Textile Dyes

INTRODUCTION Textile industry is a mother industry upon which millions of people are dependent. Out of the various sectors of textile's production, the processing segment is one of the weak sectors although weaving and garmenting also fall in the same category. Share of Indian textiles and clothing production is 3.3% of the world production and the world textile export has been growing at the rate of 5.58% per annum and clothing export 6.44% per annum. The overall textile and clothing export growth rate is around 6% per annum. We can say, the finished products are being more welcomed than the unfinished textiles. From the business strategy point of view too, the maximum value addition is possible when we go for supplying finished material as RMG fabric or the Garments. Due to the increase in the demand for clothing, the processed goods are now in large demand. CHANGE IN TRENDS There are lots of factors, which are responsible for the present day change in scenario of the wet processing of textiles. Good quality at competitive prices is becoming the key factor with trade globalisation. Hence, the wet processing sequence is accordingly being changed, keeping in mind the end objective of obtaining high quality goods at the most competitive prices. To get the best quality fabric of international standards, one needs to have the best of spinning and weaving technologies employed, before subjecting the fabric to various operations of wet processing. However, for a textile wet processor, it becomes the most challenging task to get the right shades at first time and also at competitive cost with the best quality. The high performance standards expected by the consumer are to be met with, in addition to the adherence to the norms of eco-friendly processing, which is the need today. All these characteristics lead the direction of research in textile wet processing and the various operations actually carried out in the processing industry. Many a times, it has been observed that there is a huge difference in the process sequence to be ideally followed vis-à-vis the one actually practiced at the shop floor. Otherwise, e.g., how do you explain high twisting of micro denier yarn and weight reducing the fabric made out of it with cauterization process? There are many other examples. Wet processing can be broadly divided into pretreatment, dyeing, printing and finishing. Irrespective of the various stages of operations falling under the above-mentioned classification, every single method followed finds its basis and objective of the following: • Maintenance of high quality • Cost effectiveness • Better performance • Lower effluents and minimum input cost • Minimum use of chemicals • Environment friendly process • Application of latest machinery which can guarantee the reproducibility of the product once approved. PRETREATMENT Degumming/Desizing Although, acid desizing, rot steeping and enzymatic desizing are quite well accepted methods of desizing the fabric containing starch based sizes, different types of amylase base enzymes are available in the market and as its consumption as well as production is increasing day by day, the cost of enzymes is coming down and the processors thus been able to make use of such enzymes, which are

quite specific in action with respect to concentration, pH, temperature and do not pose any danger of fiber degradation. In pretreatment, either the yarn or the fabric requires to be treated with such chemicals that subsequently are turned to be receptive to the dyes and chemicals as well as finishes. In case of silk, most of the quality goods manufacturers degum the silk in the hank form before subjecting it for bleaching and then dyeing. Since, they intend to use these yarns for woven designs; the treatment in the hank form as well as its colouration becomes handy. Besides the age-old method of degumming of silk using Marsellies soap or sometimes 501 soap, nowadays enzyme like Degummase is used to remove the sericin, the cementing agent of the silk fibroin. Many also prefer to remove the sericin to a limited extent, so that the strength of silk yarn is kept intact. Companies like Himatsingka exporting almost 100 per cent of their products - upholstery and curtain materials, have found an enviable position in this business. The Central Silk Board as well as the ministry of textiles are working on improving the quality of silk and training silk reelers so that India can not only increase the silk yield but also improve its share in the silk trade. Enzymes such as proteases are also used in wool processing. Not only are the scales removed, imparting anti-felting property, but the wool fibre is found to give improved dye uptake. Cotton fabric is normally subjected to desizing and a number of research methods talk about desizing with hydrolytic and oxidative desizing agents, plasma treatment, etc. The rot steeping, acid desizing and enzymatic desizing are the conventional hydrolytic desizing techniques, which