Annexure I Textile Wet Processing and Environmental Concerns 978-981-10-6295-7/1.pdfآ  laboratory to

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  • Annexure I Textile Wet Processing and Environmental Concerns

    Increasing environment consciousness, newer legislations to preserve environment and human rights, scarcity of water and energy has driven efforts in search of safe substitutes of the dyes and auxiliaries, and in this customer-driven market, specific needs of customers such as flame retardancy, antibacterial property, stain repellency have further increased challenges to the textile manufacturers.

    These above-mentioned 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 global trade. 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 andweaving 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 bemet 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 shop floor.

    Broadly, wet processing can be divided into pretreatment, bleaching, 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 • Improved functionality • Lower generation of waste • Minimum use or reuse of water and chemicals • Overall environment friendliness • Application of appropriate machinery and technology

    © Springer Nature Singapore Pte Ltd. 2018 K.S.K. Kumar (ed.), A Study of India’s Textile Exports and Environmental Regulations, https://doi.org/10.1007/978-981-10-6295-7

    131

    This annexure has been compiled by Dr. Zareena Begum.

  • Pretreatment

    Desizing

    Nowadays enzyme desizing is the widely used technique for the removal of size from the textile material. For cotton desizing, amylase-based enzymes are widely used; generally, these enzymes works at different pH range mostly 5–7 and the temperature range of 40–70 °C. Advantage of such enzyme is that cotton waxes get partially emulsified due to the alkalinity, and the desizing is attained almost instantly due to the high temperature maintained. Enzymes such as lipases and proteases are used in wool processing, which impart antifelting property and also improved dye uptake.

    Scouring

    Incorporation of persulphate in scouring eliminates the separate desizing stage. Simultaneous desizing and scouring can be carried out by padding fabric with potassium persulphate (2–3 gL−1) and NaOH (40–50 gL−1) and subsequently steaming in J-box. Desizing can be achieved in 1–3 min and scouring takes 90– 120 min, in J-box or 2–10 min in vapour locks machines. Many companies have introduced novel enzyme product which are used for scouring, generally these enzymes are mixture of enzymes such as pectinase, lipase. The pectinase enzyme is used to hydrolyse the pectins and remove the same from cotton causing lossening of hydrophobic waxes which are easier to get rid off in subsequent washing. The action of lipase subsequently brings about hydrolysis of such hydrophobic waxes.

    Solvent-assisted scouring makes use of 5% 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 processing of cotton goods. However, complete recovery of solvent brings limitations on its use. The continuous bleaching ranges are becoming popular as scales of operations have increased although capital investment is high. The small processor makes use of modern jiggers for scouring, bleaching and dyeing in which the goods weighing up to about 1400 kg can be processed.

    Bleaching

    Hydrogen peroxide bleaching is carried out at around pH 10.5–10.7 and temper- ature 85–90 °C so scouring and beaching can be combined together. Bleaching recipe consists of 4–10 gL−1 of 35% H2O2, 2–3 gL

    −1 stabiliser and about 1–3 gL−1 NaOH (100%), and a liquor ratio 8:1 is maintained on Jigger. In case of

    132 Annexure I

  • continuous bleaching on J-box, the recipe indicates 20 ml/L of H2O2 (35%), 4 gL−1 NaOH (100%), and 6 ml/L of sodium silicate at 90–95 °C for 15–30 min.

    From the environmental point of view and also from the point of view of permanent whiteness, chlorine-based bleaching agents are objectionable. The goods for exports are thus required to be peroxide-based which is relatively an expensive process. The general trend is to carry out initial bleaching with hypochlorite, and instead of giving antichlor treatment, the subsequent process is hydrogen peroxide bleaching of relatively lower concentration. In this case, not only the final whiteness is ensured to be permanent but objectionable chlorine-based bleaching agent can be avoided with better economy. From the point of view of exports, hydrogen peroxide has no substitute as far as bleaching is concerned. However, it is to be noted that the peroxide remains on the fabric even after thorough washing which poses specific problems in getting consistent dyeing with the dyes sensitive to oxidation such as reactive dyes. And hence, many a times the peroxide bleached goods necessitate initial rinsing of the fabric with the reducing agents more known as peroxide killers. The peroxide bleaching is also sensitive to metal ions, which initiates the accel- erated decomposition of H2O2 causing uneven bleaching which will reflect in patchy dyeing in the subsequent dyeing process. Nowadays, zeolite and bentonites which are clay materials are also used as stabilisers although they are not much in use. Bleaching with peracetic acid has been reported as eco-friendly process in which case it is produced using hydrogen peroxide and glacial acetic acid.

    However, it is to be carried out at 50–80 °C at 6–7 pH for about 20–60 min, and many a time, it is used as substitute for hypochlorite bleaching. From the envi- ronment point of view, bleaching with ozone is recommended. Ozone–oxygen mixture is highly unstable liberating hydroxy radicals giving good degree of whiteness in a short time at room temperature, the absence of harmful chemicals and use of low quantity of water are promising factor from environment point of view.

    Ozone bleaching operation could be made continuous too. Potassium perman- ganate is also used as powerful oxidising bleaching agent and said to be economic and non-polluting. One of the reported methods makes use of electrolysis of sodium chloride solution in which chlorine is liberated at the cathode which is used for bleaching. One-step scouring and bleaching process makes use of grey fabric or desized fabric to be treated with caustic soda/soda ash, stabiliser and hydrogen peroxide at 80 °C for 4–6 h. This incurs high chemical cost due to higher con- centration of H2O2.

    Mercerisation

    Mercerisation improves lustre, enhances dyeability and improves the strength and also soft feel. For cotton fabrics 50–54 Tw (22.5%), sodium hydroxide solution is used at room temperature for 45–60 s 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. Wet-on-wet mercerisation

    Annexure I 133

  • and hot mercerisation are the additional two processes. In the latter, 60 °C tem- perature 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 more preferred.

    Dyeing

    Once the fabric undergoes controlled pretreatment and brought in a ready-to-dye stage, the dyeing becomes the next challenge to get uniform and consistent shades right at first attempt. 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 quality of fabric being processed as well as quality standards required. Number of challenges faced by the dyer includes shade matching, production of solid and defect-free dyeing. The advancement in reactive and disperse dye classes is apparent in the market. Developments in reactive dyes are aimed at improving their exhaustion and fixation by improving the reactivity and decreasing their sensitivity to temperature and pH variations. Dyes showing high exhaustion are thus recom- mended for dyeing of knit goods, which is carried on winches where a lot of variation in temperature and pH occur. The reactive dyes which make use of low salt and some based on fluorine are increasingly recommended. In India, the dyes which are extensively used are as follows.

    The newly developed TULAREVS XL range, a state-of-the-art reactive system, manufactured using innovative membrane technology, to meet stringent fastness requirements (like that of M&S fastne