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Printing And Dyeing Process Of Cotton And Its Blended Fabrics

   2022-04-06 2260
First Printing and dyeing process of cotton and its blended fabrics  Cotton textile and its printing and dyeing industry is the largest, well-founded and long-established major industry in my country's textile industry, and has a history of ne

First   Printing and dyeing process of cotton and its blended fabrics  Cotton textile and its printing and dyeing industry is the largest, well-founded and long-established major industry in my country's textile industry, and has a history of nearly a hundred years of development in my country. The variety and quantity of dyestuffs used in the cotton printing and dyeing industry are the largest. Since cotton fibers contain a large number of natural impurities that can affect subsequent processing and product quality, they need to be removed by chemical methods. The binding force between cotton fiber and dye mainly depends on van der Waals force and hydrogen bond. The dyes used are mainly reactive, reduced, direct, vulcanized, etc. These dyes are relatively cheap, but the dyeing rate is not too high, especially sulfur dyes. The dye uptake rate is only about 30%, and the rest remain in the water, resulting in a high concentration of pollutants in the wastewater. The chemical fibers in cotton blended fabrics are mainly polyester, which is mainly dyed with disperse dyes. Compared with the above-mentioned dyes suitable for cotton fabrics, disperse dyes have a higher dyeing rate, but there are many fillers in the dyes, which also give Difficulties in wastewater treatment.

1. Singeing  During spinning, there are many short and loose fibers exposed on the surface of the yarn, coupled with the friction of the loom, a layer of short fibers stands on the surface of the fabric. This layer of fluff not only affects the smoothness and quality of the fabric. It is easy to be contaminated with dust, and various defects will occur in the printing and dyeing process. The purpose of singeing is to burn off the fluff on the cloth surface, make the cloth surface smooth and beautiful, and prevent uneven dyeing and printing defects due to the existence of fluff during dyeing and printing. 

Singeing is to quickly pass the grey cloth through the flame or wipe the red-hot metal surface. At this time, the fluff existing on the cloth quickly heats up and burns, while the grey cloth is relatively tight and the temperature rises slowly, and it has left before reaching the ignition point. The flame or red hot metal surface can be used to burn off the fluff without damaging the fabric.  Fabric singeing is a physical dry process without waste water. 

2. Desizing  During weaving, warp yarns are subject to greater tension and friction due to the opening and pick-up action, and warp breakage often occurs. To proceed smoothly, the warp yarns are generally subjected to sizing treatment before weaving. 

Warp sizing is convenient for weaving, but it brings many difficulties to the printing and dyeing process. It not only affects the permeability of the fabric, but also hinders the contact of chemicals and dyes with fibers, consumes more dyeing chemicals, and also affects product quality. Therefore, desizing should be carried out before the cotton cloth is refined and bleached. Desizing not only removes the size of the cotton cloth, but also removes some of the natural impurities from the cotton fibers. Desizing methods mainly include enzyme, alkali, oxidant desizing and so on. The sizing on the fabric mainly includes natural sizing (such as starch, etc.), synthetic sizing (such as polyvinyl alcohol (PVA)) and cellulose sizing (such as carboxymethyl cellulose (CMC), etc.). 

Enzyme is a protein secreted by some animals, plants or microorganisms, which has a specific catalytic effect on the decomposition of certain substances. The most commonly used enzyme in desizing is amylase, which only catalyzes the decomposition of starch. 

Under the action of hot caustic soda (sodium hydroxide), starch or chemical slurry will swell violently, and the solubility will increase, and then washed with hot water to remove the slurry. 

Under the action of oxidizing agent, starch and other slurries are oxidized and degraded until the molecular chain is broken, the solubility increases, and it is easy to remove after washing with water. The oxidants used for desizing include hydrogen peroxide, sodium bromite, persulfate, etc. 

Due to the different types of sizing on fabrics, different desizing methods and processes are required, and the quality of wastewater produced is quite different. The difficulty also increases. 

3. Scouring  After the cotton cloth is desizing, although most of the pulp and a small part of natural impurities have been removed, there are still most of the natural impurities, such as waxy substances, pectin substances, nitrogenous substances, cottonseed hulls and some oils. The presence of these impurities makes the cotton fabric yellowish and has poor permeability, which cannot meet the requirements of subsequent processing such as dyeing, printing and finishing. Therefore, scouring should be carried out after desizing. Scouring is the process of chemically removing natural impurities from cotton cloth and refining and purifying cellulose. 

The main agent for scouring is caustic soda (sodium hydroxide). In addition, commonly used scouring aids include surfactants, sodium silicate and sodium bisulfite. Caustic soda can saponify fatty acid esters in waxy substances, and fatty acids generate sodium salts, which are converted into emulsifiers, and can emulsify and remove waxy substances that are not easy to saponify. In addition, caustic soda can hydrolyze pectin substances and nitrogen-containing substances into soluble substances and remove them. The cottonseed husk swells and becomes soft during the alkaline cooking process, and then after washing and rubbing, the cottonseed hull disintegrates and falls off. Surfactants can reduce surface tension and play the role of wetting, cleaning and emulsifying. Under the action of surfactant, the scouring liquid wets the fabric and penetrates into the fabric, which helps to remove impurities and improve the scouring effect. Sodium sulfite can make the lignin in cottonseed hulls into soluble sodium lignosulfonate and help the removal of cottonseed hulls. Sodium silicate has the ability to absorb iron in scouring solution and impurity decomposition products in cotton fibers, which can prevent rust spots or redeposition of impurity decomposition products on cotton fabrics, and help improve the water absorption and whiteness of cotton fabrics. 

Due to the large use of caustic soda, surfactants and other additives in the scouring liquid, the pH value of the residual liquid is high and the concentration of organic pollutants is high. 

4. Bleach  After the cotton cloth is scouring, although impurities are removed to a certain extent, there are still residual pigments, and the appearance is not white, which affects the color and luster of dyed and printed fabrics, so bleaching is required. 

The purpose of bleaching is mainly to remove natural pigments and give cotton the necessary whiteness. At the same time, it can also further remove other impurities remaining on the cotton and improve its wetting performance. 

There are two types of bleaching agents used in cotton bleaching: reducing type and oxidizing type. Reduction bleaching agents destroy pigments through reduction, but the effect is unstable. After bleached fabrics are placed in the air for a long time, the damaged pigments will be re-oxidized and recolored, so reducing bleaching agents are rarely used. Commonly used oxidative bleaching agents are mainly sodium hypochlorite, hydrogen peroxide, sodium chlorite, etc. Under certain conditions, they can decompose to produce oxidative groups, destroy the chromophoric groups in the fabric pigments, and cause them to lose their color and play a bleaching role. 

Because the bleaching requires less chemicals and removes impurities, the pollutant content and chromaticity of the wastewater discharged are low. 

5. Mercerizing  Mercerizing is a process of padding concentrated alkali on cotton fabrics under a certain tension state. After the fabric is mercerized, the dimensional stability is improved, and the shrinkage rate is reduced; the breaking strength is improved, and the breaking elongation is reduced; the adsorption capacity of dyes and moisture is improved; and it has a good gloss.

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