Desizing

Desizing is done in order to remove the size from the warp yarns of the woven fabrics. 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 material present on the 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.

 If  the  fabric  is  woven  from  sized yarn, desizing is essential before subjecting it to other treatments. For this, the  fabric  must  be  soaked  in  0.5% aqueous  solution  of  amylase  enzyme  for 8  hours  ensuring  that  it  is  completely immersed  in  the  solution.  After  the  size has been removed, the fabric is subjected to a hot and cold water wash.

The factors, on which the efficiency of size removal depends, are as follows:

• Type and amount of size applied
• Viscosity of the size in solution
• Ease of dissolution of the size film on the yarn
• Nature and the amount of the plasticizers
• Fabric construction
• Method of desizing, and
• Method of washing-off
   

Different methods of desizing are:

• Enzymatic desizing
• Oxidative desizing
• Acid steeping
• Rot steeping
• Desizing with hot caustic soda treatment, and
• 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.

1.Enzymatic desizing 

Enzymatic desizing consists of three main steps: application of the enzyme, digestion of the starch and removal of the digestion products. The common components of an enzymatic desizing bath are as follows:

 

• Amylase enzyme
• pH stabilizer
• Chelating agent
• Salt
• Surfactant, and
• Optical brightener 

               Advantages Of Enzymatic desizing

• No damage to the fibre
• No usage of aggressive chemicals
• Wide variety of application processes, and
• High biodegradability

               Disadvantages enzymatic desizing 
Lower additional cleaning effect towards other impurities, no effect on certain starches (e.g. tapioca starch) and possible loss of effectiveness through enzyme poisons.


2.Oxidative desizing 

Oxidative desizing can be effectede  by hydrogen peroxide, chlorites, hypochlorites, bromites, perborates or persulphates. Two important oxidative desizing processes are:  the cold pad-batch process based on hydrogen peroxide with or without the addition of persulphate; and the oxidative pad-steam alkaline cracking process with hydrogen peroxide or persulphate. 

                 The advantages offered by oxidative desizing are

• Supplementary cleaning effect
• Effectiveness for tapioca starches
• No loss in effectiveness due to enzyme poisons.

Some disadvantages of oxidative desizing include possibility of fibre attack, use of aggressive chemicals and less variety of application methods.

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.