De-sizing Process and its types

De-sizing Process

A process carried out to remove the sizing material (like starch) from the cloth. 

Objective of De-sizing

             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.

Importance of De-sizing

                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.

Enzymatic desizing

What are enzymes?

Name

·         Greek, meaning “In Yeast”

Nature

·         Bio-catalyst (proteins)

Structure

·         Usually large & complicated

Properties

·         Specific action

·         Thermo ability

·         Low energy of activation

·         Narrow working pH arrange

Classification

·         According to origin

·         According to action

·         According to structure

·         According to working temperature

Sources of Amylases

·         Malt

·         Bacteria

·         Pancreas

Requirements of desizing:

• Desizing materials
• Desizing methods
• Desizing machine

Desizing processes

Desizing, irrespective of what the desizing agent is, involves impregnation of the fabric with the desizing agent, allowing the desizing agent to degrade or solubilise the size material, and finally to wash out the degradation products. The major desizing processes are:

• Enzymatic desizing of starches on cotton fabrics
• Oxidative desizing
• Acid desizing
• Removal of water-soluble size

Enzymatic desizing

Enzymatic desizing is the most widely used desizing process of degrading starch size on cotton fabrics using enzymes. Enzymes are complex organic, soluble bio-catalysts, formed by living organisms that 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 a large quantity of the substance it acts upon..

Amylases is the enzyme that hydrolyses and reduce the molecular weight of amylose and amylopectin molecules in starch, making it water soluble enough to be washed off the fabric.

Effect of PH and Temperature on enzyme desizing

Effective enzymatic desizing require strict control of pH, temperature, water hardness and electrolyte addition Activity of enzymes increase with temperature; however, above a critical temperature, enzymes are deactivated. The effectiveness of enzymes exhibit a maximum at certain temperatures, usually 40 -75 0C. Bacterial enzymes are the most thermally stable and can be used up to 100 0C under special stabilizing conditions. Certain salts increase the activity of specific enzymes. Pancreatic amylase is ineffective without the addition of salt. A combination of sodium chloride

Exhaust Process

                           Enzyme                    4 g/l

                            NaCl                        4 g/l

                           Wetting Agent         1 g/l

            Continuous Process

1. Saturate fabric with a solution containing:

Bacterial Amylase 0.8 - 1.0 %

Wetting agent 0.1 - 0.2 %

Sodium Chloride 10 %

2. Hold:

Open-width Range: Steam 1 to 2 minutes at 200 to 212 ⁰F.

Rope Range: Store in J Box for 20 to 30 minutes at 170 to 180 ⁰F. or

over night at room temperature.

3. Thoroughly rinse with hot water, 175 0F or hotter.

           

Oxidative desizing

In oxidative desizing, the risk of damage to the cellulose fiber is very high, and its use for desizing is very rare. Oxidative desizing uses potassium or sodium persulfate or sodium bromite as an oxidizing agent.

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 cotton fabrics.

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:

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 some what 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.