12

CHAPTER 2

LITERATURE REVIEW

2.1 INTRODUCTION

Wool is a natural protein fibre that grows from the follicles of the

sheep’s skin. Throughout history, wool fibre has enjoyed widespread

popularity as an apparel fibre and in interior textiles. Today wool remains a

premium, sought after apparel fibre with an impressive set of technical

attributes. Similarly, for carpets, rugs and home textiles, wool remains the

benchmark of quality and performance with which other fibres are compared.

Efforts are also being made to utilize wool for technical applications. The

unique physical and chemical characteristics of this fibre provide a range of

unique properties like warmth and coolness, breathability, moisture

absorption and buffering, resilience, low odour and odour absorption,

softness, flame resistance, biodegradability and recyclability to the apparel

fabrics.

2.2 MORPHOLOGICAL STRUCTURE OF WOOL

Wool has complex morphological structure (Figure 2.1). The fibre

contains two major components in its structure namely cuticle, the

hydrophobic outer layer of the fibre with scales and cortex, the inner layer of

the fibre which contains micro fibrils. Medium fine and coarse wools contains

a third component in their fibre structure called medulla, which is the central

canal filled with air running along the fibre axis. The cuticle and cortex differ

13

in confirmations in their polymer chains as well as amino acid compositions.

The cortical cells are found to be higher in α- helical content as compared to

the cuticle cells, which has increased disorder content.

Figure 2.1 Morphological structure of wool fibre

2.2.1 Cuticle

The cuticle, which protects fibre, can be further divided in to

epicuticle, exocuticle A, exocuticle B and endocuticle based on their amino

acid composition (Figure 2.2). The outer surface of the cuticle scale cells is

covered by a very thin membrane called the epicuticle (about 3 nm thick),

which is visible by means of the transmission electron microscopy. The outer

surface of the epicuticle cells with sulphur rich keratin is coated by covalently

bound thin fatty acid layers, which impart several characteristic to the wool

fibre like hydrophobic nature, resistant to acid, alkali, oxidation, reduction

enzymes and microorganisms. The cuticle is also responsible for the

shrinkage of wool materials during usage due to the overlapping of scales

present on them (Jhonson et al 2003).

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Figure 2.2 The cuticle structure of wool fibre

The exocuticle, just below the epicuticle is referred to as the ‘A’

layer, having a distinctly higher cystine rich component (35% amino acid

residues) than the rest of the exocuticle known as ‘B’ layer of about 15%

cystine content. Below the exocuticle, forming the reminder of the scale

structure is the endocuticle and a thin layer of intercellular cement. The

endocuticle has low cystine content (approximately 3%) and is mechanically

the weakest component of the cuticle structure. The endocuticle component of

the scales swells considerably more in water than does the exocuticle

component due to their big difference in their cystine components and

consequent cross-linking of the protein structure. This explains the

pronounced projection of the scales from the body of a wool fibre in water as

observed by scanning electron microscope. It also explains the tendency of

felting of wool to be greatest in water, where entanglement is most likely

because the scale structure of the fibres is protruding at its maximum. The

cuticles of the wool do not display any molecular ordering as indicated by an

absence of optical birefringence for the scale structure. The cuticle is the

barrier to the sorption of large molecules such as dyes, by the bulk of the fibre

and protects the cortex from physical damage (Feugalmann 1997, Hearle

2002).

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2.2.2 The Cortex

The mechanically the most important component of wool is the

cortex, the main shaft of the fibre, which is covered by the cuticle. The cortex

is made up of cortical cells, which is fully formed fibres, are elongated cells

of few µm and about 100 µm in length packed tightly together and oriented

parallel to the axis of the fibre. Each cortical cells is separated from its

neighbour by a cell membrane complex approximately 25 nm thick made up

of the modified membranes of the original living cells with intercellular

addition provided by the laid down intercellular cement (Feugalmann 1997).

Under the electron microscope, the cortical cells are seen to consist

of long, uniform filaments known as micro fibrils oriented parallel to the axis

of wool fibre. The micro fibrils are about 7.3 nm in diameter and are set of

11nm apart in wet fibres. In cortical cell, the micro fibrils are grouped

together in units of approximately 0.5 µm in diameter called macro fibrils. In

each macro fibrils, the micro fibrils are separated by a cystine rich matrix.

Within the each macro fibrils, the micro fibrils are separately related to each

other and act as a single mechanical unit (Rogers 1959).

The cortex of the wool fibres has been differentiated by dye

staining techniques into ortho cortex and para cortex. The ortho cortex is

more heavily stained by some dyes than the para cortex (Horio and Konda

1953, Mercer 1953). Electron microscope examination of the ortho cortex

shows that the macro fibrils in this region are separated by a thin layer of

intramacro fibrillar material, with the micro fibrils are packed in a ‘whorl’

like configuration in the macro fibrils. The micro fibrils in para cortical cells

tend to be nearly hexagonally packed with no material present between the

macro fibrils. In the fine wool like merino, the ortho and para cortex form a

bilateral structure in which the para cortex is always on the inside of the crimp

16

wave and ortho cortex on the outside of the wave (Rogers 1959, Feugalmann

1997).

2.2.3 Medulla

When the keratin fibre emerges from the dermal papilla of the

animals, various cells grow to the surface and solidify as solid keratinous

fibre. During keratinisation, the vacuole in the cell invaginates and form air

pockets called medulla, which appear dark under the microscope. The

medulla forming cells do not differ in any obvious way from those destined to

form the cortex. The difference appearing subsequently is that in the typical

cortical cell the intercellular synthesis of fibrous keratin fills the entire cell

body with fibrils, whereas, in the medullary cell such synthesis fails to occur.

The chemical resistance of the contents of the medulla is similar to that of

non-keratinous constituents of the cortex (Mercer 1953). Depending on the

volume of medulla, the wool fibre can be classified as hetero, hairy and

kempy fibres. Hetero fibres are with medullary volume of 5-50% of the fibre

volume. It is hair if medullary volume is 51-80% and kempy if it is above

80%. This character of the fibre play very important role in the tensile

strength, torsional rigidity, spinning, dyeing, friction, resiliency, etc.

2.3 CHEMICAL NATURE OF WOOL

Wool is chemically a polypeptide polymer made up of 18 amino

acids with α- keratin arranged in helical structure. Apart from long helical

structure, the wool polymer chain also has free thiol groups, amino acid

residue with amide group and protonated carboxyl acid group side chains

(Church et al 1997). The different amino acids present in wool and their

nature of side chains are given in Tables 2.1 and 2.2 (Rippon 1992).

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Table 2.1 Amino acids of wool with reactive side chain groups

Amino acids Structure Nature of Side Chain

Serine

Polar

Threonine

Polar

Tyrosine

Polar

Aspartic acid

Acidic

Glutamic acid

Acidic

Histidine

Basic

Arginine

Basic

Lysine

Basic

Methionine

Sulphur containing

Cystine

Sulphur containing

Tryptophan

Heterocyclic

Proline

Heterocyclic

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Table 2.2 Amino acids of wool with hydrophobic side chain groups

Amino acids Structure Nature of Side

Chain

Glycine

Hydrocarbon

Alanine

Hydrocarbon

Phenylalanine

Hydrocarbon

Valine

Hydrocarbon

Leucine

Hydrocarbon

Isoleucine

Hydrocarbon

The wool amino acids are classified in five groups: “acidic” amino

acids, “basic” amino acids, ‘polar’ amino acids with hydroxyl groups, sulfur-

containing amino acids, and amino acids with hydrophobic groups (non

reactive) in the side chain. The total amount of amino acids with reactive side

chain groups is 5395 µmol/g, and of amino acids without reactive side chain

groups is 3450 µmol/g. The sum of these two figures, after subtraction of the

concentration of C-terminal amino acids (10 µmol/g), gives a figure of

8835 µmol/g for the concentration of peptide groups in the peptide chains of

wool.

Like all proteins, wool is amphoteric and contains both cationic and

anionic groups. The cationic character is due to the protonated side chains of

19

arginine, lysine, and histidine, and to the small number of free amino groups

at the ends of the peptide chains. The amino groups of lysine, histidine, the

amino end groups, and the thiol groups of cysteine are important sites for the

covalent attachment of reactive dyes. Anionic groups are present as

dissociated side chains of aspartic and glutamic acids and as carboxyl end

groups. The side chains, which in wool account for a considerable proportion

(50%) of the protein material, interact with each other, thereby stabilizing the

peptide by forming links between the chains and rings within a chain

(Zhan 2005).

