Indian Journal of Fibre & Textile Research Vol 26, March-June 2001, pp. 93-100

Environment-friendly dyeing processes for cotton

R B Chavana

Department of Textile Technology, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India

Environment-friendly dyeing of cotton with reactive dyes has been critically reviewed. Major developments are in terms of innovations in dyes and dyeing processes for high dye bath exhaustion and fixation with a view to reduce the total quantity of colour in the effluent. The use of bifunctional reactive dyes having high exhaustion and fixation properties, use of low-salt reactive dyes, machinery developments for dyeing at low liquor ratio, pad troughs with reduced volumes, re­placement of urea with dicyandiamide, econtrol process, etc are some of the approaches for ecofriendly colouration of cot­ton with reactive dyes. The uses of environment-friendly reducing systems for dyeing of cotton with vat and sulphur dyes are also briefly discussed.

Keywords: Bifunctional reactive dye, Cotton, Sulpur dye, Vat dye, Waste minimization

1 Introduction During the first half of the twentieth century, the

textile chemical industry focussed its energies and resources on product and process innovations. As a result, a phenomenal improvement in product quality was observed. Unfortunately, little attention was paid to the consequences that the introduction of new chemicals and new processes might have on the eco­logical balance of the environment. Thus, by dumping chemical effluents the eco-balance of nature was dis­turbed slowly. In recent years, the realization of the need for controlling pollution through industrial efflu­ents has grown and all efforts are being made by gov­ernments the world over to draw up or to tighten the legislation pertaining to the controls on the types and extent of pollutants that could be passed on to nature'.

The textile industry is highly water intensive and is one of the major contributors to water pollution in India. Textile processing includes sizing, desizing, scouring, bleaching, mercerization, dyeing, printing and finishing. Each process uses various chemicals that are let out into effluents.

The world-wide fibre consumption, major dye classes used for the dyeing of cotton and their esti­mated annual consumption, and the types of chemi­cals found in the effluent stream are given in Tables 1-3 respectively. It is seen from Table 1 that the con­sumption of cotton is almost half of the world's total fibre consumption and it is expected to maintain this position during the current century. The dye con­sumption pattern (Table 2) indicates that in the next

a Phone: 6591406; Fax: 0091-011-6581103; E-mail: rbchavan@hotmail.com

five years, the sulphur, direct, vats and indigo dyes will remain constant, azoics will continue to decline, and reactive dyes will show increase2

• The lack of growth of dye classes other than reactives may be due to the following factors:

• High price and limited supply in case of vat dyes. • Environmental problems associated with vat, sul­

phur and azoics. • Technological obsolescence in case of direct and

azoics.

Table 3 shows the chemicals present in effluent streams of various dye classes. The effluent streams of typical dye houses have been shown to have dye­stuffs to the extent of 10-50 mg/L with BOD and COD loads as high as 200-3000 mg/L and 500-5000 mg/L respectively' . This paper critically reviews dyeing processes for cotton, particularly with reactive dyes, with the objective of minimizing the effluent problems.

2 Reactive Dyes As shown in Table 2, the use of reactive dyes is

predicted to rise by 50% over a period of 1992-2004. Presently, most of the reactive dye manufacturers are concentrating on addressing the following environ­mental problems associated with the reactive dyes:

• Colour in the effluent. • Minimization of chemical usage.

2.1 Colour in the Emuent

Removal of colour from the effluent is one of the expensive approaches for the end of pipe technology.

94 INDIAN J. FIBRE TEXT. RES., MARCH-JUNE 2001

Table I-World-wide fibre consumption

Year Consumption (1000 tonnes) Cotton Regenerated

1990 18700 2700 (49%)

1995 20700 2500 (49%)

2000 23400 2400 (49%)

Table 2-Estimated annual consumption of cellulosic dyes

Dye

Sulphur Direct Vat Indigo Azoic Reactive Total

1988a

90000 74000 36000 12000 28000 60000 300000

Usage per annum (tonnes) 1992 20046

70000 60000 21000 12000 18000 109000 290000

70000 68000 22000 12000 13000 178000 354000

, Does not include China, India and Eastern Europe. b Figures from Business Research Service Ltd.

