain alkyl sulphate or sulphonate can be modified (toned down)by mixing it with a more weakly anionic poly(oxyethylene) sulphate or with a nonionic agent.

Some levelling agents operate both by complexing and by competition. For example, inthe application of acid dyes a weakly cationic agent may be used to complex with the dyeand an anionic agent may also be used as a competing agent. This combination is moreversatile because unlevelness may arise from different mechanisms. Unlevelness arising fromprocess or equipment variables can often be controlled by dye–agent competition, whereaslocalised dye uptake variations generally respond better to dye–agent complex formation.Evidently, in this combined system the balance of properties is highly critical. In particularthe oppositely charged surfactants must not mutually precipitate; hence the more weaklyionic ethoxylates are of particular interest, since the oxyethylene assists solubilisation of anycomplex so formed. A purely nonionic agent may also be used to prevent coprecipitation ofthe ionic types. Amphoteric agents, in a sense, fall within this combined system.

Theoretical considerations are clearly useful in formulating suitable levelling agents.Nevertheless, a good deal of empiricism is always involved in formulating well-balanced agentsfor specific dye–fibre systems. Table 10.36 shows the general types of levelling agents nowbeing offered and their uses; more detail is given in Chapter 12 relative to each class of dye.

Many, but not all, levelling agents promote migration of dye in addition to retardingdyeing, such agents will obviously be a further aid to level dyeing. In some cases, however,

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higher concentrations of levelling agent are needed to obtain significant migration and thismay interfere unduly with dye sorption. Levelling agents are also widely used as strippingagents, either alone for non-destructive desorption or together with reducing agents such assodium dithionite for destructive stripping. When used for this purpose, their hydrophilic–hydrophobic balance is not as critical as when they are used simply as levelling agents. Thushigher concentrations are often used in order to maximise rather than optimise desorptionof the dye.

It should not be overlooked that electrolytes can play an important part in levelling andretardation. In recent times the use of bolaform electrolytes (section 10.1), cyclodextrins(section 10.3.1) and liposomes (section 10.3.4) as complexing agents has been proposed.

Table 10.36 Levelling agent types and their uses

Recommended for use with

Type of levelling agent Substrate Dye classes

Nonionic Cotton Direct, vat, azoicWool, nylon Milling acid, metal-complexPolyester Disperse

Nonionic/anionic Polyester DisperseWool, nylon Milling acid, metal-complex

Nonionic/cationic Wool Acid, metal-complex, reactive, chrome

Anionic Wool, nylon AcidCotton DirectPolyester Disperse

Weakly anionic Polyester Disperse

Anionic/cationic Wool, nylon Acid, metal-complex

Cationic Acrylic BasicWool, nylon Acid, metal-complex, reactive

Weakly cationic Wool, nylon Acid, metal-complex, chrome

Cationic/polymeric Cotton Vat, sulphur

Amphoteric Wool Acid, metal-complex, reactive, chrome

THICKENING AGENTS, MIGRATION INHIBITORS AND HYDROTROPIC AGENTS

10.8 THICKENING AGENTS, MIGRATION INHIBITORS ANDHYDROTROPIC AGENTS USED IN PRINTING AND CONTINUOUS DYEING

Most, if not all, textile printing and continuous dyeing processes entail the use of auxiliariesthat considerably increase the viscosity of the application medium compared withconventional batchwise dyeing processes, the aim being to facilitate and stabilise the localapplication of colour prior to its actual fixation to the fibre. Such auxiliaries are generallyknown as thickening agents in printing and as migration inhibitors in padding operations.

chpt10(2).pmd 15/11/02, 15:44645


They are characterised by undergoing marked macromolecular swelling in solution due tosolvation (hydration in aqueous systems). While the principal role of thickening agents is toincrease the viscosity of print pastes or pad liquors, certain other properties are also ofimportance, such as stability and rheology of the print paste, adhesion and brittleness of thedried thickener film, the effect on colour yield and penetration, ease of preparation andremoval, and not least cost. A succinct account of these factors and of theirinterrelationships is available elsewhere [342]. The following discussion is restricted torheology of print pastes, since an understanding of the basic principles of fluid flow isessential in appreciating the fundamental mechanism taking place during printing. Thenumber of recent publications dealing with rheology, particularly in relation to specific typesof thickening agents, is evidence of its importance, both as a concept and in currentresearch [343–352].

The essential fact concerning thickening agents is that they are viscoelastic, exhibitingproperties associated with both fluids and solids and showing what is known as pseudoplastic(non-Newtonian) flow behaviour [352]. This is best understood by comparison with simpleliquids such as water or alcohol, which show Newtonian flow behaviour. The apparentviscosity of Newtonian liquids does not change when a shear stress is applied (curve A inFigure 10.48). All thickening agents, however, are highly viscous in a static state butapparently show reduced viscosity when a shearing force is applied. This is indeed theirmodus operandi: they must flow under shear to allow transfer through the screen, thenresume high viscosity when the shear is removed so that colorants remain where they havebeen deposited.

5 1510Shear stress/kPa

2

4

6

0

10

8

Rat

e of

she

ar/s

–1 ×

103

DC

B

A

AB, CD

NewtonianShear thinningThixotropic

Figure 10.48 Typical flow curves demonstrating behaviour of viscous liquids [342]

Most thickening agents are of the shear thinning type represented by curve B, theapparent viscosity progressively decreasing as the shear rate is increased. It is important thatthis change is reversible, viscosity returning to its original level as soon as the shear isremoved. In some cases, shear thinning may not begin until a certain critical shear has beenapplied (curve C). Thixotropic fluids (curve D) show time-dependent effects in thatapparent viscosity depends on both the rate and duration of shear, the return to originalviscosity being delayed. The opposite of shear thinning is shear thickening, often referred to

chpt10(2).pmd 15/11/02, 15:44646

647

as dilatant behaviour; such behaviour is clearly not suitable for textile printing. Analternative method of representing flow behaviour is shown in Figure 10.49.

Shear stress/Pa

102

10

103

105

104A

ppar

ent v

isco

sity

/mP

asDilatant

Newtonian

Shear thinning

Figure 10.49 Relationships between apparent viscosity and shear stress [350]

Print pastes may be thickened by any of the following methods [342]:(a) a relatively low concentration of a long-chain thickening agent(b) a relatively high concentration of a shorter-chain thickener or one having a highly

branched structure(c) an emulsion(d) a finely dispersed solid such as bentonite (derived from clay).

The first two methods, particularly the first, are most frequently used today; combinations ofthese methods are also possible.

Thickening agents can be of natural or synthetic origin. Various natural gums andstarches have been used traditionally in many printing styles. The materials from which theyare extracted are valuable sources of foodstuffs, so availability and cost can depend onfluctuating demand from the food industry. The properties required of an ideal thickenercan be summarised as follows [352]:(1) Compatibility with colorants and other auxiliaries(2) Adequate solubility and good swelling properties in cold water(3) Good washing-off properties(4) High degree of purity and conformity to standard(5) Non-dusting(6) Biodegradable(7) Non-toxic(8) Manufactured from replenishable raw materials.

10.8.1 Natural thickeners

Natural thickeners are derived from plants by extraction from part of the plant itself or from aplant secretion; their biosynthesis is now a possibility. These products are generallypolysaccharides and are thus closely related to cellulose. They consist of homo- or


chpt10(2).pmd 15/11/02, 15:44647


heteropolymers of simple hexoses, most commonly glucose, mannose or galactose [353].Linear and branched segments are normally present, the degree of branching being importantin relation to the technical properties of the product. Polysaccharides bear some structuralsimilarities to anionic polyelectrolyte dispersing agents (section 10.6.1) and sizing agents(10.5.2). In particular, the nature of the side groups (mainly, though not always hydroxy orcarboxyl groups) has a decisive effect on viscosity and other technical properties. Some, suchas native starch, are used as extracted from their sources; others, such as starch ethers, arederived by introducing substituents or undergoing controlled hydrolysis to lower their viscosity.As with other polymeric auxiliaries already discussed, their detailed structure is still notcompletely understood and the formulae given are only indicative of their structures.

Although native starch is less important nowadays as a thickening agent for textileprinting, some starch derivatives still make a significant contribution. Starch has twocomponents, both of which are made up of linked α-glucoside units (10.106). In amylose,which accounts for some 20–30% of the polymer and has a relative molecular mass in therange 2–6 × 105 the α-glucoside units are linked in a linear 1,4 arrangement (10.107).Cellulose (10.108), by contrast, consists of β-glucoside chains. In amylopectin (Mr 4.5 × 104

to 4 × 105) the linear α-1,4-linked main chain is randomly branched at the 6-position every15–30 glucose units to give an α-1,6-anchored side chain (10.109).

O

HO OH

OHOH

CH2OH

O1

23

4 5

6

10.106

α-Glucoside unit

O

HO

OHOH

CH2OH

O

O

OHOH

CH2OH

O

O

OH

OHOH

CH2OH

n

10.107

Amylose

O

HO

OHOH

CH2OH

O

OHOH

CH2OH

O

O OH

OHOH

CH2OH

O

n

10.108Cellulose

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649

The amylose component is substantially crystalline, forming helical structures that uncoilin an aqueous solution. It can also aggregate to give a gel or precipitate, an undesirablephenomenon known as retrogradation. Amylose is completely hydrolysed by the β-amylaseenzyme. Amylopectin is substantially amorphous, having a globular structure that canexpand considerably in aqueous solution. Its branched chains give rise to a much morestable solution, substantially free from retrogradation, and it is much more resistant to theaction of β-amylase. Starches containing little or no amylose are known as ‘waxy starches’.

The properties of starch can be improved from a printing viewpoint by conversion toBritish Gum (10.110). This is done by a dry roasting treatment at 135–190 °C, acceleratedby trace quantities of acid, to give random hydrolysis of the 1,4-links to decrease the chainlength but with the formation of 1,6-links (branching). The effect is to increase thesolubility and stability although reducing characteristics, which can affect certain susceptibledyes, are enhanced by formation of more aldehyde end groups. Control of the hydrolysis andbranching reactions yields a varied range of products.

O

OHOH

CH2OH

O

OHOH

CH2

O

OHOH

CH2OH

O

OHOH

CH2OH

O O

O

O O

O

10.109

Amylopectin

O

HO

OHOH

CH2OH

O

OHOH

CHO

O

OHOH

CH2OH

O

OHOH

CHO

O

OHOH

O O O

O

CH2

O

O O

British gum

10.110


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Besides roasting, other methods of modifying starch are available. Etherification andesterification, to give starch ethers and starch esters, are both practised although the ethers,being resistant to hydrolysis in acidic or alkaline media, are much the more important asthickening agents for textile printing. The starch may be first partially decomposed beforeetherification and the degree of etherification itself may be varied. The most importantproducts are the carboxymethyl (10.111), hydroxyethyl (10.112) and methyl (10.113)starches (the structures illustrated use the glucose unit as a model, showing the primaryhydroxy group substituted). The degree of alkylation is said to be low or high depending onwhether it is less or greater than 0.3 substituents per glucose (or other) unit; the productsare termed modified starches if the degree of substitution is low and starch derivatives if it ishigh. Crossbonded starches can be obtained by treating, for example, a starch ether of lowdegree of substitution with bifunctional agents such as ethylene oxide, propylene oxide,epichlorohydrin or phosphates. The corresponding derivatives of cellulose can also be madeand used as thickening agents if the chain length is appropriate. The steric hindrance effectof the substituents gives thickening agents of improved all-round properties and certainderivatives have ousted their parent products in terms of commercial importance.

O

HO OH

OHOH

CH2OCH2COOH

10.111

O

HO OH

OHOH

CH2OCH2CH2OH

10.112

O

HO OH

OHOH

CH2OCH3

10.113

Galactomannans are another source of natural thickeners. Structurally related to starchesthey are polysaccharides composed of main-chain mannose and side-chain galactose units asin 10.114. Typical values are: locust bean gum (m = 3, n = 375) and guar gum (m = 1, n =440). The distribution of galactose units varies with the source, as shown schematically in10.115 [354]. Amongst other things, this distribution has an influence on ease ofdispersibility. For example, warm water is required to effect complete dispersion of locustbean gum (the 1,4 form) but guar gum (the 1,2 form) disperses readily in cold water becauseof decreased molecular association arising from the greater frequency of side-chainsubstitution. As with the starches, modified gums can be obtained. In particular,etherification improves the cold water dispersibility of locust bean gum. In Table 10.37various derivatives of galactomannans are listed together with their main applications [354].

Locust bean gum forms an interesting and unusual crosslinked complex by association ofcis-dihydroxy groups in the mannose chains with borate ions, diagrammatically representedin structure 10.116. This complex forms a gel, which has been made use of in printing withvat dyes in a two-stage fixation process. The crosslinks are relatively weak, being in a stateof dynamic equilibrium, and are ruptured in the presence of hydrotropes such as glycerol.

The alginates derived from seaweed are of great importance as thickening agents. Theseare based on alginic acid (10.117; n = 60–600) of which the major commercial salt issodium alginate, although calcium (particularly in mixture with sodium), magnesium andammonium alginates, as well as amine salts, are also available. Their exceptionally low

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651

M M M M M M M M M M M


G G G


G G G


G G G G


G G G G G G


G G G G G G G G G G G

10.115

Galactomannan-1,5 (m = 4) Cassia gum from Cassia tora/obtusifolia seeds

Galactomannan-1,4 (m = 3) Carob gum from Ceratonia siliqua seeds

Galactomannan-1,3 (m = 2) Tara gum from Cesalpinia spinosa seeds

Galactomannan-1,2 (m = 1) Guar gum from Cyamopsis tetragonoloba seeds

Galactomannan-1,1 (m = 0)

Polymannose

O

OHOH

O

O

OHOH

CH2OH

O O

OHO

OHOH

CH2OH

O

CH2

mn

10.114D-Galactomannoglycan


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Table 10.37 Composition and applications of galactomannan derivatives [354]

Galactomannan Chemical variant Main applications

-1,2 Unmodified Carpet printing/dyeing: acid, metal-complex dyes

Depolymerised Carpet printing/dyeing: acid, metal-complex dyesCotton, viscose: vat, direct, azoic dyesPolyester: disperse dyesNylon: acid, metal-complex dyesAcrylic fibres: basic dyes

Hydroxyethylated Carpet printing/dyeing: acid, metal-complex dyesCotton: African prints with azoicsPolyester: disperse dyesNylon: acid, metal-complex dyesAcrylic fibres: basic dyes

Hydroxypropylated Carpet printing/dyeing: acid, metal-complex dyes

Additionally depolymerised Sizing

Carboxymethylated Carpet printing/dyeing: acid, metal-complex dyesCotton: vat, limited reactive dyes

-1.4 Hydroxyethylated Carpet printing/dyeing: acid, metal-complex dyesCotton: African prints with azoicsPolyester: disperse dyesNylon: acid, metal-complex dyesAcrylic fibres: basic dyesWool, silk: acid, metal-complex dyes

Carboxymethylated Cotton, viscose: vat dyesWool, silk: acid dyes

-1,5 Hydroxypropylated

Additionally depolymerised Sizing

Carboxymethylated Carpet printing/dyeing: acid, metal-complex dyes

Additionally depolymerised Cotton, viscose: vat dyesWool: acid dyes

O

O

HOCH2

O

O

O

H

HO

O

O

O

CH2OHO

H

H

O B O

10.116

_

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reactivity with reactive dyes is a special advantage. This is a result of replacement of theprimary hydroxy groups by carboxyl groups. As well as being non-reactive towards reactivedyes, the ionised carboxylate anions repel dye anions in alkaline or neutral media.Carboxylated polymers form gels with multivalent metal ions. This behaviour, like the locustbean gum borate complex mentioned earlier, has been exploited in the two-stage flashageing process for vat dyes in printing. Alginate esters (such as the hydroxypropyl ester)have also been used.

O

HO

OHOH

COOH

O

OHOH

COOH

O

O OH

OHOH

COOH

O

n

10.117

Alginic acid

Other natural polysaccharides used as thickening agents include gum arabic, gumtragacanth and xanthan gum, but these are of diminishing significance nowadays.

Research and development in the area of natural thickeners continues to be active.Detailed studies of the rheology of starch derivatives, alginates, cellulose ethers andvegetable gums [343,344] have shown that thickeners with low solids content form loosenetworks with low convolution density whilst those with a high solids content show higherconvolution densities. This results in differences in tackiness, shear sensitivity andviscoelastic properties, emphasising the major influence of flow properties and viscosity onthe quality of prints obtained. Starch carbamate esters and carboxymethyl carob gums ofvarying degrees of substitution have been evaluated in the vat printing of cotton [355,356].As the degree of substitution of the carboxymethyl carob gum was increased, the colourstrength of the prints increased. Such thickening agents, either alone or in combination withalginates, showed shear thinning behaviour that became thixotropic after storage. Alkali-modified starches have also been assessed in vat printing [357,358]. The alkali-treatedstarches showed higher aqueous solubility and so were more easily removed during washingoff, giving a fabric with a softer handle. Improved colour yields could also be obtained.

It has been shown that carboxymethylcellulose thickeners can effectively replaceemulsion systems in the application of pigments [359,360]. Research in Russia has beendirected towards finding a carboxymethylcellulose thickener for reactive printing that wouldbe efficient and economical, the latter resulting from the fact that low concentrations giveexcellent printing properties that cannot be achieved with other known thickeners [361].The rheology of carboxymethylcellulose thickeners, including storage for up to seven days,has been studied in conjunction with their use in reactive printing [351], indicating suitableconditions for achieving optimum penetration, depth of colour, sharpness of outline andevenness of the prints.

A review of uses of galactomannan thickening agents, of which some 23 000 tons wereused worldwide in 1986, is available [354]. The use of borate ions to crosslink locust bean orguar mannose chains has been mentioned already. It has been shown that addition of borax


chpt10(2).pmd 15/11/02, 15:44653


to dry guar gum delays the development of viscosity, making it easier to prepare large stocksfor continuous dispensing equipment [362]. The oxidative treatment of carob gum withsodium hypochlorite resulted in improved rheological properties and increased aqueoussolubility [363]. No mention was made, however, of possible environmental problems arisingfrom the use of hypochlorite.

When applying reactive dyes, it is essential to use a thickener that does not react with thedyes, alginates being by far the most used. The rheological behaviour of alginate thickenershas been studied [346], showing that the gradient rule is not obeyed across the whole rangeof shear rates. Alginate thickeners may deteriorate on storage, notably as a result ofbiological attack. The significance of this for printing performance [364] and the additionand effects of bactericidal preservatives (sodium o-chlorophenate or chlorinated m-cresol)have been investigated [364]. Certain preservatives have a marked effect on the huesobtained, this being greater with dry heat than with saturated steam fixation, butformaldehyde does not give these effects [364].

The almost exclusive use of alginates with reactive dyes is threatened by uncertainty oversourcing of raw material and by drastic fluctuations in price and quality [365]. Hence therehave been sustained efforts to find alternatives. In one study of various possibilities [366] asynthetic acrylic thickener was chosen, but another investigation [367] showed thatalthough good results can be obtained with synthetic thickeners they cannot fully replacealginates because of poor fastness to rubbing. It has been claimed [368] thatcarboxymethylated derivatives of cellulose, guar gum or starch are quite suitable for reactiveprinting and actually give better performance. Presumably the ionised carboxymethyl groups,like the carboxyl groups in sodium alginate, inhibit reaction with the reactive dyes. Indeed,it has been confirmed that carboxymethylated guar gum derivatives do not react withreactive dyes, and that both carboxymethylated and unsubstituted guar thickeners givesatisfactory results with vinylsulphone reactive dyes, especially for pale grounds and fineoutlines [365].

10.8.2 Synthetic thickeners

Polymers based on acrylic acid have been known since the 1930s but it was not until the late1970s that the use of thickening agents based on them came into prominence in textileprinting [369]. Typical repeat units are shown in 10.118 and 10.119. Commercial linearproducts represented by 10.118 can have n = 50–750 but crosslinked grades of higherrelative molecular mass are also available. The products represented by 10.119 cover a rangeof n values from 3200 to 30 000. Only the longer-chain grades are of significant interest fortextile printing in the form of their sodium or ammonium salts. However, the scope for

CH2 CH CH CH2

C

CO OH

O OHn

10.118

CH2 CH CH2

CO OH

CH

COHO

n

10.119

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655

CH2 CH CH2

CO OH

CH CH2 CH

CO OH

CH CH2 CH CH2 CH CH2 CH CH2

C CO OH O OH

CH CH2 CH CH2

CO OH

100 100

100

100 100

10.120

forming copolymers and crosslinked variants is virtually limitless. A schematicrepresentation of a crosslinked copolymer, using acrylic acid and divinylbenzene at a molarratio of 100:1, is shown in 10.120 [370].

It is important to make a clear distinction between acrylic binders used in pigmentprinting and acrylic thickeners. Binders are generally copolymers and usually contain anintegral crosslinking agent [371]. The thickeners also find their greatest use in pigmentprinting. Their biggest drawback is their sensitivity to electrolytes, although this is less of aproblem in pigment printing than in printing with dyes. The sensitivity of poly(acrylic acid)to electrolytes can be reduced by copolymerising with acrylamide [371], although onlyrelatively small proportions can be incorporated before a deterioration in thickeningefficiency occurs. Two important and interrelated parameters for acrylic thickeners arerelative molecular mass and degree of crosslinking. Simply increasing the molecular mass oflinear poly(acrylic acid) yields thickeners that give stringy pastes unsuitable for use inprinting. Hence a degree of crosslinking is necessary to minimise stringiness by decreasingthe water solubility and promoting dispersibility. Figure 10.50 illustrates the effect ofcrosslinking for three acrylic acid polymers of the same molecular mass [371]. The balanceof molecular mass and degree of crosslinking influences other properties, such as degree ofpenetration, levelness of ground colours and sharpness of the print.

These products are usually supplied to the printer as partially neutralised polyacids.Further neutralisation is carried out by the printer when making up the print pastes. Thisneutralisation is often a critical process. For certain applications, as with resin-bondedpigments, neutralisation is carried out with ammonia. This has the advantage that duringsubsequent baking the ammonia is driven off to liberate the free polyacid, which thencatalyses activation of the resin binder. In other cases neutralisation is carried out with non-volatile alkalis such as sodium hydroxide. It is particularly important to use the latter inreactive printing, since ammonia would be evaporated off during fixation leading to alowering of pH and consequently poor fixation. Moreover, reactive dyes can be deactivatedby reaction with ammonia to form their non-reactive amino derivatives.

The commercial success of acrylic thickeners in pigment printing is attributable to the


chpt10(2).pmd 15/11/02, 15:44655


Vis

cosi

ty

A B

C

ABC

Highly crosslinkedModerately crosslinkedSlightly crosslinked

Thickener concentration

fact that they can be designed to give properties very similar to those of emulsionthickenings. These were previously the only systems used in pigment printing and they aredealt with in section 10.8.3. It is important to realise that an acrylic thickener intended foruse with pigment systems may be unsuitable for use with dyes. This is because commercialthickeners, available as solutions, emulsions, liquid dispersions or powders, often containadditional chemicals to improve their stability and performance in particular systems. Forpigment systems, for example, the thickener may also contain additives (surfactants orpolyelectrolyte dispersing assistants), such as the ammonium or sodium salt of linearpoly(acrylic acid) (10.121), which not only modify the behaviour of the acrylic thickener butalso assist dispersion of the pigment [371]. Surfactant additions are undesirable withreactive dyes because they promote colour bleeding, whilst the ammonia is undesirablebecause of deactivation of reactive groups, the lowering of pH that occurs by its evolutionduring the fixation process and the subsequent difficulty in washing-off of the residualthickener, now bereft of its solubilising ammonium ions.

Figure 10.50 Effect of crosslinking on thickener efficiency [371]

CH2 CH

CO O M

n

10.121

M = Na or NH4

_+

A major drawback of synthetic thickeners when used with dyes is their sensitivity toelectrolytes. Most soluble dyes behave as highly ionised electrolytes and disperse dyescontain anionic polyelectrolyte dispersing agents unless they have been formulated withnonionic systems specifically for use with acrylic thickeners. Consequently there is a loss ofviscosity; this can be quite pronounced although it depends on circumstances, particularlyon the dye concentration. As already mentioned, this can be alleviated to some extent bycopolymerisation with acrylamide during manufacture. Otherwise it is necessary to try toeliminate all electrolytes from the system or to increase the concentration of thickener. Suchmeasures have their limitations in practice, however. Alternative synthetic thickening

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657

CH2 CH2 CH CH

C CO O O

n

10.122

agents include poly(vinyl alcohol) and copolymers of maleic anhydride with alkenes(10.122).

A detailed comparative study of the rheological properties of four acrylic thickenersvarying in relative molecular mass from 1.25 × 106 to 4 × 106 and of two crosslinkedethylene-maleic anhydride copolymers has been published [345]. In respect of someproperties, comparisons were also made with a starch ether and an alginate. Amongst otherfactors, the influence of molecular mass was demonstrated (Figure 10.51), showing that thehigher the molecular mass of the acrylic polymer, the less the amount of thickener requiredto achieve a given viscosity. Nevertheless, earlier comments in relation to the stringiness oflinear acrylic polymers should be borne in mind, i.e. factors other than viscosity need to beconsidered. Viscosity develops as water is absorbed, causing swelling and rearrangement ofthe polymer chains, a process that is assisted by, indeed is critically dependent on,neutralisation. Figure 10.52 gives a schematic representation of this swelling and alsoillustrates the similarity in behaviour with oil-in-water emulsions [371]. It is important thatthe degree of crosslinking is the optimum required to maintain the polymer in this swollenstate and prevent it from dissolving, which would result in loss of desirable properties.

0.2 0.6 1.0 1.4Thickness concentration/%

10

20

30

40

50

Vis

cosi

ty/P

a s

Mr 4 × 106

Mr 3 × 106

Mr 1.75 × 106

Mr 1.25 × 106

Acrylicthickeners

Crosslinked ethylene–maleic anhydride copolymers

No 1No 2

Figure 10.51 Comparison of rheological behaviour of acrylic and copolymeric thickeners [345]

Dispersedpolyacid/pH 4

Neutralised,swollen

polyacid/pH 9

Kerosene emulsionin water

Base

Figure 10.52 Schematic diagram illustrating swelling of polyacid thickener and comparison with oil-in-water emulsion [371]


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A novel concept [371] was evaluated to seek a replacement for alginates in printing withreactive dyes. This approach utilised copolymers of poly(acrylic acid) onto which starch hasbeen grafted by free radical polymerisation, the free radical initiator being a potassiumpersulphate redox system. Both native starch and starch pre-oxidised with sodiumhypochlorite were included in the study. It was found that an acrylic–starch graft copolymereffectively replaced 25% sodium alginate, whilst a copolymer of poly(acrylic acid) andoxidised starch replaced 50% sodium alginate. A detailed rheological study was presentedbut the evaluation lacked economic analysis.

10.8.3 Emulsion thickeners

When immiscible liquids are emulsified the viscosity increases and this can be exploited toprepare thickenings for textile printing. The emulsions used contain a hydrocarbon solvent(usually white spirit), surfactant(s) and water; the oil phase must account for at least 70% ofthe total volume [342]. The first pigment printing systems introduced in the late 1930s werewater-in-oil emulsions (that is, at least 70% of the product was water) in which typicalsurfactants were ethoxylated alcohols, acids or amides with a low degree of ethoxylation,perhaps 5–8 oxyethylene units per molecule, morpholine/oleic acid or lauric, palmitic andstearic acid esters of sorbitol. Later, manufacturers developed oil-in-water emulsions forwhich appropriate surfactants are higher ethoxylated alcohols, acids or amides, or a widevariety of alkylaryl types. For any type of emulsion, the HLB of the emulsifying agent(s) isclearly of great importance.

The size of the droplets in an emulsion is inversely related to its viscosity, typicaldiameters ranging from 100 to 7000 nm. Theoretically no more than 75% of oil can beincorporated in an aqueous emulsion, assuming uniformly spherical droplets, but distortiondue to packing allows significantly higher proportions of oil phase to be added. Presumablythe oil droplets are stabilised by a surrounding layer of like charges, the type and strength ofthe charge depending on the surfactant(s) used. Consequently the stability of the emulsiontends to be impaired by any additions that reduce the charge on the droplets.

Emulsion thickeners can be mixed with low concentrations of either natural or syntheticthickeners, especially when applying fibre-substantive dyes rather than pigments; theseadditions act as film formers, taking the place of the binder used with pigments to increaseretention of the dye by the substrate prior to fixation.

10.8.4 Continuous dyeing

The foregoing discussion has concentrated on the use of thickening agents in textileprinting. Similar types of product are used to thicken pad liquors in continuous dyeingprocesses, although then they are normally described as migration inhibitors rather thanthickening agents. All polysaccharides mentioned previously, especially the alginates, locustbean, guar and xanthan gums, modified starches and celluloses, can be used in continuousdyeing, but by far the most widely used are the alginates. Concentrations tend to besignificantly lower than in printing since the dyeing process requires a lower viscosity,permitting rapid and complete penetration into the fabric during padding. In addition to thealginates, polyacrylates (10.8.2), polyacrylamides and polyethoxylates are also used [373].Polyethoxylates are mixed polyglycol ethers of fatty alcohols with ethylene oxide (or

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propylene oxide), the nonionic block copolymers described in section 9.6 that operate byvirtue of a low cloud point (about 25 °C). The functions of these products are two-fold: (a)they should favour the uniform application of dye by padding; and (b) they shouldeffectively inhibit any tendency of the dye to migrate during the subsequent intermediatedrying process. In the absence of an inhibitor, dye liquor tends to migrate towards hotterregions of the fabric during drying, causing either patchiness or an undesirable two-sidedeffect. Whilst viscosity plays an important part in facilitating the uniform absorption of dyeliquor, it is less effective than coagulation for inhibiting migration. The polyacrylates areparticularly effective [373].

In continuous dyeing agents to assist rapid wetting and even penetration of the fabric areinvariably added to pad liquors along with the migration inhibitor. Many types of surfactantcan be used, including phosphate esters (which are particularly effective [373]),sulphonates, sulphates, sulphosuccinates and sulphated ethoxylates. Care should be taken toensure that the type and concentration of product chosen do not depress the degree offixation of the dyes. Wetting agents are rarely used in printing since they would tend topromote bleeding or haloing of printed areas.

Methods available for assessing migration inhibitors have been reviewed [374,375].Factors influencing dye migration during the drying phase include fibre type, liquor pick-up,drying temperature, running speed, type of dyes and the various pad liquor additives present.Those additives intended to increase substantivity and agglomeration of the dyes tend toinhibit migration. Interaction between dyes is demonstrated by the finding [376] that, usingan alginate migration inhibitor on 50:50 polyester/cotton, the migration of an individual dyecan be inhibited by the presence of other dyes in the mixture that have larger particle sizesor a greater tendency to flocculate. However, increasing the concentration of migrationinhibitor progressively neutralises this tendency. When the concentration is high enough,specificity of particulate migration is eliminated as the migration of dyes in the mixtureapproaches zero.

Polyacrylamides (10.123) of chain length (n) 7000 to 14 000, which is higher thannormally suitable for migration inhibitors, are useful pad liquor additives [373] in that theyincrease liquor pick-up and sometimes colour yield, notably with pigments. When used withazoic dyes on cellulosic fabrics, they can eliminate the need for the intermediate dryingprocess. In a study [377] using polyacrylonitrile saponified with alkali to form amide groupscapable of being crosslinked with formaldehyde, it was found that migration during dryingdecreases as the degree of crosslinking is increased, this being attributed to the increasedstructural density of the polymer films.

CH2 CH

CO NH2

n

10.123

The continuous dyeing of polyester/cotton blends inevitably results in staining of thecotton by disperse dyes, this effect being greatly influenced by the chemical nature andconcentration of the migration inhibitor, the dyebath pH and the chemical nature andconcentration of the dyes, whereas the presence of wetting agent or neutral electrolyte doesnot have much influence [378]. Normally in the dyeing of polyester/cotton blends a higher


chpt10(2).pmd 15/11/02, 15:44659


dyeing temperature minimises staining of cotton, since the higher temperature facilitatesmigration of disperse dye from the cotton to the polyester. Migration inhibitors can negatethis effect, however, so it is important to establish the optimum agent concentrationnecessary to prevent migration but avoiding excessive concentrations that could lead toincreased staining of the cotton.

It has been shown that xanthan gum is an effective migration inhibitor for the applicationof water-soluble chemicals, leading to uniform distribution and more reproducible fixation[379]. Although this work was specifically concerned with the application of a soluble flameretardant to polyester, suitability for the application of reactive dyes or resin finishes is alsoclaimed.

10.8.5 Hydrotropic agents

In many printing and some continuous dyeing processes the colour yield of dyes can often beimproved, sometimes markedly, by the use of an auxiliary that tends to increase the aqueoussolubility of the dye, particularly when using highly concentrated pastes or liquors that tendto lose moisture under adverse conditions. There is clearly an analogy here with themechanism of solubilisation discussed in section 10.6.2, since the hydrotrope acts as anamphiphilic bridge between the dye solubilisate and the aqueous medium. Surfactants cantherefore function as hydrotropes and are sometimes used as such in continuous dyeingprocesses. Hydrotropes with much less powerfully surface-active properties are more suitablefor use in printing, where surfactants are normally avoided because their concomitantpowerful wetting properties would promote bleeding and haloing of the print.

By far the most important of these compounds are urea (10.124) and thiourea (10.125).However, dye–fibre systems are so varied that many hydrotropes are of interest underspecific conditions. Typical examples that have been mentioned [380] include:triethanolamine (10.126), N,N-diethylethanolamine (10.127), sodium N-benzy1sulphanilate(10.128), sodium N,N-dibenzy1sulphanilate (10.129), ethanol, phenol (10.130), benzylalcohol (10.131), resorcinol (10.132), cyclohexanol (10.133), ethylene glycol, glycolic acid(10.134), 2-ethoxyethanol (10.135), diethylene glycol (10.136), 2-ethoxyethoxyethanol(10.137), 2-butoxyethoxyethanol (10.138), thiodiethylene glycol (10.139) and glycerol(10.140), the last-named in particular being useful with practically all classes of dyes.

OC

H2N

H2N

10.124

Urea

SC

H2N

H2N

10.125Thiourea

CH2CH2OHN

HOCH2CH2

HOCH2CH2

10.126Triethanolamine

CH2CH2OHN

CH3CH2

CH3CH2

10.127

HN SO3Na

H2C

10.128

N

CH2

CH2

SO3Na

10.129

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Most hydrotropic agents, though not surfactants in the usual sense of the word, dosignificantly lower the surface tension of water and this is an important prerequisite for theirsolubilising action. Hydrogen bonds, together with weaker dipolar and van der Waals forces,contribute to this interaction, the active centres in the hydrotropic molecules being proton-donating groups (such as hydroxy, amino and amido groups) and proton-accepting atoms(such as the nitrogen atom of a tertiary amine). For this reason hydrotropes can also beadded to the diluent system in the manufacture of certain dyes, particularly liquid brands, inwhich they not only increase the apparent solubility of the dye but also help to prevent theformation of surface skin. Another compound that has entered this area is 1-cyanoguanidine(10.141) popularly known as dicyandiamide. The main mechanisms whereby hydrotropesare believed to function have been mentioned above. However, there has been a good dealof discussion regarding possible mechanisms, mainly in connection with the interaction ofurea with reactive dyes. Useful reviews of this topic are available [381,382].

OH

10.130

Phenol

CH2OH

10.131

OH

HO

10.132

Resorcinol

OH

10.133

Cyclohexanol

HOCH2COOH

10.134

CH3CH2OCH2CH2OH

10.135

HOCH2CH2OCH2CH2OH

10.136

CH3CH2OCH2CH2OCH2CH2OH

10.137

CH3CH2CH2CH2OCH2CH2OCH2CH2OH

10.138

HOCH2CH2SCH2CH2OH

10.139

HOHC

CH2OH

CH2OH

10.140

Glycerol

NHC

HN

H2N

CN

10.141

Dicyandiamide


chpt10(2).pmd 15/11/02, 15:44661


The growth of environmental awareness has certainly impinged on this area, restrictingthe use of hydrotropes that are volatile during steaming or dry heat treatments, as well asthose suspected of endangering health. These problems are particularly acute with urea,widely used for a long time and considered an essential hydrotropic assistant in printing andcontinuous dyeing with reactive dyes. This is considered in more detail in section 12.7.1.

10.8.6 Environmental aspects

Reference to Table 10.10 in section 10.5.2 indicates printing to be responsible for some 10–20% of the total pollution load from the textile wet processing of cotton. Other substrates,as well as continuous dyeing, also make significant contributions. Useful reviews of theenvironmental aspects of textile printing generally [383,384] and of pigment printing inparticular [385,386] are available. There are two aspects to be considered:(1) airborne pollution from volatile components during steaming and dry heat treatments(2) aqueous effluent pollution from washing-off and washdown procedures.

In both cases there are two basic methods of control:(1) elimination or minimisation of offending products, which may involve total or partial

substitution by more benign products, or a redesigned process that does not require thattype of auxiliary

(2) remedial treatment of exhaust gases or effluent.

In any case, it is always sound practice to optimise the concentration of an auxiliary so thatno more than necessary is applied. This aim of using minimum quantities is assisted by thetrend towards more efficient thickeners. For example, in the 1960s it was common forthickeners to be made up to a 20–30% concentration, whereas today 10% is more commonand it is predicted that this will fall to an average of about 5% within the next decade [383].

Airborne pollution can arise from volatile fractions during drying, as well as dry heatcuring or heat setting treatments. This aspect has been associated mostly, but notexclusively, with the use of emulsion thickeners in pigment printing, considerable volumesof organic compounds being released during drying and curing. Consequently, emulsionprinting is now seldom carried out in the major developed countries [383]. Those acrylicthickeners that contain organic solvents or hydrocarbons are also sources of airbornepollution, but thickeners free from such additives are now available [386]. One method ofdealing with the volatile emissions is incineration, but this is very costly. Scrubbing is highlyefficient with water-soluble volatiles but not with hydrocarbons: hence it is best to avoidusing polluting materials. A detailed account of the ecological factors in pigment printing isavailable [385], indicating the obligation to minimise or eliminate emissions of residualmonomers, formaldehyde and unwanted solvents. Furthermore, alkylphenol ethoxylatesformerly used as emulsifiers can be replaced by fatty alcohol ethoxylates or, to some extent,by anionic surfactants. Pigment printing also offers ecological benefits over other systems inthat it saves time and energy, and washing-off is unnecessary [385].

In general (excluding pigment printing), however, the thickeners or migration inhibitorsare present in waste waters, both from washing-off the dyeings or prints and from washing-down of equipment. The usual three methods of treatment (chemical degradation,bioelimination and recycling) have to be considered for dealing with them. Natural

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thickeners are not toxic in themselves but, in parallel with the analogous sizing agents(Table 10.11 in section 10.5.2), they show high chemical and biological oxygen demand[354,383,387]. Galactomannans [216,217,354] in particular are thought to have a goodfuture in respect of environmental impact. Since the naturally occurring polymers presentthe least difficulties in environmental terms, it is thought that their derivatives will becomeless important [383].

The treatment and disposal of these wastes is not only expensive but entails the loss ofpotentially useful materials. Thus, as with sizing agents, recycling is an attractiveconsideration [388,389]. The basic principle is precipitation of the thickener in a suitableaqueous solvent system followed by isolation, often by ultrafiltration. Dyes can be separatedfrom the thickener either continuously or discontinuously [388]. In a detailed study [389],the efficiency of precipitation of high-, moderate- and low-viscosity alginate thickeners infour solvent systems was compared with that of carboxymethylated galactomannans orcelluloses and four synthetic thickeners, giving the results in Table 10.38. Thus all types ofalginate thickener can be isolated quantitatively from the wash water and recycled withoutdifficulty. These recycled thickeners are claimed [388,389] to give rheological properties andprinting results equivalent to those of the original products. Pure acrylic thickeners can alsobe precipitated almost quantitatively [389].

Table 10.38 Precipitation (%) of printing thickeners using solvents [389]

AqueousThickener Methanol Ethanol Acetone methanol

Low-viscosity alginate 100 100 100 100Moderate-viscosity alginate 100 100 100 100High-viscosity alginate 100 100 100 100

CarboxymethylgalactomannanLow-viscosity >90 >90 >90 70–90High-viscosity >90 >90 >90 70–90

CarboxymethylcelluloseLow-viscosity >90 >90 >90 55–90High-viscosity >90 >90 >90 55–90

Synthetic thickener 1 70–90 70–80 70–95 <20Synthetic thickener 2 50 50 50–70 <20Synthetic thickener 3 0 0 0 0

Poly(acrylic acid) >90 >90 >90 >90

Natural thickeners are prone to biological attack and degradation, leading to a loss ofthickening efficiency. It is common practice to add a small amount of a bactericide toprotect against attack during storage [383,387,390]. About thirty classes of compounds havebeen listed as bactericides [387], but phenol derivatives (including chlorinated phenol,m-cresol or o-phenylphenol) and formaldehyde have proved particularly suitable, addedeither by the manufacturer of the thickening agent or by the printer during formulation ofthe stock thickener. As bactericides, such compounds are by definition toxic and hence


chpt10(2).pmd 15/11/02, 15:44663


environmentally undesirable if not actually prohibited in some countries. However, it hasbeen pointed out [383] that they are used only at very low concentrations (usually below0.1%) that are just about sufficient for bacterial efficiency. Together with the thickener theyare then washed out with copious amounts of water, entering the effluent at such highdilution that they no longer have bactericidal action and can even be biologically eliminatedduring effluent treatment.

Mention has already been made of the environmental undesirability of many hydrotropes(section 10.8.5) and this particularly applies to urea, the most widely used. This is discussedin more detail in section 12.7.1 in connection with reactive dyes. Suffice to note that theefficiency of urea as a hydrotrope is proving difficult to equal in every respect withsubstitutes that are environmentally acceptable [382].

10.9 TREATMENTS TO ALTER DYEING PROPERTIES OR ENHANCEFASTNESS

Treatments to improve the intrinsic fastness of dyeings have a long and prolific history. Mostof these are aftertreatments, although some are concurrent with the dyeing process as in theapplication of UV absorbers to enhance light fastness. The treatments described in thissection are essentially concerned with improving fastness properties or fibre stability.Treatments to modify the handle or other fibre properties are excluded, these being dealtwith in section 10.10. Furthermore, such treatments are not an essential integral part of thedyeing process, thus excluding the afterchroming of chrome dyes and the oxidation ofsulphur and vat dyeings. Many processes have been developed for improving fastness butrelatively few are still in commercial use today. Their efficacy has to outweigh the extraprocessing cost and time. The emphasis will be on processes of current commercial orresearch interest. An excellent comprehensive review covering the period 1880–1980 isalready available [391]. Of growing interest, at least in the research sector, are fibrepretreatments aimed primarily at modifying dyeing properties, although these may havesecondary benefits in terms of improved fastness. There has always been a good deal ofinterest in pre- and aftertreatments amongst textile chemists and colourists, largely onaccount of the innate interest of the chemistry and the challenges involved. Commercialprocessors have been less enthusiastic, however, since these variations involve additionalcosts and scheduling difficulties.

10.9.1 Pretreatments

One of the earliest fibre pretreatments for improving the dyeability of cotton is of coursemercerisation (section 10.5.4). However, more recent research interest in this area has beengenerated by environmental concerns about reactive dyeing, aiming to enhancesubstantivity for the modified fibre so that higher absorption and fixation are obtained. Thisresults in less dye (hydrolysed or still active) in the effluent. A further objective is tominimise the usage of electrolyte in the application process. This area has been thoroughlyreviewed [392,393].

At the risk of over-simplification, most of the pretreatments covered by Lewis andMcIlroy will be summarised in Tables 10.39 to 10.42. Their review [393] and the originalworks cited in it should be consulted for details, of course. The great majority of processes

chpt10(2).pmd 15/11/02, 15:44664

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involve increasing the nitrogen content (i.e. the basicity) of the cellulose, there being ananalogy here with the better dye-sorption properties of wool that naturally contains suchbuilt-in nitrogen. Table 10.39 presents structural modifications of cellulose not entailing theapplication of a polymer. Most of these reactions involve amination in one form or another.In fact interest in the amination of cellulose began in the 1920s, long predating currentenvironmental concerns regarding reactive dyes. Hence it is not surprising that some of theearliest processes are environmentally suspect. Nevertheless, more acceptable processeshave evolved, making cellulose more substantive to anionic dyes and enabling much lesselectrolyte and non-alkaline fixation conditions to be adopted for reactive dyeing.

It has been pointed out that aminoethylcellulose produced by a two-stage dry processusing 2-aminoethylsulphuric acid can even react covalently with hydrolysed vinylsulphonedyes, thus enhancing fixation and making washing-off easier [394]. Another cationicquaternary compound that has been used [395] is 3-chloro-2-hydroxypropyltrimethyl-ammonium chloride (10.142). This compound and four analogous reactive cationic agents(10.143) have been proposed [396], prepared by reaction of the appropriate amine withepichlorohydrin.

The cationisation of cotton cellulose using these agents takes place in two stages(Schemes 10.49 and 10.50), although from a practical viewpoint these occur concurrently ina single process by exhaustion for 20 minutes at 80 °C in the presence of sodium hydroxideas the base catalyst. In the first stage an epoxide is formed in the presence of alkali. In thesecond stage this epoxide reacts with a hydroxy group in the cellulose. The cationisedcotton could be dyed with anionic dyes; only CI Acid Red 127 was used in this research.The degree of substitution of the cellulose and the amount of dye absorbed both decreasedas the length of the hydrocarbon chains attached to the nitrogen atom was increased.Regardless of hydrocarbon chain length, the light fastness was slightly higher on cotton thanon nylon or wool dyed with the same dye. Fastness to washing decreased with increasinglength of the hydrocarbon chains. The cationised cotton showed enhanced antibacterialproperties, the potency increasing with increasing hydrocarbon chain length [396].

TREATMENTS TO ALTER DYEING PROPERTIES OR ENHANCE FASTNESS

CH3 N CH2

CH3

CH3

CH

OH

CH2Cl

Cl

10.142

+

_

R2 N CH2

R1

R3

CH

OH

CH2Cl

Cl

R1 = R2 = R3 = CH3

R1 = R2 = R3 = CH3CH2

R1 = R2 = R3 = CH3CH2CH2

R1 = R2 = R3 = CH3(CH2)4

R1 = R2 = CH3R3 = CH3(CH2)13

10.143

_

+Trimethylamine:

Triethylamine:

Tripropylamine:

Tripentylamine:

Dimethyltetradecylamine:

chpt10(2).pmd 15/11/02, 15:44665


R N CH2

R

R

CH

OH

CH2Cl

R N CH2

R

R

CH

O

CH2Cl

R N CH2

R

R

CH

O

CH2

Cl

HO

Cl

Cl

+ Cl

+

_

_+

_ _

+

_

_

Scheme 10.49

R N CH2

R

R

CHO

CH2

[cellulose]HO

R N CH2

R

R

CH

OH

CH2 O [cellulose]Cl

Cl

++

_

+

_Scheme 10.50

Table 10.39 Pretreatments to modify cotton cellulose by substitution reactions [393]

Pretreatmentreaction Method Comments

Amination Tosylation, followed by treatment All amination treatments give with amines (Scheme 10.51) improved dyeability

p-Nitrobenzoyl chloride, followed by reduction of nitro (Scheme 10.52)

Acetylaminobenzenesulphonyl chloride in nitrobenzene or chloroform, followed by hydrolysis of the amide (Scheme 10.53)

2-Chloroethylamine and alkali-treated cellulose (Scheme 10.54)

Sodium 2-aminoethylsulphate in More efficient in producing aqueous alkali (Scheme 10.55) 2-aminoethylated cellulose.

Relatively inexpensive and doesn’t require organic solvent

Sodium borohydride added to a Gives white aminated cotton. sodium 2-aminoethylsulphate Borohydride reduces yellow pad liquor Schiff bases formed by reaction

of aldehyde groups with amino groups

Polymerisation of ethyleneimine on Improved dyeability fibre by various methods

Diethylaminoethylation 2-Chloroethyldiethylamine Dyeable in absence of salt hydrochloride (Scheme 10.56) with direct, reactive or

acid dyes

Continued on next page

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667

Cl SO2 CH3OH[cellulose]

O SO2 CH3[cellulose]

CH3O3S[cellulose] NH2 NH4

+

+ HCl

2 NH3

++

Scheme 10.51


Table 10.39 Continued

Pretreatmentreaction Method Comments

Esterification Inorganic or organic acids Improved dyeability

Esterification and 3-Chloropropionyl chloride, Primary, secondary, tertiary amination followed by amine (Scheme 10.57) and quaternary derivatives

can be produced. Dyeable with reactive dyes, neutral to slightly acidic without salt

Amination Epoxides in alkali, including ethylene Glycidyltrimethylammonium oxide, propylene oxide, glycidol chloride marketed to enhance (2,3-epoxypropan-l-ol). Scheme 10.58 dyeability with direct and shows glycidyltrimethylammonium reactive dyes chloride

Introduction of 1,1-Dimethyl-3-hydroxyazetidinium Dyeable with reactive dyes at quaternary N groups chloride in presence of strong pH 7 without salt, giving

alkali by pad–bake (Scheme 10.59) extremely high fixation

Acylation Nicotinoyl-thioglycolate and alkali by Dyeable with monochlorotriazine pad–bake (Scheme 10.60) reactive dyes at pH 3

without salt

Amination N-Methylolacrylamide in presence Improved dyeability with of Lewis acid catalyst. Further dichlorotriazine dyes at pH 5 modifications possible by without salt, giving 99% addition to double bond (Scheme 10.61) fixation

Amines with durable press resins Some improvements in dyeability, especially with direct dyes, but light fastness can be a problem

chpt10(2).pmd 15/11/02, 15:44667


[cellulose] OH C NO2Cl

O

C NO2O

O

[cellulose]

C NH2[cellulose]

O

+

+ HCl

reduction

O

Scheme 10.52

[cellulose] OH Cl SO2 NHCOCH3

[cellulose] O SO2 NHCOCH3

[cellulose] O SO2 NH2 CH3C

HO

O

H2O

+

+ HCl

+

Scheme 10.53

[cellulose] OH [cellulose] O CH2CH2NH2ClCH2CH2NH2 + HCl+

Scheme 10.54

chpt10(2).pmd 15/11/02, 15:44668

669

[cellulose] OH

[cellulose] O CH2CH2NH2

NaOH

+ H2NCH2CH2OSO3Na

+ Na2SO4 + H2O

130°C15 min

Scheme 10.55

NH

CH3CH2

CH3CH2

CH2CH2Cl HO [cellulose]

CH2CH2N

CH3CH2

CH3CH2

O [cellulose]

NaOH

Cl

+ NaCl + H2O

++_

Scheme 10.56

[cellulose] OH Cl C CH2CH2Cl

O

[cellulose] O C

O

CH2CH2Cl

[cellulose] O C

O

CH2CH2Cl [cellulose] NR3 HO C CH2CH2Cl

O

NaOH

Cl Cl

+ NaCl + H2O

+

+ NHR3100°C

++

_

+

_

Scheme 10.57

CH3 N CH2

CH3

CH3

CHO

CH2[cellulose]HO

CH3 N CH2

CH3

CH3

CH

OH

CH2 O [cellulose]

Cl

++

+

_

Cl_

Scheme 10.58


chpt10(2).pmd 15/11/02, 15:44669


HN

H3C

H3C

H2C O

CH2Cl CH

CH2

NH3C

H3C

CH2CH OH

Cl

++

_

Scheme 10.59

N

C

O

S CH2COONa

HO [cellulose] N

C

O

O [cellulose]

HS CH2COONa

N

C

O

S CH2COONa

N

C

O

OH

HS CH2COONa

HO

HO

+ +

+ H2O +

_

_

Scheme 10.60

[cellulose] OH HO CH2 NH C CH

O

CH2

[cellulose] O CH2 NH C CH

O

CH2

[cellulose] O CH2 NH C CH2CH2

O

NH2


O

NHCH3


O

N(CH3)2


O

N(CH3)3X


O

NHCH2CH2OH

+

ZnCl2, 150 °C

+ H2O

Reaction with Product

Ammonia

Methylamine

Dimethylamine

Trimethylamine.HX

Ethanolamine

+ _

Scheme 10.61

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671

A somewhat different substitution reaction involved treating cotton with an aqueoussolution containing 10 ml/l carbon disulphide, 1% sodium hydroxide and 0.5% wetting agent[397]. This thiocarbonate pretreatment was followed by dyeing with a direct dye in thepresence of ammonium persulphate as free radical initiator, giving higher wet fastness thanon alkali-treated or untreated cotton. This was explained in terms of involvement of thethiocarbonate groups in the formation of covalent bonds between cellulose and the directdye by a free radical mechanism.

Table 10.40 represents those pretreatments requiring application of a polymer, thefundamental objective again being to increase basicity by incorporation of amino groups. Apoint of interest is use of the cationic polymer Hercosett 125 (Hercules), formed bycondensation of adipic acid and diethylenetriamine to give a polyamide subsequentlypartially crosslinked with epichlorohydrin, a resin more commonly associated with shrink-resist treatments of wool. Once again, improvements in dyeability are usually claimed butretaining good light fastness appears to be a common problem. Fibre-reactive quaternaryammonium compounds with chlorohydrin functionality have also been evaluated as epoxidepretreatments for wool/cotton blends in an attempt to facilitate union dyeing using anionicdyes for wool [398]. Since alkaline conditions were required, the integrity of the wool waspreserved by treatment at ambient temperature for 3 hours at pH 11.

Cotton grafted with 2-vinylpyridine followed by quaternisation using an excess of an alkylbromide or epichlorohydrin showed markedly increased exhaustion with direct dyes [399].Grafting alone gave a substantial effect, with further slight improvements being conferred bythe quaternisation. Improved fastness to washing was also claimed.

Table 10.40 Pretreatments to modify cellulose using amino-containing polymers [393]

Pretreatment polymer Method Comments

Chitosan Cationic polymer applied by Improved coverage of immature exhaustion from acidic solution fibres, increased exhaustion but

decreased fastness unless further treated with fibre-reactive quaternary compound

Sandene (Clariant) Cationic polymer applied by Enhanced dyeability with anionic exhaustion under alkaline and reactive dyes, the latter conditions applied under neutral or slightly

acidic conditions. Reduced light fastness and marked dulling with some dyes

Hercosett 125 (Hercules) Pad-dry-cure for 3 min at 100 °C. Dyeable neutral without salt; good reactive cationic polymer Scheme 10.62 represents the results with some high-reactivity formed by condensation reactive and nucleophilic sites dyes (dichlorotriazine and of adipic acid and that may exist on the surface of difluoropyrimidine) but not with diethylenetriamine, then the treated fibre some other types partially crosslinked with (monochlorotriazine and epichlorohydrin dichloroquinoxaline). Washing

fastness very good but light fastness lower

Continued on next page


chpt10(2).pmd 15/11/02, 15:44671


Table 10.40 Continued

Pretreatment polymer Method Comments

Thiourea derivative Scheme 10.63 Build-up good for dichlorotriazine of Hercosett dyes. Dyes of other types gave (isothiouronium salt) good results up to about 2%

applied depths

Ethylenediamine derivative Scheme 10.64 About 95% fixation of low- or of Hercosett high-reactivity dyes under slightly

acidic conditions without salt, but light fastness still inferior

Polyepichlorohydrin and Polymerisation of epichlorohydrin Good yields with direct dyes using dimethylamine in carbon tetrachloride with only 2 g/l salt. Excellent build-up

boron trifluoride/ether catalyst, with most reactive dyes; only 10% then reaction with dimethylamine. of normal salt usage needed for Applied to cotton by exhaust low-reactivity dyes and none for method or pad–dry. highly reactive types. Washing Scheme 10.65 fastness very good but light

fastness impaired.

CH2N

CH2

CH OH

NH NH2

CH2N CH

OH

CH2 N NH CH2 CH

OH

CH2 NH

[cellulose] OH

[cellulose] O CH2 CH

OH

CH2 N

Cl

HO

H

2 H

2 HO

Azetidinium cation

Secondary amino

Tertiary amino, secondary hydroxy

Cellulose hydroxy

Covalent link to cellulose

+

_

_

+

_

+ + +

+

Scheme 10.62

CH2N

CH2

CH OH CS

NH2

NH2

CH2N CH CH2

OH

S C

NH2

NH2

ClCl

++

_

+

_

Azetidinium cation Thiourea Isothiouronium salt

Scheme 10.63

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673

CH2N

CH2

CH OH H2N CH2CH2 NH2

NH CH2 CH

OH

CH2 NH CH2CH2 NH2

CH2N

CH2

CH OH

NH CH2 CH

OH

CH2NHCH2CH2NH

CH

HO

CH2

CH2 NH

CH2CH

CH2

NHO

Cl

Cl

Cl Cl

ClCl

H2NCH2CH2NH2

+

++

_

+

+

_

_

+

_

+

_

+

_

+

Scheme 10.64

O

H2C CH

CH2Cl

CH2 CH O

CH2Cl

HN

CH3

CH3

CH2 CH O

CH2 NH CH3

CH3

CCl4

Cl

BF3 , EtOEt

n

_

+

Epichlorohydrin

n

Scheme 10.65

Certain pretreatments depend on introducing sulphur rather than nitrogen, these beingsummarised in Table 10.41. So far these appear to have been much less successful thannitrogenous treatments.

A more radical approach to pretreatment reverses the conventional reactive dyeingconcept by preparing a reactive cotton cellulose capable of reacting covalently with suitabledyes containing, for example, aliphatic amino groups. In an initial attempt, cotton was


chpt10(2).pmd 15/11/02, 15:44673


Table 10.41 Pretreatments to modify cellulose using sulphur-containing agents [393]

Pretreatment reaction Comments

Cotton treated with bis(2-isocyanatoethyl) The greater the sulphonium content, the greater disuphide in dimethylformamide at 80 °C, the uptake of direct dyes. Not an environmentally followed by reduction with tri-n-butylphosphine acceptable pretreatment, however. in methanol containing 10% water. This gives the 2-mercaptoethylcarbamyl ester, which is treated with methyl iodide to form sulphonium salts. (Scheme 10.66).

Alkali-treated cellulose immersed in an Dyeings with direct, reactive, sulphur and disperse arylsulphonium salt solution. dyes at pH 5 showed improved colour strength

but detailed results were not reported.

Linen esterified with thioglycolic acid to Increased colour yields obtained with reactive dyes, incorporate thiol groups, these being more but not enough to give adequate solidity on strongly nucleophilic than hydroxy groups. linen/wool blends.

[cellulose] O C NHCH2CH2SH

O

[cellulose] O C NHCH2CH2

O

S CH3

[cellulose] O C NHCH2CH2

O

S CH3

CH3

CH3I

CH3I

I

+

_Scheme 10.66

[cellulose] O CH2 NH C CH

O

CH2

N

N

N

HNNH

H2NCH2CH2NH

R D (SO3 )n

[cellulose] O CH2 NH C CH2CH2NHCH2CH2NH

O

N

N

N

NH HNR D (SO3 )n

+

_

_Scheme 10.67

treated with N-methylolacrylamide (Scheme 10.61). This is capable of Lewis acid catalysedetherification of hydroxy groups in cellulose and alkali-promoted Michael addition tonucleophiles [393]. The modified cotton was reacted with aminoalkylated triazine dyes(Scheme 10.67) at pH 10.5 in the presence of 80 g/l salt. In contrast to conventionalhalotriazine reactive dyes, these are stable to hydrolysis under these conditions. However,

chpt10(2).pmd 15/11/02, 15:44674

675

build-up proceeded only up to a limit, beyond which the absorbed dye appeared to act as aresist agent, preventing further uptake. In any case, this process still required the addition ofa high concentration of salt.

This approach was subsequently improved considerably by treating cotton with2,4-dichloro-6-(2′-pyridinoethylamino)-s-triazine (Scheme 10.68). One of the chlorosubstituents reacts under mild conditions with cellulose and the reactivity of the otherchloro is reduced [393], giving a fibre containing monochlorotriazine residues that can reactcovalently with aminoalkylated dyes. Pad–batch pretreatment gave better results thanexhaustion, enabling application of the dye at pH 9 in absence of salt and giving similarfastness to conventional dyeing. A highly significant advantage is that full use is made of thedye applied because no dye hydrolysis can occur, resulting in shorter washing-off times and areduction in coloured effluent. However, the process requires special dyes and at present nocommercial range of these exists. Furthermore, there could be hydrolysis problems with thepretreating agent or with the pretreated fibre. Any unlevelness in the pretreatment processwould clearly be disastrous in commercial terms.

[cellulose] OH

N N

NCl

Cl

NHCH2CH2 N

N N

NO

Cl

NHCH2CH2 N[cellulose]

Cl

Cl

++

_

_

+

Scheme 10.68

A rather more perverse activity of researchers concerns their attempts to make cottondyeable with disperse dyes so that, for example, polyester/cotton blends could be dyed orprinted with a single dye class. This inevitably means invoking symbiotic chemistry; if cottonis to become dyeable with hydrophobic disperse dyes, it must itself be made morehydrophobic. This carries with it the risk that the very physical properties that make cottonso desirable, and for which it is incorporated into blends with hydrophobic fibres, arenegated. Thus the moisture regain of a blend of polyester with hydrophobic cotton may besuch that it has no advantage over a fabric made entirely from polyester microfibre. It isperhaps significant that moisture regain is hardly mentioned by researchers devisingtreatments for making cotton more hydrophobic. Little is also said regarding light fastness,although good fastness to washing is sometimes claimed [400].

Processes devised to make cotton hydrophobic are summarised in Table 10.42. Theseprocesses are undoubtedly successful in conferring substantivity for disperse dyes butattaining compatibility within a range of dyes across the entire colour gamut and on fibreblends of various blend ratios could be a problem. In addition, ester bonds can be saponified


chpt10(2).pmd 15/11/02, 15:44675


during washing, typically losing 30% of the effect after one wash and 60–70% after fivewashes. Hence commercial exploitation is subject to severe limitations.

Table 10.42 Pretreatments to make cotton more dyeable with disperse dyes [393,400]

Pretreatment reaction Comments

Benzoylation with benzoyl chloride. Proposed in the 1920s to confer easy-care properties. In 1976 process patents introduced for making cotton acceptable to disperse dyes in transfer printing.

Water-soluble acylating esters sodium Applied by padding with alkali and baking at 200 °C. benzoylthioglycolate and benzoylsalicylate More acceptable for health and safety reasons (Scheme 10.69). than the lachrymatory agent benzoyl chloride.

Aliphatic acid chlorides. Increasing the length of the aliphatic chain increased uptake of disperse dyes; maximum uptake needed at least eleven C atoms per molecule.

Aromatic substituents in cellulose confer Benzoylation required a degree of substitution of greater disperse dye substantivity than 15–25% for optimum substantivity; 30–40% was aliphatic substituents. needed to give good fastness to washing.

Graft copolymerisation of styrene on partially Higher graft yields of polystyrene gave higher carboxymethylated cotton using colour yields with disperse dyes. gamma radiation.

C SCH2COONa

O

HO [cellulose]

C O

O

[cellulose]

HO

+

+

_

Sodium benzoylthioglycolate

HSCH2COONa

C O

O

HO [cellulose]

C O

O

[cellulose]

HO

+

+

_

Sodium benzoylsalicylate

NaOOC

HO

NaOOC

Scheme 10.69

chpt10(2).pmd 15/11/02, 15:44676

677

Since wool dyeing is characterised by much higher levels of exhaustion than are typicalfor the dyeing of cellulosic fibres, as well as much lower concentrations of electrolytes,interest in pretreatments for wool is less concerned with the environmental aspects ofdyeing processes. The motivation to modify wool is usually to increase or decrease dyeabsorption so that, for example, differential dyeing effects can be obtained [401]. As well asoverdyeing fabrics containing some predyed wool, possibilities in this area include the use ofprechromed and unchromed wool, pretreatment with a cationic agent to give increased dyeuptake or pretreatment with a syntan (section 10.9.4) to give a dye-resist effect. Attemptsaround 1970 to commercialise the prechromed/unchromed system faltered as a result ofpoor reproducibility. In any case, nowadays there would be environmental questions to beaddressed regarding chromium in the effluent. Pretreatment with a cationic agent or syntanhas economic benefits over the spinning of predyed wools in the production of heather-effect yarns.

Wool has been pretreated with glucose-derived crown ethers of the type shown in 10.144and 10.145 [402]. Crosslinking occurs by interaction of amino groups in wool keratin withthe glucosidic hydroxy groups in the crown ether. Increased dyeability with reactive dyes wasachieved (Figure 10.53), a primary objective of the research being the low-temperaturedyeing of wool.

10.144

H2CO

CH2

H2C CH2

O OCH HC

CH HCO O

H2CO

CH2

H2C CH2

HCCH

CHO

O HOR

HC OCH HCH2C

HO OH OR

CH2

OH

R = H or CH3

Bis-glucopyranoside-18-crown-6 ethers

O O

OO O

O

O O

O O

O O O O

MeO OMe

O O

OO O

O

O O

O O

O O OMe

MeO O O

Bis-glucopyranosido-24-crown-8 ethers

10.145


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Wool dyeing at low temperatures has been enhanced by utilising the Maillard, Streckerand modified Strecker (P-Mannich) reactions as pretreatments [403,404]. In a sense, theinteraction of wool with the glucose crown ethers already mentioned made use of theMaillard reaction. This is the reaction between the oxo group of an aldose molecule and thefreely accessible amino groups of an amino acid or protein molecule, leading to the ultimateformation of the tautomeric forms of an N-substituted 1-amino-1-deoxy-2-ketose via theseries of reactions shown in Scheme 10.70 [403]. This reaction is associated with thebrowning observed when cooking saccharide-containing foods. Hence it is necessary tocontrol the conditions of application to wool so that yellowing or browning does not occur;30 minutes at 90 °C gave satisfactory results [404]. The Strecker reaction is similar exceptthat a glucose-cyanohydrin is used (Scheme 10.71). The modified Strecker or P-Mannichreaction typically uses dimethyl phosphite in place of the cyanohydrin [404], thus beingmore acceptable on health and safety grounds for use in a dyehouse (Schemes 10.72 and10.73).

30 60 90 110 130Dyeing time/min

80

20

60

100

40Dye

abs

orpt

ion/

%

A

B

A

B

Wool treated with glucosido crown etherUntreated wool

Figure 10.53 Rate of absorption of Lanasol Red G (Ciba; CI Reactive Red 83) by wool [402]

C

CHOH

H O

CHOH

CHOH

CHOH

CH2OH

CHOH

CHOH

CHOH

CHOH

CH2OH

CHOH

NH R

CHOH

CHOH

CHOH

CHOH

CH2OH

CH

N R

R NH2

CH

CH

CH

CH

O

CH

NH

HO

HO

OH

CH2

OH

R

COH

CHOH

CHOH

CHOH

CH2OH

CH

NH R

C

CHOH

CHOH

CHOH

CH2OH

CH2

NH R

O

+

R = protein

Aldose Aminatedaldose

Schiffbase

Aldoseamine

Enol Keto

Amadori products

Scheme 10.70

chpt10(2).pmd 15/11/02, 15:44678

679

C

CHOH

CHOH

CHOH

CHOH

CH2OH

OHCHOH

CHOH

CHOH

CHOH

CHOH

CH2OH

CN

CH

CHOH

CHOH

CHOH

CHOH

CH2OH

CN

NH (CH2)4 CH

C

[wool]

O

NH

[wool]

[wool]

C

CH

O

NH

(CH2)4

[wool]

H2N

NaCN

++

lysine residue

40°C

Glucose Cyanohydrin

Scheme 10.71

OC

CH3

CH3

OHP

CH3O

CH3O

CH3OP

HCH3O

O

CH3

P

OH

CH3OCH3

O

CH3O

H2N(CH2)4CH

CH3

P

NH(CH2)4CH

CH3OCH3

O

CH3O

COOH

NH2

COOH

NH2

+

+L-Lysine

Dimethyl phosphite

N-Phosphonomethyl-L-lysine

Scheme 10.72

C

CHOH

CHOH

CHOH

CHOH

CH2OH

OH

[wool]

C

CH

O

NH

(CH2)4

[wool]

H2NOHP

CH3O

CH3O

CH3OP

HCH3O

O

CHOH

CHOH

CHOH

CHOH

CHOH

CH2OH

P

CH3O

OCH3O

CH

CHOH

CHOH

CHOH

CHOH

CH2OH

P

CH3O

OCH3O

NH(CH2)4 CH

C

[wool]

O

NH

[wool]

lysine residue

80°C

+

+

D-GlucoseDimethyl

phosphite

Scheme 10.73

When the Maillard reaction was evaluated using 10 g/l glucose for 30 minutes at 90 °Cand 20:1 liquor ratio the fibre diameter increased by 3.5%; xylose gave an increase almosttwice as much but showed some yellowing. In this process accessibility of the fibre for dyemolecules is increased, since the glucose molecules penetrate between the peptide chains.The reaction also introduces primary alcoholic groups, making the wool more dyeable with


chpt10(2).pmd 15/11/02, 15:44679


the reactive dyes normally used on cellulosic fibres, as well as with bromoacrylamide reactivedyes. Satisfactory dyeing was achieved at pH 5 and up to 80 °C on glucose-treated wool.Penetration was rapid, enabling the dyeing time or temperature to be decreased. The build-up and fixation of reactive dyes were increased. The Strecker reaction utilising glucose–cyanohydrin required only 20 minutes at 40 °C, whereas the modified Strecker reactionusing glucose–dimethyl phosphite needed 30 minutes at 80 °C. All treatments gaveimproved dyeability, the order of efficacy being:

Modified Strecker > Strecker > Maillardalthough differences were relatively slight. Dichlorotriazine dyes showed increased yieldafter the modified Strecker reaction. There was no decrease in light fastness. Treatment withdimethyl phosphite alone also improved dyeability but not to the same extent as glucose–dimethyl phosphite. Penetration of the peptide chains by glucose–dimethyl phosphite isshown in Scheme 10.74. The process is thought to occur in the following stages [404]:(1) Penetration of dimethyl phosphite into the fibre, accompanied by decomposition of salt

linkages and elimination of structural water in a multi-step hydration process. New saltlinkages are formed between cationic groups in wool keratin and anionic dimethylphosphite.

(2) Penetration of glucose molecules into the loosened structure, causing further increasein accessibility of the reactive sites.

(3) Formation of covalent bonds between dimethyl phosphite, glucose and amino groups inwool keratin, stabilising the loosened fibre structure.