2.3.1 Chemical Bonding in Wool

The wool polymer is stabilized by different types of inter and intra

bond formation between the side groups of individual peptide chain as shown

in Figure 2.3. The bond formation in wool polymer can be broadly classified

into covalent cross-links and non-covalent interactions (Rippon 1992). From

the top downwards, it shows hydrophobic interactions between phenyl rings,

an ionic bond between an arginine residue and a lysine residue, hydrogen

bonds between a glutamine residue and a serine residue, a disulfide bridge

between two cysteine residues, and an isodipeptide bridge between glutamic

acid and lysine. The disulfide bridge plays an important part in stabilizing the

wool fiber, leading in particular to its relatively high wet strength, moderate

swelling, and insolubility. A second covalent bridge is provided by the

isodipeptide N (β-aspartyl lysine), which provides an additional stabilizing

effect in the cornified cell envelopes of the cortex and cuticle. The so-called

salt bridge is due to the electrostatic interaction between cationic and anionic

side-chain groups. Hydrogen bonds are formed in proteins between as

acceptors, especially in the helical rod domains of the keratin filaments.

Hydrophobic effects stabilize the aggregation of non-polar side chains, so

reducing the area of the interface with water. The three-dimensional structure

20

of the wool proteins is stabilized both by the hydrophobic effect and by a

wide variety of electrostatic interactions between amino acid constituents

(Zhan 2005).

Figure 2.3 Covalent and non-covalent interactions in wool

2.3.2 Chemical Reactivity of Wool

The chemical reactivity of wool is largely due to the amino acid

cystine, which may be readily oxidised, reduced, and hydrolysed to give a

21

variety of complex reaction products. As cystine is a diamino acid, it can

combine with adjacent polypeptide chains to form a disulphide cross-link.

This component of the keratin structure contributes largely to the physical and

mechanical properties of the wool fibre.

Wool is more resistant to acids than alkalis. This is because acids

hydrolyse the peptide groups but do not damage the disulphide bonds. The

disulphide bonds are very important to keep the wool polymer intact.

Although the polymer system of wool is weakened under acid conditions, the

fibre does not dissolve, but it is vulnerable to further degradation. It is

therefore important that wool be neutralised after any acid treatment (Lewis

1992).

Wool dissolves readily in alkaline solutions, which hydrolyse the

disulphide bonds, hydrogen bonds, and salt linkages and cause the polymer

chains to separate from each other. Prolonged exposure causes hydrolysis of

the peptide bonds of the polymer chains, leading to polymer fragmentation

and complete destruction of the fibre (Zhan 2005).

2.4 PHYSICAL PROPERTIES OF WOOL RELATED TO

HANDLE

2.4.1 Fibre Diameter

Fibre diameter is the most important property of wool since it

determines its suitability for certain end-uses. Wool fibre can be broadly

classified into three types based on diameter. Fine wool fibre has the diameter

range of 18-25µ without any medullation or with less than 1% medullation.

Sheep breeds like Merino, Rambouillet etc., produce this type of wool. This

wool is used for producing fine apparel cloths like suiting, knitted garments,

shawls etc. Medium fine or carpet wool normally has the diameter range of

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25-40µ with 10-40% medullated fibres. The presence of medullated fibres

gives resiliency and springiness to the wool. Hence, this wool is mainly used

to produce carpets, knitted goods, blankets, wall hangings etc. The coarse

wool has the diameter range of above 40µ with 40-70% medullation. This

type of wool is brittle and has the rough feel. This wool is mainly used to

produce coarse blankets, floor mats etc.

The coefficient of variation of diameter (CVD), which is

determined from the fibre diameter distribution, is also important parameter

affecting the performance of wool processing. A good approximation can be

calculated using the formula CVD (%) = 10.5 + 0.5 × D (where D is the mean

fibre diameter in microns). For wools in the 20-24 microns range, about 95%

of sale lots will have CVD between 19 and 26%. In terms of processing

performance and product quality, a change in CVD of 5% is equivalent to a

change of 1 micron in mean fibre diameter. For example, 21 micron wool

with a CVD of 20% can be a substitute for 20 micron wool with a CVD of

25% (Wood 2003).

An indicator, the so-called comfort factor (CF) has been developed

for the level of prickle in fabrics: it is the percentage of fibres finer than

30 microns. A related indicator, the prickle factor (PF), is simply PF = 100% -

CF The lower the CF (or the higher the PF) the greater the risk of prickle

occurring, and a CF of under 95% (PF over 5%) is generally regarded as

producing an uncomfortable fabric (Wood 2003). The risk of prickle is low in

fine merino wools with the mean fibre diameter of 18 to 21 microns. At the

other extreme, in carpet wool blends, which are commonly in the

30-38 microns prickle is obviously not an issue. However, the medium fine

wool with diameter range of 25-30 microns variation can affect the comfort of

wearers by the way of prickliness (Wood 2003).

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2.4.2 Wool Fibre Bending

Considering a fibre as a flexible rod, the force required to bend it

varies as the square of the diameter. This means that a fibre with a diameter of

40 microns requires four times the force as a 20 micron fibre to bend it to the

same extent (assuming the same length of fibre is involved). The physics of

fibre bending, which is governed by this simple rule, is very important in the

flexibility of yarns, and the handle and drape of fabrics (Wood 2003).

For fabric prickle, the mechanism is the buckling of the fibre ends

protruding from the fabric surface, which is described by Euler’s theory

(Naylor et al 1997). Under compression, fibre ends remain vertical until a

certain threshold force is reached, but they are unable to sustain a larger force

and hence they eventually buckle. The threshold force at which buckling will

occur is proportional to E.D4 / l2, where E is the Young’s modulus (i.e.

stiffness) of the fibre material, D is the fibre diameter and l is the length of the

protruding fibre end. This relationship indicates that buckling is highly

dependent on the fibre diameter – the coarser the diameter the higher is the

buckling force. However, increasing the length of the protruding fibre ends

has the opposite effect - the buckling force is reduced (Naylor et al 1997).

2.4.3 Crimp of Wool

Crimp is the fibre characteristic that largely determines the bulk,

resistance to compression and lustre of wool. The bulk of a wool is related to

its crimp characteristics (i.e., the crimp spacing or crimp frequency and its

amplitude), and measures its ability to fill space and have a springy handle.

Bulk is a particularly important property in selecting wools for knitwear and

machine-made carpets, where good cover is desirable in the pile. The bulk test

is mainly used to assess the suitability of wools for woollen processing into

carpets and knitwear. Crimp promotes cohesion between fibres so that higher

24

bulk wools are generally easier to process. In addition, because air is trapped

in the spaces between crimped fibres, products made from high bulk wools

tend to provide good thermal insulation and are warm (Feughelman 1997).

2.4.4 Exothermic Nature of Wool

Wool as such is exothermic in nature because of the various

reactive groups in its structure. When it is wet, it is able to release heat.

Therefore, in high humidity condition in winter, it absorbs the moisture and

provides us the warmth. Warmth property of the woolen product is also due to

entrapped air in the fabric structure, which insulates/prevents our body

temperature to move out along with the exothermic nature of the wool (Rama

Rao and Gupta 1992).

2.4.5 Stress-Strain Property of Wool

Wool has a unique form of stress-strain curve, depicted in

Figure 2.4. One of the key features is the long flat yield region, where the

fibre extends at constant stress. It has been shown that this mechanical feature

is associated with the conversion of α helical rods to extended structures

known as β-pleated sheets (which are also strongly hydrogen bonded). The

α-β conversion is an important feature of wool behaviour. At the same time as

the helices are extended, the matrix is also stretched. The retractive force of

the strained disulphide-bond network, much like rubber, assists recovery of

the fibre when the stress is released. This ability of wool to change

configurations reversibly (i.e. its high degree of elasticity and resilience)

compensates for the relatively low tensile strength of the fibre. The Young's

modulus of wool in the Hookean region is about 1.4 GPa (Feughelman 1997,

Parthasarathy and Patni 1982).

