Cotton fabric bleaching

Textiles. Sorbitol sequesters iron and copper ions in strongly alkaline textile bleaching or scouring solutions (see Textiles). In compositions for conferring permanent wash-and-wear properties on cotton fabrics, sorbitol is a scavenger for unreacted formaldehyde (252) and a plasticizer in sod-resistant and sod-release finishes (253). 

Most cotton is bleached with peroxide, by far the greater proportion by continuous methods. Synthetic fibres seldom require bleaching, but where it is necessary either peroxide or chlorite bleaches are recommended. Most regenerated cellulosic fabrics are only bleached... 

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. 

Cotton and Cotton-Polyester. Cotton is the principal fiber bleached today, and almost all cotton is bleached. About 80-90% of all cotton and cotton-polyester fabric is bleached with hydrogen peroxide. 

The best laundry procedure for 100% cotton and 65/35 polyester/cotton fabric is to presoak the fabric for 3 hours in the washing machine filled with warm water (70 liters at 50°C) to which 280 mL of chlorine bleach has been added. After the 3 hours, empty the machine, and give the fabric a single warm wash (at 50°C). Less than 0.1% chlorpyrifos residue remains on both cotton and polyester/cotton when the bleach soak pre-treatment is used however, when a single wash, without the bleach pre-soak is used, approximately 40% of the chlorpyrifos residue remains on the cotton fabric and 28% on the polyester/cotton blend. 

In processing cotton fabric, TEX-WET 1158 will greatly improve the penetration and wetting of bleaching solutions, such as hydrogen peroxide. When used in the dyeing process, TEX-WET 1158 will completely and quickly wet-out the goods—and will stabilize the dye bath. 

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. 

Venn obtained very little levoglucosan from the pyrolysis of raw cotton, whereas washing with water increased the yield to 28 %, and further prepurification by treatment with dilute alkali and acid provided a yield of 38%. Holmes and Shaw found that a cotton fabric purified by scouring and bleaching provided more tar and less char on vacuum pyrolysis than the reference materials. A Russian patent" claims that the heat resistance of cellulose is greatly increased by lowering the content of alkali metals to 0.03 — 0.05%. 

The aim of accelerated bleaching is to adopt a continuous bleaching by reducing the bleaching time and chlorine consumption with the help of more reactive bleach liquor. However, the accelerated bleaching may lead to degradation and yellowing of the cotton fabric. 

The optimum concentration of hydrogen peroxide depends on number of factors namely liquor ratio, temperature and class of fibre. In the batch process using kiers about 2-4% (o.w.f.) hydrogen peroxide is sufficient for cotton fabrics with a liquor ratio of 10 1 to 20 1. In the continuous process, the cotton fabrics are saturated with bleach bath containing 1 -2% (o.w.f) hydrogen peroxide. Very high concentration may damage the fibre. 

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. 

Figure 10-1. Reflectance of a bleached cotton fabric (A), after adding a blue tint (B) and after adding a fluorescent brightener (C) [1].

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

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