Imaginative use has been made of triazine and sulphatoethylsulphone reactive dye chemistryin the application of pretreatments to nylon [405]. The concept resembles that used tomake cotton cellulose reactive before dyeing with aminoalkylated dyes, as discussed earlier(Schemes 10.67 and 10.68). In this case, nylon becomes the reactive partner bypretreatment with a reactive multifunctional crosslinking agent:(a) 1,3,5-tris(acryloyl)hexahydro-s-triazine (10.146) or(b) the trifunctional reactive compound XLC (10.95), already discussed as a shrink-resist

reactant for wool (section 10.5.5).

The pretreated nylon then undergoes covalent fixation of dyes containing aminoalkylgroups. Interestingly, nylon treated with XLC showed markedly lower substantivity andreactivity with conventional dyes. If the pretreated nylon was reacted with ammonia,however, creating amino functionality at the reactive sites, normal reaction with aconventional reactive dye was restored [405].

Many of the syntan aftertreatments described in section 10.9.4 can be applied aspretreatments instead, mainly to confer dye-resist effects. This aspect is dealt with in section10.9.4 rather than here.

10.9.2 Ultraviolet absorbers

The use of polyester and nylon upholstery fabrics in automobiles results in their prolongedexposure to sunlight at high temperatures and humidities. This has created a demand fordyeings of very high light fastness and for these dyed fibres to have exceptional resistance tophotodegradation. Although the selection of dyes of suitably high fastness under these

chpt10(2).pmd 15/11/02, 15:44680

681

CH

C

NH

C

HN

CH

C

NH

C

HN

CH

C

NH

CH

2O

H

O

O

(CH

2)4

NH

3

O

O

(CH

2)4

NH

3

O

CH C

HN C

NH

CH C

HN C

NH

CH C

HN

CH

2O

HO

O

(CH

2)4

H3N

O

O

CH

2C

O OO

CH

C

NH

C

HN

CH

C

NH

C

HN

CH

C

NH

CH

2O

H

O

O

(CH

2)4

O

O

(CH

2)4

NH

3

O

NH

3

CH

2OH

CH

OH

CH

OH

CH

OH

CH

OH

CH

OH

P

CH

3O

OC

H3O

CH

2OH

CH

OH

CH

OH

CH

OH

CH

OH

CH

OH

P

CH

3O

OC

H3O

CH C

HN C

NH

CH C

HN C

NH

CH C

HN

CH

2O

HO

O

(CH

2)4

H3N

O

O

CH

2C

O OO

+

+

_

+++

+ _

Ser

ine

Lysi

ne

Asp

artic

aci

d

Ser

ine

Lysi

ne

Lysi

ne

Sch

eme

10.7

4


chpt10(2).pmd 15/11/02, 15:44681


extreme conditions is of primary importance, additional protection can be achieved usingso-called ultraviolet absorbers, these being colourless aromatic compounds with a highpropensity to absorb the troublesome UV radiation. Such products are usually applied duringdyeing and confer protection to both dyes and fibres. Although the main interest in UVabsorbers centres on polyester and nylon, they are also recommended for use on other fibres,e.g. polypropylene, silk, wool (particularly bleached wools that have a tendency tophotoyellowing) and cotton. More recently, due to the increased incidence of skincarcinomas induced by excessive exposure to sunlight, UV absorbers are being used togetherwith modifications of fabric construction to give increased resistance to the passage ofharmful rays through the fabric to the skin [406–408].

The principle involved is the same as that used in skin protection creams (that go underthe commonly used misnomer of sun protection screens), as well as finding extensive use inplastics and surface coatings to enhance their durability to sunlight exposure. Two usefulreviews are available [409,410]. Table 10.43 gives a comparison of the components of solarradiation. This illustrates very clearly the apparent paradox that those components presentin least quantity nevertheless pack the greatest punch in terms of photon energy. Thus outof a total photon energy value of 1328 kJ/mol UV radiation of wavelengths 280–400 nmaccounts for 1065 kJ/mol (80%), despite the fact that it represents only 6% of the totalradiation intensity. This is the radiation that is primarily responsible for the degradation offibres and colorants. This is also the region, or at least part of it, in which UV absorbers haveto function. A useful definition [410] states that a UV absorber is a molecule that may beincorporated within a host polymer in order to absorb ultraviolet radiation efficiently andconvert the energy into relatively harmless thermal energy, without itself undergoing any

CN

CH2N

H2C

N CH2

C

C

O

CH

O

CHCH2

O

CH CH2

10.146

CH2

Table 10.43 Intensity of global radiation (sum of direct and scattered radiation)at the earth’s surface and its classification (summer, vertical incidence) [409]

Radiation intensity

Regions of Mean photonsolar radiation Wavelength (nm) (W/m2) (%) energy (kJ/mol)

UV-B 280–320 5 0.5 400UV-A 320–360 27 2.4 350

360–400 36 3.2 315Visible 400–800 580 51.8 200Infrared 800–3000 472 42.1 63

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irreversible chemical change or inducing any chemical change in the host macromolecules.Thus UV absorbers do not simply scavenge UV radiation preferentially and becomethemselves expended in the process, in the way that gas-fume fading inhibitors operate(section 10.9.3). UV absorbers convert electronic excitation energy into thermal energy by arapid but reversible intramolecular proton transfer mechanism [410].

The fundamental nuclei of most of these compounds are represented by o-hydroxy-benzophenone (10.147), o-hydroxybenzotriazole (10.148) or o-hydroxybenzotriazine(10.149). All of these structures exhibit keto–enol tautomerism, the keto form beingfavoured on irradiation. The characteristics of these skeletal structures can be modified byintroducing substituents to make them more suitable for specific purposes. For examplewater-insoluble UV absorbers are preferred for application with disperse dyes for polyester,whereas water-soluble sulphonated derivatives are more suitable for use with anionic dyeson nylon. Thermal stability will be required if such agents are to be incorporated into moltenpolymers or intended for pad–thermosol application to polyester. Typical variants, togetherwith other types of protective agents, will be mentioned later when dealing with specificfibres. Even certain dyes can act as UV protective agents, particularly in heavy depths, butother dyes may catalyse fading or fibre degradation [411]. Derivatives of certain dyes withbuilt-in UV absorber moieties have been produced specifically with the aim of obtaining dyesof exceptionally high light fastness. For example, CI Disperse Yellow 33 (10.150; R = H) canbe converted to a benzophenone derivative (10.151) and a benzotriazole analogue (10.152)of the important polyester dye CI Disperse Yellow 42 (10.150; R = Ph) is also readilyavailable [410]. In some instances there is also a need for an antioxidant; these are discussedbelow in connection with nylon additives.

OH C OH

O

OH

hν

10.147

Enol Keto

HO

C

O

N

N

N

HO

N

N

N

O

hν

10.148Enol Keto

H

N

N

N

R1

R2

HO

R3

N

N

N

H O

R3

R1

R2

hν

10.149Enol Keto


chpt10(2).pmd 15/11/02, 15:44683


Polyamides are characteristically prone to photochemical and thermal degradation,although much depends on the structure and purity of the polymer and the types andconcentrations of additives present. Although the titanium dioxide delustrant is an effectiveUV absorber, it is also a sensitiser of the photodegradation of nylon. The decomposition isinitiated mainly by free radicals from sensitising impurities. Although most polyamides areresistant to photochemical attack below 40–50 °C, they can degrade fairly rapidly at highertemperatures, particularly with higher concentrations of titanium dioxide present and whendyed in pale or medium depths. Certain metal-complex dyes, however, can act asphotostabilisers, especially in full depths. UV absorbers afford protection to nylon only atblack-panel temperatures below 40–50 °C. Since degradation is largely the result ofoxidative free-radical attack, protective effects are shown by antioxidant addition, eitheralone or in conjunction with a UV absorber. Both agents may be incorporated during fibremanufacture or can be applied during dyeing or finishing. Some commercial formulationscontain both types of agent.

Water-soluble UV absorbers are preferred for nylon, particularly sulphonated benzotriazolesof the general type represented by 10.148, with halo, sulpho or sulphonated arylalkylsubstituents in the benzotriazole nucleus and alkyl, alkoxy or sulpho groups in the phenolicnucleus. A specific example is provided by structure 10.153. Sulphonated derivatives of thedihydroxybenzophenone structure 10.147 are also suitable. One study of the influence of UVabsorbers on light fastness involved a selection of nine acid dyes typical of those used in thedyeing of nylon carpets. The efficacy of four water-soluble agents applied by exhaust dyeing(10.154–10.156 and an undisclosed structure of the anionic o-hydroxyphenylbenzotriazoletype) and three water-insoluble types applied from solution in tetrachloroethylene(10.157–10.159) was evaluated [412]. Some of the UV absorbers significantly improved lightfastness, but others gave significantly lower ratings. Overall, the behaviour of the UV absorberswas dye- and hue-specific [412]. Conversely, in another investigation it was claimed thatstabilisers applied to nylon during dyeing can markedly improve fibre stability and lightfastness, independently of hue and depth of shade [413].

HN SO2

O2N

HN R

10.150

HN SO2

O2N

HN

C

O

HO

OCH3

10.151

N

N

N

Cl

NH

O2N

CH3

HO

O2S NH

10.152

chpt10(2).pmd 15/11/02, 15:44684

685

The additional protection given to nylon by antioxidants has already been mentioned.Since the need is to protect against oxidation by free radicals, antioxidants are essentially oftwo types: peroxide decomposers and radical scavengers. Reviews of these products areavailable [409,410,413]; these should be consulted for details of the mechanisms involved.Peroxide decomposer types include compounds of manganese(II) or copper(I) andcopper(II) complexes, such as azomethine bridge derivatives of the type represented by10.160, of which numerous water-soluble or water-insoluble variants are possible [409].These products have a catalytic action and are therefore used in very small amounts.

N

N

N

HO CH

CH3

SO3Na

CH2CH3

10.153

C

O

HO

OCH3

SO3Na

HO

CH3O SO3Na 10.154

C

O

HO

OCH3

SO3Na

10.155

N

N

N

HO

CH310.156

C

O

HO

O(CH2)7CH3

10.157

C

CH3CH2O

O

N CH N

CH3

10.158

C

O

HO

OCH3

10.159

YN N

HC CH

O O

Cu

X

R

X

R

R = H, halo, hydroxy, alkyl, alkoxy X = H, sulpho, carboxyl, sulphonamide Y = alkylene, cycloalkylene linkages10.160


chpt10(2).pmd 15/11/02, 15:44685


Conversely, radical scavengers have to be used in larger amounts because they lack theregeneration capability of catalytic types. The principal product types [409] are stericallyhindered phenols (10.161) and sterically hindered amines (10.162). The latter are importantbecause they act not only as scavengers but also as peroxide decomposers [410]. Furthercompounds include the semicarbazides 10.163 and 10.164, applicable by continuous andexhaust methods respectively. The product 10.165 is suitable only for continuousapplication. Most radical scavengers are suitable only for thermal stabilisation [410],photostabilising scavengers being restricted to transition-metal chelates, especially nickel(II)dithiocarbamates (10.166), and hindered amines such as 2,2,6,6-tetramethylpiperidine(10.167). Water-soluble triazine derivatives of hindered amines (10.168) have also beenused [410].

HO A

C

H3C

H3C

CH3

C

H3C

H3C

CH3

R

X

10.161

R = H, halo, alkyl, alkoxyX = H, sulpho, carboxyl, sulphonamideA = triazine or (CH2)nCONH linkages

AN

N

N

N

R

HN

HN

Y

Y

X

X

10.162

R = H, oxy, hydroxy, acyl, alkyl, alkoxyA = O or NHX = H, sulpho, carboxylY = alkylene, arylene, substituted arylene (identical or different)

CH3

CH3

H3C

H3C

NHN

H3C

H3C

C

HN

O

(CH2)6 NH

C

HN

O

N

CH3

CH3

10.163

NH HNC C

O

NH

O

HNN N

H3C

H3C

CH3

CH3

SO3Na

10.164

CH2 H2C

N

OH

10.165

SC

S

Ni

S

CS

N N

CH3CH2CH2CH2

CH3CH2CH2CH2

CH2CH2CH2CH3

CH2CH2CH2CH3

10.166

N

H

10.167

H3C CH3

H3C CH3

N

N

N

Cl

NH HN SO3NaNH

10.168

CH3H3C

CH3H3C

chpt10(2).pmd 15/11/02, 15:44686

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Water-soluble UV absorbers are preferred on bleached wool, mainly to retard thephotoyellowing process. The sulphonated o-hydroxybenzotriazole type (10.169) isparticularly effective [414]. When applying such products by the exhaust process it is best toavoid the use of sulphates as these lower the exhaustion, as does a pH higher than 5.5[415,416]. These products are claimed to be fast to hand washing, shampooing and drycleaning [416]. Nevertheless, polymeric UV absorbers have been developed with theintention of improving on the limited fastness of conventional types [417]. These havetypical UV absorber moieties attached to an acrylate, methacrylate or methacrylamidepolymer, examples of relevant monomers being 10.170–10.172.

N

N

NX

HO

X

R

10.169

R = alkyl, alkoxy, sulphoX = H, halo, sulpho, sulphonated arylalkyl

C

O

HO

OCH2OC

C

O

CH2

H3C

10.170

N

N

N

HO

C

(CH2)4CH3H3C

CH3

H2C NC

C

O

H3C

CH2

CH3

10.171

N

N

N

HO C

CH3H3C

CH3

CH2CH2CH2

CO

O

CH

CH2CH2

CH2CH2

CH OC

CH

O

CH2

10.172

Polyester fibres generally show high resistance to photodegradation providing exposure inthe 280–320 nm range is avoided, so the main reason for using UV absorbers on this fibre isto enhance light fastness, particularly to the standards required for automobile furnishings.Indeed, marked improvements generally only become evident with prolonged exposure athigh ambient temperatures [409]. Water-insoluble benzophenones and benzotriazoles, suchas 10.173 and 10.174 respectively, are widely used. These products, however, have onlymoderate fastness to sublimation. Where a high setting temperature or pad–thermosolapplication is involved, o-hydroxyphenylbenzotriazoles of greater molecular mass offerhigher fastness to sublimation [409]. Water-insoluble mono- (10.149) or bis(o-hydroxy-


chpt10(2).pmd 15/11/02, 15:44687


phenyl)triazines are also of interest. Derivatives of o-hydroxyphenylpyrimidine andbenzoxazin-4-one have also been patented [409]. In a recent series of papers [418,419], theeffectiveness of the following UV absorbers on polyester was demonstrated: 10.147, 10.156,10.175 and a benzotriazole of undisclosed structure.

C

OHO

CH3O

OH

OCH3

10.173

N

N

N

HO C

CH3H3C

CH3

CH3

Cl

10.174

N

N

N

HO C

CH3H3C

CH3

CCl

CH3

CH3

H3C10.175

Benzyl salicylate (10.176) at an applied concentration of 2% o.w.f. is reported [420] tohave given good protection from fading to two anthraquinone acid dyes on silk.

Water-soluble UV absorbers can be used on cotton to improve the photochemicalstability of the fibre or to protect the skin from UV radiation but this does not generallyimprove the light fastness of dyeings. From a chemical viewpoint, bifunctional reactive UVabsorbers (10.177) for cotton are of particular interest [421]. The vat dye 10.178 containingtwo imidazole rings is patented [422] for use by dyeing or printing on cellulosic fibres toimprove solar protection both to the wearer and to the fibre. The use of 10.177 incombination with 10.178 is also patented [421]. Normal untreated cotton with a sunprotection factor of 5 can be improved to factor 28 after applying 10.178 and to factor 187when this dye is used in combination with the reactive UV absorber 10.177.

C

O

O

CH2

HO

10.176

NHC

C

O

HN

O

NH N

NN

NaO3S

OCH2CH3

Cl

HN SO2CH2CH2OSO3Na

10.177

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10.9.3 Gas-fume fading inhibitors

Not all disperse dyes on cellulose acetate fibres are resistant to oxides of nitrogen that maybe present in the atmosphere. Susceptible dyes, usually those containing primary amino orsecondary amino groups, can undergo quite profound changes in hue depending on thereactivity or basicity of the susceptible group(s). The primary general mechanism of fading isbelieved to be the formation of N-nitrosamines [423]. The problem is best avoided by usingdyes that do not fade, but this may not always be possible as these tend to be more costlyand less easy to apply. Some protection can be obtained by treatment of susceptible dyeings,either during or after dyeing, with colourless agents that react preferentially with oxides ofnitrogen. Such agents are known as gas-fume inhibitors. Since they act as scavengers of theacidic oxides of nitrogen, they need to be more basic in character than the dyes they protect.Many such basic compounds, generally applied from aqueous solution or dispersion, havebeen proposed [424].

The most popular and efficient are substantive to the fibre; typical examples are N,N′-diphenylacetamidine (10.179), which tends to yellow on exposure to oxides of nitrogen, andparticularly the diphenylated diamines such as N,N′-diphenylethylenediamine (10.180),which does not yellow. Non-substantive inhibitors applied by padding and drying, such astriethanolamine (10.126) and melamine (10.181), have also been used despite the fact thatthey are removed on washing. The demand for and commercial availability of gas-fumeinhibitors have declined.

NH

N N

HN

Cl

Cl

O

O O

O

10.178

NH

N

C CH3

10.179

NH

CH2CH2

HN

10.180

H2N

N

N

N

NH2

NH210.181

Atmospheric ozone has also been reported as causing fading of certain dyes in somecountries [425,426]; diallyl phthalate (10.182) used as a carrier in the dyeing of cellulosetriacetate fibres, is said to be an effective ozone inhibitor [427]. Nylon, especially when dyedwith certain amino-substituted anthraquinone blue acid dyes, can also be susceptible toozone fading [428,429]. Selection of ozone-resistant dyes is obviously the best counteractivemeasure, although hindered phenols (10.161) and hindered amines (10.162) are said toprovide some protection.


chpt10(2).pmd 15/11/02, 15:44689


10.9.4 Aftertreatments and resist treatments for acid dyes

By far the most important aftertreatment for acid dyeings is the so-called syntan processwidely used on nylon, which has superseded the classic full back-tan process. This involvedtreatment of the dyed nylon first with ‘tannic acid’ (a highly complex gallotannin orpolygalloylated glucose, such as structure 10.183), which hydrolyses to give digallic acid(10.184) or gallic acid (10.185) [391,430], followed by further treatment with potassiumantimonyl tartrate (tartar emetic; 10.186). It has been replaced on grounds of its high cost,instability to hot alkali, undesirable effects on fabric handle and light fastness, changes incolour (usually dulling) during treatment, and diffusion and degradation of the antimonyltannate complex during subsequent steaming or dry heat setting, as well as for health andenvironmental reasons. Its presumed mechanism of action is complex [391,430].

C

C OCH2CH

O

OCH2CH

O

10.182

CH2

CH2

C

O

O CH

CH

CH

O

CH

CH O

OC

O

OH

OH

OH

OC

O

HO

HO

HO

HO

HO

O

C O

HO

OH

OH

CH2

OC O

HO

OH

OC

O

OH

OH

OH

C

O

OH

OH

O

CO

OH

OH

HO

10.183

O

C

OHOHO

C

O

HO

OH

OH

OH

10.184

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691

The synthetic tanning agents (syntans) that have superseded the back-tan have theadvantage of being applied in a single process but they tend to be polyphenolic compoundslike the naturally derived gallotannins. In fact, compounds of this class have been describedin section 10.6.1 as condensation products of formaldehyde with sulphonated phenols,naphthols or naphthylamines. Structures 10.100–10.103 are typical of these compoundsalthough, as mentioned earlier, they form a large and varied group. The precise structuraldetails of typical commercial syntans for nylon are hidden within a plethora of patentliterature, much of which pertains to their use as tanning agents for leather [391]. Inaddition to the products mentioned in section 10.6.1 analogous sulphur-containingproducts, such as those derived from thiophenols, and heavy-metal phenolates were formerlyused.

A brief simplified mechanism whereby syntans are thought to operate can be described, atthe risk of incompleteness. Their rapid sorption by the fibre under the (usually acidic)conditions of application is largely the result of electrostatic attraction between negativelycharged sulphonate groups and protonated amino groups in the fibre. Hydrogen bondingbetween uncharged polar groups, and hydrophobic bonding between the nonpolar moietiesin the syntan and the fibre, create conditions for the formation of complexes. Maximumimprovement in wet fastness results when the complexes are formed at the fibre surface,since any treatment leading to diffusion of the syntan complex into the fibre tends to yieldlower fastness. Hence a ‘barrier effect’ seems to be involved. Possibly multilayers can beformed via a Brunauer, Emmett and Teller (BET) mechanism. Some syntans will give amodest improvement in the wet fastness of disperse dyes on nylon but even with acid dyesthe response to the syntan treatment in terms of improved wet fastness varies markedly fromdye to dye.

Empiricism has guided the screening of syntans for nylon. Both the affinity of the syntanfor the fibre and its diffusion rate are important, and the required values are obtained bybalancing the degree of sulphonation and range of molecular sizes present. Moreover, manyof these products are used not only as aftertreating agents for the improvement of wetfastness, but also as blocking agents to inhibit absorption of dye either partially or completely– for example, in the dyeing of nylon/wool blends to restrain the usually preferential uptakeof dye by the nylon, or to produce resist effects, particularly in printing. The balance ofproperties required in the syntan may therefore vary somewhat and is influenced by thecharacteristics of the dye–fibre system for which it is intended.

Studies on nylon, using commercial syntans of undisclosed structure [431–433] showedthat the absorption of syntan increased with decreasing pH, providing further evidence forelectrostatic interaction. Evidence was also found for the BET mechanism, indicating thatother forces are also operating, such as hydrophobic interaction. Uptake of syntan increasedwith applied concentration and increasing temperature, this being attributable to the higher

C OH

OH

OH

HO

O

10.185

C

CH

O O

OH

CH OH

CO O

[SbO]

K

10.186

_+

_+


chpt10(2).pmd 15/11/02, 15:44691


kinetic energy of the syntan molecules and the greater extent of fibre swelling. Syntanuptake was greater on nylon microfibres (greater surface area) than on conventional nylonstaple fibres, but the fastness to washing was inferior on microfibres. Application of acationic agent subsequent to the syntan treatment gave an additional significantimprovement in fastness, particularly where multiple washes were involved [433].Unsulphonated 1:2 metal-complex dyes showed less desorption of dye than disulphonatedtypes after 1–3 washes but greater loss after 5–10 washes. The cost and inconvenience ofthese two aftertreatments, however, would not be justifiable under commercial conditions.

Similar commercial syntans have been evaluated in dye–resist pretreatments on wool[434]. Evidence for both kinds of agent–fibre interaction was again found. The resist effectimparted to the wool was specific to the type of anionic dye applied, giving high resistance tofour hydrophilic dyes and low resistance towards two hydrophobic dyes. Syntan desorptionoccurred during dyeing, indicating that these agents are relatively weakly attached to thefibre. The desorbed syntan had a restraining effect on the uptake of all six dyes examined.

Section 10.9.1 includes an account of nylon pretreatments based on chemistry normallyassociated with reactive dyes for cellulosic fibres. Similar chemistry has been used to developalternatives to syntans, both as aftertreatments to improve fastness and as pretreatments togive dye-resist effects. The trifunctional reactive compound XLC (10.95), already discussedas a shrink-resist reactant for wool (section 10.5.5), is hydrolysed under alkaline conditionsto the active vinylsulphone form (10.187). This trifunctional crosslinking agent has beenevaluated to improve the fastness of nylon dyed under mildly acidic conditions with speciallyprepared anionic aminoalkylated triazine dyes [435]. Such dyes do not bond covalently withnylon, but aftertreatment with this trifunctional reactive agent results in a significant degreeof covalent bonding with a consequent improvement in wet fastness. A cationicaminoethyisulphonyl dye (10.188) was prepared and applied to nylon at the optimum pH 10,followed by treatment with the parent sulphatoethylsulphone crosslinker (10.95). In thisway, the dye became fixed covalently via the agent to terminal amino groups in nylon,resulting in improved fastness to washing [435].

Traditional syntan treatments are rather ineffective for improving the wet fastness ofanionic dyes on wool, mainly because of weakness of the interaction between syntan andfibre. It is therefore not surprising that covalent reactive systems have been explored to findmore effective aftertreatments and dye-resist treatments for this fibre. In an initial study

N

N

N

NH HN

Cl

CHSO2 SO2CH CH2H2C

10.187

NNH2NCH2CH2SO2

N

CH2CH2

CH2CH3

N

X

10.188

_

+

chpt10(2).pmd 15/11/02, 15:44692

693

aimed at producing a system that would overcome the level dyeing problems associated withreactive dyes on wool [436], a specially prepared aminoalkylated dye (not fibre-reactive) wasapplied together with hexamethylenetetramine, from which formaldehyde was released(Scheme 10.75) to give covalent bonding between the amino groupings in the dye and inthe fibre (Scheme 10.76). This process was successful up to a point but gave lower fixationwhen dyeing in full depths. Certain chromogens also showed marked changes of hue.

+ 6 H2O 6 HCHO + 4 NH3H2C N

H2C

CH2H2C

N

CH2

N

CH2

N

Scheme 10.75

[dye] CH2CH2NH2

[dye] CH2CH2NHCH2NH(CH2)4[wool]

+ HCHO + H2N(CH2)4[wool]

Scheme 10.76

This approach was subsequently improved [437] by dispensing with the formaldehydeprecursor and adding a trifunctional crosslinking agent to the dyebath after dye absorptionwas complete. The crosslinking agent was 1,3,5-tris(acryloyl)hexahydro-s-triazine (10.146).Scheme 10.77 represents an idealised reaction for the formation of dye–agent–wool linkages.Since reaction takes place in a random manner, however, the derivatives 10.189–10.192could equally well be produced. Products 10.191 and 10.192 need to be minimised, becauserespectively they are linked only to the dye or to the fibre [437].

CN

CH2N

H2C

N CH2

C

C

O

CH CH2

O

CH

O

CHCH2

CH2

CN

CH2N

H2C

N CH2

C

C

O

CH2CH2NH(CH2)4[wool]

O

[dye]CH2CH2NHCH2CH2

O

[dye]CH2CH2NHCH2CH2

2 [dye]CH2CH2NH2 + + H2N(CH2)4[wool]

Scheme 10.77


chpt10(2).pmd 15/11/02, 15:44693


Subsequently, two more cost-effective bifunctional crosslinking agents, both com-mercially available, were used to give the same quality of dyeings [438], these being N,N′-methylene-bis-acrylamide (10.193) and a quaternised precursor (10.194). Idealised reactionswith functional groups in the dye and in wool are given in Schemes 10.78 and 10.79.Although these bifunctional reagents are cheaper than the trifunctional product, dyeing hasto be carried out at pH 8 as their reactivities are lower than that of tris(acryloyl)hexahydro-s-triazine. However, this lower reactivity allows the bis-acrylamide to be added at thebeginning of the dyeing process. The quaternised derivative is a more effective crosslinkingagent, possibly because of its greater substantivity for wool under slightly alkaline dyeingconditions, but it cannot be added at the beginning of dyeing because of its cationiccharacter.

There has been active interest in exploring the potential of reactive triazines as dye-resistagents for wool. A model structure for reactive dye-resist agents (10.195) has been proposed[439], where Z is a suitable high-reactivity group, NH is a bridging group, Ar is an aryl groupand [SO3]– is a solubilising and anionic dye-repelling group. Four dichlorotriazinecompounds (10.196–10.199) based on this model structure were found to be effective as

CN

N

N

C

C

O

CH2CH2X[wool]

O

[dye]NHCH2CH2

O

[dye]NHCH2CH2

10.189

X = NH or S

CN

N

N

C

C

O

CH2CH2X[wool]

O

[wool]XCH2CH2

O

[dye]NHCH2CH2

10.190

X = NH or S

CN

N

N

C

C

O

CH2CH2NH[dye]

O

[dye]NHCH2CH2

O

[dye]NHCH2CH2

10.191

CN

N

N

C

C

O

CH2CH2X[wool]

O

[wool]XCH2CH2

O

[wool]XCH2CH2

10.192

X = NH or S

CH2 CHCONHCH2NHCO HX [wool]

[dye]CH2CH2NHCH2CH2CONHCH2NHCOCH2CH2X [wool]

[dye]CH2CH2NH2 + +

10.193

X = NH or S

CH CH2

Scheme 10.78

chpt10(2).pmd 15/11/02, 15:44694

695

dye-resist agents, although full white resist effects were difficult to achieve [439]. Pursuingthis line of research further, several more complex naphthylamine-based dichlorotriazinedye-resist agents (10.200–10.206) were evaluated [440]. These agents each contained atleast one dichlorotriazine reactive moiety as well as a sulphonated naphthylamine systemthat enabled the anionicity of the product to be varied. Using the trisulphonated dye CIAcid Red 27 (10.207), the resist efficiency improved significantly with increasing degree ofsulphonation of the agent and with increasing relative molecular mass, enabling lowerconcentrations of the more effective agents to be used. The upper limit of resist achievedwas 98.6%.

R3N CH2CH2CONHCH2NHCOCH2CH2 NR3

CH2 CHCONHCH2NHCOCH

HX [wool]

[dye]CH2CH2NHCH2CH2CONHCH2NHCOCH2CH2X [wool]

A

10.194

10.193

+[dye]CH2CH2NH2

X = NH or S A = anion

+ 2 NHR3 A

A_ + + _

+ _

_

CH2

Scheme 10.79

Z NH Ar [SO3 ]n

10.195

_

NH

N

N

N

Cl

Cl

SO3Na

10.196

NH

N

N

N

Cl

Cl SO3Na

NH

N

N

N

Cl

Cl

10.197

NH

N

N

N

Cl

Cl

OH

COONa

10.198

NH

N

N

N

Cl

Cl

SO3Na

SO3Na

HO

10.199

NH

N

N

N

Cl

Cl

C

HN

O

SO3Na

10.200


chpt10(2).pmd 15/11/02, 15:44695


NH

N

N

N

Cl

Cl

C

HN

O

NaO3S

SO3Na

10.201

NH

N

N

N

Cl

Cl

C

HN

O

SO3Na

SO3Na

NaO3S10.202

NH

N

N

N

Cl

Cl

C

HN

O

C

HN

O

SO3Na

10.203

NH

N

N

N

Cl

Cl

C

HN

O

C

HN

O

SO3Na

NaO3S

10.204

NH

N

N

N

Cl

Cl

SO3Na

NaO3S

NH

N

N

N

HN

Cl

SO3Na

10.205

chpt10(2).pmd 15/11/02, 15:44696

697

A rather different approach [441] to obtaining resist effects on wool involved evaluationof the relatively simple compound sulphamic acid (10.208). From a comparison of curingtemperatures over the range 100–150 °C, it was found that only by curing at 140 °C orhigher was the sulphamic acid bound sufficiently to resist desorption during dyeing. Thereare several possible routes of decomposition of sulphamic acid [441], leading to theformation of various sulphonic acids and sulphonamides, as well as the possibility of scissionto form sulphur trioxide and ammonia (Scheme 10.80). The reactions of sulphamic acidwith wool are mainly through hydroxy groups and to a much lesser extent amino groups inthe fibre (Scheme 10.81). Thus the dye-resist effect is provided by the presence on themodified fibre of anionic sulphate and sulphamate groups [442]. This approach was alsoexamined on silk [443]. Compared with wool, silk has a much smaller proportion ofnucleophilic amino groups but a substantial content of hydroxy groups with whichsulphamic acid reacts preferentially. Excellent dye-resist effects were obtained with acid, 1:2metal-complex and reactive dyes, better than when a commercial dichlorotriazine reactant(similar to 10.196) was used.

10.9.5 Aftertreatments for direct and reactive dyes on cellulosic fibres

The fact that the aftertreatment of direct dyes has a long history is not surprising since wetfastness within this class is not particularly good. Their prime advantages are ease ofapplication and economy compared with dyes of higher fastness (reactive, sulphur or vat) –hence the continued search for highly effective aftertreatments that improve wet fastness

NH

N

N

N

Cl

Cl

SO3Na

NaO3S

NH

N

N

N

HN

Cl

NaO3S

SO3Na

10.206

HNN

NaO3S

O SO3Na

SO3Na10.207

CI Acid Red 27

H2N S OH

O

O

H3N S O

O

O10.208

+ _


chpt10(2).pmd 15/11/02, 15:44697


without excessive additional cost. Some of the aftertreatments used for this purpose havealso been applied to reactive dyes, as will be described later.

One of the earliest aftertreatment processes employed diazotisation of free amino groupsin the adsorbed dye, followed by coupling (development) with a suitable component such asa phenol, naphthol or amine; 2-naphthol, m-phenylenediamine, resorcinol and 1-phenyl-3-methylpyrazol-5-one (10.209) were particularly popular. Obviously, this was only possiblewith dyes containing a diazotisable amino group, an example being CI Direct Yellow 59, theclassic primuline (a mixture of structures 10.210a and b), which was converted on the fibrefrom a greenish yellow to a bluish red by diazotisation and development with 2-naphthol.The reverse approach, application of a dye containing a phenolic group and aftertreatmentwith a solution of a diazotised amine, such as p-nitrobenzenediazonium chloride, was alsoused.

HO S NH2

O

O

H2N S OH

O

O

HO S NH S OH

O

O

O

O

H2N S OH

O

O

H2N S OH

O

O

H2N S NH S OH

O

O

O

O

H2N S OH

O

O

NH

SO2

SO2

HN

O2SNH

H3N S O

O

O

+ + NH3

+ + H2O

+ 3H2O3

NH3 + SO3

+ _

Scheme 10.80

[wool] OH [wool] O

[wool] NH2

HO3S NH2 SO3

[wool] NHHO3S NH2 SO3+

+

NH4

NH4

_

_ +

+

Scheme 10.81

NNO

CH3

10.209

chpt10(2).pmd 15/11/02, 15:44698

699

Formaldehyde aftertreatment was employed to link together pairs of amino-substituteddye molecules by a methylene bridge (10.211). The required reactivity of the sites in the dyetowards formaldehyde was ensured by pairs of o- and p-directing electron-donating groupsprovided by resorcinol, m-phenylenediamine or 3-aminophenol.

N

C

S

NH2

C

S

N

H3C

SO3Na

N

C

SC

S

N

H3C

SO3Na

N

C

S

NH2

a

b

10.210

CH2

NH2 H2N

N

H2N

N

Ar

N N

Ar

NH2

10.211

A third approach utilised copper salts, especially copper(II) sulphate, in conjunction withdyes containing chelatable groupings such as salicylic acid or o,o′-dihydroxyazo moieties.Indeed, special ranges of ‘copperable’ direct dyes, for which the treatment with copper(II)sulphate was really part of the dyeing process rather than an optional aftertreatment, wereintroduced. In the past the main use of this chelation treatment was to enhance lightfastness, but it is little used for this purpose nowadays.

Since direct dyes have anionic structures, many cationic surfactants such as quaternaryammonium compounds were used as aftertreatments to form surfactant–dye complexes ofreduced aqueous solubility and therefore higher wet fastness. The improved fastness relatedonly to non-detergent agencies such as perspiration and water, however. In soap-basedwashing processes the stronger interaction between the anionic soap and the cationic agenttended to cleave the dye–cation complex, thus effectively negating the aftertreatment evenafter a single mild wash. The aftertreatment often brought about changes in hue andreduced light fastness, although the latter could sometimes be countered by a combined orsubsequent treatment with a metal salt such as copper(II) sulphate, as described in thepreceding paragraph.

All the aftertreatment processes so far described have declined considerably incommercial significance and are now rarely carried out. Nevertheless, their commonprinciple of creating on the fibre a dye–agent complex of larger size, reduced solubility and a


chpt10(2).pmd 15/11/02, 15:44699


correspondingly lower rate of desorption has survived in today’s use of resin-type fixingagents. The principle of cationic aftertreatment, in particular, has seen noteworthydevelopment.

The earliest polymeric cationic aftertreatments stemmed from the development of crease-resist finishes for cellulosic fibres. One such, promoted specifically for its colour fastnessimprovements when applied as an aftertreatment to direct dyeings, was a condensationproduct of formaldehyde with dicyandiamide (Scheme 10.82). Many similar compoundsfollowed, such as condensation products of formaldehyde with melamine (10.212),poly(ethylene imine) with cyanuric chloride (10.213) and alkyl chlorides with poly(ethyleneimine) (10.214; R = alkyl).

n H2N C

NH

NH

CN

NH2 C

N

NH

CN

CH2+ n HCHO + n HCl

n

+ n H2O

Cl

Scheme 10.82

N(CH2OCH3)2

N

N

N

(CH3OCH2)2N

(CH3OCH2)2N10.212

NH(CH2CH2NH)nH

N

N

N

H(NHCH2CH2)nNH

H(NHCH2CH2)nNH10.213

RNH(CH2CH2NH)nCH2CH2NH3 Cl

10.214

+ _

These condensates were an improvement on the simpler cationic surfactants mentionedearlier. Their interaction still relies on electrostatic bonding between agent cation and dyeanion; hence the major weakness remains fastness to washing with anionic detergents. Thislimitation of conventional cationic agents has been overcome by the development of bi-, tri-and tetra-functional agents which carry reactive groups capable of forming covalent bondsby reaction with other suitable groups in the dye and/or the hydroxy groups in the cellulosicfibre. Studies on the functionality and reactivity of various multifunctional crosslinkingagents [442] led to the selection of 1,3,5-tris(acyl)hexahydro-s-triazines (10.215) as beingparticularly effective, their efficiency arising from the lability of the chloro substituents andthe strongly polarising effect of the carbonyl groups.

The tris(acryloyl) derivative (R is CH=CH2) was subsequently developed as a fixingagent for use with the Basazol(BASF) dyes in the printing of cellulosic fibres, in conjunctionwith urea as hydrotrope [444]. It has also been shown [445] that aftertreatment with 1,3,5-tris(acryloyl)hexahydro-s-triazine or the tris (β-chloropropionyl) derivative improves the wet

chpt10(2).pmd 15/11/02, 15:44700

701

fastness of direct dyes, providing the dyes contain suitable nucleophilic groups. The samecompounds have useful properties with reactive dyes (other than the Basazols) in givingincreased colour yield through fixation of hydrolysed dye, an aspect discussed in more detailbelow.

The basic mechanism of action of multifunctional fixing agents has been well described[446] (Figures 10.54 and 10.55). However, the multifunctional products are no better thanmonofunctional types when used with conventional direct dyes that do not contain suitablenucleophilic groups through which to link the fixing agent. Direct dyes suitable for formingadditional bonds have been termed reactant-fixable dyes and are mostly of the copper-complex type. Thus brightness of shade is limited.

A range of bifunctional, trifunctional and tetrafunctional fixing agents was developed[446,447] for use with a selected range of copper-complex (Indosol) dyes. The bifunctionaltype, which reacts only with the dye, was applied in a fresh bath at about 60 °C and gavefastness to washing at 50 °C through the formation of an extensive dye–agent complexwithin the fibre. The trifunctional type additionally forms covalent bonds with cellulose andis applied at 40 °C for about 15 minutes, followed by addition of alkali to bring aboutreaction; this confers a higher degree of fastness to washing at 60 °C even with deep shades.

CN

N

N

C

C

O

R

O

R

O

R

CH C(Cl)

CH(Cl) CH2Cl or CH2CH2Cl

10.215

R = CH2 CH2

+

+

+

+

Tetrafunctional type

Trifunctional type

Bifunctional type

Monofunctional type

Figure 10.54 Fixing agents showing various degress of functionality [446]


chpt10(2).pmd 15/11/02, 15:44701


Tetrafunctional reactant resins confer the highest fastness, even to washing at the boil.Typical polymers are made by the reaction of an amine such as diethylenetriamine withcyanamide, dicyandiamide (especially), guanidine or biguanide. A cationic polymer of thistype is applied with an N-methylol reactant such as dimethyloldihydroxyethyleneurea andan acid-liberating catalyst such as magnesium chloride to give a commercial product sold asa cationic reactant resin. This formulation is applied to the dyeing by padding, at whichstage a dye–agent complex is believed to be formed. The fabric is then treated at 175–180°C, resulting in covalent reaction between the cationic agent and the N-methylol groups aswell as crosslinking of cellulose chains by the N-methylol reactant, conferring not onlyexcellent wet fastness but also improved crease resistance and good dimensional stability.

The multifunctional agents offer an alternative aftertreatment for reactive dyeings [446],if problems arise from the presence on the fibre of unfixed hydrolysed dye, normally removedby ‘soaping’. This process is invariably lengthy, expensive and not always fully effective.Multifunctional cationic reactants will react with unfixed (still reactive) dye and withhydrolysed (hydroxy-containing) dye. Tri- and tetra-functional reactants will further fixthem by covalent bonding to the fibre. Improved wet fastness can also ensue whentetrafunctional products are used, including considerably improved resistance to hydrolysisof the dye–fibre bond and better fastness to treatments involving chlorine or perborates.Deleterious effects on hue and light fastness still have to be carefully considered in theselection of dyes for aftertreatment, however.

Most of the polymeric cationic products available [448] are based on the types describedin Table 10.44. The ideal aftertreating agent must fulfil many requirements [448]. There is ahigh demand for:– improved fastness to washing and other wet treatments– low price

+

+

+

+

Tetrafunctional type

Trifunctional type

Bifunctional type

Monofunctional type–O3S–D

–O3S–D

–O3S–D

–O3S–D

O

O

HO

O

HO

HO

cellulose

Figure 10.55 Modes of reaction of the various fixing agents [446]

chpt10(2).pmd 15/11/02, 15:44702

703

– applicability by exhaustion– applicability with various dye classes and on various fibres and blends– stability to electrolytes– capability to minimise migration of hydrolysed reactive dyes– stripping and overdyeing performance– ease of cleaning of stained machinery– biological elimination.

In addition, the ideal agent must not:– cause significant changes of shade of dyeings– increase chlorine retention or free formaldehyde content– impair the handle, hydrophilicity, light fastness, gas-fume fastness or sewability of the

fabric– be toxic to humans or irritate the skin.

Table 10.44 Classes of polymeric cationic agents for aftertreatment of dyed cellulosic fibres [448]

Amide–formaldehyde condensates No defined structure

Polybiguanides CH2 NH C NH

NH

C

NH

NH2 CH2

X n

Poly(ethylene imine)NH CH2 CH2 NH2 CH2 CH2

Xn

Quaternised poly(ethylene imine) NH CH2 CH2 N CH2 CH2

Xn

CH3

CH3

Quaternised polyheterocycles

HC

H2CN

CH2

CH

H2C CH2

H3C CH3

Xn

Quaternised polyacrylamides

CH2 CH

CO NH CH2 CH2 N CH3

CH3

CH3 Xn


chpt10(2).pmd 15/11/02, 15:44703


Amines can give rise to chloramines during hypochlorite bleaching (Scheme 10.83). Inaddition to increasing AOX values, this can result in cellulose oxidative degradation byhypochlorite on subsequent hydrolysis of the chloramines (Scheme 10.84) [448].

Cationic fixing agents remain associated with the dyes on the fibre and are only partiallyremoved by subsequent washing. Skin irritation is therefore an important consideration andin this respect they appear to have negligible influence. In common with cationicsurfactants, however, they are readily absorbed by protein material and show moderatelyhigh toxicity to aquatic organisms, although interaction with excess anionic surfactant canconsiderably reduce this problem. In vitro they can show poor biodegradability, BOD valuesbeing typically not more than 10–20% of COD values, which are usually in the region of 40–400 mg/g oxygen. However, they are highly absorbed by activated sludge, resulting in about95–98% bioelimination accompanied by about 80% actual biodegradation.

Studies of the effectiveness of cationic polymers for aftertreatment of dyeings continue toappear, although not all researchers disclose the detailed structures of the polymersevaluated. One comparison involved three types of commercial fixing agents for improvingthe fastness of six direct dyes on cotton, the agents being described as a dicyandiamide–formaldehyde condensate, a cationic nitrogenous polymer and a long-chain polyamine-typeproduct. The long-chain polyamine imparted higher wet fastness than the dicyandiamidetype, but fastness characteristics were found to be influenced by the number and position ofsolubilising and hydrogen bonding groups in the dye structures [449].

A polyacrylamide with a molecular mass of 1.87 × 105 was prepared by polymerising a 5%w/v aqueous solution of acrylamide monomer in the presence of 0.15% w/w benzyl alcoholand 0.025% w/w potassium persulphate for 45 minutes at 80 °C. This product was effectivein preventing the bleeding of direct dyes and hydrolysed reactive dyes from dyed cotton,which was simply dipped in a 1% solution of the polyacrylamide and dried in air [450].

Ecological factors, as well as the improvement of wet fastness, were taken intoconsideration in a study of viscose dyed with direct dyes (CI Direct Yellow 126, Red 83:1and Blue 90) and aftertreated with three cationic agents described as monofunctional,bifunctional and trifunctional [451]. For a system to be biodegradable, the BOD5/COD ratioshould be at least 0.5. For the systems examined, this ratio ranged from 0.00 to 0.38. Thus,all of them proved difficult to treat, biodegradation taking at least 20 days. It was difficult to

R NH2

NH

R

R

R NHCl

NCl

R

R

+ HClO + H2O

+ HClO + H2O

T < 40°C

T < 40°C

Scheme 10.83

R NH2

NH

R

R

R NHCl

NCl

R

R

+ HClO+ H2O

+ HClO+ H2Oheat

heat

Scheme 10.84

chpt10(2).pmd 15/11/02, 15:44704

705

decide which of the three agents was least hazardous in environmental terms. Thebifunctional agent gave the best overall performance, since it was not more environmentallyhazardous than the others and gave the smallest colour change, together with good fastnessto washing at 60 °C.

There has been interest recently in comparing formaldehyde-free, low-formaldehyde andconventional N-methylol fixing agents with direct [452] and reactive [453] dyes.Advantages are claimed for formaldehyde-free products in giving less dye bleeding, lesschange in shade and excellent fastness to light and wet treatments [453].

The effect of conventional cationic aftertreating agents in improving the fastness of directdyes can be further enhanced, albeit to only a small extent, by subsequent treatment with asyntan [454]. This may result from the formation of a larger electrostatically linked complexbetween the anionic syntan and the cationic fixing agent at the fibre surface, having lowaqueous solubility and slow diffusion behaviour. Traditionally, cationic fixing agents have beenapplied to direct dyeings at neutral or slightly acidic pH. There is now some evidence [455]that they can be somewhat more effective if applied under alkaline conditions. The reasons forthis remain speculative and more than one mechanism may be operating.

10.9.6 Aftertreatments for sulphur dyes on cellulosic fibres

The fastness of sulphur dyes to the increasingly severe conditions of washing currentlydemanded, especially in the presence of peroxide-containing detergents, can sometimes beimproved by an alkylation treatment. The best-known products for this purpose are adductsof epichlorohydrin with either ethylenediamine (especially) or ammonium salts. Typicallythe procedure involves the application of 2–3% o.w.f. of a proprietary alkylating agent with1–2% o.w.f. sodium carbonate (pH 10 and 30–40 °C), raising the temperature to 90–95 °Cand allowing the reaction to reach completion at this temperature, which it does after 10minutes. This treatment usually replaces the traditional oxidation treatment, except for dyeshaving a distinctly yellow leuco form.

There has been interest recently in extending the use of cationic fixing agents normallyused with direct dyes to sulphur dyes. Five cationic polymers markedly improved the fastnessto washing of solubilised sulphur, leuco sulphur and insoluble sulphur dyes when the cationictreatment was applied by a simple exhaust method typical of their application to direct dyeings[456]. The polymers were similarly effective when applied to leuco sulphur dyeings by pad–dryand pad–flash cure methods [457]. The mechanisms operating were thought to resemble thosewith direct dyes, i.e. the formation of electrostatically linked dye–agent complexes and/or theformation of a peripheral layer of polycations restricting dye mobility. The use of a reactive cat-ionic agent has also been examined [458], this being of a type used successfully as a pretreat-ment to enhance dyeability. Washing fastness of reoxidised, solubilised sulphur dyes whenaftertreated by an exhaust method was markedly improved. It was shown that the cationicaftertreatment could replace the usual reoxidation procedure, giving enhanced fastness.

AGENTS FOR FIBRE LUBRICATION, SOFTENING, ANTISTATIC EFFECTS, SOIL RELEASE

10.10 AGENTS FOR FIBRE LUBRICATION, SOFTENING, ANTISTATICEFFECTS, SOIL RELEASE, SOIL REPELLENCY AND BACTERICIDALACTIVITY

Most of the title products are finishing agents rather than dyeing or printing auxiliaries.Although in principle they could be applied before, during or after coloration (with the

chpt10(2).pmd 15/11/02, 15:44705


exception of lubricants applied during textile manufacture), economic reasons firmly put thepreference on conjunct application with colorants wherever possible. Whilst not dyeingauxiliaries as such, their inclusion often has a bearing on choice and behaviour of dyes aswell as on possible problems encountered during coloration (compatibility or otherwise ofthe components) and afterwards (fastness properties). Many of these products have usefulactivity in more than one function. Thus an agent (and especially a composite commercialproduct) promoted chiefly as a softener may also have antistatic, yarn lubricating and soilrelease properties, and it is useful to bear this diversity of function in mind.

10.10.1 Fibre lubricants

The main consideration here is lubricants applied during wet processing although someconsideration will also be given to those included during textile manufacture. The basicrequirement of a lubricant is that it should form a thin uniform protective coating aroundthe fibres to lower their surface friction and flexural rigidity, thus assisting a process such asweaving or minimising the formation of durable creases during rope dyeing, particularly athigh temperatures. In general practice a lubricant may, and usually does, contain more thanone component, a typical composition comprising base lubricant(s), antistatic agent andsurfactant(s). HPLC provides a useful means for the analytical separation and identificationof the components of a lubricant formulation [459].

From the point of view of subsequent wet processing, it is usually preferable that fabricsbe free from fatty substances such as oils and waxes. From the weaver’s viewpoint, however,it is usually contended that such fatty lubricants are essential to minimise wear of machineparts and to prevent yarn abrasion and breakages, more especially where high-speed weavingprocesses are concerned. Weaving lubricants of this kind are usually applied with the sizingagent (section 10.5.2) and they must operate in conjunction, one of the main functions ofthe lubricant being to increase the film-forming properties of the size polymer(s). However,such fatty lubricants are difficult to remove and may complicate subsequent desizing,bleaching and coloration processes. This practice of using fatty lubricants has beenquestioned in recent times, based on evidence that such hydrophobic agents can actuallyimpair the performance of the size as well as creating difficulties in desizing [460–463].

This is the main reason why surfactants are often used with fatty lubricants, to improvecompatibility with size polymers and to assist emulsification and removal. However, argumentshave been advanced [463] for the use of carefully selected surfactants as lubricating agents.The fatty or hydrophobic moiety of a surfactant produces a low coefficient of friction betweensurfaces to which it is applied and so acts as a lubricant, whereas the hydrophilic moiety makesfor built-in ease of subsequent removal. In addition, the use of inorganic salts such as sodiumsilicate and sodium carbonate, as routinely used in detergent formulations, enhances theperformance of the surfactant as a lubricant, a typical formulation comprising 0.6% surfactant,0.6–1.0% sodium silicate and 0.5% sodium carbonate. It is claimed that such a compositionperforms just as well as a fatty lubricant in sizing and yet it is much easier to remove duringsubsequent scouring or desizing. Anionic and nonionic surfactants were compared butunfortunately no details of structure were disclosed. Clearly a good deal of development workwould be required to evaluate a range of surfactants varying in hydrophobicity, in order toobtain optimum performance with a specific size composition and substrate. This is essential in

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any case, irrespective of whether the surfactant is to replace a traditional lubricant or is insupport of one.

Compatibility with other components becomes a critical factor when lubricants are usedin the dyebath. The properties of the ideal dyebath lubricant have been summarised asfollows [464]:(1) excellent fibre-to-fibre and fibre-to-metal lubrication;(2) economical;(3) no effect on other physical properties such as handle, water-repellency and absorbency;(4) a foam suppressant or deaerating agent;(5) no effect on reproducibility of dyeing;(6) no effect on fastness properties;(7) non-yellowing;(8) easily washed out; and(9) biodegradable.

When formulating a dyebath lubricant, particular attention must be paid to the type ofsubstrate (hydrophilic or hydrophobic) and to substantivity of the lubricant for that substrate.Another important consideration is ionicity of the lubricant in relation to the components ofthe dyebath, since this has such an important bearing on compatibility (clearly, anioniclubricants should be avoided when basic dyes are used). Solubility and/or dispersibility inrelation to dyebath composition and the conditions of dyeing (temperature, pH, liquor ratio)are also important, since the overall hydrophobic/hydrophilic balance has a major influence oncompatibility. The behaviour of the lubricant during drying can be just as critical as duringdyeing. Many lubricants promote undesirable thermomigration of disperse dyes on polyesterduring high-temperature drying and heat setting, leading to lower wet fastness.

Natural products such as animal fats and vegetable oils still constitute an important shareof the lubricants market although synthetic types are gaining acceptance [464]. Natural fatsand oils include saponified fatty acids, fatty esters, fatty alcohols and fatty amides. Variousanionic groups are suitable, including carboxylate, phosphate, phosphonate, sulphate orsulphonate, the last-named being the most widely preferred. Esterification of the fatty acidsis particularly useful. For example, ethoxylation with ethylene oxide enables products ofsubtly graded character to be produced, depending on the degree of hydrophobicity of thefatty acid and the degree of ethoxylation. Sulphonates offer greater stability at higher pHand ionic strength but they can generate troublesome foaming.

Synthetic-based lubricants include polyacrylates, acrylamide/acrylic acid copolymers,emulsified paraffin oils and waxes, modified silicones. The acrylates generally combineexcellent solubility in the dyebath with fabric lubrication and such polymers can be designedto cover a wide range of solubility, rinsibility and lubricating performance. Ionicity can bevaried, whilst acrylic esters offer scope for incorporating other functionalities along thepolymer chain. Acrylic esters also allow the degree of anionicity to be varied. Acrylamidegroups generally result in increased stability, especially in acidic media where the amidogroups are partially protonated and thus mildly cationic, whilst in neutral media they behavesubstantially as nonionic moieties.

Paraffin hydrocarbons of molecular mass 300 to 700 (i.e. 20–50 carbon atoms) requireemulsification with surfactants. They show good resistance to oxidation and yellowing.


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However, the stability of these systems is critically dependent on effective emulsification.Viscosity is usually low, a concomitant of this being that the emulsifier constitutes asubstantial proportion of the system. Ideally, the emulsifier should contribute to thelubricating power of the product. Related to the paraffins are the even more hydrophobicpolyethylenes, primarily used as sewing lubricants or handle modifiers. These polymers maybe difficult to wash out, although their ionic character can be varied according to whetheranionic, cationic or nonionic emulsifying systems are chosen. Paraffin waxes andpolyethylenes have an advantage over paraffin oils in that they are terminated with a mildlyionic functionality that assists their emulsification and, coupled with their high molecularmass, increases substantivity. This tendency, however, can make them difficult to wash outand leads to compatibility problems in dyeing. They are perhaps best regarded as softeners.

Alkoxylated polysiloxanes are a relatively new class of dyebath lubricants. They havepractically no substantivity for the substrate, yet combine adequate lubrication with watersolubility and easy rinsability. If the silicones contain primary hydroxy groups, these can bemodified by esterification, phosphation, phosphonation, sulphation, sulphonation orcarboxylation. These anionic substituents confer substantivity for various substrates withoutlosing rinsability. Anionic organic sulphates and sulphonates probably offer the best overallproperties for dyebath lubricants, whilst other types can be more suitable for selectedapplications [464].

Certain problems associated with lubricants

The manufacture of textile fabrics for automotive trims normally requires the application ofa spin finish, a considerable percentage of which is still present in the finished fabric [465].Residual lubricants can give rise to ‘fogging’ on the interior of car windows, caused bycondensation on the glass of volatile components present: other similar products, such asantistats and softeners, may also be implicated. Fogging is measured by reflectance and isexpressed as the ratio of the reflectance values of a glass plate before and after exposure tofogging (DIN 75021), a high value representing a low degree of fogging. Low-foggingcompounds typically give values of at least 90%; some measured values [465] are shown inTable 10.45. Chemical type is not a direct indicator of propensity to fogging, since volatilityvaries with hydrophobic/hydrophilic balance as determined by chain length and structuraltype on the one hand and the degree of polar character, including ionicity, on the other.

Yellowing can occur with certain lubricants during drying or ageing. This phenomenonhas been investigated on wool and acrylic fibres and blends of them lubricated with a

Table 10.45 Fogging values of lubricants [465]

Lubricant chemical type Fogging value (%)

Alkylbenzene 36Poly(ethylene glycol) 86Fatty acid polyglycol ester No. 1 55Fatty acid polyglycol ester No. 2 95Poly(ethylene/propylene glycols) >95Phosphate monoester/diester >90

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cationic fatty acid condensation product, a cationic nitrogenous condensation product, aquaternary stearylalkylamino ester, a nonionic oxyethylated carbonate/phosphate and apolyglycol carbonate ester formulated with a nonionic surfactant [466]. Yellowing did notoccur when the lubricant manufacturer’s instructions regarding concentration and dryingtemperature were followed. In some circumstances, however, such as radio-frequency drying,cationic compounds can induce the formation of sparks that may result in yellowing,indicating a need to reduce the power of the dryer and/or to use a lower concentration oflubricant.

As already mentioned, some lubricants can be difficult to remove by washing andsurfactants are often added to overcome this problem [463]. Lubricants can impair fastnessproperties, particularly those of disperse dyes. They may influence the uptake of dyes eitherpositively or negatively, although seldom seriously except where it results in unlevelness. Forexample, knitting oils can increase the yield of relatively oleophilic reactive dyes on cottonand yet with highly hydrophilic types they may cause dye-resist effects [467].

When considering the environmental aspects of lubricants, it is necessary to bear in mindthat commercial formulations are often more or less complex mixtures and that even smallamounts of toxicologically questionable components can lead to restrictions on use,depending on local regulations. Thermally stable, readily biodegradable esterified oilemulsions can offer a compromise solution, being environmentally preferable to unmodifiedoils, fats and waxes, although problems can still arise in high-temperature processes such astexturising or heat setting, as a result of cracking reactions leading to the formation ofharmful residues or condensates [468]. In order to overcome this problem of cracking,attention must be paid to the susceptible rupture points in the molecule. This has been donein the development [468,469] of a range of ecologically friendly lubricants by usingcarbonate ester bridging groups to combine fatty alcohols or their ethoxylates withpoly(ethylene glycol) (10.216). Such products are readily water-soluble, self-emulsifiable,thermally stable compounds with excellent cracking behaviour. Their lubricity is such thatthey can be used in amounts 50–80% lower than normal. They show excellentbiodegradability and skin compatibility properties. Emission values are 10–20 times less thanwith comparable esters or mineral oils. Environmentally friendly lubricants containingalkylglucoside with emulsifier (1:2) or amine oxide with emulsifier (1:2) have also beenclaimed [470].

R O (CH2CH2O)x C (OCH2CH2)y

O

O C (OCH2CH2)y

O

O R

10.216

10.10.2 Antistatic agents

Antistatic agents are useful to prevent garments accumulating charges of static electricity,particularly those made entirely from hydrophobic fibres. Whilst this is desirable fordomestic wear, it takes on added importance in many other sectors, when working inexplosion hazard areas, with micro-electronic components, in electronic data processingenvironments, near textile dust filtering systems, or in certain areas of military and spacetravel technology. Antistatic treatments may be impermanent (easily removed on washing)


chpt10(2).pmd 15/11/02, 15:44709


or have a degree of durability. They operate through their hydrotropic properties, creating amoist microclimate that discourages the build-up of static, or by having some degree ofconductivity, thus dissipating any localised static charge into a wider area of lower potential.

Impermanent antistats

Surfactants may exhibit a degree of hydrotropy and thus function as antistatic agents. Theyare often used for this and for their emulsifying properties in conjunction with fibrelubricants, or may be used alone in a dual capacity as lubricant and antistat. Suitably activesurfactants can be found amongst all four ionic types, some typical examples being [471]:– quaternary ammonium derivatives of fatty acids– polyethoxylated quaternary ammonium derivatives– quaternised alkylenediamines– alkyl sulphates, chlorides or phosphates– mixtures of mono- and di-esters of phosphoric acid– long chain amine oxides– polyethoxylated and polypropoxylated nonionics.

Nonionic and cationic types are generally preferred, usually on grounds of fibre compatibility,higher hygroscopicity and higher oil solubility. The quaternary ammonium derivatives of fattyacids in particular impart static protection at a low level of application (i.e. 0.25%), whilst thepolyoxyethylated versions give higher solubility in water. The nonionic and cationic typesdominate particularly in spin finishes [471]. However, the cationic types may createsubsequent problems if anionic surfactants are used at the washing stage.

Impermanent antistatic agents that are not surface-active include triethanolamine,glycerol in combination with potassium acetate, as well as inorganic salts such as lithiumchloride [471].

The major requirements for impermanent antistats are [47l]:(1) effective at low levels of humidity;(2) low volatility;(3) non-corrosive;(4) low toxicity; and(5) no yellowing.

In addition, if used in spin finishes they must be thermally stable, oil-soluble and exhibit lowmigration on the fibre.

Durable antistats

There are two types of durable antistat. The first (so-called external antistatic agent) isapplied at some stage after fibre manufacture whilst the second type is incorporated duringmanufacture. The former type exhibits varying degrees of durability and is not usually asdurable as the latter.

External antistatic treatments generally involve the formation of a crosslinked polymercontaining ionic and/or hygroscopic groups, cationic polyelectrolytes containing poly(ethyleneoxide) units being widely used. Many crosslinking agents are suitable to insolubilise hydrophilic

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components within a fibre, including poly(ethylene glycol) dihalides, epoxy compounds,cyanuric chloride, derivatives of piperazine, hexahydro-s-triazine, polyaziridines, poly-acrylamide and polyamines [471].

Casein modified by graft copolymerisation with esters of acrylic acid and methacrylic acidhas been traditionally used for the coating of leather. Casein modified in this way by graftingwith ethyl acrylate [472] showed good film-forming properties on textile materials,particularly as an antistatic agent in the backing of carpets and as an additive in pigmentprinting. This polymer satisfied ecological requirements and its antistatic properties weresubstantially enhanced by incorporating electroconductive materials such as carbon blackand powdered metals.

In another evaluation [473], durable antistatic properties were obtained using water-soluble quaternary ammonium polyamines applied to polyester by padding and curing at 160°C. These products were prepared according to Scheme 10.85. Epichlorohydrin wascondensed with poly(ethylene glycol) 600 (known for its hygroscopicity and antistatic effect)using boron trifluoride as catalyst to give the poly(ethylene glycol) diglycidyl ether. This wasthen reacted with the highly electroconductive tetra-ethylenepentamine to give a water-soluble polyamine. Finally, in order to prevent gelation and to improve the stability inaqueous solution, this polyamine was cationised as the phosphate salt.

An obvious means of increasing conductivity is to incorporate metals into the fabric.Thus fabric can be sprayed with a liquid resembling metallic paint, containing micron-sizedmetallic particles such as copper incorporated into a binder such as a polyester, epoxy oracrylic resin. During curing the metallic particles come into contact with one another, thus

H2C

O

CH CH2Cl CH

O

CH2ClCH2

H2C

O

CH CH2 O(CH2CH2O)nCH2CH2O CH2 CH

O

CH2

H2C

O

CHCH2O(CH2CH2O)n+1CH2CHCH2NH(CH2CH2NH)4CH2CHCH2(OCH2CH2)n+1OCH2CH

OH OH

CH2

O

OHPO

OH

OH

HOCH2 CHCH2O(CH2CH2O)n+1CH2CHCH2NH2(CH2CH2NH2)4CH2CHCH2(OCH2CH2)n+1OCH2CH CH2OH

OHOHOH OH

OPO

O

O

OPO

O

OH

BF3

H2N(CH2CH2NH)3CH2CH2NH2

+ HO(CH2CH2O)nCH2CH2OH +

__

__

+ +

_

Scheme 10.85


chpt10(2).pmd 15/11/02, 15:44711


providing excellent conductivity throughout the material [474]. The presence of copper,however, may present environmental problems when waste material of this kind requiresdisposal.

An unusual method of imparting antistatic properties to acrylic fibres involvessaponification of some of the nitrile groups to give amide and carboxyl groups, using sodiumhydroxide under exhaust (80 °C), pad–batch (varying within 15–120 minutes at 160–60 °C)or pad–steam conditions. The pad–steam method in particular provides convenient surfacemodification of acrylic fibres, giving antistatic properties together with improved soil-releaseand improved dyeing properties with basic dyes [475]. This method may be substrate-specificsince acrylic fibres vary widely in their sensitivity to alkali, some being quite easily discoloured.

Methods of incorporating durable antistats during fibre manufacture include:– incorporation of a hydrophilic component, such as a surfactant– formation of a lattice of highly conductive components within the fibre.

This approach is outside the scope of the present chapter but details may be found elsewhere[471,476,477].

10.10.3 Softeners

It is difficult to define unequivocally the quality of fabric handle or softness/firmnessdifferences, since this involves many factors. It is often linked with lubrication, especially assimilar products are often used for softening and lubrication. Whilst experienced assessorscan be quite remarkable in the extent to which they can grade and assess softeners simply bymeans of a highly developed tactile sense, more objective methods are clearly desirable forscientific investigations. Since many factors combine in producing an overall sense ofsoftness, it is not surprising that objective determination of softness involves more than oneparameter of measurement. The details of assessment are outside the scope of this chapter,but descriptions and discussions are available elsewhere [478–481]. Suffice it to say herethat the Kawabata system has acquired considerable importance in quantifying variousaspects of fabric handle.

The oils and waxes described as lubricants in section 10.10.1, as well as talc, can be usedas softeners but have now been superseded by more effective products. These may be non-reactive or reactive and may be cationic, anionic, nonionic or amphoteric. Although manycompounds have been patented, by far the most important are cationic quaternaryammonium compounds and various silicones. Until quite recently the field was led by thecationic types but there is now evidence that aminofunctional polysiloxanes have becomethe most important product group [482].

Before dealing with the detailed chemistry of softening agents, it is useful to considersome general factors. The desirable properties of an ideal softener can be summarised asfollows [482]:– easy to handle (suitable for pumping, stable on dilution)– low foaming, stable to shearing, free from deposits on rollers– compatible with other chemicals– good exhaustion properties– applicable by spraying

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– stable to high temperatures, non-volatile especially in steam– no effect on shade or fastness– no yellowing– easily biodegradable– non-toxic, dermatologically harmless, non-corrosive– no restrictions for transport or storage.

No single product satisfies all the above requirements. Commercial softeners are usuallymore or less complex mixtures, available as aqueous emulsions with a solids content of 15–25%. As well as the softener component(s) other additives are also present, including theemulsifying and/or dispersing agents necessary. It cannot be over-emphasised that emulsionstability is just as important as softener effectiveness, since it is crucial to the performance ofthe product. Nonionic emulsifying agents are especially useful, showing all-roundcompatibility with other substances even if they are different in ionicity. Considerationneeds to be given to cloud point phenomena and stability to electrolytes when usingnonionic surfactants. In such cases the emulsifying system itself may be a mixture.

Softener emulsions are available in different commercial grades. Semimicro-emulsionshave an average particle size of <0.1 µm, allowing for penetration to the fibre core andgiving excellent distribution of the softener. Micro-emulsions have an average particle size of<0.01 µm, giving the substrate an excellent inner softness and a distinctive surfacesmoothness without looking greasy [482]. Micro-emulsions have good stability and adecreased tendency for deposition on rollers. Critically optimised products are particularlynecessary when using micro-emulsions on high-speed equipment where high shearing forcesare developed [482]. Irrespective of the basic chemical type, the fabric handle can bemodified by varying the hydrophilic–hydrophobic balance of the softener. Thus increasinghydrophobicity by incorporating a longer alkyl chain imparts an increasingly greasy handle tothe fabric.

The ionicity of the softener influences its substantivity for different fibres. Whilst this canbe significant in padding applications, it is of primary importance for application by exhaustmethods. Cationic softeners are highly substantive to acrylic fibres, whilst anionic productsare substantive to wool and nylon. Nonionic softeners, if water-soluble, have very lowsubstantivity for any substrate and are easily lost by washing. Insoluble types may besubstantive to polyester or cellulose acetate, as well as wool and nylon. The degree ofsoftening with the nonionic products is only moderate but their nonionic nature makesthem convenient for application simultaneously with any class of dyes or fluorescentbrightening agents. The low-substantivity agents are preferable for repeat application indomestic washing.

There has been a trend towards so-called multifunctional softeners in which account istaken of various other factors such as hydrophilicity, sewability, antistatic behaviour,lubrication and shearing stability [482]. For example, in jet dyeing a product will require, inaddition to softening power:– extremely high emulsion stability to cope with high pump and nozzle shearing forces at

high temperatures– a suitable defoaming system– good substantivity to ensure adequate exhaustion.


chpt10(2).pmd 15/11/02, 15:44713


Conversely, if good sewability is required, suitable products need to have good lubricationbehaviour.

Quaternary ammonium compounds and other cationic softeners

Organic cationic softeners, as opposed to silicone derivatives, are usually quaternaryalkylammonium compounds, the most important over many years being dimethyldistearyl-ammonium methosulphate (10.217), on grounds of economy and availability. There areobviously many possible variants of such structures but typically the long-chain substituentsare within the range C16 to C22 and may be fully or partially saturated [483]. These softenerspossess aqueous solubility, substantivity for various fibres and, to some extent, antistatic andwater-repellency properties. These properties can be modified by varying the substituents onthe quaternary nitrogen atom. The softening effect is especially good. The balance ofproperties can be controlled more precisely using analogous ethoxylated or propoxylatedamines (10.218), in which the degree of ethoxylation can also be varied. These are moreexpensive but provide high-quality industrial softeners. Further compounds available includequaternised imidazolines (10.219) and diamides or diurethanes containing a protonatedamino group (10.220; R = alkyl or alkoxy).

CH3

N

(CH2)16CH3

(CH2)16CH3CH3

10.217

+

CH3SO4

(CH2CH2O)nH

N

CH2CH2NH

CH2CH2NHCH3

C

O

R

C R

O

CH3SO4

10.218

_

+

HN N CH2CH2NH

R1

C R2

OCH3SO4

10.219

+

_ R C

HN

O

CH2CH2

H

CH2CH2

HN

NH

C

O

R

X10.220

+

_

The specific properties of such compounds obviously depend on the precise nature of thesubstituents but in general the degree of softening decreases as follows: dialkyldimethyl-ammonium compounds > imidazolinium salts > aminoalkyl diamides or diurethanes [480].In the case of the quaternised compounds the preferred anions are methosulphate orethosulphate, since these have a less corrosive effect on steel vessels than the chloride salts.These compounds generally show maximum cation activity at around pH 3.5 but are usuallyapplied at higher pH values. In some cases the cationic softening agent is mixed with anonionic surfactant to serve as a lubricant/antistat, this being particularly common in fabricsofteners for domestic use. It is also worth noting that most quaternary ammoniumcompounds have a degree of antibacterial activity.

