Desizing Process in Textile Industry : Enzymatic desizing of starches on cotton fabrics

 

Desizing

Objective:

To remove the size applied to the grey fabric as a part of weaving process, so that the fabric becomes free from the added matter and it can be effectively processed further.

·       Chemically Starch is a glucose polymer in which glucopyranose units are bonded by alpha-linkages in which Amylose is linear chain and amylopectin branch chain are present.

·       Both of these are insoluble in water but can be solubilized by the hydrolysis of long-chain compounds into shorter one.

·       This under suitable conditions starch is hydrolysed and finally it is converted into glucose which is a soluble form.

·       Indie sizing hydrolysis is controlled or carried out up to Dextrin (C6H12O5)_l stage only.

·       The sizing is a chemical process and it can be controlled e.g. if concentration of chemical in desizing bath is higher, the process can be completed in short duration and vice versa.

·       Desizing can be carried out in continuous or batch wise manner.

 

Methods / classification of Desizing

1.       Hydrolytic method

a.       Rot steeping

b.       Acid desizing

c.       Enzymatic desizing

2.       Oxidative method

a.       Chlorine

b.       Chlorite

c.       Bromite

3.       Novel method

a.       Solvent desizing

b.       Low temperature plasma temperature

c.       Desizing 2000

 

 Below are Hydrolytic method process

1.       Rot steeping

·       This is oldest and cheapest method.

·       No special chemicals are used.

·       The grey cotton fabric is first stepped or immersed in water at 40°C and then passed through padding mangle to give 100% expression.

·       Then it is piled in a tank and allowed to stand for 24 hours.

·       During the storage the microorganisms naturally present in water secret starch liquefying enzymes which hydrolysed the starch present in the size.

·       Then the fabric is washed to remove the hydrolysed starch.

·       The actual process is fermentation and precaution should be taken that fermentation is not too active otherwise cellulose itself may be attack and strength loss may occur.

Advantages:

·       - Most economical and easy process.

·       - No specific chemicals and machinery is required.

Disadvantages:

·       - Low efficiency

·       - Due to longer time production is less

·       - Degradation of cellulose and due to uncontrolled fermentation

·       - It requires a large floor space.

 

2.       Acid desizing

·       In this method cotton fabric is treated with dilute sulphuric acid (H2SO4) or hydrochloric acid (HCL)

·       The acid hydrolyses of starch present in the size fabric.

·       It attacks the polymer chain of starch and does lower down the molecular weight of starch which is converted into soluble form.

·       The concentration is used is 5 to 10 gpl or 0.5 to 1% at temperature of about 40°C.

·       The treatment is continued for 3 to 4 hours. The acid immersed fabric is squeezed evenly and is batched and weight on a roller.

·       The weight roll is covered with polythene film and kept aside in a storage room with the role in rotating condition so that it remains uniformly weight during the sizing action for required period.

·       Then the fabric is washed in open width soaper till it gets free from acid and the hydrolysed product.

·       Precaution must be taken that acid treated fabric roll is covered properly and kept rotating during storage time.

·       If any part of the fabric dries up before washing, cellulose will be formed and tendering of fabric may occur.

Advantages:

·       -The process is economical

·       - As compared to rot stripping it requires less time, thus production is more.

·       - It does not require specific condition of temperature and pH. It can be done at room temperature and thus the energy consumption is less.

Disadvantages:

·       - There are chances of tendering of cellulose.

 

3.       Enzymatic desizing

·       Enzymes are organic biocatalyst. This are highest specific both in a reaction catalyst and their choice of reactance. i.e. substrate

·       Deciding enzymes like amylase liquefy the starches in the fabric and these are classified on the basis of source they are obtained.

·       Desizing enzymes –

Animal sources –

-          Veveral

-          Degomma

-          Pancreatic

-          Novofermosal

Vegetable sources –

-          Malt Extract (e.g. Diastafore, Diastace, Maltostace)

-          Bacterial Extract (e.g. Raddase, Arey, Take)

·       The feature of enzyme deciding process is the specific nature of the action of the enzymes which are active under certain condition of concentration, pH and temperature.

·       The optimum concentration, pH and temperature at which different enzymes are effective as desizing agent are given as follows:

 

Enzymes	Concentration (gpl)	Temperature (°C)
Malt Extracts	3-20	50-60
Pancreatic	1-5	50-60
Bacterial	0.5-1	60-70