Environment-friendly dyeing processes for cotton

Transcription

1 Indian Journal of Fibre & Textile Research Vol 26, March-June 2001, pp Environment-friendly dyeing processes for cotton R B Chavana Department of Textile Technology, Indian Institute of Technology, Hauz Khas, New Delhi , 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: ; Fax: ; 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 increase 2 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, sulphur 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 mg/l with BOD and COD loads as high as mg/l and 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 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.

2 94 INDIAN J. FIBRE TEXT. RES., MARCH-JUNE 2001 Table I-World-wide fibre consumption Year Cotton Consumption (1000 tonnes) Regenerated Wool Synthetic Total (49%) (49%) (49%) (39%) (41%) (42%) Dye Table 2-Estimated annual consumption of cellulosic dyes Sulphur Direct Vat Indigo Azoic Reactive Total 1988 a Usage per annum (tonnes) , 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 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 Dye Vat Reactive Direct Disperse Acid Metal complex Chrome Sulphur Table :>----Chemicals in effluent streams 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 sites in the fibre 3.4. 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-

3 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 manufacturers 5. 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 p sulphoethyl 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 Washing orr Two or even three reactive groups in a dye molecule 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 Dye Liquor Wastage Minimization of dye liquor wastage is a major environmental 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 Dyeing at Low Liquor R'ltio The average consumption of water per kilogram of finished fabric is around 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-

4 96 INDIAN 1. FIBRE TEXT. RES., MARCH-JUNE 2001 Table 4--Homo- and hetero-bifunctional reactive dyes Reactive dye Dye structure Homobifunctional ProciOll Supra (Zcneca) Procion HE (Zencca) Procion H-EXL 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 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)

5 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% 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 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 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- IS. f actory d yemg ' II 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 as t h at m pa d -steam process Low-salt Reactive Dyes Substantial quantities of electrolytes, such as sodium 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 gil. This corresponds to g salt per kg of dyed cotton. During effluent treatment, only a small quantity of salt is removed. Thus, a major 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 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 requiring 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 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 reactivity, 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 Reactive group V Reactive group I Cbromophore I Fig. l--general structure of Cibacron LS dyes

6 98 INDIAN J. FIBRE TEXT. RES., MARCH-JUNE 2001 I kg dye Property Conveolional dyes 60 % fix.tion /600 g on the fibre ~ 400 g inlo the wa.~1e water 8 /800 Cihacrom LS (bi~.ctlve) 80 'Yo fixation 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 Benefit High fixation rate Low dye concentration In effluent Low electrolyte concentration Low salt effluent and cost/time saving Good wash off Ecologically selected culling agents Halogen-free chromophores No use of amines listed in MAKIIIAIIA2 High reqeatability of dyeing Less water and energy Low BOD values No AOX in effluent No health risk to dyers No risk of ban 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), NaHS0 4, 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 stability 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 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 neutralization 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 Sulphur Dyes Sodium sulphide is commonly used for the reduction 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 acidic ph, gaseous H 2 S is

7 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 Replacement of Sodium Sulphide 23, 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 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 Yhanbatte 25 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 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. References I Vaidyanathan T, Joshi S B & Pai K P, Asia Pacific Tech Monitor, 15 (1998) 9. 2 Phillips D, J Soc Dyers Colour, 112 (1996) 183.

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