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From a detailed study of three quaternary ammonium softeners based on hydrogenatedtallow oil (10.221–10.223), adsorption of these softeners by cotton and acrylic fibres isshown in Figure 10.56. Adsorption of the three softeners by cotton did not vary greatly,apparently following a Donnan equilibrium mechanism. On the acrylic substrate a saturationpoint was reached at about 2 µmol of each softener per gram of fabric. A twenty-memberpanel using tactile sensation found no significant difference between the products forsoftness on either fibre. Measurement of bending rigidity indicated the following results forthe same quantity of applied softener:

on cotton fabric: imidazolinium quat > ester quat or alkyl quaton acrylic fabric: ester quat or imidazolinium quat > alkyl quat.

Figure 10.56 Adsorption of cationic softeners by cotton and acrylic fabrics at 30 °C [483]

0.2 0.4 0.80.6Softener concentration/%owf

0.6

0.2

0.4

0.8

Ads

orbe

d so

ftene

r/%

owf

A

I

E

Q

Q

Q

EI

A

QQ

Q

0.2 0.4 0.80.6Softener concentration/%owf

0.15

0.05

0.10

0.20

Ads

orbe

d so

ftene

r/%

owf

Softener adsorption on cotton fabric [NaCl] = 1.25 g/l, 30 oC, pH 4.5–5.0

Softener adsorption on acrylic fabric [NaCl] = 1.25 g/l, 30 oC, pH 4.5–5.0


chpt10(2).pmd 15/11/02, 15:44715


A problem associated with quaternary ammonium softeners is that they increase thehydrophobic nature of the fibre and thus interfere with rewetting, this effect beingcumulative with each successive application of the softener. The rewettability of a cottonfabric treated with the above three softeners is shown in Figure 10.57, wettability beingaffected somewhat less by the alkyl or ester quat than by the imidazolinium quat. Antistaticeffects on the acrylic fabric are illustrated in Figure 10.58, showing wide variations betweenthe softeners at low concentrations [483]. Surprisingly, none of these softeners inducedsignificant thermomigration of three disperse dyes on polyester [483].

Micro-emulsion formulations of cationic softeners are available and are claimed to havesuperior fibre penetration properties [484]. Figure 10.59 shows the sorption of a micro-

0.1 0.2 0.3 0.4 0.5 0.6Softener concentration/%owf

50

20

30

10

40

Rew

etta

bilit

y/%

A Q

E Q

I Q

Figure 10.57 Effect of cationic softeners on the rewettability of cotton fabric [483]; 20 °C, 60% relativehumidity

E–Q A–Q I–Q E–Q A–Q I–Q E–Q A–Q I–Q0.20.1 0.4

Softener concentration/%owf

50

20

30

10

40

Sem

i–di

scha

rge

time/

s

Figure 10.58 Effect of cationic softeners on the antistatic behaviour of acrylic fabric [483]; 20 °C, 60%relative humidity

chpt10(2).pmd 15/11/02, 15:44716

717

0.25 1 42 3Time/min

60 oC40 oC32.5 oC25 oC

1.0

1.5

2.5

0.5

3.0

2.0

Sor

ptio

n/g

per

kg c

otto

n

Figure 10.59 Sorption kinetics of micro-emulsion cationic softener formulation at differenttemperatures on cotton fabric [484]. Initial concentration 3 g/kg fibre, liquor ratio 15:1

CH2CH2OH

N

(CH2)nCH3

(CH2)nCH3CH3

CH3SO4

10.224

+

_

CH3(CH2)16 C

NHCH2CH2

O

N

(CH2)16CH3C

CH2CH2NH

O

N

CH3

10.225

+

CH3SO4–

emulsion formulation of dimethyldistearylammonium (10.217) chloride. Similar results wereobtained with micro-emulsion formulations of N,N-dialkyl-N-2-hydroxyethyl-N-methylammonium methosulphate (10.224) and N,N-bis(stearamidoethyl)-2-methyl-imidazolinium methosulphate (10.225), although the highest rate constant values wereobserved with the imidazoline derivative and the lowest with the dialkylhydroxyethyl-methylammonium methosulphate (Table 10.46). Cotton washed 50 times in hard waterabsorbed the softeners rather more strongly than unwashed cotton. It is thought that thismay be due to adsorption by the cotton of inorganic salts from the hard water and of acidicresidues from the detergent used.

Yellowing can occur with quaternary cationic softeners and this limits their use on whitefabrics. This problem can be overcome to some extent, provided drying or fixationtemperatures are not too high, using so-called pseudo-cationic softeners [482]. Theseproducts are analogous to the so-called weakly cationic surfactants described in section 9.5.


chpt10(2).pmd 15/11/02, 15:44717


Silicone softeners

Poly(dimethyl siloxane) (10.226) represents the simplest of the silicone softeners, which areusually applied from emulsion. This polymer, however, is unreactive and is not substantive tofibres. Therefore it is not fast to washing and is little used nowadays. More durable softenerscan be derived by modification of this fundamental siloxane. The so-called ‘conventionallyreactive’ silicones were the first to be introduced, typical examples being those containingactivated silanic hydrogen (10.227) or silanol (10.228) groups. Such polymers react withcrosslinking agents (10.229 being a typical example) during a curing process with anorganometallic catalyst, typically a zinc or zirconium alkanoate. A crosslinked polymernetwork of high molecular mass is formed on the fibre, this being the mechanism necessaryto achieve a soft finish of high durability.

Table 10.46 Rate constants for sorption of cationic softeners at various temperatures onunwashed and washed cotton [484]

Rate constants

Temperature Unwashed WashedSoftener (°C) cotton 50 times

Dimethyldistearylammonium (10.217) chloride 25 0.42 0.72

32.5 0.55 0.7940 0.64 1.1460 0.83 1.70

Dialkylhydroxyethylmethylammonium methosulphate (10.224) 25 0.46 0.57

32.5 0.50 0.6340 0.55 0.7160 0.61 0.81

Bis(stearamidoethyl)methylimidazolinium methosulphate (10.225) 25 0.73 0.95

32.5 0.80 1.1040 0.91 1.1660 1.03 1.47

CH3 Si O

CH3

CH3

Si O Si

CH3

CH3

CH3

CH3

CH3n

10.226

CH3 Si O

CH3

CH3

Si O Si

H

CH3

CH3

CH3

CH3n

10.227

HO Si O

CH3

CH3

Si O Si

CH3

CH3

OH

CH3

CH3n

10.228

CH3 Si OCH3

OCH3

OCH3

10.229

chpt10(2).pmd 15/11/02, 15:44718

719

The next development arose in the late 1970s with the introduction of aminofunctionalsilicone softeners. These contain aminoalkyl groups attached to the poly(dimethyl siloxane)backbone, leading to improved orientation and substantivity for the fibre [482]. Thisfavourable orientation, illustrated in Figure 10.60, leads to an extremely soft handle, oftendescribed as ‘supersoft’. These polymers are highly cost-effective, very small amounts beingrequired to obtain the desired properties, thus giving both economic and environmentalbenefits. Indeed, such are the advantages of these products that they have assumed adominant role in the marketplace. Mainly based on a single type of amino group, more than90% of all commercially available aminosilicone softeners are in fact aminoethyliminopropylsilicones (10.230) [485].

The softening effect of silicones results from their lubrication behaviour that affects boththe surface and the interior of the fibre. The behaviour of polysiloxanes of the 10.230 typecan be varied by adjusting the average values of x and y and the range of chain lengthspresent. Further variations are possible by varying the R groups. In view of the technical and

Negative zeta potential of cellulosic fibre

Figure 10.60 Attachment and orientation of an aminofunctional polysiloxane on a cellulosic fibre[482]

R Si O

CH3

CH3

Si O

CH3

CH3

Si O

R

(CH2)3

Si R

CH3

CH3

NH

CH2

CH2

NH2

x

y

10.230

R = OH, CH3, OCH3


chpt10(2).pmd 15/11/02, 15:44719


commercial importance of the aminofunctional polysiloxanes, it is worth exploring somestructure/property relationships [485–489] even though there is as yet no conclusivecorrelation available for the most important aspect, that of softness as distinguished bytactile sensation.

The presence of the aminofunctional group appears to be necessary to obtain a ‘supersoft’handle. In a study [488] of nineteen variants of the aminoethyliminopropyl polysiloxanestructure (10.230) differing in amino group content (0 to 1.5 milli-eq./g), viscosity (300 to21 000 mPas) and emulsion particle size (50 to 350 nm), good agreement was found betweentest parameters and amino group content, for which there was a strong substrate dependence.The cotton fabric softness showed a marked dependence on amino group content, giving anoptimum value at about 0.3–0.4 milli-eq./g polysiloxane. Polyester/cotton blends (65:35 to35:65) gave an optimum at about 0.2 milli-eq./g with a strong dependence on amino content,whereas a 100% polyester fabric showed a much weaker dependence together with a relativelyhigh optimum at 0.5–0.6 milli-eq./g. Emulsion particle size and viscosity played subordinateroles, although it is known that very low viscosities (indicating short polymer chains) and verysmall particle sizes can be detrimental to softness effects under certain conditions.

Epoxyfunctional siloxanes are also useful as softeners. These may be derived frompolysiloxane (10.231) or from aminopolysiloxanes (10.232). Further possibilities are repre-sented by the polyalkoxylated epoxyfunctional silicones (10.233) and polyalkoxylatedaminofunctional silicones (10.234). However, it has been pointed out [485] that thereaction of epichlorohydrin with aminopolysiloxanes is not very specific, since primary andsecondary amine groups are usually randomly epoxidised resulting in viscous products that

CH3 Si O

CH3

CH3

Si O

CH3

CH3

Si O

CH3

CH2

Si CH3

CH3

CH3

CH

O CH2

x

10.231

y

CH3 Si O

CH3

CH3

Si O

CH3

CH3

Si O

CH3

R

Si CH3

CH3

CH3

NH

CH2

CH

O CH2

x

10.232 y

CH3 Si O

CH3

CH3

Si O

CH3

CH3

Si O

CH2

CH3

Si O

CH3

(CH2)n

Si CH3

CH3

CH3

(OCH2CH2)a(OCH2CH2CH2)bOCH3

CH

CH2O

x y z

10.233

CH3 Si O

CH3

CH3

Si O

CH3

CH3

Si O

CH2

CH3

Si O

CH3

(CH2)n

Si CH3

CH3

CH3

(OCH2CH2)a(OCH2CH2CH2)bOCH3

CH2CH2NHCH2CH2NH2

x yz

10.234

chpt10(2).pmd 15/11/02, 15:44720

721

are difficult to emulsify. Thus they do not offer a significant improvement over the parentaminopolysiloxanes.

Promising products, offering a variety of structural possibilities, are obtained by acylationof aminopolysiloxanes [485]. Suitable acylating agents include anhydrides, lactones and car-bonates (Scheme 10.86), of which acetic anhydride is the most economical. The optimumcombination of effects is obtained by 30–70% acylation; more than 70% substitution canreduce the softening effect to the level associated with conventional poly(dimethylsiloxane). No significant differences have been observed with respect to handle, whitenessor water absorbency, depending on whether acylation is achieved using acetic anhydride,butyrolactone or ethylene carbonate as the acylating agent. A slight decline in softness ofhandle is observed with the acylated products compared with that from normal amino-polysiloxanes but this is compensated for by better whiteness, water absorbency and soilrelease properties. A major drawback of the standard aminoethyliminopropyl polysiloxanes istheir tendency to show yellowing, resulting from the formation of chromogenic species byoxidative thermal decomposition of the aminofunctional group. Acylation largely overcomesthis problem and also gives improvements in water absorbency and soil-release performance.

CH2CH2CH2OHC

O

(CH2)3NHCH2CH2NSi

CH3C

O

(CH2)3NHCH2CH2NSi

OCH2CH2OHC

O

(CH2)3NHCH2CH2NSi

C

OCH2

CH2

CH2O

CH2

CH2O

C

O

O

CH3C

O

O

CH3C

OSi (CH2)3NHCH2CH2NH2

H

H

H

Scheme 10.86

Further fine tuning of the properties of aminopolysiloxanes can be achieved [485] bysubstitution of the siloxane units with primary, secondary or tertiary amino functions(10.235). Whiteness, water absorbency and soil release properties are improved withincreasing degree of amino substitution from primary through to tertiary amines (Figure10.61). The improvement in whiteness compared with the aminoethylimino referenceproduct is particularly noteworthy, this being attributed to retardation of the oxidativethermal degradation because of the protective effect of alkylamino substitution. However,only the secondary amines show softness values as good as those of the aminoethyliminoreference. The cyclohexylamino function, in particular, gives rise to a most useful


chpt10(2).pmd 15/11/02, 15:44721


Handle Whiteness Waterabsorbency

Soilrelease

ReferenceNHRNH2

PrimaryNH2

SecondaryNHR

TertiaryNR2

Figure 10.61 Effect of aminofunctional polysiloxanes on the physical properties of treated cottonfabric [485]

chpt10(2).pmd 15/11/02, 15:44722

723

combination of effects: it gives optimum whiteness, as well as improved water absorbencyand soil release (both properties superior to those of acylamino derivatives). In view of theseresults, it is not surprising to learn that hindered secondary amino substituents (10.236) alsogive rise to non-yellowing softeners [490].

An interesting attempt has been made to formulate a theoretical mechanism for thesoftening and fibre-substantivity characteristics of aminopolysiloxanes [489]. Theiroutstanding softening effects are the result of exceptionally high mobility of the polymersegments, this lubrication behaviour affecting both the surface and the interior of the fibre.Polyether chains are more flexible than polyalkyl chains, having a bond length of 1.41 Å anda bond angle of about 110° (Figure 10.62), thus providing a lower barrier to rotation.

H2C

CN

C

CH2

CH

O

CH3

CH3H

H3C

H3C

(CH2)3

Si

CH3

OO Si CH3

CH3

CH3

SiO

CH3

CH3

SiCH3

CH3

CH3x

y

10.236

C C

C

C

C

O

O

Si Si

1.63Å

1.41Å

1.54Å

ca. 110o

ca. 110o

ca. 144o

Typical compound

H3C—CH2—CH3 2.74

H3C—O—CH3 1.06

H3Si—O—SiH3 0.32

Bond dimensionsBarrier to rotation(kcal/mol)

Figure 10.62 Molecular dimensions of alkane, ether and siloxane chain segments [489]


chpt10(2).pmd 15/11/02, 15:44723


Polysiloxane chains have a longer bond length and a larger bond angle, giving rise to aneven lower barrier to rotation.

These softening agents have a binary nature; on the one hand there is the relativelyhydrophobic polysiloxane backbone of the polymer, whilst on the other there is theaminoethyliminopropyl group capable of varying degrees of protonation. Increasedprotonation gives enhanced hydrophilicity and an increasing number of positive electrostaticcharges. The hydrophobic backbone confers substantivity for hydrophobic fibres, whereasthe protonated amino groups provide for electrostatic attraction to negatively charged fibres.These relationships are illustrated diagrammatically in Figures 10.63 to 10.68. In the case ofpoly(dimethyl siloxane) on cotton (Figure 10.63), the fibre–water hydrogen bonding forcesare favoured and the weak polymer–fibre forces are inadequate to provide uniformdeposition of a polymer film over the fibre surface. Thus the lubricating action to decreasefibre–fibre friction is inadequate and the handle of the finished fabric is firmer thanuntreated cotton.

Dimethyl siloxane group

Cotton fibre surface

Figure 10.63 Poly(dimethyl siloxane) attached to cotton by weak polymer-fibre interactions [489]

Attachment to the cotton surface of an aminofunctional silicone containing relatively fewpartly protonated amino substituents is illustrated in Figure 10.64. The strong interactionbetween these groups and hydroxy groups in the cellulose does bring about some orientationof the silicone polymer segments close to the fibre surface but coverage of the latter isincomplete. The lubricating action of the polymer film, though more effective than in Figure10.63, is somewhat limited and the fabric handle is not ‘supersoft’. Figure 10.65 shows themuch more effective distribution over the fibre surface of an aminofunctional siliconecontaining the optimum proportion of partly protonated amino substituents. Coverage isuniform and complete and the length of the silicone polymer segments between theanchoring amino substituents is sufficient to permit their high flexibility to contribute tooptimum lubrication and a ‘supersoft’ handle. In the case of an aminofunctional siliconecontaining an excessive proportion of partly protonated amino groups (Figure 10.66),coverage is complete but the thin silicone film composed of short and inflexible polymersegments between the anchoring points is inadequate to provide a ‘supersoft’ handle.

Figure 10.67 indicates the probable distribution of a silicone containing the optimumcontent of aminoethyliminopropyl groups when applied to a polyester fibre surface. In thiscase the attachment is through hydrophobic polymer–fibre interaction and the mobility ofthe silicone chain segments is increased by electrostatic repulsion between neighbouringcationic groups. Dependence of softness of the treated polyester fabric on the proportion of

chpt10(2).pmd 15/11/02, 15:44724

725

Dimethyl siloxane groupAminoethyliminopropyl group (partly protonated)


Figure 10.64 Aminoethyliminopropyl silicone attached to cotton by too few cationic amino groups[489]



Figure 10.65 Aminoethyliminopropyl silicone attached to cotton by the optimum proportion ofcationic amino groups [489]



Figure 10.66 Aminoethyliminopropyl silicone attached to cotton by too many cationic amino groups[489]


Polyester fibre surface

Figure 10.67 Probable orientation of an aminofunctional silicone on the surface of polyester [489]


chpt10(2).pmd 15/11/02, 15:44725


partly protonated amino groups present is less sensitive than in the case of cotton and theoptimum level of softness is reached at a higher proportion on polyester than on cotton.These concepts are extended in Figure 10.68 to the behaviour of an aminofunctionalsilicone applied to a polyester/cotton blend fabric. Hydrogen bonding and coulombic forcesof interaction between the partly protonated aminoethyliminopropyl groups of the siliconeand hydroxy groups at the cellulose surface are reinforced by hydrophobic interactionbetween the dimethyl siloxane units and the polyester fibre surface. The mobility of thesilicone polymer segments is favoured by electrostatic repulsion between neighbouringcationic groups and the optimum degree of softness is achieved at a relatively low proportionof these groups, somewhat less than the corresponding optimum on cotton.

Although fabrics made from microfibres generally have a softer handle and better drapethan those from conventional fibres, these properties can be further improved to asignificant extent by the application of silicone softeners, the best results being obtainedwith aminofunctional polysiloxanes [491].

Miscellaneous softening treatments

Many other products can be used as softeners but are less important commercially becauseof greater cost and/or inferior properties. Examples are anionic surfactants such as long-chain (C16–C22) alkyl sulphates, sulphonates, sulphosuccinates and soaps. These have ratherlow substantivity and are easily washed out. Nonionic types of limited substantivity anddurability, usually applied by padding, include polyethoxylated derivatives of long-chainalcohols, acids, glycerides, oils and waxes. They are useful where ionic surfactants wouldpose compatibility problems and they exhibit useful antistatic properties, but they are morefrequently used as lubricants in combination with other softeners, particularly the cationics.


Polyester fibre surface


Figure 10.68 Probable orientation of an aminofunctional silicone between the surfaces of componentfibres in a polyester/cotton blend [489]

chpt10(2).pmd 15/11/02, 15:44726

727

Some amphoteric softeners such as amino acids (10.237) and sulphobetaines (10.238) aremore effective and durable than the nonionic types but less durable than the cationics;moreover, they tend to be expensive. Other amphoteric types include the zwitterionic formsof quaternised imidazolines (10.239); long-chain amine oxides (10.240) also exhibitsoftening properties.

R

HN N CH2CH2OCH2CH2 C

O

O

10.239

+

_

R NH2 CH2CH2 C

O

O10.237

+

_

R N (CH2)n

R

R

S O

O

O10.238

+ _

R N

R

R

O

10.240

+ _

There are reactive softeners, some of which are N-methylol derivatives of long-chainfatty amides (10.241) while others are triazinyl compounds (10.242). The N-methylolcompounds require baking with a latent acid catalyst to effect reaction, whereas dichloro-triazines require mildly alkaline fixation conditions. The N-methylol compounds aresometimes useful for combination with crease-resist, durable-press, soil-release and water-repellent finishes. In this context, the feasibility of using silane monomers such as methyltri-ethoxysilane (10.243), vinyltriethoxysilane (10.244), vinyltriacetylsilane (10.245) andepoxypropyltrimethoxysilane (10.246) in crosslinking reactions to give crease-resistproperties and softness simultaneously has been investigated [492].

O

C

NH HN CHOCH2

O

R

10.241

N

R

RN

N

N

Cl

Cl

10.242

H3C Si OCH2CH3

OCH2CH3

OCH2CH3

10.243

CH Si OCH2CH3

OCH2CH3

OCH2CH3

H2C

10.244

CH Si C

C

C

H2C

O CH3

H3C O

O

CH3

10.245

CH2CH2CH2SiH3CO

OCH3

OCH3

10.246

CH

O

CH2

Softening treatments of a rather different nature include biofinishing enzyme treatments tomodify the fabric surface. This has been dealt with already in section 10.4.2. Even moreesoteric is the use of so-called telluric treatments using minerals (microliths) of preciselydefined lithological and metamorphic properties. A detailed account of these complexmaterials is available [493]. In essence, an enzyme is micro-encapsulated within the mineral


chpt10(2).pmd 15/11/02, 15:44727


microlith. Under the action of strong mechanical forces this crystal structure is broken open,progressively releasing the enzyme. The process thus combines mechanical surface erosion ofthe textile with biochemical modification.

Environmental aspects of softening treatments

This account is concerned with environmental aspects of the application of softeners ratherthan their manufacture, although a discussion of environmental factors involved in theproduction of quaternary ammonium softeners is available [494]. Environmental aspects ofcationic quaternary ammonium salts, nonionic surfactants and amphoteric compounds havebeen dealt with already in section 9.8.1. With regard to cationic quaternary softeners (Figure10.56), it has been reported that the ester 10.221 can be considered to be biodegradable, inwhich respect it is superior to the tetra-alkylammonium salt 10.222 and the imidazoliniumsalt 10.223 [483]. A paradox has also been pointed out [494]: as the water solubility ofquaternary compounds increases so does the rate of biodegradation and the fish toxicity, sothat the requirement for the maximum rate of biodegradation can only be met by developingproducts that are more toxic to fish. Fish suffer through the interference of pollutants withgill breathing. In some mammals, however, there is a possibility of developing asubcutaneous toxicity that can cause neuromuscular problems and ultimately possible death.In this sense, quaternised silicone polymers can become highly toxic if certain neuronaldistances are met, in the sense of a lock and key fit between them.

The environmental compatibility of silicone softeners is generally favourable [495,496].The discussion here concerns only the silicone component of the formulation and not thesupporting emulsifying system. For the most part this is nonionic, preferably based on linearethoxylated fatty alcohols, although alkylphenol ethoxylates are still used in some countries[496]. The salient points regarding the environmental influence of silicones can besummarised as follows:(1) Silicones are a minor part of discharges to waste waters.(2) Although highly resistant to biodegradation by micro-organisms, poly(dimethyl

siloxane) derivatives are very effectively degraded via natural chemical processes [495]such as catalysed hydrolysis and oxidation during soil contact to produce siloxanols andsilanols of lower molecular mass (Scheme 10.87). These are then susceptible to bothbiological and abiotic decomposition, ultimately oxidising to natural silica.

CH3 Si O

CH3

CH3

Si O

CH3

CH3

Si CH3

CH3

CH3

HO Si O

CH3

CH3

H CH3 Si OH

CH3

CH3x y

soil

water+

y = mainly 1Scheme 10.87

(3) Silicones are ecologically inert, having no effect on aerobic or anaerobic bacteria. Thusthey do not inhibit the biological processes taking place during waste water treatment.

(4) Non-volatile silicones do not bioconcentrate in aquatic media. Their large molecularsize prevents them from passing through the membranes of fish or other aquaticcreatures. They readily become attached to particulate matter and are effectively

chpt10(2).pmd 15/11/02, 15:44728

729

removed by the natural cleansing process of sedimentation. Elimination rates fromsewage sludge are very high.

(5) Silicones give insignificant BOD values. Tests on aquatic plant and animal life revealedno measurable adverse effects even under highly exaggerated conditions. No significantchange in growth rates of algae, plankton or other marine organisms has been found.

(6) Silicones have not been found to pose a threat to insects or birds.(7) Volatile silicones are broken down by oxidative chemical processes on entering the

atmosphere. The partially oxidised degradation products are less volatile and these arescrubbed out of the atmosphere by rain or deposited on the ground to be further dilutedand degraded, the final products being natural silica, carbon dioxide and water.

(8) Volatile methylsiloxanes degrade quickly, the atmospheric lifetime being 10 to 30 days,and have no potential to interfere with the ozone layer.

(9) There is no risk of forming compounds that contribute to AOX values.(10) Formaldehyde can be produced only if the degradation temperature exceeds 200 °C and

even then the amounts produced are significantly less than those from carbon–carbonpolymers containing methyl groups.


10.10.4 Soil-release, soil-repellent and water-repellent agents

Agents to protect against soiling were developed following the increasing use of hydrophobicfibres, particularly nylon and polyester, since experience demonstrated the tenacity of oilystains and oil-bound dirt for these fibres. Durable press cotton fabrics also tended to soilmore easily than untreated fabrics. The subject of soiling and soil removal is more complexthan might at first appear [476,497–500] and involves such aspects as soil resistance, soiladsorption, detergency, soil removal and soil re-deposition. We are concerned essentiallywith soils attracted to, and bound mainly at the fibre surface, as opposed to particulate dirttrapped within the interstices between fibres in the yarn. The primary objective is to modifythe fibre surface (a) to increase the resistance of the fibre to soiling in the first place (soilrepellency) and (b) to ensure that any soil that is deposited is more weakly bound and ishence more easily removed in washing (soil release). Most soils, as expected from apredominantly hydrophobic interaction, are held mainly by nonpolar bonding, althoughelectrostatic forces may come into play with, for example, coloured anionic stains from foodand beverages.

The essence of any soil-resistant treatment is to render the surface of the fibres morehydrophilic. It also helps if the coating of the fibre is such as to reduce surface irregularityand surface energy. Whilst the two aspects of soil repellency and soil release are interrelated,the actual balance of these properties varies from finish to finish according to requirements.In carpet treatments for example, which are normally given a shampoo rather than washed,the emphasis must be on repellency, whereas soil release becomes of much greaterimportance in textiles that are frequently washed.

Early soil-release agents, applied particularly to resin-finished cellulosic goods, werewater-soluble polymers, many being related to thickeners (section 10.8) such as starch,hydroxypropyl starch, sodium carboxymethylcellulose, methylcellulose, hydroxyethyl-cellulose, alginates, poly(vinyl alcohol) and poly(vinylpyrrolidone). These functionedessentially as temporary barriers and ‘preferential reservoirs’ for soil, which was thus easilyremoved along with the finish in subsequent washing, when they then helped to minimise

chpt10(2).pmd 15/11/02, 15:44729


re-deposition. Obviously these finishes were only temporarily effective. More durablefinishes have been developed and these are generally classified in three groups according towhether they feature (a) carboxyl groups, (b) oxyethylene and/or hydroxy groups and (c)fluorocarbon moieties. The fluorocarbon finishes in particular have also been developed aswater-repellent treatments.

The carboxylated polymers [476,499] include acrylic, methacrylic or maleic acid polymers(all obviously anionic in character) applied mainly from aqueous emulsion and particularlyin combination with crease-resist or durable press resins. This type of chemistry has alreadybeen discussed in section 10.8.2. A particularly common example is the copolymer of acrylicacid with ethyl acrylate (10.247). In general the best balance of properties is obtained with75–85% ethyl acrylate (y) and 25–15% acrylic acid (x), with an average chain length ofabout 1300 (x + y) units; 65–85% ethyl acrylate with 35–15% methacrylic acid is alsosuitable. When the content of the acidic comonomer increases above about 30% thedurability to washing tends to decrease, whilst longer chains tend to give a stiffer handle[499].

CH2 CH CH2 CH

C CO OH O OCH2CH3x

y

10.247

Soil-release products containing oxyethylene or hydroxy groups may be anionic ornonionic. Many less durable water-soluble polymers have been mentioned already, such asthe hydroxy-containing finishes poly(vinyl alcohol), starch, and derivatives of starch orcellulose. When applied together with N-methylol reactants, as in easy-care finishing, theygive more durable soil-release properties. Typical of the oxyethylene-containing compoundsare poly(ethylene glycol) and poly(ethylene oxide) adducts of carboxylic acids, amines,phenols and alcohols, which may be combined with hydroxy-reactive functional agents asused in easy-care finishes, such as N-methylol reactants or isocyanates.

Essentially nonionic soil-release agents comprise polyesters, polyamides, polyurethanes,polyepoxides and polyacetals. These have been used mainly on polyester and polyester/cellulosic fabrics, either crosslinked to effect insolubilisation (if necessary) or by surfaceadsorption at relatively low temperature. Polyester soil-release finishes have been mostimportant, particularly for polyester fibres and their blends with cellulosic fibres. Thesefinishes, however, have much lower relative molecular mass (1000 to 100 000) thanpolyester fibres and hence contain a greater proportion of hydrophilic hydroxy groups. Theyhave been particularly useful for application in laundering processes. These essentiallynonionic polymers may be given anionic character by copolymerising with, for example, thecarboxylated polymers mentioned earlier; these hybrid types are generally applied withdurable press finishes.

Polyfluorinated chemicals now dominate in the fields of oil-repellent and water-repellentfinishes. The earlier so-called conventional polyfluorinated products were of the typerepresented by poly(N-methylperfluoro-octanesulphonamidoethyl acrylate)(10.248) [499].Such products presented a shield of closely packed fluoroalkyl groups at the fibre–airinterface, thus giving low-energy surfaces with excellent oleophobicity. These showedexcellent resistance to oil-based stains but were less satisfactory as soil-release agents during

chpt10(2).pmd 15/11/02, 15:44730

731

washing. The soil-release properties were subsequently considerably improved bycopolymerising these conventional fluorochemicals with hydrophilic moieties [476,498,499]to give so-called hybrid block copolymers represented schematically by A–B–A–B–A–,where A represents the perfluoroalkyl-containing segment and B the hydrophilic segment. Atypical example [498,499] is represented by structure 10.249, which is composed ofalternating perfluorinated units of the type shown in structure 10.248 with hydrophilicoxyethylene moieties derived from the thiol-terminated copolymer of tetra-ethylene glycoldimethacrylate and hydrogen sulphide (10.250).

CH2 CH

CRO

CH2 CH

CRO

CH2 CH

CRO

CH2 CH

CRO

OCH2CH2 N

SO2(CF2)7CF3

CH3

n

10.248

R =

H CH CH2 S CH2 CH

CO R

CH3

C O (CH2CH2O)4

O

C CH

O

CH2

CH3

S CH2 CH H

CO R

310

3

10.249

HS CH2 CH C

CH3

O

O

(CH2CH2O)4 C CH

O

CH2

CH3

S H

10

10.250

Once again these are only average schematic structures, in this case representing a blockcopolymer of alternating segments. The hydrophilic segments in themselves show nosignificant oil repellency and are not very effective as soil-release agents, yet whenincorporated into such hybrid structures they considerably improve the soil-releaseproperties without inhibiting the inherent soil-repellency of the perfluorinated segments.This is said [499] to result from the capability of these hybrid polymers to orient a specificmoiety at the surface, depending on the polarity of the fibre–environment interface. Thus inair the fibre–air interface is dominated by the closely packed perfluoroalkyl chains,promoting good oil repellency, whilst in aqueous wash liquors it is the hydrophilic segmentsthat orient at the fibre–liquid interface, thus enhancing soil release. In either case the non-active moiety is said to be collapsed below the surface. In this way, the lowest interfacialenergy, with respect to the particular environment, is attained in both cases.

The example used here incorporated a perfluorinated polyacrylate and a poly(oxy-ethylene) hydrophilic moiety. Other fluorochemical and hydrophilic moieties can be usedproviding they display similar alternating surface orientation characteristics with respect toair and water. The essential character of the hydrophilic unit is that it should have polargroups capable of strong interaction with water, preferably by hydrogen bonding; examplesare hydroxy, carboxyl and ether oxygen. Usually C5–C18 perfluoroalkyl groups are used, but


chpt10(2).pmd 15/11/02, 15:44731


individual products may contain a mixture of homologues. Thus there is tremendous scopefor designing a great variety of these complex copolymers.

It is now opportune to consider the structure–property relationships of fluorochemicalfinishes in more detail [501,502]. Water repellency depends mainly on reducing the criticalsurface energy of the fabric surface. This parameter must be less than that of the wetting

Table 10.47 Critical surface energies for low energysurfaces [502]

Chemical groups Surface energyon the surfaces (mN/m) at 20 °C

–CF3 6–CF2– 18–CH3 21–CH2– 31

Table 10.48 Surface tension of a range of liquids and surface energies of arange of textile fibres [502]

Surface tension Surface energyLiquid (mN/m) at 20 °C Textile fibre (mN/m) at 20 °C

Water 72 Nylon 46Peanut oil 40 Wool 45Olive oil 32 Cotton 44Petrol 26 Polyester 43n-Octane 22n-Heptane 20Fluorocarbons 10–15

liquid in order to create a physico-chemical barrier against penetration of the fabric by theliquid. Table 10.47 gives critical surface energies for various chemical groups at the surfaceof a fibre from which it can be seen that the CF3 group is by far the most effective forlowering this surface energy. Table 10.48 lists surface tension values of some liquids andsurface energy values of various fibres.

It is evident from these two tables that a high density of CF3 groups at the fibre surfacewill lower the critical surface energy sufficiently to create a barrier to penetration of all theliquids listed, particularly against water. It is also evident that perfluoroalkyl groups areessential to guarantee resistance to oily liquids. Thus the presence of CF3 terminal groups iscrucial. Equally important, however, is the overall structure of the molecule [502]; theperfluorinated segments should be long enough to maximise the CF3 group density on thesurface. It is this aspect that polyacrylic and polyurethane supporting structures have beenable to satisfy, the longest chains producing the lowest surface energies (Figure 10.69). Theresults shown in Table 10.49 show how an increase in the perfluorinated segment lengthgradually enhances the resistance to oils and, to a lesser extent, to water. It is theseconsiderations that have led to the preferred use of perfluoro-octyl or longer perfluorinated

chpt10(2).pmd 15/11/02, 15:44732

733

1 2 4 6 12108

(x+1) in acrylic polymer

10

15

5

25

20

Sur

face

ene

rgy/

mN

m–1

Figure 10.69 Change in critical surface energy with length of the perfluoroalkyl groups in acrylicpolymers [502]

Table 10.49 Oil and water repellency of cotton fabrics treated withperfluorinated acrylic polymers [502]

Acrylic polymer 1% Polymer applied to printed cotton

CH2 CH

CO O

n

RCH2

Perfluorinated Oil repellency Spray testgroup R (AATCC 118) (ISO 4920)

–CF3 0 50–CF2CF3 3–4 70–(CF2)2CF3 6–7 70–(CF2)4CF3 7–8 70–(CF2)6CF3 7–8 70–(CF2)8CF3 8 80

segments. Silicone emulsions, by comparison, typically give a surface tension of 25 mN/mand hence act only as water repellents.


A rather more novel yet logical development in fluorochemicals has been the emergence offluoro-silicone hybrid polymers [503,504]. A series of products (10.251–10.253) has beenextensively evaluated [503] for various properties, including liquid and solid surface energies,micelle formation, wetting, contact angle, film-release and antifoam behaviour. A similar typeof product is tridecafluoro-octyltriethoxysilane (10.254). This has been applied to polyesterand to cotton fabrics, fixation being achieved by drying at ambient temperature [504].

chpt10(2).pmd 15/11/02, 15:44733


For application of these fluorochemical finishes to textile fabrics, an extremely importantfactor is their formulation into suitable aqueous emulsions or dispersions. The quality of theformulation has a critical influence on stability during storage and application, as well as theefficacy of treatment and durability [501,502]. In particular, the choice of surfactant(s) foremulsifying or dispersing must ensure good stability with freedom from deposition on rollers,yet must not impair the water and oil repellency of the finished fabric. No individual productfulfils all requirements; hence specifically formulated products are available for certain fibretypes.

Application is mainly by padding followed by curing at 150–180 °C, although minimumadd-on techniques such as slop padding, spraying and foam application have been successful.They can also be applied by discontinuous methods, such as exhaust or dip-spin [501].

These fluoropolymers are also used as the basis of so-called stain-blocking treatments,applied especially to nylon floorcovering and upholstery [505–508]. In general, thefluorochemical is used in conjunction with an anionic syntan resist agent of the typedescribed in section 10.9.4. The latter functions by blocking the cationic protonated aminesorption sites in nylon. Thus the fluoropolymer repels oil-based soils and facilitates theirremoval during cleaning, whilst the syntan inhibits electrostatic interaction between thecationic sites and many coloured anionic substances in food, drinks and human/animalexcreta. The two product types may be applied later in the dyehouse, in which case the

R1 Si O

CH3

CH3

Si CH2CH2(OCH2CH2)x

CH3

R2

OR3

CH2CH2CF3

CH2CH2CF3

CH2CH2CF3

CH2CH2CF3

CH3

CH3

CH2CH2CF3

CH2CH2CF3

CH2CH2(CF2)3CF3

CH2CH2(CF2)3CF3

CH3

CH3

H

H

H

H

CH3

CH3

R2R1 R3

2 10.253

x

7

12

7

12

7

12

F3C(CF2)5CH2CH2 Si OCH2CH3

OCH2CH3

OCH2CH310.254

Si O

CH3

R n

10.251

R O Si O

O

RCH2CH2CF3

CH2CH2(CF2)3CF310.252

R = or

chpt10(2).pmd 15/11/02, 15:44734

735

effect is less durable. For maximum efficacy the two component types must be carefullychosen after much empirical screening. Fluorochemicals having perfluoroalkyl groupscontaining 10–12 carbon atoms and yielding an overall concentration level of 200–800 ppmof fluorine on the weight of fibre appear to be optimal [508]. The syntans are typicallyformaldehyde–phenolic condensation products; one product has the structure 10.255, whichis interesting in that all the hydroxy groups have been converted into alkylaryl ethers [508].

Polymers of methacrylic acid or maleic acid, either alone or as a blend or copolymer withthe sulphonated aryl–formaldehyde condensation products, have also been evaluated asstain-blocking chemicals [508,509]. An interesting development is the use of a polystyrene–maleic acid copolymer, this being unusual because of the absence of sulphonic acid groups[508,510]. Although the maleic and methacrylic acid polymers do not have the durability ofthe conventional syntans, they have the advantage that they are non-yellowing.

Although stain-blocking treatments were originally developed for nylon, there has been agood deal of emphasis over the last decade on extending their use to wool carpets [511–515]. Whilst syntans similar to those used on nylon are also suitable for wool, largeramounts are required to block the greater number of dye sites in wool [512].

Environmental aspects of fluorochemical finishing agents

In certain circumstances, organofluorine compounds can lead to the generation of AOXvalues, although a satisfactory method of measuring specific AOF values has yet to bedeveloped [516]. Typical results of the environmental analysis of twelve fluorochemicals areshown in Table 10.50.

10.10.5 Bactericidal and insecticidal agents

There are three areas to consider:(1) The use of insecticidal agents on wool to prevent attack by moth and beetle larvae.(2) The use of bactericides to prevent biodegradation of chemicals such as thickening

agents.(3) The use of bactericides to inhibit bacterial activity on textiles.

Insecticidal agents for wool

Lists of the principal types of insect that lay their eggs in wool are available [11,517].Damage to the fibre is caused by the larvae that emerge from these eggs. Hence any

CH3(CH2)x O S

O

O

O

CH2

(CH2)yCH3

NaO3S CH2OCH3

O (CH2)zCH3

10.255 x, y, z = 1–5


chpt10(2).pmd 15/11/02, 15:44735


C

C

C

C

C

ClCl

CH

CH

Cl

Cl

Cl

Cl

CH

CH

CH

CH

O CH2

10.256

Dieldrin

HN

Cl

O

NaO3S

Cl

CNH

O Cl

Cl

10.257

Sulcofenuron

insecticide used must be effective against these larvae. Since these products have noinsecticidal effect on insects that do not consume the wool, it seems likely that suchproducts act only through the digestive tract of the insect larvae. In addition to health andsafety considerations, fastness properties also need to be taken into account. Fastnessrequirements on carpets are not so stringent as on machine-washable wool. Environmental,as well as health and safety factors, have resulted in an almost total ban on the use ofdieldrin (10.256), the first compound to be used for this purpose. It proved to be toxic toman, animals, fish and birds, and was highly persistent in the environment.

Table 10.50 Environmental analysis of fluorochemical agents at a product concentration of1 g/l [516]

BacterialProduct Solids AOX value COD value BOD5 value toxicityno. content (%) (mg/l) (mg/l) (mg/l) (% resistance)

1 21.2 0.18 275 140 26 2 40.9 5.6 620 15 18 3 20.3 0.18 250 60 21 4 18.7 <0.05 345 30 66 5 5.3 1.3 305 160 6 6 17.0 <0.05 300 55 23 7 18.5 0.10 510 45 20 8 18.0 0.06 365 90 29 9 32.3 19.0 805 335 2710 10.1 0.08 220 5 4211 23.4 <0.05 425 55 5012 31.2 0.06 355 85 54

Environmental factors coupled with the relatively small size of the market are actingrestrictively against several other products that were formerly used. In an excellent review ofthis subject [517], the compounds that have found use are divided into two categories. Thefirst category comprises those compounds that were developed specifically as woolmothproofing agents, most of these being anionic multichlorinated aryl compounds; some ofthese, for the reasons cited above, are no longer available. One of the best known issulcofenuron (10.257). The sulpho group confers water solubility and exhaustion behavioursimilar to those of an acid dye. This product is relatively expensive but has very good

chpt10(2).pmd 15/11/02, 15:44736

737

fastness to washing and light. Sulcofenuron, up to a concentration of 4 g/kg wool, is one ofonly three types of insect-resist agent permitted for the GuT scheme of ecolabelling [518].

Chlorphenylid (10.258) has also provided insect-resist treatments but AOX generationduring manufacture has led to the withdrawal of several such products since 1989. Thechloromethylsulphonamido group requires at least pH 10 for aqueous solubility as thesodium salt. On acidification during application a dispersion of the free sulphonamide isformed and this is absorbed mainly through weak nonpolar interactions and hydrogen bonds.Combined chlorphenylid/sulcofenuron and chlorphenylid/flucofenuron (10.259) productswere formerly used but have been withdrawn since 1989. Chloromethylsulphonamido-trichlorobenzene (10.260) has also been withdrawn since that date. Thus of the productsdeveloped specifically as insect-resist agents for wool only the sulcofenuron type remains inany significant use.

O

Cl Cl

ClCl

NH

O2S

Cl

CH2Cl10.258

Chlorphenylid

HNC

NH

O

ClCl

F3C CF310.259

Flucofenuron

Cl

Cl

NHSO2CH2Cl

Cl 10.260

O

CH C

O

HC CH

C

H3C CH3

CH C

Cl

Cl

F

NC

O

10.262

Cyfluthrin


Compounds in the second group were originally developed as pesticides for agricultural use.These products have proved efficaceous and amenable to formulation as wool insect-resistagents. Most of them are pyrethroids, including permethrin (10.261), cyfluthrin (10.262) andcyhalothrin (10.263). A further effective compound is the hexahydropyrimidine derivative10.264. Permethrin and cyfluthrin (together up to 210 mg/kg wool) are the other two types ofinsect-resist agent permitted for the GuT ecolabelling scheme [518].

O

CH2 O C

O

HC CH

C

H3C CH3

CH C

Cl

Cl

10.261

Permethrin

chpt10(2).pmd 15/11/02, 15:44737


These synthetic pyrethroids mimic natural counterparts, of which the most important ispyrethrin 1 (10.265). Unfortunately, the natural products lack the photochemical andhydrolytic stability necessary for use as wool insect-resist agents. The synthetic productshave the required stability, yet retain the low mammalian toxicity and low environmentalretention of the natural products. Permethrin, however, is toxic to aquatic life and istherefore subject to increasingly severe discharge limits. There is some evidence thatpermethrin is less effective against larvae of a certain beetle. This can be compensated for byusing a combination of permethrin with the hexahydropyrimidine derivative 10.264. Somepossible alternative pyrethroids have been mentioned [517] as development products(10.266–10.269).

O

CH C

O

HC CH

C

H3C CH3

CH C

Cl

CF3

NC

O

10.263

CyhalothrinCl NH

Cl

C

O N

N

O

O

O

CH3

CH310.264

CH3

O

OH2C CH CH CH CH2 C

O

HC CH

CH3H3C

CH C

Cl

Cl

10.265

Pyrethrin I

O

CH C

O

HC CH

C

H3C CH3

CH

NC

O

C

Br

Br

10.266

DeltamethrinO

CH C

O

CH

NC

O

CH

Cl

H3C CH3

10.267

Fenvalerate

chpt10(2).pmd 15/11/02, 15:44738

739

Other chemicals evaluated but not yet adopted commercially include organophosphoruscompounds, triphenyltin compounds, quaternary ammonium salts, imidazoles, benzi-midazoles, carbamates and the precocene anti-juvenile hormones [517]. Although none ofthe above has found use as an insect-resist agent, several have been used as antimicrobialagents for textiles.

Vinylsulphone fibre-reactive, insect-resist agents have been described, in which theinsecticidal moiety is an organophosphorus grouping (10.270; X = O or S). Thevinylsulphone group, by virtue of its nucleophilic addition reactions with wool keratin,confers excellent fastness. An interesting feature of these products is that they do not act asinsecticides on wool until they become activated by hydrolysis of the ester bond duringdigestive processes within the insect.

A development reported recently [519] involves reduction of the cystine disulphidebonds in wool with either thioglycolic acid or tetrakis(hydroxymethyl)phosphonium chlorideto form thiol groups, followed by crosslinking with bifunctional reactive dyes. This gaveimproved insect resistance but had adverse effects on physical properties such as strength,shrinkage and stiffness, thus limiting the potential of the process for commercial use.

Despite these interesting developments, it has been pointed out [11,517] that because ofthe relatively small market and the costs of registration and ecotoxicological testing, it iscurrently unlikely that novel agents designed specifically for wool could be marketedeconomically. Any further advances are likely to be spin-offs from agricultural pesticidedevelopments.

Application of these agents is best carried out from the dyebath to achieve the highestfastness ratings. This may not always be possible, however, and alternative stages (duringscouring or application of spinning lubricants) are available [11,517]. Particular care is

O

CH C

O

HC C

C

H3C CH3

NC

O

CH3

CH3

10.268

Fenpropathrin

O

CH C

O

HC C

C

H3C CH3

NC

O

CH C

CH3

CH3

10.269

Cyphenothrin

H2C CHSO2CH2CH2

C O

O

CH2CH2S O P X

OCH2CH3

SCH2CH2CH3

10.270X = O or S


chpt10(2).pmd 15/11/02, 15:44739


necessary in the choice and application level of insect-resist agents when applying them tofibre blends, since their partition behaviour between the component fibres varies.Pyrethroids, for example, tend to partition in favour of nylon in wool/nylon blends [520]. Itis not surprising, given their aquatic toxicity, that these agents are under continualenvironmental scrutiny [521]. In order to comply with minimum discharge requirements, itis obviously helpful to be able to apply the minimum levels needed for adequate functionalityand this has led to the development of appropriate machinery and methods [522–524].

Bactericides for addition to fibres and other polymers

Bactericides can be added to microbially nutritious polymers, such as certain size polymers,dispersing agents and thickening agents, in order to protect them against biodegradation.For the same reason, they may be incorporated into man-made fibres for geotextiles andawnings. They may be applied to medical fabrics, hosiery, underwear and sports clothing forreasons of hygiene, in order to prevent infection, promote healing or prevent thedevelopment of odours. A comprehensive index is available [525]. Although this indexcovers all uses of antimicrobials, there is a section devoted to agents for textiles, in whichthe following are listed:– Ammonium zirconium carbonate– 1-Capryl-2-hydroxyethylimidazoline (10.271)– Cis-N-[(trichloromethyl)thio]-4-cyclohexene-1,2-dicarboximide (10.272; Captan)– 2,2′-Dihydroxy-5,5′-1-dichlorodiphenylmethane (10.273; Dichlorophene)– Diiodomethyl-p-tolylsulphone (10.274)– Dimethylaminopropylricinoleamidobenzyl chloride (10.275)– Lauryl/stearyltrimethylammonium bromide/chloride (10.276)– Myristylamine (10.277)– Sodium 2-mercaptobenzothiazole (10.278)– 2,4,4′-Trichloro-2′-hydroxydiphenyl ether (10.279; Triclosan)– Zinc 2-pyridinethiol-1-oxide (10.280; Zinc pyrithone).

N

N

CH2CH2OH

C (CH2)8CH3

O

10.271

C

N

C

O

O

S C Cl

Cl

Cl

10.272

Captan

CH2

OH

Cl

Cl

HO10.273

Dichlorophene

H3C SO2 CH

I

I

10.274

Cl CH2 NH

C (CH2)7CH

O

CHCH2CH(CH2)5CH3

OCH2CH2CH2NCH3

CH310.275

chpt10(2).pmd 15/11/02, 15:44740

741

The environmental implications of adding biocides to polymers must always be borne inmind since, by definition, all effective biocides are more or less toxic. Thus addition of abiocide may render a normally biodegradable thickener less so. Hence biocides should beused at as low a concentration as possible. Natural size polymers, thickening agents anddispersing agents are particularly attractive targets for microbial attack, but degradation bythis route is more rapid wet than dry. Thus printing pastes and liquid disperse dyes aresensitive targets. In the case of printing pastes, biological degradation on storage can lead toa significant loss of viscosity and rheological malfunction. Sensitive thickening agents, forexample, can be protected by incorporating a biocide, usually at less than 0.1%, which is justabout sufficient for effectiveness [383].

It has been suggested [383] that manufacturers of thickening agents will cease toincorporate preservatives in their products. The printer will then be responsible for selectingand using, in just the required amount, a preservative that is still just tolerable under theconditions of use. Formaldehyde has been widely used for this purpose but is nowecologically undesirable. Phenolic compounds such as sodium pentachlorophenate, o-phenylphenate or chloro-m-cresol have also been used. Such nucleophilic compounds canadversely affect the yield and hue of certain reactive dyes [390]. If absolutely minimalquantities are used, just sufficient for the required bacterial efficiency, it is possible for theseresidues to be washed out with much water to give extremely dilute waste waters. Thesetraces no longer exert significant bacterial action and may even be biodegradable underthese conditions.

Interest in the application of biocides to textiles has increased in recent years. They may insome cases be applied during manufacture of the fibre (in melt spinning) or as a finishingtreatment. Although numerous papers on this subject have been published, many areunfortunately of little chemical interest as they disclose little other than commercial names.Poly(ethylene glycol) (HO[CH2CH2O]nH) crosslinked with dimethyloldihydroxyethyleneureahas been reported to give fabrics with antibacterial properties suitable for nonwoven protectivesurgical apparel [526].

Triclosan (10.279) kills a wide range of bacteria that cause food poisoning, dysentery,cholera, pneumonia, tetanus, meningitis, tuberculosis and sore throats. It also prevents thedevelopment of bacterially related odours and kills the yeasts responsible for candida ulcers

CH3

N

CH3

CH3R

X

10.276

R = lauryl / stearylX = bromide / chloride

+

_

CH3(CH2)13NH2

10.277

MyristylamineS

C

N

S Na

10.278

+_

Cl O Cl

Cl

OH

10.279

Triclosan

N O

S

N O

SZn2+

10.280

__

Zinc pyrithone


chpt10(2).pmd 15/11/02, 15:44741


and athlete’s foot. This compound can be incorporated during fibre manufacture to givedurable antibacterial properties [527–529]. Despite the widespread use of Triclosan intoothpastes and acne creams, it is reported that it can cause allergic dermatitis in susceptibleindividuals, especially when used in products for the feet [525].

Poly(hexamethylenebiguanide hydrochloride) (10.281) has been used in the sanitisationof swimming pools. For textiles, it has been formulated into a finish capable of providing arange of antibacterial fabrics from medical products to odour-free socks [530,531]. Thisagent is mainly of interest for cotton; the polymeric cationic structure exhibits highsubstantivity for the negatively charged surface of the fibre. Application is by padding,optimally at pH 7–8, and the performance can be improved by subsequent wet-on-wetpadding with an anionic fixing agent. No elevated curing temperature is required andapproximately 1% of the antimicrobial agent on the weight of cotton is optimal forbactericidal performance. The product has a long history of use as a bactericide, exhibits lowtoxicity and is environmentally acceptable, being bioeliminated by adsorption.

CH2CH2CH2CH2CH2CH2 NH C NH

NH

C NH

+NH2 Cln

10.281

average n = 12_

Novel bis-quaternary compounds have been reported for improving the microbialresistance of wool [532]. These products are described as a new class of bis-quaternaryammonium surfactants known as gemini quaternary ammonium compounds or bis-quats.They consist of two saturated hydrocarbon chains and a complex polar group consisting oftwo quaternary ammonium salts linked through an alkane spacer chain containing amideand optional disulphide bonds. Bacterial efficacy on wool was confirmed for two products:N,N′-bis(N-dodecyl-N,N-dimethylglycine)-1,4-diaminobutane dihydrochloride (10.282)N,N′-bis(N-dodecyl-N,N-dimethylglycine)cystamine dihydrochloride (10.283). Theseagents were applied at 0.0025–0.5% on the weight of wool by exhaustion from an aqueoussolution at 40 °C.

NHCH2

H3C

H3C

C

O

N

H3C(CH2)11

CH2CH2CH2CH2 N

(CH2)11CH3

C

O

CH2NHCH3

CH3Cl Cl10.282

+ +

_ _

NHCH2

H3C

H3C

C

O

N

H3C(CH2)11

CH2CH2

(CH2)11CH3

C

O

CH2NHCH3

CH3

SS CH2CH2 N

Cl Cl10.283

+ +

__

chpt10(2).pmd 15/11/02, 15:44742

743

The bacterial resistance of silk fibroin can be improved by treatment at low pH values inaqueous solutions of metal ions and at high pH values in solutions of metal–aminecomplexes, using untreated silk or silk pretreated with an aqueous solution of tannic acid(10.183). Suitable metals include Cu, Zn, Ni, Fe and Ag [533]. It is to be expected thattreatment with such metal compounds will be subject to restrictions in environmentallysensitive areas.

Antimicrobial agents may adversely affect the light fastness of nylon dyeings or causeyellowing of the fibre. The six antimicrobial treatments listed in Table 10.51 have been

Table 10.51 Comparison of antimicrobial finishes on nylon [534]

Finish Chemical class(es) Structure(s)

1 Silicone quaternary ammonium salt 10.2842 Silicone quaternary ammonium salt 10.2853 Organo-tin 10.2864 Phenolics (mixture) 10.273/10.2875 Phenolic and organo-tin 10.279/10.2886 Organo-tin and quaternary ammonium salt 10.288/10.289

H3C N CH2CH2CH2

(CH2)17CH3

CH3

Si OCH3

OCH3

OCH3

Cl

10.284

+

_

H3C N CH2CH2CH2

(CH2)9CH3

(CH2)9CH3

Si OCH3

OCH3

OCH3

10.285

Cl

+

_

H3C(CH2)3 Sn H

(CH2)3

(CH2)3

(CH2)3CH3

Sn

(CH2)3CH3

(CH2)3CH3HO C

O

CH CH

C

O

O

10.286

+_ _+

H3C

H3C

Cl O

OH

Cl Cl

10.287

H3C(CH2)3 Sn O

(CH2)3

(CH2)3

Sn (CH2)3CH3

(CH2)3CH3

(CH2)3CH3

10.288

H3C

H3C

R N CH3

CH2

CH3 Cl

10.289

R = n-alkyl_

+


chpt10(2).pmd 15/11/02, 15:44743


evaluated in relation to these two factors, each being applied at the manufacturer’srecommended concentration [534]. The effect on light fastness varied with each dye andonly a few dyes were studied; some dyes were more sensitive to specific antimicrobialfinishes. Overall, Finish 3 had the smallest effect on light fastness, followed by Finish 5.Finishes 6 and 4 gave the greatest reduction in light fastness, Finish 4 also causingappreciable discoloration of the undyed fibres.

10.11 FOAMING AND DEFOAMING AGENTS

10.11.1 Foaming agents

The idea of using a matrix of foam to transfer chemicals and colorants to textiles had itsorigin in the growing need to conserve thermal energy and water in the aftermath of the so-called oil crisis and recession of the early 1970s. In a sense, it was an antidote to the feverishwork carried out in the late 1960s on solvent dyeing. Foam dyeing began with an elegantshort-liquor (about 1.5:1) dyeing process developed in 1972 mainly for garment dyeing inrotary machines. The application of foams to textiles has been widely investigatedsubsequently, although the degree of commercial acceptance has proved limited.Nevertheless, potential exists for the foam-based application of lubricants, stiffening agents,waxes, size polymers, mercerising liquors, durable press resins, water/oil repellents, softeningagents, shrink-resist resins, dyes and print pastes. Although continuous dyeing and printing,mainly of carpeting, has attained significant commercial use, most foam processing isconfined to the application of finishes where concentration tolerances and evenness ofapplication are much less critical. A major account of the properties of foams and theirgeneral industrial applications includes a chapter devoted to textile applications [535].

Apart from reproducibility and uniformity of application, one of the main problemsassociated with foam application is dissolving or dispersing relatively large quantities of theprincipal and auxiliary agents in a very small volume of water, followed by the difficulty ofmaintaining compatibility of the components and the density and stability of the ‘loaded’foam under such conditions. For example, resin finishing can typically require thedissolution of about 600 g of resin, softener and catalyst in only 400 g of water. Nevertheless,foam processing does offer advantages, notably in the conservation of water and energy andthe reduction of effluent problems.

The foam matrix used in textile wet processing is a stabilised air-in-water system. A foamcannot be made with pure water, however; a foaming agent, usually a surfactant, is neededto give a reasonably stable honeycomb matrix of air cells, each enclosed by a thinviscoelastic film of liquid. Clearly, a reduction in surface tension is one important factor inthe creation of foam; others include the elasticity and viscosity of the film walls between thebubbles, and the size and uniformity of the bubbles themselves. Drainage, by gravity, of theliquid from the film walls leads to instability of the foam and tends to a maximum with largerspherical bubbles. In a system consisting of variously sized bubbles, the smaller ones tend tocoalesce into the larger, which are thus further increased in size and become less stablebecause of the increased propensity for liquid drainage. Thus, for maximum stability, thebubbles should be as small and uniform as possible, leading to minimum diffusion of air frombubble to bubble, and maximum entropic and electrical double layer repulsion. The idealstate is not attainable in practice, however, and all foams are unstable to some degree. Nor is

chpt10(2).pmd 15/11/02, 15:44744

745

perfect foam stability particularly required in textile application, since at some stage duringthe process collapse of the foam is generally desirable to ensure maximum deposition ofchemicals and/or colorants.

The foaming propensity of surfactants generally reaches a maximum at the critical micelleconcentration, beyond which there appears to be little further contribution to foam density.Foam stabilisers are also added in some cases. The two important steps in the foamtreatment of textile materials are generating the foam and applying it to the substrate:(1) Generation is generally by high-speed rotors, with metered air and liquid flows and

monitoring to control the density of the foam.(2) Application of controlled amounts of foam to the substrate is by knife-on-roller, knife-

on-blanket, floating knife, horizontal pad or furnishing roller with doctor blade, or bysqueegee across a printing screen.

Subsequent collapsing of the foam is generally by collapse onto the fabric (controlled tosome extent by the chemicals used), by vacuum suction of the foam into the fabric, or bymeans of a pad nip.

The most important auxiliary used is, of course, the foaming agent. In theory anysurfactant that will form a stable foam can be used. In practice the choice is usually betweenanionics (generally cheaper), nonionics, or a mixture of both. Consideration must be givento overall compatibility as well as to foaming characteristics: for example, anionic agentsshould generally be avoided when applying cationic products. Long-chain alcoholsulphonates and ethoxylates, as well as sulphates and sulphosuccinates, have been used; atypical selection is given below, the first three being anionic and the others nonionic:– sodium lauryl sulphate– ammonium lauryl sulphate– sodium dioctylsulphosuccinate– lauryl alcohol poly(oxyethylene)– decanol poly(oxyethylene)– tridecanol poly(oxyethylene).

Particularly effective is a mixture of anionic and nonionic agents, such as a mildly anionicsulphated alcohol ethoxylate with a nonionic alcohol ethoxylate. Ideally, foaming agentsshould:– generate consistent foam easily– show optimum and uniform wetting– cover a wide range of wettability so as to be adaptable for different situations– show little or no effect on colour fastness– be compatible with the other components– be biodegradable.

The main function of the foam stabilising agent is to reinforce the intercellular film wall bycontributing rheological characteristics of viscoelasticity. The increased viscosity may alsoassist handling. The aim, as so often with auxiliaries, is to achieve an optimum balance. If thebubbles are too thin and wet too quickly they will collapse prematurely, whilst too stable a filmcould hinder uniform application. Examples of products used as foam stabilisers includethickening agents such as the polysaccharides, hydroxyethylcellulose, methylcellulose,

FOAMING AND DEFOAMING AGENTS

chpt10(2).pmd 15/11/02, 15:44745


carboxymethylcellulose, poly(vinyl alcohol) and poly(acrylic acid), as well as other compoundssuch as sodium tripolyphosphate, sodium hexametaphosphate (detergent ‘builders’) anddecanol. Ideally, foam stabilisers should increase the stability of the foam to the optimumcontrollable level whilst also allowing for subsequent controllable collapse. They should becompatible with the other components and effective at various concentrations, givepseudoplastic solutions, and should not affect the drape and handle of the fabric. Electrolytescan have positive or negative effects on foam stability [535]; for example, a low concentrationof phosphate ions increases the stability of a sodium laurate foam, whereas sodium chloridedecreases the stability.

10.11.2 Defoaming agents

All chemical systems, to varying degree, tend to reduce free energy and foams are noexception. Hence foams are thermodynamically unstable, yet their stability varies fromalmost instantaneous collapse to prolonged persistence. Although foam can be useful, thereare still many circumstances where its presence and persistence is enough of a nuisance tocreate a need for foam-destruction products, known as defoaming agents or antifoams. Anauthoritative reference work on the theory and applications of defoaming is available andthis includes a chapter on applications in textile dyeing [536]. Reference 535 contains achapter on the science and technology of silicone antifoams.

Just as foams are stabilised by decreasing the rate of liquid drainage from the film walls,they can be destabilised by increasing this drainage, resulting in thinning and eventualrupturing of the film. Defoaming agents generally effect this by two mechanisms, the basicobjective being to displace foam-stabilising substances from the liquid-air interface [537]:(1) Spreading alone is sufficient with light foams of high blow ratio (those having a high

air-liquid ratio); in this case surfactants of relatively low surface tension (i.e. powerfulsurfactants) will spread over the large surface area of the intercellular film and displacethe surfactants that are tending to stabilise the foam.

(2) Denser foams of low blow ratio additionally require penetration of the thicker aqueousfilm by the defoaming agent. Such a defoamer consists of an emulsified hydrophobicsubstance, which when added to the foaming system disperses fine droplets of insolublehydrophobic material within the liquid lamellar walls, thus entering the liquid-airinterface, aided to some extent by the solubilising action of the foaming agent(s). Thiscreates a weak link as a result of high interfacial tension, the foam then tending torupture at the interface between defoamer and foamer. In practice, the system is a finelybalanced one requiring careful formulation of the composite defoaming agent.

The main requirement of an effective defoamer [537] is that the agent should be insolublein the foaming system and should have a high rate of spreading. Spreading will be favoured ifthe defoamer has a lower surface tension than that of the foaming system. The interfacialtension between defoamer and foaming system must be high, but not so high as to inhibitspreading. A low degree of attraction between defoamer and foaming system (i.e. a highinterfacial tension) is achieved by nonpolar defoamer systems that do not contributepositively to the surface viscosity of the lamellar walls.

For maximum efficiency the defoamer should be added to the system as soon as foaming

chpt10(2).pmd 15/11/02, 15:44746

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becomes troublesome, since its active life-span is inevitably limited by the system’sthermodynamic instability. Its principal action is on the liquid-air interface; therefore there islittle point in adding it before sufficient liquid-air interfaces are formed. Secondly, theinsoluble active ingredient of the defoamer, given the nature of the system in which it isworking, must eventually become more or less solubilised, or at least emulsified, into the bulkliquid system, thus losing its activity and in some circumstances actually promoting foaming.

Defoamers are generally anionic or nonionic systems and fall into two groups. The firstgroup consists of water-soluble surfactants with polar and nonpolar moieties. Thesecompounds are effective only over a narrow range of conditions, functioning simply asspreading agents, and are seldom used alone. Such surfactants are readily absorbed into thebulk of the foaming system where, not surprisingly, they contribute to the foaming problem.This system is more frequently used, however, as the vehicular or ‘carrier’ basis of the secondgroup of defoamers, which are much more widely used. These more active defoamers areemulsions of water-insoluble silicones or organic-based compounds of low volatility and highspreading power.

The general requirements for an ideal defoamer can be summarised [536]:– eliminates existing foam as well as preventing further foam from forming– easy to disperse in the dyebath– does not react or interact with dyes or auxiliaries present in the bath– chemically stable under dyeing conditions– no deposition on fabric or machinery causing spotting or staining of the fabric– no colour– no odour– stable to storage– safe both to humans and to the environment.

The active organic-based defoamers include:(1) Fatty acids, their glycerides and other esters, including fats, waxes and oils such as

mineral and vegetable oils; fatty alkylamines and acylamides. Alkaline earth metal oraluminium salts of fatty acids tend to leave deposits on machinery [536].

(2) Higher alkanols, including the isomeric octyl alcohols (2-octanol and 2-ethylhexanol),cyclohexanol, lauryl and cetyl alcohols. Aliphatic alcohols have relatively poor foamcontrol and have an odour that can be nauseous [536].

(3) Polyglycols, especially poly(propylene-1,2- or -1,3-glycol). Poly(oxyethylene) andpoly(oxypropylene) block copolymers have relatively poor foam control [536].

(4) Insoluble alkyl esters of phosphoric acid, especially tributyl phosphate. These phosphateesters have relatively poor foam control [536].

Certain limitations of organic defoamers can be minimised by judicious formulation ofmixtures. The following system is said to overcome some of the drawbacks associated withaluminium salts of fatty acids [536]:– 87% Paraffin oil– 6% 2-Ethyl-n-hexanol– 4% Aluminium distearate– 3% Phosphoric acid esterified with polyethoxylated p-nonylphenol.


chpt10(2).pmd 15/11/02, 15:44747


In the case of an aliphatic alcohol such as iso-octanol, a similar level of defoaming activitytogether with a much less offensive odour can be achieved by using a propoxylated ester of abranched aliphatic acid, such as propylene-1,2-glycol mononeodecanoate (10.290).

H3CCH2CH2CH2CH2CH2 C C

CH3

CH3O

O

CH2 CH

CH3

OH10.290

The active ingredients in silicone-based defoamers have traditionally been a poly(alkylsiloxane), especially poly(dimethyl siloxane) (10.226), and silica (SiO2); the latter may bechemically bonded to the polysiloxane to render its surface hydrophobic. Some ‘spotting’problems have been experienced with these defoamers owing to incompatibility of theantifoam emulsion with certain dye dispersions, especially at high rates of shear in the high-temperature dyeing of polyester with disperse dyes. This created a poor reputation for theearly silicone antifoams in jet dyeing. The problems arose by destabilisation of the emulsion,resulting in cracking out of hydrophobic components which caused staining or spotting ofthe fabric, together with a drastic loss of defoaming action. The poly(dimethyl siloxane)products exert little foam suppression power alone. Their foam inhibition properties onlybecome fully developed when combined with finely divided, hydrophobic silica particles[538].

Further improvements have been made to the emulsion system and to the siliconecomponents themselves. Improved derivatives of poly(dimethyl siloxane) include [537] blockcopolymers with poly(oxyethylene) and poly(oxypropylene) segments represented schematic-ally by structure 10.291. The solubility and other characteristics of these alkoxylated silicones(silicone polyglycols) can be adjusted by varying the proportions of dimethyl siloxane andoxyalkylene units. A specific advantage claimed for these compounds in high-temperaturedyeing is their inverse solubility, analogous to the cloud point effect of nonionic surfactants. Asthe dyebath temperature approaches its maximum the solubility of the defoamer decreases,thus helping to maintain its effectiveness, whilst the increase in solubility on cooling aftercompletion of the dyeing is claimed to overcome potential problems of subsequent spotting.The four most commonly used alkoxylated silicones are represented by structures10.292–10.295 [536], where R is typically a methyl group, a is typically 1 and G is typicallyrepresented by formulae such as –(CH2)3(OCH2CH2)x(OCH2CH2CH2)yOR or–(OCH2CH2)x(OCH2CH2CH2)yOR, where R is an end-capping group such as methyl.

A more recent development is the use of hybrid fluorochemical silicones, such asnonafluorohexyl-substituted siloxanes of the type represented by structure 10.296 [503].

CH3 Si O

CH3

CH3

Si O

CH3

CH3

Si O

CH3

Si CH3

CH3

CH3

R1(CH2CH2O)a(CH2CH2CH2O)b R2x y

10.291

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749

These copolymers have been mentioned already in section 10.10.4 as versatile and highlyeffective stain-resist, oil- and water-repellent finishing agents.

Antifoams are generally supplied for textile use as carefully formulated, relatively diluteaqueous emulsions; this ensures that the hydrophobic phase is uniformly distributed withinthe foaming system and also helps to safeguard against overdosing, with the attendantdanger of spotting. A typical emulsion generally contains emulsifying agent(s) andthickening agent(s) in addition to water and the insoluble hydrophobic defoaming agent.For emulsification, the most common system [537] is a combination of a low-HLB surfactantwith one of high HLB value, such as glyceryl monostearate or sorbitan monostearate withpoly(ethylene glycol) monostearate. Obviously, the surfactants selected need to be low-foaming types and should provide an optimum level of emulsification, since over-emulsification will tend to negate the activity of the defoamer by inhibiting its interactionwith the foaming system. The size of the defoamer droplets, largely determined by theemulsification system and the degree of comminution during manufacture, is critical inrelation to the efficacy of the product; too small a size gives inadequate activity and if it istoo large the stability of the emulsion is adversely affected. The optimum droplet sizegenerally appears to be in the range 2 to 50 µm [537].

The function of the thickening agent is to increase the viscosity and so contribute to thestability of the product, the aim again being to attain an optimum level of viscosity.Thickening agents that do not gel at the high temperatures used in textile processing areessential; hydroxyethylcellulose, alginates and synthetic poly(acrylic acid) derivatives may beused. A small amount of a bactericide such as methyl p-hydroxybenzoate is often added tosafeguard against biological degradation during storage, particularly in the case of naturalthickening agents.