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Figure 2.4 Stress-strain curve of fine wool

2.4.6 Permanent Set

Set is a very interesting and unique property of wool due to its

chemistry. When wool is stretched for a considerable point of extension and

kept at a particular temperature and humidity, it sets on that length (Astbury

and Woods 1934). This can be overcome only when the wool is taken to a

higher temperature and humidity condition. This is because the disulphide

bond of wool is elongated and slightly displaced and left in that condition and

rearrangement of hydrogen bonds (Farnworth 1957). Woollen fabrics are

treated for set in a stenter at 120˚C followed by steaming for 15-20 minutes so

that the fabric takes up the definite dimension and maintains it throughout the

life. The setting of wool can also be done with the boiling aqueous solution of

sodium bisulphate solution at the pH of 6-10 (Farnworth 1957).

2.5 WOOL PREPARATORY TREATMENTS

The shorn wool from sheep contains grease, a waxy substance and

lot of acquired impurities like skin flakes, suint, sand, dirt and vegetable

26

matter. These impurities are removed before the fibre is subjected to spinning

operations. The following processes are employed for the above such

purposes.

2.5.1 Scouring of Wool

Scouring is the first process that raw wool goes through, to remove

the dirt, grease, and other impurities. The scouring of wool is normally carried

out with hot solution of sodium carbonate and non-ionic surfactants

(Dominguez et al 2003). The scouring loss depends on the grease content of

wool. Generally, fine wool fibres exhibit 50% weight loss compared to 5-10%

in the case of carpet and coarse wools. The grease that is removed (lanolin)

from wool is considered as a valuable byproduct and can be use in the

manufacture of many cosmetics and pharmaceuticals. Lanolin consists of a

highly complex mixture of esters, alcohols, and fatty acids (Anderson 1960).

The scouring wool needs large quantities of water and leads to environmental

problem. Another problem related to wool scouring is fibre entanglement.

These problems have renewed interest in solvent scouring using

trichloroethylene.

2.5.2 Carbonising of Wool

Scouring does not always eliminate vegetable matter such as burrs

and seeds. Heavily contaminated wool must go through a process known as

carbonising. If burrs are not removed at this stage, they can cling to the wool

fibres and not be noticed until the process is complete. The process of

carbonising is the treatment of the vegetable matter with sulphuric acid and

heat. The wool is steeped in the sulphuric acid solution, which causes the

burrs to break up. The wool is then subjected to heat, which converts the

disintegrated material into carbon. The burrs are then finally removed, firstly

by crushing and then shaken out of the wool by a machine rotating at high

27

speed. Any remaining impurities are thus blown out of the fibre. Following

carbonising, the wool should be rinsed and neutralised by a wet process. Such

neutralisation should be carried out immediately after baking, otherwise fibre

damage will occur during storage of the wool in such an acidic state (Lewis

1992).

2.5.3 Bleaching of Wool

Wool is generally bleached with an alkaline peroxide solution.

However, there are some studies in literature, which shows that wool can be

bleached using hydrogen peroxide in acidic condition with Prestogen W, a

peroxide stabilizer from BASF containing organic acid salts (Gacen and

Cayuela 2000), as well as a mixture of citric acid and sodium acetate

(Karunditu et al 1994). Hydrogen peroxide and peroxy compounds damage

wool fibres, due to progressive oxidation of disulfide bonds ultimately

forming cysteic acid.

2.5.4 Dyeing of Wool

Acid dyes, metal complex dyes, metal containing acid dyes and

reactive dyes are used for dyeing wool. Wool fibre has large number reactive

groups due to its heterogeneous side chain. Wool is needed to be dyed at

boiling condition because of its higher glass transition temperature. Several

research studies are reported in which wool is subjected to enzyme

(Cardamone and Damert 2006), UV light (Xin et al 2002) and plasma

treatments (Kan 2006, Wakida et al 1993) in order to dye it in lower

temperatures.

Chromium either used in metal complex dyes or used as a mordant,

is present in the majority of dyed wools. The threat to chrome dyeing systems

from environmental legislation appears to have been reduced by the

28

widespread adoption of optimized dyeing methods (Clarke and Steinle 1995)

and, in many cases, the installation of improved dye-house effluent treatment

plants. The major dyestuff manufacturers have responded to the concerns over

chromium by promoting alternative metal-free dyes. New reactive dyes (e.g.

Lanasol CE – Ciba) and optimized ranges of metal-free acid dyes (Sandolan

MF – Clariant) have been developed to achieve a balance of economy and

performance comparable to chrome dyes in targeted applications such as

piece and hank dyeing, especially bright fashion shades. Basolan AS (BASF)

inhibits loss of bulk in package dyeing and damage to the dyed wool. This

type of technology is finding use in a number of applications, particularly the

dyeing of wool-polyester blends at temperatures up to 120˚C where the

protective effect is claimed to be better than older style formaldehyde-release

agents (Cookson et al 1995).

2.6 WOOL FINISHING

Finishing is done to wool materials to enhance inherent unique

properties and to impart several new functional characteristics like shrink

resistance, handle, microbial resistance, and water repellency etc. The

finishing steps depend on the type of material namely woolen, worsted and

knitted fabrics. The conventional steps involved in the wool finishing are as

follows.

2.6.1 Woolen Materials

The woven wool fabric is subjected to scouring and milling

operation followed by drying in the stenters. During this process stains or oil

grease present in the fabric are removed. The milling operation gives

dimensional stability and bulkiness to the fabric. This process is followed by

raising and shearing operation for producing soft pile on the surface of the

fabric. Finally, woolen materials are steam pressed (Anon 2009, Anon 2006).

29

2.6.2 Worsted Materials

For this type, the materials are subjected to the process called

crabbing in which fabrics are wound in a roller and rotated slowly with

application of steam by which they are allowed to relax and stabilize the

latent strains with in the fabric that are introduced during the pre processing

operations. The fabric is then steam pressed. Continuous steam relaxation

tables are used to relax the processing strains remaining in the fabric. The

fabric is steamed in the continuous moving belts (usually with vibration and

overfeed) to promote shrinkage (Anon 2009, Anon 2006).

2.6.3 Knitted Materials

The same procedures mentioned above are used for woolen and

worsted knitwear but the treatment is gentle compared to woven material due

to the delicacy of the knitted products. The milling of woolen knitted material

is not done in milling machine like woven materials but with rotating type

washing machine using gentle detergents (Anon 2009, Anon 2006).

2.7 FUNCTIONAL FINISHING OF WOOL

Apart from the conventional finishes, the current trends of research

revolve around the production of functional textiles with antimicrobial, handle

improvement, shrink resistant, easy care, deodour, freshness (aroma) finish,

flame retardant and smart textiles with electrical and thermal conductivity

properties.

2.7.1 Shrink Resistant Finish for Wool

This is the most commonly used finish for wool, which draw the

attention of most of researchers on wool finishing. Even now, several research

works are going in this field to find the most appropriate method. This finish

30

is especially most important for any wool materials because of the scaly

structure of the wool cuticle. Before going to the different methods of

finishes, it is worth to know about the causes of shrinkage in wool materials.

There are two types of shrinkage that wool products are prone to: Relaxation

shrinkage occurs due to removal of stresses and strains introduced during

various manufacturing and handling processes involved in production, which

is largely reversible and another type of shrinkage is ‘Felting Shrinkage’

which is occurred during actual usage of the material. The major cause of this

type wool shrinkage is overlapping of surface cuticle scales and this process is

called as "differential frictional effect”. This differential friction effect arises

from the ratchet-like nature of the wool scales, with the pointed scale edges

all pointing towards the tip. Movement of the fibres against any surface in the

root to tip direction results in contact with the "smooth" surface of the scale

and a low resistance to sliding (Brady 1997).

On the other hand, movement in the opposite direction brings

contact with the pointed scale edges protruding from the fibre and a stronger

resistance to movement (Makinson 1979). In any wool materials, adjacent

fibres are randomly aligned and when wool fibres are subjected to any form

of severe mechanical action in the presence of water, a progressive and

irreversible entanglement of the fibres occurs which is referred as "felting".