are widely followed. However, the recent trend is to use the size paste based on acrylic and PVA formulations which are easily washable and the quantity of starch in the size paste is greatly reduced so that BOD in the effluent is accordingly diminished. Different types of enzymes mainly "amylase based" are in the market, which being concentration specific, pH specific and temperature specific, are most satisfactorily used. Some of the enzymes require sodium and potassium chloride to increase their activities, whereas the heavy metal ions such as mercury, copper, zinc, iron are strong inhibitors for the action of enzymes of amylase type. Although the enzymes - pancreatic, bacterial and malt type work in the range of 6-7, 5-7 and 4.6 - 5.2 pH range and the temperature 4055ø, 60-70ø and 40-50øC respectively, special desizing agent such as Finogene DH 250 N, (Korean company) works in the range of pH 5-9 and the advantage of such enzyme is that the cotton waxes get partially emulsified due to the alkalinity, which can be removed in subsequent operations. Thermozyme, an enzyme stable at high temperature is known to bring about desizing almost instantly at higher temperature. Oxidising type of desizing agents are energy saving, acting on all the sizes and offer shortening of the process sequence. Sodium bromite and potassium persulphate are quite important oxidative agents. Scouring Although soda boil is so far the best method of scouring, the efforts are on to make use of pectinase and lipase together as one shot chemical in which case, it is understood that while pectinase will loosen the hydrophobic waxes due to the hydrolysis of pectins and the lipase can bring about the hydrolysis of waxes.The persulphate could be incorporated in the scouring process, eliminating separate desizing stages. Simultaneous desizing and scouring can be carried out by padding the cloth in 2-3 gpl potassium persulphate and 40-50 gpl NaOH and subsequently steaming in J box. While desizing can be achieved in 1-3 mins, scouring takes 90 to 120 mins, in J box or 2-10 minutes in vapour locks machines. Of all these methods, acid steeping, enzyme and bromite desizing and simultaneous scouring and desizing with persulphate are the cheapest as far as cost considerations are concerned. As far as scouring operation is concerned, although a number of attempts have been made at scouring the fabric using Lipase type of enzymes to hydrolyse the triglycerides which are hydrophobic in nature, there has been no substitute really worth mentioning to the soda boil, which can give expected absorbency.

The pectinase enzyme is used to hydrolyse the pectins and remove the same from cotton causing loosening of the hydrophobic waxes which are easier to get rid of in subsequent washing. The action of lipase subsequently brings about hydrolysis of such hydrophobic waxes and thus nowadays, the pectinase and lipase are getting increasing importance in the process of scouring. One research study indicates the pretreatment of the fabric with chloroform followed by pectinase treatment. But one can understand the limitation of application as chloroform is used here to dissolve non-cellulosic impurities. Solvent assisted scouring makes use of five per cent solvent, which is emulsified and also can be coupled with bleaching and desizing. In this case the hydrogen peroxide not only acts as bleaching agent but also as an oxidative desizing agent. In other words, alkaline scouring till today remains the most suitable way of scouring. Solvent scouring is of theoretical importance so far as cotton is concerned. However, in case of silk and wool, solvent such as perchloroethylene is used, provided sophisticated machinery is available for total recycling of the solvent. The carbonisation of wool after solvent treatment is done with H2SO4. Kier and J boxes are used for scouring of cotton fabric with soda boil at 4-5 per cent on the weight of the fabric with wetting agent upto 3 gpl. Sodium sulphite (about 1%) brings down the scouring time in the pressure boil which otherwise requires 8-12 hours at 30 PSI (1.5-2 Kgs/Cm2). Open width scouring though initially was carried out on the jiggers, now pad roll process can also be used. The recently introduced continuous bleaching range of Benninger is becoming popular although capital investment is high. The small processing units can make use of modern jiggers for scouring, bleaching and dyeing in which the goods weighing upto about 1,400 kg (3,000 mts bottom weight) can be processed. Key points Increasing environment consciousness leading to eco-friendly textile processing has driven our R&D efforts in search of safe substitutes for the dyes and finishes. The specific needs of the customers such as flame-retardancy or anti-bacterial properties have further created challenges in this field. A lot of dynamism is thus going to be apparent in the coming years with efforts to improve market shares. Mercerisation improves lustre, enhances dyeability, and