CH3 Si O

CH3

CH3

Si O

CH3

G

Si O

CH3

Si CH3

CH3

CH3CH3m n

10.292

G Si O

CH3

CH3

Si O

CH3

G

Si O

CH3

Si G

CH3

CH3CH3m n

10.293

Ra Si O Si O

CH3

CH3

Si O

CH3

G

Si CH3

CH3

CH3

n

m

4–a

10.294

Ra Si O Si O

CH3

CH3

Si O

CH3

G

Si G

CH3

CH3

n

m

4–a

10.295

Si O

CH3

CH2CH2CF2CF2CF2CF3

n10.296


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Typical formulations of commercial composite antifoams have been detailed [536,537].There are many products on the market but evaluation of their relative efficacy depends onthe foaming problems to be overcome. Not only does the chemical type of the activedefoamer have to be considered, but its state within the emulsion and the intrinsicproperties of the emulsion are also of crucial importance. Methods of evaluating defoamershave been described [539,540].

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123; 34 (1997) 207.420. P Y Wang, Y P Chen and P Z Yang, Dyes and Pigments, 30 (1996) 141.421. Ciba, German P, DE 19 613 671 (1996).422. Ciba, German P, DE 19 613 251 (1996).423. F M Rowe and K A J Chamberlain, J.S.D.C., 53 (1937) 268.424. V S Salvin, W D Paist and W J Myles, Amer. Dyestuff Rep., 41 (1952) 297.425. V S Salvin and R A Walker, Text. Res. J., 30 (1960) 381.426. V S Salvin, Amer. Dyestuff Rep., 53 (Jan 1964) 12.427. A Murray and K Mortimer, Rev. Prog. Coloration, 2 (1971) 67.428. J Bowles, A Püntener and J R Aspland, Text. Chem. Colorist, 26 (Mar 1994) 17.429. J C Haylock and J L Rush, Text. Res. J., 46 (1976) 1; 48 (1978) 143.430. J Shore, J.S.D.C., 87 (1971) 3, 37.431. S M Burkinshaw, K D Maseka, D J Marfell and R Gordon, AATCC Internat. Conf. & Exhib. (Oct 1994) 151.432. S M Burkinshaw and K D Maseka, Dyes and Pigments, 30 (1996) 21.433. R S Blackburn and S M Burkinshaw, J.S.D.C., 114 (1998) 77; 115 (1999) 102.434. S M Burkinshaw and N Nikolaides, Dyes and Pigments, 15, No 3 (1991) 225; 16, No 4 (1991) 299.435. D M Lewis and Y C Ho, Dyes and Pigments, 30 (1996) 301; 31 (1996) 111.436. X P Lei, D M Lewis and Y N Wang, J.S.D.C., 108 (1992) 383.437. D M Lewis, Y N Wang and X P Lei, J.S.D.C., 111 (1995) 12.438. X P Lei, D M Lewis and Y N Wang, J.S.D.C., 111 (1995) 385.439. B D Jeon, M T Pailthorpe and S K David, Dyes and Pigments, 20, No 2 (1992) 109.440. J Haarer and H Höcker, Text. Res. J., 64 (1994) 480.441. B D Jeon, M T Pailthorpe and S K David, Dyes and Pigments, 19, No 2 (1992) 99.442. B A Cameron and M T Pailthorpe, Text. Res. J., 57 (1987) 619.443. 1 N Supriyatna and S K David, Dyes and Pigments, 18, No 4 (1992) 297.444. G Lützel, J.S.D.C., 82 (1966) 293.445. M Kamel, M M Kamel and M A El-Kashouti, Amer. Dyestuff Rep., 60 (Mar 1971) 33, (Apr 1971) 44.446. T Robinson, Melliand Textilber., 68 (1987) 137, E61.

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447. J Hook and A C Welham, Dyer, 172 (Aug 1987) 10.448. H Fischer, Textilverediung, 25 (1990) 54.449. N Bhattacharya, B A Doshi, A S Sahasrabudhe and P R Mistry, Amer. Dyestuff Rep., 79 (Mar 1990) 24.450. S R Shukla and M Mathur, Indian Text. J. 150 (Dec 1994) 66.451. V Golab, S Jeler and J Donlagic, IFATCC Congress (1996) 374.452. Y Yiqi and E F Carman, Amer. Dyestuff Rep., 85 (Oct 1996) 39.453. R Raikar, M K K Kaimal, A V Afini and S Varadarajan, BTRA Scan, 27 (Feb 1996) 1.454. S M Burkinshaw and G W Collins, J.S.D.C., 114 (1998) 12.455. R S Blackburn, S M Burkinshaw and G W Collins, J.S.D.C., 114 (1998) 317.456. S M Burkinshaw and G W Collins, Dyes and Pigments, 29 (1995) 323; 33 (1997) 1.457. S M Burkinshaw, G W Collins and R Gordon, AATCC Internat. Conf. & Exhib. (Oct 1995) 169; (Oct 1996) 296.458. S M Burkinshaw, F E Chaccour and A Gotsopoulos, Dyes and Pigments, 34 (1997) 227.459. C D Taylor, H A Anderson and D L Brydon, J. Text. Inst., 85 (1994) 35, 44, 49.460. R P Walker and W S Perkins, Text. Res. J., 55 (1985) 667.461. D M Hall, E C Mora and B M Callaway, Text. Res. J., 57 (1987) 614.462. M D Strauss and D A Pettey, Text. Asia, 18 (May 1987) 47.463. H L Thomas, P N Brock and G W Moore, Amer. Dyestuff Rep., 86 (Aug 1997) 19.464. L Barringer, Amer. Dyestuff Rep., 83 (Sep 1994) 68.465. R Niestagge, Text. Horizons Supplement, 10 (Oct 1990) 7.466. I Müllejans, K Schäfer and H Höcker, Textilveredlung, 31 (May–Jun 1996) 100.467. M S Elliott and D Whittlestone, J.S.D.C., 110 (1994) 266.468. W Dohrn, K Winck, O Singendonk, H Ulmer and W Pölzl, Melliand Textilber., 77 (1996) 770, El70.469. W Dohrn and K Winck, Melliand Textilber., 79 (1998) 130, E28.470. R Gutmann, Chemical Fibers Internat., 46 (1996) 123.471. V K Joshi, Man-made Textiles in India, 39 (1996) 245.472. E Rybicki and B Mielicka, Fibres and Textiles in Eastern Europe, 4 (Jul–Dec 1996) 112.473. H S Park, S J Kim and H W Rhee, Sen-i Gakkaishi, 53(1997) 249.474. Anon, High Performance Textiles (Nov 1996) 5.475. K Sen, P Bajaj and S Rameshbapu, Melliand Textilber., 72 (1991) 1034, E416.476. S B Sello and C V Stevens, in Handbook of fibre science and technology; Vol. II, Part B, Chemical processing of fibres

and fabrics, functional finishes, Ed. A M Lewin and S B Sello (New York: Marcel Dekker, 1984).477. W Löbel, Melliand Textilber., 76 (1995) 916, E227.478. H V Patel, SDCANZ 13th Internat. Symp. (Oct 1992).479. J J Kim, H Hamouda, I Shalev and R L Barker, Text. Chem. Colorist, 25 (Aug 1993) 15.480. K J Baumert and P C Crews, AATCC Internat. Conf. & Exhib. (Oct 1994) 140.481. A Bereck, S Dillbohner, B Weber, D Riegel, J Mosel, J M Pieper and A Brakelmann, J.S.D.C., 113 (1997) 322.482. K Nostadt and R Zyschka, Colourage, 44 (Jan 1997) 53.483. B Nogues, J Soler and J Valldeperas, 15th IFATCC Congress, Lucerne (Jun 1990).484. F J Carrion, J.S.D.C., 110 (1994) 234.485. H J Lautenschlager, J Bindl and K G Huhn, Textil Praxis, 47 (1992) 460; 48 (1993) 438; AATCC Internat. Conf. &

Exhib. (Oct 1993) 271; Text. Chem. Colorist, 27 (Mar 1995) 27.486. A Bereck, D Riegel, C Kuma, C Rant, J Bindl, P Habereder, K G Huhn, H J Lautenschlager and G Preiner,

Melliand Textilber., 74 (1993) 1263, E416.487. A Bereck, D Riegel, A Satz, B Weber, M Münter, J Bindl, P Habereder, K G Huhn, H J Lautenschlager and G

Preiner, Textilveredlung, 31 (1996) 241.488. A Bereck, B Weber, D Riegel, J Bindl, P Habereder, K G Huhn, H J Lautenschlager and G Preiner, Textilveredlung,

32 (1997) 135.489. A Bereck, D Riegel, B Weber, J Mosel, J Bindl, P Habereder, K G Huhn, H J Lautenschlager and G Preiner,

Textilveredlung, 32 (1997) 138.490. A van der Spuy, Dyer 180 (Oct 1995) 11.491. A J Sabia, Text. Chem. Colorist, 26 (Aug 1994) 13.492. K Oh and K Yeh, Text. Asia, 25 (May 1994) 36.493. S Fornelli, Melliand Textilber., 77 (1996) 230, E49.494. P Hardt, Melliand Textilber., 71 (1990) 699, E326.495. G Chandra, Text. Chem. Colorist, 27 (Apr 1995) 21.496. P Habereder, Melliand Textilber., 78 (1997) 352 (1996) E105.497. S Smith and P O Sherman, Text. Res. J., 39 (1969) 441.498. P O Sherman, S Smith and B Johannessen, Text. Res. J., 39 (1969) 449.499. T F Cooke, Text. Chem. Colorist, 19 (Jan 1987) 31.500. M L Srivastava, Text. Dyer and Printer, 20, N0 13 (Jul 1987) 17.

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501. B Sahin, Internat. Text. Bull., Dyeing/Printing/Finishing 42 (Jan–Mar 1996) 26.502. J M Corpart and A Dessaint, Melliand Textilber., 78 (1997) 625, El35.503. M J Owen and H Kobayashi, Surface Coatings Internat., 78 (Feb 1995) 52.504. D Knittel and E Schollmeyer, Melliand Textilber., 79 (1998) 362, E96.505. G R Turner, Text. Chem. Colorist, 19 (Sep 1987) 69.506. P W Harris and D A Hangey, Text. Chem. Colorist, 21 (Nov 1989) 25.507. T F Cooke and H D Weigmann, Rev. Prog. Coloration, 20 (1990) 10.508. A S Johnson, B S Gupta and C Tomasino, Amer. Dyestuff Rep., 83 (Jun 1994) 17.509. R C Buck, Textilveredlung, 33 (Mar–Apr 1998) 57.510. DuPont, USP 5 001 004 (1991).511. I Holme, Dyer, 175 (Jun 1990) 14.512. P E Ingham, Australian Text., 9 (Nov–Dec 1989) 27.513. A Fisher, Australian Text., 11 (Sep–Oct 1991)16.514. Anon, Australian Text., 14 (Jan–Feb 1994)56.515. I Holme, Wool Record, 153 (Dec 1994) 33.516. R Teichmann, Textilveredlung, 31 (1996) 251.517. D M Lewis and T Shaw, Rev. Prog. Coloration, 17 (1987) 86.518. B J McCarthy and B C Burdett, Rev. Prog. Coloration, 28 (1998) 61.519. J L Burtness and B M Gatewood, AATCC Internat. Conf. & Exhib. (Oct 1995) 324.520. R J Mayfield, J.S.D.C., 101 (1985) 17.521. J Haas, SDCANZ 13th Internat. Symp. (Oct 1992) 37.522. D A Allenach, Wool Record, 151 (Mar 1992) 35.523. Anon, Internat. Carpet Bull., 246 (Feb 1994) 7; Wool Record, 154 (Mar 1995)25; Canadian Text. J., 111 (Dec 1994/

Jan 1995) 28.524. Anon, Wool Record, 155 (Jul 1996) 55.525. M Ash and I Ash, The index of antimicrobials (Aldershot: Gower, 1996).526. R S Jinkins and K K Leonas, Text. Chem. Colorist, 26 (Dec 1994) 25.527. J Luesby, Financial Times, 24 Oct 1996.528. Coville Inc., High Performance Textiles (Nov 1996) 3.529. J W McCurry, Text. World, 147 (Jan 1997) 52.530. J D Payne and D W Kudner, AATCC Internat. Conf. & Exhib. (Oct 1995) 341; Amer. Dyestuff Rep., 85 (June

1996) 26.531. J D Payne, Text. Chem. Colorist, 28 (May 1996) 28; J.S.D.C., 113 (1997) 48.532. M R Infante, M Diz, A Manresa, A Pinazo and P Erra, J. Applied Bacteriology, 81 (1996) 212.533. W Chen, T Koyama, K Hanabusa and H Shirai, J. Soc. Fibre Sci. Tech. Japan 51 (1995) 176.534. D G Johnson and B M Reagan, Text. Chem. Colorist, 22 (Apr 1990) 21.535. Foams: theory, measurements and applications, Ed. R K Prudhomme and S A Khan (New York: Marcel Dekker,

1995).536. Defoaming: theory and industrial applications, Ed. P R Garret (New York: Marcel Dekker, 1993).537. D N Willig, Amer. Dyestuff Rep., 69 (Jun 1980) 42.538. R E Patterson, Text. Chem. Colorist, 17 (Sep 1985) 181.539. W L Magee and P Habereder, AATCC Internat. Conf. & Exhib. (Oct 1994) 129.540. TEGEWA Working group on dyeing auxiliaries, Melliand Textilber., 77 (1996) 153.

REFERENCES

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760 FLUORESCENT BRIGHTENING AGENTS

760

CHAPTER 11

Fluorescent brightening agents

John Shore

11.1 INTRODUCTION

Partly from an association of whiteness with cleanliness and partly for aesthetic reasons toachieve a more pleasing contrast with coloured goods, from the earliest times people havesearched for methods of producing ever whiter textiles. The oxidative bleaching of vegetablefibres on exposure to air and sunlight, as well as the reductive bleaching of animal fibres bythe action of fumes from burning sulphur, have been known for thousands of years. Naturalblue minerals such as lapis lazuli (section 2.10.2), ground up and applied at very lowconcentrations, have been used for centuries to mask any residual yellowish colourremaining after bleaching, the overall greyish effect being perceived as ‘white’. Todaywhiteness is achieved by a combination of chemical bleaching and the use of so-calledfluorescent brightening agents (FBAs).

Fluorescent hydroxy-substituted derivatives of coumarin (benzo-α-pyrone) occurnaturally, including aesculetin (6,7-dihydroxy) in horse chestnut, daphnetin (7,8-dihydroxy)in daphne and umbelliferone (7-hydroxy) in spurge laurel. Paul Krais in 1929, in the firstrecorded application of an FBA to textiles, carried out tests on half-bleached linen using anaqueous extract of aesculetin-6-glucoside (11.1) from horse chestnut bark [1]. The additionof blue-violet light to the total light reflected from the fabric produced a ‘whiter than white’effect. Unfortunately, aesculetin showed poor fastness to light and washing. Nevertheless,Krais’s work stimulated research in this area and in 1934 Paine and Radley patented the useof 4,4′-bis(benzoylamino)stilbene-2,2′-disulphonic acid (11.2) in banknotes [2]. The firstcommercial bis(triazinylamino)stilbene derivative (11.3) was introduced by I G Farben in1940 as a brightener for cotton.

O OHO

OCH

OCH

CH

CH CH

OH OH

CH2OH

HO

11.1Aesculetin-6-glucoside

HO3S

HN C

O

CHHC

SO3H

NHC

O

11.2

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After the Second World War, development of synthetic FBAs was extremely rapid.Several hundred commercial products, representing a wide variety of chemical types, havesince been marketed, with FBAs probably corresponding to approximately 10% of worlddemand for dyestuffs. Several excellent books and reviews of the chemistry, application andproperties of FBAs have appeared [3–13].

FBAs are used to brighten not only textile materials but also paper, leather and plastics.They are important constituents of household detergent formulations. More specialisedareas of application include lasers, liquid crystals and biological stains. By far the mostimportant uses for FBAs, however, are in applications to textiles and paper. Much of whatfollows will be concerned with these two categories.

Descriptive terms such as ‘fluorescent whitening agents’, ‘optical brighteners’ and ‘opticalbleaches’ have all been used for the products described in this chapter as FBAs. Many ofthese terms have validity and the term ‘fluorescent brightening agents’ is preferred here onlybecause it has been adopted in the indexes of Chemical Abstracts.

An FBA is a strongly fluorescent substance that absorbs ultraviolet radiation, emits lightin the blue-violet region of the visible spectrum and is substantive to the substrate for whichit is intended. The product should be applicable without undesirable side-effects, such asstaining or subsequent photosensitisation of degradation, on any other substrate that may bepresent. The treated material should retain its properties during its working life and underthe conditions in which it will be found. For commercial success the product will also needto be priced attractively and supplied to the customer in a form that is convenient andpractical to use. The marketed product and the active brightener that it contains must notexhibit toxicity problems nor create an environmental hazard.

11.2 MODE OF ACTION OF A FLUORESCENT BRIGHTENER

All dyes absorb light. Fluorescent dyes re-emit the absorbed energy as light of longerwavelengths. An FBA is a fluorescent chemical that absorbs in the ultraviolet region of thespectrum and emits blue-violet light. A typical FBA shows maximum absorption at a

NaO3S

HN

CHHC

SO3Na

NH

N

N

N

OH

HN

N

N

N

NH

HO

11.3

CI Fluorescent Brightener 113

MODE OF ACTION OF A FLUORESCENT BRIGHTENER

chpt11(2).pmd 15/11/02, 15:46761


wavelength between 340 and 380 nm and emits visible light at a maximum emissionbetween 425 and 450 nm. When describing a substrate treated with an FBA, the terms‘remitted’, (reflected plus emitted) and ‘remission’ (reflectance plus emission), which takeinto account the fluorescence component of the total light emitted from the brightenedsubstrate, are preferred to ‘reflected’ and ‘reflectance’.

When present on a substrate an effective FBA increases the apparent reflectance of thearticle in the blue-violet region of the spectrum. The treated material remits more light inthe visible region than does an untreated white sample and thus appears ‘whiter than white’.These effects are indicated in Figure 11.1, which illustrates the importance of thoroughpreparation of the substrate to be brightened. Curve C represents the remission of anunbleached ‘dirty’ fabric. On treatment with an FBA this material is brightened but thetreated sample (curve D) may be less bright than a clean but unbrightened fabric (curve A)and much less bright than the same fabric after brightening (curve B). The details of fabricpreparation and bleaching processes are discussed fully elsewhere [14]. In some casesbrightening and bleaching can be carried out simultaneously and these possibilities will bediscussed later in this chapter.

Clean brightened article

Clean unbrightened article

‘Dirty’ unbrightened article‘Dirty’ brightened article

Fluorescence

A

B

C

D

300 400 500 600 700

Wavelength/nm

Rem

issi

on

Figure 11.1 Remission of brightened and unbrightened fabric

An efficient FBA must absorb strongly in the ultraviolet region and must also re-emit amajor proportion of the absorbed energy as visible light, that is, it must have a highfluorescence efficiency. Although fluorescence can occur from the α-bonds of many organiccompounds, strong fluorescence is associated with π-bonded electrons. All FBAs thereforecontain a considerable number of conjugated double bonds.

Processes occurring during absorption and fluorescence are shown in Figure 11.2, where

chpt11(2).pmd 15/11/02, 15:46762

763

S0, S1, S2, … represent so-called singlet states in which all the electrons have paired spins,and T1, T2, … represent triplet states in which two electrons have unpaired spins. Theenergy levels of both ground (S0) and activated states (S1, S2, …) are subdivided intovibrational and rotational energy levels. The vibrational energy levels are shown in Figure11.2. Differences in rotational levels are very small and can be ignored for the presentdiscussion.

Thermaldeactivation

Intersystem

crossing

T2

T1

S0

S2

S1

S0

Abs

orpt

ion

Fluo

resc

ence

Inte

rnal

con

vers

ion

Pho

spho

resc

ence

Inte

rsys

tem

cro

ssin

g

Inte

rnal

conv

ersi

on

Ene

rgy

Figure 11.2 Absorption and fluorescence processes

When an FBA absorbs a photon of light an electron is raised from the ground state (S0)of the molecule to one of its activated singlet states (S1, S2, …). Transitions from a singlet toa triplet state are quantum-mechanically ‘forbidden’. Absorption occurs when the moleculeis in its ground state, the vibrational level of the activated state reached by absorption beingdecided by the size of the quantum of energy (E) involved (E = hc/λ, where h is Planck’sconstant, c the velocity of light in a vacuum and λ the wavelength). Vibrational energylevels are extremely close to each other and vibrational energy is lost very rapidly (withinabout 10–12 s) before fluorescence occurs, when the molecule returns from the lowestvibrational level of the activated state (S1) to one of the vibrational levels of the groundstate (S0) whilst simultaneously emitting a photon of light. Fluorescence lifetime is typicallyabout 10–9 s. Energy can also be lost from the activated singlet state by non-radiativeprocesses (internal conversion) or by ‘forbidden’ intersystem crossing to give the tripletstate. The triplet state in turn can lose energy, returning to the ground state byphosphorescence or by a further radiationless intersystem crossing. Phosphorescence alwaysoccurs at a longer wavelength than fluorescence because the energy difference between T1and S0 is less than that between S1 and S0. FBAs do not exhibit significant

MODE OF ACTION OF A FLUORESCENT BRIGHTENER

chpt11(2).pmd 15/11/02, 15:46763


phosphorescence. For a more detailed discussion of the principles of fluorescence the readeris referred to books by Lumb [15] and Lakowicz [16].

Typical absorption and fluorescence spectra are shown in Figure 11.3. Since energy is lostin the activated state (S1) before fluorescence, the emission maximum always occurs at alower wavenumber than the absorption maximum. The difference, which is termed theStokes shift, can be calculated approximately from the absorption spectrum using thePestemer rule [17,18]. This rule states that the Stokes shift is 2.5 times the half-bandwidthat the absorption maximum.

Stokesshift

Absorption

Half-bandwidth

Fluorescenceemission

Wavenumber

Rel

ativ

e ab

sorp

tion

or e

mis

sion

Figure 11.3 Typical absorption and emission curves for an FBA (polar solvent)

For the following reasons the Stokes shift of an FBA should not be too large:(1) The closer the maximum wavelength of light absorption of an FBA approaches the

visible region, the greater is the energy content of sunlight at the earth’s surface that isavailable for fluorescence excitation and the greater the potential fluorescence. AnFBA with a Stokes shift of 60 nm or less would have a maximum absorption at 370 nmor longer and it would still show maximum fluorescence in the blue region of thespectrum.

(2) Although absorption and fluorescence spectra are not always (or even normally)symmetrical, a smaller Stokes shift reduces the chance of significant fluorescence in thegreen or yellowish green regions of the spectrum. Green or yellowish green fluorescencereduces whiteness.

A possible method for predicting absorption bandwidths of chromogenic molecules or FBAsusing PPP–MO theory (section 1.5) has been devised. It is based on the empirical linearrelationship stated by the Pestemer rule. Thus theoretical Stokes shifts are computed by thePPP–MO method and related to bandwidths. The requisite MO parameters for various typicalabsorption bands have been developed for use in these calculations. Reasonable correlationbetween calculated and experimental half-bandwidth data was found, suggesting that thisapproach has practical potential in predicting colour tone and brightness intensity [19].

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The quantum efficiency of fluorescence of a molecule is decided by the relative rates offluorescence, internal conversion and intersystem crossing to the triplet state. Up to thepresent time it has proved impossible to predict these relative rates. Thus, whilst it is nowpossible to calculate theoretically the wavelengths of maximum absorption and of maximumfluorescence of an organic molecule, it remains impossible to predict which molecularstructures will be strong fluorescers. Design of new FBAs still relies on semi-empiricalknowledge plus the instinct of the research chemist.

EVALUATION OF FBAs: MEASUREMENT OF WHITENESS

11.3 EVALUATION OF FBAs: MEASUREMENT OF WHITENESS

Almost all man-made fibres destined for sale as white goods are producer-brightened by themanufacturer and white textiles are almost always laundered using detergents containingcellulose-substantive FBAs [20]. To evaluate an FBA it is necessary both to apply theproduct to the desired substrate and to measure the whiteness of the treated material.Measurement of the fluorescence intensity of the FBA-treated substrate provides usefuladditional, although different, information.

Visual judgement of whiteness is highly subjective. Many factors, such as the observer’sage, sex and colour perception, and even the hue of ‘white’ materials normally encounteredin the observer’s country, help to decide on individual preference for either a red-violet or ablue-green shade of white.

Instrumental measurement of whiteness has been the subject of much research. Theparameters needed for unambiguous characterisation in the assessment of whiteness and tintof fluorescent substrates have been reviewed [21]. The importance of seeking goodcorrelation between different instruments is stressed [20]. Various trials have demonstratedthat it is possible to adjust modern instruments used to measure the optical characteristics ofFBA-treated samples of paper so that the results agree with a standard deviation of the orderof one CIE whiteness unit [22].

Many whiteness (W) formulae have been proposed. All are based on CIE colour spaceand the X,Y,Z tristimulus values. Three of these equations are those of Berger [23](Equation 11.1), Stensby [24] (Equation 11.2) and the CIE 1982 formula (Equation 11.3).

3 3W B G R� � � (11.1)

� � �3 3W L a b (11.2)

� � � � �800( ) 1700( )n nW Y x x y y (11.3)

The R,G,B values of the Berger formula measured by tristimulus colorimeters are linearlyrelated to the X,Y,Z tristimulus values of the CIE system. The Stensby formula incorporatesthe L,a,b tristimulus values of the Hunter system. In the CIE 1982 formula, xn and yn are thechromaticity coordinates of the D65 (2° or 10° observer) light source.

Information on the hue of whiteness is provided by dividing CIE colour space, in theneighbourhood of the D65 achromatic point, into a series of parallel strips corresponding tovariations in hue of whiteness. The principle is illustrated in Figure 11.4. According to this

chpt11(2).pmd 15/11/02, 15:46765


system, a neutral white has a nuance (NU) value of zero. Greener shades have valuesbetween 0 and +5, whereas violet shades have values between 0 and –5. The NU value canbe calculated from chromaticity data. Typical values giving a result corresponding to thejudgement of a standard observer are shown in Equation 11.4.

� � � �NU 1132 725 115.45x y (11.4)

The brightened specimen under test must be carefully prepared and meticulous attention tothe instrumentation and state of equipment is necessary if reliable data are to be obtained.The subject is complex and has been well reviewed by Griesser and co-workers [25,26].

D65 achromaticpoint

NU valuesnegative

Moreviolet

NU valuespositive

Greener

0.33

0.280.28 0.31

y

x

Figure 11.4 Colour space and hue of whiteness

To an average observer a sample of white material brightened with a low concentration ofan FBA will exhibit a bluish tone corresponding to a dominant wavelength in the vicinity of467 nm. As the amount of FBA present in the substrate is increased this hue changes. Atfirst the sample may become slightly more violet but as the maximum possible whiteness isapproached there is a distinct change in hue towards green until the sample becomesoverloaded with FBA, whiteness falls and the material is perceived as coloured rather thanwhite. Typical effects are illustrated in Figure 11.5.

In industry a direct comparison for strength between two FBAs is frequently required.Where both brighteners contain the same active component, or where they give a closelysimilar shade of white, such a comparison presents little difficulty. Where the two productsyield quite different shades of white, however, comparisons between them are usuallymeaningless. A typical situation is illustrated in Figure 11.6.

At concentrations below that yielding the maximum whiteness achievable there is anapproximately linear relationship between whiteness and the logarithm of the concentration

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767

of FBA present on the substrate. If two FBAs are similar in hue the straight line portions ofthe graph are almost parallel and a single-strength relationship can readily be calculated. Iftwo FBAs differ in hue, however, their relative effectiveness changes with the degree ofwhiteness, products that give greener hues generally being more effective at lowconcentrations.

Methods for the instrumental measurement of whiteness are well established but visualcomparison remains important, even in well-equipped laboratories. Some degree ofquantification is achieved by the method of paired comparisons, in which a panel ofobservers is presented with pairs of FBA-treated samples and asked to decide, withoutundue delay, which is the brighter. The total of positive scores can be used as a measure ofwhiteness and the results presented graphically as shown in Figure 11.7. Although time-

0

–ve

+ve

Whiteness

NU

val

ue

Brightened with green-hued FBA

Brightened with violet-hued FBA

Figure 11.5 Variation of hue with whiteness

Log (concn of FBA on weight of substrate)

Whi

tene

ss

Violet-hued FBA 2

Green-hued FBA

Violet-hued FBA 1

Figure 11.6 Dependence of whiteness on concentration

EVALUATION OF FBAs: MEASUREMENT OF WHITENESS

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consuming to carry out, a paired comparison series produces results that can be regardedwith considerable confidence and that usually correlate well with a comparison based on theCIE whiteness formula.

Brightened with FBA 1

Brightened with FBA 2

Log (concn of FBA on weight of substrate)

No.

of p

ositi

ve s

core

s

Figure 11.7 Presentation of results of a paired comparison

11.4 GENERAL FACTORS INFLUENCING FBA PERFORMANCE

Apart from needing to be cost-effective, a good FBA must be capable of producing a highlevel of whiteness. As the amount of FBA on a substrate is increased, whiteness increasesuntil a maximum value is reached (Figure 11.6). Further application of FBA results in lowerwhiteness. On polyester the fall in whiteness with increasing concentration of FBA isapparently not accompanied by a fall in total fluorescence [27]. On cotton both whitenessand total fluorescence fall, the decrease in whiteness occurring before that in fluorescence[28]. In general the main cause of the fall in whiteness with increasing concentration ofFBA is an increase in aggregation of the FBA on the substrate, resulting in a shift influorescence hue. The effect is shown in Figure 11.8. Not surprisingly, FBAs that give agreenish hue at concentrations below the maximum whiteness tend to produce a lowermaximum white than those exhibiting a violet hue. Other factors such as substantivity are ofconsiderable importance, of course. A computer-based expert system has been devised as anaid to selection of the most suitable FBAs for specific applications to various fibres using theprocessing equipment available in any given finishing works [29].

The presence of salts and additives can have an important influence on the performanceof an FBA. Traces of transition-metal ions such as iron and copper have an adverse effect onfluorescence [30], but this can be controlled using conventional polyphosphate or EDTA-type sequestering agents [31]. Other salts, even sodium sulphate or sodium chloride, havebeen claimed to enhance the fluorescence of FBAs in solution [32]. Apart from the normal

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effect of such electrolytes in minimising electrostatic repulsion between anionic FBAs andthe negatively charged surface of cellulose, their influence on the whiteness of a brightenedsubstrate is doubtful.

Surfactants, not surprisingly, exert a highly significant influence on the fluorescence ofFBAs in solution. This effect is associated with the critical micelle concentration of thesurfactant and may be regarded as a special type of solvent effect. Anionic surfactants havealmost no influence on the performance of anionic FBAs on cotton, but nonionicsurfactants may exert either positive or negative effects on the whiteness of the treatedsubstrate [33]. Cationic surfactants would be expected to have a negative influence, but thisis not always so [34]. No general rule can be formulated and each case has to be consideredseparately.

The influence of additives needs to be kept under constant consideration whenformulating commercial brands of FBAs. Liquid formulations are of considerable commercialimportance and it is often necessary to use solvents such as diethylene glycol, poly(ethyleneglycol) and alkoxylated alkylphenols in order to achieve stable solutions. Surfactants canhelp to stabilise FBA-resin bath combinations but in other circumstances they can adverselyaffect the performance of the formulation, not only in its capacity to brighten the substratebut also in the desired end-use of the brightened fabric. For example, nylon brightened bythe pad–thermosol method using a liquid formulation containing a large amount of analkoxylated polymer cannot be used as a substrate for colour printing unless previouslywashed. Without a wash-off, diffuse prints are obtained.

Violet or bluish violet dyes can be used in combination with FBAs for shading purposes.These shading dyes are used sparingly at no more than 2% of the weight of FBA applied.They are of particular importance when the material to be brightened is slightly yellow. Theshading dye converts the pale yellow hue of the substrate to a perceived grey, enhancing theeffectiveness of the FBA. These effects can be considerable. Many types of violet or bluishviolet dye may be used. Typical examples include crystal violet (CI Basic Violet 3) used inthe brightening of paper stock or cotton, CI Disperse Violet 28 and CI Acid Violet 43applied with cotton brighteners. Disperse dyes are also selected for shading with disperseFBAs on polyester and with basic FBAs on acrylic fibres.

Wavelength

Rel

ativ

e flu

ores

cenc

e

Low concentration FBA

Medium concentration FBA

High concentration FBA

Figure 11.8 Effect of FBA concentration on fluorescence hue

GENERAL FACTORS INFLUENCING FBA PERFORMANCE

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On slightly yellow cotton the addition of a shading dye is particularly convenient, sincethe substrate becomes bleached during the wash-wear cycle. Detergent formulations containFBAs that are substantive to cotton and loss of shading dye during washing is unlikely tohave a noticeable effect on the perceived whiteness of the article. Shading dyes are oflimited use with thoroughly prepared and well-bleached cotton. The importance of theinherent degree of whiteness of cotton in determining its response to finishing processes hasbeen emphasised. The effects of dry heat on peroxide-bleached cotton fabrics treated withbrighteners that varied in substantivity were investigated. Remission spectroscopy was usedto analyse the origins of thermal yellowing [35].

FBAs applied in combination may show a synergistic effect. Synergism is, at present, onlyof commercial interest with polyester brighteners. This phenomenon is discussed further insection 11.10. Conversely, the presence of a trace impurity in an FBA formulation maygreatly reduce its effectiveness. In industrial laboratories much time and effort is expendedin developing processes to minimise the content of such impurities or even to eliminatethem completely.

11.5 CHEMISTRY AND APPLICATIONS OF FBAs

FBAs are available for application to all types of substrate. Thus there are anionic FBAs forapplication to cellulosic materials in the presence of added salts, anionic types forapplication to nylon or wool in the presence of acid, cationic types for acrylic fibres, dispersetypes for polyester, and so on. Brighteners such as CI Fluorescent Brightener 104 (11.4),capable in principle of being fixed to wool or cellulosic fibres by reaction with nucleophilicgroups in the substrate have been reported but have never achieved commercial importance.Such FBAs in their reactive forms show diminished fluorescence because the presence oflabile halogen atoms leads to quenching. Hydrolysis or reaction with the fibre isaccompanied by development of fluorescence but an aftertreatment step is essential toensure that all the active chloro substituents present have been removed from the FBAabsorbed by the substrate [36]. The chemical structures of FBAs are many and variedalthough, of course, they all contain some sort of extended π-electron system. In thefollowing discussion all the main chemical types are mentioned, but mainly in terms ofapplication rather than chemistry of preparation.

11.6 BRIGHTENERS FOR CELLULOSIC SUBSTRATES

The earliest FBAs were developed for application to paper. Even today larger tonnages ofbrightener are marketed for application to cellulosics than to any other substrate.

11.6.1 FBAs for cotton

Brighteners are applied to cotton by methods similar to direct dyes. By far the most commonare triazinyl derivatives of diaminostilbenedisulphonic acid (DAS) of general formula 11.5,where M is an alkali metal, ammonium or alkylammonium cation. Examples of groups R1and R2 are shown in Table 11.1. Most suppliers of FBAs market such compounds, oftencalled DAST brighteners. Products in this class have sometimes been marketed because thesupplier needed to offer something different for commercial reasons, or to avoid infringing acompetitor’s patent, rather than for any real technological necessity.

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Table 11.1 Important FBAs of type 11.5 used to brighten cotton

Substituents R1 and R2 Substantivity Application

NH

SO3Na

NH

SO3Na

Lower Padding

NH

SO3Na

NaO3S

N O

NH

SO3Na

N(CH2CH2OH)2

NH

SO3Na

NHCH2CH2OH

NH SO3Na N O

NH N(CH2CH2OH)2

NH NCH2CH2OH

CH3

NH OCH3 Higher Exhaust

NaO3S

HN

CHHC

SO3Na

NH

N

N

N

Cl

N

N

N

Cl

N

N

O

O

11.4

CI Fluorescent Brightener 104

MO3S

HN

CHHC

SO3M

NH

N

N

N

R1

N

N

N

R1

R2

R2

11.5

BRIGHTENERS FOR CELLULOSIC SUBSTRATES

chpt11(2).pmd 15/11/02, 15:46771


SO3Na

O2N

CH3

SO3Na

ON

CH3

SO3Na

HN

CH3

OH

NaO3S

NO2

CHHC

SO3Na

O2NNaO3S

H3C

N N

SO3Na

CH3

O

SO3Na

H2N

HC CH

NH2

NaO3S

2 H 2 H

NaOCl orair/catalyst

self-condensation

11.8

11.7

11.6

Reduction

DAS

Scheme 11.1

DAS (11.7) is synthesised from 4-nitrotoluene-2-sulphonic acid (11.6) by the routeoutlined in Scheme 11.1. An important factor in the preparation of DAST brighteners inthe purity necessary for good performance is the purity of the DAS used as starting material.At one time DAS made in this way contained significant amounts of yellow azoxycompounds similar to 11.8, which formed the main components of the obsolescent dye SunYellow (CI Direct Yellow 11) made by the partial reduction and self-condensation ofintermediate 11.6. Today the major manufacturers supply DAS essentially free from theseundesirable impurities [37].

It is almost impossible to give a comprehensive list of all the FBAs of type 11.5 that haveappeared on the market since the first (11.3) of them was patented in 1940, but severalimportant commercial products are shown in Table 11.1. The less water-soluble productshave been widely used in the past as brighteners for detergent formulations and are generallyused to brighten cotton by exhaustion. The more soluble, less substantive types are usuallyapplied by padding in continuous bleaching, as a white ground for printing or in resinfinishing of woven goods for white garments or household textiles. Application methodshave been well described by Williamson [9].

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Where the groups R1 and R2 as defined in Table 11.1 are both derived from amines,variation of the amine has little influence on the hue of the white obtained. In general theseproducts give slightly violet shades of white and the main technical justification for theexistence of so many different structures in this class is the need for different levels ofsubstantivity. The compound in Table 11.1 where R2 is methoxy gives a distinctly violettone. All these products have light fastness values in the range 3–4 on cotton. In principle,all the more substantive types may be applied in conjunction with a hydrogen peroxidebleach. Their stability towards chlorine bleaches such as sodium hypochlorite varies, butthey are all essentially unstable towards these reagents [4].

Nine DAST-type FBAs, including six of those listed in Table 11.1, were exhausted ontocotton fabric at two applied concentrations (0.05% and 0.5% on weight of fibre). Values ofexhaustion, fluorescence intensity and whiteness index were determined, as presented for0.05% o.w.f. in Table 11.2. Results at the lower concentration facilitated more precisecomparisons between FBAs because of self-extinguishing of fluorescence and greatervariability in exhaustion at the higher level [38]. Structures containing disulphonatedanilino groups linked to either diethylamino or morpholino via the triazine rings werecharacterised by high solubility, poor exhaustion and relatively disappointing increases inwhiteness and fluorescence intensity. Conversely, the combination of unsulphonated anilinoand methoxy substituents yielded high fluorescence and whiteness values in spite of onlymoderate exhaustion, especially at the higher concentration. Average performance wascharacteristic of those products containing monosulphonated anilino and hydroxyalkyl-amino groups.

The treated fabrics were exposed for various times in an Atlas Weatherometer and typicalvalues of loss in fluorescence intensity and whiteness are given in Table 11.3. Relativelyhigher photostability was shown by the unsulphonated anilinotriazines, especially ifassociated with alkoxy substituents. The least stable combinations were disulphonatedanilino groups with either diethylamino or morpholino i.e. those exhibiting poor exhaustionand low whiteness index in Table 11.2. As in the latter table, the presence ofmonosulphonated anilino and hydroxyalkylamino substituents on the triazine rings resultedin average levels of performance.

A detailed study of the photostability of the DAST-type agent CI Fluorescent Brightener85 on cellophane film was carried out recently. The initial fading reaction is a photo-sensitised trans–cis isomerisation of the stilbene grouping. The subsequent oxidative attackon the molecule is concentrated on the vulnerable ethene linkage at the centre of thismoiety [39].

Substituted triazinyl derivatives of DAS are usually chosen for pad–dry–bake application tocotton in conjunction with an easy-care or durable-press finish. In these mildly acidicconditions (pH about 4) the FBA must show appreciable resistance towards the catalyst(usually magnesium chloride) necessary to cure the resin. The less substantive products in theupper half of Table 11.1 are important in this respect, as are compounds of type 11.9 where R= OCH3 or CH3NCH2CH2OH. It is likely that the hydroxyethylamino groups present inmany of these compounds participate in condensation reactions with N-methylol groups in thecellulose-reactant resin. The performance of an FBA applied in conjunction with a resin finishcan be modified and improved by careful formulation of the pad liquor but this lies beyond thescope of the present chapter. Alternatively, FBA and resin can be applied in two separatesteps; most DAST-type brighteners would be suitable if applied in this way.


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Table 11.2 Exhaustion, fluorescence and whiteness shown by type 11.5 FBAs on cotton [38]

Increase in Increase influorescence whiteness

Exhaustion (%) intensity at index (ASTM)Substituents R1 and R2 at 0.05% o.w.f. 0.05% o.w.f. at 0.05% o.w.f.

NH OCH3 80 0.98 43

NH N(CH2CH2OH)2 100 0.95 36

NH NCH2CH2OH

CH3

100 0.88 33

NH

SO3Na

NHCH2CH2OH 60 0.88 32

NH OCH2HCOH

CH3

80 0.83 31

NH

SO3Na

N(CH2CH2OH)2 80 0.78 28

NH

SO3Na

NaO3S

N O 40 0.82 26

NH SO3Na N(CH2CH2OH)2 80 0.76 25

NH

SO3Na

NaO3S

N(CH2CH3)2 40 0.69 19

11.6.2 FBAs for paper

The paper industry is the second most important user of FBAs after the detergent industry,most of the products applied to paper being of the DAST type.

Paper may be brightened during preparation, the FBA being added to pre-bleached pulpbefore the paper sheet is laid down, or during a subsequent sizing operation. Approximatelyone-third of the total FBAs used are applied to pulp and two-thirds at the sizing stage. AnFBA selected for addition to the pulp must show high substantivity at low temperature,otherwise there would be excessive loss of brightener with the waste water from the process.Resistance towards acidic conditions as low as pH 3 can also be important. Fillers used in

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775

Table 11.3 Loss in fluorescence and whiteness of type 11.5 FBA-treated cotton on exposure to xenonlight [38]

Effect of 40 AATCC fading units

% Loss in % Loss influorescence whiteness (ASTM)

Substituents R1 and R2 at 0.05% o.w.f. at 0.05% o.w.f.

NH

SO3Na

NaO3S

N(CH2CH3)2 70 92

NH

SO3Na

NaO3S

N O 58 84

NH SO3Na N(CH2CH2OH)2 49 75

NH

SO3Na

NHCH2CH2OH 49 73

NH

SO3Na

N(CH2CH2OH)2 44 73

NH NCH2CH2OH

CH3

46 69

NH N(CH2CH2OH)2 45 67

NH OCH2HCOH

CH3

44 66

NH OCH3 44 65

papermaking, such as alum, chalk or china clay, may cause loss of fluorescence and the typeand quantity of FBA added may have to be adjusted accordingly.

For use from the size press it is necessary for the FBA to be compatible with the chosensize, such as starch, casein or urea-formaldehyde resin. Since sizes tend to be yellowish andto absorb ultraviolet radiation, brighteners are generally less effective in sized paper.

The choice of FBAs and their methods of application to paper is highly complex, beingalmost as much an art as a science [40]. Examples of important FBAs for use with paper arelisted in Table 11.4. This list is far from exhaustive, however, and there are other importantproducts of type 11.5 used as FBAs for paper.


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11.6.3 Preparation of DAST-type FBAs

A major reason for the importance of DAST brighteners is their essentially straightforwardmanufacture from readily available and inexpensive intermediates. Products with widelydifferent substituents and hence showing quite different application properties are easilyprepared in a three-step, one-pot synthesis starting from diaminostilbenedisulphonic acid(11.7). The process is illustrated in Scheme 11.2.

By suitable choice of reaction conditions the chloro substituents of cyanuric chloride(11.10) can be replaced in a stepwise fashion. In the first step DAS reacts with cyanuricchloride at a temperature in the 0–20 °C range, ideally at pH 5–6. In the second step anamine or alcohol (R1H) reacts within the range 20–50 °C under neutral or slightly alkaline

NaO3S

HN

CHHC

SO3Na

NH

N

N

N

R

N

N

N

R

N(CH2CH2OH)2

N(CH2CH2OH)2

11.9

Table 11.4 Important FBAs of type 11.5 used to brighten paper

Substituents R1 and R2 Application

NH N(CH2CH2OH)2 Pulp

NH

SO3Na

N(CH2CH2OH)2 Pulp and size press

NH SO3Na NCH2CH2CN

CH2CH2OH

Pulp and size press

NH

SO3Na

NaO3S

N(CH2CH3)2 Size press

NH2 N(CH2CH2OH)2 Size press

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NaO3S

NH2

CHHC

SO3Na

H2N

ClN

NN

Cl

Cl NaO3S

HN

CHHC

SO3Na

NHN

NN

NN

N

Cl

Cl

Cl

Cl

NaO3S

HN

CHHC

SO3Na

NHN

NN

NN

N

R1

Cl

Cl

R1NaO3S

HN

CHHC

SO3Na

NHN

NN

NN

N

R1

R2

R2

R1

+ 2step 1

11.10

2 R1H step 2

2 R2H

step 3

11.7

Scheme 11.2


conditions. The third step with another amine or alcohol (R2H) is completed within therange 50–100 °C under alkaline conditions (pH 8–9). The exact conditions selected for thesecond and third steps depend on the nature of the attacking nucleophiles (R1H and R2H),as well as the substituents already present in the chlorotriazine intermediates.

Scheme 11.2 illustrates the conventional sequence for the manufacture of DASTbrighteners. However, it is not always necessary and may not be desirable for DAS to be thenucleophile selected for the first step. In principle the three nucleophiles can be reacted inany order, but it is preferable for the most nucleophilic amine to react last in order to avoidforcing conditions during removal of the last remaining chloro substituents. Alkylaminesreact more readily than alcohols, thus ensuring that alkanolamines yield theirhydroxyalkylaminotriazine derivatives.

The mechanism of these bimolecular nucleophilic substitution reactions is shown inScheme 11.3 for the reaction between a primary amine and the intermediatedichlorotriazine. A corresponding scheme can be drawn for reaction of a secondary amine,an alcohol or any other nucleophile in any of the replacement steps. It follows from thismechanism that the rate of reaction depends on:(1) the nucleophilicity of the attacking species, the more nucleophilic reagents reacting

more quickly or under milder conditions(2) the electronegativity of the substituent R1.

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The more electronegative the substituent, the more rapid the reaction or the milder theconditions required. Thus the reaction rate decreases in the order: PhO > MeO > EtO >PhNH > NH2 > MeNH > EtNH > Me2N > OH [4].

It also follows that protonation of the triazine ring makes it more susceptible to attack bynucleophilic reagents unless the reagent itself is also protonated. If the triazine ring remainsunprotonated when a nucleophilic base, such as an alkylamine, is present as its acid salt thereaction is slower, of course. Cyanuric chloride itself is a very weak base that becomesprotonated only under strongly acidic conditions. Thus step 1 in Scheme 11.2 can be carriedout in aqueous solution even at pH 2 without risk of undesirable hydrolysis of cyanuricchloride, water being an extremely weak nucleophile.

The manufacture of several important brighteners containing alkoxytriazine groups, suchas the DAS derivative of highest substantivity in Table 11.1, does not follow theconventional sequence. The first step involves reaction of cyanuric chloride with excessmethanol and excess acid acceptor, usually sodium bicarbonate. Under acidic conditions thisreaction takes a quite different course and can become dangerously violent (Scheme 11.4).

N N

NR1 Cl

Cl

R2H2N

N N

N

Cl

HN R2R1

N N

N

Cl

R1

Cl

HN R2

H

+ HCl

+slow

_ +

Scheme 11.3

N N

NCl Cl

Cl

N N

N Cl

Cl

H3CO

N N

N

OH

OHHO

NaHCO3

+ excess CH3OH

acidconditions

+ 3 CH3Cl(g)11.10

Cyanuric chloride

Cyanuric acid

Methyl chloride gas

Scheme 11.4

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779

In the preparation of other DAST brighteners it may be advantageous to avoid reactingDAS with cyanuric chloride in the first step. It is difficult to suppress the reactivity of thesecond chloro substituent completely and undesirable by-products of the general type 11.11can be eliminated if DAS is made to react with a dichlorotriazine intermediate in the secondstep. Very careful control of the reaction conditions, especially in steps 1 and 2, is alsonecessary in order to avoid formation of partially hydrolysed by-products such as structures11.12 and 11.13.

Unsymmetrical products of type 11.14 derived from DAS have been described but theseare much more complicated and expensive to prepare than those with symmetricalstructures. They have never become commercially important.

N N

N

R2

NHHN

SO3NaNaO3S

HCCHHC CH

SO3Na NaO3S

NH HN

N

N

N N

N

N

R2

R1

R2

R1

11.11


NaO3S

HN

CHHC

SO3Na

NH

N

N

N

N

N

N

HO

R

R

OH

11.12

HO

N

N

N

R1

R2

11.13

NaO3S

HN

CHHC

SO3Na

NH

N

N

N

N

N

N

R2

R1

R3

R4

11.14

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11.6.4 Speciality FBAs for cotton

Two commercially important brighteners for cotton are not of the DAST type.The distyryldiphenyl 11.15 is mainly of value for brightening cotton during laundering but

it can also be used to brighten cotton by exhaust application. It has high aqueous solubilityand has been recommended for use in combination with resin finishes, although its stabilityat the padding stage is suspect. It has found further uses in the brightening of polyester/cotton and nylon/cotton blends. Both components of a nylon/cotton blend are brightenedand on polyester/cotton the cellulosic component is brightened without any undesirablestaining of the polyester. Its light fastness on cotton is 4, which is slightly superior to that ofDAST brighteners. Compound 11.15 is also resistant to hypochlorite bleach but on cotton ithas limited fastness to washing in soft water. The effect of humidity on the photostability ofCI Fluorescent Brightener 359, a distyryidiphenyl structure similar to 11.15, has beenstudied on cellophane film recently. A kinetic analysis of fading rates under these conditionsindicated participation in a bimolecular oxidative mechanism [41].

The manufacture of brightener 11.15 is shown in Scheme 11.5. The firstchloromethylation stage can only be accomplished safely in a plant specially designed to

Scheme 11.5

ClCH2 CH2Cl

CH2 P

OCH2CH3

OCH2CH3

OCH2PO

OCH2CH3

OCH2CH3

CHO

SO3Na

CH HC

CH

HC

NaO3S

SO3Na

2 P(OCH2CH3)3

+ 2 CH3CH2Cl

11.15

Base

+ 2 H2O2 HOCH2Cl

ZnCl2

Biphenyl

Triethyl phosphite

O

CH2Cl

CH2ClZnCl2HOCH2ClHCHO + HCl

11.16

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ensure that the highly carcinogenic by-product bis(chloromethyl) ether (11.16) formed fromformaldehyde and hydrochloric acid does not escape. Otherwise the preparation proceedswithout undue difficulty.

A specific advantage of sulphonated distyrylarene brighteners such as 11.15 and similarstructures is that the ethene double bonds are readily oxidised during effluent treatment,leading to the formation of soluble acids of low relative molecular mass (Scheme 11.6). Thusit can be inferred that these brighteners and their degradation products are relativelyinnocuous and are unlikely to accumulate in the environment.

BRIGHTENERS FOR CELLULOSE ACETATE AND TRIACETATE FIBRES

CH HC

CH

HC

NaO3S

SO3Na

C

O

OH

C

HO

O

COOH

SO3Na

11.15

2

+

O2 + H2O

free-radical conditions

Scheme 11.6

The introduction of heterocyclic rings as terminal groups in the 4,4′-positions of thestilbene nucleus intensifies the fluorescence of the conjugated system and shifts thefluorescence maximum to a longer wavelength. The vic-triazole 11.17 is a premium productand cotton brightened with it has a light fastness of 5. This structure is stable to bleachingwith hypochlorite or chlorite. It has adequate fastness to washing and a liquid formulationhas been marketed for use in combination with a resin finish. It is also of some importanceas a component of household detergents. Unfortunately, it is also expensive and its main useis probably as an FBA for nylon, on which fibre it gives better value for money. Itspreparation is shown in Scheme 11.7.

A more soluble derivative of compound 11.17, the tetrasulphonated analogue 11.18, hasbeen recommended for application to cotton in combination with a resin finish. Unlike DAST-type FBAs under these conditions, compound 11.18 is compatible with resin formulationscontaining zinc nitrate as latent acid catalyst. The brightness achieved is not high, however.

Many other products of a variety of structures have been patented for the brightening ofcellulosic substrates. The reader is referred to the reviews mentioned earlier for furtherinformation.

11.7 BRIGHTENERS FOR CELLULOSE ACETATE AND TRIACETATE FIBRES

Cellulose acetate and triacetate fibres are brightened with disperse-type FBAs, includingderivatives of 1,3-diphenylpyrazoline (11.19). These form a commercially important group ofFBAs. If suitably substituted they can be applied to substrates other than acetate andtriacetate. The commercially more important products of this type are used to brightennylon and acrylic fibres. Their preparation and other aspects of pyrazoline chemistry arediscussed in section 11.8. Examples of pyrazolines used to brighten acetate and triacetate

chpt11(2).pmd 15/11/02, 15:46781


SO3H

H2N

HC CH

NH2

HO3S C

H3C

O

C

CH

O

N

HOSO3H

NH

HC CH

HN

HO3S

H2N

NH2

SO3Na

NH

HC CH

HN

NaO3S

N

N

HC

CHN

HO

SO3Na

N

HC CH

N

NaO3S

N

N

N

N

CH3COONa

11.7

+ 2

1. Diazotise2. Na2SO3, then acid

Alkyl nitrite(acid or alkoxide)

CH3OH / H2O / H2NCONH2 / (CH3CO)2O11.17

N

OH

Scheme 11.7

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fibres include the sulphonamide 11.20 and the sulphone 11.21, the former giving greenishtones and the latter violet effects.

A pyrene derivative (11.22), a naphthalimide (11.23) and benzoxazoles of smallermolecular size are also used and these are discussed in more detail in section 11.10. Thenaphthalimide ring system is highly stable, leading to products with good light fastness andstability to chlorite. Their main disadvantage, however, is relatively low fluorescenceefficiency, which is primarily a result of low molar extinction coefficients. Strongerfluorescence arises when there is an electron-donating group such as methoxy (as in 11.23)or alkylamino in the 4-position, in which case the absorption and emission properties areassociated with intramolecular charge transfer involving the donor group and the electron-withdrawing peri-carbonyl groups.

BRIGHTENERS FOR CELLULOSE ACETATE AND TRIACETATE FIBRES

NaO3S

N

CHHC

SO3Na

N

N

N

N

N

NaO3S

SO3Na

11.18

NN

11.19

1,3-Diphenylpyrazoline

NN

Cl SO2NH2

11.20N

NCl SO2CH3

11.21

N

NN

OCH3

OCH3

11.22

N

CH3

OO

OCH311.23

chpt11(2).pmd 15/11/02, 15:46783


Several interesting analogues of structure 11.23 were synthesised recently. Thesederivatives of 4-methoxynaphthalimide contained a triazine ring with an unsaturatedpolymerisable substituent capable of addition copolymerisation with other vinyl or acrylicmonomers. Such brighteners can be incorporated into the synthesis of polymeric finishesand show exceptional durability to organic solvents and wet treatments [42–44].

11.8 BRIGHTENERS FOR NYLON

Disperse brighteners of the types used to brighten cellulose acetate, triacetate or polyesterfibres can be used to brighten nylon. In practice, however, disperse types are little used andnylon is usually brightened with sulphonated compounds akin to acid dyes. Silk andpolyurethane fibres can also be brightened with the FBAs normally used on nylon. Chiefamongst these are products of the DAST type already discussed in section 11.6.3. Two seriesof DAST derivatives of the general structure 11.5 were evaluated recently, in which the R1groups were morpholino and the R2 groups either arylurea or arylthiourea (11.24; Ar =aryl). These products showed high substantivity for both cotton and nylon, the ureido (X =O) or thioureido (X = S) residues providing scope for hydrogen bonding with polar groupsin these substrates [45].

NaO3S

HN

CHHC

SO3Na

NH

N

N

N

N

N

N

N

NH

N

HN

O O

C

C

X

NHAr

X

HN Ar11.24

Exhaust brightening of nylon is usually combined with a reduction bleach based onsodium dithionite. Important DAST-type FBAs suitable for this process are exemplified bythe four most cotton-substantive compounds listed in Table 11.1. The methoxy-substitutedderivative can be applied to nylon successfully at pH 6–6.5 but those with three NH groupsattached to each triazine ring are best applied at pH 4–5. All these products show a lightfastness rating of only about 3 on nylon. If a reductive bleach is unnecessary the DASTbrightener 11.25 can be applied to nylon from an alkaline scouring bath. Good whites areachieved but the light fastness of this tetrakis(anilino) derivative is suspect on nylon.

chpt11(2).pmd 15/11/02, 15:46784

785

The premium FBAs 11.15 and 11.17 already mentioned are of importance on nylonbecause of their superior fastness properties. As on cotton, the distyryldiphenyl structure11.15 has slightly higher light fastness than the DAST-type brighteners. In contrast to itsperformance on cotton, however, it has excellent fastness to washing on nylon.

The light fastness of the vic-triazole 11.17 on nylon is 4–5; as on cotton this issignificantly superior to that of the DAST derivatives. Unlike the DAST types, the vic-triazole is also stable towards a sodium chlorite bleach. Applied to nylon in combinationwith sodium chlorite, compound 11.17 can give exceptionally high whiteness and excellentfastness properties.

Nylon can also be brightened using an anionic derivative of 1,3-diphenylpyrazoline, suchas FBA 11.26, 11.27 or 11.28. Although these pyrazolines give excellent whites whenapplied to nylon by exhaustion, they are usually less cost-effective than the DASTbrighteners. For continuous application by pad–thermosol or pad–acid shock methods thesituation is reversed, however, and the pyrazoline FBAs are commercially important in thissector.

NaO3S

HN

CHHC

SO3Na

NH

N

N

N

N

N

N

NH

NH

HN

HN

11.25

BRIGHTENERS FOR NYLON

11.26N

NCl SO2CH2CH2SO3Na

11.27N

NCl SO2CH2 C SO3Na

CH3

CH3

11.28

NN

Cl

Cl

CH3

SO2CH2CH2SO3Na

chpt11(2).pmd 15/11/02, 15:46785


On nylon these three pyrazolines (11.26–11.28) have light fastness values in the range 3–4, slightly superior to the DAST types. Light fastness in the wet state is generally lower,however, and the pyrazolines suffer more in this respect than the DAST brighteners.Pyrazolines 11.26 and 11.27 in particular have very poor light fastness values of 1–2 in thewet state. The two electron-withdrawing chloro substituents in compound 11.28 have theeffect of improving light fastness, especially in the wet state, but at the expense of a loss insolubility and slightly more difficult formulation and application. All three pyrazolines giveviolet-toned whites on nylon, compound 11.28 being slightly less violet than the other two.

Electron withdrawal by the 3,4-dichloro substituents and the lactone ring in the 3′,4′-positions of the diphenylpyrazoline derivative 11.29 enhances light fastness in both the dryand wet states. Thus the fastness rating of compound 11.29 is slightly higher than those ofcompounds 11.26–11.28 in the dry state and significantly higher on wet nylon. Pyrazoline11.29 is also capable of giving brilliant whites of a pleasing bluish hue. It is complicated tomanufacture, however. Increasing substitution also reduces solubility and thus adverselyaffects pad liquor stability, although this problem can be solved by suitable formulation. Padliquor formulation can be especially important in the pad–thermosol process, where thebrightened nylon fabric is often intended as a prepared white ground for colour printing withacid dyes. If too much surfactant has been applied together with the FBA in the pad liquor,‘bleeding’ from the fabric at the printing stage can give an unacceptably blurred appearanceto the printed design.

11.29

NN

CH2SO3Na

Cl

Cl

CH3O

O

The general method for the preparation of diphenylpyrazolines is shown in Scheme 11.8,in which X is a suitable leaving group, usually chloro but sometimes dialkylamino. Thisreaction normally proceeds readily, although pH control may be important. Preparation ofthe substituted ketone and hydrazine intermediates needed for the synthesis may involvelengthy and complicated sequences. Further reactions are often required to modify thesubstitution in ring B after formation of the pyrazoline ring. The preparation of compound11.26 shown in Scheme 11.9 illustrates one of the simpler instances.

Derivatives of 1,3-diphenylpyrazoline have been used to brighten cellulose acetate(section 11.7) and acrylic fibres (section 11.11.1) as well as nylon. There has been muchstudy of the effects of substituents on application properties and some general rules can beformulated:(1) The greater the electron-withdrawing character of substituents in ring A, the greener

the hue of brightening.(2) The greater the electron-withdrawing character of substituents in ring B, the more

violet the hue of brightening.(3) An electron-donating group in the 4-position of the pyrazoline ring has a slight

hypsochromic effect, but an electron-withdrawing group has a bathochromic effect.(4) Light fastness is improved by the introduction of electron-withdrawing substituents into

ring A, but is adversely affected by electron-donating substituents.

chpt11(2).pmd 15/11/02, 15:46786

787

C

CH

O

CH X

HN

H2N

A B A B+base

NN

Scheme 11.8

Cl Cl C

CH2CH2Cl

O

Cl C

CH2CH2Cl

O

HN

H2N

SO2CH2CH2OH

Cl SO2CH2CH2OH

Cl SO2CH2CH2SO3Na

AlCl3

+

+

base

1. H2SO42. Na2SO3

11.26

Chlorobenzene

NN

NN

Scheme 11.9

BRIGHTENERS FOR NYLON

Theoretical explanations for the effects of substituents on the hue of diphenylpyrazolinebrighteners have been published by Güsten and co-workers [46,47]. In practice almost allcommercially important diphenylpyrazoline FBAs have the general structure 11.30, in whichSO2R is a sulphone or sulphonamide grouping.

SO2R Cl

CH3

Cl

Cl11.30

Ar = orNN

Ar

chpt11(2).pmd 15/11/02, 15:46787


Since pyrazoline FBAs tend to stain cotton they cannot be used to brighten nylon/cottonblends, which require an FBA of the DAST type, the distyryldiphenyl 11.15 or the vic-triazole 11.17. The two most important of these are probably the methoxy-substitutedDAST product in Table 11.1 and the distyry1diphenyl derivative. By careful adjustment ofthe dyebath pH these products will exhaust onto both nylon and cotton to give a good solidwhite. Nylon can also be brightened by incorporation of a thermally stable FBA in the melt,the FBA being added to the polymer before extrusion or shaping. Structure 11.31 is typicalof the compounds selected for this purpose. The benzoxazole ring system is particularlyimportant in intensifying the fluorescence of the stilbene system and conferring high fastnessto light.

11.9 BRIGHTENERS FOR WOOL

Wool is naturally yellower than other textile substrates and bleached wool gradually becomesyellow again when exposed to sunlight or other ultraviolet sources. The effect of light onwool is complex and depends on the conditions of exposure. The problem of yellowing isaccentuated if the wool is left wet in sunlight. Since FBAs absorb ultraviolet light theyaccelerate this photo-initiated yellowing. The rate of yellowing of bleached wool increaseswith decrease in wavelength of the incident radiation, 300 nm being the approximate lowerlimit of ultraviolet reaching the earth from the sun. In contrast, the most importantwavelengths for yellowing FBA-treated wool are within the range 340–420 nm. Maximumfree-radical formation and acceleration of yellowing occurs in the region 350–410 nm, wherebrightener excitation takes place [48].

For satisfactory whiteness on wool, it is essential for the fibre to be well scoured andbleached, either oxidatively with hydrogen peroxide or by reduction using stabilised sodiumdithionite. Brightener is usually applied together with the dithionite bleach. To achieve thehighest possible whiteness, the wool should first be scoured to remove natural waxes andother contaminants, then bleached with peroxide and finally treated with FBA during asecond bleach with dithionite.

If wool containing a reducing agent is exposed to irradiation, the rate of yellowing isslower than on untreated wool. Thiourea dioxide is particularly effective in this regard,especially when used in conjunction with formaldehyde. Although thiourea dioxide retardsthe yellowing of wool treated with an FBA, it does not prevent destruction of the latter. Itwas shown that this reducing agent minimises yellowing by reducing the coloured productsformed on photodegradation of the FBA and certain amino acid residues in the substrate[49].

The FBAs used to brighten wool are mainly DAST types and pyrazolines of the aciddyeing type already discussed in section 11.8. Examples include the three most cotton-substantive DAST brighteners listed in Table 11.1, although on wool these give light

HC

CHO

NCH3C

CH3H3C

11.31

chpt11(2).pmd 15/11/02, 15:46788

789

fastness ratings of only approximately 2. The pyrazolines 11.26–11.28 have light fastness of3–4 on dry wool, but very poor light fastness in the wet state. The coumarin derivative 11.32is sometimes used on wool and can give exceptional brilliance, but unfortunately its lightfastness is only 1. The important fluorescent coumarin derivatives almost invariably containan electron-donating substituent in the 7-position, for example a dialkylamino group as incompound 11.32. Furthermore, an electron-withdrawing substituent in the 3- or 4-position,such as a cyano group, leads to shifts of the absorption and emission bands to longerwavelengths [50].

O(CH3)2N

CH3

O

11.32

Although wool is most often brightened using FBAs containing stilbene or pyrazolinefluorescent systems, such compounds degrade rather quickly on exposure to sunlight andalso sensitise photodegradation of the wool. Degradation on the surface of wool appears toinvolve interaction of the stilbene with wool keratin and this is not necessarily a photo-oxidative process. There is good evidence that adsorbed stilbene derivatives can sensitisethe formation of singlet oxygen, which then reacts with indole rings (tryptophan residues) inwool. Like the pyrazolines, stilbenes have the facility to self-destruct in photochemicalprocesses [51].

Novel fluorescent anionic surfactants of the types 11.33 and 11.34, where R representsalkyl groups of various lengths, have been applied to wool in order to study their distributionand effects on the physical and chemical properties of the fibre. Sections of the treated fibreswere examined under a fluorescence microscope. The intercellular and cell remnant regionsappeared to be the preferred locations of the adsorbed surfactants, but the distributionpattern was dependent on the length of the R chain of the surfactant and the conditions ofapplication to wool [52].

BRIGHTENERS FOR WOOL

OHO

CH3

O

RKO3S

11.33 11.34

NN

R SO3NH4

Fluorescence quenching studies of these alkylated FBAs in aqueous solution were carriedout using spectroscopic techniques. For the higher members of each series, plots offluorescence quantum yield against agent concentration showed a sharp decrease influorescence intensity at a specific concentration, in contrast to the smoothly decreasingcurve characteristic of the lower alkyl or unsubstituted FBAs. The kinetics of fluorescence

chpt11(2).pmd 15/11/02, 15:46789


quenching were consistent with the formation of micelles from the more surface-activehigher alkyl derivatives [53].

Long-lived transient species have been detected during laser flash photolysis of solutionsof the disulphonated distyryldiphenyl derivative 11.15. These transients are readilyquenched by reducing agents but their yield is enhanced in the presence of oxidising ions.Such species are believed to be radical cations formed following monophotonic photo-ionisation. Transient quenching is observed in the presence of indole, tryptophan andtryptophyl peptides. These results are indicative of the sensitised photodegradation of woolkeratin in the presence of FBAs of the distyrylarene class [54].

11.10 BRIGHTENERS FOR POLYESTER FIBRES

Much research has focused on the development of better brighteners for application topolyester. Huge numbers of patents have appeared and it is impossible to cover all thechemical variations in this chapter. Many of the more important commercial products andchemical types are discussed here but the reader is referred to published reviews [5,6,10,11]for more detail.

Although polyester is always brightened with disperse-type products, the methods ofapplication vary. FBAs are marketed for incorporation in the polymer mass, for exhaustapplication with or without carrier and for use in the pad–thermosol process at atemperature within the range 160–220 °C. Most products are applicable by more than onemethod, although none can be applied satisfactorily by all methods and cost-effectiveproducts introduced in the 1950s still remain important today.

In general and as expected, brighteners of relatively small molecular size are most suitablefor application by exhaustion. Less volatile compounds of larger molecular size tend to bepreferred for pad–thermosol application or for incorporation in the polymer mass.Commercially important for exhaust application are the previously mentioned pyrenederivative 11.22, the naphthalimide 11.23, the bis(benzoxazolyl)ethene 11.35, thebis(benzoxazolyl)thiophene 11.36, the distyrylbenzene 11.37 and the stilbene bis(acrylicester) derivative 11.38. Products of the 11.35 type show excellent light fastness but onlymoderate fastness to sublimation. In view of this volatility they can be used in the transferprinting of polyester.

O

NH3C

HC

CH

N

O

CH311.35

O

NSO

N

11.36

HC

CN

CH HC

CH

NC11.37

chpt11(2).pmd 15/11/02, 15:46790

791

CH HC

HC CHC

O

CH3CH2O

HC

CH C

O

OCH2CH3

11.38

Polyester is brightened more effectively by exhaustion either in pressurised equipment at125–130 °C or at the boil in the presence of a carrier. Small amounts of carrier may be addedto assist levelling in the high-temperature process but the use of carriers is increasinglydeprecated for environmental reasons. Commercially satisfactory results are obtained at theboil in the absence of carrier using rapidly diffusing FBAs of relatively small molecular size,such as the naphthalimide 11.23 or the benzoxazole derivatives 11.35 and 11.36. PolyesterFBAs that are suitable for exhaust application are normally stable to sodium chloritebleaching, although the pyrene derivative 11.22 and the bis-ester 11.38 are exceptions.

Most of the FBAs used to brighten polyester by exhaustion may also be appliedsuccessfully by the pad–thermosol method at a baking temperature up to 190 °C. Attemperatures higher than this the more volatile brighteners sublime and give poor yields.Compounds suitable for use with a baking temperature that exceeds 190 °C include thepyrene derivative 11.22, the benzoxazole 11.31, the distyrylbenzene 11.37 and the stilbenebis-ester 11.38. The temperature used during the baking stage in this process dependslargely on the equipment available to the finisher. Thus FBAs showing optimumperformance at temperatures ranging from 160 to 220 °C all have a niche in the market.Shorter baking times, leading to greater throughput of brightened fabric, are possible at thehigher temperatures, although energy costs are greater. Products capable of giving goodwhiteness at relatively low thermosol temperatures appear to be gaining in importance [12].

