Desizing refers to the process of removing size from fabrics.
1. Introduction to sizing for desizing purposes
In the weaving process , the warp yarns are subject to greater tension and friction, and are prone to breakage. In order to reduce warp breakage and improve weaving efficiency and gray fabric quality, the warp yarns need to be sizing before weaving. This causes the fibers in the yarn to stick together and form a strong sizing film on the yarn surface, making the yarn tight and smooth. Smooth, thus improving the breaking strength and abrasion resistance of the yarn.
The sizing rate is related to fiber quality, yarn count, density, and loom type. It is generally 4% to 8%. Line fabrics can be without sizing or have a sizing rate of 1%. Below, the sizing rate of compact fabrics (such as poplin) can reach 8% to 14%. In recent years, the warp sizing rate of high-speed looms has exceeded 14%.
General rules:
The finer the yarn count and the higher the density, the higher the warp sizing rate;
Air jet looms require a higher sizing rate than ordinary looms
The slurry used can be divided into natural slurry (starch, sodium alginate) and modified slurry Materials (soluble starch, carboxymethyl cellulose), synthetic slurry (polyvinyl alcohol PVA, polyacrylate PA).
The presence of sizing on the original cloth is not conducive to subsequent scouring and bleaching processes. The desizing process is the basis of fabric pre-treatment. This process can remove most of the sizing on the original fabric to facilitate scouring and bleaching processing, and can also remove some natural impurities.
2. Desizing method
The commonly used desizing method is relatively There are many types of desizing, including enzyme, alkali, acid and oxidant desizing, etc. The appropriate desizing method can be selected according to the type of original cloth, sizing composition, desizing requirements and factory equipment. After desizing, it must be washed with hot water in time, because impurities such as starch decomposition products will re-condensate on the fabric, seriously hindering subsequent processing.
Enzyme desizing
Enzyme is a biological catalyst that is A protein secreted by animals, plants or microorganisms (bacteria, mold), which has a specific catalytic effect on the decomposition of certain substances.
The catalytic efficiency of the enzyme is high, the action conditions are gentle, and it does not require severe conditions such as high temperature and pressure, and it works quickly. However, enzymes have specificity, that is, an enzyme can only catalyze one type of chemical substance or type of chemical substance.
For example
Amylase has an efficient catalytic effect on the hydrolysis of starch and can be used for desizing fabrics sized with starch and modified starch. Amylase has a high desizing rate (up to 90%) and will not damage cellulose fibers. However, amylase only has a desizing effect on starch sizing and has no desizing effect on other natural sizing and synthetic sizing.
According to the source of enzymes, they are divided into animal enzymes, plant enzymes and microbial enzymes. According to their catalytic properties, they are divided into oxidoreductases, hydrolases, lytic enzymes, transferases, etc.
Principle of desizing:
Enzymes are a type of protein with special catalytic ability and have specific functions for the decomposition of certain substances. Catalysis.
For example
The desizing effect of amylase is that it can catalyze the hydrolysis of starch macromolecular chains; and produce smaller molecular weight, Some low molecular compounds with lower viscosity and higher solubility (dextrin, maltose and glucose) are then washed with water to remove the hydrolyzate to achieve the purpose of desizing.
Amylase is used for desizing, which changes the reaction process of starch hydrolysis, reduces the activation energy and increases the collision rate between reaction molecules, thereby making the hydrolysis of starch rapid. conduct.
Features:
Simple process and easy operation.
The starch slurry is completely removed without damaging the fiber.
It has no desizing effect on chemical slurry;
It cannot remove the oil agent in the slurry and the natural impurities on the original cloth. .
Alkali desizing
PVA-based mixed pulp (including Starch) and PA slurry use this method.
