Desizing is the process of removing the size material from the warp yarns in woven fabrics. Sizing agents are selected on the basis of type of fabric, environmental friendliness, ease of removal, cost considerations, effluent treatment, etc.
Natural Sizing Agents
Natural sizing agents are based on natural substances and their derivatives:
- Starch and starch derivatives; native starch, degradation starch and chemical modified starch products
- Cellulosic derivatives; carboxymethlycellulose (CMC), methylcellulose and oxyethlycellulose
- Protein-based starches; glue, gelatin, albumen
Synthetic sizing agents
- Modified polyesters
- Polyvinyl alcohols(PVA)
- Styrol/maleic acid copolymers
Desizing, irrespective of what the desizing agent is, involves impregnation of the fabric with the desizing agent, allowing the desizing agent to degrade or solubilise the size material, and finally to wash out the degradation products. The major desizing processes are:
- Enzymatic desizing of starches on cotton fabrics
- Oxidative desizing
- Acid desizing
- Removal of water soluble sizes
Enzymatic desizing is the classical desizing process of degrading starch size on cotton fabrics using enzymes. Enzymes are complex organic, soluble bio-catalysts, formed by living organisms, that catalyze chemical reaction in biological processes. Enzymes are quite specific in their action on a particular substance. A small quantity of enzyme is able to decompose a large quantity of the substance it acts upon. Enzymes are usually named by the kind of substance degraded in the reaction it catalyzes.
Amylases is the enzyme that hydrolyses and reduces the molecular weight of amylose and amylopectin molecules in starch, rendering it water that is soluble enough to be washed off by the fabric.
Effective enzymatic desizing require strict control of pH, temperature, water hardness, electrolyte addition and choice of surfactant.
In oxidative desizing, the risk of damage to the cellulose fiber is very high, and its use for desizing is increasingly rare. Oxidative desizing uses sodium or potassium persulphates or sodium bromite as an oxidizing agent.
Cold solutions of dilute sulphuric or hydrochloric acids are used to hydrolyze the starch, however, this has the disadvantage of also affecting the cellulose fiber in cotton fabrics.
Removal of water-soluble sizes
Fabrics containing water soluble sizes can be desized by washing using hot water, perhaps containing wetting agents and a mild alkali. The water replaces the size on the outer surface of the fiber, and absorbs within the fiber to remove any size residue.