Therefore, it is necessary to find suitable alternatives . In exhaust dyeing, the use of reactive dyes is the ma­jor source of concern. The vat, sulphur and azoic dyes exhibit a high degree of exhaustion and the insoluble unfixed dye can readily be removed as a part of the primary flocculation process. Whereas in case of re­active dyes, sometimes as high as 30% dye remains unbound during primary treatment and needs to be treated either on site as a secondary process after biological oxidation or at a municipal sewage works. The adsorption of hydrolyzed reactive dye on biomass is not as efficient as of other water-soluble dye classes. However, the amount of hydrolyzed dye in the effluent can be minimized through process inno­vations.

Unrelenting the government pressure for regulating industrial effluent discharge has led to the develop­ment of new reactive dyes, machinery and processes to minimize waste and colour in the effluent. The de­velopment of bifunctional reactive dyes is important from this point of view.

2.1.1 Bifunctional Reactive Dyes Bifunctional reactive dyes consist of two reactive

groups capable of forming covalent bonds with the fibre. Dichloro- and difluro-chloropyrimidine dyes have also been considered bifunctional as these dyes

Wool Synthetic Total

2000 14900 39300 (39%)

2000 17300 42600 (41%)

2000 20500 48500 (42%)

Table :>----Chemicals in effluent streams

Dye

Vat

Reactive

Direct

Disperse

Acid

Metal complex

Chrome

Sulphur

Chemicals in effluent streams

Residual dyestuff (5 - 20 %) Reducing agents Oxidising agents Detergents Salt

Residual dyestuff (20 - 50 %) Salt Alkali Detergent

Residual dyestuff (5 - 20 %) Salt Dye fixing agen ts

Residual dyestuff (I - 20 %) Acid Dispersi ng agents Leveling agents Reducing agents Alkali

Residual dyestuff ( I - 20 %) Organic acid

Residual dyestuff (2 - 5 %) Organic acid Leveling agent

Organic acid Heavy metal

Residual dyestuff (30 - 40 %) Sodium sulphide Alkali Salt

have two reactive centres which may react with two si tes in the fibre3

.4. However, the reactive centres in these dyes are in such a close proximity that the reac­tion of both the centres, to a considerable extent, seems to be remote possibility, mainly because of the stearic hindrance and availability of reactive sites in the fibre at such close distances. Dyes with two sepa-

CHA V AN: ENVIRONMENT-FRIENDLY DYEING PROCESSES FOR COTTON 95

rate reactive groups can, in true sense, be called bi­functional. These bifunctional dyes can be of two types: (i) those consisting of two similar reactive groups (homobifunctional reactive dyes) and (ii) those with two different reactive groups (heterobifunctional reactive dyes). Table 4 shows various homobifunc­tional and heterobifunctional reactive dyes presently marketed by main dyestuff manufacturers5

.

ICI developed the Procion Supra range with two monochlorotriazine groups per molecule. The idea behind this approach was to produce a range of high­temperature dyes for exhaust dyeing with substan­tially increased exhaustion and fixation as compared to that of the corresponding products carrying only one triazinyl group. This approach led to better dye utilization and resulted in less hydrolyzed dye and less effluent. Later, ICI developed bifunctional Pro­cion HE dyes which also consist of two monochloro­triazine groups. These dyes have become very popular for exhaust dyeing of cellulosics due to their high de­gree of exhaustion and fixation.

In 1980, Sumitomo launched the first range of het­erobifunctional dyes which carry two different reac­tive groups under the Sumifix Supra trade mark. Su­mitix Supra dyes, prepared by combining two differ­ent reactive systems (monochlorotriazine and p­sulphoethyl sui phone), possess the advantages of both these groups6-9, namely

• The chemical bond between the vinylsulphone group and the fibre is very stable to acid hydroly­sis and hence these dyes have excellent stability of dyed goods during storage.

• The electrophilic property of the cynuric group permits selection of a wide range of chromo­phores having good fastness to light, perspiration and chlorine.