As the fibres are worked together, the profile of the fibres favours movement

in the direction of the root-end of the fibre and prevents them from returning

to their original position. This behaviour is similar to that of a ratchet

mechanism, as the relative movement is unidirectional. Eventually the fibres

become more and more entangled until they are completely jammed together

and wool "felt" is formed. This effect of felting is positively used to produce

the bulky woolen materials by the process called as milling.

31

2.7.2 Mechanisms of Shrink Resist Finish

The basic mechanism of this type of finish is to remove the surface

scales present in the wool fibre (approximately 10% of the total weight of the

fibre) partly/completely, which are the major cause of shrinkage. The scales

can be removed using variety of chemicals, enzymes and combination of

chemical and mechanical methods using either oxidative or reductive

techniques. The most widely used method is the use of chlorine/ chlorine

salts, which disintegrates keratin molecules in the cuticle and removes the

surface scales by oxidation. The action of this oxidative chemical is not

restricted to the surface scale but also disintegrates keratins in the cortex of

the wool leading to loss in tensile strength and weight and also leads to AOX

(Absorbable Oxy Halides) formation in the effluents (Brady 1997). Several

methods using milder oxidative chemicals like permonosulphuric acid,

permanganate, peracetic acid, persulphate and hydrogen peroxide (arranged in

decreasing order of reactivity) with zero AOX were developed instead of

chlorine-based process (Denning et al 1994). To improve the effectiveness of

the above processes, the treated materials were coated with cationic polymers

like epichlorine (Chlorine-Hercosett process) and polyurethane (Basolan 2448

process) etc. The coated polymers apart from increasing the wet strength of

wool prevent the adjacent scale edges from interacting and causing felting

during usage (Makinson 1979).

Efforts have been made to use only polymer based shrink resistant

finishes with out the use of any oxidative chemicals. The polymers like

Synthappret BAP and Impranil DLV (Bayer), Protolan 367 and Terolin 352

(Rotta) can be applied by pad-dry cure process to wool. However, the

hydrophobicity of the fibre surface has created difficulties in obtaining

satisfactory adhesion of polymer during shrink resist treatments (Byrne 1996).

Over all, this finish introduced an important advantage of machine

32

washability and tumble dryability (Total easy care wool) to the wool materials

like knitted goods and worsted fabrics. Now the lots of total easy care wool

products are available and giving competitive edge to the synthetic apparels in

the market.

2.7.3 Enzyme Based Shrink Resistance and Handle Improvement

Processes for Wool

With the advent of biotechnology, several enzyme based shrink

resistance and handle improvement processes have been developed for wool

materials. Most of them utilize the proteolytic properties of enzymes like

protease (especially serine proteases) capacity to degrade the cuticle cells of

the wool fibre. This protease enzyme treatment partially or completely

removes the wool cuticle to impart shrink resistance. This process is

considered as environmentally safe process compared to the chlorine-based

process commonly used in wool industry. The protease enzyme treatment

with lower concentration can be used to enhance the handle related properties

of wool like softness, reduced prickliness, reduced pilling etc (Bishop et al

1998, Heine and Hocker 1995).

It is well documented that papain, a proteolytic enzyme form plant

origin was used for shrink proofing of wool in earlier years. However, the

wool had to be pretreated with reducing agent like sodium sulphite to achieve

effective shrink resistant property before enzyme treatment. The efficiency of

the proteolytic enzyme papain in conferring shrink-resistance to wool tops

and woven fabrics has been enhanced by pretreatment of the wool with

lipase/sodium monoperoxyphthalate/sodium sulphite (El-Syed et al 2001).

The proteolytic enzymes from bacterial sources are commonly used

for imparting shrink resistance and handle to wool. This enzyme is applied on

wool in alkaline pH. A pretreatment with alkaline peroxide before the enzyme

33

treatment is necessary for effective activity of the enzyme on wool

(Cardamone et al 2004). The enzyme activity on wool depends on the pH of

the treatment, Temperature, Time and concentration of enzyme (Shen et al

1999, Brier 2000). The wool fabric is subjected to an alkaline protease

(Perizym-AFW) treatment, which improved shrink resistance, whiteness,

pilling behaviour, dyeability and washability. This process is called as

Lanazym process, which is based on purely enzymatic anti felting finishing of

wool (Brier 2000). Levene et al (1997) found that sodium bisulphite

pretreatment enhanced the activity of alkaline bacterial protease enzyme and

had conferred better shrink resistance to wool tops and fabric. Among

enzymes, Esperase was found to be most active while causing the least

damage. Wool yarn spun from Novolan-L enzyme treated wool tops has

lower number of neps, lower number of breaking through spinning, low

co-efficient of hairiness, where the knits have softer feel with better drape,

improved pilling performance and dimensional stability.

One of the major problems associated with the use of this enzyme

is the excessive weight and strength losses in the treated wool materials (Shen

et al 1999). The enzyme due to its high reactivity and relatively small size

((MW approximately 14–20 kDa for most proteases) penetrates into the

cortex of the wool and hydrolyze the proteins and cell membrane complexes,

which leads to higher weight loss and strength loss in the treated wool

materials (Jovancic et al 2001). The surface of the enzyme treated wool is not

smooth and uniform due to the uneven damage to the wool cuticle. Hence, it

is necessary to give softening treatment with silicone or cationic-based

finishing agent.

2.7.3.1 Immobilized protease enzymes treatment

The activity of the enzyme can be restricted to the outer surface of

cuticle in order to minimize the loss of weight and strength. For this purpose,

34

the native enzymes were immobilized with polymers. A commercial protease,

Esperase, was covalently linked to Eudragit S-100, a reversible soluble–

insoluble polymer by carbo-diimide coupling. When compared to the native

enzyme, the immobilized form presented a lower specific activity towards

wool but a higher thermal stability at all temperatures tested. The optimum

pH of the immobilized protease was shifted towards the alkaline side by about

one pH unit while there was no change in optimum temperature between the

free and immobilized protease. The immobilized protease exhibited a good

storage stability and re-usability (Silva et al 2006).

Similarly, proteases were modified with the soluble polymer

polyethylene glycol (PEG) in the bio-finishing process of wool fibres, to

target enzyme action to the outer parts of wool fibres. Different proteolytic

enzymes from Bacillus lentus and Bacillus subtilis in native and PEG-

modified forms were investigated and their influence on the modification of

wool fibres morphology surface, chemical structure, as well as the hydrolysis

of wool proteins, the physico-mechanical properties, and the sorption

properties of 1:2 metal complex dye during dyeing were studied. SEM images

of wool fibres confirmed smoother and cleaner fibre surfaces without fibre

damages using PEG-modified proteases. Modified enzyme products have

exhibited beneficial effect without severe fibre damage measured in terms of

tensile strength and weight loss of the fibre. Meanwhile the dye exhaustion

showed slower but comparable level of dye uptake at the end of the dyeing

(Jus et al 2007, Silva et al 2005).

In order to limit the action of protease within wool cuticle, the wool

was pretreated with alkaline peroxide in the presence of high concentration of

common salt. The presence of salt in the outer surface of wool improved the

susceptibility of the outer surface scale protein for enzyme degradation

(Lenting et al 2006).

35

2.7.3.2 Chemeoenzymatic processes

Shrink proof finish was given to wool fabric by pretreating it with

dicyanamide activated hydrogen peroxide followed by protease enzyme

application at room temperature. It has been shown that the enzyme action

was limited to surface of the wool and the shrink resistance obtained was

comparable with other processes in industry due to room temperature

treatment. This process gave the added advantage of bio polishing to wool

(Cardamone 2006).

Cardamone et al (2004) inferred that pretreatment of woven and

knitted wool fabric of various weights with peroxy carboximide, ruptured

peptide linkages and cystine disulphide cross-linkages through hydrolysis and

oxidation reaction. Proteolytic enzyme treatment biopolished and controled

the shrinkage without appreciable loss in strength and elastic recovery of

wool. The addition of a protease enzyme in a oxidizing or reducing bleaching

bath shortened the bleaching time by half for the same whiteness index

(Levene 1997). Treating of wool with a haloperoxidease (hydrogen peroxide

+ halide) and a proteolytic enzyme resulted in improved shrink resistance

(Levene 1997). In presence of hydrogen peroxide, wool cuticle degrading

enzyme called Bacillus cereus strain NS-11 modified the cuticle components

preferentially without damaging the inner components of wool fibre

(Takatsuka et al 1998).

