Bleaching of Cotton Fabrics

Cellulase-producing bacteria include CelMomonas, Clostridium, Pseudomonas, Streptomyces, Thermonospora, and Ruminococcus, but bacterial cellulases digest cotton fibers to lower levels compared to amorphous celluloses (Ray et al., 2006). Cellulases have two functionally distinct domains in their structure, namely catalytic domain and cellulose substrate binding domain, linked by an interdomain, glycosylated linker peptide at either the N or the C terminal of the protein cellulose binding domains exhibit different affinities, specificities, some binding to crystalline cellulose, while others restrict themselves to the disordered regions. Catalytic domain has an active site in the shape of a tunnel or cleft where hydrolytic reactions take place (Cavedon et al., 1990 Lee et al., 1996). 

Structure of cellulose and catalysis of cellulase enzyme on cotton structure. 

Conventional bleaching methods have been reviewed in many occasions, (Menezes and Chaudhari, 2005 Maekawa et al., 2007), oxidative chemical pretreatments are effective in degrading 

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Maekawa, M., Hashimoto, A., and Tahara, M. (2007). Effects of pH in hydrogen peroxide bleaching of cotton fabrics pretreated with ferrous sulphate.. Text Res. L 77(4), 222-226. 

Spicka, N., Forte-TavJer, P, 2013a. New combined bioscouring and bio-bleaching of cotton fabrics. Mater. Technol. 47 (4), 409 12. 

It has also been reported that spent dyebath water can be re-used by decolouri-sation with ozone. Such decolourised liquors could be successfully used in the dyeing of polyester. The dyeing of cotton fabric with reactive dyes can be carried out using a modified dyeing method. Ozonated dyebaths can be re-used for the bleaching of cotton fabric and even for fabric whitening and rinsing, provided the effluent is extensively decolourised. 

Buschle-Diller G, Yang X D, Yamamoto R (2001), Enzymatic bleaching of cotton fabric with glucose oxidase . Text. Res. J., 71(5), 388-394. 

Scouring and bleaching of cotton fabrics are also attractive targets for enzyme-based processes, due to the severe environmental impact of these processes. These processes are also very energy demanding. Raw cotton contains about 10% of impurities. Pectins, waxes and coloured components can all be partially removed from raw cotton by enzymatic treatments but the residual seed coating remains a problem. 

The bleached woven cotton fabric of 98 gm" density was cut into equal-sized square pieces of 15 mmxl5 mm. The samples were immersed in a colloidal solution bath for 1 min, then squeezed thoroughly and dried at 60°C for 5 min. 

Anionic softener for fabric finishing can be combined with all types of filling agents softener with stabilizing effects in peroxide bleaching of cotton yarns. 

PLEX HT is a blend of chelating agents which has the ability to dissolve iron rust from cotton and cotton blend fabrics in strong caustic solutions such as are used in saturation of cotton fabric prior to continuous peroxide bleaching. It also has the ability to keep the chelated iron from redeposition as the pH drops during subsequent rinsing. 

A comparison of typical bleaching recipes of cotton fabric for batch bleaching in different equipment with hydrogen peroxide is summerised in Table 7.1. When making-up a bleaching bath, naturally the quality of cotton, degree of pre-treatment, liquor ratio, equipment used and temperature must all be taken into account. 

Mercerization improves dimensional stability of cotton woven fabrics [63]. When knitted fabrics are compared with respect to their relative openness, temperature increases can be said to improve mercerization because when the goods are bleached and then mercerized, the fabric becomes more dense. However, when unbleached fabrics are mercerized, the fabrics become more open [64]. Mercerization also gives moderate improvement in crease recovery of cotton fabrics [65] as well as some protection against the decrease in tensile strength caused by easy-care finishing. 

Glanair. [Bruce Chem.] Liq. rqrlace-ment for sodium tripolyphmphate in scouring and bleaching of cotton and polyester/cotton fabrics. 

Ibrahim, N. A., Abd Allah, S. Z., Hassan, T. M., and Borham, H. A. T. (2005). Economical and ecological bio-treatment/half bleaching of cotton-containing knit fabrics on industrial scale, Poivm. Plastic... 

Peracetic acid (Steiner N, 1995, Evaluation of peracetic acid as an environmentally friendly alternative for hypochorite . Textile Chemist Colorist 27 (8), pp. 29-32) as well as a waterless bleaching system that uses oxygen gas (Mowbray J 2008, Light fantastic , Ecotextile News No. 17 Aug/ Sep, pp. 22-24) have been reported as alternate and enviromnent-friendly methods for bleaching cotton. Ozone also can be used as another waterless process to bleach cotton. Bleaching removes residual impiuities and changes the natural color of cotton fabric to clear white rather than the off-white (i.e. various shades of yellow). 

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