Desizing methods include enzymatic desizing, oxidative desizing, acid steeping, rot steeping (use of bacteria), alkali desizing (with hot caustic soda treatment), hot washing with detergents.
Why need sizing and desizing?
The sizing process is carried out before weaving to increase the abrasion resistance, smoothness, and antistatic properties of warp yarn. Nowadays, warp yarn sizing is widely used in high-speed looms, like air-jet looms, water-jet looms.
After sizing, the slurry penetrates between the fibers and partially adheres to the surface of the warp. While improving the performance of the yarn, the slurry contaminates the dyeing and finishing liquid and hinders the chemical interaction between the fiber and the dyeing material, making it difficult to perform dyeing and finishing.
What are the desizing methods?
Desizing methods vary according to the different sizing agents in fabrics. There are mainly four desizing methods.
1. Alkali Desizing
Under the action of hot lye, the solution will swell and change from a gel state to a sol state. And the adhesion to the fiber becomes loose, then it is easy to wash the sizing agent off.
The solubility of binders, like CMC and PA, will be increased in hot lye, and they can be easily removed from the fabric after washing with water.
Hot caustic soda solution can remove part of natural impurities, especially suitable for cotton cloth with more cottonseed hulls.
Suitable for all the sizing agents and low desizing rate (50%～70%)
The fabric must be fully washed after the desizing process, and the lotion must be constantly changed.
2. Enzymatic Desizing
It catalyzes the hydrolysis of starch macromolecular chains to generate some low-molecular compounds with smaller molecular weight, lower viscosity, and higher solubility. And then the hydrolysis products are removed by water.
Amylase is a biochemical catalyst. And pancreatic amylase and bacterial amylase are commonly used. The main components of these two enzymes are α-amylase. This amylase can promote the cleavage of the glycoside bonds in the long-chain starch molecules and produce dextrin and maltose. These two items can be easily washed off from the fabric. The desizing solution of amylase is preferably near neutral. And sodium chloride, calcium chloride, etc. are often added as activators to improve the activity of the enzyme. After the fabric is impregnated with amylase solution, the starch can be fully hydrolyzed by stacking it at 40～50℃ for 1～2 hours. Bacterial amylase is more heat-resistant than pancreatic amylase. So after the fabric is soaked with enzyme solution, a fast process of steaming for 3 to 5 minutes can also be used to create conditions for a continuous desizing process.
2.2 Features: specificity, high efficiency, mild conditions, environmental protection
3. Acid Desizing
3.1 Principle of desizing:
Acid can hydrolyze starch and other sizing binders to a certain extent and turn them into soluble products, which can easily fall off the fabric.
There are not many domestic applications. Because it is easy to damage the fiber when used. And it can not be used alone and must be used in combination with alkaline desizing or enzyme desizing.
4. Oxidant Desizing
4.1 Principle of desizing:
The oxidant makes the sizing agents oxidized and degraded until the molecular chain is broken. Then the solubility is increased, and the sizing chemical is removed by washing with water
This desizing method is especially suitable for PVA sizing paste.
4.2 Oxidant types:
Sodium bromite, hydrogen peroxide, sodium perborate, hypochlorite, etc. are all oxidants. Sodium bromite (NaBrO2) and hydrogen peroxide (H2O2) are commonly used in production.