Enzyme pretreatment on wool fabric decreased the resistance of the

fibre to dye diffusion and so it increased the adsorption rate constant and

decreased in the apparent activation energy for the dyestuff when compared to

untreated fabric (Riva et al 2002). Enzyme pretreatment of wool fabric with

trypsin increased exhaustion of natural dye like crocin, beta-carotene,

curcumin, chlorophyll and carmine without change in fastness property

(Liakopoulou et al 1998, Tsatsaroni and Liakopoulou 1995). Levene and

36

Shakkour (1995) found that a chlorination pretreatment followed by an

alkaline protease treatment on wool fibre made it clearly descaled wool fibre

with enhanced luster. The handle of the fibre can be retained in the post

softening treatment.

El Sayed et al (2002) described an enzyme-based process to

improve the felting resistance of wool top. In this process, lipase was used in

the pretreatment step, glutathione reductase in the reduction step, and papain

in the aftertreatment step. The lipase removed lipids from the outer surface of

wool; glutathione reductase reduced the disulfide bonds in wool keratin

together with nicotine amide adenine dinucleotide phosphate in the reduced

form and papain smoothened the wool scales. Wool fibres treated with this

system showed good felting resistance as compared with untreated wool, but

still was inferior to that treated with the chlorine/Hercosett process.

2.7.3.3 Application of acid protease

The activity of alkaline protease enzymes on wool is depending on

the pH of the treatment. The enzyme action is taking place between pH of

8–9.5 and a slight variation in the above pH range leads to deactivation of the

enzyme. This problem is further aggravated by the fact that weak acids are

rapidly released from wool during alkaline protease treatment due to the

protein hydrolysis action of enzyme. Hence, during alkaline protease

treatment on wool, the drop in pH towards acid side is evident and an

optimum alkaline pH is maintained by intermediate addition of mild alkali to

retain the enzyme activity. In order to avoid this problem, wool was treated

with Aspergillus usamii L86 enzymes including acid protease and CMCase in

acidic pH (Yu et al 2001) to improve the shrink resistance and handle.

37

2.7.3.4 Application of transglutaminase

Transglutaminase enzyme has the ability to incorporate primary

amines and to graft peptides (containing glutamine or lysine residues) into

proteins. These properties enable transglutaminase to be used in the grafting

of a range of compounds including peptides and/or proteins onto wool fibres

altering their functionality. Cortez et al (2007) investigated the

transglutaminase mediated grafting of silk protein on wool and its effects on

wool properties. The grafting leads to significant effect on the properties of

wool yarn and fabric, resulting in increased bursting strength as well as

reduced values of felting shrinkage and improved fabrics softness. The

transglutaminase enzyme was applied on the protease enzyme treated wool

material in order regain the strength loss The transglutaminase reactivity

involves the transferase-mediated, acyltransfer reaction between glutamine

and lysine with the formation of carboxylamide groups of peptide-bound

glutamine in wool keratin (Cardamone 2007).

2.7.4 Antimicrobial Finishing

Textiles have long been recognized as media to support the growth

of microorganisms such as bacteria and fungi. These microorganisms are

found almost everywhere in the environment and can multiply quickly when

basic requirements such as moisture, nutrients and temperature are met. Most

synthetic fibres, due to their high hydrophobicity are more resistant to attacks

by microorganisms than natural fibres. Proteins in keratinous fibres like wool

can act as a nutrient and energy sources under certain condition. Soil, dust,

solutes from some textile finishes can also be nutrient sources for

microorganisms (Purwar and Joshi 2004).

38

The growth of microorganisms on textiles is needed

To avoid cross infection by pathogenic microorganism

To control the infestation by microbes;

To arrest metabolism in microbes in order to reduce the

formation odour; and

To safeguard the textile products from staining, discolouration

and quality deterioration. (Ramachandran et al 2004).

2.7.4.1 Bacteria

Bacteria, a type of microbes, are tiny creatures that individually are

too small to be seen with the naked eye; however, they can have a major

impact on the human life. In order to survive, bacteria need nutrients such as

food or water. Food can be in the form of skin cells and dust, while water is

extracted from humid air, body perspiration, other body fluids, or a wet

textile. Bacteria are single cell or unicellular creatures that appear in three

forms: spherical (cocci), rod (bacilli), and spiral (spirillum), and they can be

arranged in pairs, clusters, or chains. Bacteria contain a cytoplasmic

membrane and a rigid layer (McCall et al 2001). The cytoplasmic membrane

is the internal structure that consists of a nucleoid and ribosome. The nucleoid

is the DNA of the cell and the ribosome translates genetic messages to

proteins. The rigid layer, also called the surface layer, consists of the capsule,

cell wall, and plasma membrane. The capsule protects the cell wall and

maintains the overall shape of the cell. The plasma membrane is used to

transport ions, nutrients, and waste. Some forms of bacteria have appendages

that consist of pilus and flagellum. Pilus allows bacteria to attach to other

cells, and flagellum provides the motility for the cell (Levinson and Jawetz

2002).

39

2.7.4.2 Type of bacteria

Bacteria can be identified as either gram-positive or gram-negative,

which can be distinguished by the content and structure of their cell wall

through a staining procedure called gram-stain (McCall et al 2001). If the

bacteria remain purple after the procedure, they are gram-positive; whereas, if

pink appears, they are gram-negative (Brooks et al 2001).

2.7.4.3 Gram-positive bacteria

Gram-positive bacterium contains peptidoglycan and teichoic acids.

Peptidoglycan comprises 90% of the cell wall and is made of amino acids and

sugar (McCall et al 2001). Teichoic acids are responsible for the antigenic

determinant of the organism. One example of Gram-positive bacteria is

S. aureus that appears in pairs, short chains, or grape-like clusters. It ranges in

size from 0.5 µm to 1.0 µm. The temperature for growth ranges from

35-40 °C. S. aureus is the major cause of cross-infection in hospitals and

makes up 19% of total surgical infections (Huang and Leonas 2000).

S. aureus also can cause boils, skin infections, pneumonia, and meningitis

especially in a debilitated person (Prescott et al 2002). It is also responsible

for scaled skin and toxic shock syndromes.

2.7.4.4 Gram-negative bacteria

Gram-negative bacteria are similar to Gram-positive bacteria except

they have an additional layer of outer membranes attached to their

peptidoglycan layer by lipoproteins (McCall et al 2001). The outer layer is

made of lipopolysaccharide and porin. Porin is used to transport low

molecular weight substances. One example of Gram-negative bacteria is

Klebsiella pneumoniae (K. pneumoniae). Its shape is like a bacillus, and it

appears as single, pair, or short chains. K. pneumoniae is the major cause of

40

urinary tract infection, septicemia, and pneumonia in people with

compromised immune systems. The bacteria can be transmitted via the fecal-

oral route, mouth, throat and air to the lungs (Brooks et al 2001). The

symptoms of K. pneumoniae include fever, difficulty breathing, chest pain,

and bloody stool. Another example of Gram-negative bacteria is Escherichia

coli (E. coli). E. coli is shaped like a bacillus and lives in the intestines of

humans. It can spread through the handling and eating of raw food. The

symptoms of E. coli are severe diarrhea especially in children and kidney

damage. Gram-negative bacteria are harder to reduce than Gram-positive

bacteria because of the extra cell wall on Gram-negative bacteria (Murray

et al 1988). The study results of Murray et al (1988) confirmed that

Gram-negative bacteria (i.e., E. coli and Pseudomonas aeruginosa) are harder

to reduce than Gram-positive bacterium (i.e., S. aureus).

2.7.4.5 Spread of bacteria

Bacterium cannot move from one location to another location by

itself; instead, a carrier such as blood, perspiration, alcohol, shed skin, or dust

must transport it. The carrier can be transported by either liquid or air. Liquid

transport provides a wet or moist transfer of bacteria. The air transport is a dry

transport that occurs in the air, usually through vents (Leonas and Jinkins

1997). Air transport of bacteria can occur through the air movement of dust,

particles, and shed skin. It has been reported that humans can shed an average

of 1,000 bacteria per minute, which can be a major source of contamination in

hospital environment (Whyte et al 1976).

