Desizing

Desizing

Desizing is done in order to remove the size from the warp yarns of the woven fabrics.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.
Object:
1. To eliminate the water repellent nature of sized cloth. 2. To increase the absorbency. 3. To reduce the consumption of chemicals in subsequent process.
Impotance of desizing:
The importance of de-sizing is mainly because of water repellent nature of “Sizing materials”. So, it is important to remove the size material before conducting any coloration or finishing process.
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
Types of desizing:
a)Hydrolytic desizing * Rot steeping * Alkali steeping * Acid steeping * Enzymatic desizing
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b)Oxidative desizing- * Bromide desizing * Chloride desizing * Ammonium per sulphatedesizing

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. * Enzymatic desizing : Enzymes are solubilizing bio catalysts, mainly proteins, thermo labile (readily changed or desized by heat) and highly specific in their action. A small quantity of enzyme is able to decompose a large quantity of the substance it acts upon.Enzymatic desizing consists of three main steps: application of the enzyme, digestion of the starch and removal of the digestion products.

Classification of Enzyme For Textiles:
Enzymes for textiles use can be classified as follows-

1. Amylase enzyme(Starch)
2. Cellulaseenzyme(Cellulose)
3. Pectinaseenzyme(Protein)
4. Catalase(H202)
The common components of an enzymatic desizing bath are as follows: * Amylase enzyme * pH stabilizer * Chelating agent * Salt * Surfactant, and * Optical brightener

Types of Enzyme :

Enzyme | SL | Animal | Vegetable | 1 | Viverel | SL | Malt-extract(α-ß amylase) | SL | Bacterial(α-amylase) | 2 | Novofermosal | 1 | Dastator | 1 | Rapidase | 3 | Degomma | 2 | Diastace | 2 | Biolase | 4 | Pancreatic(α-amylase) | 3 | Malto stage | 3 | Bactolase | | | 4 | Malto ferment | | |

Malt Extract: Malt Extract Enzyme are obtained by extraction of freshly germinated barely.
Bacterial: The fermentation of Bacterial Enzyme is obtained from bacillus substituent which grows on rice and breaks down the starch into water soluble dextrose.
Condition of desizing of different enzyme:

SL | Enzyme | Temp | PH | | Required cons. | 1 | Malt- Extract | 55-60 | 6-7.5 | | 3.0-20 gm/l | 2 | Bacterial | 60-75 | 5.5-7.5 | | 0.5-1.0 gm/l | 3 | Pancreatic | 50-55 | 6-7.5 | | 1.0-3.0 gm/l |

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.

Acid desizing
Cold solutions of dilute sulphuric or hydrochloric acids are used to hydrolyze the starch, however, this has the disadvantage of damaging cellulose fiber in cottonfabrics.

b)Oxidative desizing :
Oxidative desizing can be effected 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. Advantage: * Supplementary cleaning effect * Effectiveness for tapioca starches * No loss in effectiveness due to enzyme poisons.
Disadvantage:
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 (surfactants) and a mild alkali. The water replaces the size on the outer surface of the fiber, and absorbs within the fiber to remove any fabric residue.

Assessment of size removal:
Iodine test:
A drop of iodine solution placed on a test specimen resulting in a characteristic blue color is the universal test for identifying starch. It can be used as a qualitatively test to show whether all the starch was removed. Absence of the blue color signifiesthat all the starch has been removed. The intensity of the color is somewhat related to how much is left. Usually, if the color is faintly perceptible, the remaining starch will come out in the scouring and bleaching steps that follow.

Conventional Method:
In this method we first take the weight of the sized fabric, let it be W 1 . Then desize the fabric, dry & take the weight, let it be W 2 . After that the fabric is treated with 3gpl (35%) HCl at 70 0 C for 30 min. dry & take the weight of the fabric. Let it be W 3 .
Total size = W 1 -W 3 .
Residual size = W 2 -W 3 .
Desizing Efficiency = (Total size – Residual size)/Total size X 100.