Principle of desizing:
Under the action of hot caustic soda, starch and chemical pulp undergo strong expansion and their adhesion to the fiber changes. It changes from gel state to sol state. The solubility of chemical slurry increases in hot alkali. It can be easily washed away from the fabric through effective water washing. At the same time, hot caustic soda solution can remove some natural impurities, especially suitable for cotton seed hulls. Lots of cotton. Most of the alkali used for desizing is spent alkali from scouring or mercerization, which is low-cost and does not damage the fiber. Therefore, this process is widely used in printing and dyeing factories. The desizing rate of alkali desizing is about 50~70%, and the remaining slurry can only be further removed during scouring.
Features:
Low cost, waste alkali solution in pre-treatment can be used;
It helps to remove part of the fiber symbiosis of the remover of cottonseed hulls, reducing the refining burden, and has good whiteness and permeability;
It has an effect on all types of slurries ;
The desizing rate is not high and the stacking time is long, which hinders the continuous production;
Alkali has no effect on PVA slurry. Due to chemical degradation, the viscosity of the washing liquid is high, and the washing should be sufficient to prevent the slurry from re-contaminating the fabric.
Acid desizing
Mostly used for cotton desizing, rarely used Used alone, it needs to be used in conjunction with other desizing methods. Such as alkaline-acid desizing or enzyme-acid desizing, etc.
Desizing principle:
Under appropriate conditions, dilute sulfuric acid has no decomposition effect on PVA and PA slurries, but it can The starch is gradually hydrolyzed and converted into products with higher water solubility, which are eventually washed away by water.
Features:
Suitable for cotton containing impurities, can remove a large amount of mineral salts and heavy metal ions, and improve the whiteness;
It will cause certain damage to cotton, so when handling, T and C should not be violent;
Starch is not fully hydrolyzed and the desizing rate is not high.
Oxidant desizing
Oxidant desizing is mainly used for PVA and Desizing of its mixed pulp.
Desizing principle:
Strong oxidants such as perhydrochloric acid, hydrogen peroxide, sodium bromate, etc. It has the function of breaking the macromolecules of various slurries, making them easy to wash off from fabrics.
Features: The oxidant has fast desizing speed, high efficiency, uniform texture, and a certain bleaching effect.
However, strong oxidants also have an oxidizing effect on cellulose, so the process conditions should be controlled to maintain the strength of the fiber as much as possible.
Common desizing methods for fabrics
●Cotton fabrics: Commonly used desizing methods include enzyme desizing and alkali desizing. , oxidant desizing and acid desizing.
●Viscose fabric: Desizing is the focus of pre-treatment of viscose fabrics. Viscose gray fabrics are generally coated with starch slurry, so BF7658 amylase is often used for desizing. The desizing process is the same as Cotton fabric
●Tencel: Tencel itself has no impurities. During the weaving process, sizing mainly composed of starch or modified starch is applied. Enzyme or alkali-oxygen one-bath method can be used for desizing.
●Soy protein fiber fabric: desizing using amylase
●Polyester fabric (desizing and refining): polyester itself does not contain Impurities only exist in a small amount (less than 3%) of oligomers during the synthesis process, so they do not require intensive pre-treatment like cotton fibers. Generally, desizing and refining are carried out in one bath. The purpose is to remove the oil added during fiber weaving, the slurry added during weaving, coloring dyes, and travel and dust contaminated during transportation and storage.
●Polyester-cotton blended and interwoven fabrics: The sizing of polyester-cotton fabrics mostly uses a mixed slurry of PVA, starch and CMC. The desizing method generally uses hot alkali desizing or oxidant desizing. Pulp.
●Elastic woven fabrics containing spandex: The physical and chemical properties of spandex should be taken into consideration during pre-treatment to minimize damage to spandex and maintain the relative stability of the elastic fabric shape. The desizing method generally uses enzyme desizing (open-width relaxation treatment).
3. Evaluation of desizing quality
The desizing effect is based on the desizing effect. The pulp ratio is expressed as follows:
In production, the desizing rate is generally required to be above 80% or The residual pulp is less than 1% relative to the cloth weight, and the remaining residual pulp can be further removed during the scouring process.
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