Desizing

If singeing can be considered as the last operation of preparations before chemical processes, desizing is the first one of preparatory processes necessary for dyeing and printing. Sizing materials are applied to yarns, particularly warp yarns, before they are woven into cloth. These form a protective coating over the yarns and keep them from chafing or breaking during weaving. It is usually not necessary to apply sizing to yarns that are used in knitting.

Desizing is the process of removing the size materials from the warp yarns in fabrics. Most of sizes must be removed before the fabric can be dyed satisfactorily. Residual size prevents the yarns and fibers from wetting quickly and can affect dye absorption in either batch or continuous dyeing. Most synthetic sizes are water soluble by design so that they can be easily washed from the fabric by a hot water or in the scouring process. Typical synthetic sizes are polyvinyl alcohol, acrylic copolymers and carboxmethy cellulose.

Starch is also common in size formulations .Starch is not very soluble in water and must be chemically degraded in order to remove it from the fabric. Starch used in sizing is often modified to improve its properties and removability in desizing. Oils and waxes are also often used with sizing agents to increase the softness and pliabilty of the yarns .However, lubricants added to size formulations to enhance the fabric manufacturing process may be more difficult to be removed than the size itself. Virtually, all size formulations contain lubricants derived from natural fats and waxes. And complete removal of these lubricants is required before the fabric can be dyed. The desizing step removes mostly size and not much lubricant. Most of the lubricant is removed in the scouring process.

Fabrics containing only water-soluble sizes can be desized by using hot water perhaps containing wetting agents and a mild alkali. Fabrics that contain starch are usually desized with enzymes. Enzymes are complex organic substances that catalyze chemical reactions in biological processes. They are formed in the living cells of plants and animals. A particular enzymes catalyzes a very specific reaction. An enzyme is usually named by the kind of substance degraded in the reaction it catalyzes. Thus, enzymes which hydrolyze an reduce the molecular weight of starch are called amylases because they hydrolyze the amylose and amylopection molecules in starch. The hydrolysis of starch catalyzed by amylaes enzymes produces starch fragments which are soluble enough to be wasted from the fabric .Cotton and rayon yarns are most often sized with water-insoluble starches. Enzymes are used to break the starch into smaller water-soluble molecules, these degradation products are then easily removed by washing before the cloth is scoured, this is a necessary step because some of the starch degradation products in the presence of the sodium hydroxide used scouring form strong reducing agents that can cause colour changes in dyed materials.