Sizing has been considered as an 'invention of the devil' by some dyers and finishers because it is the main source of many processing problems. Warp yarns are coated with sizing agents prior to weaving in order to reduce their frictional properties, decrease yarn breakages on the loom and improve weaving productivity by increasing weft insertion speeds. The sizing agents are macromolecular, film forming and fibre bonding substances, which can be divided into two main types: natural sizing agents which include native and degraded starch and starch derivatives, cellulose derivatives and protein sizes; and synthetic sizes which include polyvinyl alcohols, polyacrylates and styrenemaleic acid copolymers. Starch-based sizing agents are most commonly used for cotton yarns because of being economical and capable of giving satisfactory weaving performance.
Other products are also used, either alone or in combination with starch sizes, when the higher cost can be offset by improved weaving efficiency. Some auxiliaries are also used in sizing for various functions and include softening agents, lubricating agents, wetting agents, moistening agents, size degrading agents, and fungicides. The desizing procedure depends on the type of size. It is therefore necessary to know what type of size is on the fabric before desizing. This can easily be determined by appropriate spot tests.
The sizing material present on warp yarns can act as a resist towards dyes and chemicals in textile wet processing. It must therefore be removed before any subsequent wet processing of the fabric. The factors on which the efficiency of size removal depends are as follows:
- Viscosity of the size in solution.
- Ease of dissolution of the size film on the yarn.
- Amount of size applied.
- Nature and the amount of the plasticizers.
- Fabric construction.
- Method of desizing.
- Method of washing-off.
Different methods of desizing are:
- Enzymatic desizing.
- Oxidative desizing.
- Acid steeping.
- Rot steeping (use of bacteria).
- Desizing with hot caustic soda treatment.
- Hot washing with detergents.
The most commonly used methods for cotton are enzymatic desizing and oxidative desizing. Acid steeping is a risky process and may result in the degradation of cotton cellulose while rot steeping, hot caustic soda treatment and hot washing with detergents are less efficient for the removal of the starch sizes.