2.7.4.6 Microorganisms and their infections on skin

There are different types of microorganisms. Bacteria and fungi are

very important in the textile point of view to give antimicrobial treatment.

Table 2.3 gives the brief information about the microorganisms and the

disease caused by them (Gupta and Bhaumik 2007).

41

Table 2.3 The diseases causing microorganisms

Microorganisms Disease or conditions caused

Gram-positive bacteria

Staphylococcus aureus Pyogenic infections like sepsis, necrotizing fasciitis, cellulitis, septic arthritis, pneumonia, and conjunctivitis

Staphylococcus epidermis and Corny bacterium ditheroides

Body odour

Brevibacterium ammoniagenes Bacterial pneumonia

Mycobacterium tuberculosis Tuberculosis

Gram Negative bacteria

Pseudomonas aeruginosa Infections of wounds and burns

Escherichia coli Infections in urinogenital tract

Fungi

Candida albicans Diaper rash

Epidermothyton floccosu Infections of skin and nails

Trichophyton interdigitale Athletes’ foot

Trichophyton rubrum Chronic infection of skin and nails

Healthy intact skin is a highly effective barrier against the ingress

of microorganisms. Different species of microorganisms are present on

different parts of the body. Commonly found skin bacteria include

staphylococcal species (e.g. Staphylococcus epidermidis), micrococci,

corynebacteria and propionibacteria. These organisms tend to aggregate

around hair follicles where they can be numerous. For example, micrococci

may number around 60 per cm2 on the forearm, but up to 500 000 per cm2 in

the axillae. Many diseases of the skin are caused by staphylococcal (often

Staphylococcus aureus) and streptococcal (e.g. Streptococcus pyogenes)

organisms.

42

The major skin infections are due to the gram positive

staphylococcus aureus. This bacterium is most often found in the nose and

some times on the skin especially in the hospital staffs and patients (Levinson

and Jawetz 2002). S.aureus is also found regularly on clothing, bed linens and

other fomites in human environment. The S.aureus infection can lead to

impedigo, furuncles, carbuncles, paranychia, cellulites, foliculitis and surgical

wound infections (Williams 2003, Levinson and Jawetz 2002, Brooks et al

2001).

Other bacterial infections, less common than those caused by the

Gram-positive cocci, include infections due to mycobacteria; tuberculosis is

caused by Mycobacterium tuberculosis, which generates the condition lupus

vulgaris, characterized by red/brown plaques often on the face and neck.

Other opportunistic pathogens like Mycobacterium fortuitum can infect skin

and causes surface lesions (Vigo and Benjaminson 1981).

Fungal skin infections are also common and range in severity. Two

types of causative organisms are common: Candida species, which are yeasts;

and the dermatophytes that are multicellular. Candida albicans is an

opportunistic pathogen causing thrush, especially where the normal bacterial

flora is disturbed (e.g. following antibiotic therapy). Dermatophyte infections

target the keratinised tissues of the body – hair, nails and stratum corneum

and are commonly known as hypopigmented areas on tanned skin in summer

(malassiezia furfur), brownish spot in skin (Tinea nigra) athlete’s foot (Tinea

pedia), ringworm (Tinea corporis) and ‘jock itch’ (Tinea cruris). It is clear

that many of the above infections vary in their severity. Consequently, the

damage caused to the skin barrier integrity will vary with severity of the

infection (Vigo and Benjaminson 1981).

43

2.7.4.7 Staphylococcus aureus and Atopic dermatitis

Atopic dermatitis (AD) is a chronic inflammatory skin disease with

increasing prevalence over the last few decades. Various factors are known to

aggravate the disease. In particular, wool and synthetic fabrics with harsh

textile fibres, aggressive detergents and climatic factors may exacerbate AD.

Cutaneous super infection, particularly with Staphylococcus aureus, is also

recognized as an important factor in the elicitation and maintenance of skin

inflammation and acute exacerbations of AD. The severity of AD correlates

with S. aureus colonization of the skin (Haug et al 2006). Hauser et al (1985)

showed that, in comparison to healthy individuals, AD patients have

significantly more S. aureus colonization, even in non-lesional skin.

Moreover, in lesional skin of AD patients, S. aureus colonization is

100-1,000 times higher than in non-lesional skin, and the degree of

colonization is associated with disease severity.

2.7.5 Antimicrobial Finishing Methods

Antimicrobial agents were used on textiles thousands of years ago,

when ancient Egyptians used spices and herbs as preservatives in mummy

wraps (Seong et al 1999). An antimicrobial finish is a method used to reduce

the spread of microorganisms by either killing or inhibiting their growth

through contact with the fabric surface (Huang and Leonas 2000). A durable

antimicrobial finish must be able to

(a) control bacteria or fungi on the cloth

(b) remain effective over the lifetime of the treated article

(c) durable to washing and bleaching

(d) have no risk of adverse dermal or systematic affect

44

(e) provide no detrimental effects on fabric properties such as

yellowing, hand, and tensile strength

(f) coexist with other finishes, such as softeners and resins

(g) have a low environmental impact (i.e., free of heavy metals,

formaldehyde, phenolic, organic halogens) (Payne and Kudner

1996, Seong et al 1999).

Several processes have been used to apply antimicrobial finishes to

fabrics. The pad-dry-cure process is the primary process used to apply

antimicrobial agents to textiles (Yang et al 2000). This finish is more

important to woolen materials like carpets, home textiles and knitted goods.

The researchers of Osaka University, Japan conducted a study to determine

the distinctive bacteria binding properties of the cloth materials. Based on the

result, they suggested that wool cloths could be good carriers of S.aureus and

P.aeruginosa than the cotton. Hence, they suggested that woolen goods

should be covered with cotton cloths to minimize the spread of pathogens

(Takashima et al 2004).

2.7.6 Antimicrobial Finishing Agents

Current antimicrobial agents that are used in either industry or

academia have three mechanisms: controlled-release, regeneration and

barrier-block.

2.7.6.1 Controlled-release mechanism

The controlled-release mechanism is the most commonly used of

the antibacterial agents. The agent is released gradually in enough quantities

to kill or inhibit the growth of bacteria. Examples of antibacterial agents that

use controlled-release mechanism are gentamincin and triclosan.

45

2.7.6.1.1 Gentamincin

Gentamincin is an aminoglycoside antibiotic complex. Each

component consists of five nitrogens per mole of gentamincin base

(Figure 2.5). Gentamincin is widely used in hospitals as an antibiotic and acts

by inhibiting bacteria protein synthesis. Cho and Cho (1997) conducted a

study on a dual functional finish using the antibiotic gentamincin as an

antibacterial agent and a fluorochemical as a blood repellency agent on

surgical gown fabric. The findings showed a 98% reduction of Gram-positive

S. aureus and Gram-negative K. pneumoniae on both 100% cotton and

55/45% woodpulp/polyester spunlaced nonwoven fabrics.

Figure 2.5 Structure of gentamincin

2.7.6.1.2 Triclosan

Triclosan is a diphenyl ether derivative known as 5-Chloro-2-(2,4-

dichlorophenoxy) phenol (Huang and Leonas 2000). The chemical structure is

given in Figure 2.6. It is a broad-spectrum antibacterial/antimicrobial agent

that has been incorporated into personal care products such as toothpaste,

soaps, deodorants, antiperspirants, body washes, detergents, cosmetics,

antimicrobial creams, lotions and hand soaps. Triclosan interrupts the

cytoplasmic membrane of the bacteria that in turn interferes with the

metabolic function of the cell. Once triclosan is inside the bacterium cell, it

can poison a specific enzyme that the bacterium needs to survive. It has

46

bacteriostatic activity against a wide range of both Gram-positive and Gram

negative bacteria. Some of the negative features of triclosan are that it may

cause cancer in humans and creates skin irritations (Bajaj and Sengupta

1992). Huang and Leonas (2000) examined triclosan as an antibacterial finish

along with a fluorochemical repellent finish on nonwoven fabrics. They found

that a 0.25% add-on of triclosan was sufficiently high to inhibit bacterial

growth of S. aureus. However, bacterial resistance to triclosan has been well

documented and is of great concern. When exposed to sunlight, triclosan

breaks down into 2,8-diclorodibenzo-p-dioxin which is a toxic substance.