improves the strength and also the soft feel. For the cotton fabrics 50 to 54oTw (22.5%) sodium hydroxide solution is used at room temperature for 45 to 60 seconds in which case caustic pick up varies from 25 to 40%. Normally 10% of the caustic is consumed in the operation on the weight of the fabric and 75% of the caustic is recovered. For the viscose rayon 8 - 10% caustic soda is to be used the parameters of mercerisation followed by scouring and washing should be so adjusted that the final fabric should be almost neutral. Wet on Wet mercerisation and hot mercerisation are the additional two processes. In the latter 60oC temperature is used causing the even effect on the skin and core of the fabric and the better dyeability can thus be obtained. Hot mercerisation can make use of shorter machine. The efficiency of such mercerisation operation can reflect in barium activity number or deconvolution count, the former method is however preferred to latter. DYEING The proper selection of dyes and machines as well as the dyeing aids are the key factors to get right first time products. The shade matching, the production of solid shades and defect free dyeing are the day to day challenges faced by the dyers. Once the fabric undergoes controlled pretreatments and brought in a ready to dye stage, the dyeing becomes the next challenge to get uniform and consistent shades right at the first attempt (RFT). The proper selection of dyes and machine depends upon number of factors. The first and the topmost is the nature of the fibre, the variety and the quantity of fabric being processed as well as quality standards required. Number of challenges faced by the dyer includes shade matching, production of solid shade and defect free dyeing. With the general knowledge of the

nature of the dyestuff and performance properties as well as their applicability on various fibres, the selection of a given class of dye is not a big problem. However, depending upon structure of individual dyes, its chormophores and auxochromes, hues and substantivity differ. e.g. as a class, reactive dyes react with the cellulosic fibres and thus said to be giving vary good performance properties. But they suffer many a times with poor extent of fixation, problem of hydrolysis, sensitivity to metal ions, poor chlorine bleach fastness etc. The use of salt in reactive dyeing further adds TDS to the effluents. The advancement in this class of dyes are thus aimed at improving their exhaustion and fixation by improving the reactivity and decreasing their sensitivity to temperature and pH variations. HE and ME dyes showing high exhaustion are thus recommended for dyeing of Knit goods, which is carried out on winches where a lot of variation in temperature and pH occur. While the former is homo bi-reactive system, the latter one is hetero bi-reactive system and their fixation claimed to be as high as 85%. The reactive dyes of LS type which make use of low salt and some based on fluorine are increasingly recommended. While dyeing the compound shades, the rate of dyeing as well as compatibility of dyes have to be taken into consideration so that the shades do not go off the tone. The vat dyes are the costliest class of dyes, although they offer best of performance properties and many a times in polyester/cotton blends, disperse- reactive and disperse-vat are recommended. In such blends selection of disperse dyes is also equally important which are stable to alkali so that, once the polyester component is dyed, the vat pigment mechanically deposited is reduced in alkaline blank vat solution. While the vat dye thus gets fixed on the cotton component the additional reduction clearing treatment is totally eliminated in this case. The recent trend is also to cationise cotton or the cellulosic material and apply the reactive dyes in acidic conditions so that the protonated or cationised cotton adsorbs the reactive dye anions at the enhanced rate and subsequent fixation in alkaline medium gives very high shade build up. Number of quaternary amines and cationising agents are used towards this purpose and in many cases reactive dye systems can eliminate the use of salt. This not only reduces, the TDS of the effluent, but also drastically reduces unfixed dye being drained out in the effluent. Polyester fibre dominating the apparel scene, it is but natural that one chooses Disperse dyes. While they are being selected, one needs to take into consideration their energy class, diffusion number, migration rating etc. The dyeing is based on diffusion-controlled mechanism and thus for the compound shades, it is highly necessary that these dyes are selected from the same subclass so that in general the performance properties and shade build up are not drastically affected. Serilene brand of VX dyes of Yorkshire chemicals, UK offers most compatible range of disperse dyes in which case shade prediction and matching become relatively easy and their edge in performance can be apparent from