Introduction to sizing
During 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 may not be sizing or the rate may be below 1%. Compact fabrics (such as poplin) may have a sizing rate of up to 8%~14%. In recent years, the warp sizing rate of high-speed looms has exceeded 14%.
General law:
The finer the yarn count and the higher the density, the higher the warp yarn 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), modified slurry (soluble starch, carboxymethyl cellulose), and 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.
Desizing method
There are many commonly used desizing methods, including enzyme, alkali, acid and oxidant desizing. 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, 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 the nature of their catalytic action, they are divided into oxidoreductases, hydrolases, lyases, transferases, etc.
Desizing principle:
Enzyme is a type of protein with special catalytic ability, which has a specific catalytic effect on the decomposition of certain substances.
For example: the desizing effect of amylase is that it can catalyze the hydrolysis of starch macromolecular chains; and generate some low molecular compounds (dextrin, maltose and glucose) with smaller molecular weight, lower viscosity and higher solubility; and then remove them by washing with water. Hydrolyzate products to achieve the purpose of desizing.
When amylase is used for desizing, it changes the reaction process of starch hydrolysis, reduces the activation energy and increases the collision rate between reaction molecules, so that the hydrolysis of starch proceeds rapidly.
Features: Simple process and easy operation. The starch slurry is completely removed without damaging the fibers. 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
This method is used for PVA-based mixed slurries (containing starch) and PA-based slurries.
Principle of desizing: Starch and chemical pulp undergo strong expansion under the action of hot caustic soda, and the adhesion with fibers becomes loose, changing from gel state to sol state. The solubility of chemical pulp increases in hot alkali. Through effective water washing, It is easy to wash off fabrics, and hot caustic soda solution can remove some natural impurities, especially suitable for cotton fabrics containing a lot of cotton seed hulls. 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, can use waste alkali liquid in pre-treatment; helps to remove part of fiber symbiosis in cottonseed hull remover, reduces refining burden, has good whiteness and permeability; has effect on all types of slurries; degrades The slurry rate is not high and the stacking time is long, which hinders the continuity of production; alkali has no chemical degradation effect on PVA slurry, the viscosity of the washing liquid is high, and the washing should be sufficient to prevent the slurry from contaminating the fabric again.
Acid desizing
It is mostly used for desizing cotton cloth and is rarely used alone. It needs to be used in conjunction with other desizing methods. Such as alkaline-acid desizing or enzyme-acid desizing, etc.
Principle of desizing: Under appropriate conditions, dilute sulfuric acid has no decomposition effect on PVA and PA slurries, but can gradually hydrolyze starch and transform it into products with higher water solubility, which are eventually washed away by water.
Features: Suitable for impurity-containing cotton, it can remove a large amount of mineral salts and heavy metal ions and improve whiteness; it will cause certain damage to cotton, so T and C should not be violent during treatment; starch is not fully hydrolyzed, and the desizing rate is not high.
Oxidant desizing
Oxidant desizing is mainly used for desizing PVA and its mixed pulp.
DesizingTreatment: 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 fiber strength as much as possible.
Common fabric desizing methods
Cotton fabric: Commonly used desizing methods include enzyme desizing, alkali desizing, oxidant desizing and acid desizing.
Viscose fabrics: 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 that of cotton fabrics.
Tencel: Tencel itself has no impurities. During the weaving process, a slurry 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, but there is a small amount of oligomers (less than about 3%) in the synthesis process, so it does not require intense pre-treatment like cotton fiber. 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.
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).
Desizing quality assessment
The desizing effect is expressed as desizing rate, which is calculated as follows:
In production, it is generally required that the desizing rate is above 80% or that the residual pulp is below 1% relative to the cloth weight. The remaining residual pulp can be further removed during the scouring process.
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