Figure 2.6 Structure of triclosan

2.7.6.2 Regeneration mechanism

With the regeneration mechanism, an antibacterial chemical finish

is applied to the fabric and is continually replenished by bleaching agents

during laundering. An example of this mechanism is MDMH. Monomethylol-

5,5-dimethylhydantoin (MDMH). MDMH is a hydantoin derivative (Sun and

Xu 1999). The chemical structure is shown in Figure 2.7. In the past, MDMH

was used as a renewable disinfectant for swimming pools (Sun and Xu 1999).

It was also known to provide durable and regenerable antibacterial activity for

fabrics that contain cellulose. The MDMH, which would kill the cell

membrane, is covalently bonded with the fabric and laundered with chlorine

bleach to replenish the antibacterial activity of the fabric. When MDMH was

used as a regenerated antibacterial finishing agent, it showed significant

47

inhibition on both the gram-positive and gram-negative bacteria as well as

maintaining the tensile strength of both fabrics after 20 repeated washings.

Figure 2.7 Structure of MDMH

2.7.6.3 Barrier-block mechanism

Barrier-block mechanism inhibits bacteria through direct surface

contact. The antibacterial agent can bond covalently to the fabric to form a

strong and durable bond. Agents with the barrier-block mechanism do not

leach and have fewer problems in skin irritation than agents with the

controlled-release mechanism (McCall et al 2001). In addition, they require

less add-on (i.e., less antibacterial agents) on fabric. Examples of agents with

the barrier-block mechanism are polyhexamethylene biguanide (PHMB),

chitosan and quaternary ammonium compounds (QAC).

2.7.6.3.1 Polyhexamethylene biguanide (PHMB)

PHMB is the basic compound of Reputex 20. PHMB is an oligomer

(Figure 2.8) with an average of 12 biguanide per molecule (Huang and

Leonas 2000). It has been used in swimming pools, cosmetics, and the food

industry for many years because of its activity against Gram-positive and

Gram-negative bacteria, fungi, and yeast (Payne and Kudner 1996). PHMB

antibacterial activity is through the biguanide functional group, which disrupts

the bacterial cell membrane (Huang and Leonas 2000).

48

Figure 2.8 Structure of PHMB

2.7.6.3.2 Chitosan

Chitosan is a derivative of chitin, which is the second most

abundant natural polymer (Figure 2.9). Its structure is very similar to that of

cellulose except an amino group replaces one of the hydroxyl groups.

Chitosan can destroy bacteria by converting its amino group into an

ammonium salt in dilute acid solutions. Quaternary ammonium salt of

chitosan can destroy the cell wall of the microorganism by connecting to its

negatively charged protoplasm (Kim et al 1998). Chung et al (1998b) used

chitosan as an antibacterial finish along with a durable press finishing agent

on 100% cotton fabrics and found that antibacterial activity remained to a

level of 80% after 10 repeated launders. Lee et al (1999) evaluated chitosan as

an antibacterial agent along with a blood repellent finish. Chitosan was

applied to wool as a shrink proofing polymer. Owing to the hydrophobic

nature of wool, treatment with chitosan required pretreatments so that

polymer can adhere to the surface (Pascula and Julia 2001). It is reported that

the wool was oxidized with potassium permanganate prior to the application

of chitosan. Although chitosan had conferred durable antimicrobial ability to

wool, the handle of the treated wool fabric was adversely affected (Heish et al

2004). Wool fabric was finished with chitosan along with henna natural dye

to give dying and antimicrobial finishing (Gridev et al 2009).

49

Figure 2.9 Structure of chitosan

2.7.6.3.3 Quaternary ammonium compounds

Quaternary ammonium compounds (QAC) particularly those

containing chains of 12-18 atoms (Figure 2.10) have been widely used to

impart antimicrobial finishing to wool. These compounds carry a positive

charge at nitrogen atom in solution and inflict a variety of detrimental effects

on microbes, including damage to cell membranes, denaturation of proteins

and disruption of cell wall structure. (Gao and Cranston 2008). The

attachment of QAC compounds to wool is believed to be predominantly by

ionic interaction between the cationic QAC and anionic fibre surface. Hence,

the carboxylic group containing glutamyl and aspartyl residues in wool is

reaction site for QAC. The wool treated with 5% cetyl pyridinium chloride

showed durable antimicrobial activity for 10 launderings (Zhu and Sun 2004,

Zhao and Sun 2006). Attempts have been made to covalently attach QAC on

to wool. Wool specific QAC like N-dodecyl-amino-betaine-2-

mercaptoethylamine hydrochloride (DABM) has been synthesized. DABM

can react with wool by means of its thio group, either with cystine-S-

sulphonate residue (bunte salt) of bisulphate pretreated wool or disulphide

bond of cystine wool, forming an asymmetric disulphide bond. Such covalent

attachment of the quaternary ammonium compounds provides antimicrobial

activity (Diz et al 1997, Diz et al 2001).

50

Figure 2.10 Structure of QAC

2.7.6.4 Nano silver

Silver is one of the strongest bactericides. Silver is one of non-toxic

and safe antibacterial agents to the human body, and can kill many harmful

microorganisms. The mechanism of antibacterial action of silver ions is

closely related to their interaction with proteins, particularly at thiol

(sulfydryl, –SH) groups, and are believed to bind protein molecules together

by forming bridges along them. Since the proteins behavior often like as

enzymes, the cellular metabolism is inhibited and the microorganism dies

(Ki et al 2007). Wool can be imparted antimicrobial and antiodour properties

using commercially available silver nanoparticles with out affecting the

handle (Anon 2008).

2.7.6.5 Natural dyes and antimicrobial finishing

There are several studies in literature in which natural dyes like

Quercus infectoria, curcumin etc (Han and Yang 2005, Singh et al 2005) have

been used to give antimicrobial finishing to the wool materials. Wool fabrics

with a deodorant function were prepared by acid mordant dyeing with copper

(II) nitrate. Carboxylate groups in the wool fibre and the dye work together to

fix the copper ion with the deodorant ability against mercaptan (Kobayashi

et al 2002). The Wool Mark Company of Australia through their SPT process

microencapsulated tea tree oil and used an antimicrobial agent. Wool

materials treated with tea tree oil microcapsules have been subjected to

51

international standard laboratory anti-bacterial testing and shown to have

significant properties of protection.

2.8 COMFORT FINISHES FOR WOOL

Comfort to wear is very important property for any textile material.

Various type of tactile, moisture and thermal interactions between the

clothing material and the human skin determine the comfort level of a person

at a given environment condition while engaged in a specific level of activity.

Apart from tactile and thermal comfort, protection from microorganisms, bad

odour, freshness and scented finishes give textiles additional comfort for

wearing. Wool is considered as comfort and breathable fibre due to its high

moisture vapour capacity and regain, it can adjust different climates and

comfortable for sit long periods due to buffering of microclimate by wool.

Figure 2.11 gives the moisture regain of wool, cotton and polyester with

different RH condition. For wool the following comfort finishes are given.

Moisture Regain Vs RH

0

10

20

30

40

0 20 40 60 80 100

RH

Moi

stur

e R

egai

n

PolyesterCottonWool

Figure 2.11 Moisture regain of wool, cotton and polyester Vs. Relative

humidity

52

2.8.1 Softness Finishes to Wool

The main problem associated with wool products is prickliness

during wear, which can be reduced by giving softness finish. This problem is

more intense in the knitted wool garments. Different studies in this regard

indicate that softness of wool fibre depends on the fibre diameter. Fine and

super fine wool having diameter less than 19µ show less prickliness than the

higher micron medium and coarse fibre. It has also been predicted that the

presence of damaged broken fibre ends due to processing in wool materials

were responsible for prickliness.

Cationic softening agents and silicones can be applied to reduce the

prickliness of wool. Different methods like treatment with Aloe vera extract,

microcapsules containing moisturizer, chemical agents, which reduces the

fibre micron to import softness have been reported. Similarly, Wool fibres

were treated with proteolytic and lipolytic enzymes alone or in combination

with oxidative chemicals for imparting softness in addition to shrink

resistance character (Shah et al 1971, Das and Gita 2006, Barkhuysen et al

2005).