• The presence of two reactive groups of different reactivity results in dyes that are less sensitive to dyeing conditions. For instance, these dyes can be applied over a wide range of temperature (60-80°C) and accordingly the shade reproducibility is improved. This is a useful feature, particularly when the temperature distribution in a dye bath is not so uniform .

Looking into the various advantages of bifunctional reactive dyes, it is not surprizing that the reactive dye manufacturers have increasingly opted for reactive dyes with two or even more reactive groups in the dye

molecule. For example, Ciba Geigy has introduced Cibacron C range of dyes which have an aliphatic vinylsulphone group combined with either a mono­flourochlorotriazine group or an aromatic vinylsul­phone grouplD.

2.1.2 Washing orr Two or even three reactive groups in a dye mole­

cule do not guarantee 100% fixation efficiency and thus a thorough wash off procedure is essential, par­ticularly in heavy shades. Even if 99% fixation of re­active species (sometimes claimed for some reactive dyes such as Cibcron C applied by pad-batch process) occurs, there will still be a need to some form of wash off to remove hydrolyzed dye impurity present in the original standardized dyestuff. Perhaps the answer may be the design of more effective after-treatment agents to interact with the hydrolyzed dye during the finishing process, allowing the small amount of hy­drolyzed dye to be chemically immobilized in the fi­bre 2.

2.1.3 Dye Liquor Wastage Minimization of dye liquor wastage is a major en­

vironmental consideration in a continuous dyeing to reduce colour in the effluent. At the end of every dyeing, there is liquor left in the pad trough as well as in the stock tank prepared as a precaution against run­ning out before the dyeing is completed. A latest de­velopment from Monforts (Matex 3 bowl padder) has the feasibility to decrease the pad liquor volume to 15 litres as the fabric comes to the end of the run. Ramisch Kleinwefers has also developed a padder with a minimum trough volume of 10 litres.

2.2 Minimization of Chemical Usage Some of the approaches to minimize the use of

chemicals are:

• Dyeing at low liquor ratio. • Right-first-time approach. • Process innovations in continuous dyeing. • Use of low-salt reactive dyes.

2.2.1 Dyeing at Low Liquor R'ltio The average consumption of water per kilogram of

finished fabric is around 80-100 litres. Lowering of liquor ratio brings down the volume of water used and the waste generated. Apart from the easier handling of lower volume of effluent, the dosing of chemicals and auxiliaries in the dye bath is done on the basis of gllitre of liquor. This significantly reduces the quanti-

96 INDIAN 1. FIBRE TEXT. RES., MARCH-JUNE 2001

Table 4--Homo- and hetero-bifunctional reactive dyes

Reactive dye

Homobifunctional

ProciOll Supra (Zcneca)

Procion HE (Zencca) Procion H-EXL

Drimarene XN (S)

Kayacelon React (KYK)

Selected Remazol dyes (HOE)

Remazol Black B (HOE)

Hctcrobifunctional

Sumifix Supra (NSK)

Diamara SN (Mitsubishi Kasci Hocchsl)

Cibacron C (CGY)

Remazol Brilliant Red SBB

(HOE) Remazol SN/S (HOE)

Dye structure

N ~ N O-fjH~ IrNH~NH~ 'it- NH - R

No/" N..,... .. Ct (I

0 .- NH -,'N)- NH-oNH~)-NH-O N~H N~

CI (I

CHA V AN: ENVIRONMENT-FRIENDLY DYEING PROCESSES FOR COTION 97

ties of chemicals and auxiliaries and finally the efflu­ent load. In reactive and vat dyeing systems, a change of liquor ratio from 1: 10 to 1:5 brings about a de­crease in pollution load by about 40%.

2.2.2 Right-first-time Approach Carefully following the dyestuff manufacturer's

recommendations for salt, alkali usage, temperature, time, etc. ensures the optimum fixation levels and right-first-time production, thereby avoiding the need to make shading additions. The computer colour matching should help in this regard.

