Desizing Process and Types Used in Textile

Desizing in Textiles

Desizing is the first wet-processing textile finishing technology employed to remove the sizing material from the fabric.

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Objectives of Desizing

To remove starch from the fabric.
To increase the absorbency of the fabric.
To increase the luster of dyeing and printing.
To perfect the action of scouring and bleaching.
To make the fabric ready for the subsequent process.

Common Size Materials

Starch: Corn, Potato, Sago, Hydroxy ethyl starch.
Natural gums: Tragacanth, locust beam, etc.
Proteins: Glue, Gelatin.
Synthesis sizing agent: PVA, PVC, PAN, PAA.
Additives: Defoamers, Thinner, Tallow, Oils, and waxes.

Types of Desizing

1. Hydrolytic Method

Rot steeping
Alkali steeping
Acid steeping
Enzymatic steeping

2. Oxidative Method

Bromite desizing
Chlorite desizing
Ammonium persulfate desizing

Rot Steeping

Starch is removed by rotting. This is the very oldest method when men did not know the use of enzymes or other special chemicals.
No special chemicals are required.
In this method, the fabric is passed through a padding mangle containing warm water of 40-60℃, and the fabric is kept for prolonged stocking.
If the warm water temperature is 40℃ then the fabric is stored for 24 hours stocking and if the warm water temperature is 60℃ then the fabric is stored for 12 hours.
Pectin and organisms are naturally present in water-produced starch-liquefying enzymes which remove the starch. However, starch is not completely removed and other size materials are removed.
To remove more starch, more time is required. But if the fabric is allowed to be stocked for a very long time then enzymes may attack the cellulose. After washing, desizing is completed.
Here the temperature is mainly to be controlled.

Alkali Steeping

In this method, the starch is removed by alkaline hydrolysis. The fabric is treated with 0.4-0.6% caustic soda solution at 60℃ to 70℃ and stored for 8 to 10 hours.

Acid Steeping

In this method, 0.5-1.0% dilute sulphuric acid solution or HCL acid at 40-60℃ is used for 4-6 hours to hydrolyze the starch from the sized fabric.

Hydrolytic Desizing

Enzymatic Steeping

This method is the traditional process and is most commonly used nowadays. An enzyme is one kind of biocatalyst. Its molecular weight is high but different from the chemical catalysts. The action of enzymes is controlled by the temperature, concentration, time, and pH of the solution. Most enzymes have no action at temperatures above 75℃.

Enzymes

Enzymes are living organisms.
It is based on protein.
Its main function is the fermentation of starch.
Soluble in water but insoluble in acid and alcohol.
There is no risk of hydrocellulose formation during pilling.

Types of Enzymes: Mainly Two Types

Cellulose enzymes (degrade cellulose and if more time then cellulose is damaged and converted into glucose).
Amylase enzymes: Outstanding features are- they only hydrolyze starch but do not attack cellulose. So it is used for desizing.

Types of Extraction of Enzymes

Malt extract/malt preparation enzymes.
Bacterial extract enzymes.
Pancreatic Enzyme Analysis.

1. Malt Extract/Malt Preparation Enzymes

Malt enzymes are obtained by extraction of corns. Concentration, temperature, pH and are very important for the efficiency of enzymes. If the temperature is raised to the upper limit it becomes destroyed (not useable). Best result at 60℃ and destroyed at 75℃.

Trade Names:

Disastafor
Ferment D
Terhyd, M.D., etc

2. Bacterial Extract Enzymes

Fermented rice produces bacteria → Extract → Grow → Enlarge → Prepared.

It is best at 70-75℃ and destroyed at 95-100℃

Trade Names

Rapidase
Biolase
Bactolase
Gelatase
Aery

3. Pancreatic Enzyme Analysis

Pancreatic (animal) enzymes are prepared from the pancreas of slaughtered animals. A characteristic feature of the pancreatic is that it forms a milky emulsion when mixed with water, and is capable of modifying starch, albumen, pectins, and fats.

Trade Names

Fermosol
Novofermosol DS
Degonmma etc
Viveral

Conditions for Desizing with Different Extracted Enzymes

Enzymes	Temperature	pH	Required Concentration
Malt Extract	50-60℃	6-7.5	3-20 g/L
Bacterial	60-70℃	5.5-7.5	0.1-1 g/L
Pancreatic	50-55℃	6.5-7.5	1-5 g/L

Desizing Machines

Padding Mangle to Impregnate the Cloth (One Dip One Nip)

Process: In the machine, the fabric is passed through the guide roller and then the fabric is passed through two immersed rollers in which (alkali) desizing is kept.

One Dip, One Nip

The fabric is passed through two squeezing rollers i.e. one rubber and the other ebonite. When the fabric is passed between rollers, desizing occurs i.e. nip is done. As the fabric at first dips in the tank and then is squeezed/ nipped by the roller, this process is one dip and nip process.

Padding Mangle to Impregnate the Cloth (Two Dip, Two Nip)

Process: Fabric is passed through a guide roller and then an immersion roller. It is passed through a squeezing roller and then again immersion or dip in the desizing liquor. Then the fabric is passed through a squeezing roller.

Two Dip, Two Nip

In this machine, the fabric is dipped twice.

Enzymatic Desizing Process/Continuous Desizing Process

The continuous process of cotton desizing by enzymatic process is as below:

Recipe:

Enzyme = 3-5 g/L
Time = (for stronger solution 5 min) = 20-30 min
Temperature = 60-65℃
Common salt = 5-10 g/L
pH = 6-7

The whole process of desizing is completed in the following stages.

1. Preparation of Desizing Mixture

Desizing mixture usually contains the following agents-

Water
Wetting agent
Salt
Acid or alkali
Chelant if necessary
Malt extract enzymes.

The above chemicals dissolve completely in the mixture.

2. Padded in Padding Mangle

The desizing or enzyme is mixed with a sufficient amount of water and added to the padding mangle. Then the sized fabric in rope form is immersed into the desizing liquor by an immersion roller, the temperature of the liquor is 50-60℃, and the rate of desizing mixer consumption should be regulated so that the mixer level remains constant.

Continuous Desizing

3. Storing

After padding the material, the fabric is squeezed and passed over the winch roller. The fabric is stored in the storage bath by folding it for (3-4 hours) or 5 minutes at room temperature. Here the malt is extracted with a starch material and decomposes into glucose.

4. Washing

The fabric is brought into the washing mangle by a guide roller to remove the short-chain sugars because these are water-soluble. After that, the fabric is squeezed and dried.

Temperature, concentration, and pH of the enzyme and the mixture must be controlled, otherwise, the desizing will be uneven.

Controlling Points of the Desizing Machine

Pipelines
Faults bottom
Steam line
Multitubular heater
Inlet outlet pipe
Bulb
Temperature
pH

Desizing Test

After desizing following test is done to ensure the desizing

Potassium Iodide (KI) - 0.24 gm
Lodine (1) - 0.13 gm
And water to make a 100 ml solution.

If one drop of this solution falls on the sizing material then the following incident may occur-

Deep blue: present of starch
Violet- partially degraded starch (dextrin)
Brown - completely degraded dextrin.

Ahmedabad