Page 92 - Basic Principles of Textile Coloration

COTTON PROCESSING 81

same basic principles apply. For bulk production of woven goods, fully continuous
preparation of open-width and rope forms is common. A woven cotton fabric may
undergo singeing, desizing, alkali boiling and bleaching treatments, followed
possibly by mercerisation, before it is ready for dyeing (Section 2.4). Knitted
materials do not require singeing or desizing. Because knitted fabric stretches
easily, handling with minimum tension is essential to avoid excessive elongation.
There are therefore many different types of equipment. For more specialised, low
volume materials, and in commission dyehouses that dye only small lots,
preparation is often a batch process carried out in the dyeing machine.

5.4.2 Preliminary processes for woven cotton fabric

The first process in the preparation of woven cotton materials is singeing. The full-
width fabric passes rapidly through a flame issuing from a long gas burner. This
burns off the short fibre ends projecting from the fabric surface without any
damage to the fabric. These exposed fibres arise from the abrasion of the yarns
during weaving. The result is a much smoother fabric surface.

The fabric is then ready for desizing, the removal of the sizing material covering
the warp yarns [7]. Warp size consists of a film-forming polymer and hydrophobic
lubricants; its effective removal is essential for rapid and uniform wetting of the
cotton fibres. A sizing formulation must coat each warp yarn with a flexible
polymer film and lubricants, and good adhesion of the film to the yarn surface is
vital. Starch-based products are widely used for sizing cotton warps, since their
chemical similarity to cellulose ensures good film adhesion. However, although
starches have excellent sizing characteristics and low price, they are often difficult
to remove by simple washing. The fabric is therefore usually treated with a
suspension of an enzyme capable of depolymerising the starch molecules to the
point at which they become water-soluble.

Like cellulose, the terminal glucose units of the starch have a reducing action.
Unless the starch is effectively removed from the cotton material, it will reduce
the oxidising agent used in bleaching, increasing its consumption.

Because of the sensitivity of enzymatic systems, desizing proceeds at a
temperature and pH where the enzyme has high activity. The fabric is then well
washed to remove the starch degradation products. Alternatively, the starch size
may be degraded by oxidation with hydrogen peroxide in alkaline solution.

It is now common practice to size warps of cotton/polyester with polyvinyl
alcohol. This polymer is water-soluble and easily removed by simple washing in hot