2.2.3 Process Innovations in Continuous Dyeing Continuous dyeing of cotton with reactive dyes is

carried out by cold pad-batch, pad-dry-thermofix, pad-steam and pad-dry-chemical pad-steam processes. These involve the use of various chemicals such as urea, soda ash , caustic soda, salt and sodium bicar­bonate which cause loading of the effluent to various extents.

2.2.3.1 Reduction in the Use of Urea In pad-dry-bake process, 200 gIL urea is used. Such

a large quantity of urea decomposes during curing, giving fowl smell. There is also an increase in nitro­gen content of effluent, affecting biological treatment. The environmental impact of urea can be reduced by replacing part of the urea with dicyandiamide (e.g. by using 60 giL urea and 15 gIL dicyandiamjde) instead of 200 gIL urea in a pad-bake process to produce sat-. f d ' II IS actory yemg .

2.2.3.2 EcontroI Process Zeneca in collaboration with Monforts has recently

developed an interesting variant to reduce the chemi­cal usage in continuous dyeing using highly reactive dichlorotriazinyl Procion MX dyes and sodium bicar­bonate. The method involves the fixation of dye under controlled relative humidity conditions of 25-30% in hot flue dryer. The dyeing is carried out continuously using pad-dry-wash sequence. The fixation is as high

h · d 12 as t at m pa -steam process

2.2.4 Low-salt Reactive Dyes Substantial quantities of electrolytes, such as so­

dium chloride and sodium sulphate, are used for the dyeing of cotton with reactive dyes. Exhaust dyeing with 10: 1 liquor-to-material ratio needs a salt concen­tration of 30-80 gIL. This corresponds to 300-800 g salt per kg of dyed cotton. During effluent treatment, only a small quantity of salt is removed. Thus, a ma-

jor quantity of salt enters the environment on dis­charge of effluent treated water. High salt concentra­tion in effluent has the fallowing disadvantages:

• Rivers and lakes get polluted with effluent con­taining high salt concentration.

• Fresh water organisms can have toxic effects. • In the regions with scarce fresh water resources,

such water has to be used for irrigation . • If salt concentration is too high, soil may become

overloaded with salt, making the land infertile and ultimately useless.

2.2.4.1 Approach to Minimize Salt Load There are following three approaches to minimize

salt load:

• Dyeing at low liquor ratio. • Using dye classes other than reactive, e.g. vat re­

quiring low salt concentration for dyeing. • Using reactive dyes which give high exhaustion

and fixation at low salt concentration.

Ciba Speciality Chemicals has introduced low-salt Cibacron LS 13 dyes which require only 20 gIL salt, whereas the conventional reactive dyes require 60-80 giL salt for exhaustion. The dyes have the general structure as shown in Fig. 1. The main features of these dyes are given below:

• Bifunctional reactive dyes have medium reactiv-ity, but high dye affinity and high dye fixation .

• Stable dye-fibre bond • Less dye to be removed from fabric after dyeing. • Rinsing step is much faster. • Less water is needed for wash off. • Less dye effluent.

As the salt requirement is reduced to 1,4 of that re­quired for conventional dyes, the saving in cost is ob­served. Another benefit is that rinsing step after dye application is much faster and less water is needed. Since the Cibacron LS dyes have a higher fixation rate than the conventional dyes (Fig. 2), this leads to lower dyestuff concentration in the effluent.