pale to dark shades. Now a days, trend is to shorten dyeing cycles and many a times one bath dyeing of the binary and ternary blends is attempted. Disperse dyes applicable in alkaline conditions are also offered by some of the manufacturers which are most suitable in polyester cotton blends where the cotton component requires alkalinity for application of reactive, vat or sulphur dyes. One of the methods developed makes use of simultaneous disperse-acid dyeing and finishing of polyester/cotton bland .Initially the method was developed for one bath acid dyeing -resin finishing of cotton/cellulosic material. The selection of acid dye however is quite critical and it should have abstractable-H in its amino group substituent .Finishing agent like DMDHEU gets attracted to cotton to one end while anchoring or holding acid dye molecules on the other end. Dyeing was quite fast and so called non substantive acid dyes can be applied on cotton simultaneously resulting in improvement in crease recovery angles. When the same system is extended to polyester/cotton blend for disperse-acid-dyeing-cum resin finishing from single bath by pad - thermosol technique, it was found that polyester and cotton component can be dyed with respective dyes, and heat-setting as well as finishing take place almost simultaneously. Washing- off problem in reactive dyes is always a challenging task which impairs the fastness properties of the dyes if hydrolysed dye is not fully washed out. Though non-ionic detergent is

recommended for the same, the recently introduced Bio rinsing making use of Bylase RP / Assist RP, the enzyme which decolorises selectively the hydrolysed dyestuff from the fabric and also the unfixed reactive dyestuff in unexhausted bath provides a great promise. This reduces the consumption of water, energy and time as it is claimed to be quite efficient washing- off process. The use of cationic dyefixing agent is quite common in the case of direct dyed material which is obvious as they have very high solubility in water imparting poor wash fastness properties. However, due to the presence of unfixed hydrolysed dye on the fabric as a result of incomplete washing-off, it becomes essential even in case of the reactive dyed material, to subject it for the treatment of cationic dye fixing agent. The hydrolysed dye acting as a direct dye being held on the fabric by hydrogen bonding, complexes with cationic dye fixing agent and brings about fixation of dye, improving its wet rubbing fastness at least by half a grade. In case of garment dyeing, in case of the post finishing process, reactive dyes can be used provided the garments so finished are already subjected during finishing with certain nitrogen containing additives. These additives used during finishing render the dyeability to the finished garment and upto about 85% dyeability can be regained even after finishing the garment at the intermediate level, which otherwise is known for displaying only 15-20% dyeability. This phenomenon of additives improving the finished goods dyeability works similar to process of Cationisation and requires almost similar conditions as used in dyeing of cationised cotton. From the environment point of view the general trend is to make use of non-dusting powders of the dye and thus granules are preferred to dusting powders. Higher concentrates of dyes are also used to decrease the volume being handled. As the automation is increasingly accepted and high quality standards become pivotal part of the business, the most modern machines with auto dispensing systems are coming in use .To cater to such modern machine with colour kitchen and auto dispensing systems, liquid dyes become an important and essential formulation. Presently the Dispersol and Procion T dyes are supplied in liquid formulation and so also some of the soluble sulphur dyes. However, very soon most of the dyestuffs would be required to be supplied in liquid formulation, so that most modern automatic and microprocessor based systems of dyeing can be made use of, especially in which case the shade reproducibility is of utmost importance. The Procion T dyes are acid fixable liquid reactive dyes based on phosphoric acid condensation products and they also carry the potential of making the fabric flame retardant while being the integral part of the fabric. To continoue to read more articles on Textile, Fashion, Apparel, Technology, Retail and General please visit www.fibre2fashion.com. If you wish to download/republish the above article to your website or newsletters then please include the "Article Source”. Also, you have to make it hyperlinked to our site. Fibre2fashion has emerged as a distinctive B2B platform for global Textile, Apparel, Fashion and Retail and allied industries. Fibre2fashion.com offers business solutions, news, articles and information that help to survive and sustain in the most hostile and competitive business environment. Article Source: http://EzineArticles.com/?expert=Gaurav_Doshi