Desizing Process in Textile Industry

Desizing:
Sizing materials are added to increase the strength of warp yarn. It needs only weaving process. For knitting as well as non-woven process, no need the sizes on the fabrics thus no need to desizing. During wet processing, various chemicals are applied and before starting these processes we must be sure of maximum absorption of these chemicals. For this, the sizing materials are removed by desizing. We can say, the main aim of desizing is the elimination of the size material from the fabric. If the sizing materials cover the warp yarns, it will lead to uneven dyeing, printing or finishing. If not taken out completely, it will affect washing and rubbing fastness of the fabrics. The removal of hydrophobic part of the sizes is often especially problematic. It is not possible to remove all sizing agents during desizing, but are expected to be stabilized or emulsified in the alkaline scouring. The total material present in the cotton fiber is up to 20% of the fiber weight including that of 4-12% natural impurities. In the process of desizing, not only sizing agents, but also some natural impurities are eliminated from fibers. Desizing depends upon the sizing agents, adhesions between sizing materials and fabric materials, allowing the desizing agent to degrade or solubilise the size material and finally to wash out the degradation products.
Desizing in J-box
Fig: Desizing in J-box (Image courtesy: http://nptel.ac.in)
Methods of desizing in textile wet processing:
In desizing process of cotton fabric can be accomplished by physical, chemical or combination of physical and chemical mechanism. In desizing, the starches and polymers that are applied which are insoluble, are converted into water soluble corn pound to ease their removal. This is accomplished by transferring the starch into their simple sugars or simple water soluble polymers.

Mainly desizing method are two types:

1. Hydrolytic method:
  • Rot steeping
  • Alkali steeping
  • Enzymatic desizing process
  • Washing with hot water
  • Acid de-sizing process
2. Oxidative desizing:
  • Chlorine desizing
  • Chlorite desizing
  • Bromite desizing
  • Peroxide desizing
Rot steeping desizing:
Rot steeping is very oldest method of desizing when man cannot know the use of enzymes or other special chemicals. In this desizing process, starch is removed by rotting. In this process fabric is stored at 40-60oC for overnight in water. It removes starch and water soluble impurities with natural reaction. The main problems in rot steeping desizing are low efficiency due to longer treatment time and degradation of cellulose due to cross-infections of mildew if the fermentation process is not properly controlled.

Alkali steeping desizing:
In this method the starch is removed by the alkaline hydrolysis. The fabric is treated with 0.4-0.6% caustic soda (NaOH) solution at 60oC to 70oC temperature and stored for 8 to 10 hours. In alkali steeping care must be taken that, goods do not dry up, otherwise, it causes partial concentration of alkali.

Acid steeping or de-sizing process:
In this method, 0.5-1.0% dilute sulphuric acid solution or HCl acid at 40-60oC temperature is used for 4-6 hours to hydrolyzed the starch from the sized fabric. Dilute acid attacks the polymer chain of starch and due to chain cleavage of starch molecule short water soluble or dispersible chain segments are formed. In this desizing process the amount of size removed is comparatively greater but it is risky process, due to hydrolytic degradation will occur which fall the strength of cotton.

Enzymatic desizing process:
Among all desizing process, enzymatic desizing is very popular and effective. 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, which 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 large polymer molecules. Amylases are the enzymes that hydrose and reduce the molecular weight of amylase and amylosepectin molecules in starch, rendering it water soluble enough to be washed off the fabric.

These enzymes, as vary through their source, they also vary according to their application condition.

Enzyme
Temperature (oC)
Concentration (g/L)
pH value
Pancreatic
50-55
1.0-5.0
7.0-7.5
Malt extract
55-60
3.0-20.0
5.5-6.5
Bacterial
60-65
0.5-3.0
5.5-7.5
Effective enzymatic desizing requires strict control of pH, temperature, water hardness, electrolyte addition and choice of surfactant.

The oxidative desizing:
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 potassium or sodium persulfate or sodium bromite as an oxidative agent. The most important aspects of oxidising agents are that they can be applicable to wide range of fabrics, the size content of which is often not known.

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