I Chromophore I v

I Cbromophore I V

Reactive Reactive group group

Fig. l--General structure of Cibacron LS dyes

98 INDIAN J. FIBRE TEXT. RES., MARCH-JUNE 2001

Conveolional dyes 60 % fix.tion

Cihacrom LS (bi~.ctlve) 80 'Yo fixation

/600 g on the fibre

I kg dye

~ 400 g inlo the wa.~1e water

8 /800 g on Ihe fibre

llcgdye

~ ~ 2008 into ~ the W8~c wet",

Fig. 2-Higher fixation rate of Cibacron LS dyes

Table 5--Ecological benefits of Cibacron LS

Property Benefit

High fixation rate Low dye concentration In

effluent

Low electrolyte concentration Low salt effluent and cost/time saving

Good wash off Less water and energy

Ecologically selected cUlling Low BOD values agents

Halogen-free chromophores No AOX in effluent

No use of amines listed in No health risk to dyers MAKIIIAIIA2 No risk of ban

High reQeatability of dyeing Less shading and striQQing

Ecological benefits of Cibacron LS dyes are sum­merized in Table 5. The manufacturer claims that Ci­bacron LS is a range of compatible dyes specially de­veloped for outstanding producti vity, repeatability and superior environment safety in exhaust dyeing. Thus, Cibacron LS dyes are an example of innovative ecological product design that contributes to a safer and healthier environment. These dyes address some urgent environmental problems simply by not creating them in the first place.

3 Alternative Reducing Systems for the Dyeing of Cotton with Vat and Sulphur Dyes

3.1 Vat Dyes Vat dyes are applied by usi ng sodium hydrosulphite

as reducing agent and sodium hydroxide as an alkali. Some byproducts of sodium hydrosulphite are acidic in nature, necessitating over dosing of sodium hy­droxide over stiochiometric requirement. A few of the byproducts formed are sulphur compounds like Na2S and NaHS etc., which pollute air through the forma­tion of H2S. At the same time, the salts of sulphur in the form of sulphate and su lphites (Na2S0), NaHS04,

Na2S04, Na2S20 3) contaminate sewage, lower its pH

and show corrosive action on concrete pipes. Other problems associated with the use of Na2S204 are its cost and low storage stabili ty l4. To overcome these problems, attempts were made by several researchers to develop alternate reducing systems which are eco­friendly in nature. Such new systems include electro­chemical reduction, use of organic reducing agents

. like hydroxy acetone, iron pentacarbonyl compounds and iron (II) complexes. In electrochemical reduction method, the dye is directly reduced by contact be­tween dye and electrode, though a reducing agent must be added to the reduced dye bath to ensure cor­responding stability of the reduced liquor.However, the dyestuff requirement to produce a specific shade is higher.Hydroxy acetone (CH3-CO-CH20H) is re­ported as a reducing agent for the dyeing of indigo and other vat dyes. This reducing system is bio­compatible and gives 20% higher indigo uptake along with less consumption of assisting chemicals. The use of iron pentacarbonyl compounds, though proposed, has not been discussed in detail 15-20.

Iron (II) salts have been widely used since ancient times to reduce vat dyes 21 through a technique known as copperas method in which FeS04 and Ca(OH) 2 are used. But the dye bath produced in this way resulted in bulky sediments due to poor solubility of Fe(OH) 2.

It has been found that Fe(OHh, produced via reaction of iron (II) salts with NaOH can be complexed and taken into solution with the help of gluconic acid to get desired reduction potential. The dye reduction and dyeing is carried out at 60°C. It is claimed that the system is environment friendly as during the neutral i­zation of the effluent, the free Fe(OHh is produced which gets converted to Fe(OH) 3 and acts as f1occu­lant. Thus, the problems associated with the use of Na2S204 as a reducing agent have been eliminated in this new technique 15. This concept of the use of iron (II) complexes is further extended by Chavan and Chakraborty based on alkanol amines as single ligand system for dyeing of cotton with indigo, and double ligand system based on alkanol amine and organic acid for vat dyeing. It is reported that such iron (II) complexes not only totally replace Na2S20 4, but the vatting and dyeing can be carried out at room tem­perature 22.

3.2 Sulphur Dyes Sodium sulphide is commonly used for the reduc­

tion in the application of sulphur dyes on cotton. Re­sidual sodium sulphide acts as contaminant in the ef­fluent. Sodium sulphide causes no marked odour nui­sance above pH 9 but in ac idic pH, gaseous H2S is

CHA V AN: ENVIRONMENT-FRIENDLY DYEING PROCESSES FOR COTTON 99

liberated,which gives giving fowl smell of rotton eggs and is toxic when inhaled. Its odour threshold value is 10 ppm.

