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Glimpses of Indian textile wet processing scenario -I

Dr M D Teli

A number of factors are responsible for the present day change in scenario of the wet processing of textiles. Good quality at competitive prices is becoming important with trade globalisation. Hence, the wet processing sequence is accordingly being modified, keeping in mind the objective of obtaining high quality goods at the most competitive rates.

For international quality fabric, 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 a challenging task to get the shades right first time and also at competitive cost with the best quality. The high performance standards expected by the present day consumer are to be met with, in addition to the adherence to the norms of eco-friendly processing, which is the need of the day. All these characteristics navigate 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 large difference in the process sequence to be ideally followed vis-a-vis the one actually followed at the shopfloor. Otherwise, eg, how do you explain high twisting of microdenier yarn and weight reducing the fabric made out of it with causticisation process? There are many such examples.

Broadly, wet processing can be divided into pretreatment, dyeing, printing and finishing. Irrespective of the various stages of operations falling under this classification, every single method followed finds its basis and objective of:

• 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

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 40-55ø, 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

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.

To be continued

(The author is head, department of fibres and textile processing technology University Institute of Chemical Technology, University of Mumbai, Matunga,India. website: textileudct@yahoo.com)