Many other compounds have been marketed as polyester brighteners for application byexhaustion or in the pad–thermosol process. No account would be complete withoutmention of the important class of coumarin disperse FBAs, of which structure 11.39 is atypical example. Many commercial brighteners for polyester contain one or two benzoxazolegroups, including compounds 11.31, 11.35, 11.36, 11.40 and 11.41.

BRIGHTENERS FOR POLYESTER FIBRES

N

N O ON

CH3 11.39

O

NH3C

H3C

HC

CH C

O

OCH3

11.40

chpt11(2).pmd 15/11/02, 15:46791


Polyester brighteners typically show excellent fastness properties. Light fastness is usually5–6, the pyrene derivative 11.22 being an exception with light fastness of only 2–3.Although this compound gives a distinctly greenish brightening tone it is capable ofproducing remarkably brilliant whites and remains an important commercial product.

Yellowing of white textiles in the presence of gas fumes (nitrogen oxides) has becomeimportant in recent years. The yellowing is often attributable to the formation of quinonoidcompounds arising from reaction between the oxides of nitrogen and antioxidants such asdi-t-butyl-p-cresol 11.42 present in packaging materials. Reinehr and Schmidt have shownthat several polyester FBAs yellow in the presence of exceptionally high concentrations ofnitrogen oxides, but they were unable to detect any significant yellowing at concentrationslikely to be approached in practice [55].

Synergistic effects can often be observed with polyester brighteners and formulatedmixtures of brighteners are increasing in importance. For example, mixture productscontaining the pyrene derivative 11.22 with either the naphthalimide 11.23 or thebenzoxazole 11.35 have been marketed. This property may be exploited either to increasethe maximum whiteness achievable or to attain a desired level of whiteness by applying alower concentration of the synergistic mixture. The subject has been discussed by Martiniand Probst [56], but the mechanism by which the synergy operates is not completelyunderstood.

In a recent evaluation of this phenomenon, the whiteness indices given by elevenindividual brighteners on polyester were compared with those of their binary mixtures invarious ratios. In many cases the whiteness performance of a mixture was markedly superiorto that shown by the individual components [57]. A more specific investigation wasconfined to a series of benzoxazole FBAs. Their fluorescence spectra and fluorescencelifetimes were determined individually and in mixtures. The relationships betweenmolecular structure and photophysical properties were discussed [58].

Much polyester fibre intended for curtain net or other white goods is sold in pre-brightened form. The brightener is added to the polymer melt before or during extrusion, soit must exhibit high thermal stability. Two important FBAs used in this way are thetriazolylcoumarin 11.43 and the bis(benzoxazolyl)stilbene 11.44. The latter structureprovides brilliant whiteness with a violet tone, more pleasing to most observers than theslightly greenish hue given by compound 11.43. The coumarin, however, is stable under theconditions of condensation polymerisation and can be incorporated before polymermanufacture. The benzoxazole is not entirely stable and this has to be added to the polymergranules immediately before extrusion. Polyester brightened with the coumarin has a lightfastness rating greater than 7, making it very important commercially. The presence of the

H3C OH

C

CH3H3C

CH3

C

CH3

CH3

H3C

11.42

O

N

N

O

11.41

chpt11(2).pmd 15/11/02, 15:46792

793

phenyl and naphthotriazolyl groupings, contributing π-electron mobility in positions 3 and 7respectively, markedly amplifies the fluorescence intensity, as well as conferring exceptionalphotostability.

ON ON

N

11.43

O

N

HC

CH

N

O

11.44

The photochemical fading of disperse dyeings on polyester is retarded if an FBA has beenapplied by mass pigmentation before extrusion. The higher the concentration of FBApresent, the greater is the protective effect on the light fastness of the disperse dyes [59].Studies of the mechanism of photochemical decomposition of DAST-type brighteners haveshown that stilbenes may act not only as sensitisers to produce singlet oxygen, but also asphysical and chemical quenchers [60]. Owing to their capacity to self-destruct by suchmechanisms, certain types of FBA can be preferentially destroyed in the presence of thephotochemically more stable disperse dye molecules [59].

Since the structures of polyester FBAs are so varied, the reactions employed in theirsynthesis are also diverse. The organic chemistry can be complex and the intermediatesrequired are often difficult to prepare. A full discussion is beyond the scope of this chapter.The reader is referred, in the first place, to the reviews mentioned in the introduction forfurther information [3–13]. A summary of the more important methods of manufacturefollows.

Those polyester FBAs containing a benzoxazole group are usually prepared from theappropriate o-aminophenol and carboxylic acid (11.45; Y = OH) or one of its derivatives, asshown in Scheme 11.10. The reaction proceeds via an intermediate amide and it can beadvantageous to start from an acid derivative such as the acid chloride (11.45; Y = Cl) orester (11.45; Y = OEt), which are both more effective acylating agents. The preparation ofcompound 11.36, shown in Scheme 11.11, illustrates this process, but the optimumconditions for ring closure vary considerably from one structure to another. The article byGold contains a valuable and detailed summary [4].

Formation of the oxazole ring is not always the last step in synthesis of the brightener.Unsymmetrical compounds that contain both a benzoxazole group and an ethene linkagecan be prepared by the anil synthesis [51], in which a compound possessing an activatedmethyl group reacts with a Schiff base. The preparation of brightener 11.31 is an illustrationof this method (Scheme 11.12).


chpt11(2).pmd 15/11/02, 15:46793


SC C

O

HO

OH

O

NH2

OH

O

NSO

NH3BO3+trichlorobenzene

150–220 °C

11.362

Scheme 11.11

O

N

CH3

(CH3)3C

NH2

NCH

HCCH

N

O

(CH3)3C

+

Benzoxazole intermediateSchiff base

KOH/dimethylformamide 40–60 °C

11.31Aniline

+

Scheme 11.12

NH2

OH

C

O

Y R NHC

R

OOH O

N

R+

11.45

2-Aminophenol Acylating agent Amide Benzoxazole

Scheme 11.10

chpt11(2).pmd 15/11/02, 15:46794

795

Most of the important class of coumarins used as polyester FBAs are made via 7-amino-3-phenylcoumarin (11.46), which can be prepared by the Pechmann procedure from m-aminophenol. Conversion of intermediate 11.46 to FBAs may be achieved in various ways,two of which are shown in Scheme 11.13.

The naphthalimide 11.23 is manufactured from acenaphthene by sulphonation, oxidationto the naphthalic anhydride derivative and conversion to 4-methoxy-N-methylnaphthal-imide as outlined in Scheme 11.14.

A Michaelis-Arbusov rearrangement followed by a Wittig-Horner reaction is involved inpreparation of the distyrylbenzene derivative 11.37, as shown in Scheme 11.15. Precautionsmust be taken in the first stage to minimise formation of the carcinogenic by-productbis(chloromethyl) ether 11.16. The stilbene bis-ester 11.38 can be made by a similarprocedure, or alternatively by the reaction of ethyl acrylate with 4,4′-dibromostilbene in thepresence of a palladium-based catalyst (Scheme 11.16), a synthesis that yields the requiredtrans form of the brightener.

The important bluish mixing component 11.22 for whitening polyester is made byFriedel-Crafts acylation of pyrene (Scheme 11.17). This tetracyclic hydrocarbon is notunlike anthracene in its susceptibility to substitution reactions. The most stable bondarrangement in pyrene appears to be that shown as form 11.47a, which contains threebenzenoid (b) rings. Canonical form 11.47b, containing only two such rings, contributes to alesser extent (Scheme 11.18). In all monosubstitutions, pyrene is attacked initially at the3-position, corresponding to the α-positions in anthracene or naphthalene.

Brightener structures of only moderate molecular size are of interest for white grounds inthe transfer printing of polyester fabrics. Derivatives of 6-acetamidoquinoxaline with anelectron-donating substituent (X) in the 2-position (11.48) were prepared by convertingquinoxalin-2-one to 2-chloro-6-nitroquinoxaline and condensation with amines (X =RNH), alcohols (X = RO) or phenols (X = PhO), followed by reduction and acetylation(Scheme 11.19). The nitro intermediates (11.49) are also of interest as low-energy dispersedyes for polyester [61].

FBAs for incorporation in the polymer melt are usually sold as the pure brightenerwithout diluents. Most polyester FBAs, however, are supplied in the form of an aqueousdispersion. Considerable care is required in formulating these dispersions; not only must thedispersion be stable in transportation and storage, but the dispersing agents selected mustnot adversely affect the properties (such as light fastness) of the goods treated with thebrightener. An FBA must be correctly formulated if it is to succeed commercially.

White polyester/cotton fabrics represent a substantial segment of the market. Such blendscan be brightened either by exhaustion or continuously by pad–thermosol or pad–steamprocesses. Suitable brighteners are selected from those intended for use on polyester orcellulosic substrates. Most polyester/cotton fabrics are woven constructions and it isessential to desize them before application of an FBA. Fabrics produced for sale as whitegoods must be chemically bleached before, during or after FBA treatment. In order toachieve the most solid white effects both fibre components of the blend require a brightener.The disperse and anionic brighteners selected must be compatible in hue. It is commonpractice, however, to brighten only one of the blend components for less critical end-uses.

In a recent detailed evaluation of CI Fluorescent Brightener 393 on polyester, thisproduct was incorporated into the polymer melt. The prebrightened fibre was blended withcotton and fabric knitted from these yarns was scoured and bleached. It was demonstrated


chpt11(2).pmd 15/11/02, 15:46795


Scheme 11.13

OHH2N

CH

CCH3CH2O O

CHO

O OH2NO ON

NH2

N

N

NN

O O

C

C

O

H3C

N

OH

O ONHH2N

O O

N

NN

H3C

+

Inert solvent

Lewis acidcatalyst, heat

11.46

1. Diazotisation2. Tobias acid

Copper-containing catalystInert solvent, air

11.43+

1. Diazotisation2. Na2SO3, then acid

Ring closure

11.39

chpt11(2).pmd 15/11/02, 15:46796

797

SO3H

OO O

SO3H

NO O

OCH3

CH3

Na2Cr2O7 / H2O

heat in anautoclave

1. CH3NH2

2. CH3OH / NaOH

11.23

Scheme 11.14

ClCH2 CH2Cl

CH2 CH2 P O

OCH2CH3

OCH2CH3

PO

OCH2CH3

OCH2CH3

CHO

CN

HC

CN

CH HC

CH

NC

HCHO

HCl / ZnCl2

2 P(OCH2CH3)3

+ 2 CH3CH2Cl

2base

11.37

Benzene

Ethyl chloride

Triethyl phosphite

Scheme 11.15


chpt11(2).pmd 15/11/02, 15:46797


Br HC

CH BrCCH

OCH2CH3

O

H2C

HC

CH HC

CH

HCC

O

CH3CH2O CH C

O

OCH2CH3

+ 2

Ph3P/Pd(OAc)2/NaOAc dimethylformamide

11.38

Ethyl acrylate

Scheme 11.16

N

N

N

Cl OCH3

OCH3

N

NN

OCH3

OCH3

AlCl3+

11.2211.47

Pyrene

Scheme 11.17

b

b

b

b

b

11.47a 11.47bScheme 11.18

that if an effective cotton bleaching process is applied in combination with producer-brightened polyester, the final degree of whiteness is sufficiently high to avoid the use of acotton-substantive fluorescent brightener [62].

If a padding process is used to brighten a polyester/cotton blend, both the disperse andanionic brighteners may be applied from the same pad bath, even when a resin finish isapplied simultaneously to the cellulosic component of the blend. Similarly, both types of

chpt11(2).pmd 15/11/02, 15:46798

799

FBA may be applied by exhaustion from the same bath. If the polyester portion of the blendis to be bleached with sodium chlorite, the cotton is usually brightened in a second stepsince most FBAs for cotton are destroyed by sodium chlorite. Both types of FBA arenormally compatible with a hydrogen peroxide bleaching process.

11.11 BRIGHTENERS FOR ACRYLIC FIBRES

At one time disperse-type FBAs, such as pyrazoline, coumarin or naphthalimide derivatives,were commonly used to brighten acrylic fibres. Today all the important brighteners for thesefibres are cationic in character and can be divided into two main categories:– type A: products that are oxidised by sodium chlorite– type B: products that are stable to sodium chlorite.

Type B brighteners can be applied in the absence of bleach, of course, but show their bestresults when applied simultaneously with sodium chlorite, where they are capable of givingexceptionally high whiteness.

Acrylic fibres are usually brightened from an exhaust bath in the presence of diluteorganic acid. Application of brightener by a padding method, such as pad–roll or pad–steam,can be used but is uncommon. When these fibres are brightened from an exhaust bath,careful control of application conditions is necessary. Most acrylic fibres have a glass-transition temperature in the region of approximately 80 °C and the rate of absorptionaccelerates rapidly with increase in temperature over this critical region. Too rapid anincrease in temperature can lead to unlevel absorption of the FBA.

As an alternative to oxidative bleaching with sodium chlorite, acrylic fibres may be givena reductive bleach using sodium bisulphite in the presence of oxalic acid. This method is

N

N

O

H

N

N

O

H

O2N

N

N

Cl

O2N

N

N

X

O2N

N

N

X

H2N

N

N

X

CH3CONH

HNO3 POCl3

HX

reduction

acetylation

11.48

11.49

Quinoxalin-2-one

Scheme 11.19

BRIGHTENERS FOR ACRYLIC FIBRES

chpt11(2).pmd 15/11/02, 15:46799


necessary with Courtelle (Courtaulds) because this fibre would be damaged by chloritebleaching. Both types of acrylic brightener can be applied with a bisulphite bleach.

11.11.1 Type A products

This category mainly consists of derivatives of 1,3-diphenylpyrazoline, such as compounds11.50–11.52. None of these substituted pyrazolines shows significant resistance to oxidativebleaching. The fluorescence stemming from the central ring disappears owing todehydrogenation to the corresponding pyrazole (Scheme 11.20). The sulphones 11.50 and11.51 are marketed as aqueous solutions of their formate salts. They produce violetbrightening effects of light fastness 4 and are capable of producing excellent whiteness. Thesulphonamide 11.52 gives greener effects and is incapable of producing the levels ofwhiteness attainable using either 11.50 or 11.51. In order to suppress the violet tone slightlyand to achieve a higher visual level of whiteness from a given amount of FBA, the sulphonetypes are sometimes formulated with a small amount of a shading dye.

The general method for the preparation of diphenylpyrazoline FBAs has already beendiscussed (Scheme 11.8). As a further illustration, the synthesis of the sulphone 11.51 isshown in Scheme 11.21.

Scheme 11.20

Cl X

Cl X

oxidative bleach

+ H2O

H2C CH2

NN

HC CH

NN

SO2CH2CH2OCHCH2NH(CH3)2

CH3

HCOO

11.50

X = _+

SO2CH2CH2CONHCH2CH2NH(CH3)2 HCOO

11.51

X =+ _

SO2NHCH2CH2CH2N(CH3)3 Cl

11.52

X =+ _

Cationicgrouping

Anion

Cationicgrouping

Anion

Cationicgrouping

Anion

chpt11(2).pmd 15/11/02, 15:46800

801

Cl

CH2CH2Cl

O

HN SO3H

H2N

Cl SO3Na

Cl SO2Cl

Cl SO2Na H2C CHCONHCH2CH2N(CH3)2

Cl SO2CH2CH2CONHCH2CH2N(CH3)2

Cl SO2CH2CH2CONHCH2CH2NH(CH3)2

SOCl2

Na2SO3

HX

HCOOH

HCOO

+

base

+

11.51

+ _

NN

NN

NN

NN

NN

Scheme 11.21

11.11.2 Type B products

The benzimidazoles 11.53 and 11.54, both of which gave greenish brightening effects, wereformerly used widely in the chlorite bleaching of acrylic fibres. The first of them to beintroduced, the bis(benzimidazolyl)furan 11.53, gave excellent whites of light fastness 4 buta strongly acidic dyebath was recommended to give the best results. More recently,compounds 11.53 and 11.54 were supplanted by the improved benzofuranyl (11.55) andbenzoxazolyl (11.56) benzimidazole derivatives, which give neutral shades of white withlight fastness ratings slightly higher than the bis-benzimidazole 11.53. They are also easier toapply and are capable of producing a higher level of whiteness.

For a time the parent compound 11.57, easier to prepare than its methylsulphonylderivative 11.55, was also marketed. This was capable of producing brilliant whites onacrylic fibres that were exceptionally violet in tone. If violet brightening effects of good lightfastness are required they can be achieved in combination with sodium chlorite using the

BRIGHTENERS FOR ACRYLIC FIBRES

chpt11(2).pmd 15/11/02, 15:46801


coumarin 11.58, which has been well-established for some years. Naphthalimide derivativessuch as compound 11.59 can be used to obtain greenish shades of white on acrylic fibres incombination with a sodium chlorite bleach, but the effects are generally inferior to thoseproduced by the preferred benzimidazoles 11.55 and 11.56.

Acrylic fibres can also be brightened during manufacture by gel application during thewet spinning process. Special FBAs have not been developed for this purpose. Products suchas the pyrazoline sulphone 11.50 and the benzofuranyl-benzimidazole 11.55 are suitable forthis application.

Some interesting organic chemistry is involved in the synthesis of chlorite-resistantbrighteners for acrylic fibres. None of these compounds is easy to make and methods forpreparation of the starting materials can be complex. Much manufacturing know-how isinvolved. One route for introduction of the benzimidazole nucleus into structure 11.55 isshown in Scheme 11.22. Preparation of the chemically rather simpler benzoxazole groupingin product 11.56 is shown in Scheme 11.23.

O

N

NH N

N

CH3

CH3X11.53

+

_

O

N

N

CH3

CH3 X 11.54_

+ NN

N

N

CH3

CH3

OH3CO SO2CH3

X11.55

_

+

N

N

CH3

CH3

O

N

H3CO

X11.56

_

+

N

N

CH3

CH3

OH3CO

X11.57

_

+ O O

N

N

NN

N

NCH3CH2

CH3

CH3

X

11.58

_

+

H3CO

N

O

O

N

N

CH3CH3

CH3

CH3

X

11.59

_

chpt11(2).pmd 15/11/02, 15:46802

803

Synthesis of the coumarin derivative 11.58 containing two isomeric triazolyl rings isindicated in Scheme 11.24. The substituted pyrazolyl derivative of naphthalimide 11.59 isprepared by a reaction sequence somewhat similar in principle to that already shown inScheme 11.14, using 4-amino-1,3,5-trimethylpyrazole in the penultimate step followed byquaternisation.

11.12 BRIGHTENERS IN DETERGENT FORMULATIONS

The largest single commercial use of FBAs is in domestic detergents. Detergent technologyis continually changing; modified, improved or even chemically new FBAs are still appearingon the market. The combination of large-volume sales by a limited number of detergentmanufacturers with several suppliers of FBAs competing for the available business ensuresthat prices remain low.

In the 1960s, FBAs for both cotton and nylon were incorporated into householddetergents. Today FBAs for nylon are of negligible significance for the detergent industry.FBAs that are capable of effectively brightening polyester from a household wash at anacceptable laundering temperature (≤60 °C) remain undiscovered. Fibre types other thanthe cellulosics are essentially ignored from the viewpoint of FBA selection in the context ofhousehold detergents.

There are fundamental differences in approach to the selection of FBAs for eitherhousehold detergents or textile finishing. Brighteners in detergent formulations are intended topreserve the whiteness of fabrics that already contain FBAs during many successive wash and

O

C

O

Cl H2N

H2N SO2CH3

O

NH

NH3CO SO2CH3

O N

N

CH3

CH3

SO2CH3H3CO

(CH3)2SO4

CH3SO4

+

base

11.55_

+

H3CO

Scheme 11.22

BRIGHTENERS IN DETERGENT FORMULATIONS

chpt11(2).pmd 15/11/02, 15:46803


NH2

OHH3CO

NH

N

Cl3C

O

N

N

NH

O

N

N

N

CH3

CH3

(CH3)2SO4

CH3SO4

+

base

_

+

H3CO

H3CO

Scheme 11.23

C

C

CH3CH2 N

CH3 O

OH

O

N

O

NN

NHH2N

O

N

O

NN

NH

C

CCH3 N

CH3CH2 N OH

O O

N

N

NN

CH3

N

NCH3CH2

CH3

CH3SO4

+

1. Ring closure2. (CH3)2SO4

11.58

_

+

Scheme 11.24

chpt11(2).pmd 15/11/02, 15:46804

805

wear cycles, whereas textile finishers apply FBAs to unbrightened material. If too much FBA ispresent during household washing then most of it is wasted. Using excessive amounts couldeven lead to deterioration in whiteness, as an excess of FBA builds up after several washes. Iftoo little FBA is used, the goods will gradually suffer a loss in whiteness although it could beseveral months before this is noticed by the detergent user. Typically a household detergentpowder contains 0.02–0.05% FBA, although the trend is towards the use of less FBA withactivated oxidative bleaching agents that are effective at lower washing temperatures.

The washing conditions and the type of surfactant present in household detergents varyfrom one part of the world to another. In some countries washing temperatures can be as lowas 30 °C, whereas in others they can be as high as 90 °C. Chlorine-containing bleaches areroutinely added to the wash in some countries but in others very rarely, if at all. Theintensity of sunlight to which the washed goods are exposed during drying greatly influencesthe rate of fading of the FBA and this obviously varies considerably throughout the world.Since household detergents are marketed directly to the public, much attention has beengiven to packaging, physical appearance and handling of the various formulations available.Discoloration or development of odour on storage of the product, for example, would inhibitsales whatever the actual performance of the product in the wash. All these considerationsinfluence the choice of type and quantity of FBA to be incorporated into a formulation.Furthermore, wash loads normally contain a variety of textile articles and an FBA designedto brighten cellulosic fibres must not adversely affect other fibre types present under theconditions of use of the detergent into which it has been incorporated.

Safety in use and environmental impact are increasingly important factors for theselection of FBAs used in household detergents. The waste water from a household wash isdischarged directly into the municipal drainage system. There is no opportunity fortreatment of this effluent before disposal, in contrast to washing processes operated underfactory conditions. The total quantity of waste waters from domestic washing is a majorcontribution to the effluent load on municipal effluent treatment facilities. Accordingly, theFBA and other components in a household detergent must be innocuous in theenvironment and free from toxic hazard.

Evaluation of an FBA for use in household detergents is a lengthy and costly procedure.The information obtained from a standard washing test on unbrightened cotton is valuablebut does not go far enough. Products must also be screened by measuring the build-up ofwhiteness during a series of successive washes using a detergent formulation containing thesmall proportion of FBA usually present in practice. Further tests continue on pre-brightened textiles in machine washing cycles and in field trials. Extensive toxicity andenvironmental test protocols must also be followed. Although many different FBAs havebeen found acceptable for use in household detergent formulations, only a few productsremain important today.

The structures of typical examples have been mentioned already, such as the substantiveDAST-type product 11.60, the distyryldiphenyl 11.15 and the vic-triazole 11.17. CertainFBAs, including these three compounds, can exist in relatively purer, near-colourless β-crystal modifications or in less pure yellowish crystalline forms. They are most easilyprepared in the yellower form but incorporation of this material into a detergent formulationleads to unacceptable discoloration of the powder. Today, these products are supplied in anear-colourless form that may be prepared, for example, by heating an aqueous alkalinesuspension of the yellowish material together with a co-solvent.


chpt11(2).pmd 15/11/02, 15:46805


NH

N

N

N

N

CH3

HOCH2CH2

NH

SO3Na

HC CH

NaO3S

HN

N

N

N

HN

N

H3C

CH2CH2OH

11.60

The DAST brightener 11.61 is the most important FBA used in detergent formulationsand is probably the cheapest to manufacture. It shows excellent performance attemperatures of 60 °C and above but relatively poor solubility in cold water compared withthe alternatives specified above. If it is to perform satisfactorily in low-temperature washing,it must be supplied in a finely divided form so that it will dissolve adequately during a typicalhousehold washing treatment. The necessary particle size can be achieved in various ways,one of which is wet milling in the presence of excess salt.

NH

N

N

N

N

NH

SO3Na

HC CH

NaO3S

HN

N

N

N

HN

N

O O

11.61

DAST-type FBAs may contain by-products (such as 11.13) derived from hydrolysis of oneor more of the chloro substituents in cyanuric chloride (11.10). One such troublesome by-product is 2,4-bis(anilino)-6-hydroxy-s-triazine (11.62). Not only is this compoundenvironmentally undesirable, it may also interact with certain bleaching agents and itspresence can lead to the development of unpleasant odours on storage of a detergent

chpt11(2).pmd 15/11/02, 15:46806

807

powder. The proportion of this triazine present as an impurity in a brightener such as 11.61can be kept to a minimum by careful control of the reaction conditions during manufacture.Alternatively, it can be extracted from the FBA using hot alkali.

N N

NHN NH

OH11.62

The instability of DAST-type brighteners towards chlorine-containing bleaches has beenmentioned already. They also show limited stability towards per-acids. As recommendedwashing temperatures have tended to fall in recent years, a bleach consisting of sodiumperborate activated by addition of tetra-acetylethylenediamine (11.63) has become animportant component of household detergent formulations. This system is effective attemperatures as low as 40–50 °C. Since the FBA may be sensitive to the activated oxidant,however, in some formulations it is necessary to protect compounds such as 11.60 or 11.61by encapsulating either the brightener or the activator, if adequate shelf-life is to bemaintained.

N CH2CH2

C

C

O

CH3

O

CH3

N

C

C

O

CH3

O

CH3

11.63

The so-called ‘super brighteners’ 11.15 and 11.17 are generally much more stable towardsactivated per-acid bleaching systems. Both products, and especially the vic-triazole 11.17,offer higher light fastness than the DAST brighteners. The distyryldiphenyl 11.15 is aneffective FBA when applied from a wash liquor at a temperature below 50 °C, but showspoor performance at higher temperatures and poor washing fastness in soft water. The vic-triazole 11.17 is effective at all temperatures but is expensive. Nevertheless, it may provecost-effective in tropical climates where washed textiles can fade severely during drying.Compound 11.15 is particularly effective in enhancing the brightness of the detergentpowder itself, although this in no way indicates its performance in the wash. Table 11.5summarises the advantages and drawbacks of the four major FBAs discussed above.

The poor performance of the distyryldiphenyl derivative 11.15 at higher washingtemperatures is a serious drawback in some countries. In an attempt to overcome thisdisadvantage, product 11.15 has been marketed in admixture with an analogous FBA(11.64) derived from 4-chlorobenzaldehyde-3-sulphonic acid (see Scheme 11.5). This muchless soluble variant is highly effective at high washing temperatures.

Where resistance to chlorine bleaches such as sodium hypochlorite is required, thenaphthotriazole 11.65 can be used. Formerly, this FBA was extremely important for use in


chpt11(2).pmd 15/11/02, 15:46807


detergents but today it is much less so. It has the advantage of brightening both cotton andnylon from the wash bath.

In the absence of hypochlorite bleach, pyrazolines such as the sulphonamide 11.20 andthe ester 11.66 give brighter whites than the naphthotriazole 11.65 on nylon garments in thewash. Especially brilliant whites with a somewhat greenish tone are given by compound11.66 but this FBA tends to stain polyester goods under wash bath conditions. Thesulphonamide 11.20 gives less intense whites but stains polyester less. Neither pyrazolinederivative is effective on cotton, so they are not much used in detergent formulationsnowadays.

Domestic detergents in liquid form have become increasingly popular in recent years.This trend has created problems in the choice of suitable FBAs, as it is more difficult todevise liquid formulations of adequate storage stability. Liquid detergents containing FBAscan cause yellow ‘specking’ faults on the washed goods, which can be a serious problem. Ithas been claimed [63] that the use of an FBA such as 11.65 or 11.67, which contain onlyone sulphonic acid group in their stilbene residue, ameliorates the ‘specking’ problem.Classical approaches to the preparation of stilbenes or their symmetrical disulphonates arenot applicable to the synthesis of their unsymmetrical monosulphonated analogues. Several

Table 11.5 Advantages and disadvantages of FBAs in detergent formulations

Product Advantages Disadvantages

11.61 Low price Unstable towards hypochlorite and activated perborateEffective at all temperatures

11.60 Effective at all temperatures Unstable towards hypochlorite and activated perborate

11.15 Stable in bleaching Poor performance above 50 °CGood light fastness Poor wash fastness

11.17 Effective at all temperatures High priceStable in bleachingExcellent light fastness

Cl HC

CH

NaO3S

HC

CH

SO3Na

Cl

11.64

N

N

N

HC

SO3Na

CH

11.65

Cl C

OCH3

O

11.66

NN

chpt11(2).pmd 15/11/02, 15:46808

809

condensation routes provide viable opportunities to introduce a single sulphonic acid groupinto the stilbene nucleus but these procedures do add substantially to the cost ofmanufacture [64].

NH

N

N

N

NH

N SO3Na

HC CH

HN

N

N

N

HN

N

OO

11.67

The disulphonated DAST derivative 11.25 containing four anilino groups per molecule iseffective in liquid detergent formulations and much cheaper to manufacture than themonosulphonated DAST brightener 11.67, which was withdrawn from the market in thelate 1980s. It has been necessary to purify compound 11.25 specially for use in detergents, inorder to eliminate traces of residual unreacted aniline as far as possible, owing to the toxicproperties of this impurity.

11.13 ANALYSIS OF FBAs

Qualitative analysis of FBAs is best carried out by thin-layer chromatography (TLC). Silicagel is the usual stationary phase for the TLC of FBAs. Various eluants are available andthese can be chosen according to the chemical nature of the FBA under test. Suitablestandards are required. The plethora of possible brighteners of the DAST type, togetherwith the various impurities present in such products, can cause difficulties in theiridentification by TLC. The technique can be used quantitatively, although costlyinstrumentation is required and considerable care must be taken in preparing and handlingchromatograms. Chapters by Theidel and Anders in the book edited by Anliker and Müller[7] contain valuable information on the analysis of FBAs by TLC. More recently, Lepri andDesideri have described methods for the TLC identification of FBAs in detergentformulations [65].

If suitable standards are unavailable (for example, if the FBA has not been encounteredpreviously) the active agent must first be isolated and purified. The pure compound can becharacterised by the usual techniques, including elemental analysis and infrared, n.m.r. andmass spectroscopy. Final proof of structure demands synthesis of the FBA indicated by theanalytical data. Once again, difficulties may be encountered with compounds of the DAST

ANALYSIS OF FBAs

chpt11(2).pmd 15/11/02, 15:46809


class. Optical densitometry using a Shimadzu CS 9000 flying spot scanner has beenevaluated for the analysis of anionic DAST-type brighteners [66].

Once the FBA has been identified, ultraviolet absorption spectroscopy affords a rapid andaccurate method of quantitative analysis. Care must be taken when interpreting the spectraof stilbene-type compounds, since trans to cis isomerisation is promoted by ultravioletradiation. Usually, however, a control spectrum of the trans isomer can be obtained beforethe compound undergoes any analytically significant isomerisation. FBAs are often marketedon the basis of strength comparisons determined by ultraviolet spectroscopy.

FBAs can also be estimated quantitatively by fluorescence spectroscopy, which is muchmore sensitive than the ultraviolet method but tends to be prone to error and is lessconvenient to use. Small quantities of impurities may lead to serious distortions of bothemission and excitation spectra. Indeed, a comparison of ultraviolet absorption andfluorescence excitation spectra can yield useful information on the purity of an FBA.Different samples of an analytically pure FBA will show identical absorption and excitationspectra. Nevertheless, an on-line fluorescence spectroscopic method of analysis has beendeveloped for the quantitative estimation of FBAs and other fluorescent additives presenton a textile substrate. The procedure was demonstrated by measuring the fluorescenceintensity at various excitation wavelengths of moving nylon woven fabrics treated withvarious concentrations of an FBA and an anionic sizing agent. It is possible to detectremarkably small differences in concentrations of the absorbed materials present [67].

High-performance liquid chromatography (HPLC) is being used increasingly to identifyFBAs, to investigate product purity and for process control. HPLC has many attributes thatTLC lacks, including greater sensitivity, better resolution and discriminatory power.Quantitative analysis can be carried out conveniently and rapidly using HPLC, providingthe constitution of the FBA is known and a pure sample is available for calibration.Drawbacks of this approach, however, include the fact that samples have to be runsequentially rather than in parallel, substantially increasing the time for analysis. Care isneeded to minimise the risk of cross-contamination caused by carry-over from one sample tothe next.

Although HPLC quickly became established for the analysis of organic compounds inmany fields, the development of test procedures for textile dyes and FBAs took place moreslowly. This is attributable to the number and variety of chemical classes represented andthe fact that these structures may be anionic, cationic or nonionic. Attempts to devisegeneral eluant systems to cope with this diversity in solubility characteristics met withconsiderable difficulties. Gradient elution systems using mixed solvents in variousproportions could be used but the techniques were complicated and time-consuming.Multichannel detection of peak wavelengths using a variable wavelength UV-Vis detector iseffective in enabling simultaneous monitoring of different components in mixtures [68].Chromatographic problems associated with the separation of anionic dyes and FBAs wereattributed to undesirable properties of silica-based packing materials in the column andbetter results were found using a chemically inert copolymer of styrene and divinylbenzene[69]. The addition of di-t-butyl-p-cresol (11.42) as an antioxidant was useful in ensuringchemical stability of oxidation-sensitive dyes and FBAs during extraction and analysis [70].Established techniques of HPLC analysis are available for estimation of the relatively fewFBAs that are widely used in detergent formulations [71,72].

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811

REFERENCES 1. P Krais, Melliand Textilber., 10 (1929) 468. 2. C Paine and J A Radley (ICI), BP 442 530 (1934). 3. A K Sarkar, Fluorescent whitening agents (Watford: Merrow, 1971). 4. H Gold in The chemistry of synthetic dyes, Vol. 5, Ed. K Venkataraman (New York: Academic Press, 1971). 5. D Barton and H Davidson, Rev. Prog. Coloration, 5 (1974) 3. 6. A Dorlars, C W Schellhammer and J Schroeder, Angew.Chem.Internat., 14(1975)665. 7. Fluorescent whitening agents, Eds. R Anliker and G Müller (Stuttgart: Thieme, 1975). 8. R Zweidler and H Hefti, Kirk-Othmer encyclopedia of chemical technology, 3rd Edn., Vol. 4 (New York: Wiley-

Interscience, 1978) 213. 9. R Williamson, Fluorescent brightening agents (Amsterdam: Elsevier, 1980).10. I H Leaver and B Milligan, Dyes and Pigments, 5 (1984) 109.11. A E Siegrist, H Hefti, H R Meyer and E Schmidt, Rev. Prog. Coloration, 17 (1987) 39.12. T Martini, Textilveredlung, 23 (1988) 2.13. B J Maier, Text. Asia, 21 (July 1990) 80.14. W S Hickman in Cellulosics dyeing, Ed. J Shore (Bradford: SDC, 1995) 81.15. Luminescence spectroscopy, Ed. M D Lumb (New York: Academic Press, 1978).16. J R Lakowicz, Principles of fluorescence spectroscopy (London: Plenum Press, 1983).17. M Pestemer, A Berger and A Wagner, Textilveredlung, 19 (1964) 420.18. X Qian, Y Zhang, K Chen, Z Tao and Y Shen, Dyes and Pigments, 32 (1996) 229.19. C Lubai, C Xing, H Yufen and J Griffiths, Dyes and Pigments, 10, No 2 (1989) 123.20. O Pitzurra, Melliand Textilber., 77 (1996) 331.21. R Griesser, Col. Res. Appl., 19 (1994) 446.22. J A Bristow, Col. Res. Appl., 19 (1994) 475.23. A Berger, Die Farbe, 8 (1959) 187.24. P S Stensby, Soap/Cosmetics/Chem. Specialities, 43 (July 1967) 80.25. R Griesser, Rev. Prog. Coloration, 11 (1981) 26.26. J Rieker, R Griesser and C Puebla, Textilveredlung, 31 (1996) 64.27. I Soljacic, A M Grancaric and K Weber, Textilveredlung, 10 (1975) 492.28. A M Grancaric and I Soljacic, Melliand Textilber., 62 (1981) 876.29. J R Aspland, J S Davis and T A Waldrop, Text. Chem. Colorist, 23 (Sep 1991) 74.30. I Soljacic and R Cenko, Melliand Textilber., 60 (1979) 1032.31. I Soljacic, D Kalovic and A M Grancaric, Textil Praxis, 46 (1991) 331.32. I Soljacic and K Weber, Textilveredlung, 9 (1974) 220.33. I Soljacic, A M Grancaric and B Luburic, Textil Praxis, 39 (1984) 775.34. K Seguchi, Y Ebara and S Hirota, Yukagaku, 34 (1985) 17.35. W Schürings, Textilveredlung, 29 (1994) 260, 291.36. R S Davidson, G M Ismail and D M Lewis, J.S.D.C., 104 (1988) 86.37. R Zweidler, Textilveredlung, 4 (1969) 78.38. R P Hurd and B M Reagan, J.S.D.C., 106 (1990) 49.39. H Ikuno, M Okuni, M Komaki and T Nakajima, Text. Res. J., 66 (1996) 464.40. Anon, Paper (June 1980) 33.41. H Ikuno, M Komaki and T Nakajima, J. Soc. Fibre Sci. Tech. Japan (1994) 87.42. I Grabtchev and T Philipova, Dyes and Pigments, 27 (1995) 321.43. I Grabtchev, P Meallier, T Konstantinova and M Popova, Dyes and Pigments, 28 (1995) 41.44. I Grabtchev and T Konstantinova, Dyes and Pigments, 33 (1997) 197.45. B R Modi, N M Naik, D N Naik and K R Desai, Dyes and Pigments, 23 (1993) 65.46. H Strahle, W Seitz and H Güsten, Z. Naturforsch., 316 (1976) 1248.47. H Güsten and G Heinrich, Ber. Bunsen-Ges., 81 (1977) 810.48. I H Leaver and G C Ramsay, Text. Res. J., 39 (1969) 730.49. R S Davidson, G M Ismail and D M Lewis, J.S.D.C., 103 (1987) 308.50. P Moechli, Dyes and Pigments, 1 (1980) 3.51. R S Davidson, G M Ismail and D M Lewis, J.S.D.C., 103 (1987) 261.52. L A Holt and I W Stapleton, J.S.D.C., 104 (1988) 387.53. L E Aicolina, I H Leaver and I W Stapleton, Dyes and Pigments, 11, No 3 (1989) 213.54. K J Smit and K P Ghiggino, Dyes and Pigments, 13, No 1 (1990) 45.55. D Reinehr and E Schmidt, J.S.D.C., 102 (1986) 258.56. T Martini and H Probst, Melliand Textilber., 65 (1984) 327.57. S Yongjia and R Shengwu, Dyes and Pigments, 15, No 3 (1991) 183.

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58. Z Zhu, H Tian, X Zhang and S Ren, Dyes and Pigments, 21, No 4 (1993) 235.59. I N Bykova, Tekstil. prom., 8 (Aug 1987) 55.60. H Oda, N Kuramoto and T Kitao, J.S.D.C., 97 (1981) 462.61. D W Rangnekar and P V Tagdiwala, Dyes and Pigments, 7, No 6 (1986) 445.62. P L Moriarty, AATCC Internat. Conf. & Exhib. (Oct 1994) 297.63. J Wevers, L A Halas and P R Peltre, USP 4 559 169 (1985).64. R V Casciani et al., AATCC Internat. Conf. & Exhib. (Oct 1991) 93.65. L Lepri and P G Desideri, J. Chromatog., 322 (1985) 363.66. U Denter, K Tiroke, D Knittel and E Schollmeyer, Melliand Textilber., 73 (1992) 834.67. S Cleve and E Schollmeyer, Textilveredlung, 30 (1995) 18.68. J C West, J. Chromatog., 208 (1981) 47.69. P C White and A M Harbin, Analyst, 114 (1989) 877.70. B B Wheals, P C White and M D Paterson, J. Chromatog., 350 (1985) 205.71. B P McPherson and N Omelczenko, J. Amer. Oil Chem. Soc. (1980) 388.72. G Micali, P Curro and G Calabro, Analyst, 109 (1984) 155.

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813

813

CHAPTER 12

Auxiliaries associated with main dye classes


12.1 INTRODUCTION

The aim in this chapter is to summarise the properties of auxiliaries normally used with eachof the main dye classes. Where these agents have been dealt with earlier, the emphasis hereis on application behaviour. Chemical details are included, however, for those auxiliariesthat have not yet been mentioned; emphasis is given to the auxiliaries used rather than toprocessing details.

12.2 ACID DYES

Only the products associated with acid and premetallised dyes are dealt with in this section.The auxiliaries used with mordant dyes are covered in section 5.8. Anionic acid dyes,applied principally to wool and nylon, vary widely in their fastness and level-dyeingproperties (section 3.2.2); in general, the higher the wet fastness of a dye the more difficultit is to apply evenly. Hence it is not surprising that the use of auxiliaries with acid dyes isrelated mainly to level-dyeing properties. There are two basic aspects:(1) controlling the pH to give a satisfactory dyeing rate and ultimate exhaustion(2) using auxiliaries to give additional levelling, either through a competitive mechanism that

exerts further control on absorption or through the promotion of migration and diffusion.

Temperature provides another means of control although this is rarely the only techniqueemployed. The control of pH is of particular importance, as the optimal pH varies withdifferent types of acid dyes. This can be seen in Table 12.1, which shows the pH valuesgenerally required to give 80–85% exhaustion [1]. However, in some cases, either bymodification of the dye type or by addition of certain auxiliaries, different pH values fromthose listed may be used.

Table 12.1 Dyebath pH values to give 80–85% exhaustion [1]

Dye class pH

1:1 Metal-complex dyes 2.0–2.5Levelling acid dyes 2.5–3.5Chrome dyes 4.0–5.0Milling acid dyes 4.5–5.5Disulphonated 1:2 metal-complex dyes 4.5–5.5‘Super-milling’ acid dyes 5.0–6.0Monosulphonated 1:2 metal-complex dyes 5.0–6.0Unsulphonated 1:2 metal-complex dyes 5.5–6.5

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814 AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES

Levelling acid dyes and particularly 1:1 metal-complex types generally require anexceptionally low pH in order to promote exhaustion and levelling; up to 3% o.w.f. sulphuricacid is most commonly used for levelling acid dyes, although hydrochloric, formic andphosphoric acids are also effective. In the case of conventional 1:1 metal-complex dyes it isessential to use a sufficient excess of acid over and above the typical 4% o.w.f. sulphuric acidnormally absorbed by the wool, otherwise there may be a tendency towards tippy dyeingsand lower wet fastness. The actual excess required depends on applied depth and liquorratio [2]; typical recommendations are given in Table 12.2.

Table 12.2 Amounts of sulphuric acid used with conven-tional 1:1 metal-complex dyes [2]

Sulphuric acid (96% solution)(% o.w.f.)

Liquor ratio <1% dye >1% dye

10:1 4.7 520:1 5.4 630:1 6.1 740:1 6.8 850:1 7.5 960:1 8.2 10

Such high concentrations of strong acid may cause fibre damage at the boil. After dyeingit is essential to ensure that the acid in the fibre is adequately neutralised. Hence formic acid(8–10% o.w.f.) is sometimes used instead, a further advantage being that it leads to lesschromium in the effluent. If the dyes are modified to have one or more of the three waterligands replaced by colourless inorganic complex anions such as hexafluorosilicate (SiF6)2–

ligands, their dyeing behaviour is markedly altered. This facilitates dyeing at pH 3.5–4.0with formic acid and an amphoteric auxiliary (a mixture, said to be synergistic, of quaternaryand esterified fatty amine ethoxylates, polyaddition compounds of fatty amine ethoxylatesand fluorosilicates) [3,4].

The use of sulphamic acid (12.1) has been recommended, resulting in a shift of pH from1.8 to between 3.0 and 3.5 as the temperature approaches the boil, thus giving rise to lessfibre damage. Typically, 6% o.w.f. sulphamic acid is added, together with an auxiliary andsodium sulphate. The change in pH arises as a result of hydrolysis of the sulphamic acid togive ammonium bisulphate (Scheme 12.1)[2,5].

NH2SO3H + H2O NH4HSO4

12.1

Scheme 12.1

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815

With the so-called ‘half-milling’ or intermediate levelling dyes, values in the range 1.8–3.5 would lead to too rapid a rate of exhaustion with consequent risk of unlevel dyeing. Forthese dyes, the optimal pH is 4.0–5.5, generally achieved using up to 2% o.w.f. acetic acid.Milling acid dyes and 1:2 metal-complex types are highly responsive to acid. Hence thetendency with these dyes is to use a pH-shift system (section 10.1), starting from neutral orslightly alkaline conditions and progressively decreasing the pH to the required level asdyeing proceeds. A hydrolysable organic ester or a latent-acid salt, such as ammoniumsulphate or ammonium acetate, may be used, often with ammonia to give a higher initialpH. Whilst such techniques do not damage nylon, initially alkaline conditions can lead tosome degradation of wool. Hence for wool it is preferable to choose dyes showing lowsubstantivity at pH 7–8 but exhausting well at pH values of 6.2 or lower [2].

Figure 12.1 shows the ease with which wool is damaged in highly acidic, neutral oralkaline dyebaths [3,4]. The least damage occurs at pH 3.5–4.0, slightly lower than theisoelectric point of pH 4.5–5.0 (section 3.2.2). These considerations have led to thedevelopment of processes by which milling dyes and 1:2 metal complexes can be applied atpH values close to the isoelectric range. An effective surfactant-type levelling or retardingagent must then be used to counteract the high rate of exhaustion promoted by this degreeof acidity [1–4,6–10].

pH2 4 63 5 8 91 107

Increasing hydrolysis of amide groups

Increasing hydrolysis of cystine links

Dyeing with 1:1 metal-complex dyes

Region of least wool damage

Isoelectric range

Figure 12.1 Wool hydrolysis and region of least damage as a function of dyebath pH at the boil [3,4]

In general, rather less acidity is required on nylon than on wool for application of thesame combination of dyes. However, there has been some discussion regarding the bestmeans of controlling pH [11]. In some cases, starting at pH 7 or higher with ammoniumsulphate or acetate can lead to variations in pH at the end of the process, withconsequential variations in performance. Better end-point control is achieved by starting atpH 6.0–6.5 using a sodium dihydrogen phosphate/disodium hydrogen phosphate buffer andensuring a slow rise of temperature. The improved consistency of dyeing may offset thehigher cost of the phosphate buffer. In some regions, however, the use of phosphates isregarded as environmentally sensitive.

ACID DYES

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A neutral electrolyte, usually 10–20% o.w.f. sodium sulphate or sodium chloride, is oftenadded with acid dyes to aid levelling. This action results from the competition for the dyeingsites in the fibre provided by this high concentration of inorganic anions. Ultimately theseare replaced by the dye anions as a result of their higher affinity. Electrolytes are lesseffective as levelling agents in near-neutral dyebaths, however, since under these conditionsthe cationic charge on the fibre is too low to attract simple inorganic anions and dyesorption is generally through nonpolar rather than electrostatic forces. Nevertheless, it isstill common to add an electrolyte when applying these dyes, although it functions primarilyto boost exhaustion through a common-ion mechanism rather than as a levelling agent.

The use of surfactant-type levelling agents is of importance with acid dyes on wool andnylon, especially with dyes of relatively high wet fastness. Anionic surfactants act bycompeting for the cationic sites and are mainly used to counteract fibre-orientedunlevelness due to physical and chemical irregularities in the fibre. Strongly cationicquaternary compounds readily form complexes with acid dyes, but may precipitate them ifused alone. Weakly cationic ethoxylated tertiary amines do not suffer from this disadvantageand are of great importance in minimising unlevelness associated with rapid dye uptake.Combinations of anionic and weakly cationic types, carefully chosen according to theprinciples described in section 10.7, are of particular importance since they counteract bothtypes of unlevelness. An incompatible combination of dyes is one that does not build up ontone because of the sequential sorption of individual components. A well-chosen levellingagent, or a combination of suitable agents, can effectively convert such a mixture into acompatible one.

Amphoteric levelling agents, combining the properties of anionic and weakly cationicagents in the same molecule, have attained increasing importance [6–10]. Originallydeveloped for the application of reactive dyes on wool, amphoteric agents have beenexploited with acid dyes, particularly for dyeing at pH 4–5. They are especially suitable forthe ranges of 1:2 metal-complex dyes containing ionic solubilising groups (carboxyl orsulpho) rather than the nonionised but polar groups (such as sulphonamide or sulphone) intraditional dyes (sections 3.2.2 and 5.1). These are often cheaper to manufacture and offerbetter wet fastness; their development and exploitation owed much to the use of amphotericbetaine levelling agents [12,13]. Although the behaviour of amphoteric agents has beenstudied with metal-complex and acid dyes on both wool and nylon, their main focus ofinterest has remained the application of reactive dyes to wool. For this reason, therefore,their application and mechanism of action are considered in more detail in section 12.7.2.Suffice to say here that the mechanisms observed are generally applicable to both reactiveand acid dyes, including metal-complex types.

By using more than the optimal amount of dye-complexing agent required for effectivelevelling, some of these products can be used as stripping aids either alone for partial non-destructive stripping or in combination with oxidising agents (such as sodium dichromateand sulphuric acid) or reducing agents (such as sodium formaldehyde-sulphoxylate orsodium dithionite) for more drastic destructive stripping.

There has been long-standing interest in the so-called low-temperature dyeing of wooland, to some extent, of nylon. Generally this implies dyeing at 60–90 °C, most commonly at80–85 °C, rather than at the traditional boil. This approach results in energy savings withboth fibre types but is particularly attractive to wool dyers because it results in less damageto the fibre than when dyeing at the boil. Although some earlier methods involved the use

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of solvents such as benzyl alcohol or chlorinated hydrocarbons, now unacceptable onenvironmental, health and safety grounds, methods involving surfactants continue togenerate interest, even though this does not result in widespread commercial use. Nonionicsurfactants have been favoured [2,13], including ethoxylated alcohols, ethoxylatednonylphenol [2] and polyglycol esters and ethers [14]. Amphoteric auxiliaries have alsogiven effective results. On nylon dyed at 75 °C with acid dyes, the best results were obtained[15] with the lauryl-substituted member of the fatty acylamidoethyl-N,N-dimethylglycinebetaine series indicated in structure 12.2. Compounds of general formula 12.3 have beenfound effective with acid dyes on wool [16].

R C

HN

O

CH2CH2 N CH2COO

CH3

CH3

12.2

R = lauric CH3(CH2)10 or palmitic CH3(CH2)14

_+

R1 C

HN

O

R2 N

R4

R3

12.3

R1R2R3, R4

===

long-chain alkylshort-chain alkyleneshort-chain alkyl

The most effective auxiliaries for low-temperature dyeing processes are generally thosethat result in accumulation at the fibre surface of an auxiliary-rich phase of high dyeconcentration. This implies the formation of an auxiliary-dye complex at a concentration asclose as possible to the critical micelle concentration and hence usually close to the limit ofsolubility for the system. This complex then migrates into the fibre; depending on theapplied depth, dyeing time may need to be extended. In order to meet these criteria, thenonionic types selected should have a low degree of ethoxylation, typically 4–10 ethyleneoxide units per molecule. This can lead to a high degree of auxiliary-dye specificity withconsequent implications for the compatibility of dyes in mixtures, as well as possibleproblems of precipitation as a result of low cloud point phenomena. In spite of the potentialattractions, such low-temperature dyeing methods have attained only limited commercialuse.

It is interesting to note that products effective in the low-temperature dyeing of wool andnylon tend to be effective in overcoming dyeability variations attributable to fibreirregularities, such as tippy or skittery dyeing in wool and barry dyeing in nylon. Theamphoteric agents have proved to be particularly efficaceous in this respect, playing animportant part in facilitating level dyeing with sulphonated 1:2 metal-complex dyes, whichare otherwise rather prone to tippy or skittery dyeing. The phenomenon of barriness in nylonfabric dyeing has been reviewed [17,18] and the factors discussed in section 10.7 in regardto the action of levelling agents are pertinent here. Mixtures of auxiliaries are particularlyeffective, such as a sulphonated anionic with a fibre-substantive cationic type based on analiphatic amine [18]. Care should be taken that such mixtures are compatible, eitherthrough the use of ethoxylated components and/or the addition of a solubilising ethoxylatednonionic agent.

In a study of ethoxylated ethylenediamine derivatives (12.4) in the application of acid dyesto nylon, covering a range of ethoxylation from 40 to 180 units per molecule (average

ACID DYES

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n = 10–45), the best initial restraining effect together with the highest uptake of dye atequilibrium was obtained with 180 ethylene oxide units (average n = 45) [19]. This highlyethoxylated ethylenediamine was found to increase dye uptake when incorporated into nylon 6granules as an antistatic agent [20].

CH2CH2N

(OCH2CH2)n

(OCH2CH2)n

N

(CH2CH2O)n

(CH2CH2O)n

H

HH

H

12.4

The use of liposomes as complexing agents in the application of premetallised acid dyes towool has been investigated [21–24]. Liposomes are lipid structures containing aqueouscompartments surrounded by bilayer membranes. However, the methods as yet available forthe preparation of these agents are hardly practical in dyehouse terms (section 10.3.4).

It is possible to increase colour yields on wool by the use of protease or hydrolase enzymes(section 10.4.2). Although some improvement in yield was observed at temperatures as lowas 50 °C, the increase was insufficient to be of commercial interest, but yields with enzymeat 85 °C were close to those obtained without enzyme at 100 °C [25].

The presence in dyehouse effluents of typical dye-complexing metal ions is an environ-mentally sensitive issue, such metallic contamination arising mostly from the decompositionof metal-complex dyes [26]. The synthetic complexing agent cucurbituril (section 10.3.2)can be used to selectively extract such metal ions from the effluent.

Continuous dyeing with acid dyes is most frequently carried out on loose fibre, tow orslubbing before yarn manufacture. Resilient fibres such as wool can cause problems at theimmersion stage and during subsequent steaming, leading to unlevel results characterisedchiefly by tippy or frosty effects. Similar effects can be observed with pile fabrics such ascarpets owing to differing degrees of penetration of the pile. These defects are usuallyovercome using a hydrotropic agent such as urea with surfactant auxiliaries [9,12,27,28].Certain anionic surfactants are claimed to be effective, particularly sodium dioctylsulpho-succinate and its 2-ethylhexyl and 1-methylheptyl isomers [27,28]. The mechanism involvesformation of an agent-dye complex that wets the fibres evenly and forms a uniform filmaround them. The surfactant creates a foam during steam fixation, thus assisting theuniform transport of the dye throughout the fibre; the complex subsequently breaks downand the dye is then uniformly fixed.

Detailed accounts of the printing of wool with acid dyes and metal-complex types areavailable [2,29]. Typical formulation details for print pastes are given in Table 12.3. Ahydrotrope such as urea or thiourea is used to increase solvation of the dyes and to act as ahumectant, thereby enhancing fixation. Additional solvents, such as thiodiethylene glycol(12.5) or sec-butylcarbinol (12.6), may also be added [2]. Locust bean or guar derivativesare used as thickening agents, either alone or in combination with water-soluble Britishgum; high solids content is preferred for fine line effects and low solids content for largerblotch prints, because of better levelling and freedom from crack marks.

For generation of acidic conditions, a non-volatile acid such as citric acid (12.7), or anacid donor such as ammonium tartrate (12.8) or ammonium sulphate, is preferred. An acidor acid donor is not used with 1:2 metal-complex dyes of high neutral-dyeing affinity,

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however, since this may lead to destabilisation of the print paste, aggregation, specking andunlevelness. Wool or the modified natural thickeners present may tend to promotereduction of certain sensitive azo dyes; to counteract this a small amount of sodium chloratemay be added to the print paste. Defoamers and surfactants to prevent frosting may also berequired.

In discharge printing a reducing agent is also required. The most widely used is zincformaldehyde-sulphoxylate (CI Reducing Agent 6), since this functions in the weakly acidicpH range and thus gives less fibre damage. There can be problems in washing out theunsulphonated arylamines produced by reduction of certain azo dyes [2]. Sodiumformaldehyde-sulphoxylate (CI Reducing Agent 2) may give excessive fibre damage since itrequires an alkaline medium. The water-insoluble calcium formaldehyde-sulphoxylate (CIReducing Agent 12) may cause screen blockage and inadequate penetration, althoughcommercial formulation as a 30% dispersion is said to give better results. The calcium saltmay be applied in admixture with the sodium salt. Thiourea dioxide is rarely chosen becauseof its low solubility (only 37 g/l at 20 °C) [2].

The washing-off of prints is best carried out with anionic polycondensation products ofarylsulphonic acids [29] since these can improve the wet fastness of anionic dyes.

Table 12.3 Typical formulation for the printing of wool withacid dyes [2]

Concentration (g/l)

Dye xUrea 50–100Thiodiethylene glycol 50Wetting agent 5–10Antifoam 1–5Acid or acid donor 10–30Thickener (10–12%) 500Water to give 1000

HOCH2CH2SCH2CH2OH

12.5

HOCH2HC

CH3

CH2CH3

12.6

HO C

CH2

O

CHO

OH

O

HO C

O

CH2

C

12.7

C

CH

CH

HO

HO

CO ONH4

O ONH4

12.8

ACID DYES

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12.3 AZOIC COMPONENTS

There are three main demands for auxiliaries in the application of azoic components [30–32]:(a) the composition of the naphtholate solution(b) the composition of the diazo solution (developing bath)(c) aftertreatments to develop hue and maximum fastness.

These will be considered first in relation to batchwise application, followed by variationspertinent to continuous dyeing and printing. The discussion relates solely to cotton, by farthe most important substrate for these dyes; application to other cellulosic substrates followsgenerally similar principles, the main difference being in product concentrations.

12.3.1 Composition of the naphtholate solution

A primary requirement for naphtholate preparation is soft water; otherwise, insolublecalcium or magnesium naphtholates will be formed. If soft water is not available then asequestering agent must be added, the sodium hexametaphosphate, EDTA or NTA types(section 10.2) being suitable. A little alcohol is generally added during the initial pasting anddissolving of the naphthol. Given water of suitable quality, the naphtholate bath inbatchwise dyeing then usually contains the following additions:– alkali, most often sodium hydroxide, although in certain circumstances (particularly with

regenerated cellulosic or bast fibres) sodium carbonate or trisodium orthophosphate maybe used [31]

– a protective colloid (dispersing agent) and perhaps a wetting agent– formaldehyde– electrolyte, either sodium chloride or sodium sulphate.

The purpose of the alkali is to convert the insoluble free naphthol into its colloidally solublesodium salt. An excess of sodium hydroxide is generally needed but too much will tend topromote hydrolysis of the amide groups present in most azoic coupling components. Theactual amount required varies with the naphthol and processing conditions; themanufacturer’s detailed literature must be consulted.

The protective colloid/wetting agent may be a single anionic agent; Turkey Red Oil, forexample, combines both functions but is prone to form a precipitate in hard water. Onlyanionic types are suitable, since nonionic and cationic types generally cause precipitation[31]. Most protective colloids are of the following types:(a) lignosulphonates(b) protein-fatty acid condensates(c) sulphonated condensates of aromatic compounds, especially of phenols and naphthols

with formaldehyde.

The chemistry of these product types has been described previously (section 10.6.1). Theanionic polyelectrolyte helps to stabilise the colloidal solution of the naphtholate, through amechanism similar to that already described. Where the protective colloid itself does not giveadequate wetting of the fabric a suitable wetting agent, which in batchwise dyeing mustfunction well in the cold, should be added; the alkylnaphthalenesulphonate types are suitable.

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The formaldehyde plays an important role in counteracting the tendency of amide-containing naphtholates to hydrolyse at high pH values to the o-carboxynaphthol, whichcouples to give coloured by-products of inferior fastness. Its protective action is in additionto that provided by the excess alkali and its use is recommended with most naphthols,exceptions being yellow acetoacetarylamides where coupling is inhibited. Formaldehydeoperates through the reversible formation of a 1-methylol derivative at 40–50 °C, but attemperatures above 50 °C this derivative reacts with a second molecule of naphtholate togive a non-coupling dinaphthylmethane compound (Scheme 12.2) [30].

OH

CONHR

OH

CONHR

CH2

HO

OH

CONHR

OH

CONH

CH2

R

HCHO

Naphtholate Methylol derivative

High temperatureNaphtholate

Insoluble methylene compoundScheme 12.2

The naphthols used in batchwise dyeing are moderately substantive and their exhaustionis improved by electrolyte addition, with consequent improvement in yield and fastnessproperties. The amount of electrolyte required varies with the substantivity of the naphthol,the depth applied, liquor ratio and substrate quality, but generally ranges from 10 to 40 g/lsodium chloride or sodium sulphate. Higher amounts are required for heavier depths of low-substantivity naphthols in long liquors. With high-substantivity naphthols on substrates thatare difficult to penetrate and short liquor ratios, treatment is begun in a salt-freenaphtholate solution and electrolyte is added later. After application of naphthols bybatchwise techniques, excess surface naphthol is usually minimised or removed byhydroextraction, suction, squeezing or by rinsing in 10–50 g/l electrolyte and 0.3–0.6 g/lsodium hydroxide solution.

Batchwise application of naphthols is generally carried out at 20–30 °C. Although ahigher temperature may be chosen to improve the penetration of difficult substrates, it

AZOIC COMPONENTS

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should not be allowed to rise above 50 °C; substantivity decreases with increasingtemperature. In continuous dyeing, however, in order to ensure uniformity of uptake fromthe pad bath, the requirement is for minimal substantivity. Hence application temperaturesare generally high (80–95 °C) and naphthols of low to medium inherent substantivity areused. These factors indicate a need for the following modifications to the auxiliaryformulations used:(a) formaldehyde should be omitted due to formation of the non-coupling

dinaphthylmethane derivative at temperatures higher than 50 °C(b) electrolyte should be omitted(c) the amount of wetting agent can be reduced, or it may be omitted if the higher

temperature together with the protective colloid provide sufficient wetting.

Naphtholates in the form of ready-to-use solutions are now available [31,32]. These areformulated to contain the essential auxiliaries, are quite stable on storage and offer thefollowing advantages:– no dissolving or boiling necessary– no dust during weighing and preparation and therefore cleaner working conditions– shorter times for setting up the process.

12.3.2 Composition of the diazo solution or developing bath

This bath is essentially a dilute solution of a diazonium salt produced either by thediazotisation of an arylamine (Fast Colour Base) or by simply dissolving a stabiliseddiazonium compound (Fast Colour Salt). Soft water is desirable but not essential. Generaladditions for batchwise dyeing with Fast Colour Bases include acid, sodium nitrite andpossibly ice, together with a dispersing agent. Hydrochloric acid is the most widely used acidto effect dissolution of the base and activation of the sodium nitrite so as to bring aboutdiazotisation. Temperatures must be kept low (5–15 °C) to avoid decomposition of therelatively unstable diazonium salt (section 4.3.1); hence ice is often added to the solution. Adispersing agent is used to ensure the fine and uniform dispersion of the azoic dye as it isformed during coupling. Only nonionic types such as fatty alcohol ethoxylates (section 9.6)are suitable, as anionic or cationic types may cause precipitation [31].

Once diazotisation is complete the excess hydrochloric acid must be neutralised beforethe diazonium salt is coupled with the naphthol, usually by addition of an alkali-bindingagent. The agent most commonly used is sodium acetate, which by reaction with thehydrochloric acid produces acetic acid, so that the resultant mixture of acetic acid andsodium acetate acts as a buffer. The acetic acid/sodium acetate balance must be adjusted tosuit specific needs related to the reactivity or coupling energy of the system (section 4.4),giving a pH varying from 4 to 5.5 for those having high coupling energy to 6–7 for thosewith low coupling energy. Examples of azoic diazo components and their relative couplingenergies are given in Table 7.2 of reference [30]. Sometimes buffer systems utilising sodiumdihydrogen phosphate and disodium hydrogen phosphate or sodium bicarbonate arepreferred.

When Fast Colour Salts are used hydrochloric acid and sodium nitrite are obviously notrequired, although some Fast Colour Salts do need an addition of acetic or formic acid. Thenonionic dispersing agent is still necessary but as most Fast Colour Salts contain an alkali-

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binding agent (aluminium sulphate, zinc sulphate, magnesium sulphate or, in a few cases,chromium acetate) to give the required pH, the only electrolyte additions to the developingbath are to correct any local variations in pH.

In some cases, as in the batchwise application of diazo components, it may be advisable toadd electrolyte to the developing bath to inhibit bleed-off of low-substantivity naphthols.Otherwise the auxiliaries for batchwise and continuous application of diazo components areessentially the same.

Fast Colour Bases in the form of ready-to-use solutions or dispersions are now available[31,32]. As can be seen from their advantages listed below, their use has implicationsregarding the addition of auxiliaries:– the formulations are resistant to freezing temperatures and stable in use– all except one product can be diazotised without the addition of ice provided the bath

temperature does not exceed 20 °C– diazotisation is complete in 2–5 min compared with up to 30 min for conventional Fast

Bases– no nitrous gases are formed during diazotisation– these formulations already contain a dispersing agent and no further addition is needed– no dusting or formation of lumps as during dissolution of conventional Fast Bases.

12.3.3 Aftertreatments to develop hue and maximum fastness

After the coupling (development) process is complete the goods are rinsed, acidified andgiven an alkaline soaping treatment. This not only substantially removes surface dye but alsobrings about a process of aggregation of dye molecules within the fibre, thus developing thefull potential of hue and fastness. A combination of Marseilles (olive oil) soap (3–5 g/l) withsodium carbonate (batchwise 1–2 g/l, continuous 2–3 g/l) is traditionally used. Apolyphosphate sequestering agent is needed if the water is hard. A second wash with anonionic surfactant is also required.

The conventional technique in printing is to apply the naphthol by padding as describedfor continuous dyeing, followed by printing with the diazo componenents using celluloseether, locust bean or guar derivatives as thickening agents. In other respects the auxiliariesand general processing requirements are similar to those described above. A different systeminvolves application of the naphthol coupling component and a stabilised diazonium salt inthe same print paste followed by neutral steaming to effect development; a starch etherthickening agent is recommended for this process [29]. In certain resist styles aluminiumsulphate is applied by printing onto naphthol-treated fabric; this brings about a localisedreduction in pH that inhibits coupling during subsequent application of the diazocomponent, thus giving rise to a resist effect [29].

Overall, the application procedures for azoic dyeing are quite complex since many factorsmust be taken into account, such as:– the specific naphthol and diazo components selected, with regard to molecular

characteristics, substantivity and applied depth– the application method, e.g. continuous or batchwise, paying attention particularly to

liquor ratio and substantivity in the latter case– the substrate, including unmercerised or mercerised cotton, causticised regenerated

cellulosics or bast fibres.

AZOIC COMPONENTS

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All the above criteria influence the concentrations of the various components required.Some indication of how they are affected can be gleaned from reference [30]. It is importanttherefore to consult detailed information from the supplier of the naphthols and bases. Suchinformation available on disc for use on a personal computer has been provided for thebatchwise dyeing of cellulosic yarns [31] and the continuous dyeing of cellulosic fabrics [32].

The stripping of fully developed azoic dyeings can often be carried out using a hotsolution of sodium hydroxide (1.5–3 g/l), sodium dithionite (3–5 g/l) and a surfactant;addition of anthraquinone (0.5–1 g/l) generally increases the effectiveness of the process.Yellow azoic dyeings are resistant, however, and can only be partially stripped [30]. On theother hand, stripping of naphtholated material before it has been coupled with the diazocomponent can be done quite effectively in boiling alkali.

12.4 BASIC DYES

There are two major characteristics of basic dyes applied by exhaustion techniques to acrylicfibres:(1) below the glass-transition temperature (about 80 °C) exhaustion is very slight,

becoming much more rapid at temperatures only a little above this(2) very little, if any, migration occurs at temperatures up to 100 °C.

Consequently the rate of dyeing, and hence levelness, are very difficult to control; thedegree of difficulty varies from fibre to fibre, generally tending to a maximum for readilydyeable fibres with a high glass-transition temperature. Owing to the sensitivity of somebasic dyes to alkaline hydrolysis, these dyes vary in their response to dyebath pH, againdepending on fibre type (section 3.2.4). The pH must be controlled to within 4.0 to 5.5 inorder to obtain reliable, reproducible results across the range of dyes and fibres. Hence inthe conventional batchwise application of basic dyes to acrylic fibres, auxiliaries must fulfiltwo functions:(1) to give the required pH(2) to control the rate of sorption in the critical temperature region and, as far as possible,

to promote migration.

A buffer system is preferred for the control of pH, the most common one being the relativelycheap acetic acid/sodium acetate system, although a simple addition of acetic acid may beadequate with water that does not show a significant pH shift on heating.

The major variables are undoubtedly the rate of temperature rise and the use of retardingagents to control level dyeing. A review of acrylic fibres and their processing is available[33]; here we are concerned only with the essential chemistry of the auxiliaries used,particular emphasis being given to retarding agents.

Cationic types of retarding agent are especially important. These function essentially ascolourless cationic moieties competing for the anionic sorption sites in the fibre. Quaternaryammonium compounds (section 9.5) largely predominate; their fundamental structure(12.9) offers the possibility of varying up to four substituent groups around a quaternarynitrogen atom, and hence the variety of possible structures is enormous. A range of thesecompounds examined for their retarding effect in the application of basic dyes [34] givessome idea of the possibilities (Table 12.4). The selection of a retarder depends on several

chpt12(2).pmd 15/11/02, 15:46824

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factors, however, of which the most important are the rate and extent of sorption of theretarder compared with those of the dyes. The dyeing kinetics of basic dyes in mixtures arenow universally denoted by compatibility values covering the range from 1 to 5 [35–38].Simply varying the substituents in otherwise structurally similar dyes can change theircompatibility values [37]. The sorption properties of quaternary ammonium compounds canbe varied and characterised similarly, as seen from the examples shown in Table 12.5 [39],which were obtained using a titration-spectrophotometric method [40].

The type of associated anion has only a minor effect on the properties of a cationicretarder. General practical experience [39,41] suggests that optimal control is achieved if the

Table 12.4 Structures of some typical retarding agents [34]

Substituents in quaternary ammonium compound 12.9

R R1 R2 R3 Anion X

C12H25 (dodecyl) CH3 CH3 CH3 ClCoco* CH3 CH3 CH3 ClC16H33 (hexadecyl) CH3 CH3 CH3 ClC18H37 (octadecyl) CH3 CH3 CH3 ClTallow** CH3 CH3 CH3 ClCoco* CH3 CH3 CH3 CH3SO4Coco* CH3 CH3CH2 CH3 CH3SO4Coco* CH3 C6H5CH2 CH3 Cl

* Consisted of approximately 47% C12 and 18% C14 with lesser amounts of C8, C10,C16 and C18 hydrophobes.