3.2.1 Replacement of Sodium Sulphide 23,24

3.2.1.1 Sodium Hydrosulphite, Sodium Formaldehyde Sul­phoxylate and Dithioglycollic Acid

Because of the restrictions on effluent disposal , many efforts have been made to find products which would effectively reduce sulphur dyes to their sub­stantive form and yet give minimum problems at the end of dyeing. Sodium hydrosulphite used together with caustic soda or sodium carbonate is suitable for certain sulphur dyes, notably the blues. However, other sulphur dyes are partially destroyed by this re­ducing agent and do not give dyeings of correct hue or colour value. Products based on sodium formalde­hyde sulphoxylate, used together with alkali, although are stable than sodium hydrosulphite, have the same disadvantage, in that they are unsuitable for applying many sulphur dyes. Products such as dithioglycollic acid can be used effectively together with caustic soda. However, this reducing agent is so stable that although it does not give rise to inorganic sulphides in the effluent, it has high chemical oxygen demand, which, in some cases, makes it more of a problem than sodium sulphide it replaces.

3.2.1.2 Glucose Glucose has long been known as reducing agent for

sulphur dyes. Sulphur black and Indocarbon dyes were almost always reduced with glucose in the one­stage printing process . In exhaust dyeing, however, glucose at first gave unsatisfactory results, especially in open dyeing machines like jigger and winch. This was because the dye yield highly depended on tem­perature. Only at constant temperature (above 90°C), reasonable results were obtained. A considerable im­provement is achieved when the dyeing is carried out under strongly alkaline conditions with the addition of Stabilisol S liquid. In exhaust dyeing at temperature above 90°C, the results obtained with solubilized sul­phur dyes in the presence of glucose are virtually the same as those obtained with sodium sulphide. Glu­cose and caustic soda together with Satbilisol S liquid have also proved successful in continuous dyeing. Good results are obtained with all solubilized sulphur dyes. In principle, the water-insoluble sulphur dyes can also be applied using glucose, caustic soda and Stabilisol S liquid, but first they must be dissolved by boiling up.

Chavan and Yhanbatte25 obtained glucose by acid hydrolysis of molasses and cane sugar. On the basis of detailed investigations of various parameters, such as concentration of glucose, caustic soda, temperature and time, they concluded that at dyeing temperature of 90°C, glucose gives colour yield equivalent to that obtained with sodium sulphide. Century mill at Mum­bai also established, on commercial scale, that sodium sulphide can be totally replaced with glucose obtained from hydrolysis of starch.

Other sugars and related substances like hydroxy acetone have been examined in laboratory, but none of them were found to offer any advantage over glu­cose.

3.2.1.3 Mercaptoethanol Of the organic sulphur compounds studied, ~­

mercaptoethanol has aroused particular interest. It is marketed by BASF under the name Molleskal SF as a substitute for sodium sulphide in the leather industry.

Solubilized sulphur dyes can be applied using mer­captoethanol and caustic soda both by exhaust method and one-bath pad-steam process. The yields in some cases are slightly poorer than those obtained with so­dium sulphide but are generally acceptable. The ad­vantage with Molleskal SF is that there is no sulphide in the effluent and absolutely no smell during dyeing. The drawback is the high cost of dyeing.

4 Conclusions Statistics show that the world-wide consumption of

cotton is going to increase due to its comfort proper­ties, cost effectiveness and environment friendliness. Whereas among the various dye classes suitable for colouration of cotton, the use of reactive dyes is going to increase in comparison to direct, azoic, vat and sul­phur dyes. Some of the approaches adopted for eco­friendly colouration of cotton with reactive dyes in­clude: the use of bifunctional reactive dyes having high exhaustion and fixation properties, low-salt re­active dyes, machinery developments for dyeing at low liquor ratio, pad troughs with reduced volumes, replacement of urea with dicyandiamide, econtrol process, etc. Research is in progress for replacement of sodium hydrosulphite in vat dyeing and sodium sulphide in sulphur dyeing. The use of iron (II) com­plexes for vat dyes and glucose for sulphur dyes seem to be promising.

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