What is Desizing?
Desizing (desize) refers to the process of treating fabrics with acids, alkalis, enzymes, etc., to remove the slurry rhyme added to the warp yarns during weaving. The purpose is to facilitate subsequent processing such as refining. Before fabric weaving, warp yarns are generally subjected to sizing treatment (the warp yarns are dipped in the sizing solution and then dried) to make the fibers in the yarn stick together and form a layer of film on the surface of the yarns, which is convenient for weaving. Cotton fabrics are generally sizing with starch or modified starch sizing or polyvinyl alcohol and polyacrylic acid (ester) sizing. The slurry is also sizing with lubricants, softeners, preservatives and other auxiliary agents.
Desizing refers to the process of removing the size on the fabric. The warp yarns of fabrics such as cotton, viscose and synthetic fibers are mostly sizing before weaving. In the dyeing and finishing process, the size will affect the wettability of the fabric and hinder the chemical contact with the fiber. Therefore, fabrics are generally desizing first. Cotton fabric desizing has the effect of removing some impurities in the fiber; synthetic fiber fabrics can sometimes be desizing simultaneously during the scouring process.
The method of desizing of various fabrics varies with the size used for sizing. The following four methods are commonly used.
Hot water desizing method
After the fabric is immersed in hot water, it is kept in the desizing tank for more than ten hours to keep the slurry swelled and easy to wash away with water. This method has a good desizing effect for fabrics that use water-soluble sodium alginate, cellulose derivatives, etc. as sizing agents.
For fabrics sizing with starch, stacking them for a long time at 25-40°C, allowing them to ferment and degrade naturally, can also obtain the desizing effect.
Lye desizing method
Starch can swell under the action of sodium hydroxide (caustic soda) solution. Polyacrylic acid polymer is easier to dissolve in lye. The waste sodium hydroxide solution in the scouring or mercerizing process can be used as a desizing agent. The concentration is usually 10~ 20 g/l. After the fabric is soaked in lye, it should be stacked for 6-12 hours at 60～80℃; the cotton fabric can also be desizing with alkali and acid. The method is to desizing with alkali and then wash with water and then padding with a concentration of 4～6. The gram/liter dilute sulfuric acid is stacked for several hours to further promote the hydrolysis of starch and wash away the inorganic salt impurities in the cotton fiber.
It is mainly used to decompose starch slurry on fabrics, with high desizing efficiency. Amylase is a biochemical catalyst, and pancreatic amylase and bacterial amylase are commonly used. The main components of these two enzymes are α-amylase, which can promote the cleavage of the glycoside bonds of the long-chain starch molecules to produce dextrin and maltose, which can be easily washed off the fabric. The amylase desizing solution is preferably near neutral, and sodium chloride, calcium chloride, etc. are often added as activators in use 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 continuous desizing process.
Oxidant desizing method
A variety of oxidants can be applied. The fabric is immersed in an alkaline solution of hydrogen peroxide with a concentration of 3 to 5 g/liter, and then steamed for 2 to 3 minutes, which can promote the degradation of starch and polyvinyl alcohol, and has a certain bleaching effect on the fabric.
When desizing with sodium bromite, the fabric is padded with a sodium bromite solution with a pH of 9.5 to 10.5 and an effective bromine concentration of 0.5 to 1.5 g/l, and stacks it at room temperature for about 20 minutes. The fabric sizing with vegetable, starch or polyvinyl alcohol has a good desizing effect. Ammonium persulfate or potassium persulfate also has a good desizing effect, but it is easy to make the cellulose fiber brittle.