** Consisted of approximately 48% oleyl, 27% cetyl and 13% stearyl, with minorquantities of others.

R1

N

R3

R2R X

12.9

+ _

BASIC DYES

Table 12.5 Compatibility values of retarding agents [39]

Substituents in quaternaryCompatibilityammonium compound 12.9valueassigned by

R R1 R2 R3 experiment

C14H29 CH3 C14H29 CH3 1.0C6H5CH2 CH3 C14H29 CH3 2.5CH3 CH3 CH3 C14H29 3.0C6H5CH2 CH3 C7–9H15–19 CH3 5.0C6H5CH2 CH3 C6H5CH2 CH3 >5.0

chpt12(2).pmd 15/11/02, 15:46825


retarder has a compatibility value equal to or slightly lower than that of the dyes, so that itwill tend to be absorbed by the fibre either at the same rate as the dyes or somewhat morequickly. If the compatibility value of the retarder is significantly lower than that of the dyes,then there is a very real tendency for it to act as a blocking agent (with attendant problems),whilst if its compatibility value is much higher its efficacy is impaired.

Once the substituents in the quaternary ammonium compound have been selected,consideration must be given to the applied concentration of retarder. Acrylic fibres varysignificantly in the number of anionic sites available for sorption of cations but it is generallyassumed that maximum likelihood of level dyeing occurs when the number of cations in thesystem (retarder as well as dyes) is just enough to saturate the anionic sites in the fibre.Thus the amount of retarder needed to achieve this theoretical saturation will vary fromfibre to fibre, and also depends on the applied concentrations of the dyes. More retarder willbe needed for fibres of high saturation value and for lower applied depths; the actualquantities required to satisfy the given conditions are generally specified by the dyemanufacturers. However, the use of these theoretical quantities can lead to lower degrees ofdye exhaustion within normal dyeing times. In any case level dyeing is not just simply afunction of the ionic dye–fibre system but involves many other aspects, especially physicalfactors such as substrate form and machinery efficiency. It may well be that in a givenpractical situation there may be little or no level dyeing problem, so why use any moreretarder than is necessary to ensure a level dyeing under practical conditions? Experiencesuggests that much less than the theoretical amount of retarder will often be adequate andthis will help to alleviate any problems due to saturation if subsequent reprocessing for shadecorrection is needed.

In addition to having an effect on the rate of dyeing, cationic retarders will assistmigration to an extent that depends on the fibre and the substantivity of the dyes. Retarderstend to diffuse more quickly than dyes and to be absorbed at lower temperatures (typically65–70 °C, compared with 80–85 °C), although the magnitude of these effects will depend onthe structure and properties of both retarder and dye. In some cases, such as hank dyeing onmachines with poor circulation or inadequate temperature control, it may be preferable touse a retarder that almost totally restrains the uptake of dye until the top temperature hasbeen reached, after which dye sorption takes place gradually.

A useful general classification of cationic retarders according to their properties has beengiven [42]:(a) strongly cationic with a strong blocking effect(b) moderately cationic with a weak blocking effect(c) weakly cationic with no blocking effect(d) products with little or no retarding effect but giving some levelling.

Products in groups (b) and (c) allow for greater safety margins and give optimal exhaustioncurves in bulk practice, although they may be more expensive than products in group (a).The main need for retarding activity arises during the critical exhaustion phase as thetemperature increases from about 80 °C to the boil. Therefore some cationic retarders havebeen designed to hydrolyse progressively in this temperature region, so reducing theretarding activity in the later stages of dyeing and safeguarding against blocking effects.Subsequent shading and redyeing are then less problematical. Furthermore, the amounts ofhydrolysing retarder used are perhaps less critical than with their non-hydrolysing

chpt12(2).pmd 15/11/02, 15:46826

827

counterparts, although more may be needed initially to obtain an equivalent retarding effect.A combination of hydrolysing and non-hydrolysing types may be used in somecircumstances.

On 100% acrylic materials the quaternary ammonium retarders are used almostexclusively. Other types have been evaluated, however. For example, saturated alkylamines(RNH2; R = C10, C12, C14 and C16 hydrophobes) were found to be just as effective as thequaternary types although other factors, such as aqueous solubility at the optimal dyebathpH and resistance to subsequent discoloration, favour the quaternary compounds [34]. Onthe other hand, bis(hydroxyethyl)cocoamine (12.10) had relatively little effect and theamphoteric carboxymethyldimethylcocoamine (12.11) none at all, although dimethyl-cocoamine oxide (12.12) was quite an effective retarder [34]. Other cationic compoundsused [43,44] have included alkylpyridinium salts, imidazoles and imidazolinium salts,alkyldiamines, alkylpolyamines, as well as sulphonium and phosphonium derivatives.

Polymeric cationic retarders that contain up to several hundred cationic groups permolecule have been proposed [45–47]. The early types [45,46] were described asquaternised polyamines (section 9.5) of relative molecular mass 1000–20 000, as comparedwith 300–500 for conventional quaternary ammonium compounds. Polyacrylamides ofmolecular mass 2500–780 000 have been evaluated more recently [47].

These agents, because of their large molecular size, do not diffuse into the fibre but arestrongly adsorbed at the fibre surface, reducing its anionic potential. They retard the dyeingrate far more than does an equal concentration of a conventional quaternary agent, but donot assist migration. Some of these products can adversely affect the compatibility of dyes asa result of selective behaviour but are said to be free from blocking effects. They do notinterfere with crimp development as conventional retarders sometimes do and areparticularly effective in giving superior coverage of bicomponent fibres. Since they are notabsorbed into the fibre but concentrate their activity at the surface, these polymers areeffective at much lower concentrations than are conventional types and this favours theircost-effectiveness. Their retarding action is sustained throughout the dyeing cycle.

Nevertheless, it does appear that the molecular mass of the polymer needs to be optimisedin relation to the type of dyes used [46,47] With polyacrylamides [47] for example, a dye ofsmall molecular size responds best to a retarder of large molecular size, and vice versa. Theseeffects have been explained on the basis of the relative ease of diffusivity of the dyes throughthe polymeric retarder to reach the fibre surface. Thus a dye of small molecular size needs theextra resistance to diffusion of a retarder of great molecular mass, whilst such a retarder wouldpresent too effective a barrier to diffusion of a dye of larger molecular size. This relationshipclearly has major implications for the selection of dyes to achieve close compatibility inmixture recipes. Despite the claimed advantages and the prediction as long ago as 1973 thatpolymeric retarders would rapidly become the preferred choice for dyeing acrylic fibres with ahigh content of dye sites [45], they have not attained a commercially significant role.

RCOCO N

CH2CH2OH

CH2CH2OH

12.10

RCOCO N CH2COO

CH3

CH3

12.11

_+CH3

N

CH3

RCOCO O

12.12

BASIC DYES

chpt12(2).pmd 15/11/02, 15:46827


Retarders of opposite ionic charge to the dyes can be used [33,36,48–51]. Anionicretarders function by forming a thermally labile complex with the dye and thus lowering thesubstantivity of the dye for the fibre. Undesirable precipitation of this complex, which is oneof the drawbacks of the system, can be inhibited:(a) by using excess anionic agent(b) by using an anionic agent that contains two or more sulphonate groups so that the

resultant 1:1 complex retains solubility(c) by incorporating a nonionic agent as an antiprecipitant.

Examples include sodium dinaphthylmethanesulphonates (section 10.6.1) andpolyethoxylated alkylarylsulphates (section 9.4). Polymeric types, such as polystyrenesulphonate, have been tried but do not seem to offer any advantages.

The advantages and disadvantages of anionic retarders can be summarised as follows.The advantages include:(a) the system is compatible with anionic dyes and anionic dispersing agents in the dyeing

of fibre blends(b) there is no blocking of dyeing sites in the fibre(c) they have no adverse effects on the bulkiness of certain bicomponent fibres(d) they promote good migration of dyes(e) they can be used as stripping agents to reduce the depth of colour in reprocessing.

The disadvantages include:(a) to prevent precipitation the quantity of anionic retarder should increase with increasing

quantity of dye (the opposite of the situation with cationic retarders) and this conflictswith requirements for promoting exhaustion; hence exhaustion of dye when applyingmedium or heavy depths is poor

(b) they show less levelling during the exhaustion stage(c) the use of cationic softeners in the dyebath is not possible.

In practical terms the disadvantages outweigh the advantages, thus limiting the importanceof anionic retarders.

Electrolytes such as sodium chloride and sodium sulphate tend to retard dyeing[36,52,53] through preferential adsorption and subsequent displacement by the dye of themore mobile sodium ions, although the effect is relatively weak even compared with theweaker cationic retarders. Nevertheless, the use of up to 10% o.w.f. sodium sulphate incombination with a cationic retarder may enable the amount of the latter to be reduced byup to 20–30% [36]. The limitations of electrolytes, apart from this lower effectiveness, arethat they reduce the final uptake of dye, their effectiveness decreases with increase intemperature and their effect is greatest with fibres containing weakly anionic groups such ascarboxylate, rather than stronger ones such as sulphonate. Cationic softeners for acrylicfibres are sensitive to the presence of electrolytes, although sulphate-tolerant softeners maybe used.

The retarding effect of electrolytes in the application of basic dyes to acrylic fibresincreases with increasing concentration of salt up to a certain level. Increasing theconcentration beyond this point has no further effect on exhaustion with certain univalentanions, whilst with multivalent types there is an increase in dye sorption (Figure 12.2)

chpt12(2).pmd 15/11/02, 15:46828

829

[54,55]. These results have led to the conclusion that ionic mechanisms alone do notentirely explain the complex interactions that occur between basic dyes and acrylic fibres.Hydrophobic interaction also plays an important part and it has been demonstrated [55]that multivalent anions such as sulphate or phosphate can enhance the hydrophobicinteraction, thereby also increasing dye sorption in some circumstances. Whilst such resultsare of interest in terms of dyeing theory, it is extremely doubtful whether there will ever bepractical interest in exploiting the use of electrolytes at such high concentrations.

Salt concentration in dyebath

Dye

sor

ptio

n on

fibr

e A

B

A

B

Represents sulphates and phosphatesRepresents chlorides, bromides and nitrates

Figure 12.2 The effect of salt on the equilibrium sorption of basic dyes on acrylic fibres [54,55]

BASIC DYES

The use of dye-solubilising agents such as urea or thiourea is more usually associated withcontinuous dyeing or printing. However, such compounds have also been investigated for theireffects in exhaust dyeing [56,57]. These compounds increase the rate of exhaustion and exerttheir maximal accelerating effect at low temperatures (e.g. 80 °C) and low dye concentrations.Despite the higher acceleration factor at 80 °C as compared with 100 °C, the ultimate yields at80 °C are lower than at 100 °C (Figure 12.3) [57]. When dyeing at 100 °C or above it isdifficult to see any commercial reason for making these additions, particularly in view ofenvironmental concerns regarding such compounds.

Continuous dyeing of acrylic fibres with basic dyes [50,58,59] generally requires the useof saturated steam for fixation. As in batchwise dyeing there is a need to maintain anoptimum pH of 4.5–5.0. If a sodium acetate/acetic acid buffer were to be used the aceticacid may volatilise in the steam, leading to development of alkalinity; hence it is usual toutilise a non-volatile acid such as citric acid (12.7) or tartaric acid (12.14). The thickeningagent for use with basic dyes must not be anionic, a useful choice being galactomannan-based locust bean gum. Hydrotropes and fibre-swelling agents assist dye solubilisation andfixation; compounds used include thiodiethylene glycol (12.5), dicyanoethylformamide(12.15) and potassium thiocyanate (KSCN). A nonionic wetting and solubilising agent mayalso be useful.

In general, for the pad–steam application of basic dyes to acrylic fibres, the auxiliaries areselected to increase the solubility of the dyes with minimal retardation and maximalimprovement of fixation. In a detailed statistical study along these lines [60], it was

chpt12(2).pmd 15/11/02, 15:46829


C

CH

CH

HO

C

HO

O OH

O OH

12.14 NC

O

H

CH2CH2CN

CH2CH2CN

12.15

Thiourea concentration/g l–1 Thiourea concentration/g l–1

0.4 0.8 1.2 1.6

50

60

70

80

90

100D

ye e

xhau

stio

n/%

Dye

exh

aust

ion/

%

0.4 0.8 1.2 1.6

50

60

70

80

90

100Dyeings at 80 oC Dyeings at 100 oC

1.52.02.53.0

Dye concn/% owf

1.52.02.53.0

Dye concn/% owf

Figure 12.3 Effect of thiourea on exhaustion of CI Basic Yellow 21 (12.13) by acrylic fibres [57]

N

CH

CH3H3C

CH3

CH

CH3

N

Cl

12.13

CI Basic Yellow 21

concluded that the following represented the optimal composition of a mixed auxiliaryformulation:50% of a solubilising agent based on a branched-chain fatty alcohol with 8 units ethylene

oxide per molecule32% of a short-chain fatty alcohol with 2.5 units ethylene oxide per molecule14% of a plasticising agent for acrylic fibres12% of an ethoxylated long-chain alcohol (C10–C18) with 15 units ethylene oxide per

molecule12% of a short-chain fatty alcohol that increases the fixation of basic dyes.

In addition to the conventional dyeing of acrylic fibres, there is considerable interest in so-called gel dyeing of acrylic filaments during the manufacturing process after extrusion. Fromthe viewpoint of auxiliary usage this is outside the scope of the present work, but a usefulaccount of the factors involved is available [61].

Basic dyes are used to dye acid-modified polyester fibres, in which case there is usually lessneed for a retarding agent. Glauber’s salt is often added, however, to guard against hydrolyticdegradation of these fibres [62]. Cotton modified according to Scheme 12.3 can then be dyedwith basic dyes, although commercial exploitation of such a process is unlikely [63].

chpt12(2).pmd 15/11/02, 15:46830

831

In direct printing of acrylic fabrics a typical stock print paste [29] may contain thefollowing components by mass:

0.5% citric acid (12.7)1–2% dicyanoethylformamide (12.15)

3% thiodiethylene glycol (12.5)7% acetic acid (30%)

50–60% locust bean thickener

Dioctyl phthalate (12.16), caprolactam (12.17) and urea together with resorcinol (12.18)are also said to act as fixation assistants [64].A further addition of an anionic thickeningagent such as carboxymethylcellulose [29] can act as a levelling agent when printing largeblotches. A wash-off with anionic surfactant is usually given.

CH

CH

CH CH

CH

O

OHOH

CH2OHC

Cl

O

CH

CH

CH CH

CH

O

OHOH

CH2 O

C

O

CH

CH

CH CH

CH

O

OO

CH2 O

C

O

CH2

CH

O

O

C

CH

O

CH

CH

SO3Na

OHCH

SO3Na

HO

acylation

NaIO4oxidation

NaHSO3

Scheme 12.3

C

C

O

O

O

O

(CH2)7CH3

(CH2)7CH3

12.16

C

HN

H2CCH2

CH2

CH2

CH2

O

12.17

OH

OH

12.18

BASIC DYES

chpt12(2).pmd 15/11/02, 15:46831


Discharge white styles are obtained using either formaldehyde-sulphoxylate or the weakertin(II) chloride as reducing agent; crystal gum or British gum are recommended thickeningagents, together with potassium thiocyanate as a fibre-swelling agent [29]. For coloureddischarges tin(II) chloride is the recommended reducing agent, since formaldehyde-sulphoxylate would reduce the illuminant basic dyes; other additions are generally as fordirect printing. Discharge styles, after steaming and rinsing, are given a clearing treatment at40 °C in 1 ml/l ammonia (25%) and 1 g/l sodium dithionite [29], followed by rinsing andsoaping with anionic detergent at 60–70 °C.

Non-destructive partial stripping techniques for basic dyes on acrylic fibres are carried outat 100 °C (or higher if possible) using, for example, 1–10% o.w.f. anionic retarder and 1 g/lacetic acid (60%), or 1–5 g/l Marseilles (olive oil) soap. Destructive stripping requiresacidified (pH 5.5–6.0) sodium hypochlorite, followed by an antichlor treatment in sodiumdithionite or sodium bisulphite. In some cases a preliminary boiling treatment in 5 g/lmonoethanolamine and 5 g/l sodium chloride is said to improve the effect of the strippingtreatment.

12.5 DIRECT DYES

Direct dyes represent one of the simplest dyeing systems, usually requiring only anelectrolyte as an essential auxiliary for their application. Nevertheless a surfactant maysometimes be added to assist wetting and levelling, as well as a sequestering agent, sincemany direct dyes are sensitive to hard water. Control of pH may also be desirable. Certaintraditional dyes require aftercoppering as part of their application procedure, whilst it isusual to aftertreat direct dyeings to improve their wet fastness properties. The dyeing ofpolyester/cellulosic blends with direct and disperse dyes requires application at temperatureshigher than 100 °C.

An up-to-date account of the application of direct dyes is available [30]. The main areato be considered in the batchwise application of these dyes is the use of either sodiumchloride or sodium sulphate to promote exhaustion, although the sulphate can give rise tocalcium sulphate deposits in hard water. Direct dyes vary enormously in their response toelectrolyte; in general the more highly sulphonated dyes require greater amounts of salt.This is in line with the behaviour of dyes according to the universally used SDCclassification [30,65,66], whereby dyes are allocated to three application classes. Class Adyes are generally the most soluble and least sensitive to salt, hence necessitating substantialadditions of electrolyte to boost their low exhaustion values. It is advisable with class A dyesto add electrolyte to the rinsing water to inhibit the otherwise copious bleed-off of dye intothe water. For this purpose magnesium sulphate may be more efficient than sodium saltssince it can form the less soluble magnesium salt of the dye, but the acceptability of this willdepend on whether magnesium can be tolerated in subsequent processing. Dyes in classes Band C are generally less soluble and are so responsive to electrolyte that salt must be addedgradually over the dyeing cycle as otherwise the rate of strike will be so rapid as to giveunlevel, poorly penetrated dyeings and there may even be salting out of the dye in thedyebath. More salt is needed in longer liquors, and for heavier depths.

Dyes having the same CI generic name but made by different manufacturers may alsorequire different amounts of electrolyte to be added to the dyebath, according to the amountof electrolyte present in the commercial formulation. A typical instruction is to use from 0 to

chpt12(2).pmd 15/11/02, 15:46832

833

20 g/l salt depending on the factors described above. Electrolyte may influence migration aswell as exhaustion [67], an optimal concentration of electrolyte being found for maximalmigration of class A and B dyes, whilst the migration of class C dyes decreases withincreasing amounts of salt. As mentioned above, sodium chloride and sodium sulphate arethe electrolytes most commonly used in practice and it is generally accepted that they exerttheir effect by means of the common ion effect. There is another aspect, however;electrolytes also modify the structure of water around the hydrophobic groups in dyemolecules and around the surface of the fibre, creating a new order in solution as a result ofsolvation. This enables dye molecules to approach more closely to the fibre surface withinthe influence of short-range interactive forces [68].

Numerous electrolytes have been investigated in fact, although some of the researchwork is seriously limited by having been carried out with only a few dyes, sometimes justone. In an investigation of the relative effects of Zn, Mn, Cd, Sr, Al and Ce nitrates [68], itwas found that the size of the cation, as well as its charge, played a part in the sorptionprocess: saturation values and sorption rates increased with increasing size of the cation. Asimilar effect has been observed for the series: LiCl < NaCl < KCl [69], whilst in binarymixtures of these electrolytes [70] the larger cation has a strongly promotional effect on theactivity of the smaller cation (Figure 12.4). It has to be admitted, however, that theseexperiments were carried out at temperatures lower than is typical in commercial dyeingwith direct dyes.

Mention has been made above of the use of magnesium sulphate to prevent bleeding ofclass A dyes during rinsing. With carefully selected dyes [71], magnesium salts caneffectively replace the conventional sodium salts during dyeing. An optional mixture of

Concentration of dye in solution/(mol/l) × 104

2 6 10 14

2

4

6

8

10

Con

cent

ratio

n of

dye

on

fibre

/(m

ol/k

g) ×

102

t1/2 (min1/2)2 6 10

0.05

0.10

0.15

0.20

0.25

0.30Equilibrium absorption at 34 oC Rate of dyeing at 40 oC

Mt/

M∞

KCl (0.1 mol/l)NaCl (0.1 mol/l)LiCl (0.1 mol/l)

KCl (0.05 mol/l) + NaCl (0.05 mol/l)LiCl (0.05 mol/l) + NaCl (0.05 mol/l)

Figure 12.4 Equilibrium absorption isotherms at 34 °C and rate of dyeing curves at 40 °C for CI DirectBlue 1 on viscose in the presence of electrolytes singly and in binary mixtures [70]

DIRECT DYES

chpt12(2).pmd 15/11/02, 15:46833


organic magnesium salts has been formulated. Since precipitation could be a problem, thisproprietary mixture also contains a polymeric sequestrant. The rationale behind this processwas to eliminate the conventional inorganic electrolytes from the dyeing process, since thesehave chronic toxic effects on freshwater organisms above certain concentrations. Inaddition, the usual electrolytes are quite difficult to remove from effluent. It is claimed thatmagnesium salts are used at lower concentrations and are easily removed from the effluentby precipitation.

For environmental reasons, other attempts have been made to reduce the amount ofconventional electrolyte added. Lowering the liquor ratio will in itself reduce the amount ofelectrolyte required. In one commercially feasible system [72], a range of direct dyes wassuccessfully screened to select members that could be applied efficiently to give 95–100%exhaustion using significantly less electrolyte than usual. Thus at applied depths up to 2–3%,only 2–5 g/l salt is required; navy and black dyeings can be produced with only 7.5–10 g/lsalt compared with the conventional 25 g/l addition.

Ultrasonic irradiation has been shown in laboratory studies [73] to increase dyeexhaustion, enabling salt levels to be reduced. However, it seems doubtful whether thehigher effectiveness is sufficient to merit development to overcome the problems involved inscaling-up the ultrasonic equipment to bulk-scale processing. For example, in oneexperiment using 5% salt at 65 °C, ultrasound treatment increased the dye exhaustion from77% to 82%.

A sequestering agent is usually necessary in hard water to prevent the formation ofsparingly soluble calcium and/or magnesium salts. These can lead to uneven deposits oflower fastness on the surface of the fabric as well as reduced yields due to precipitation inthe dyebath. Polyphosphates are particularly useful in this respect. Organic sequesteringagents such as EDTA must be avoided with metal-complex direct dyes as they tend toextract the metal from the dye molecules, resulting in a change in hue and a significantlowering of fastness, although they can be used safely with unmetallised dyes. In a ratherunorthodox approach to the use of sequestering agents, ethylenediaminetetra-methylphosphonic acid (EDTMP; 12.19) applied as a pretreatment for cotton at ambienttemperature was shown to increase the exhaustion of CI Direct Red 79 (12.20) applied at100 °C in the presence of salt [74]. The maximum effect was achieved with 3 mg ofsequestering agent per g of cotton. On dyeings carried out for 30 minutes, this gavesurprising improvements in exhaustion from 35% to 45% (for 10 mg dye per g cotton) andfrom 17% to 29% (for 40 mg dye per g cotton). Rate of dyeing was apparently increased, too.Only this one dye and one sequestering agent were examined, however. It was found thatabout 35% of the tetraphosphonate was absorbed by the cotton. It was postulated that thephosphonate groups are only partially ionised in the neutral dyebath and that the presenceof the nitrogen atoms favours hydrogen bonding between nonionised phosphonate groupsand anionic sulpho groups in the dye molecule (12.21). Increased fastness to washing wasalso claimed [74].

Some direct dyes are sensitive to reduction or hydrolysis under alkaline conditions,particularly if temperatures above 100 °C are used (section 3.1.3); pH 6 is frequentlyfavoured for stability and this can usually be achieved using ammonium sulphate. A few dyesgive optimal results under alkaline conditions, using sodium carbonate or soap; the tetra-amino dye CI Direct Black 22 (12.22) is an example. Whether or not an addition is neededwill depend on whether alkali is already present in the commercial brand.

chpt12(2).pmd 15/11/02, 15:46834

835

HO P CH2

O

OHCH2CH2N

HO P CH2

O

OH

CH2

CH2

P

P OH

O

OH

OH

OH

O

N

12.19

EDTMP

N

NaO3S

NaO3S

OH

N

H3C

OCH3

HNC

NH

O

H3CO

CH3

NN

SO3Na

SO3Na

HO

12.20

CI Direct Red 79

NaOS

O

O

[dye]

OP

N

O

OH

H

12.21

N

H

N

H

SO3Na

NaO3S

N N

N

O O

N

NaO3SSO3Na

NN

NH2

H2N

H2N

NH2

12.22CI Direct Black 22

DIRECT DYES

Levelling and wetting agents for direct dyeing are mostly ethoxylated adducts, such asalkylaryl ethoxylates, although anionic types such as alkylarylsulphonates, phosphate estersand alkylbenzimidazoles are also marketed. Care should be exercised in the use of suchagents; there is spectrophotometric evidence [75] that they interact with dyes, leading tolower exhaustion. Since this interaction is dye-specific, there may be problems with mixturesof dyes. Such auxiliaries also add quantitatively to the COD value of the effluent [76].Various other auxiliaries have been investigated but their commercial use is either limited or

chpt12(2).pmd 15/11/02, 15:46835


non-existent: urea, pyridine, amyl acetate, gelatin, carboxymethylcellulose [77].Cyclodextrins [75,78] may have some potential for the future (section 10.3.1), althoughtheir action with direct dyes is dye-specific [75].

There is research interest in the potential to pretreat cotton with reactive compounds(section 10.9.1) followed by application of direct dyes or nucleophilic amino-containing dyesto give increased fixation and/or fastness. Attempts have also been made to apply suchreactive compounds simultaneously with direct dyes [79]. Despite the inroads made byreactive dyes, there still remains considerable interest in the application of direct dyes, asevidenced by the introduction of the Optisal system by Clariant [80–82]. This is a carefullydesigned package of selected metal-free direct dyes, that require little salt to give highexhaustion and are stable up to 130 °C, together with a cationic formaldehyde-free fixationagent applied as an aftertreatment. The dyes may also be applied isothermally [81].Environmental advantages are claimed, including high exhaustion, less pollution of effluentand low salt usage.

Various techniques are available for the application of direct dyes by semi-continuous andcontinuous methods, such as pad–jig, pad–batch, pad–steam, pad–dry and pad–thermofix.The major problem arises from the high substantivity of direct dyes for cellulosic substrates,making it very difficult to avoid tailing problems. Hence concentrated brands of dyes havingminimal electrolyte content are preferred; of these, the class B dyes offer better operatingproperties. The main methods of controlling uniform uptake remain careful selection of dyesfor compatibility, speed of padding and the rate of supply of padding liquor. Low solubility ofthe dyes may also be a problem; use of a hydrotropic agent such as urea improves thesolubility of certain dyes and may also improve fixation, particularly in dry fixation processes.The washing-off process after fixation may be combined with an aftertreatment to improvethe wet fastness and avoid undesirable bleeding of dye. Treatment with durable-press resinsand with cationic products, particularly of the multifunctional reactant type, is especiallyuseful here. The aftertreatment of conventional direct dyeings to improve fastness to lightand particularly to wet treatments, using copper(II) sulphate, formaldehyde, diazotisationand coupling techniques or cationic fixing agents, has been described in section 10.9.5 andwill not be discussed further here.

An interesting, if little-used, method of overcoming dye substantivity problems at thepadding stage involved the use of certain amines, particularly those containing carboxyl orhydroxy groups such as structure 12.23 [83], in combination with 1:1 copper-complex directdyes. A low-stability amine–copper–dye complex was formed. The complex diffused readilyinto the fibre and reverted to dye and amine during steam fixation, the amine beingsubsequently removed during washing-off. Careful selection of the amine, or mixture ofamines, was necessary to achieve the desirable balance of properties. It is interesting tocompare this use of an amine and 1:1 metal-complex direct dyes with the similar requirementfor such dyes in the much more recent Indosol (Clariant) process (section 10.9.5).

HOCH2CH2NHCH2CH2NHCH2CH2OH

12.23

Direct dyes are of limited interest for printing because of their restricted wet fastness,resulting in cross-staining of whites or pastel-dyed grounds when the prints are subsequentlywashed. Somewhat better results can be achieved by treating the prints after steam fixation

chpt12(2).pmd 15/11/02, 15:46836

837

with, for example, a cationic fixing agent, durable-press resin or, in the case of chelatabledyes, copper(II) sulphate as already described (section 10.9.5). Most azo direct dyes willdischarge easily with reducing agents and can therefore be applied as dyed grounds fordischarge print styles, although the limitations described above with regard to wet fastnessare especially pertinent here.

Partial non-destructive stripping of untreated direct dyeings can be accomplished with analkaline solution of soap or synthetic detergent. Destructive stripping using a reducing agentsuch as sodium dithionite is also effective except with stilbene-type dyes. Aftertreated dyeingsmay additionally require a treatment to counteract the aftertreatment; for example, coppereddyeings can be treated with a sequestering agent such as EDTA and dyeings treated with asimple, non-reactive cationic agent may respond to treatment with an anionic detergent.

DISPERSE DYES

12.6 DISPERSE DYES

Disperse dyes as a class are peculiarly sensitive to the influence of auxiliary agents, both asregards the quality and stability of the dispersion and the response of the dyes during thevarious coloration processes. Essential auxiliaries in batchwise dyeing include dispersingagents and chemicals to control the pH. Supplementary auxiliaries termed ‘carriers’ may beneeded under certain circumstances to accelerate the otherwise inadequate rate of dyeing.Aftertreatment of the dyeings to remove surface dyes is important in many cases, as are theconditions of drying and finishing since these can influence fastness properties.

12.6.1 Dispersing agents

The essential chemistry of dispersing agents has been discussed in section 10.6.1, where itwas noted that different considerations may apply at the comminution stage of dyeformulation compared with maintaining the stability of the dispersion during subsequentcoloration processes. The dyeing of polyester at a temperature in the region of 120–135 °Cin beam and package dyeing machines places severe demands on initial dispersion qualityand subsequent stability under adverse conditions. Jig dyeing with a high concentration ofdye in a very short liquor (as for navy blues and blacks) can also be the source of dispersionstability problems.

The crux of the problem lies in the inherent thermodynamic instability of all dyedispersions, there being an overall tendency of fine particles to undergo Ostwald ripeningwith the consequent formation of larger particles. Although disperse dyes are generallyconsidered to be virtually insoluble in water they are, in colloidal terms, sparingly soluble;indeed a low degree of solubility seems to be a necessary prerequisite for dyeing to take placefrom an aqueous medium. It is this limited solubility that favours Ostwald ripening. Thedetailed colloid chemistry of dispersions with particular reference to these phenomena hasbeen thoroughly discussed [84]. The solubility of disperse dyes normally increases withtemperature and dispersing agent concentration, although these effects vary enormouslyfrom agent to agent and from dye to dye.

Most dispersing agents are of the anionic polyelectrolyte type, comprising varioussulphonated condensation products of aromatic compounds and lignosulphonates (section10.6.1). Increasing understanding of lignin chemistry with consequent improvements inmanufacture, enabling lignins to be more economically and reliably ‘tailored’ for specific

chpt12(2).pmd 15/11/02, 15:46837


end-uses, currently favours the use of these products, although by no means exclusively.Solid brands of disperse dyes contain a significant proportion of dispersing agent addedduring formulation; liquid brands contain rather less as they do not have to withstand thethermal and mechanical rigours of spray drying [85] and do not require redispersing at thedyebath preparation stage. Despite this, it is still advisable to add extra dispersing agent atthe dyeing stage, more being required with liquid dyes to compensate for their lowerintrinsic content. At the grinding stage of dye manufacture lignosulphonates with a highdegree of sulphonation generally perform better. Less sulphonated types tend to give betterstability at high dyeing temperatures [86], since they are more readily adsorbed onto andretained by the hydrophobic surfaces of the dye particles.

The complexity of the relationships within a disperse dye system is well illustrated inFigure 12.5. The particle size distribution in a disperse dyebath and any transformationstaking place during dyeing, including the three successive phases of heating up, maintainingtop temperature and subsequent cooling, can exert critical effects on rate of dyeing, finaldegree of sorption and levelness. Whilst microscopy techniques are undoubtedly useful instudying dispersions, much more detailed information is obtained from photon correlationspectroscopy using a Coulter counter [88]. Figure 12.6 shows an example of particle size

Molecular solution of dye

Micellar

solubilisation

Hydrotropy

Molecules

Dissolving

Crystal formationfrom solution

Crystal

Crystal formationfrom dispertion

Crystallites withdispersant

Displacement

Dispersing agent system

Micelle

Crystallisation

Agglomeration

Aggregation

Figure 12.5 Model of the disperse dye system [87,88]

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839

transformations taking place in a specific dispersion of CI Disperse Orange 13 on exposureto three different temperature profiles.

The addition of auxiliaries, such as additional dispersing agents, levelling agents, carriersand electrolytes, brings about further changes that may be beneficial or otherwise,depending on circumstances. A method based on a ternary diagram (Figure 12.7)representing the relationship between dyeing properties and the concentrations of threedispersing agents used simultaneously in the dyebath has been described [89]. Of course,any ranges of concentration can be chosen that are appropriate for the dispersing agentspresent. The scheme is suitable not only for studying particle size distribution using a laserparticle sizer as in this instance, but also for examining effects such as solubilisation, rate ofdissolution, diffusion coefficient, dyeing rate and colour difference. Figure 12.8 shows howthese various factors can be compared in a single diagram [89].

An alternative approach to evaluating the efficiency of dispersing agents depends on thepartition effect [90]. In this simple and practical method a sample of the aqueous dispersionis extracted with a water-immiscible solvent (e.g. chloroform, methylene dichloride,monochlorobenzene, dimethyl phthalate, tetrachloroethylene) in which the dye but not thedispersing agent is soluble. Unprotected disperse dye particles dissolve immediately in thesolvent layer. Dye particles protected by a sheath of dispersing agent are more hydrophilicand therefore favour the aqueous layer. The rate of extraction of a disperse dye from theaqueous layer into the solvent depends on the stability of the dispersion and the extractionconditions. However, there must be limits to the applicability of this method to the study ofphase transitions during the dyeing process.

The influence of dispersant structure on the thermal stability of dye dispersions has beenillustrated as in Figure 12.9. Thermal stability appears to be related to the relative numbersand strengths of the adsorbing and stabilising groups in the dispersing agent. Partial blocking

0.4 0.8 1.2 2 3.2 5 8 12.6 20Particle size/µm

10

20

30

40

50

Volu

me/

%

A

B

C

Treatment A 70 oC 100 oC

Treatment B

Treatment C

5 min 100 oC10 min 70 oC10 min

70 oC 130 oC20 min 130 oC60 min 70 oC15 min

70 oC 130 oC20 min 130 oC180 min 70 oC15 min

Figure 12.6 Effect of temperature changes on the particle size distribution in a dyebath containing0.6% CI Disperse Orange 13 [88]

DISPERSE DYES

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Agent 1

Agent 3 Agent 2

0.5 0.1

0.4

0.3

0.2

0.1

0.2

0.3

0.4

0.5

0.5 0.4 0.3 0.2 0.13 4 5

7

8

1

2 6

9 10

Figure 12.7 Basic points of the ternary diagram in which individual parameters were determinedexperimentally; agent concentrations are quoted in g/l [89]

Agent 1

100

045

100

17

Agent 3 Agent 2

3 4 5

7

8

1

2 6

9 10

0

2515

28

47

63

360

54

62

59

9657

57

1007

253

0

058

100100

57

68

51

544

56

61

36

25100

43

5

25

4243

68

15

80

6159

78

83

Rate of dissolution (mg/l min)

Rate of dyeing (mg/g min)

Solubilisation at 60 min (mg/1)

BASF diffusion test (m2/s)

Colour difference (∆E)

Figure 12.8 Overall dependence of observed parameters in dyeing with CI Disperse Orange 21 in thepresence of a mixture of three agents [89]. The value within each symbol represents a percentage ofthe maximal effect (= 100%) for that factor

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of phenolic groups in a lignosulphonate, for example, reduces the thermal stability by anamount that corresponds quite well with the lower concentrations of residual phenolic andcarboxyl groups. The degree of adsorption can be determined by equilibriating a known massof dye with a surfactant solution of appropriate concentration at constant temperature. Thedye particles are then separated by filtration or centrifugation, followed by UV analysis ofthe supernatant liquid to determine the concentration of dispersant remaining. The amountof surfactant adsorbed per gram or per unit area of the dye particles is then calculated fromthe difference between the initial and final concentrations of the surfactant [91].

From the environmental viewpoint there are two important problems associated withdispersing agents:(1) dustiness of powder brands(2) inadequate and slow biodegradability.

Since disperse dye powders supplied to the dyer have been treated already to render themessentially non-dusting, dustiness problems are mainly of concern during dye manufacture[92]. Nevertheless, some dyes remain inadequately treated, or after an initially adequatetreatment they may deteriorate during storage, thus giving rise to hazards in handlingassociated with excessive dustiness. Methods of assessing dustiness have been reviewed [92].

Since dispersing agents are not significantly absorbed by the fibre, they remain in theexhaust dyebath and are discharged to effluent. As a result of their polymeric nature and thepresence of stable benzenoid rings, most lignosulphonate and formaldehyde-naphthalenesulphonate dispersing agents are only bioeliminated to about 30% [92–94].They may also contain small amounts of residual starting materials that are toxic to fish[92]. More complete elimination can be achieved by precipitation with heavy metal salts orcationic surfactants, but this leads to problems of disposal of solid wastes. Dispersing agentsbased on mixtures of sodium salts of arylcarboxylates are claimed to offer superiorbioeliminability (70%) and to show markedly improved application properties comparedwith traditional dispersing agents [93,94].

Dyebath pH exerts a marked influence on the efficacy of lignosulphonate dispersing agents,since this factor determines the degree of dissociation of phenolic and carboxylic acid groups,influencing the extent to which they are able to interact with the dye molecule. In general, thelowest pH that can be tolerated by the system (dye, fibre and auxiliaries) tends to give the

Dye crystal

x

x

Dye crystal

Moderate thermal stabilityVery good thermal stability

x

x

x x x

Dispersant adsorbing group

x Dispersant stabilising group

Figure 12.9 Dependence of dispersion thermal stability on dispersing agent structure [91]

DISPERSE DYES

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greatest dispersion stability during heating of sodium lignosulphonates [91]. The situation issomewhat different with amine salts of lignosulphonates, since these differ from the sodiumsalts in their degree of ionisation. For example, when the pH is lowered from pH 7 to 4, moreamino groups are protonated, thus increasing the proportion of ionised sulphonate groupspresent. The solubility of the dispersant is increased and so the adsorption of agent by thedisperse dye particles becomes more difficult. However, the overall situation is complexbecause lignin derivatives vary in their content of phenolic and carboxylic acid groups andthese exhibit a range of pKa values according to their location within the macromolecule.

Some azo dyes are susceptible to reduction under unfavourable conditions [95]. The leaststable dyes tend to be those containing electron-withdrawing groups, such as nitro, chloro orcyano, ortho to the azo linkage. This instability to reduction is minimised by dyeing at theoptimal pH, usually pH 4–5, in the presence of air, and by minimising the dyeing time athigh temperature. Hence, under appropriate conditions, instability is not a serious problem.Decomposition is favoured, however, by various factors:(1) pH values greater than 6(2) the absence of air (anaerobic dyeing conditions)(3) the presence of fibres containing reducing groups, such as wool or cellulose(4) the presence of reducing metal ions, such as copper(I) or iron(II), in the water supply(5) dispersing agents containing phenolic groups(6) conditions that tend to maintain the dye for longer periods in the liquor, such as slower-

dyeing substrates (low-porosity sewing threads, for example) and auxiliaries that tend tosolubilise the dye too much.

Lignosulphonate dispersing agents tend to promote this reduction of sensitive dyes, muchmore so than the naphthalenesulphonic acid condensation types, probably owing to thepresence in lignin of catechol residues and other easily oxidised functional groups [95](structures 10.104 and 10.105). Commercial lignins vary considerably in their detailedconstitution, however, and consequently in their reducing power. In certain cases theproblem can be ameliorated by adding an oxidising agent (such as sodium dichromate) tothe dyebath, but the effects can be variable and difficult to control. In printing applications,where steam fixation can have a pronounced reductive effect, stronger oxidising agents suchas sodium chlorate are often added to the print paste. In theory the reductive tendency oflignosulphonates can be counteracted by chemical blocking of the active phenolic groupsbut this impairs the dispersing properties of the product [91,95]. Significant improvementscan be achieved by replacing the conventional sodium ion in the lignosulphonate salts byother cations [91]; lithium is effective in this respect, but the most promising salt appears tobe that of triethanolamine. This compound additionally acts as a chelating agent and soprotects against the catalytic influence of iron(II) and copper(I) ions.

Since high concentrations of electrolyte can adversely affect dispersion stability, low-saltformulations of dispersing agents have been developed [95]. These also help to minimise thelowering of viscosity by electrolytes with certain synthetic thickening agents in printingapplications.

In some cases it is necessary to choose dispersing agents that give minimal staining of thesubstrate. This applies particularly when dyeing nylon since anionic dispersing agents havesignificant substantivity for this fibre under acidic conditions. In general, lignosulphonateshave a greater propensity to stain than have the naphthalenesulphonic acid condensationproducts.

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12.6.2 pH control and sequestering agents

Although many disperse dyes give good results over an extensive pH range (pH 2–9 forexample), some will only give satisfactory results over a narrower acidic range (pH 2–6) anda few require careful control to within pH 4 to 5.5. Since practically all dyes give goodresults at pH 5, this region tends to be regarded as the standard for exhaust dyeingconditions. A simple addition of acetic acid will be satisfactory where water quality permits;otherwise a buffered system is preferred. EDTA (section 10.2.1) is widely used to counteractthe effects of metallic impurities, which not only affect the hue and fastness of a fewsusceptible dyes but may also catalyse dye reduction and promote deterioration of dispersionproperties, as described above.

In spite of the traditional preference for dyeing at about pH 5, the past decade has seenthe promotion of polyester dyeing methods under alkaline conditions at pH 9.0–9.5 [96–98].Various advantages are claimed for alkaline dyeing conditions. These include the benefits ofeconomy and convenience that arise from dyeing at a pH closer to those used inpreparation, including bleaching, mercerising of polyester/cotton and caustic weightreduction of polyester, as well as the clearing of surface dye after dyeing. This approach mayeliminate the need for neutralisation or slight acidification after alkaline treatments. Furtheradvantages claimed include improved handle of the substrate, more effective solubilisationand removal of oligomer, less frequent and easier cleaning of machinery and possibleavoidance of reduction clearing. Alkaline conditions facilitate the simultaneous applicationof disperse and reactive dyes to polyester/cellulosic blends.

Apart from the essential primary requirement of selecting disperse dyes that are stable topH 9.5 at least, or preferably higher to ensure a safety margin, the choice of auxiliaries forthis process is critical. The main requirement is a buffer system having sufficient reservecapacity to maintain pH 9 throughout most of the dyeing process. This is more difficult thanmight be assumed, since the polyester fibre and the oligomers present are partiallyhydrolysed by alkali at a high dyeing temperature to form carboxyl and other groups thatcause a gradual lowering of the pH. Consequently, dye manufacturers have introducedalkaline dyeing ‘packages’ comprising a selected range of stable disperse dyes together with apurpose-designed auxiliary system to maintain the required pH. Little has been publishedabout the detailed composition of such systems. However, they are claimed to contain morethan just a suitable buffering system. For example, one such auxiliary is claimed [96] to bedesigned to (a) stabilise the dyes, (b) provide adequate buffering, (c) chelate metal ions and(d) assist dissolution of oligomers, whilst a second auxiliary is offered for use where there isan unusually high content of oligomer.

Attention must be given to dispersion stability and to oligomer control. This is becausecertain types of dispersing agent have inferior efficiency under alkaline conditions and moreoligomer is released into alkaline liquors. Unless the oligomers are adequately dispersed orsolubilised they may contribute to dye dispersion problems and may be deposited onto thefibre or machinery. Hence, careful thought must be given to the selection of dispersing orsolubilising agents.

With regard to the buffer system, an extensive range of amino acid derivatives applied incombination with an alkali have been claimed [99]. From this extensive list, primarypreference is given to N,N-bis(hydroxyethyl)glycine (12.24) in combination with sodiumhydroxide. However, N,N-dimethylglycine, N-methylglycine and N-methylalanine are alsolisted as preferred compounds, whilst other possible alkalis include sodium carbonate,

DISPERSE DYES

chpt12(2).pmd 15/11/02, 15:46843


sodium bicarbonate or borax. Another system depends on the formulation of a mixture of aphosphonate and a polycarbonic acid in combination with sodium hydroxide, borax, sodiumcarbonate or bicarbonate [100]. Whichever buffer system is used, extensive empirical trialsare required to determine the balance and concentration level needed to ensure stability ofpH during the specific conditions of use.

CH2C

HO

O

N

CH2CH2OH

CH2CH2OH

12.24

12.6.3 Electrolytes

Electrolytes are unnecessary for the application of disperse dyes alone. Nevertheless,electrolytes will be present when applying disperse dyes together with direct or reactive dyesin the dyeing of fibre blends. In particular, the high concentration of salt often used withreactive dyes can have an adverse effect on dispersion stability and may also interfere withthe stability and/or efficacy of other auxiliaries, particularly those based on emulsionsystems. These effects are often attributed to the destabilising influence of inorganic ions onthe forces of attraction between disperse dye particles and dispersing agents. Manufacturerstake these effects into consideration in marketing ‘electrolyte-stable’ formulations ofdisperse dyes or auxiliaries.

12.6.4 Levelling agents

It is necessary to distinguish clearly between levelling agents and dispersing agents. Theprimary function of a dispersing agent is to maintain a stable dispersion. Since most of theseagents enhance the low water solubility of disperse dyes they may improve level dyeing,although they vary significantly in this effect. Maximal dispersion stability is usually attainedwith agents that maintain dye particles at constant size and minimal solubility. Henceprimary dispersing agents seldom enhance levelling; different auxiliaries are added wherelevelling action is needed. These are invariably anionic or nonionic surfactants and theytend to solubilise the dye much more effectively. Some anionic levelling agents are able topromote dispersion stability but nonionic types have a destabilising effect and great care istherefore required in selection.

It is useful to consider how levelling agents can adversely affect dispersion stability. Asdyebath temperature increases, thermal effects tend to cleave the film of dispersing agentprotecting the dye particles. High shear rates in jet dyeing machines and additives such aselectrolytes, fibre lubricants or sizes, as well as oligomers from the fibre, can contribute tothis effect. Commercial batches of the same dye brand may behave differently according tothe initial dispersion quality of the dye. A dispersion of a vulnerable dye then tends todeteriorate, resulting in crystallisation and agglomeration. The types of precipitation thatcan occur have been described [87,88,101] and are illustrated diagrammatically in Figure12.5 (section 12.6.1).

Suspended dye crystallites tend to agglomerate, eventually forming larger crystals. Thedissolved dye molecules are able to diffuse into the fibre, but under adverse conditions

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845

crystallisation is favoured. Once seeded, the crystals may grow in size whilst retaining theiroriginal form, or they may undergo a transformation from the original thermodynamicallymetastable form to a more stable but less soluble form. On the other hand, such crystals maynot form until the incompletely exhausted dyebath is cooled after the dyeing process; thisproblem may be avoided by blowing off the dye liquor at 125–130 °C. Precipitation byagglomeration tends to predominate with those dispersing agents that do not enhance dyesolubility significantly, whereas crystallisation is more prevalent with levelling agents ordispersing agents having greater solubilising power. It is interesting that surfactant additionsmay be used during dye synthesis in order to obtain the dye in the optimal form for isolationand subsequent milling; for example, very fine crystals may clog filters, whilst thin needle-like crystals tend to mill more easily than platelets.

Anionic levelling agents, especially the polyelectrolyte dispersing agents describedpreviously, are generally preferred as the primary addition [102], particularly where it is desiredto promote level dyeing by control of exhaustion during the heating phase of dyeing; higherconcentrations have a greater retarding effect. Few of these anionic products promote dyemigration, a characteristic that is useful if a more powerful levelling action is required.

Nonionic surfactants, on the other hand, tend to solubilise the dye much more effectivelyand thus contribute to level dyeing both by a retarding effect and through the promotion ofmigration. Consequently they are generally more powerful levelling agents than anionicproducts although their effects are much more dye-specific. The dye-specific effects onretarding and restraining have been well-publicised [103–107], although the full extent ofthe variations is often overlooked in the industry. The restraining effects of a typicalnonionic agent, a nonylphenol with an average of 20 ethylene oxide units per molecule, onfive commercially important disperse dyes are illustrated in Figure 12.10. The spread ofresults between CI Disperse Blue 56 (only slightly affected) and CI Disperse Yellow 42(much more affected) should be noted. Other nonionic auxiliaries would yield differenteffects and dyes may behave differently in mixtures compared with their response whentested in isolation. The retarding effects of the same agent on CI Disperse Blue 56 andYellow 42 applied as a green 1:1 mixture are shown in Figure 12.11.

Agent concentration/g l–1

1 5 10

0.2

0.4

0.6

0.8

1.0

Rel

ativ

e dy

e up

take

afte

r 12

0 m

in

Blue 56Yellow 5Red 60Red 82Yellow 42

CI Disperse

Figure 12.10 Relative dye uptake values for five disperse dyes on polyester at various concentrationsof a nonylphenol 20 EO levelling agent [107]

DISPERSE DYES

chpt12(2).pmd 15/11/02, 15:47845


This complexity of response has critical implications for reproducibility of dyeing,particularly within the context of right-first-time dyeing. Careful prior evaluation andoptimisation of each recipe, followed by consistent bulk use and monitoring, are essential forgood reproducibility. Instrumental colour difference measurements are particularly useful forevaluating and monitoring responses [107,108]. On the other hand, a study of four differentpolyethoxylated sorbitan esters indicated that there were no significant differences betweenthem in terms of desorption of disperse dyes from polyester [109]. Desorption, however, isonly one aspect of level dyeing and migration.

A major problem with nonionic agents arises from their inverse solubility. Thus an agentwith a low cloud point may increase dye precipitation, although once again the effect is dye-specific. Published data suggest that a nonylphenol with a low degree of ethoxylation, havinga cloud point of about 40 °C, should not be used as a disperse dye levelling agent [110]. Aproduct of type 12.25, having a cloud point of about 105 °C, should be satisfactory for dyeingat any temperature up to 100 °C but should be avoided at higher temperatures. On the otherhand, a carefully selected nonionic agent may be mixed with an anionic agent to raise itscloud point. For example, a mixture of the fatty acid ethoxylate 12.26 with 7–10% sodiumdodecylbenzenesulphonate [110] has a cloud point of about 150 °C and is suitable for use inhigh-temperature dyeing.

90Temperature 100 110 120 130 130 oC

1

2

3

1

2

3

Dye

upt

ake/

mg

dye

per

g po

lyes

ter

30Time 60 90 120 min at 130 oC

CI Disperse Yellow 42

CI Disperse Blue 56

Agent/g l–1

1510

Agent/g l–1

1510

Figure 12.11 Rate of dyeing curves for CI Disperse Blue 56 and Yellow 42 in admixture on polyester atvarious concentrations of a nonylphenol 20 EO levelling agent [107]

CH3(CH2)15(OCH2CH2)17OH

12.25

CH3(CH2)7CH CH(CH2)7 C

(OCH2CH2)14OH

O

12.26

Surprisingly, other investigators were unable to confirm the adverse effect of nonionicsurfactants of low cloud point in the high-temperature dyeing of polyester, even in thepresence of electrolytes [111]. This was probably because of the rather low concentrationsused. Adducts containing a C18–C20 hydrophobe and a decaoxyethylene hydrophile, as well

chpt12(2).pmd 15/11/02, 15:47846

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as sorbitan ester ethoxylates were shown to be particularly effective levelling agents. Certaindyes were found to be sensitive to nonionic additives under the stringent conditions of thelaboratory dispersion tests carried out in the absence of fibre, but it was pointed out thatthese systems still performed satisfactorily in actual dyeings [111].

An alternative type of levelling system contains a mixture of ethoxylates with aliphaticesters [112]. This combination exerts a retarding effect on many dyes during heating up toabout 100–110 °C, especially if the dyes are present in low concentration. At highertemperatures this retarding effect is increasingly offset by the accelerating effect of thealiphatic esters. This temperature-dependent interaction is said to improve the compatibilityof combinations of dyes applied with this system.

The adverse effect of nonionic adducts of low cloud point can be avoided by the use ofhybrid agents of the ethoxylated anionic type, variously and confusingly referred to as‘modified nonionic’, ‘modified anionic’ or ‘weakly anionic’ types. Thus Mortimer [113] hasproposed the use of products of the ethoxylated phosphate type (12.27). In this structure, R,as well as the degree of ethoxylation (n) may be varied to optimise the overall HLB value.The numerous ether groups are said to enhance the dye-solubilising and levelling capacity,whilst the polyphosphate grouping exerts several useful effects [113]. These compounds:(1) are sufficiently anionic to avoid most of the disadvantages of conventional nonionic

agents with regard to high-temperature instability and the lack of an electrical doublelayer of value in dispersion stability

(2) behave similarly to more orthodox polyphosphate sequestrants, thus offering someprotection from hard water and other trace metal impurities

(3) maintain effective stability in high concentrations of electrolyte(4) offer possibilities for pH control by varying the nature of M(5) are fully effective at pH 4–5, the most useful pH range for application of disperse dyes,

whereas conventional nonionic types are said to become less effective as levellingagents at pH values less than 7.

R (OCH2CH2)n PO

O

O M

O M

x

12.27

RnxM

= hydrophobe= typically 10–20= typically 1–3= H, alkali metal or organic base

Hence these agents are sophisticated multifunctional auxiliaries, which can take the place ofseparate additions of levelling agent, sequestering agent and a buffer to control the pH.Nonionic surfactants can be beneficial in minimising the redeposition of the sparinglysoluble polyester oligomers that are released from polyester fibres during high-temperaturedyeing.

Ethoxylated multi-ester compounds (so-called ‘oligo-soaps’) have been promoted recentlyas dispersing/levelling agents [114]. These contain a multi-branched hydrophobe with

DISPERSE DYES

chpt12(2).pmd 15/11/02, 15:47847


pendant carboxyl groups that are esterified with poly(ethylene glycol). Thus they are similarin structural class to a mono-ester (so-called ‘mono-soap’) of structure 12.28, aconventional condensate of a fatty acid of high molecular mass with poly(ethylene glycol)but the multi-ester has a much higher relative molecular mass (Table 12.6). A micelleformed from an ethoxylated mono-ester is an aggregate of several molecules, whereas theindividual molecule of an ethoxylated multi-ester is said to behave as a micelle, thusexhibiting a much lower critical micelle concentration. The thermal stability of a multi-esteris much greater because this macromolecular structure resists thermal agitation to a muchgreater degree. This maintains a more stable dye dispersion at high temperature underconditions of high shear. Further advantages include:(1) solubilisation of the dye takes place at a lower temperature(2) strike rates at lower temperatures in the dyeing cycle are much slower(3) solubilisation of oligomer and acrylic size(4) low foaming.

R C

(OCH2CH2)n

O

OH

12.28

R = fatty alkyl group

Table 12.6 Comparison of characteristic properties of mono-ester andmulti-ester compounds [114]

Mono-ester Multi-ester

Relative molecular mass Small LargeCritical micelle concentration 0.400 g/1 0.004 g/1Foaming tendency High LowOligomer solubilisation No YesSize solubilisation No Yes

12.6.5 Carriers

Although polyester or cellulose triacetate fibres are normally dyed at high temperatures,their blends with wool are still dyed at or near the boil. In such cases an auxiliary termed acarrier must be used to promote adequate exhaustion of disperse dyes by the ester fibrewithin a commercial dyeing time. Even in high-temperature dyeing, there are occasionswhen the usual maximum temperature (around 130 °C for polyester) cannot be used, aswhen dyeing qualities of texturised polyester that suffer loss of crimp at 130 °C. Carriers arethen used to assist more rapid and complete exhaustion, using smaller amounts than at ornear the boil. Carriers are sometimes employed to promote migration of unlevel dyeings.

The active component of a carrier formulation is generally a nonionic compound of Mr150–200 containing a benzenoid ring system. A comprehensive review listed the classes ofcompounds used together with their general properties, ideal requirements and themechanisms that have been proposed for carrier action [115]. Carrier compounds fall intofour main classes: phenols, primary arylamines, aryl hydrocarbons and aryl esters. Major

chpt12(2).pmd 15/11/02, 15:47848

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representatives of these in commercial use include o-phenylphenol, biphenyl, methyl-naphthalene, trichlorobenzene, methyl cresotinate, methyl salicylate (sometimes mixed withphenyl salicylate), butyl benzoate, ethers of 2,4-dichlorophenol, diethyl or diallyl phthalateand N-alkylphthalimide derivatives. Benzaldehyde has been used for the dyeing of aramidfibres [116,117].

Over the last decade the use of carriers has declined markedly and continues to do so,essentially for health, safety and environmental reasons [118–121]. In some countries theseproducts are now virtually banned. Nearly all carrier compounds exhibit all or some of thefollowing: toxicity, physiological irritancy or poor biodegradability (Table 12.7). Typicalpollution loads for comparable high-temperature and carrier methods are given in Table12.8.

Table 12.7 Chemical and biochemical oxygen demanddata for various types of carrier chemical [118]

Carrier type COD (mg/l) BOD5 (mg/l)

o-Phenylphenol 1000–2000 200–800N-Alkylphthalimide 1000–2100 100–200Arylcarbonate ester 900–1900 700–800Methyl cresotinate 800–1700 200–800Dichlorobenzene 500–1000 0Trichlorobenzene 300–1000 0

Table 12.8 Chemical and biochemical oxygen demand data for high-temperature and carrier dyeing methods [120]

Polyester Liquor BOD5 COD BOD5: Harmfuldyeing method ratio (mg/l) (mg/l) COD factor*

High-temp. 40:1 584 140.2jet dyeing 40:1 165 722 1:4.4 72.2

Carrier dyeing 40:1 200 2043 1:10.2 408.6on the winch 20:1 189 1888 1:10 188.8

* Harmful factor = g COD per kg of dyed goods

Harmful effects from carrier dyeing can arise in three ways:(1) residual carrier in the dyebath contributes to effluent pollution and may be

environmentally harmful(2) carrier that is volatilised during dyeing or subsequent heat setting becomes an

atmospheric contaminant(3) residual carrier in the fibre can be a health hazard, as well as causing an unpleasant

odour on heating or during storage.

The degree of carrier action is important in practice and varies considerably. For example,the phthalates have little action on polyester but are efficient on cellulose triacetate, for

DISPERSE DYES

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which they are the most widely used compounds. Ortho-phenylphenol and the chlorinatedbenzenes are generally powerful carriers for polyester, whilst methylnaphthalene andparticularly butyl benzoate are less powerful, Although all carriers tend to promote theexhaustion of dyes, some degree of dye-specific behaviour results from the respectivehydrophobic/hydrophilic balance of dye and carrier [102,109].

Some carriers, such as o-phenylphenol, tend to lower the light fastness of many dyes ifcarrier residues remain in the dyed fibre; others, such as the chlorobenzenes, have no effecton this property. Similarly, carrier residues differ considerably in odour. A dry heat treatmentat 160–180 °C after dyeing, to volatilise the residual carrier, is the best method of minimisingproblems with light fastness and odour. The steam volatility of a carrier and its toxicity tohuman and plant life need careful consideration. For example, o-phenylphenol has relativelylow volatility in steam and traditionally has been used in machines open to the atmosphere.The chlorinated benzenes, on the other hand, are readily steam-volatile and are toxic, soshould not be used in machines where volatilised carrier is likely to condense (for example,on a cooler lid) since drops of condensate may cause ‘carrier spots’ if they fall onto thefabric. Biphenyl is relatively non-toxic to river life but is not readily biodegradable;methylnaphthalene, also of low toxicity, is moderately biodegradable, but halogenatedbenzenes are both toxic and difficult to biodegrade. Some carriers such as chlorinatedbenzenes and butyl benzoate are relatively efficient in promoting migration; others, such aso-phenylphenol, are less so. When dyeing a blend such as polyester/wool it is useful toconsider the extent to which the carrier will promote migration of dye to polyester so as tominimise staining of the wool.

All the carrier compounds mentioned above have little or no solubility in cold water.They are therefore used in the form of emulsions, many being marketed as ‘self-emulsifiable’liquids that form stable emulsions on dilution in the dyebath. The choice of emulsifyingsystem is very important, not only from the viewpoint of emulsifying the active carriercomponent, but also to ensure stability of the emulsion under dyebath conditions andcompatibility with dyes and dispersing agents, as well as efficacy of carrier action. Thus twocarriers of identical active components but with different emulsifying systems may well differappreciably in behaviour. Two typical formulations [122] are given in Table 12.9, both beingcompletely solubilised concentrates that on dilution in the dyebath give stable emulsions ofgood dyebath compatibility. The weakly anionic ethoxysulphates and ethoxyphosphates areespecially useful emulsion bases for carriers. A small amount of a simple organic solventsuch as ethanol may also be added to improve stability.

Most commercial carriers are used in the dyebath at concentrations within the range 1–8g/l depending on active strength of the carrier concentrate, applied depth, liquor ratio and

Table 12.9 Typical examples of carrier emulsions [122]

Formulation 1 Formulation 2

90% Diethyl phthalate 40% Phenyl salicylate10% Ethoxylated castor oil 40% Methyl salicylate

(40 mol ethylene oxide) 20% Ethoxylated nonylphenol(20 mol ethylene oxide)

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other dyeing conditions. Although carriers exhibit dye-specific properties to some extent aparticular carrier will generally have an optimal concentration in the dyebath to givemaximum dye yield; higher concentrations will tend to solubilise the dye to such an extentthat colour yield is depressed.

12.6.6 Aftertreatments and thermomigration

Cellulose acetate and nylon dyed with disperse dyes are usually given a simple rinse with orwithout a synthetic detergent (anionic or nonionic) after dyeing. Most cellulose triacetate issimilarly treated; however, some full depths are given a clearing treatment to remove surfacedye so as to improve the fastness properties. Such a clearing treatment is more generallyimportant with polyester dyeings. It most frequently takes the form of a reduction clear using1–2 g/l sodium dithionite in alkaline solution. For triacetate the preferred alkali is ammonia(1–2 ml/l of s.g. 0.800) at temperatures up to 60 °C. Polyester will tolerate more severeconditions; hence the alkali is usually 1–2 g/l sodium hydroxide used at temperatures up to70 °C, or even higher in continuous ‘short-dwell’ processes. This treatment works mostly byreductive fission of azo dyes and by converting anthraquinone dyes to their soluble leucoforms. It is also advantageous to use 1–2 g/l of a nonionic surfactant in the reduction clearto assist solubilisation of the reduction products and in some cases their thorough removal isensured by a subsequent treatment with a nonionic detergent alone. Fatty acid ethoxylatesof the type mentioned earlier (structure 12.26) are excellent nonionic agents for use inreduction clearing.

This process, however, is not only expensive in itself, but creates additional expensethrough the need to deal with an environmentally unacceptable effluent. There is also thecost and inconvenience in carrying out two changes of pH: first from the acidic dyebath tothe alkaline reduction clear, followed by neutralisation of the substrate after the reductionclear. It is not surprising, therefore, that reduction clearing is nowadays avoided as much aspossible. One possibility is to use specialised dyes that can be cleared with alkali alone(section 4.9.2): this avoids the environmental nuisance of the reducing agent but still leavesalkali and the need for two pH changes.

Alternative reducing agents are still sometimes proposed and evaluated. A detailedcomparison of five reducing agents has been reported: sodium dithionite, thiourea dioxide,iron(II) chloride/gluconic acid, sodium hydroxymethanesulphinate and hydroxyacetone[123]. Results of fastness tests on black polyester dyeings variously aftertreated are given inTable 12.10.

Hydroxyacetone must be used at temperatures above 80 °C on account of its sluggishaction. Nevertheless it did not give adequate improvement of fastness to washing. It giveshigh COD values and has an unpleasant smell. The reducing power of sodiumhydroxymethanesulphinate, even with anthraquinone as activator, is insufficient under theseconditions. It did not give adequate improvement of washing fastness. Iron(II) chloride hasthe environmental advantage that it does not contain sulphur but the gluconic acidcomplexing agent results in relatively high COD values. Improvement of washing fastnesswas inadequate. Only thiourea dioxide gave results as good as sodium dithionite. It is threetimes more expensive but causes only half the sulphur pollution of dithionite. The relativeusefulness of these two reducing agents really depends on the dyeing process. In the winch,the slow production of active species from thiourea dioxide is a disadvantage when working

DISPERSE DYES

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with the counterflow. On the other hand it could be an advantage in jet dyeing machines,although this only arises if reduction of the dye occurs relatively quickly. In closedmachines, sodium dithionite is more effective.

Several reduction clearing auxiliaries have been introduced under commercial brandnames, their composition in many cases not being revealed. At least one of these is designedto be used under acidic conditions [124]. Advantages claimed for this product include: nopH changes needed, low COD, high biodegradability, low toxicity, with further savings oftime and water consumption. Moreover, since the agent is added directly to the exhaustdyebath any residual dye present is decolorised before discharge to effluent. Although highlyeffective with the majority of dyes, in a few cases (e.g. CI Disperse Yellow 29, Violet 35 orBlue 56) a higher concentration is needed.

Polyester dyed with disperse dyes generally shows excellent fastness to wet treatments andrubbing after thorough reduction clearing and drying at low temperature (below 120 °C),irrespective of the dyes used. However, cost-effective production and the requirements forfabric dimensional stability demand the use of a combined drying and heat setting treatmentat temperatures in the range 150–210 °C, most frequently at 180 °C. This causes some dyesto migrate from the core of the fibre to the surface, thus tending to negate the effect ofreduction clearing. This surface dye is a potential source of lower fastness to rubbing and wettreatments [125–127], although the extent to which this occurs depends greatly on the dyeand its applied depth. This phenomenon has been termed ‘thermomigration’, its effect onfastness varying considerably because of the generally adverse influence of surfactants,lubricants, softeners, antistats and so on. Similar problems occur on cellulose triacetate. Amethod for assessing the influence of auxiliaries on thermomigration has been published[128]. This is carried out with 1/1 standard depth CI Disperse Blue 56 (or other suitabledyeings) and the fastness to an ISO C02 washing test is determined after reduction clearingand stentering at specified temperatures. A detergent-based, rather than soap-based,washing test would be more critical [129].

The mechanisms operating during thermomigration in the presence of a surfactant havebeen evaluated experimentally [130]. The overall mechanism consists of four main processes:

Table 12.10 Fastness of black polyester dyeings after various reduction clearingtreatments [123]

Fastness properties

Perspiration

Reduction clear Washing at 60 °C Acidic Alkaline

Untreated control 1–2 2–3 2–3Hydroxyacetone 2–3 4–5 4–5Sodium hydroxymethanesulphinate 2–3 4–5 4–5Same, with anthraquinone activator 3 4–5 4–5Iron(II) chloride/gluconic acid 3 4–5 4–5Thiourea dioxide 4–5 4–5 4–5Sodium dithionite 4–5 4–5 4–5

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(1) extremely rapid attainment of equilibrium between dye in the surfactant layer and dyein the surface zone of the fibre

(2) rapid diffusion of the dye molecules from the interior of the fibre towards the surface(3) slower diffusion of surfactant molecules into the substrate phase(4) eventual formation of a composite dye–fibre–surfactant phase in the surface region.

In fact thermomigration readily takes place in the absence of surfactant, albeit usually to alesser extent. In this case only the second of the above processes takes place. The term‘surfactant’ in this model can be interpreted broadly to include any residual surfactant,reduction clearing assistant or applied finish, such as an antistat, lubricant or softener.

No general correlations exist between the degree of thermomigration and the structure ofa dye, its molecular mass, diffusion coefficient or fastness to sublimation. Nevertheless, to alimited extent, in a series of disperse dyes of closely related constitution there does appear tobe some relation between the hydrophobic–hydrophilic balance of the dye molecule and itssusceptibility to thermomigration. In such restricted series of related dyes, thermomigrationdecreases with increasing hydrophobicity [130]. This could be related to the strength ofdye–fibre hydrophobic bonding, stronger bonding tending to limit migration of dye to thefibre surface. There seems to be a relationship between thermomigration and the degree ofinteraction between dye and surfactant, more specific interaction leading to greaterthermomigration [130]. The degree of interaction in this work was deduced from specificconductivity measurements.

It is important to recognise that the degree of thermomigration in itself is not necessarilyindicative of any practical implications that may show up in fastness tests [129]. Forexample, CI Disperse Blue 60 thermomigrates to an appreciable extent, as measured bysolvent extraction of the dyed fibre after stentering. Nevertheless, in most wet fastness testsit still gives excellent results simply because it has relatively low substantivity for adjacentfibres (especially nylon) in wet fastness tests. Conversely, a dye may show very little actualthermomigration yet give poor wet fastness after stentering on account of its highsubstantivity for nylon under the conditions of test. Direct measurements of dye diffusionbehaviour generally have little practical significance in themselves, unless they can berelated to the effects on fastness properties. Problems arising from thermomigration are bestavoided by selecting dyes that show acceptable washing fastness after heat setting treatmentand by ensuring that all surfactants from dyeing and afterclearing are completely rinsed out.Careful choice of finishing agents and finishing conditions is also important.

In heat setting and curing, for example, temperature has a greater effect than time inpromoting thermomigration [131]. Thus improved fastness to rubbing and wet treatmentsmay be achieved using a selected durable press/softener finish (incorporating a rapidlyreacting resin/catalyst system) giving the required finish effect at 140–160 °C. The longercuring time required at a lower temperature has a less deleterious effect than a highertemperature (such as 180 °C) for a shorter time. Another means of minimising the effects ofthermomigration is to apply certain mildly reducing chemicals after dyeing. The applicationby padding of a polysiloxane and an organotin catalyst along with any other finishing agents[131,132] gives rise to a reducing effect during subsequent dry heat treatment that iscapable of decomposing many dyes brought to the surface by thermomigration. Suchproducts do not work successfully with all dyes and finishes, however, and can confer adegree of water repellency that is not always desirable or even acceptable.

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The presence in polyester fibres of polymerisation by-products (oligomers) can give rise toproblems, particularly if their concentration is greater than normal. With polyesters based onethylene glycol and terephthalic acid, the amount of oligomer is normally between 1.4 and1.7%, consisting mainly of the cyclic trimer of ethylene terephthalate with smaller amounts ofa pentamer, a dimer containing diethylene glycol residues and traces of other compounds.Significant migration of such oligomers from within the fibre can occur at dyeing temperaturesof 110–135 °C, leading to deposits on the fibre and/or machine surfaces and sometimes also tointerference with dispersion stability, since dispersed oligomer particles form potential nucleifor the crystallisation and agglomeration of disperse dyes. Discharge of the spent hot dyeliquors without prior cooling is the best way of avoiding oligomer problems. Reduction clearingwill normally remove any deposits from the fibre surface. Deposits on machine surfaces mustbe removed by regular cleaning at high temperature with strong solutions (5 g/l) of sodiumhydroxide together with thermally stable surfactants and solvents.


