Why starch?

In order to increase the wear resistance, smoothness and antistatic property of the warp in weaving, improve its strength and holding force, and improve its weavability, sizing processing should be carried out before weaving, also known as sizing (protecting the warp), especially for the high-speed looms (air jet looms, water jet looms) that are widely used at present.

Warp sizing rate is generally 4 ~ 8% of the cloth weight, and tight fabrics such as poplin are higher, reaching 8 ~ 14%.

Note: Knitted fabrics are not starched.

Why desizing?

After sizing, the slurry penetrates between the fibers and partly attaches to the surface of the warp. While improving the performance of the yarn, the slurry pollutes the dyeing and finishing processing fluid, hinders the chemical interaction between the fibers and dyeing and chemical materials, and makes it difficult to carry out dyeing and finishing processing.

A slurry commonly used for desizing

The paddles used for sizing the warp yarns are natural sizing materials such as starch, wild starch, seaweed gum, gum, etc., chemical sizing materials such as polyvinyl alcohol (PVA), polyacrylic acid (PAA), cellulose preparations such as carboxymethyl cellulose (CMC), polyester and so on. Other ingredients such as preservatives, softeners, hygroscopic agents and anti-wear agents are also added to the slurry.

Starch pulp is mostly used for cellulose fiber fabrics, such as cotton fabrics, hemp fabrics, etc., chemical pulp is mostly used for synthetic fiber fabrics.

What are the desizing methods?

The method of desizing all kinds of fabrics varies with the size used for sizing, and the following four methods are commonly used.

1. Alkaline desizing

Alkali desizing principle:

1) Under the action of hot lye, the slurry will swell, change from gel state to sol state, and the adhesion with the fiber becomes loose, easy to wash down.

2) The solubility of CMC, PA class slurry in hot lye increases, and then washed, it is easy to remove from the fabric;

3) Hot caustic soda solution can remove part of the natural impurities, especially suitable for cotton cloth containing more cottonseed shells.

Suitable for all sizes, low desizing rate (50% ~ 70%)

Note: The washing of the fabric after desizing must be sufficient, and the lotion must be constantly changed

2. Enzyme desizing

Enzyme desizing principle:

The macromolecular chain of starch is catalyzed to hydrolyze, and some low-molecular compounds with low molecular weight, low viscosity and high solubility are generated. Then the hydrolysis products are removed by water washing to achieve the purpose of desizing.

Amylase is a kind of biochemical catalyst, commonly used are pancreatic amylase and bacterial amylase. These two enzymes are mainly composed of α-amylase, which can promote the glycoside bond of starch long chain molecules to break, and produce dextrin and maltose, which are easily washed from fabrics. The amylase degrout should be near neutral, and sodium chloride and calcium chloride are often added as activators to improve the activity of the enzyme. After dipping the fabric in amylase solution, the starch can be fully hydrolyzed at 40 ~ 50℃ for 1 ~ 2 hours. Bacterial amylase is more heat resistant than pancreatic amylase, so the rapid process of steaming for 3 to 5 minutes can also be used after dipping the enzyme liquid in the fabric to create conditions for continuous desizing process.

Features: specificity, high efficiency, moderate conditions, environmental protection

3. Acid desizing

Principle of desizing:

Acid can make starch and other slurry hydrolyze to a certain extent and change into soluble products, which is easy to fall off from the fabric

Domestic application is not much, because the use is easy to damage the fiber, not used alone, combined with alkali desizing or enzyme desizing

4. Oxidant desizing

Principle of desizing:

The oxidizing agent oxidizes the slurry, degrades it until the molecular chain breaks, the solubility increases, and it is removed by washing

Especially suitable for PVA slurry

Oxidizing agent:

Sodium bromite, hydrogen peroxide, sodium perborate, hypochlorite, etc. are all oxidants, and sodium bromite (NaBrO2) and hydrogen peroxide (H2O2) are commonly used in production.

Advantages and disadvantages of various desizing methods

Common fabric desizing methods

Cotton fabric: the commonly used desizing methods are mainly enzyme desizing, alkali desizing, oxidizing desizing and acid desizing.

Viscose fabric: Desizing is the focus of viscose fabric pretreatment, viscose grey cloth is generally starch, so BF7658 amylase desizing, desizing process is the same as cotton fabric.

Tensilk: Tensilk itself is free of impurities, and starch or modified starch based slurry is applied in the weaving process, which can be desized by enzyme or alkali-oxygen one-bath method.

Soybean protein fiber fabric: using amylase desizing

Polyester fabric (desizing and scouring) : Polyester itself does not contain impurities, but there is a small amount of oligomers (less than 3%) in the synthesis process, so it does not need to be strongly pre-treated like cotton fiber. Generally, desizing and refining are carried out in one bath to remove the oil added during fiber weaving and the slurry, coloring dyes added during weaving, and the travel notes and dust contaminated during transportation and storage.

Polyester-cotton blended and interwoven fabrics: the sizing of polyester-cotton fabrics is mostly made of PVA, starch and CMC mixed slurry, and the desizing method is generally made of hot alkali desizing or oxidizer desizing.

Elastic woven fabric containing spandex: pre-treatment should take into account the physical and chemical properties of spandex, minimize the damage to spandex, and maintain the relative stability of the elastic fabric shape. The desizing method is generally enzyme desizing (open-width relaxation treatment).

Desizing quality evaluation

The desizing effect is expressed by the desizing rate, which is calculated as follows:

In production, it is generally required that the desizing rate is above 80% or the residual pulp is less than 1% relative to the cloth weight, and the remaining residual pulp can be further removed in the boiling process.

What is the impact of unclean desizing on subsequent processing?

Desizing is not clean, there is pulp spot on the cloth surface, after dyeing, the formation of color difference, color flower, color fastness decline, spot stains, uneven luster and other defects, seriously affect the quality of the product.