2.8.2 Fragrance and Deodorant Finish

Fragrance infused textile materials are now marketed by various

companies and have wide acceptability among consumers. These kinds of

fabrics are also called as aromatherapy textiles because these materials apart

from giving freshness feeling to the wearer also give medicinal value like

relieving the stress, cough, allaying fear and antibacterial property. Generally,

aromatic chemical fragrance compounds and essential oils are used for

producing the scented materials (Achwal 2004). Table 2.4 gives the fragrance

of essential oils and their equivalent chemical compounds.

53

Table 2.4 Fragrance of essential oils and their equivalent chemical

compounds

Fragrance Equivalent Chemical Compounds

Citrus Lemon(Citral ,Citronellal), Orange(Mandarin oil, Decyl acetate)

Floral Carnation(Phenethylsalicylate), Gardenia (Nonyl aceetate), Geranium(Citronellol), Lilac(Anisylacetate), Lilly(Hydroxy citronellol), Rose(Rose absolute), Violet (Costus oil, methyl-2-nonenoate)

Fruity Apple(Benzyl acetate), Apricot (Allyl butyrate), Banana (Amyl acetate),Grape (Isobutyl isobutyrate), Peach (Allyl butyrate), Strawberry (Benzyl benzoate)

Herbaceous Clove (Eugenyl acetate), Minty (l-carveol,l-carvone,l-menthol)

Sweet Anise(Ethyl acetate,Methyl sorbate), Cinnamon (Cinnamaldehyde) Honey(Allyl phenoxyacetate), Sweet(Acetanisole), Vanilla (Anisyl acetate)

The major problem in this finish is durability because most of the

compounds are volatile. Two types of approaches are proposed to solve this

problem

2.8.2.1 Microencapsulation techniques

Using this technique the fragrance compounds are encapsulated and

applied to the textile materials. The slow release of the aromatic compounds

during wearing gives the necessary freshness.

54

2.8.2.2 By forming inclusion compounds

The inclusion forming compounds like β-Cyclodextrin can be

utilized to entrap the aromatic fragrance compounds. They can be fixed on the

textile material by way of chemical bonding using cross-linking agents.

Polycarboxylic acids such as 1,2,3,4-butane tetra carboxylic acid, citric acid

or polyacrylic acid can be used to fix the cyclodextrins on wool materials in

the presence of disodium hydrogen phosphate or sodium dihydrogen

hypophosphite. One of the problems associated with cyclodextrin is that in

unmodified form, it has only limited solubility and low affinity for the fibres.

Thus using pad bath and pick up of 70-80%, the transfer to the fibre will be

only 1.5% which is low for getting the desird effect(level of 3-5% is

necessary). Hence, the basic molecule is modified with functional side chains.

The following are the main modified form of cyclodextrins.

Monochloro triazine derivative

Hydroxy propyl derivative with high solubility (not reactive)

Trimethyl ammonopropyl (higher substantivity)

The cyclodextrin as such with out any inclusion compound can be

used for textile as a smell absorbent agent. The textiles thus finished absorb

unpleasant smells, body perspiration smells, cigarette smells etc (Martel et al

2002, Achwal 2004).

2.8.3 Insect Repellent Finish

This type of finish is mainly applied for carpets and to small extent

for woolen products like upholsteries; blankets. A number of well-established

insect repellent products like permethrin, pifenthrine are available. Permethrin

is chlorine based chemical compound having chemical formula C21H20Cl2O3

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and the pifenthrine is fluorine based. The following are the challenges

associated with the insect repellent finish

all the chemicals in use increasingly are less effective against

insect mutations that impart more resistance to the insect

resist agent

older products are less effective on beetle larvae

the ecotoxical properties of permenthrin and other synthetic

pyrethroids are giving rise to some concerns (Holme 2003).

Alternatively, quaternary ammonium compounds and chemicals

based on bioengineering approach like insect growth regulator technology can

be used as moth proofing agents.

2.8.4 Biopolymer Chitosan for Shrink Proof Finish

Wool fabrics were treated with the biopolymer chitosan to get

shrink resistance finish. In order to enhance chitosan absorption, the wool was

pretreated with different oxidative chemicals like hydrogen peroxide and

permonosulphuric acid. It was shown that the shrink proofing ability

depended upon the molecular mass and oxidative pretreatment. The

combination of chitosan, protease enzyme and oxidative pretreatment also

reported for shrink proofing of wool, which also imparted antimicrobial

characteristics to the treated materials. (Julia et al 2000, Jovancic et al 2001)

2.8.5 Low Temperature Plasma Treatment for Wool

The low temperature plasma (LTP) technique is used to modify

wool surface to provide hydrophilicity, shrink resistance, etc. This is regarded

as an environmentally friendly process because no chemicals are used. Plasma

is an ionized gas composed of charged and neutral particles. When this

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plasma is applied to the wool, the free electrons or other meta stable particles,

upon collision with the substrate, break the chemical bonds creating free

radicals like C=O,C-O, O-C=O, SO3-on the polymer surface. These free

radicals can undergo additional reactions depending on the gases in the

atmosphere. Thus, the substrate can be coated with different kinds of gases

without changing the fabric's bulk characteristics. Plasmas containing

CF4, C 3F6, argon, or other hydrocarbon gases can provide hydrophobicity to

the fabric. Similarly, plasmas containing oxygen and nitrogen can provide

hydrophilicity to the fabric (Rajpreet et al 2004, Kan and Yeun 2006).

Several attempts have been made to give shrink resist finish using

plasma as a pre treatment. The wool fabric was first treated with oxygen,

nitrogen, or mixture of nitrogen and hydrogen plasmas, which resulted in the

enhanced wettability of the surface due to the formation of more ionisable /

free radical groups in the fibre surface. Then the shrink resistant finish was

given using different kind of silicone polymers. The wool treated by this

process showed increased shrink resistant property compared to the untreated

one (Kim and Kang 2002).

2.8.6 Anti Pilling Finish

Woollen knitted materials are prone to pilling. Various factors

related to fibre, yarn characteristics like fibre tenacity, twist of the yarn have

been predicted for the pilling of woolen knitted materials. A number of

processes have been shown to improve the woolen garments. The knitted

goods made out of chlorinated wool have shown less pilling tendency than the

untreated one. Similarly, the treatment of woolen materials with proteolytic

enzymes reduced pills formation (Beltran et al 2006).

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2.9 CONCLUSION

Due to limited production of wool world wide, the research works

on wool are comparatively less to other types of fibres like cotton and

synthetics. Most wool researchers have taken up studies on shrink resistance

finish and still newer processes are coming. The enzyme based shrink

resistant methods could not replace the widely used chlorine based processes

of 1970’s completely due to complex application method in spite of its eco

friendly nature. The same process is proposed for improving handle of the

wool with lower enzyme concentration. The activity of widely used alkaline

protease enzyme is sensitive to pH, temperature and impurities in the enzyme

bath. Similarly, controlling the activity of enzyme on wool in alkaline pH

conditions is critical problem, which generally results in excessive strength

and weight losses. Hence, the enzyme-based processes for wool should be

further improved to get an optimum result during shrink resistant and handle

improvement finishing. The use of acid protease enzyme for above such

purposes is not fully explored.

Wool has hydrophobic outer surface cuticles, which is the

protective barrier against the attack of microorganisms. However, the cuticle

surface of wool is partially damaged or completely removed during bleaching

and enzyme treatments. Hence, wool materials also need a separate

antimicrobial finishing process. Several antimicrobial agents like

polyhexamethylene biguanide (PHMB), quaternary ammonium salts, nano

silver, natural dyes etc., have been identified for wool. Among them, silver

nanoparticle based antimicrobial finishing for wool provides protection

against wide range of microorganisms. However, this process has some

limitations like aggregation in solution, less durability on the fabric in its

native state and high cost. There is the limited research in the development of

textile specific antimicrobial finishing using silver nanoparticles. Another

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promising antimicrobial finishing process is based on natural dyes. It is

possible to provide both dyeing and antimicrobial finishing to the wool

materials using natural dyes. This process is eco friendly and cost effective

compared to silver nanoparticles. There are only limited researches related to

the use of natural dyes for providing antimicrobial finishing to wool.