The conventional method of continuous dyeing with disperse dyes is the pad–thermofixprocess [133,134], most frequently used for polyester/cellulosic blends although it can beused with 100% polyester (or cellulose triacetate) materials. The auxiliaries normally used atthe padding stage include a thickening agent as migration inhibitor and a wetting agent.Alginates and other polyelectrolytes such as polyacrylamides are popular as migrationinhibitors. Anionic sulphosuccinates are suitable wetting agents; since cloud point problemsdo not arise in continuous dyeing to the same extent as in batchwise processes, nonionicethoxylates may also be used, often fulfilling a dual role as wetting and levelling agents. Anaddition of acetic acid to give pH 5–6 is usually adequate when applying disperse dyes alone,although certain processes may demand selection of dyes stable at higher pH, as in thecombined alkaline application of disperse and reactive dyes to polyester/cellulosic blends. Insome processes, too, the use of hydrotropes such as polyglycols and their esters, as well as ofurea and related compounds, can be useful to enhance the degree of fixation.

12.6.8 Printing

Printing with disperse dyes is generally carried out using a thickening agent and an aciddonor to maintain a low pH during steam fixation. High-solids thickeners such as crystalgum or British gum give optimal sharpness of outline but suffer from the disadvantage offorming brittle films [29]. Hence low-solids thickeners such as alginates and locust beanethers, which form more elastic films and are more easily removed in subsequent washing-off, are preferred. Further additions may include a fixation accelerator (hydrotropes such asurea, thiodiethylene glycol, cyclohexanol, dicyanoethylformamide) or a carrier and anoxidising agent, such as sodium chlorate or sodium m-nitrobenzenesulphonate, to inhibit thepossible reduction of susceptible dyes during steaming. The dispersing agent present in thedisperse dye formulation can have an influence on printing problems involving loss ofviscosity of the print paste and reduction of some azo dyes. Loss of print paste viscosity isparticularly associated with synthetic thickeners. Nonionic formulations of disperse dyeswere developed to counteract this problem; in these brands the usual anionic dispersingagent was wholly or substantially replaced by a nonionic system.

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Freedom from electrolytes is also desirable for much the same reasons. The nature of thecations associated with the sulphonate groups in lignosulphonate dispersing agents isimportant, since it affects the hydrophilicity of the agent, this aspect having the primaryinfluence on the absorptive behaviour of the dispersant. The concentration of ionisedfunctions affected by the nature of these cations correlates directly with conductivity,printing paste viscosity and azo dye reduction [91]. A detailed investigation of threeinorganic (Na, K, NH4) and six amine salts of lignosulphonates showed the triethanolaminesalt in particular to offer the greatest benefits. The results of these conductivity and printpaste viscosity measurements are shown in Table 12.11. Apart from the beneficial effects ofthe triethanolamine salt on print paste viscosity, this agent showed the least sensitivity topH, did not promote reduction of azo dyes and was an effective sequestrant for dissolvediron and copper ions.

Table 12.11 Conductivity and print paste viscosity data for various salts of ligno-sulphonate dispersant [91]

Conductance(m.mhos) at 5% Print paste*

Lignosulphonate dispersant viscosity (cps)cationic salt concentration at 25 °C

Nonionic control – 71 000Conventional low-sulphonated Na salt 9.80 1 800Low-electrolyte (Na) lignosulphonate 5.26 29 000Dimethylamine salt 4.75 23 000Trimethylamine salt 4.34 27 500Triethanolamine salt 3.31 41 000

* Viscosity sample: 8g dispersant in 970 ml water at pH 7 added to 30 g synthetic thickening

For polyester, the washing-off process to remove unfixed dye and thickening agent isgenerally a reduction clear as described in section 12.6.6. A simple wash-off with nonionicsurfactant must be used on cellulose acetate or triacetate, although a mild reduction clearmay be preferable on triacetate.

Discharge effects on acetate are carried out by overprinting dyed grounds with a thickenedpaste containing the reducing agent thiourea dioxide and thiodiethylene glycol; a disperse dyestable to these reducing conditions may be added to a similar paste to give a contrastilluminated effect. Similar effects may be produced on polyester by printing a discharge(reducing) paste onto fabric that has been padded with dye; reduction of the discharge areasand simultaneous fixation of dye in the undischarged areas then takes place during subsequentsteaming. The discharge paste may contain a reducing agent such as zinc formaldehyde-sulphoxylate or tin(II) chloride, although special ranges of alkali-dischargeable dyes areavailable that require only alkali (section 4.9.2). Reduction-stable dyes may be added to thedischarge paste to create illuminated effects. More detailed recipes are available elsewhere.

12.6.9 Stripping

Non-destructive stripping can be carried out at dyeing temperature with surfactants, a

DISPERSE DYES

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nonionic type with a high cloud point being particularly effective. In the case of polyesterthe stripping effect can be increased markedly by adding a carrier that has migration-promoting properties, the chlorobenzenes and butyl benzoate being particularly effective,although this is now much restricted on health, safety and environmental grounds. Theefficiency of destructive stripping depends on the fibre and dye types. The most usualmethod is to use a reduction process (alkaline sodium dithionite) together with a nonionicsurfactant and, where possible, a carrier, the temperature being varied to suit the fibre. Insome cases, particularly with anthraquinone dyes, an oxidation treatment (chlorite,hypochlorite or permanganate) may be more efficient. Occasionally a sequentialcombination of oxidation and reduction treatments may have to be used.

12.7 REACTIVE DYES

12.7.1 Cellulosic fibres

As described in Chapter 7, the various ranges of reactive dyes for cellulosic fibres differconsiderably in their reactivity and the number of application procedures is bewilderinglylarge, including numerous variants within each category of batchwise, continuous, semi-continuous and printing methods. Even a specific method may require modifications to suita particular quality or form of substrate, dyeing machine or the specific dyes selected from agiven range. Fortunately certain general principles are applicable to the great majority, if notall, of these methods. The characteristics of each range of reactive dyes and details of theirapplication methods are fully described elsewhere [30], but in the working situation it isespecially important with reactive dyes to consult the dye manufacturer’s literature.

Recent years have seen considerable research into the modification of cellulose andreactive dyes, specifically to overcome some of the drawbacks of this dye–fibre system,including the limited degree of fixation in full depths, the need for alkali and relatively highconcentrations of electrolyte. This research, which is driven by environmentalconsiderations, was discussed in sections 7.10 and 10.9.1. Thus it need not be consideredfurther here.

Exhaust dyeing

The critical importance of substantivity and its overriding influence on applicationproperties has been described in sections 3.2.1, 3.3.2 and 7.5, as well as elsewhere [30]. Theessential auxiliaries used to control reactive dyes in batchwise dyeing are electrolyte andalkali. Secondary auxiliaries may include sequestering agents, mild oxidising agents toprevent reduction of certain sensitive dyes and wetting or levelling agents. The classicprocedure for dyeing with reactive dyes involves application under substantially non-reactiveconditions by exhaustion with electrolyte at a temperature selected according to thereactivity of the particular dyes used, followed by addition of alkali to enhance absorptionand, more particularly, to create the conditions through which the dyes can react covalentlywith the fibre. In the so-called ‘all-in’ process electrolyte and alkali are present togetherthroughout to bring about simultaneous sorption and reaction, though this inevitablyincreases the opportunity for hydrolysis of the dye in the dyebath. Application temperaturesvary from room temperature to the boil or even higher.

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There is as yet no official method of classifying reactive dyes according to their dyeingproperties, unlike the situation with direct or vat dyes. Nevertheless, a promising preliminaryscheme has been proposed [135] and it is worthwhile presenting the rationale here, sincethis scheme has a bearing on the use of auxiliaries insofar as they affect levelling properties.

Group 1 – Alkali-controllable reactive dyes

Dyes that have their optimal fixation temperature between 40 and 60 °C belong to thisgroup. These dyes are characterised by relatively low exhaustion in neutral solution beforethe addition of alkali. This type of dye has high reactivity and careful addition of alkali mustbe made in order to obtain level dyeing. For these reasons, the name ‘alkali-controllablereactive dyes’ has been chosen.

Typical examples of dyes belonging to this group are dichlorotriazine, dichloro-quinoxaline, difluoropyrimidine and vinylsulphone dyes.

Group 2 – Salt-controllable reactive dyes

This group includes dyes that have their optimal fixation temperature between 80 and 95°C. Such dyes show comparatively high exhaustion before fixation so it is important toensure that dyeings are level. Salt should be added portionwise at specified stages during theexhaustion process, hence they are termed ‘salt-controllable reactive dyes’.

Typical examples of dyes belonging to this group are aminochlorotriazine,bis(aminochlorotriazine) and trichloropyrimidine dyes.

Group 3 – Temperature-controllable reactive dyes

This group includes dyes that react with the fibre at 100 °C or above, without alkali present.Dyes in this group have self-levelling properties so there is no need to exercise control bymeans of dyeing auxiliaries. Good results can be obtained by controlling the rate oftemperature rise.

At present only the Kayacelon React (KYK) range of bis(aminonicotinotriazine) dyesrepresent this group.

Sodium chloride is undoubtedly the most widely used electrolyte, a particular advantagebeing its ease of dissolution. Certain dyes, such as brilliant blue phthalocyanines andanthraquinones, are susceptible to aggregation and sometimes even precipitation in itspresence, however, and in these cases sodium sulphate, which has a lesser aggregating effect,is preferred. The electrolyte should be free from calcium and magnesium salts and fromalkali to avoid premature fixation in a two-stage process. The electrolyte functions withreactive dyes in a manner similar to that with direct dyes, but as reactive dyes are morehighly sulphonated and hence less substantive than direct dyes, more salt is required toattain equivalent exhaustion. As with direct dyes, the higher the concentration of salt thegreater the uptake of dye, provided over-aggregation and precipitation do not occur. Theprimary objective is to achieve maximal exhaustion over an optimal dyeing period, takingcare to ensure level uptake since this is the only phase during which the rapidly diffusingreactive dyes can migrate. Hence electrolyte may be added in portions over the wholedyeing period.

As might be expected with highly soluble dyes, liquor ratio has a pronounced effect on

REACTIVE DYES

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exhaustion and thus on the amount of electrolyte needed, especially with dyes of lowsubstantivity. Decreasing the liquor ratio from 30:1 to 5:1 would justify lowering theelectrolyte concentration to one-sixth but in fact even less than this can be used because ofthe marked effect on exhaustion of the reduction in liquor ratio. There is another importanteffect of electrolyte: initially the internal pH of the fibre is lower than that of the dyebath, astate that tends to favour hydrolysis of the absorbed dyes; adding electrolyte tends toequalise these pH values, thus protecting against hydrolysis. In general, the longer the liquorratio, the lower the dye substantivity and the greater the applied depth, then the higher isthe concentration of electrolyte required.

Not long ago a rather cavalier attitude existed towards the consumption of electrolyte,based on the prevailing philosophy that ‘salt is cheaper than dye’ [136]. This outlook ledmany dyers to use as much as 20–30% more salt than recommended by dye manufacturers(100–150 g/l being not unknown) in an effort to secure maximum exhaustion. A graphicillustration of the total amount of salt used in reactive dyeing worldwide has been provided[137]: this is 1.8 million tons p.a., equating to 80 000 loaded rail wagons that would stretchfor 1000 km from Paris to Berlin. This tendency towards excessive salt usage has beenturned on its head, on both ecological and economic grounds. Considerable effort is nowdevoted to defining application conditions whereby the minimal amount of salt can be used,sometimes supported by computer programs supplied by dye manufacturers [136]. The trendtowards lower liquor ratios evinced by machinery developments and the increasing use ofbifunctional dyes with their higher average levels of fixation have both contributed tolowering of the amounts of salt used. Special ranges of ‘low-salt’ reactive dyes have beenmarketed, giving further emphasis to this important and worthwhile trend [137,138]. Inspite of these advances, however, it has been suggested [139] that such developments havenot proven to be totally satisfactory.

Consequently, investigations were carried out to explore the potential of cationicsurfactants, Groups IA, IIA and IIIA chlorides and carboxylate salts as alternatives toconventional electrolytes [139]. Cationic surfactants proved unsuitable as they promoted onlysurface deposition with low fixation, most of the dye being easily removed by washing. GroupsIIA and IIIA chlorides were precipitated as hydroxides under alkaline conditions and werethus also unsuitable, although amongst Group IA salts potassium and caesium chlorides gaveincreased exhaustion and fixation of CI Reactive Red 180 (12.29) with increasing saltconcentration (Figure 12.12). Fixation increased with the atomic size of the cation (Cs+ > K+

> Na+ > Li+) and equivalent fixation was achieved with about 20 g/l less of KC1 and about40 g/l less of CsCl compared with a conventional concentration of NaCl [139]. Nevertheless,although potassium and caesium chlorides promoted higher exhaustion and fixation, it isdifficult to agree that they are viable commercial replacements for sodium chloride. Significantamounts would still be left in the effluent; they will always be more costly and less readilyavailable in commercial quantities, both short- and long-term.

More promising results were observed with potassium salts of di-, tri- and tetra-carboxylicacids (Figures 12.13–12.15). Multicarboxylate salts facilitate much higher levels of dyeexhaustion and fixation than sodium chloride, sodium citrate being particularly effective(Figure 12.16). Although sodium citrate is a chelating agent, it does not appear to affectmetal-containing reactive dyes under these conditions [139].

Depending on the reactivity of the dyes used and the applied depth, the pH required forreaction with the fibre varies from 8 to 12 and in practice falls mainly between 9 and 11.

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0

1

2

4

3

Dye

fixa

tion/

mg

per

g fib

re

1.0270.6840.342 1.369 1.711Salt concentration/N

LiClNaClKClCsCl

Figure 12.12 Fixation of CI Reactive Red 180 with various concentrations of Group IA chlorides [139]

N

N

H

HN

C

O

O

NaO3S

SO3Na

SO2CH2CH2NaO3SO

12.29

CI Reactive Red 180

0

1

2

4

3

Dye

fixa

tion/

mg

per

g fib

re


KClK oxalate (12.30)

K tartrate (12.31)K phthalate (12.32)

Figure 12.13 Fixation of CI Reactive Red 180 with various concentrations of potassium salts ofdicarboxylic acids [139]

C

C

O OK

O OK

12.30C

C

O OK

C

OHH

C

OHH

O OK

12.31 C

C OK

O

OK

O

12.32

REACTIVE DYES

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0

1

2

4

3

Dye

fixa

tion/

mg

per

g fib

re


KClK citrate (12.33)K B3CA (12.34)

C

C C OK

O

OK

O

KO

O

12.34

C OHC

KO

O

OKC

O

H2C

OKC

H2C

O

12.33

Figure 12.14 Fixation of CI Reactive Red 180 with various concentrations of potassium salts oftricarboxylic acids [139]. K B3CA = Tripotassium benzene-1,2,4-tricarboxylate

0

1

2

4

3

Dye

fixa

tion/

mg

per

g fib

re


KClK BTCA (12.35)K B4CA (12.36) CH

HC

C

KO

O

C

O

OK

KO C

O

CH2

OKC

H2C

O

12.35

C C

C C OK

OKKO

KO

O O

O O

12.36

Figure 12.15 Fixation of CI Reactive Red 180 with various concentrations of potassium salts oftetracarboxylic acids [139]. K BTCA = Tetrapotassium butane-1,2,3,4-tetracarboxylate; K B4CA =Tetrapotassium benzene-1,2,4,5-tetracarboxylate

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The most widely used alkali is sodium carbonate, although sodium bicarbonate and sodiumhydroxide are also used; these reagents may be used singly or in mixtures. Sodiumbicarbonate, for example, can be used as a pH-shift agent, since the pH slowly increases onheating as sodium carbonate is formed (Scheme 12.4). The alkali induces ionisation of thecellulosic hydroxy groups, enabling the dye to react with these nucleophilic anionic sites toform covalent bonds with the fibre (section 7.3.1). It might be thought that increasingquantities of alkali would favour maximal reaction between dye and fibre, but in practice anoptimum level of alkali, rather than a maximum, has to be sought. This is because thecellulosate anion tends to repel the reactive dye anion, thus decreasing the efficiency of dyeuptake and so increasing the tendency towards hydrolysis of the dye. Consequently, the aimmust be to keep the pH as low as possible consistent with maintaining complete reactionwithin a commercially acceptable dyeing time; however, fixation conditions vary widelyaccording to the dye type and process used.

0

20

80

40

60

Exh

aust

ion/

%

20 40Dyeing time/min

Na citrateNaCl

60 80

Figure 12.16 Exhaustion profiles for CI Reactive Red 180 in the presence of 1.711N sodium chlorideor 1.711N sodium citrate [139]

2 NaHCO3 Na2CO3 + CO2 + H2O

Scheme 12.4

In certain applications sodium silicate is preferred. Replacing sodium carbonate bysodium silicate can increase yields by 10–30%, as well as giving an improvement in fastnessto washing. The increase in yield was higher with cold-dyeing than with hot-dyeing types.Cost savings were estimated at 15–35%, or 50–70% in some cases [140]. Occasionally,however, the lower alkalinity of silicate could result in greater hydrolysis with correspondingreduction in yield.

It is opportune at this point to illustrate the combined effects of electrolyte and alkali(Figure 12.17). The initial stage with electrolyte alone at neutral pH approaches equilibriumprimary exhaustion of dye; there is no fixation during this stage. Once the alkali is added (inthis case after 20 minutes), fixation of the dye begins to take place; at the same time, the

REACTIVE DYES

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alkali contributes to further exhaustion, which is termed secondary exhaustion. Thedifference between equilibrium exhaustion and fixation at the end of the process representsunfixed dye that requires washing off in order to maximise fastness properties. The profilesof these curves differ according to how the individual dyes respond to the conditions (typeand concentration of auxiliaries, pH, temperature). Once the dye is fixed, it cannot migrate.Hence the difference between the exhaustion and fixation curves represents the potentialfor levelling during the alkaline treatment.

0

50

100

Exh

aust

ion

or fi

xatio

n/%

20 40Dyeing time/min

Exhaustion curve

Fixation curve

60 80

PF

S

Primaryexhaustion

Secondaryexhaustion

PSF

Degree of primary exhaustionDegree of secondary exhaustionDegree of dye fixation

Figure 12.17 Dye exhaustion and fixation profiles in exhaust dyeing with a typical reactive dye [141]

The traditional requirement for alkalinity can be a disadvantage on both environmentaland cost grounds. Furthermore, this treatment is not fully compatible with dyeingrequirements for other fibres used in blends with cellulosic fibres, particularly polyester, thuscomplicating the dyeing of such blends. This has favoured the development of so-called‘neutral-dyeing’ reactive dyes. These do not require the addition of alkali for fixation butstill need electrolyte for exhaustion [142–144]. With these dyes reaction does not occur atneutral pH below 100 °C but fixation becomes optimum at 120–130 °C and pH 6.5–7.5. ApH of 8 is recommended for the hank dyeing of yarn and jig dyeing of fabrics, where dyeingat high temperature is not feasible. This range of dyes is the only one to fall into Group 3 ofthe classification mentioned previously; hence their levelling is controlled by temperaturerather than by additions of alkali or electrolyte.

As mentioned at the beginning of this section, the so-called ‘all-in’ method may beadopted, treatment time being saved by adding the electrolyte and alkali together, albeit atthe expense of lower fixation and (usually) inferior reproducibility. Studies ofdichlorotriazine dyes applied with various alkalis or combinations of alkalis (NaHCO3,Na2CO3/NaOH or Na2CO3/NaHCO3) have shown recently that this process can beoptimised to give enhanced dye fixation by buffering to give pH 8 or 9 [145].

Reactive dyes in general are not unusually sensitive to hard water. Nevertheless, the alkaliused in most reactive dyeing processes may precipitate calcium or magnesium hydroxide on

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Table 12.12 Metal content of regenerated cellulosic fibres [146]

Metal content (ppm)

Modal Schwarza LenzingMetal fibre modal fabric modal fabric

Copper 4.1 6.6 2.0Iron 46.0 63.0 13.0Zinc 15.0 20.0 38.0Magnesium – 22.0 9.0Lead 27.0 15.0 <0.1Manganese 2.2 1.5 0.2

Ash content (%) 0.27 0.37 0.25

O

P

OH

CHHO P OH

O

OH

NH3C CH3

12.37

HO P C

O

OH

P OH

O

OH

CH3

OH

12.38

HEDP

N CH2CH2 N

CH2

CH2

H2C

H2CC

HO

O

CHO

O

COH

COH

O

O12.39

EDTA

the substrate, to cause problems in later processes. Ideally, soft water with a pH not greaterthan 7 is preferred. Where the use of hard water is unavoidable, a sodiumhexametaphosphate sequestering agent may be used in the minimal amount needed toovercome the hardness, since excessive quantities may bring about a significant reduction indye yield. Organic sequestering agents of the EDTA type (section 10.2.1) are generally bestavoided because they often result in colour changes and reduced light fastness, althoughthey can occasionally be used successfully in minimal quantities [30]. It has been shown, atleast with some reactive dyes, that hue changes due to traces of iron or copper in modalfibres (Table 12.12) can be prevented by the use of dimethylaminomethane-1,1-diphosphonate (12.37), 1-hydroxyethane-1,1-diphosphonate (12.38), EDTA (12.39) orcertain water-soluble polymers as sequestering agents [146]. However, as discussed later, thepotential for trace metals to cause problems at the washing-off stage should not beoverlooked [30].

As mentioned previously, once reactive dyes have reacted with the fibre no levelling ispossible; hence all levelling must be achieved before the reaction has reached equilibrium.The preferred means is by controlled additions of alkali (Group 1) and electrolyte (Group 2)rather than by using a surfactant-type levelling agent. The latter causes restraining, althoughsuch products may be added in minimal amounts to aid wetting and to safeguard againstrope marks through lubricating action [30]. A particularly detailed study of the effects of arange of nonionic aromatic and aliphatic ethoxylates confirmed the negative influence ofthese agents on the fixation of reactive dyes [147–149]. Figure 12.18 shows typical results

REACTIVE DYES

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obtained with a nonylphenol ethoxylate (20 mols ethylene oxide) in dyeings of CI ReactiveRed 2 (12.40).

Time/min

20 40 60 80

0.2

0.8

0.4

0.6

Fixa

tion/

mg

per

g fib

re

Time/min

20 40 60 80

20

80

40

60

Exh

aust

ion/

%

No agent

5 g/l agent

5 g/l agent

No agent

Figure 12.18 Exhaustion and fixation curves for CI Reactive Red 2 in the presence and absence of anonionic surfactant at 30 °C [147]

N

N

H O

NaO3S

SO3Na

HN

N

N

N

Cl

Cl

12.40

CI Reactive Red 2

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However, the decrease in yield varied considerably with dye and surfactant. Themagnitude of the effect increased with increasing degree of ethoxylation of the surfactantand varied considerably from dye to dye. Interestingly, this loss of yield occurred eventhough kinetic evidence indicated that the presence of the nonionic agent decelerated therate of dye hydrolysis [149]. Spectrophotometric studies [148] showed positive evidence ofinteraction between the dye and surfactant. This interaction shifts the equilibrium betweenadsorbed dye and dissolved dye in favour of the dyebath [147]. Electrolyte and temperaturealso influenced the interaction [148]; higher temperatures tended to destabilise the dye–dyeand dye–surfactant interactions, although these interactive effects were partially maintainedat normal dyeing temperatures. Thus the possibility of using surfactants as levelling agentsfor reactive dyeing is best avoided. Similarly, any surfactants added during preparationshould be completely rinsed out before dyeing. Provided goods have been preparedthoroughly, it should be unnecessary to add wetting or levelling agents to the dyebath [30].Mention of the importance of thorough rinsing after preparation applies not only to theneed to remove surfactants but also to prevent the considerable number of dyeing faultsresulting from residual unevenly distributed alkali and residual peroxide [150].

Various attempts have been made to optimise the exhaust dyeing of reactive dyes. Forexample, some methods depend on pH control via precisely metered alkali dosing systems[151,152]. The curves in Figure 12.19 show how the fixation rate can be modifiedconsiderably by dye-specific alkali metering in order to increase the scope for levelling.Detailed attention to the precise effects of all dyeing parameters with a view to ultimate dye-specific control enables the minimal amounts of auxiliaries to be used that will give safe andeffective control. Further advantages claimed include reduced water, steam and energyconsumption together with substrate quality improvements [151–153].

Dyeing time

Fixa

tion

Conventionalfixationcurve

Meteredalkali addition

Idealfixationprofile

Figure 12.19 Control of reactive dye fixation rate by Remazol automet (DyStar) alkali addition [151]

Neps in cotton fabrics can pose a problem for reactive dyeing. Effective coverage of suchdead or immature cotton can be achieved by pretreatment with poly(ethylene imine) on the jigat 10:1 liquor ratio before the dyeing process. The cationic polymer not only gave goodcoverage of the neps but also improved the colour yield generally. In addition, wet fastnesscould be improved by a low-temperature curing treatment with a cationic polymer emulsion[154].

REACTIVE DYES

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Viscose fibres are particularly suitable for dyeing with reactive dyes, the differences incolour yield between dyed viscose and cotton varying between individual dyes anddepending on the chromogen present [155]. Salt and alkali requirements are generally loweron viscose than on cotton, although most turquoise and green hues based onphthalocyanines are applied according to special recommendations [30]. A comparison ofthree types of reactive dye applied to cotton, viscose and lyocell showed little difference incolour between these fibres [156]. The colour strength was lowest on cotton, slightly higheron lyocell and marginally the highest on viscose. It should be borne in mind that the qualityof today’s viscose is somewhat different from that produced in the 1970s: as regards responseto preparation and dyeing parameters modern viscose is generally superior [157]. Hencesome care is needed in relating current data to earlier results and many of the olderrecommendations should not be followed today. Certain dyes are susceptible to reduction,particularly under anaerobic dyeing conditions. An addition of 1–2 g/l sodium m-nitrobenzenesulphonate is a useful palliative.

Continuous and semi-continuous dyeing

There are numerous variations of procedure for the continuous or semi-continuous dyeingof cellulosic fabrics with reactive dyes, viable reasons being evinced for the promotion ofmost of the major ranges of reactive dyes. We are concerned here only with rationale in theselection of auxiliaries; details of the many processing routes can be found elsewhere[30,158–163]. Low substantivity at the padding stage is often preferred in order to minimisetailing, but in fact dyes covering the whole range of reactivities can be used. Highly reactivedyes can be fixed in shorter times at lower pH and are often easier to wash off, whilst thoseof low reactivity offer greater stability in the pad liquor. The bifunctional types give highlyefficient fixation and excellent fastness performance [30].

Many processes require only dye and alkali; for example, on cotton equal concentrations ofdye and alkali over the range 5–30 g/l may be used with a pick-up of 60–80%, whilst on viscosethe pick-up is usually higher (90–100%) and the alkali reduced to half the quantity of dye.With some aminochlorotriazine dyes both salt and alkali are used – perhaps up to 30 g/lsodium chloride and 10–15 g/l alkali. In some processes sodium silicate is preferred as the mainalkali, in order to alleviate the problem of white selvedges. This fault can occur with sodiumhydroxide, caused by neutralisation of the sodium hydroxide by carbon dioxide or other acidicatmospheric agents. A wetting agent is generally required in continuous processes to aid rapidwetting at the padding stage. A hydrotrope such as urea may be added, particularly for deepshades, to boost the solubility of less sulphonated dyes and to aid fixation through themechanism of retaining moisture by hydrogen bonding, particularly in fully continuousthermofixation techniques. If a thickening agent is needed to minimise migration, sodiumalginate is preferred since it does not interact with reactive dyes; electrolyte addition may servethe same purpose. An advantage of fully continuous procedures is that minimal quantities ofalkali and water are consumed. Two-stage continuous processes, comprising two wet-on-wetpaddings with the second pad applying the alkali, are applicable in certain circumstances.

The paramount importance of efficient preparation in producing goods of thorough anduniform absorbency for continuous dyeing cannot be over-emphasised [157,158]; allcontinuous dyeing systems are heavily dependent on this prerequisite. Typical build-upcurves on mercerised and unmercerised cotton fabrics in Figure 12.20 illustrate the

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significantly higher colour yields attained in a two-bath pad–steam process using sodiumsilicate (variant B) with vinylsulphone dyes. Recommended variant A and B chemical padformulations are shown in Tables 12.13 and 12.14, for a steaming treatment of 30–60seconds at 102–105 °C. In this process, important factors for the control of dye migrationinclude the lowest possible pick-up of dye liquor in padding, minimum addition of wettingagent, sufficient electrolyte in the chemical pad liquor and selection of vinylsulphone dyes ofhigh substantivity. If a one-bath, pad–steam process is used, the recommended variant Aand B chemical formulations are as presented in Tables 12.15 and 12.16 respectively.Steaming treatment is again 30–60 seconds at 102–105 °C.

Dye concentration/g l–120 40 60 80 100

20

40

100

60

80

Rel

ativ

e co

lour

yie

ld/%

B Bleachedand mercerisedcotton

Bleachedcottonfabric

A

B

A

Variant

Figure 12.20 Typical dye build-up curves for pad–steam process variants A and B on mercerised andunmercerised cotton [158]

Table 12.13 Chemical pad additions for two-bath salt variant A[158]

Dye concentration (g/l) <20 20–40 >40

Steaming time (s) 30 60 30 60 30 60

Sodium chloride or sulphate (g/l) 250 250 250 250 250 250Sodium carbonate (g/l) 20 20 20 20 20 20Caustic soda 32.5% (ml/l) 10 5 15 7.5 20 10

Table 12.14 Chemical pad additions for two-bath silicate variant B [158]

Caustic sodaSilicate Mass ratio Silicate solutionstrength (°Be) Na2O:SiO2 addition (ml/l) 32.5% (ml/l)

37–40 1:3.3 900 10040–42 1:3.3 760 100

REACTIVE DYES

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Table 12.15 Chemical formulation for one-bath saltvariant A[158]

30 g/l Sodium chloride or sulphate20 g/l Sodium carbonate 5 ml/l Caustic soda 38°Be if <15 g/l Dye10 ml/l Caustic soda 38°Be if >15 g/l Dye

Table 12.16 Chemical formulation for one-bath silicatevariant B[158]

95 ml/l Sodium silicate 37–40°Be 8 ml/l Caustic soda 38°Be (32.5%) if <15 g/l dye15 ml/l Caustic soda 38°Be (32.5%) if >15 g/l dye

For the pad–batch fixation of heterobifunctional Cibacron C (Ciba) dyes four alkalivariants have been suggested [159]:(1) Rapid fixation in 5–8 hours using a mixture of sodium hydroxide and sodium silicate, a

metering pump being necessary. A high concentration of alkali gives rapid fixation butstill allows excellent bath stability at temperatures up to 30 °C.

(2) Fixation in 6–12 hours with a mixture of sodium hydroxide and trisodiumorthophosphate, a metering device being necessary. This method is recommended forregenerated cellulosic fibres. This formulation contains the same total amount of alkalias method (1) with the same bath stability, but may be preferred where some bufferingcapacity is required and sodium silicate is undesirable.

(3) A different combination of sodium hydroxide (reduced amount) and sodium silicate, ametering pump not being necessary. This method demands a longer fixation time (12–24 hours). Bath stability is greater than two hours but the total amount of alkali isinadequate for application to grey goods, where the raw cotton consumes a great deal ofalkali.

(4) A mixture of sodium hydroxide and sodium carbonate, a metering pump beingnecessary. This method avoids the use of either silicate or phosphate and is popular forwoven goods and in circumstances where silicate would pose problems. Ideally thecarbonate should be free from bicarbonate. This system has less buffering capacity andgives slightly lower bath stability than methods (1) and (2).

Methods (1) and (2) are the most widely used because they offer the most reliable results inbulk-scale processing.

In pad–batch dyeing with the highly reactive chlorodifluoropyrimidine Drimarene R or K(Clariant) brands, it is equally possible to use either a weak alkali (sodium carbonate) for along batching time or a strong alkali (sodium hydroxide) for rapid fixation. It is claimed thatthe versatility of monofunctional dyes of this established type makes the more expensivebifunctional types unnecessary [160]. Detailed studies of each dye in the range led to thegeneration of a series of three-dimensional graphs from which a computer-based optimisedsystem has been developed and made available to dyers.

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The Levafix E/EA/EN (DyStar) system based on computer-centred optimisation is alsoavailable [161]. Two main alkali formulations are involved:(1) Sodium carbonate with or without sodium hydroxide(2) Sodium silicate with or without sodium hydroxide.

In general the prospect of working without silicate is regarded as a major advantage becausethe padding and washing-off of sodium silicate places considerable stresses on the machineryand on effluent treatment. However, the use of silicate in no way impairs the performance ofLevafix dyes. Six optimised alkali recipes for this system are shown in Table 12.17, whilstTable 12.18 shows the relation between pad liquor pH, batching time and pad liquorstability.

Continuous dyeing with Sumifix Supra (NSK) heterobifunctional dyes is claimed [162] togive reproducible dyeings exhibiting a high degree of fixation and good wet fastness. Thedyed fabric has an attractive handle and good appearance. The high fixation contributesadvantageously to low effluent loading. Typical pad liquor formulations and fixationconditions, in comparison with those for aminochlorotriazine and vinylsulphone dyes, aregiven in Table 12.19.

There is a general tendency to abandon the ‘classical’ continuous reactive dyeingprocesses in favour of shorter pad–dry processes that is mainly driven by environmental andeconomic factors [163,164]. Particularly pertinent to this trend is the marked decrease inaverage length of run to a given shade. The latest developments in dyeing machinery allowgreater control of the dyeing parameters and eliminate the former need for lengthy and

Table 12.17 Optimised alkali recipes for Levafix (DyStar) dyes at various applied depths [161]

Recipe Dye conc (g/l) 5 10 20 40 60 80 120

A Sodium carbonate (g/l) 5 10 20 40pH 11.3

B Sodium carbonate (g/l) 10 20 20 20 20Caustic soda 45% (ml/l) 0.2 0.35 0.65 1.3 2.15pH 11.5–12.1

C Sodium carbonate (g/l) 10 20 20 20 20 20 20Caustic soda 45% (ml/l) 0.35 0.65 1.3 2.6 4.0 5.3 7.9pH 11.8–13.2

D Sodium carbonate (g/l) 17 13.5 10.5 7 4Caustic soda 45% (ml/l) 2.0 4.3 6.3 8.6 10.6pH 12.6–13.5

E Sodium silicate 37°Be (g/l) 40 40 40 80 100 100 100Caustic soda 45% (ml/l) 2.0 3.3 5.6 8.9 11.7 13.2 14.9pH 11.5–13.0

F Sodium silicate 37°Be (g/l) 25 25 25 40pH 11.0

REACTIVE DYES

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Table 12.18 Relation between pad liquor pH, batching time and pad liquor stability for five optimisedalkali recipes and Levafix (DyStar) dyes [161]

Batching time (hours)

Pad liquor Bleached MercerisedRecipe pH stability (min) cotton cotton

Sodium carbonatewithout caustic soda A 11.3 142 21 14

Sodium carbonatewith a little caustic soda B 11.9 41 14 8

Sodium carbonate withincreasing caustic soda C 12.2 18 10 6

Increasing caustic soda,decreasing sodium carbonate D 12.7 8 5 3

Sodium silicate withincreasing caustic soda E 12.0 37 5 3

Table 12.19 Pad liquor formulations and fixation conditions for application of monofunctional andheterobifunctional reactive dyes by three continuous dyeing methods [162]

One-bath pad–thermofix One-bath pad–steam Two-bath pad–steam

Reactivesystem Temp. / time Alkali Temp. / time Alkali Temp. / time Alkali

ACT 200 °C 30 s Na2CO3 103 °C 5–10 min Na2CO3 103°C 60 s NaOH10–20 g/l 10 g/l 40°Be

20 ml/lUrea NaCl100–200 g/l 300 g/l

VS 150 °C 60 s Na2CO3 103 °C 3–8 min Na2CO3 103°C 30 s NaOH10–30 g/l 6–12 g/l 40°Be

20 ml/lUrea NaCl50–100 g/l 250 g/l

ACT-VS 180 °C 30–60 s Na2CO3 103 °C 1–5 min NaHCO3 103°C 30–60 s NaOH10–20 g/l 10–20 g/l 40°Be

10 ml/lUrea Na2CO350–100 g/l 20 g/l

NaCl100–150 g/l

ACT AminochlorotriazineVS Vinylsulphone

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expensive conditioning of machinery at the beginning of each run. Liquor wastage is a majorfactor both economically and environmentally in continuous dyeing. Downtimes up to 60minutes and waste liquor volumes up to 120 litres have been typical for traditional long runsto a shade [164]. Developments have reduced downtimes to as brief as 5 minutes andcomputerised systems inform management of the quantity and composition of pad liquors sothat wastage is minimised to about 20 litres. By adopting pad–dry–bake or simple pad–drymethods chemical padding and steaming stages are eliminated, thus benefitting theenvironment by avoiding the demand for large amounts of salt.

A typical development in this area is the Econtrol (Monforts and Zeneca) system[163,164], which consists of padding with dye, 1–2 g/l wetting agent and the alkali. Withhigh–reactivity dichlorotriazine dyes this can be sodium bicarbonate (10 g/l). No urea,sodium silicate, electrolyte or other chemicals are required. After a short air passage theuniformly squeezed fabric is passed through a hot flue in which the carefully controlledrelative humidity (25% moisture by volume) brings about fixation in two minutes. Thehumidity fulfils the function that otherwise would be provided by an environmentallyproblematical hydrotrope such as urea. This reduction of chemical consumption is indeed amajor economic and environmental benefit.

Acrylamide-based migration inhibitors are claimed to give more efficient fixation [165].A comparison of the differences between cotton and viscose has revealed [157] that viscoserequires significantly longer immersion times. For example, immersion time for thoroughlyprepared cotton can be less than one second on today’s high-speed ranges, whereas 1–1.5seconds is preferable for viscose. Contrary to some views, the addition of urea does notshorten immersion times and can in fact lengthen them [157].

Printing

Similar general considerations apply to direct printing processes as to continuous dyeing. Asingle all-in or a two-stage, pad–steam process may be used. Alginates are the preferredthickeners because other carbohydrates react with the dyes. The non-reactivity of alginates,in spite of their hydroxy groups, is thought to be due to the presence on each mannuronicunit of a carboxyl group that tends to repel the dye anions. As discussed in section 10.8.1,difficulties in obtaining alginates have led to the evaluation of alternatives, in particular thesynthetic anionic poly(acrylic acid) types that give higher colour yields but can be moredifficult to wash off. These products have been slow to replace the alginates apparentlybecause the synthetic acrylic thickeners show varying sensitivity to electrolytes. However,detailed comparative rheological studies [166] have indicated that even if electrolytesensitivity were controlled the acrylic polymers might still prove unsuitable despite thegreater fixation that they give. At the slow printing speeds necessary in these laboratorystudies the amount of paste absorbed by the fabric was always greater with the syntheticthickener, but this effect did not give higher colour yields because penetration was greater. Itwas concluded, therefore, that synthetic thickeners should not be adopted because of thehigher dye consumption that outweighs any price advantage offered by the syntheticthickener. This is expected to be even more relevant at the high printing speeds necessary incommercial production, because of the greater shear-thinning of the synthetic thickenerunder these conditions of application [166].

Emulsion thickenings, either oil-in-water or water-in-oil and including half-emulsions,

REACTIVE DYES

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have been important in the past but are now much less popular on environmental grounds.Hydrotropes and a mild oxidising agent such as sodium m-nitrobenzenesulphonate arecommonly added, the latter to protect azo dyes from reductive decomposition. Choice ofalkali depends on the reactivity of the dye. Sodium bicarbonate is usually preferred, beingcheap and offering high print paste stability with all but the most reactive dyes, but for dyesof sufficiently high stability the stronger alkalis (sodium carbonate or sodium hydroxide) maybe chosen to provide higher colour yields under more alkaline conditions. The amount ofbicarbonate may be reduced with highly reactive dyes. A pH-shift agent such as sodiumtrichloroacetate can be selected; this hydrolyses during steaming to release sodiumcarbonate (Scheme 12.5). However, another product of this hydrolysis reaction ischloroform. As well as being a volatile AOX-generating compound, chloroform ishepatotoxic and its use may be severely restricted.

Cl

C

Cl

CCl

ONa

O Cl

CH

Cl

Cl2 + H2O + Na2CO3 + CO22

Scheme 12.5

In the two-stage process the dyes are first applied without alkali, using a thickening agentsuch as sodium alginate that gels on subsequent application of alkali, usually together withelectrolyte. High concentrations of sodium silicate (Na2O/SiO2 = 1:2.1, 47°Bé) or mixedalkali solutions (for example, 185 g/kg sodium carbonate + 185 g/kg potassium carbonate +30 g/kg sodium hydroxide 32.5% or 38°Bé) are often used.

Undoubtedly of greatest concern in the printing of cellulosic fibres with reactive dyes is theessential demand for large quantities of the hydrotrope urea (typically 80–200 g per 1000 g ofprint paste). This auxiliary forms hydrogen bonds with water molecules, assisting in swelling ofthe fibre and the thickener, dissolution of the dyes and promotion of interaction between dyeand fibre [167]. The most obvious benefits provided by urea are improved levelness, diffusionand colour yield but this compound is now environmentally suspect. Various attempts toreplace urea over the last decade have led to contradictions, ranging from claims of completesuccess to the viewpoint that it has not yet been possible to find a satisfactory alternative andthe prognosis for doing so is not particularly good.

Claims have been made [168] that the two-phase flash age process is the only way toavoid urea problems. In this process, no urea or alkali is present when printing is carried outinitially, followed by padding with alkali and perhaps electrolyte (to limit migration) prior tosteaming. However, even this approach has its ecological drawbacks due to the alkali (whichmay include silicate) and electrolyte. Furthermore, not all dyes give a satisfactory response,in particular phthalocyanine turquoise blues [167]. It is possible to improve the versatility ofthe process by including small amounts (up to 50 g/l) of urea; whilst this does not attain theenvironmental objective of totally avoiding urea, it does give an improvement over moreconventional procedures. Suitable recipes are given in Table 12.20.

In conventional processes, the moisture content of the fabric at the fixation stage is about20% or even less. However, in the flash age process it is significantly greater than this andthe extent to which urea usage can be minimised is closely related to the amount ofmoisture present. Whilst traditional flash agers operated at around 50% humidity, more

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recent attempts to eliminate urea have seen increases in this level. For example, theinclusion of humidifying or moisturising equipment in front of an ager to increase moisturecontent by 30%, aided by adjustment of the viscosity of the thickener, gave satisfactoryyields in the absence of urea, even with phthalocyanine turquoise blues [167]. One means ofincreasing moisture content in the single-phase system of printing is to pre-wet by means offoam [169]. This enables the complete elimination of urea, giving additional advantages ofbetter flow properties of the print paste, shorter fixation times, reduced water consumptionand savings of thickener. In another approach, the concentration of urea could beconsiderably reduced by minor additions to the print paste of cyclodextrin, chitosanderivatives or so-called ‘superabsorbers’ based on acrylates [170].

However, after careful consideration of the various roles played by urea, it was concludedthat the prospects of finding an adequate substitute are not promising [171]. This echoesearlier views [172] resulting from a study of the fixation of reactive prints with variousconcentrations of urea by saturated (102 °C) or high-temperature (120 °C) steaming. It isimportant to remember that urea functions not just by attracting moisture but also byentering into various interactions with dyes, thickener and substrate. As regardsenvironmental aspects, the use of print paste preservatives, as discussed in section 10.8.6,should be borne in mind.

Conventional reductive discharge prints are applicable on grounds dyed with reactivedyes, especially if vinylsulphone dyes have been used, but there are difficulties with thosenon-azo blue and turquoise dyes that cede preference to resist processes [29]. Resists can beachieved by printing with a thickened paste containing a non-volatile acid (tartaric or citricacid, for example) or with an acidic salt such as sodium dihydrogen orthophosphate. Thethickening agent used must be stable to acid; hydroxyethyl or methoxyethyl cellulose ethers,locust bean gum or tragacanth are suitable. The pre-printed and dried resist areas are thenoverprinted or padded with a solution of a highly reactive dye together with the minimumamount of sodium bicarbonate.

The washing-off process

An especially important and critical aspect of the application of reactive dyes, whether bydyeing or printing, is the washing-off process necessary to remove unfixed or hydrolysed dyes,

Table 12.20 Print paste and fixation pad liquor formulationsfor two-phase flash age printing with reactive dyes [29]

Print paste recipeUrea 0–50 gSodium m-nitrobenzenesulphonate 10 gSodium alginate thickener 400–500 gWater 440–590 g

1000 g

Fixation pad recipePotassium hydroxide 185 gSodium carbonate 185 gSodium hydroxide 38°Be 30 gWater 600 g

1000 g

REACTIVE DYES

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as well as other products such as alkali, electrolyte and thickening agents. After rinsing toremove the more readily soluble products such as alkali and electrolyte, the goods are soapedat a temperature close to the boil to remove the unfixed active or hydrolysed dyes; a hightemperature is necessary to remove hydrolysed dye from the interior of the fibres. Surfactants,especially nonionic types, are often added to the soaping liquor and a sequestering agent isadded if the water is hard. Where thickening agents have been used, sufficient time must beallowed during the initial rinsing for the thickener to become hydrated and easier to remove.

This seemingly straightforward washing-off process has enormous environmental impactand has generated much research over the last few years. Washing-off accounts for as muchas 50% of the processing cost of a reactive dyeing in terms of the washing-off process itselfand subsequent treatment of the effluent. In addition to these environmental and economicconsequences, the efficiency of washing-off has a critical influence on the fastness levelsattained. Herein lies the primary rule: the extent of the washing-off sequence should be justsufficient to achieve the target fastness properties, since any further washing treatment onlyleads to unnecessary expenditure and greater effluent volumes. Seven parameters have beenlisted [173] as pertinent to the effects and efficiency of washing-off:(1) number of washing baths(2) duration of washing, varying in practice from a few seconds (continuous) to many

minutes (batchwise)(3) temperature, varying from rinsing at about 20 °C up to soaping at the boil, or higher in

special circumstances(4) liquor ratio, varying from 5:1 (continuous) to 100:1 (batchwise)(5) mechanical movement of liquor and/or goods is of major importance in influencing

liquor interchange(6) composition of wash liquor: the electrolyte and chemical content of dyeings varies

widely and these contaminants, together with any auxiliaries added to the washingbaths, will considerably influence the result

(7) the substantivity of the reactive dyes present markedly influences their washing-offbehaviour.

As a general illustration of the main effects, Table 12.21 shows the effect of a single washingtreatment of varying time and temperature on an exhaust dyeing of the monoazo J acidscarlet CI Reactive Red 123 and a continuous dyeing of the copper formazan CI ReactiveBlue 104. Both dyes contain a chlorodifluoropyrimidine reactive group, but consistentlymore dye was removed from the continuous dyeing, especially for shorter times at the lowertemperatures. Even after six successive washes at 25 °C, the removal of unfixed dye from theexhaust dyeing only increased to 56, 62 and 74% respectively from the 24, 36 and 52%shown for one wash [173]. In general, these results emphasise the importance of longertimes and higher temperatures. Further results demonstrated that electrolyte additions up to10 g/l did not markedly influence the removal of dye but showed significantly increasedeffects at 20 g/l and especially 50 g/l. Mild alkaline washing was just as effective as mildacidic washing and the addition of a nonionic agent had a favourable influence on dyeremoval. These results are specific to the two dyes evaluated. However, it is abundantly clearfrom these trends and comments, bearing in mind the wide variety of reactive dyes,substrates and machinery, that optimising the washing-off procedure in each case formaximum economy and minimum environmental impact is no easy matter.

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These difficulties have been thoroughly discussed in an exhaustive study [174].Theoretical considerations were thoroughly explored and the economic and technical meritsof four different washing systems were analysed:(1) individual washing baths(2) dyeing machines operating with continuous liquor exchange(3) continuous rope-washing machines(4) open-width washing machines.

Three key stages applicable to most washing processes were identified:(1) In the first stage, unfixed dyes, salt and alkali present in the liquor phase must be

removed and this is best done by replacing this liquor with fresh water. Sorption,desorption and diffusion processes play only subordinate roles in this stage, the keyfactors being liquor flow, mechanical action and liquor exchange. The dilution laws aregenerally applicable.

(2) In the second stage, substantial amounts of alkali and unfixed dyes are desorbed anddiffuse from the fibre pores into the liquor phase. This is the diffusion stage and therate-determining step is diffusion of the labile dye molecules out of the fibre phase.This takes time and is accelerated by higher temperatures and perhaps by mechanicalagitation of the substrate.

(3) In the final stage, the electrolyte and unfixed dye concentrations are low but furtherchanges of liquor must take place until almost all the unfixed dye molecules aredesorbed.

These three processes are not necessarily distinct consecutive stages but are majoridentifiable events that may be more or less separated or interlinked by transitional steps. Itis argued that the washing-off requirements are different in each stage. In the first stage ofliquor exchange, cold rinsing is preferred because higher temperatures offer only a slightadvantage and alkalinity may hydrolyse dye–fibre bonds. Empirically, this stage may beregarded as completed when the electrolyte content reaches about 1 g/l; the number ofliquor changes required is related to liquor ratio. A shorter liquor ratio demands morechanges and involves only slightly lower water consumption, so is without advantage at this

Table 12.21 Effect of a single washing treatment for various times at varioustemperatures on the removal of unfixed dye from exhaust or continuous dyeingswith chlorodifluoropyrimidine reactive dyes [173]

Removal (%) of unfixed dye from exhaust or continuous dyeing

Exhaust dyeing of Continuous dyeing ofCI Reactive Red 123 CI Reactive Blue 104

Washing on bleached cotton on mercerised cotton

Temperature 25 °C 70 °C 98 °C 25 °C 70 °C 98 °C

Time 15 s 24 64 84 38 80 86 1 min 36 74 88 52 90 90 5 min 52 90 92 74 92 94

REACTIVE DYES

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stage. Continuous methods are more effective, since these give direct displacement ofcontaminated liquor by fresh water, rather than mixing of them. In the second stage ofdiffusion from the fibre, hot washing is necessary to ensure the major advantage of morerapid diffusion. Individual dye properties, such as substantivity and diffusion coefficient, areparticularly pertinent here. In contrast to earlier conclusions [173], these studies involvingseveral surfactants showed no particular advantage from the use of detergents [174]. Theaddition of a detergent is best avoided if possible, since it further contaminates the effluent.It is useful to compare the economics and fastness performance, particularly in heavydepths, of a thorough washing-off against a less than complete extraction of unfixed dyesfollowed by a cationic aftertreatment.

To minimise effluent problems by treatment of liquors before discharge to effluent, it ispreferable to concentrate the residual dyes in as small a volume as possible. This implies thatremoval of unfixed dyes should be restricted to as few baths as possible. In this context dyesubstantivity is an important consideration [175]. Cold initial rinsing removes much of thelow-substantivity dye present but dyes of high intrinsic substantivity are difficult to removein this way. If the initial rinse is hot, however, even these high-substantivity products areeffectively removed, thus achieving concentration of these dyes in the initial stage andensuring easier subsequent removal. Thus it was proposed that washing-off could beimproved by using a hot rather than a cold initial wash [175]. This was substantiated infurther optimisation studies which enabled the residual dyes to be confined to two or threebaths, surfactant additions not being necessary for these water-soluble contaminants. Theseresults were confirmed in bulk processing. It was found that lower liquor ratios werebeneficial in washing-off [176] and similar control parameters were identified in continuouswashing-off [177]. The benefits of hot initial washing (60–70 °C) have been demonstratedin yarn package dyeing, emptying of the vessel after dyeing being unnecessary [178].

From the viewpoint of energy conservation, the washing temperature should be chosenaccording to the dyes present [179]. Since dyes of low substantivity are desorbed morereadily under mild conditions, a lower temperature should be selected for these dyes andhigher temperatures for dyes of higher substantivity. In an evaluation of the washing-offbehaviour of dichlorotriazine dyes from cotton at temperatures ranging from 20 to 98 °Cwith water alone, and at 98 °C with a surfactant, the surfactant-aided method was found tobe the most effective [180] and the benefits of aftertreating with cationic agents wereconfirmed.

A detailed comparison [181] of three vinylsulphone dyes included a low-substantivitymonoazo N-acetyl H acid derivative (CI Reactive Red 35), a monoazo N-acetyl J acid typeof higher substantivity (CI Reactive Orange 82) and a phthalocyanine turquoise somewhatprone to aggregation (CI Reactive Blue 21). Dyeings of these individual products weresubjected to three wash-off procedures:(1) conventional sequence: cold, cold, warm, hot, cold(2) cold/hot sequence: 30 °C, followed by several baths at 95 °C(3) hot sequence: repeated treatments at 95 °C.

Typical results are given in Table 12.22. The objective was to examine the relationshipbetween the substantivity of the unfixed dyes and their response to changes in theconditions of the washing sequence.

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The low-substantivity red dye was desorbed relatively quickly, mainly in the initial coldrinse, and the acceptable fastness rating of 4–5 was reached after three or four washingbaths. The greater difficulty of desorbing the more substantive orange dye was clearlyevident. Only after the first hot bath of the sequence was most of the unfixed dye desorbed.A fastness rating of 4–5 was reached after the 5th, 4th and 3rd baths respectively of thewashing sequences (1), (2) and (3). With CI Reactive Blue 21 seven baths were needed inthe conventional sequence (1) to give a fastness rating of 4–5. Sequences (2) and (3) weremore effective, giving acceptable fastness after four and three baths respectively. In all casesit is the hot washing sequence (without surfactant) that enables this target to be reachedquickly and with the lowest water consumption. It was confirmed that the final bath neednot be absolutely colourless after washing, nor is complete removal of unfixed dye necessaryto attain the target fastness level.

Similar results have been observed in the washing-off of reactive prints, in which it isnecessary to monitor the removal of the alginate thickener as well as desorption of theunfixed dyes [182]. Batchwise washing showed that the thickener was rapidly washed outand that elevated temperatures increased the rate of removal. No significant dwell time forthickener swelling seemed to be necessary. Significantly more effective removal of thickenerwith increasing temperature was observed in continuous washing and most of the thickenerwas eliminated in the first wash.

Table 12.22 Effects of various washing–off sequences on the rate of desorption of dyein successive washing baths and the degree of staining of adjacent material in washingtests [181]

*Colour value of wash liquor Fastness to washing

Washing sequence (1) (2) (3) (1) (2) (3)

CI Reactive Red 35 (low substantivity) W = 510 nmWashing bath 1 16 15 18 2 2 3

2 2 2 1 3 4 43 2 1 0 4 4–5 4–54 1 1 0 4–5 4–5 4–5

CI Reactive Orange 82 (moderate substantivity) W = 490 nmWashing bath 1 17 23 56 1 1 2

2 10 31 4 1–2 3 3–43 11 2 0 2 4 4–54 13 0 0 3–4 4–5 4–55 2 0 0 4–5 4–5 4–5

CI Reactive Blue 21 (phthalocyanine type) W = 664 nmWashing bath 1 14 15 33 2 2 2–3

2 3 15 6 2–3 3 43 2 2 1 2–3 4 4–54 6 0 0 3 4–5 4–55 3 0 0 4 4–5 4–5

* Colour value given by 1000 E/d, where E is extinction at the peak wavelength (W nm) and d mm isthe cell thickness

REACTIVE DYES

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Confirmation of earlier work [180] on the temperatures of washing-off of dichlorotriazinereactive dyeings on cotton revealed that surfactant and sodium carbonate together weremore effective than water alone, although there was little difference in performance betweenthe six surfactants examined [183]. However, the addition of surfactant did not appear to beessential in order to achieve adequate dye desorption and fastness to washing, 2–5 g/lsodium carbonate being sufficient. These results were later substantiated withdichlorotriazine dyeings on lyocell [184] and for aminochlorotriazine dyeings on cotton[185].

Further studies involved different alkalis (ammonia, sodium bicarbonate, potassiumhydroxide and sodium hydroxide) and unspecified buffers covering the range pH 7–13 withaminochlorotriazine dyeings on cotton. Although all these alkalis and buffer solutions werecapable of removing unfixed dyes, the effectiveness of washing-off of the systems varied;potassium hydroxide and sodium bicarbonate were the most effective and the pH 8 buffersolution was the least. In general, effectiveness increased with increasing pH but theoutstanding behaviour of potassium hydroxide and sodium bicarbonate could not beexplained in terms of pH alone. Despite varying effectiveness in terms of washing-off, all ofthe washed dyeings were virtually identical in fastness to washing. When multiple alkalinewashes were evaluated on aminochlorotriazine dyeings, potassium hydroxide and sodiumcarbonate were the most effective, although sodium bicarbonate was more attractive interms of low cost, environmental impact and fastness to washing. However, some of thesemultiple alkaline treatments showed evidence that dye–fibre bond cleavage may have takenplace in addition to removal of hydrolysed dye [185].

When washing-off in a jet machine, a combination of liquor exchange and continuousoverflow rinsing is advantageous [186]. Optimal rinsing procedures depend on machineparameters and the cost structure of the plant, but in general the most economical systemappears to be:(1) liquor exchange rinsing initially (salt removal)(2) continuous rinsing at a higher temperature (diffusion of unfixed dye)(3) liquor exchange rinsing finally (removal of dye liquor).

The objective of stage (1) is to lower the salt concentration to 1–2 g/l and this requirementdetermines the number of initial baths given before subjecting the dyeing to a highertemperature. Salt removal is accelerated by liquor changes and dilution before commencingcontinuous rinsing. The duration of continuous rinsing is adjusted according to the depth ofshade and the known diffusion properties of the dyes present. Calcium and magnesium ionspresent when washing in hard water make the unfixed dye anions more difficult to remove;poly(acrylic acid) derivatives are effective sequestering agents in mildly alkaline liquors[187].

Stripping

The stripping of cellulosic materials dyed with reactive dyes is carried out by alkalinereduction followed by hypochlorite oxidation, preceded by a boiling treatment with EDTA ifmetal-containing dyes have been used. For example, a treatment with 5 g/l sodiumcarbonate or sodium hydroxide and 5g/l sodium dithionite at the boil is followed by atreatment in 0.5–1 °Tw hypochlorite, an antichlor and thorough rinsing.

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12.7.2 Wool

Although wool can be dyed with typical reactive dyes produced essentially for cellulosicsubstrates, the adoption of reactive dyes on wool has depended on the development ofspecial ranges of dyes capable of covalent bonding under slightly acidic conditions. The useof surfactant auxiliaries is essential. The types of dyes used and the mechanisms of reactionhave been discussed in section 7.7 and elsewhere [2]. These dyes are generally similar inresponse to dyebath pH to milling acid dyes, medium or full depths being applied at pH 5.0–5.5 and pale depths at pH 5.5–6.0. At lower pH values adsorption and fixation may beirregular, whilst at higher values exhaustion is poor. Even at optimal pH fixation may still beincomplete within normal dyeing times, particularly when dyeing navy or black shades.Hence the bath is adjusted to a weakly alkaline state, which ensures complete fixation aswell as helping to remove hydrolysed dye. A typical procedure is to dye at the boil and pH 5,followed by cooling to 80 °C and adjustment of the pH to 8.0–8.5 with ammonia, afterwhich treatment is continued for some 20–30 minutes at 80 °C.

A surfactant auxiliary is necessary to prevent tippy or skittery dyeing. Cationic or nonionicproducts have been used but the most useful have been the amphoteric N-alkylbetaines andalkylamidobetaines (described in section 9.7) and ethoxylated amphoteric types represented bystructure 12.41 [188]. Compounds of this type can interact under slightly acidic conditionswith both fibre and dye. The mechanisms have been investigated in detail [188–192]. Theinitial step appears to be the formation of an auxiliary–dye complex by interaction between thequaternary group in the surfactant and the dye anion, such a complex being less soluble thanthe dye alone. As the concentration of the auxiliary is increased to an excess over thatrequired for complexing with the dye anions present, surfactant micelles are formed that tendto solubilise the auxiliary–dye complex. Unlike most levelling agents these amphotericproducts often increase the rate of wool dyeing, to an extent dependent on the chemistry andconcentration of the auxiliary and of the dye. Conversely, they may have a retarding effect ifthe concentration of auxiliary is so high as to solubilise the auxiliary–dye complex completely.These auxiliaries also tend to improve dye exhaustion at equilibrium.

(CH2CH2O)m

N

H3C (CH2CH2O)n

H3C(H2C)10H2C

H

SO3

CH3SO4

NH4

12.41

(m+n = 10–80)

+

+

_

_

The accepted explanation for this behaviour is that the zwitterionic auxiliary–dyecomplex is less electronegative than the dye anion; hence it exhibits more hydrophobiccharacteristics with increased molecular size and lower aqueous solubility. Consequently, theaffinity of the dye for the undamaged roots of the wool fibres is enhanced relative to that forthe more hydrophilic damaged tips. The auxiliary–dye complex is less sensitive to root-tipdifferences and thus gives more level dyeing. Furthermore, adsorption of the auxiliary by thefibre increases the electropositive charge on the fibre, thus increasing the attraction foranionic dyes. This will tend to further increase the rate of dyeing and is also the mechanism

REACTIVE DYES

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whereby the total amount of dye absorbed is increased. On the whole, there is little evidencethat these agents increase migration and indeed no migration can take place once the dyehas reacted covalently with the fibre.

The levelling action of betaines increases with concentration and with increasing lengthof the alkyl chain [188]; the effects on rate and total absorption disappear with productscontaining a very long alkyl chain (about C16) at a high concentration, although thelevelling properties are maintained irrespective of the rate effects. In general, the betainesbring about a greater change in sorption characteristics than do the ethoxylated amphotericcompounds of similar alkyl chain length [188]. Thus higher concentrations of the latter arenecessary to produce a similar effect; this can be an advantage, since the effects are not thenso critically sensitive to the concentration of auxiliary. Furthermore, the manufacturer canvary the balance of properties of the auxiliary by varying the length of the oxyethylenechains as well as of the alkyl group.

Using laser Raman spectroscopic techniques, it was confirmed that the primarymechanism involves ionic interaction between the anionic sulphonate groups of the dye andthe cationic quaternary group of the auxiliary, although hydrophobic interactions were alsosignificant [190]. However, investigations of the coverage of damaged wool indicated thatinteraction of the auxiliary with the fibre is mainly responsible for the improvement inlevelness, both the surface and the interior of the fibre being involved [191]. Dye–auxiliaryinteraction does not seem to play a major role in levelling out root-to-tip variations. Studieswith variously modified wools, including wool pretreated with an amphoteric agent,demonstrated that these auxiliaries mainly accelerate the rate of exhaustion on normal wool,whereas the effects on modified wools are minimal [192]. Although these agents promotedye uptake and fixation on normal wool, they do not enhance dye uptake to the same extenton Hercosett-treated wool [2].

Traditionally, the alkaline treatment given after reactive dyeing has been with ammoniaat pH 8.0–8.5 and this is still the favoured method of removing unfixed dyes, althoughsodium bicarbonate is occasionally used. Indeed, it is claimed that the lower basicity ofbicarbonate results in less fibre damage and no detrimental effect on fastness properties[193]. A specific disadvantage of ammonia is that it can cause uneven treatment in differentparts of a yarn package because of irregular swelling of the fibre [2].

Several other agents have been suggested as potential aftertreating agents for reactive-dyed wool:(1) Hexamethylenetetramine: this ammonia precursor does not cause fibre swelling and the

unfixed dyes are removed efficiently at pH 6.5, compared with pH 8.5 with ammonia,thus causing less damage to the wool. However, the hydrolysis of this compound(Scheme 12.6) results in the formation of formaldehyde and this can modify the hue ofcertain dyes [2].

(2) Sodium bisulphite: this additive reacts by nucleophilic addition to the vinylsulphonegroup of dyes of this type, decreasing the substantivity and increasing the aqueoussolubility of the unfixed dyes (Scheme 12.7).

(3) A commercially branded product believed to be sodium trichloroacetate: this is addedat a concentration of 5% to the dyebath 20 minutes before the end of dyeing. Sodiumcarbonate is formed by hydrolysis of the trichloroacetate (Scheme 12.5), accompaniedby a change of pH from 5.0–6.0 to 6.7–6.9 [2]. This reaction also releases the volatileAOX-generating chloroform, however.

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(4) Various amine salts, such as hydrochlorides of 1,6-diaminohexane, 1,12-diamino-dodecane, 1-aminododecane and a bisulphate adduct of 1,6-di-isocyanatohexane. Thiswas an attempt to introduce a final reactive step to form crosslinks between unfixed dyemolecules on the fibre [193].

H2C N

H2C

CH2H2C

N

CH2

N

CH2

N

+ 6 H2O 4 NH3 + 6 HCHO100 °C

Scheme 12.6

[dye] SO2CH CH2 [dye] SO2CH2CH2SO3Na+ NaHSO3

Scheme 12.7

Although all of these compounds were applicable in principle, treatment with sodiumbicarbonate offers a much more economical and reliable option.

The mechanism of reaction of α-bromoacrylamide dyes with wool has been investigated[194]. The application to wool of a range of heterobifunctional dyes normally used oncellulosic fibres has been promoted recently [195]. These dyes offer high exhaustion andfixation, as well as exceptional fastness. They are relatively insensitive to dyeing variablesand suitable for dyeing wool–cellulosic blends by a one-bath method, with savings in energycosts and less wool damage. The choice of auxiliaries is essentially the same as whenapplying these dyes to cotton, although the pH is varied according to whether wool or wool–cotton is to be dyed. An ammonia aftertreatment is recommended for wool dyeings. Thereactive dyeing of wool at 110 °C and pH 4.5 (ammonium sulphate and acetic acid) in thepresence of a betaine auxiliary has been described [196].

Reactive dyes can be applied to wool by printing [2,29]; suitable ranges includechlorodifluoropyrimidine, α-bromoacrylamide, sulphatoethylsulphone and aminochloro-triazine. A typical print paste recipe is shown in Table 12.23. Derivatives of locust bean or guargum, either alone or in combination with water-soluble British gum, are the preferredthickeners. Humectants, particularly urea, are essential to aid solubilisation and penetration aswell as swelling of the wool. A wetting agent (5–10 g/kg) may also be needed if chlorination ofthe goods has been less than optimum and where complete penetration of the print design isdesirable. Antifoam addition is generally necessary in machine printing. A non-volatile acidsuch as citric acid or an acid donor such as ammonium tartrate or ammonium sulphate isnormally required to give optimum pH conditions; however, vinylsulphone dyes are appliedunder neutral to slightly alkaline conditions using sodium acetate (40 g/kg). The oxidisingagent sodium chlorate is often added to counteract the reducing effect of the wool, although ifvinylsulphone dyes are printed under alkaline conditions sodium m-nitrobenzenesulphonate iseffective in protecting any reduction-sensitive dyes.

REACTIVE DYES

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The most important reducing agents used for discharge printing on wool are theformaldehyde-sulphoxylates [2]. These agents are restricted to the minimum effectiveconcentration within the range 30–180 g/kg. The print is washed in several baths containing2 g/l disodium hydrogen orthophosphate, ammonia (to give pH 9) and an anionic surfactantin successive baths at temperatures increasing from 40 to 80 °C. The printing of silk withreactive dyes, using sodium silicate as alkali and sodium alginate as thickener, has beendescribed [197]. Problems arising from the contamination of effluents with reactive dyeingand printing auxiliaries represent an important issue that has been fully discussed elsewhere[198–201].

12.8 SULPHUR DYES

The application of sulphur dyes to cellulosic substrates involves conversion to thesubstantive leuco form by alkaline reduction, followed by oxidation on the fibre to theinsoluble disulphide form (section 1.6.2). Consequently, reducing and oxidising agents areessential auxiliaries. Secondary auxiliaries include wetting agents, sequestering agents,antioxidants, electrolytes and hydrotropes. Useful reviews of developments in sulphur dyesand their application are available [30,202,203]. The concern here is with the essentials ofthe auxiliary agents used in their application. It is convenient to deal separately with theaspects of reduction and oxidation. The discussion is confined to application by dyeing sincethe use of sulphur dyes in printing nowadays is restricted mainly to sulphur blacks applied bytechniques similar to those used in vat printing [29].

12.8.1 Reduction

Sulphur dyes in the insoluble disulphide form and the CI Solubilised Sulphur brands arereduced by the dyer as part of the application procedure. In the case of the CI LeucoSulphur brands reduction has already been carried out by the manufacturer, so that they aresubstantially in a form suitable for immediate application (section 1.6.2) The chemistry ofthe reduction of sulphur dyes is complex, as is the chemistry of the dyes themselves; it hasbeen well described elsewhere [204]. It is possible to describe the state of a reduced sulphurdye in alkaline sulphide or polysulphide solution by the general formula 12.42, but there arecertain complications. In many cases the chromogen is not itself reduced, but in others,notably reddish browns, blues and navy blues based on indophenols, the chromogenic

Table 12.23 Typical print paste recipe for theprinting of wool with reactive dyes [29]

Dye x gUrea 50 gThiodiethylene glycol 50 gThickener (10–12%) 500 gAcid or acid donor 10–30 gAntifoam 1–5 gWater to 1000 g

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quinonimine grouping can be reduced (Scheme 12.8). Additionally, the value of n instructure 12.42 varies, even for a given dye. It is not surprising, therefore, that the amountof reducing agent (and alkali) required varies from dye to dye, depending on the chemistryof the dye as well as on the concentration in the formulation. Hence the manufacturer’sliterature must always be consulted for the amounts of auxiliaries to be used with particulardyes in the various batchwise, semi-continuous and continuous processes.

D Sn SXm

12.42

D = chromogenn = 0–5 m ≥ 1X = H or Na, depending on pH

N O N

H

OHreduction

oxidation

Scheme 12.8

The reduction process is invariably carried out in an alkaline medium, partly because ofthe instability of most reducing agents at low pH and partly because the acidic thiol groupsreact with alkali to give the much more soluble anionic thiolate form [204]. Traditionally themost widely used reducing agents have been sodium sulphide (Na2S) and sodium hydrogensulphide (NaHS). Technically these are still the most widely preferred, not only for theirefficacy but also because they are relatively inexpensive. Nowadays, however, they areincreasingly subject to scrutiny on environmental grounds. At least 12 g/l sodium sulphide isrequired to dissolve the water-insoluble CI Sulphur dyes. The quantity added to the dyebathvaries from dye to dye but is generally proportional to the amount of dye, with a minimum of1.5–3.0 g/l [30]. When using sodium hydrogen sulphide the quantity is generally 0.6 timesthat of sodium sulphide, but it is also necessary to add alkali (10 g sodium hydroxide or 5 gsodium carbonate per 7 g sodium hydrogen sulphide). The pre-reduced CI Leuco Sulphurdyes usually contain a mixture of sodium sulphide and sodium hydrogen sulphide, togetherwith hydrotropic and dispersing agents such as 2-ethoxyethanol, sodium 1,3-xylene-4-sulphonate, sodium p-toluenesulphonate and sodium tetralinsulphonate [202]. Thequantities of sodium sulphide and sodium hydrogen sulphide referred to above relate to thefull-strength concentrated products and must obviously be proportionately adjusted ifweaker commercial brands are available.

