Page 41 - Basic Principles of Textile Coloration

30 FIBRES AND TEXTILES: PROPERTIES AND PROCESSING

Table 1.2 shows a typical production sequence for manufacture of a cotton
jersey. A typical preparation sequence for a woven cotton fabric might include
singeing, desizing, kier boiling (alkali-boiling), bleaching and possibly
mercerisation (Section 5.4.6). All these processes remove unwanted material from
the cotton and improve its quality. In singeing, the fabric rapidly passes through a
gas flame. This burns off any short fibres projecting from the fabric surface that
abrasion has lifted from the yarns during weaving. The result is a much smoother
fabric surface. The next step for most woven fabrics is desizing. Various types of
starch are used for sizing cotton warps to reinforce them. Washing alone does not
effectively dissolve starch sizes but digestion with appropriate enzymes will
eliminate them.

The sizing washed out of a woven fabric is often discarded, so it must be cheap
and have minimal environmental impact. Washing liquors from desizing contain
substantial quantities of organic chemicals that cause an important pollution
problem. They are mostly biodegradable, but this consumes considerable amounts
of dissolved oxygen from the water, which adversely affects the local ecosystem.
Increasing government requirements for effluent treatment, to prevent high values
of this ‘biological oxygen demand’ (Section 8.5.1), have spurred developments in
water treatment, and in the use of sizes that can be recovered from the washing
liquors and re-used. For cotton/polyester warps, it is now common to use polyvinyl
alcohol (1) for sizing. This is a water-soluble polymer removed by simple washing
in hot water and then possibly recovered by hyperfiltration membrane techniques
[5]. Newer no- or low-wetting processes include solvent desizing, sizing with
molten, water-soluble polymers, and the application of size in foams.

In kier boiling (alkali-boiling) of cotton materials, the cotton is heated with
dilute NaOH solution under pressure for several hours, in the absence of oxygen
(Section 5.4.3). Small quantities of detergents and solvents are frequently added.
The objective is to eliminate the waxy cotton cuticle, the natural oils, waxes and
proteins, as well as various salts and soluble cellulose and carbohydrate derivatives.
It also helps to remove any size residues. The removal of the hydrophobic
constituents of the cotton fibres renders them much more absorbent. In many
modern plants, much faster, continuous processes of this type use higher
concentrations of NaOH. They are, however, less efficient than the classical kier
process because of the short treatment times.

The aim of bleaching is to remove any unwanted colour from the fibres. This
may be the grey or yellow tinge of a natural fibre, or it may be a consequence of
discoloration from the manufacturing processes. Bleaching is essential if high