Dyeing and finishing process of pure cotton fabric and common dyeing methods
1. Refining and bleaching natural fibers contain impurities, and various sizing agents, oils and contaminated dirt are added in the textile processing process. The existence of these impurities not only hinders the smooth progress of dyeing and finishing, but also affects the wearing of fabrics. performance. The purpose of scouring and bleaching is to apply chemical and physical mechanical action to remove impurities on the fabric, make the fabric white, soft, and have good permeability to meet the requirements of wearing and prepare for dyeing and printing.
2. Singeing: The purpose of singeing is to burn off the fluff on the cloth surface to make the cloth surface smooth and beautiful, and to prevent uneven dyeing and printing defects due to the existence of fluff during dyeing and printing. Fabric singeing is the process of passing the open width of the fabric through a high-temperature flame or rubbing over a red-hot metal surface. At this time, the fluff on the fabric surface quickly heats up and burns, while the cloth body is tighter and heats up slowly. When it reaches the fire point, it has already left the flame or red hot metal surface, so as to achieve the purpose of burning off the fluff without operating the fabric.
3. Desizing: For smooth weaving, textile mills often sizing warp yarns to improve strength and wear resistance. The slurry on the grey fabric not only affects the water absorption performance of the fabric, but also affects the quality of dyeing and finishing products, and will increase the consumption of dyeing chemicals, so the slurry should be removed before scouring. This process is called desizing. The size on the cotton fabric can be removed from the fabric by methods such as alkali desizing, enzyme desizing, acid desizing and oxidizing agent desizing. Alkali desizing causes the slurry to expand, and its adhesion to the fiber decreases, and it is removed from the fabric by washing with water. Enzymes, acids, and oxidants degrade starch, increase its solubility in water, and remove it after washing with water. Since acids and oxidants have great damage to cotton fibers, they are rarely used alone, and are often used in combination with enzyme desizing and alkali desizing.
4. Scouring: When cotton fiber grows, it is accompanied by natural impurities (pectin, waxy substance, nitrogen-containing substance, etc.). After the cotton fabric is desizing, most of the size and some natural impurities have been removed, but a small amount of size and most of the natural impurities still remain on the fabric. The presence of these impurities makes the cloth surface of cotton woven fabrics yellower and poorer permeability. At the same time, due to the presence of cotton seed hulls, the appearance quality of the cotton cloth is greatly affected. Therefore, the fabric needs to be scoured for a long time in high-temperature concentrated lye to remove residual impurities. Scouring is the use of caustic soda and other scouring aids to react with pectin, waxy substances, nitrogen-containing substances, cotton seed hulls for chemical degradation, emulsification, puffing, etc. The impurities are removed from the fabric after washing.
5. Bleaching: After the cotton fabric is scoured, because there are natural pigments on the fiber, its appearance is not white enough. If it is used for dyeing or printing, it will affect the vividness of the color. The purpose of bleaching is to remove pigments and give the fabric the necessary and stable whiteness, while the fiber itself is not significantly damaged. Commonly used bleaching methods for cotton fabrics are sodium nitrite method, hydrogen peroxide method and sodium chlorite method. The pH value of the bleaching liquid for sodium hypochlorite bleaching is about 10, and it is carried out at room temperature. The equipment is simple, the operation is convenient, and the cost is low. However, it does great damage to the fabric strength and low whiteness. The pH value of the bleaching solution for hydrogen peroxide bleaching is 10, and it is bleached at high temperature. The bleached fabric has high whiteness and stability, good hand feeling, and can also remove pulp and natural impurities. The disadvantage is that it requires high equipment and high cost. Under proper conditions, combined with caustic soda, desizing, scouring and bleaching can be completed at one time. The pH value of the bleaching solution for sodium chlorite bleaching is 4～4.5. It is carried out at high temperature. It has the advantages of good whiteness and little damage to the fiber. However, it is easy to produce toxic gases during bleaching, pollute the environment, corrode equipment, and equipment needs special It is made of metal material, so it is limited in application. Sodium hypochlorite and sodium chlorite must be dechlorinated after bleaching to prevent the fabric from being damaged due to the presence of residual chlorine in the process.
6. Mercerizing: Mercerizing refers to the processing of cotton fabrics under tension in the warp and weft directions at room temperature or low temperature, and treated with a concentrated caustic soda solution to improve the properties of the fabric. After the cotton fabric is mercerized, due to the expansion of the fiber, the longitudinal natural twist of the fiber disappears, and the cross-section becomes an elliptical shape, and the reversal of the light is more regular, thus enhancing the luster. The increase of the amorphous fixed area of the fiber increases the dye uptake rate during dyeing. The increase in the degree of orientation increases the strength of the fabric and also has a shape-setting effect. After mercerizing, be sure to use washing to remove alkali or steaming to remove alkali, or to remove alkali by washing on the floor until the fabric is neutral.