In some applications, particularly in jet and winch dyeing, there is a danger that thereducing agent may be prematurely oxidised by air. Antioxidants, added along with the dyesand the primary reducing agent at the beginning of dyeing, can be used as palliatives.Polysulphides of general formula 12.43, such as disodium tetrathionite, have been widelyused for this purpose and provide improved dyebath stability. These additives can be usedwith other reducing agents described below but are not compatible with dithionite. Anotherapproach is to add a relatively more stable alkaline reducing agent such as sodiumdithiodiglycolate 12.44 [205].

Environmental concerns are gradually curtailing the use of sulphides as reducing agents[203,206], although it has been indicated [207] that as late as 1995 some 90% of all sulphur

SULPHUR DYES

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dyes applied worldwide were still reduced by sulphides. The environmental problems arisingfrom sulphides include the toxicity of hydrogen sulphide, corrosion of the effluent drainagesystem, damage to the treatment works and the often associated high pH and unpleasantodours [208]. Sulphides cause no odour nuisance above pH 9 but at neutral or acidic pHvalues gaseous hydrogen sulphide is liberated. Neutralisation or acidification can occur inthe dyehouse or during waste stream mixing. The MAK value of hydrogen sulphide is 15mg/m3 (10 ppm), this being the maximum allowable concentration in the workplace at acontinual contamination level to avoid impairment of health, the odour threshold being0.035–0.14 mg/m3 of air [207]. Polysulphides yield free sulphur on acidification and this canlead to odours of sulphur dioxide on the dyed substrate. Nevertheless, sulphides can bequantitatively removed before discharge to the effluent, albeit at a substantial cost[208,209].

Several reducing agents have been suggested as environmentally more acceptablealternatives to the alkali sulphides. All are more expensive and exhibit other disadvantages.For example, the reduction may be more difficult to control, or a particular agent may beeffective only with a limited range of dyes. Even then the alternatives may be less effectivethan the alkali sulphides in terms of colour yield. Nor is the fact that such compounds donot generate hydrogen sulphide a guarantee of freedom from environmental problems; forexample, some give quite high COD levels. The most obvious alternative is sodiumdithionite (12.45) with alkali, the reducing agent most widely used with vat dyes (section12.9). When used with sulphur dyes, however, the process is difficult to control and somedyes may be partly destroyed through over-reduction. Nevertheless, dithionite is effectivewith CI Solubilised Sulphur and sulphurised vat dyes.

XO3S Sn SO3X

12.43

X = Na, K, H or NH4n = 1–4

CH2C

NaO

O

S S CH2 C

ONa

O

12.44

NaO S S ONa

O O

12.45CI Reducing Agent 1

Sodium hydroxide is the alkali usually used in conjunction with dithionite. Sodiumcarbonate is a possible alternative when CI Solubilised Sulphur dyes are used but isinsufficiently alkaline for the CI Sulphur brands, requiring careful control if over-reductionand the associated lower yields are to be avoided [30]. Typical concentrations are given inTable 12.24. The system of sodium carbonate and sodium dithionite used to reduce blue andblack CI Solubilised Sulphur dyes is particularly suitable for flame-retardant viscose fibresthat are sensitive to strong alkalis, since it preserves a satisfactory level of flame retardancy[30]. It is also possible to use a mixture of dithionite with sodium sulphide in alkaline media.

Sodium formaldehyde-sulphoxylate (12.46; sodium hydroxymethanesulphinate) andalkali, although more stable than alkaline dithionite, tends to share the same disadvantages

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and is rarely, if ever, used with sulphur dyes owing to associated handling difficulties,inadequate cost-effectiveness and poor efficiency [207]. Sodium dithiodiglycolate (12.44)was mentioned above as an antioxidant. Such compounds may be used as the primaryreducing agent in conjunction with alkali. Although they do not give rise toenvironmentally undesirable inorganic sulphides in the effluent, their chemical stabilityresults in a high chemical oxygen demand, often causing more problems than those arisingwith sodium sulphide [202]. This system is rarely used for much the same reasons as sodiumformaldehyde-sulphoxylate [207].

The most promising alternative to sulphides, from an environmental point of view, is theuse of the reducing sugar glucose with sodium hydroxide or sodium carbonate. This systemdoes not satisfactorily reduce all dyes, however. It is reasonably effective with CI SolubilisedSulphur brands [210], with which it may be used either as the sole reducing agent or inconjunction with sodium polysulphide, usually resulting in increased dye yields. It can beused as an additional reducing agent with CI Leuco Sulphur dyes, thus giving a lowersulphide content in the dyebath, or together with sulphide or polysulphide in the reductionof the traditional water-insoluble CI Sulphur brands [30]. The system is pH- andtemperature-sensitive; hence performance may be good on jet machines but poor on themore temperature-sensitive jigs. Typical quantities recommended for a batchwise dyeingmethod [30] at liquor ratios of 10:1 to 20:1 are 3–8 g/l glucose, 4–10 g/l sodium carbonateand 2–6 g/l sodium hydroxide, depending on applied depth, a pH of 11–12 and a minimumdyeing temperature of 90–95 °C being necessary [206].

The glucose reducing system has a characteristic odour of burning sugar that manypeople consider preferable to the odour of an alkaline sulphide bath, although others dislikeit, finding it excessively sweet and nauseous [30]. Nevertheless, the versatility of glucose-based binary systems has been emphasised [207]. The major problem with an alkalineglucose system is that it is gradually transformed into various decomposition products, thuslosing its reducing action. The intermediate by-products possess some reducing action butare not sufficiently stable. More stable decomposition products are formed if dithionite is

Table 12.24 Typical applied concentrations of sodium dithionite andalkali for exhaust dyeing [30]

Liquor ratio 10:1 1% Dye 6% Dye

Caustic soda flakes 3.5 g/l 7.5 g/lSodium dithionite 2.5 g/l 7.0 g/l

These amounts are decreased by 30–40% at liquor ratio 20:1 orincreased correspondingly at liquor ratio 5:1

Sodium carbonate 0.5 –1.5 g per g of dyeSodium dithionite 0.25–0.75 g per g of dye

HOCH2 O S ONa

12.46CI Reducing Agent 2

SULPHUR DYES

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added to the system, this being the subject of patents more than sixty years ago [207].Scheme 12.9 has been suggested to explain this effect.

C

CHOH

H O

CHOH

CHOH

CHOH

CH2OH

C

CHOH

H O

CH2OH

O

CHOH

S ONa

CHOH

CH2OH

O

CHOH

S ONa

O

CHOH

CH2OH

sodium

hydroxide

sodium

dithionite

Sodiumglyderylsulphinate

Sodiumglycerylsulphonate

Glucose

Glyceraldehyde

2 +

Scheme 12.9

The reduction potentials of various reducing agents listed in Table 12.25 show that at 50 °Cthe dithionite/glucose system has a potential that is only slightly lower than that of dithionitealone, even though glucose has the lowest potential in this series. The addition of glucosereduces the potential of sodium dithionite to the point where full colour yield is obtainedwithout the risk of over-reduction. Dyeing tests have confirmed that although sodiumdithionite alone is exceptionally concentration-sensitive, the addition of glucose gives a morestable system. Optimal colour yield and good reproducibility are obtained even if dyeingtemperature, time and concentrations of chemicals fluctuate within certain narrow limits.Similar results have been obtained with other glucose binary systems, including hydroxy-acetone or sodium formaldehyde-sulphoxylate as stabiliser.

Table 12.25 Reduction potentials of various reducing systems at50 °C [207]

Reduction potential(mV) at 50 °C

Reducing system (platinum electrode)

Sodium formaldehyde-sulphoxylate –900Sodium dithionite –850Sodium dithionite/glucose –700Hydroxyacetone –680Sodium sulphide –650Sodium polysulphide –500Glucose –200

5 g/l Reducing agent10 ml/l Caustic soda 38°Be

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The environmentally innocuous properties of reducing sugars have been claimed for thedyeing of jute with sulphur dyes [211]. This claim is based on the influence of citric acidaddition on the hydrolysis of cane sugar, giving a higher proportion of reducing sugars (Table12.26). The optimum addition is 0.04% citric acid and treatment of the coarse brown sugarfrom the sap of palm trees (so-called ‘jaggery’) is for 24 hours at ambient temperature, givinga product termed ‘liquid jaggery’. This is added at 2.5 times the mass of sulphur dye in thedyeing of jute at 100 °C for 1 hour in a liquor containing 50 g/l sodium chloride. Thisproduct can also be utilised in printing, a typical print paste formulation being shown inTable 12.27.

Table 12.26 Effect of citric acid addition on the composition ofcane sugar after hydrolysis [211]

Composition of product

Citric acid Reducing Non-reducingconcentration (%) pH sugars (%) sugars (%)

0 5.9 21 400.01 5.8 30 380.02 5.5 33 360.03 5.3 36 320.04 5.3 39 280.05 5.0 43 24

2-Mercaptoethanol (12.47) with alkali has been suggested as an alternative to sulphides[210], offering the advantages of no sulphides in the effluent and no odour from thedyebath, although the product itself can give off unpleasant and highly toxic fumes. Thisprocess is relatively expensive, with a tendency towards lower yields and a more restrictedrange of suitable dyes than when using traditional sulphides, so it has not achievedsignificant commercial use.

HSCH2CH2OH

12.472-Mercaptoethanol

CH3COCH2OH

Hydroxyacetone12.48

SULPHUR DYES

Table 12.27 Typical print paste formulation forthe application of liquid jaggery as a reducingsystem for sulphur dyes [211]

Sulphur dye 3–5 gSodium carbonate 2–3 gLiquid jaggery 7.5–12.5 gThickening agent 80 gWater (if necessary) to 100 g

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Hydroxyacetone (12.48), originally introduced for vat dyeing, has proved moderatelysuccessful with sulphur dyes. This compound requires strongly alkaline conditions, theconcentrations being critical. Colour yields are somewhat lower than with sulphide, theproduct is flammable and the dyebaths have an odour characteristic of acetone. Nevertheless,when used with CI Solubilised Sulphur dyes the effluent is free from sulphide. Mention hasbeen made already of its possible use in a binary system with glucose [207]. Hydroxyacetone isthe basis of the RD (Refine Dyeing) process of Ipose GmbH; this is a carefully optimisedsystem incorporating automatic dosing of hydroxyacetone as the sole reducing agent [212].Hydroxyacetone is suitable for application in indigo, vat and sulphur dyeing, including thecontinuous dyeing of cotton yarn with sulphur dyes or indigo, as well as the exhaust dyeing ofknitgoods [212]. These processes are described in greater detail in section 12.9.

Thiourea dioxide, also discussed further in section 12.9, has been regarded as a moreenvironmentally friendly replacement for sodium sulphide in the application of sulphur dyes[213]. In an investigation of exhaust dyeing with eight sulphur dyes applied from alkalinedyebaths, thiourea dioxide gave colour yields and fastness ratings similar to those withsodium sulphide, although in some cases slightly different hues were observed. Typicalanalyses of exhaust dyebaths are detailed in Table 12.28. On balance, thiourea dioxide ismuch less hazardous to the environment than sodium sulphide; the major improvement isthe decreased amount of oxidant required for chemical treatment of the effluent and asecond advantage is the marked decrease in sulphate ion content. Although thiourea

Table 12.28 Analysis of exhaust dyebaths after dyeing of cotton fabric with sulphur dyesusing sodium sulphide or thiourea dioxide [213]

3% CI Sulphur Yellow 1 10% CI Sulphur Blue 11

Sodium Thiourea Sodium Thioureasulphide dioxide sulphide dioxide

Colour Yellow Yellow Blue BluepH 10.2 11.4 11.2 9.1Density at 20 °C (g/ml) 1.05 1.04 1.10 1.06Permanganate oxidation (mg/l O2) 937 433 1375 850Dichromate oxidation (mg/l O2) 1853 965 2769 1692Residue dried at 100 °C (mg/l) 7720 6416 13500 8255Residue baked at 600 °C (mg/l) 6612 4924 12495 7130Loss of mass at 600 °C (mg/l) 1108 1492 1005 1125Chloride (mg/l Cl–) Trace Trace Trace TraceSulphate (mg/l [SO4]2–) 3439 933 5813 1382Sulphide (mg/l [S]2–) 128 Nil 72 NilAlkalinity (ml/l) 43 80 70 77

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889

pH value

1110 12 13 14

–600

–800

–1000

Red

uctio

n po

tent

ial/m

V

Fe–GlucIndigo

AQ derivativesSulphur dyes

Fe–TEAVat dyes

Fe–TEA = Iron–triethanolamineFe–Gluc = Iron–gluconic acid

Figure 12.21 Relationships between reduction potential and pH for various redox systems and dyeclasses [214]

SULPHUR DYES

dioxide is more expensive, calculations of processing costs must take into consideration thecost of treating waste liquors compared with those containing sulphide.

Indirect electrolysis using regenerable redox systems, first proposed for use with vat dyes(section 12.9), can also be used with sulphur dyes [214]. Sulphur dyes develop a reductionpotential of around –600 mV, compared with –900 mV for typical vat dyes (Ag/AgCl, 3MKCl). This is an important reason why a greater variety of reducing agents has beenproposed for use with sulphur dyes compared with vat dyes. There are also wider possibilitiesfor using various redox systems appropriate to the types of sulphur dye. Figure 12.21 showsthe relationships between reduction potential and pH for vat, indigo and sulphur dyes,together with relevant redox systems. This indicates that anthraquinone derivatives andiron–gluconic acid complexes are suitable for the reduction of sulphur dyes by indirectelectrolysis. Table 12.29 shows the important reaction paths involved in these two systems.

Representative anthraquinone compounds include alizarin (1,2–dihydroxyanthra-quinone), sodium anthraquinone-2-sulphonate or disodium anthraquinone-1,5-disul-phonate. Table 12.30 shows typical results obtained with various sulphur dye types. Schemes12.10 and 12.13 represent generation of the reducing species at the cathode. Schemes 12.11and 12.14 indicate the mechanism of reduction of water-insoluble CI Sulphur dyes, whilstSchemes 12.12 and 12.15 show the release of the water-soluble leuco species from the CISolubilised Sulphur precursor. Schemes 12.16 and 12.17 represent undesirable problems ofprecipitation that can occur in some cases; these may be preventable by addition of asequestering agent. Excellent results, equivalent to those of conventional processes, wereobtained [214]. This process offers advantages in terms of savings of chemicals, regenerationof the reducing species in situ, no undesirable by-products from the reducing agent, as well aspossibilities for dyebath recovery and effluent recycling.

Secondary auxiliaries used with the reducing system during preparation of the leucodyebath may include wetting and sequestering agents; the water-insoluble CI Sulphur dyesmay also require dispersing agents, The choice of wetting agent is not particularly critical asthe behaviour of most sulphur dyes is unaffected apart from one or two that can show an

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SHR R S S R2oxidant

Scheme 12.18

adverse reaction (lower yield or even precipitation) with certain nonionic surfactants.Suitable sequestering agents include sodium hexametaphosphate and EDTA [30].Electrolytes are added to assist exhaustion in batchwise dyeing. After application of theleuco dyes a thorough rinse is essential to remove any loose dye from the fibre surface.

12.8.2 Oxidation

Chemically simple thiols and thiolates are readily oxidised to disulphides (Scheme 12.18).The situation is more complex with the leuco sulphur dyes [204]. In addition to having twoor more thiol groups, they may contain other reactive functions that are potentiallysusceptible to oxidative attack, examples being primary, secondary or tertiary amino groups,hydroxy groups and aryl thioether (R–S–R) groups, as well as sulphonic acid groups in somecases. Furthermore, the high substantivity of the leuco dye for the substrate may inhibit freemovement of the dye molecules and thus prevent complete conversion of thiol groups to

Table 12.29 Reaction paths involved in anthraquinonoid and iron-complex redox systems [214]

ANTHRAQUINONE SYSTEMS

Cathodic reduction of anthraquinone derivativeScheme 12.10 AQ + 2e– AQ2–

Reduction of water-insoluble CI Sulphur dyeScheme 12.11 R–S–S–R + AQ2– 2R–S– + AQ

Reduction of CI Solubilised Sulphur dyeScheme 12.12 2R–S–SO3

– + AQ2– 2R–S– + 2SO32– + AQ

IRON-COMPLEX SYSTEMS

Cathodic reduction of iron-complex systemScheme 12.13 Fe3+L + e– Fe2+L

Reduction of water-insoluble CI Sulphur dyeScheme 12.14 R–S–S–R + 2Fe2+L 2R–S– + 2Fe3+L

Reduction of CI Solubilised Sulphur dyeScheme 12.15 R–S–SO3

– + 2Fe2+L R–S– + SO32– + 2Fe3+L

Precipitation of iron sulphidesScheme 12.16 Fe2+L + 2Fe3+L + 4S2– FeS↓ + Fe2S3↓ + 3L

Precipitation of water-soluble leuco dyeScheme 12.17 Fe2+L + Fe3+L + 5R–S– (R–S)2Fe↓ + (R–S)3Fe↓ + 2L

R–S–S–R CI Sulphur dyeR–S–SO3

– CI Solubilised Sulphur DyeAQ Anthraquinone derivativeL Ligand (gluconic acid or triethanolamine)

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891

Table 12.30 Suitability of various sulphur dye types for application with anthra-quinonoid or iron-complex redox systems [214]

Fe-GLU Fe-GLUSulphur dye types AQ pH 12 pH 13 Fe-TEA

Pre-reduced dyesCI Leuco Sulphur Black 1 + o – +CI Leuco Sulphur Black 2 + o – +

Water-insoluble dyesCI Sulphur Blue 11 + + + oCI Sulphur Green 2 + + + o

Water-soluble dyesCI Solubilised Sulphur Blue 7 + + + oCI Solubilised Sulphur Blue 10 + + + oCI Solubilised Sulphur Green 3 + + + o

Sulphurised vat dyesHydron Blue 3R Stabilosol (DyStar) + + o oHydron Blue R + + – oHydron Black CL + + – oHydron Red GGF + + + +

+ suitableo limited suitability– precipitation at high concn.AQ anthraquinonoid compoundsFe-GLU iron-gluconic acid complexFe-TEA iron-triethanolamine complex

SULPHUR DYES

disulphide crosslinks; nevertheless, it is generally assumed that disulphide or polysulphidecrosslinks are formed.

From a purely technical viewpoint, ignoring environmental concerns, the most favouredand most widely effective oxidising system for sulphur dyes is sodium dichromate acidifiedwith acetic acid. The concentration of dichromate is not critical [30], 1–2 g/l beinggenerally recommended for batchwise processes carried out at 60–80 °C, but the pH is moreimportant and should be controlled to within the range 4.5–5.5, giving good colour yield andfastness properties more reliably than all other oxidising systems. Up to 1 g/l copper(II)sulphate may be added to dichromate baths to give an improvement in light fastness, at theexpense of some dulling of shade and a harsher handle. The copper salt should only beadded to acidified and sulphide-free liquors, otherwise it may precipitate as the hydroxideand/or sulphide. The addition of copper should not be made when oxidising sulphur blacks,however, since it promotes acid tendering with these dyes. The addition of sequestering anddispersing agents to the dichromate bath may give an improvement in fastness to rubbing[30].

Dichromate oxidation does tend to give a harsher handle and a less hydrophilic fibre,tending to cause handling problems in subsequent processes such as weaving, and istherefore less suitable for yarn dyeings. The technical merits of dichromate areovershadowed by ecological considerations, however, since chromium compounds in surfacewaters pose a direct threat to health and are increasingly restricted by water treatment

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authorities. Hence oxidation by dichromate is now only carried out in certain regions of thedeveloping world. Despite considerable efforts to develop alternatives, no system so fardevised matches the versatility, efficacy, economy and reliability of acidified dichromate[30,202,204,210]. Oxidising agents that are increasingly used include (a) hydrogen peroxideor other peroxy compounds, (b) iodates or bromates, (c) sodium chlorite, or (d) sodium N-chloro-p-toluenesulphonamide (12.49). It has been reported [207] that classes (b) to (d)account for 70% of the market. However, all of these produce organic halogen compounds,giving rise to high AOX values in the effluent.

Air oxidation is obviously the most economical and environmentally friendly method butit has serious limitations. Peroxide systems are likely to become the most important methodon both economic and ecological grounds, since peroxides do not generate AOX values. Infact peroxides are already established alternatives to dichromate, especially for yarn dyeing(giving a softer handle) and for sulphur blues and blacks. About 1–2 g/l hydrogen peroxide(130 vol.) is generally used at pH 10 and 40 °C [30,210]. Careful control is necessary, sinceunder alkaline conditions oxidation is rapid and may go beyond the disulphide stage. Inmildly acidic media oxidation is much slower but adequate under batchwise conditions formost dyes except CI Sulphur Red 10. It is too slow for rapid continuous methods and thereis a danger of catalytic degradation of the fibre. Nevertheless, 1 g/l hydrogen peroxide (130vol.) with 0.8 g/l acetic acid at 50–60 °C is widely used in yarn dyeing [30]. Oxidation underneutral conditions is occasionally carried out. Electronic metering and control is particularlyuseful with peroxide systems. The use of metal-ion catalysts with peroxide has been tried;vanadates offer some promise but iron and copper salts have not proved entirely suitable[210]. Peroxide oxidation has gained a reputation for giving somewhat lower fastness towashing than that obtained with dichromate. Although true in relation to traditional washtests based on soap, tests based on the perborate-containing detergents in general usenowadays do not show this problem [210].

A concentration of 0.2–1.5 g/l potassium or sodium iodate [30,210] at 60 °C and pH 3.5–5 (preferably 3.5–4.0) gives similar hues and fastness properties to those obtained withdichromate. Although expensive, the process shows good reproducibility and does not givethe harsh handle associated with dichromate. The pH, controlled using acetic acid, iscritical; oxidation is slower at values higher than pH 4.5 and stronger acids such as formicacid lead to precipitation of iodine which is corrosive to machinery and environmentallyharmful. Higher temperatures lead to partial reduction of the iodate with consequent loss ofefficacy. Under the same conditions bromates are less effective than iodates but performequally well if a catalyst such as sodium metavanadate (NaVO3) is added [30].Metavanadate, however, is increasingly under environmental scrutiny.

Sodium chlorite is the basis of several proprietary oxidising agents for sulphur dyes. It isused at pH 10 (sodium carbonate) and 90–95 °C; careful control of conditions is importantas the reaction is rather slow. The presence of additives is necessary for the successful use ofchlorite. The proprietary formulations contain stabilisers, EDTA-type sequestering agents

H3C SO2 N Cl

Na

12.49

+

_

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and detergents or dispersing agents. Other oxidising agents that have been proposedinclude:(1) Sodium m-nitrobenzenesulphonate in alkaline media(2) Sodium salt of N-chloro-p-toluenesulphonamide (12.49; chloramine T)(3) Sodium nitrite with sulphuric acid (this carries a danger of fibre degradation)(4) Potassium or ammonium salts of peroxydisulphonic acid with acetic acid.

12.8.3 Alkylation of thiol groups and other aftertreatments

As an alternative or supplement to oxidising treatments, many leuco sulphur dyeings can betreated with alkylating agents such as those based on epichlorohydrin. This was discussed insection 10.9.6. The alkylating agent reacts with the dye thiol to yield an alkyl thioether, orwith amino groups in the dye to yield substituted amines [204]. These are generally referredto as fixation treatments and give enhanced fastness to acid storage and to wet treatmentsalthough, strictly speaking, they are still oxidation mechanisms involving the removal of anelectron from the dye thiolate. Several commercial cationic fixing agents for direct dyeingscan be used to aftertreat sulphur dyeings, giving enhanced wet fastness; these treatments areclaimed to be effective if applied in place of oxidation [215–217]. A soaping treatment afteroxidation or other fixation treatment is recommended.


Continuous methods are far more important with sulphur dyes than is batchwise dyeing interms of production volumes. There are many methods, including pad–steam, pad–sky, pad–dry–chemical–pad–steam and pad–dry–develop; summary descriptions of these processes areavailable [30]. In general the auxiliaries used, and particularly the redox chemicals, are thesame as those used in batchwise dyeing although they are usually used in higherconcentrations, especially reducing agents because they are more exposed to air incontinuous processes. Wetting and sequestering agents are generally used. Some processesincorporate hydrotropes (such as urea) and migration inhibitors. Electrolytes may be used ina chemical (reducing) pad to assist fixation but are less frequently used when padding withleuco dyes, since they can promote tailing effects.

12.9 VAT DYES

Vat dyes, like sulphur dyes, are applied to cellulosic fibres after initial conversion by alkalinereduction to the substantive leuco form, followed by reoxidation to the insoluble form onthe fibre. Consequently the major auxiliaries are, once again, reducing and oxidising agents.Ancillary products include electrolyte, wetting, dispersing, levelling and sequestering agents;thickening agents and hydrotropes may feature in continuous dyeing and printing. Anextremely important part of vat dye application is the final soaping treatment, which isessential for developing the ultimate colour and optimal fastness. Detailed accounts of vatdye application are available [30,218]. Here the reducing and oxidising stages of the processand the soaping treatment are considered separately; the main discussion deals withbatchwise dyeing, after which the requirements for continuous dyeing and printing areconsidered.

VAT DYES

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12.9.1 Reduction

Vat dyes are available mainly in their water-insoluble pigment form, although a group ofsolubilised dyes, the vat leuco esters, are supplied in a stabilised pre-reduced form that doesnot require any reduction by the dyer. The chemistry of vat dyes, and of their reduction andoxidation, is much more clearly understood than is the case with sulphur dyes. In essence,the reduction process can be represented as the addition of an electron to each of the ketogroups of the vat dye, with conversion to a conjugated dihydric quinol, the system beingreadily reversible by oxidative abstraction of electrons (Scheme 12.19). Vat dyes generallyhave larger negative reduction potentials than sulphur dyes and so require more stronglyreducing conditions. In a detailed account of reduction and oxidation processes [219],various vat dyes are shown to have reduction potentials in aqueous media varying from –770to –1000 mV, the majority being in the region –900 to –950. In contrast, sulphur dyesaverage only about –600 mV. Published reduction potentials for various reducing agents inaqueous alkaline media are shown in Table 12.31.

NH

O

HN

O

NH

O

HN

O

O

O O

O

Indigo derivative Leuco indigo derivative

Anthraquinone derivative Leuco anthraquinone derivative

+ 2e

- 2e

+ 2e

- 2e

Scheme 12.19

Table 12.31 Reduction potentials of various reducingagents measured as 5 g/l solutions in 15 ml/l causticsoda 38°Be at 60 °C [220]

Reductionpotential (mV)

Thiourea dioxide –1100Sodium borohydride –1100Sodium dithionite –970Hydroxyacetone –810Sodium formaldehyde-sulphoxylate –790

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895

Alkali is an essential component of the reducing system in order to ensure that the substan-tive ionised leuco species is formed, since the acid leuco form is usually insoluble in water andhas limited substantivity for cellulose. To be successful a reducing agent should have a lowerreduction potential than the compound it is intended to reduce but in practice there are anumber of complicating factors. For example, the reduction rate of a vat dye (section 3.1.4) iscritically dependent on the physical form of the dye as characterised by its crystal form andparticle size distribution [221,222]. Table 12.32 indicates the magnitude of this effect forvarious samples of CI Vat Green 1. The stability of the leuco dye in solution is a function ofthe concentrations of dye and reducing agent, temperature, pH and liquor ratio. The influenceof oxidation in air as it relates to different application systems has to be carefully considered.Hence the quantities of reagents suggested by the dye manufacturers can only be used aspreliminary guidelines.

By far the most important reducing system for the batchwise application of vat dyes issodium dithionite (Na2S2O4) in a solution of sodium hydroxide. Obviously the theoretical con-centrations required will depend on the number of keto groups in the dye molecule and on itsrelative molecular mass and concentration, but the reaction can be represented as in Scheme12.20 for an anthraquinonoid dye with two keto groups. The effect of air oxidation on alkaline

Table 12.32 Effect of particle size on rate of reduction of CI Vat Green 1 at 20 °C [221]

Percentage of Time (min)dye particles >1.05 µm to reduce 50% of the dye

66 20642 12022 70 6 42 2 28 1 7

CI Vat Green 1 (23%) 100 mg/lSodium dithionite 5 g/lCaustic soda 38°Be 10 g/l

O

O

ONa

ONa

+ Na2S2O4 + 4 NaOH

+ 2 Na2SO3 + 2 H2O

Scheme 12.20

VAT DYES

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dithionite must also be taken into account in practical situations and this can be representedby Scheme 12.21 [30]. In practice, more sulphite than sulphate tends to be formed. Clearly,this atmospheric oxidation results not only in loss of reducing agent but also of alkali, anotherreason why excess alkali as well as reducing agent is required. The influence of atmosphericoxidation varies enormously. Stability may be maintained for several hours in a large dyeingvessel providing the surface area of the liquor is relatively small, compared with only 30–60seconds in a padding system.

The ideal pH value for this system is between 12 and 13; below pH 12 there is anincreasing danger that the dye will either revert to its keto form or yield the leuco acid,whereas above pH 13 there is a danger that the dithionite may decompose to formthiosulphate, sulphite or even sulphide if the temperature is high enough. Specifically in thecase of indigo, however, better reproducibility is achieved when buffered to pH 10.6–11.4[223], rather than the traditional application conditions of pH 12.1–12.9. Mention has beenmade above of the need to have both dithionite and alkali present to excess in most dyeingsystems. Unfortunately this has often led to the indiscriminate use of these chemicals,particularly of dithionite, resulting in high chemical costs and excessive contamination ofthe effluent. Providing suitable analytical monitoring is available, cost savings up to 40% areachievable if the quantitative amount of dithionite is present in the system [224].Automatic dosing systems can result in both cost savings and improved quality. This isexemplified in Figure 12.22, which indicates that the conventional process of topping up ateach end in jig dyeing results in a fluctuating dithionite concentration, whereas automaticdosing enables the required minimum concentration of dithionite to be constantlymaintained.

Na2S2O4 + 2 NaOH + O2 Na2SO3 + Na2SO4 + H2O

Scheme 12.21

1 2 3 4 Ends15 30 45 60 Time/min

4

2

10

6

8

Sod

ium

dith

ioni

te/g

l–1

1 2 3 4 Ends15 30 45 60 Time/min

4

2

10

6

8

Sod

ium

dith

ioni

te/g

l–1

Conventional method Automatic dosing method

Figure 12.22 Variations in the concentration of sodium dithionite in jig dyeing using conventional andautomatic dosing methods [225]

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Table 12.33 Classification of vat dyes and chemical requirements

Group Characteristics

IK Relatively low substantivity for cellulose. Dyed at ambient temperature with a smallamount of caustic soda and a high salt concentration.

IW Higher substantivity for cellulose. Dye at 45–50 °C with more caustic soda and less salt(none for mercerised cotton or regenerated cellulosic fibres).

IN Much higher substantivity for cellulose. Dyed at 60 °C with even more caustic soda butwithout salt.

IN Special High substantivity and the maximum alkali concentration.

Table 12.34 Guideline chemical concentrations for 2% o.w.f. vat dye at 10:1 liquorratio [226]

Temperature Caustic Sodium SodiumDye group (°C) soda (g/l) dithionite (g/l) sulphate (g/l)

IK 20–25 3.6 3.0 12.0IW 45–50 4.8 4.0 12.0IN 60 8.8 5.0 –

Electrolyte is sometimes used in the application of vat dyes from alkaline dithionitedyebaths. The quantities of chemicals required vary according to conditions and dyemanufacturers’ literature should be carefully consulted, supplemented by local knowledgebased on experience. Vat dyes are generally classified into several groups (Table 12.33).Guideline recommendations for IK, IW and IN dyes applied from a 10:1 liquor are given inTable 12.34. Another source [30] indicates typical amounts as varying from:

5 ml/l Caustic soda 27% (1.35 g/l NaOH) 2 g/l Sodium dithionite

for pale depths of 1K dyes at 20:1 to:25 ml/l Caustic soda 27% (6.75 g/l NaOH) 7 g/l Sodium dithionite

for full depths of IN dyes at 10:1

Higher dyeing temperatures (up to 115 °C) are selected in some cases. These conditionsrequire a more stable reducing agent of the hydroxyalkylsulphinate type, typical concen-trations being 10–20 ml/l caustic soda 27% and 5–10 g/l reducing agent according to dyeconcentration, liquor ratio and temperature. A reduction inhibitor, such as glucose, may beadded when applying dyes that are sensitive to over-reduction [30]. In the so-calledpigmentation process, the finely dispersed vat pigments are first applied at 60–80 °C, usuallywith addition of electrolyte, followed by reduction to the vat leuco form using dithionite andalkali.

The stability and efficacy of sodium dithionite can be enhanced by addition ofpolyacrylamide, a product more frequently used as a migration inhibitor in continuousdyeing processes [227]. This biodegradable polymer allows the amount of dithionite to be

VAT DYES

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decreased, giving improved fixation with less contamination of effluent by dye anddithionite. In jig dyeing polyacrylamide helps to prevent oxidation spots and minimises end-to-end and side-to-side variations.

There are potential environmental problems associated with sodium dithionite, since ineffluent it can produce sulphite and sulphate. Although the sulphite can be oxidised quiteeasily to sulphate, this does not entirely obviate the problems since high concentrations ofsulphate can cause damage to unprotected concrete pipes. Thus there are environmentalreasons for alternative reducing agents to dithionite and several are available, althoughmainly used for special purposes. In particular, their greater stability to atmosphericoxidation makes them of special interest for continuous dyeing and printing processes,rather than for batchwise dyeing.

Thiourea dioxide (12.50) or formamidinesulphinic acid is a powerful reducing agent forvat dyes. This compound has a lower relative molecular mass than sodium dithionite andgives lower concentrations of sulphite and sulphate in the effluent, but shows certaindisadvantages. The mode of action in hot aqueous alkali is represented in Scheme 12.22,showing first the rearrangement to sodium formamidinesulphinate (12.51), which thendecomposes to yield urea and the active reducing species, sodium hydrogen sulphoxylate(12.52). Although thiourea dioxide is more stable than dithionic acid, the formamidine-sulphinate formed in alkaline media is more readily oxidised than dithionite, thus negatingany potential advantages. This agent can cause over-reduction of indanthrone vat dyes,against which inhibitors such as glucose or sodium nitrite have no palliative effect.Experimental work has indicated the possibility of using thiourea dioxide in combinationwith other compounds such as (a) sodium dithionite, formaldehyde and sodium hydroxide,or (b) saturated aliphatic ketones, but commercial exploitation has not been evident [218].

Greater commercial significance is attached to certain derivatives of sodium dithionite,including sodium formaldehyde-sulphoxylate (12.53; hydroxymethanesulphinate) (Scheme12.23) and the less important sodium acetaldehyde-sulphoxylate (12.54; hydroxyethane-sulphinate). These reducing agents have been of particular interest in printing, especially inthe flash-ageing process [228–231]. The formation of sodium formaldehyde-sulphoxylate byreaction of sodium dithionite with formaldehyde is shown in Scheme 12.23; the bisulphiteformed can be further reduced with zinc to produce another molecule of the sulphoxylate.

SC

H2N

H2N

O

O

SC

HN

H2N

O

ONa

OC

H2N

H2N

HO S ONa

12.50 12.51 12.52

NaOH H2O+

Scheme 12.22

NaO S S ONa

O O

OC

H

H

HOCH2 O S ONa

HOCH2 O S ONa

O

+ 2 + H2O +

12.53Scheme 12.23

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These reducing agents are much more stable than sodium dithionite at lower temperatures;hence they can be used to prepare stable pad liquors and print pastes. At highertemperatures, as in steam fixation treatments, they are capable of bringing about rapidreduction of vat dyes. Sodium formaldehyde-sulphoxylate was used first in conventionalsteam fixation of vat prints, although the acetaldehyde analogue was initially preferred forthe flash-ageing process. As vat dyes are invariably fixed under alkaline conditions, thesodium salts of the sulphoxylates are preferred to the basic salts of zinc (12.55) or calcium(12.56), which are unstable under alkaline conditions.

Sodium formaldehyde-sulphoxylate has been used occasionally in combination withsodium dithionite [218) but other two-component or two-phase systems based onformaldehyde-sulphoxylates have generally depended on an accelerating or catalyst system.For example, a process that has been adopted to some extent in bulk practice [232,233]comprises a strongly alkaline solution of sodium borohydride (12.57; sodium tetrahydro-borate), together with a second reducing system consisting of sodium formaldehyde-sulphoxylate and the catalyst sodium nickel cyanide. Various advantages have been claimedfor this process, although there are misgivings regarding the environmental acceptability ofsodium nickel cyanide [234]. Other accelerators used with sodium formaldehyde-sulphoxylate include sodium dimethylglyoxime complexes, anthraquinone andaminoanthraquinonesulphonic acids. Although sodium borohydride is itself a reducingagent, it generally reacts too slowly alone for use in vat systems; nor is there any evidencethat it will act as a stabiliser for sodium dithionite [235], as has sometimes been suggested.Another reducing agent, suggested [236] for both flash-age printing and batchwise high-temperature package dyeing, is trisodium nitrilotriethanesulphinate (12.58). This does notappear to have attained commercial use, however.

HO CH O

CH3

S ONa

12.54

HOCH2 O S OCa(OH)

12.56

NaBH412.57 CH2CH2N O S ONa

CH2CH2

CH2CH2

O

O

S

S

ONa

ONa

12.58

VAT DYES

HOCH2 O S OZn(OH)

12.55

Hydroxyacetone (12.48), mentioned in section 12.8.1 in connection with sulphur dyes, issulphur-free and biodegradable. This compound was originally proposed for use with vatdyes and continues to generate some interest. This agent can be used for the pad–steamapplication of vat dyes in the presence of high concentrations of sodium hydroxide (about3.5–4.5 g/l). Hydroxyacetone does not cause over-reduction of indanthrone vat dyes butdoes give different shades with carbazole dyes, compared with sodium dithionite [218].

The optimised and metered use of this product is central to the RD (Refine Dyeing) processof Ipose GmbH [212]. This process, for which both environmental and economic advantagesare claimed, is suitable for applying indigo on continuous yarn-dyeing ranges and for knitgoods

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dyeing with vat dyes that are difficult to reduce. Although colour yields are not quite as high aswith dithionite, the advantages include biodegradability, decreased chemical usage, lowerCOD values and the effluent contains no sulphide, sulphite or sulphate. This process dependson optimal machinery technology coupled with optimised computer-controlled chemicaldosing. Specific advantages in continuous yarn dyeing with indigo include [212]:(1) high quality with a good ring-dyeing effect, a prime requirement in indigo warp dyeing

for denim jeans(2) greater elasticity of yarn with fewer yarn breaks and therefore higher productivity(3) higher dye exhaustion and thus less residual dye in the effluent(4) lower chemical usage and thus 20% less waste liquor(5) liquid form of indigo for ease of application(6) minimal effluent problems as only biocompatible chemicals are used, with possibilities

for recycling of liquors and recovery of dye through ultrafiltration(7) no health hazard nor odour of hydrogen sulphide.

Typical waste water analyses and relative costings for sodium dithionite and hydroxyacetoneare given in Table 12.35.

Table 12.35 Continuous dyeing of cotton yarn with indigo using sodiumdithionite or hydroxyacetone [212]

Waste water analysis Sodium dithionite Hydroxyacetone

Dye (g/l) 1.5 0Sulphide (mg/l) 12.5 1Sulphite (mg/l) 64 1Sulphate (g/l) 19 0.2Phosphate (mg/l) 60 6*COD (mg/l) 7000–8000 700–1000Surfactant (mg/l) 18–19 1.5Sequestering agent (mg/l) 6 0

Dye and chemical cost 100% 77%Productivity 100% 140%

* COD values for hydroxyacetone are after treatment in a bioreactor; such treatmentis not possible with dithionite.

Tests using indigo have shown that reduction with alkaline hydroxyacetone is mostreproducible when the concentrations are as follows [237]:

Hydroxyacetone 5.0–12.5 ml/l or 0.054–0.135 mol/lSodium hydroxide 5.0–12.5 g/l or 0.125–0.312 mol/l

It is interesting to note that the rate of vatting of indigo with hydroxyacetone under theseconditions can be increased approximately fourfold using ultrasound, as illustrated in Figure12.23. The ultrasound causes cavitation in the indigo pigment dispersion, accelerating thedisintegration of the insoluble dye particles and thus increasing the probability of collisionsbetween reducing agent and dye molecules.

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Iron complexes have been investigated as alternatives to sodium dithionite in vat dyeing[238]. Iron(II) hydroxide Fe(OH)2 is a powerful reducing agent and this reducing powerincreases with increasing alkalinity of the medium. Alkalinity results in precipitation,however, and the hydroxide must be converted to a complex in order to maintain solution.The complex must be chosen to be reasonably stable, but not so inert that the iron(II) ioncannot exert its reducing action. Complexes with triethanolamine or gluconic acid aresuitable, the latter being favoured on environmental grounds because it does not containnitrogen. Investigation at a liquor ratio of 10:1 showed 50 ml/l caustic soda 38°Bé to beoptimum, lower concentrations of alkali producing duller and weaker dyeings.

The molar ratio of iron(II) ion to gluconic acid in the complex prepared by reactingiron(II) chloride tetrahydrate with gluconic acid was significant. Less reducing agent wasrequired for vat dye reduction at a molar ratio of 1:2 than at 1:1 but too low a concentrationof iron(II) ions resulted in paler and unlevel dyeings because of insufficient vatting of thedye pigment. It is environmentally advantageous to keep the gluconic acid concentrationlow to ensure a low COD value. A 1:1 molar ratio is probably the optimum. The formationof iron(III) ions lowers colour yield but levelness is not affected because the gluconic acidholds the iron(III) ions in solution. The use of iron(II)-gluconic acid did not cause over-reduction of sensitive dyes even when dyeing was prolonged. With most dyes the colouryield was equal to that given by sodium dithionite. Precipitation of iron(III) hydroxidefacilitates the elimination of dyes and auxiliaries from effluent after only a brief settlementtime.

The application of indirect electrolysis in conjunction with regenerable redox systems,already described in section 12.8.1 for sulphur dyeing, has attracted considerable researchinterest for the reduction of vat dyes [239–241]. In environmental terms, the advantages ofthis system for vat dyeing would be similar to those for sulphur dyeing. Figure 12.21 andTable 12.29 are essential to an understanding of the physico-chemical aspects of the process.The reduction potentials attainable with anthraquinone derivatives as mediators of theredox system are shown in Table 12.36. In general, these potentials are too low for vat dyes,as can be seen by reference to Figure 12.21.

Reaction time/s

500 1000 20001500 3000

0.5

0.25

0.75

1.0

Abs

orba

nce

at 7

10nm

A B

AB

With ultrasound

Without ultrasound

Figure 12.23 Rate of reduction of indigo with and without ultrasound [237]. 0.1 g/l Indigo, 40 °C; 2.5ml/l hydroxyacetone; 5.0 g/l sodium hydroxide, pH 12.7; 0.03 g/l anionic dispersing agent

VAT DYES

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Currently the most suitable system, that will generate potentials up to –1050 mV, is theiron-triethanolamine complex prepared from either iron(II) or iron(III) salts. Using iron(III)sulphate penta- or hexahydrate, for example, dyebaths are prepared by first dissolvingsodium hydroxide in a small amount of water, to which is added the triethanolamine.Hydrated iron(III) sulphate is separately dissolved in a small amount of water and thenadded to the alkaline triethanolamine solution until the initially precipitated iron(III)hydroxide redissolves, after which the solution is diluted to full volume to give:

g/l mol/l

Sodium hydroxide 20 0.5Triethanolamine 60 0.45Hydrated iron(III) sulphate 14 0.027

A schematic illustration of this process is shown in Figure 12.24.

Table 12.36 Peak cathodic potentials of anthraquinone derivativesmeasured using Ag/AgCl/3M KCl in 4 g/l sodium hydroxide solution [239]

CathodicAnthraquinone derivative potential (mV)

1,2,5,8-Tetrahydroxy –8851,2-Dihydroxy (alizarin) –8801,8-Dihydroxy (chrysazin) –7701,4-Dihydroxy (quinizarin) –7601,2-Dihydroxy-3-sulphonic acid –7602-Sulphonic acid (β acid) –7501,5-Disulphonic acid –7501-Amino-2-carboxylic acid –7501-Amino-2-sulphonic acid –7202,6-Disulphonic acid –560

Reducedmediator

Oxidisedmediator

Oxidiseddye (insoluble)

Reduceddye (soluble)

Heterogeneous redox reaction

Cat

hode

Figure 12.24 Schematic diagram of the cathodic reduction of the mediator that converts the insolublevat dye into its soluble leuco form [239]

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The essence of the technique is as follows [239]. The cathode reduces the mediatorwhich then reduces the dye. This mediator (the regenerable redox system) must continuallyproduce a consistent reduction potential in the dye liquor, so that no reducing agent has tobe added. The prevailing potential is defined by the Nernst Equation (12.2):

Redox pair: Oxi e Rednn � �� (12.1)

� � ��RT [Oxi]

E E lnF [Red]n (12.2)

E° = standard potential (mV) of the redox pair (under the experimental conditions)E = potential (mV) prevailing in the solutionR = molar gas constant (8.314 J/°K.mol)T = absolute temperature (°K)F = Faraday constant (96 500 coulombs)[Oxi] = concentration (mol/l) of the oxidised form of the redox pair[Red] = concentration (mol/l) of the reduced form of the redox pairn = electrochemical valency.

If the concentrations [Oxi] and [Red] are identical, the potential prevailing in solution isidentical to the standard potential E° of the redox pair under the conditions of use. If theratio of these concentrations [Oxi]/[Red] is modified by electrolysis, the potential changesaccordingly:if [Oxi] > [Red], E becomes positive and the action is one of oxidationif [Oxi] < [Red], E becomes negative and the action is one of reduction

With conventional reducing agents the ratio [Oxi]/[Red] is determined by the decreasingconcentration of reducing agent and the increasing concentration of oxidation productsresulting from its decomposition. In an electrolytic process, however, this ratio can be adjustedso as to continually regenerate the redox pair and no secondary products are formed in the dyeliquor, nor is there a build-up of unwanted oxidation products. After dyeing, the residual leucodye can be reoxidised in the exhaust dyebath and removed by filtration; thus the dye liquortogether with the mediator can be used several times, creating considerable ecologicaladvantages. The requirements of a mediator or redox catalyst system are:(1) only a small loss (if any) in activity during useful life; therefore the maximum possible

number of reaction cycles(2) rapid conversion at the electrode surface(3) no catalysis of side reactions or changes in colour(4) no affinity for the fibre(5) no reaction with the solvent(6) adequate solubility(7) non-toxic(8) no problems in effluent treatment(9) inexpensive.

VAT DYES

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A more detailed overview of the process is shown in Figure 12.25, highlighting the mainsteps of a vat dyeing process. The quotient of a pair of k values (rate constants) for areaction in the forward and reverse directions is a measure of the equilibrium constant ofthe reaction step. The reduction of the suspended oxidised form of the dye is characterisedby the pair of rate constants, k1 for reduction and k2 for reoxidation. For effective vatting ofthe dye to be achieved, k1 should be greater than k2. A measure of the affinity of the reduceddye is given by the quotient of k3 and k4; a high value of k3/k4 is characteristic of dyes withgood affinity for cellulosic fibres in their reduced form, leading to a high degree of dyeexhaustion. This can cause problems in the dyehouse resulting from rapid exhaustion of thedyebath. The quotient k8/k7 describes the stabilisation of the reduced form of the dyeadsorbed on the fibre. A high k8/k7 quotient indicates that the adsorbed leuco dye remainsstable in its reduced form, an important precondition in achieving equilibrium duringexhaustion of the dyebath. When the quotients k1/k2 or k8/k7 are small, the dyeing process isdisrupted by all oxidising agents, including air, leading to unlevelness in the dyeing. It isevident that the rates of dye reduction (k1 and k8) are of great importance in ensuringreproducibility of dyeing. The rate of reduction achieved by the iron(II)-triethanolaminecomplex is several orders of magnitude greater than that given by sodium dithionite;therefore the reduced leuco dye is effectively stabilised and the resulting dyeings are morereproducible in the case of electrochemical reduction.

Oxidationby air

Spontaneousdecomposition

Reducing agent

Reducing agent

Oxidation by air

Oxidation by air

Exhaustion Desorption

Cellulose fibre

Dyebath

Oxidised dyein dyebath

Reduced dyein solution

Reduced dyein the fibre

Oxidised dyein the fibre

Favourable: k1 > k2

k8 > k7 k8 < k7

k3 > k4k1 < k2Disadvantageous: High affinity:

k5

k1 k2

k3 k4

k8 k7

k6

Figure 12.25 Reaction scheme of the steps occurring in a vat dye reduction process [240]

The special requirements of the indigo dyeing of cotton warp yarns for denim are capable ofbeing met by indirect electrolysis systems [241]. Examples of four suitable redox systems areshown in Table 12.37. Uniform build-up of depth was observed with each successive step, theresults being at least equal to those from the conventional dithionite-based process.Apparently these processes are amenable to scaling up to bulk production levels [241].

In spite of the many reducing systems evaluated or proposed (often quite convincingly),sodium dithionite remains the agent almost universally preferred [242]; others such asthiourea dioxide, sulphinates and hydroxyacetone are only used for special purposes. Theadvantages and disadvantages of the most important reducing agents are summarised in Table12.38.

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Table 12.37 Redox recipes used in indirect electrolysis application of indigo tocotton yarn [241]

Recipe Components Concentrations

1 Iron(III) chloride 0.18 mol/lIron(II) chloride 0.06 mol/lGluconic acid 0.30 mol/lSodium heptagluconate 0.12 mol/lCalcium carbonate 0.09 mol/lCalcium chloride 0.09 mol/lSodium hydroxide to pH 11.5

2 Iron(III) sulphate hexahydrate 0.052 mol/lIron(II) sulphate heptahydrate 0.060 mol/lCalcium heptagluconate 0.129 mol/lAnionic dispersing agent 0.5 g/lSodium hydroxide 17.1 g/l pH 11.5

3 Iron(III) sulphate hexahydrate 61.3 g/lTriethanolamine 227 g/lSodium gluconate 2 g/lSodium hydroxide 69 g/l pH 13.5

4 Iron(III) chloride hexahydrate 54.0 g/lSodium gluconate 54.5 g/lCalcium chloride 11.1 g/lCalcium heptagluconate 24.5 g/lSodium hydroxide 20.0 g/l pH 11.4

It is now opportune to consider additives for reduction systems. As mentioned above, thereduction process is invariably carried out in an alkaline medium, the most common alkalibeing sodium hydroxide particularly in batchwise dyeing. In printing and certain continuousdyeing processes sodium hydroxide enhances reduction potential but impairs the stability ofthe reducing agent, increasing the danger of premature loss of reducing action, particularlyduring the drying operation prior to steaming. In these processes carbonates give greaterstability and hence are preferred [243]. There is little difference between sodium andpotassium carbonates in terms of effect on the reducing agent, but there are advantages tobe gained from using potassium carbonate to prepare relatively concentrated print pastesand pad liquors. Not only does potassium carbonate have a higher aqueous solubility at 20°C (112% w/w) than sodium carbonate (21%), but the potassium leuco salts of vat dyes arealso more soluble [29]. It is important to ensure the presence of excess alkali to counteractthat consumed by the reducing agent and the substrate, as well as any reducing agent lostthrough atmospheric oxidation of the system [30].

Electrolyte may be used to enhance exhaustion of the leuco dye, particularly in batchwisedyeing. The use of electrolyte in the essentially ‘short-liquor’ printing and continuous dyeingprocesses is seldom necessary and could be inadvisable, as it may promote tailing duringpadding. However, electrolyte plays a positive part in those continuous processes in which adispersion of the pigment form of the dye is applied first followed by separate application ofreducing agent in the so-called chemical pad. The amounts of alkali and electrolyte

VAT DYES

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obviously vary widely according to dyeing parameters (such as the liquor ratio) and the dyesused. A fully optimised computer-based program for recipe-specific calculation of quantitiesof sodium dithionite, sodium hydroxide and electrolyte has been published [244]. Theadvantage of such a program is that it enables exact quantities to be calculated, thus givingcost savings in chemicals and effluent treatment.

Levelling agents are frequently used, since the initial strike by leuco vat dyes can be rapid.A nonionic surfactant such as cetyl poly(oxyethylene) alcohol of optimum chain length(such as structure 12.25) is particularly useful because it can also act as a wetting agent.Poly(ethylene imine) derivatives of fatty alcohols behave in a similar way. On the otherhand, certain products that act through a complexing mechanism are not surface-active; forexample, one such proprietary product is based on poly(vinylpyrrolidone). Wetting anddispersing agents are also useful, particularly with inadequately prepared substrates.

Table 12.38 Summary of advantages and disadvantages of the three most important types ofreducing agent for indigo, vat or sulphur dyeing [239]

Sodium dithionite Advantages

(1) sufficient reduction potential for vat, sulphur or indigo dyeing(2) good stability of leuco vat dyebaths

Disadvantages

(1) waste water loading; inhibits biological degradation and leads to agreater oxygen demand

(2) can cause over-reduction at higher temperatures(3) special safe storage facility required

Sulphinic acid derivatives Advantages

(1) sufficient reduction potential for vat, sulphur or indigo dyeing(2) especially suitable for high-temperature dyeing methods(3) good resistance to oxidation by air

Disadvantages

(1) as for dithionite disadvantage (1)(2) potentially temperature-dependent

Hydroxyacetone Advantages

(1) biologically degradable(2) ease of dosing in liquid form(3) very good stability to storage

Disadvantages

(1) does not reach full reduction potential and thus mainly suitable forindigo and sulphur dyes

(2) persistent odour(3) limited commercial production

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Aliphatic sulphonates are widely used as wetting agents, particularly in combination withnon-surfactant levelling agents. Any surfactants used must be stable under the alkalinereducing conditions. For continuous pad application they should also have a low propensityto foam; phosphoric esters are preferred to aliphatic sulphonates in this respect and inregard to the amount of pad liquor absorbed [221]. Dispersing agents, sometimes referred toas protective colloids, help to maintain the particulate distribution of the vat dye in pigmentform and also inhibit aggregation of sparingly soluble leuco compounds; naphthalene-sulphonic acid/formaldehyde condensates are especially useful. They are also essentialadditives in the application of vat dyes by the so-called pigmentation methods, using eitherthe unreduced vat dyes themselves or, less frequently, acid leuco compounds [245].

A sequestering agent such as EDTA will prevent the formation of insoluble metal salts ofvat leuco compounds. Trace metal ions would otherwise interfere with the applicationprocess and with level dyeing. It has been reported [246] that sequestering agents of thealkylphosphonate type can retard the uptake of vat dyes. An addition of pyrocatechol ortannic acid can help to counteract the well-known propensity of certain yellow and orangevat dyes to induce fibre tendering, whilst the addition of glucose or sodium nitrite can helpto prevent the over-reduction of certain dyes.

Rinsing after exhaustion of the leuco vat dye has a marked influence on the final result,especially as regards the final hue, depth of shade and fastness properties. Figure 12.26illustrates the effect of rinsing temperature and pH on the colour yield of dyeings ofpyranthrone (12.59; CI Vat Orange 9). In general, yield decreases significantly withincreasing temperature and to some extent with increasing pH, particularly at 70 °C.Dyeings tend to be duller after rinsing at a high temperature and alkaline pH, although thiseffect varies from dye to dye. Overall, rinsing should be carried out thoroughly at a lowtemperature and at pH 7 [247]. Rinsing should be neither too brief (inefficient) nor tooprolonged (wasteful of water and processing time). A study of aftertreatments in jet andoverflow machines [248] has shown advantages from optimising the rinsing process inrelation to the concentrations of residual dithionite at the end of the dyeing stage and afterrinsing. The dithionite is measured by titration, either manually by iodometry orautomatically in an atmosphere of nitrogen according to the hexacyanoferrate method. Anominal target value (between 2 and 4 g/l) is set for the dithionite concentration at the endof dyeing, depending on dyeing machine type, fabric quality and dyes present. Starting fromthis value, a defined amount of rinsing water is used to dilute the dyebath in a given time,aiming for a final concentration of 0.5 g/l after rinsing. This procedure implies that completeremoval of dithionite is unnecessary.

12.9.2 Oxidation

Various methods have been used for the reoxidation of vat leuco dyeings; atmosphericskying, hypochlorite, chlorite and acidified dichromate are now rarely employed.Atmospheric oxidation can be difficult to control and thus uneven; with some dyes it is alsotoo slow, particularly for continuous methods. Sodium hypochlorite is used only for thosefew black dyes that tend to become dark green when oxidised with peroxide; obviouslyhypochlorite should be avoided with the many chlorine-sensitive dyes. Similarly sodiumchlorite, acidified to below pH 5 with acetic acid, can only be used with certain dyes,although with these it certainly gives rapid oxidation. Dye selectivity is also a drawback with

VAT DYES

chpt12(2).pmd 15/11/02, 15:47907


acidified dichromate, since not surprisingly its chelating potential can give rise to huevariation, especially with sensitive blues and greens. The AOX-generating potential of thechlorine-containing oxidants and the toxicity of chromium also make these compoundsenvironmentally undesirable. The most commonly used oxidising agents are hydrogenperoxide, sodium perborate and sodium m-nitrobenzenesulphonate [30]. Sodium nitriteacidified with sulphuric acid is used to hydrolyse and reoxidise the solubilised vat leuco esterdyes.

Hydrogen peroxide and sodium perborate remain the mainstay of the vat dye reoxidationprocess; the amounts to be used vary widely according to the conditions of processing.Consequently, dye manufacturers’ recommendations can only be used as guidelines and mayneed modification according to the machinery and conditions used. Many important factorshave to be considered, especially the liquor ratio, the contact frequency between liquor andgoods, the particular dyes and the depth of shade, as well as the efficiency with which thereduction liquor has been rinsed out. For example, in a long-liquor (20:1 to 30:1) winchdyeing process as little as 0.5–1.0 g/l hydrogen peroxide (130 vol.) or sodium perborate at 50°C is usually adequate; this may be increased to 3–5 g/l at 50–60 °C in short-liquor jigdyeing. The rate of oxidation can be controlled by addition of acetic acid to neutralise excess

pH of rinsing treatment

4 5 6 7 8 9

8

9

10

11

12

13

Col

our

yiel

d (K

/S)

30 oC

50 oC

70 oC

Figure 12.26 Effect of temperature and pH of rinsing on the colour yield of dyeings of CI Vat Orange 9[247]

O

O12.59

Pyranthrone

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909

alkali carried over from reduction but care should be taken not to over-acidify as this wouldtend to convert the sodium leuco compound into the less soluble acid leuco form. Someblues are exceptionally sensitive to alkaline oxidation, however, and must therefore be rinsedthoroughly before oxidation in the presence of a small amount of acetic acid. Over-oxidationcan often be corrected by again reducing the dyeing and then reoxidising under morecarefully controlled conditions.

Sodium m-nitrobenzenesulphonate has been proposed as an oxidising agent for vat dyes.It is available as a proprietary product and is claimed to react with leuco compounds morequickly than does peroxide. The solubilised vat leuco esters are most commonly hydrolysedand reoxidised to the insoluble parent dye using sodium nitrite and sulphuric acid.Alternative oxidising agents for vat leuco esters include hydrogen peroxide and ammoniummetavanadate (NH4VO3), persulphates and nitric acid [218].

12.9.3 Soaping

The final soaping at the boil of reoxidised vat dyeings must be regarded as an integral part ofthe application sequence, rather than an optional extra, since it determines thereproducibility of colour and fastness. The traditional process was to use a boiling bathcontaining 3–5 g/l Marseilles (olive oil) soap and 2 g/l sodium carbonate, usually for 10–15minutes in a batchwise process. Soaping times of less than a minute are the norm incontinuous pad–steam ranges, however, and it is then preferable to select dyes that showonly a slight change in colour during soaping [30,245]. Synthetic surfactants, particularly ofthe nonionic type, are now used instead of soap, although they contribute to an increasedeffluent load and should be used sparingly [247]. Boiling off in water alone is preferable asthe dyeings obtained are similar and in no case do they exhibit fastness ratings inferior tothose from washing with a detergent. This is clearly an important finding in view of the factthat vat dyeing effluent is already heavily contaminated [247].

Various mechanisms have been proposed to explain what happens during soaping.Investigations [249,250] suggest that dye molecules at first aligned along the fibre axis areconverted by soaping to crystalline aggregates reoriented at right angles to the fibre axis.Evidence exists, however, to show that some dyes are present as amorphous aggregates bothbefore and after soaping, whereas reoxidised indigo particles are crystalline even beforesoaping. Another possibility is that an initially metastable crystal modification of the dye isconverted by soaping into a more stable crystalline form.

12.9.4 Continuous dyeing and printing

In general, the reducing, oxidising and soaping agents for continuous dyeing are the same asthose used in batchwise dyeing, although they have to be used at higher concentrations. Insome cases there may be a preference for more stable reducing agents than dithionite, suchas sulphoxylates; in continuous dyeing dithionite is still the most widely applicable product[221], although sulphoxylates are undoubtedly preferred in printing. In continuous pigmentpadding a migration inhibitor is essential to prevent tailing, twosidedness or generalunlevelness, as well as to stabilise the pad liquor against pigment sedimentation [221]. It isimportant to select agents that promote the tendency of dye particles to agglomerate, as thisis the mechanism whereby migration is inhibited. Poly(acrylic acid) derivatives, alginates,

VAT DYES

chpt12(2).pmd 15/11/02, 15:47909


branched polysaccharides and block copolymers of ethylene oxide and propylene oxide haveproved useful. Conversely, some printing thickeners such as locust bean gum, starch ethers,guar derivatives and carboxymethylcellulose have been less successful in preventingparticulate migration in continuous dyeing, thus indicating that a mere increase in viscosityis ineffective in controlling migration [221].

In direct printing by an all-in, single-stage process the most favoured reducing system issodium formaldehyde-sulphoxylate with potassium carbonate as alkali. A thickening agent isrequired; British gum thickening may accelerate decomposition of the reducing agent butotherwise the choice of thickening agent is not critical. A hydrotrope helps to improvefixation in steam. A typical print paste recipe is given in Table 12.39. A reductionaccelerator such as an aminoanthraquinonesulphonic acid may be added. The flash-ageingmethod is a two-stage process in which the initial print paste contains dye and thickeneronly [251]. The reducing agent and alkali are subsequently applied by padding. In thisprocess exposure to air prior to steaming is minimal and the less stable sodium dithionite canbe used as the reducing agent. The thickening agent should coagulate in the presence ofalkali in order to inhibit bleeding of colour; alternatively a gelling effect can be obtained byadding a borate to a thickener such as locust bean gum, as described in section 10.8.1.Mixtures of thickeners are often formulated to attain the required degree of coagulation; ifthis is exceeded there can be problems at the washing-off stage.

Table 12.39 Typical print paste recipe for a single-stage processwith vat dyes

Amount (% w/w)

Vat dye xPotassium carbonate 15Sodium formaldehyde-sulphoxylate 8Glycerol (hydrotrope) 5Thickening agent 24Water to 100

The solubilised vat leuco esters are particularly suitable for printing. There are two mainmethods of hydrolysis and reoxidation. In the first sodium nitrite is incorporated in the printpaste and this is converted to nitrous acid by a short immersion of the printed goods indilute sulphuric acid after drying and steaming. Sodium chlorate is used in the secondmethod by incorporation into the print paste together with ammonia (to maintain alkalinityin the paste) and a steam-activated acid generator such as ammonium thiocyanate(NH4SCN); an oxidation catalyst, ammonium metavanadate (NH4VO3), is also used.

Vat dyes can be used as illuminant colours in discharge styles where the dischargedgrounds may be azoic, direct or reactive dyeings. In such cases the quantity of reducingagent has to be increased since it is needed for discharging the other dyes as well as forreducing (but not discharging) the vat dyes. A further agent, known as a leucotrope, mayalso be needed if white discharges are required; leucotropes act as discharge (or stripping)promoters and are described below.

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911

12.9.5 Correction of faults

Levelling and partial stripping may be carried out in a reducing bath using sodium dithioniteand sodium hydroxide in combination with a suitable levelling agent such as those describedearlier. Greater quantities of levelling agent can be employed to increase the stripping effect.The complexing agents are more effective than nonionic levelling agents. Certainquaternary ammonium compounds (such as structures 12.60 and 12.61) promote strippingby complexing with the leuco form of the dye. These products, often referred to asleucotropes, are used in conjunction with sodium dithionite and sodium hydroxide to givean effective stripping action.

CH3

N CH2H3C SO3

SO3

Ca2+

Cl

12.60

_

+ _

_H3C N (CH2)15CH3

CH3

CH3

Br

12.61

_

+

Poly(vinylpyrrolidone)(12.62) has an Mr range of 1–9 × 104 and a DP of 100–800. Themacromolecular chains are considerably folded in aqueous solution. The pyrrolidone ringsare oriented at right angles to the carbon chain, giving a hydrophilic series of keto groups onone side and hydrophobic propylene segments on the other [252]. This polymer readilyforms complexes with vat leuco anions in alkaline dithionite solution to give a highlyeffective stripping action.

12.9.6 Ecological considerations

Several ecological points have been mentioned above; an overall summary [220,253] will beuseful here. Certain factors in dealing with ecological problems in dyehouses are common toall textile dyes, not just vat dyes. The primary approach, of course, is precise optimisation ofall parameters so that the minimal quantity of all additives is used, thus ensuring minimumloading of the waste water. Dyeing at as low a liquor ratio as possible saves water and energy,as well as giving a lower volume of effluent. Shorter liquors give more concentrated wastewater but this need not be a disadvantage, since it may facilitate subsequent treatmentsincluding precipitation, flocculation, filtration or centrifugation. The second factor is tochoose products, both colorants and auxiliaries, that are the most environmentallyfavourable on the market; all reputable manufacturers and suppliers are working diligently inthis direction. Indeed, the chemical industry and the textile wet processing sector haveshown a responsible and increasingly effective approach to the varied and often difficultenvironmental problems that have arisen.

As already noted, vat dyeings generally give more heavily loaded waste waters than director reactive dyeings but these loads are reasonably treatable. A specific advantage of vat

CH2 CH CH2 CH

N

NH2C

H2C CH2

C

C

CH2H2C

H2C

O

O

12.62

n

VAT DYES

chpt12(2).pmd 15/11/02, 15:47911


Table 12.40 An environmental assessment of commercial brands of vat dyes [253]

Eliminability of organic halogen compounds Completely eliminated by adsorption onto sewagesludge; no organic halogen compounds in theoutlet of the treatment plant

Ecotoxicity Insoluble in water, therefore not toxic to bacteria,algae or fish; not bioavailable, so do notaccumulate in living organisms

Heavy metal impurities Range-specific differencesDispersants and additives Range-specific differences

Table 12.41 Environmental properties of reducing agents for dyeing with vat dyes [253]

Inorganic Sodium dithionite Advantages

(1) Universal and highly effective(2) Free from heavy metals

Disadvantage

Sulphite/sulphate in waste water (oxygen-consuming,may damage piping)

Organic Hydroxyacetone (other productswith improved propertiesbeing developed) Advantages

(1) Free from sulphite and heavy metals(2) Nontoxic to aquatic life(3) Readily biodegradable

Disadvantages

(1) Increased TOC/COD(2) Weaker reductive effect than dithionite

Table 12.42 Remedial action for operating with reducing agents for vat dyes [253]

Reducing agents Recommendations for dealing with the drawbacks

(1) Dithionite and derivatives Minimising the concentration by optimising

Oxidation of sulphite to sulphate (isothermal dyeing with oxidation in the dyebath for pale shades)

(2) Hydroxyacetone In certain applications dithionite can be replaced byhydroxyacetone(e.g. indigo dyeing)

(3) Mixtures of dithionite Used in optimum proportions for the particular application, it isand hydroxyacetone possible to reduce contamination of the effluent by sulphite

to the minimum

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913

dyeing is that these insoluble dyes pose few hazards. The LD50 of vat dyes, determined byoral feeding to rats, exceeds 2000 mg/kg and they are non-toxic to fish. As yet, only one vatdye (CI Vat Green 9) has given a positive Ames test response and only one other (CI VatYellow 4) has given positive results for carcinogenicity [220].

Vat dyes exhibit another important advantage compared with direct and reactive dyes:their exceptionally high exhaustion leads to very little residual dye in the effluent.Furthermore, any dye that does enter the effluent is insoluble and can easily be removed byflocculation or precipitation; it is also readily adsorbed by sludge in biological treatment.Thus vat dyes that contain halogen pose no serious threat in the waste water [253].Consequently the major contribution of vat dyes to effluent pollution arises from theircontent of dispersing agent, which may vary from one manufacturer to another. Thedifference between powder and liquid brands is pertinent here. Powder dyes, for example,can give a COD value of about 1500 mg/g, of which only 40% is from the dye and 60% fromthe dispersing agent. For liquid dyes the corresponding value is 600 mg/g, of which 50% isfrom the dye and only 25% from the dispersing agent and hydrotrope; the hydrotrope isreadily biodegradable [220]. Thus, as far as the dyes themselves are concerned, the situationis summarised in Table 12.40. There are, of course, important contributions to effluentpollution from levelling agents, sequestrants, antifoams, migration inhibitors or thickeningagents applied in conjunction with these dyes.

Since the almost universal reducing agent is sodium dithionite, there is a serious problem ofsulphite and sulphate ions in the effluent, the sulphite usually being mostly oxidised tosulphate. Directly added electrolyte may be present but this is usually less important than withdirect or reactive dyes. Hydroxyacetone has the advantage of biodegradability, although itcontributes to the organic carbon content. As already mentioned, there are environmentalbenefits to be gained from using mixtures of sodium dithionite and hydroxyacetone [253]. Theoverall situation with regard to reducing agents is summarised in Tables 12.41 and 12.42.

The oxidation process using hydrogen peroxide poses no serious environmental problem.As indicated in section 12.9.3, there are obvious advantages to be gained from a purelyaqueous boil rather than soaping with a surfactant. By employing an optimised isothermaldyeing process including metering of the dyes, reoxidation can be carried out in the exhaustdyebath giving savings in water, time and energy, as well as ensuring that all sulphite fromthe dithionite is oxidised to sulphate before discharge to effluent [253].

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