Fibers, yarns, or fabrics that have not been dyed and finished contain some impurities. Common impurities include natural impurities and artificial impurities.
Natural impurities refer to impurities already existing in natural textile fibers, such as waxy substances, pectin, pigments, nitrogenous substances on cotton and linen fibers, sericin on silk, suet and sheep sweat wool, etc. Artificial impurities refer to additives and foreign impurities contaminated during textile processing, such as size, oil, and oil.
The existence of these impurities not only makes the textile feel rough and the whiteness is low, but also the water absorption is poor. The most important task of textile pretreatment is to remove these impurities through scouring and bleaching so as to facilitate their subsequent processing. In addition to scouring and bleaching, pretreatment processing also includes some treatment processes aimed at improving the quality of textiles. It must be completed before other dyeing and finishing processes, such as singeing, mercerizing, heat setting, scouring, degumming, etc. There are many kinds of textiles and different forms, so their pretreatment processes are also different.
Pretreatment of cotton and cotton-type woven fabrics: singeing, desizing, scouring, bleaching, mercerizing.
Cotton and cotton-type woven fabrics mainly refer to pure cotton fabrics and polyester/cotton, dimension/cotton, and other blended fabrics. Because this fabric contains cotton fiber components, there are many natural impurities, and the pretreatment process is relatively complicated. The basic content mainly includes singeing, desizing, scouring, bleaching, mercerizing, etc.
1- singeing
Cotton and cotton-type woven fabrics are made of short fibers, and fluff of different lengths will form on the surface, affecting the smoothness and beauty of the fabric but also causes defects in subsequent processing, so roasting is required to remove this layer of fluff. Singeing is carried out on a singeing machine, and the most commonly used singeing machine is a gas singeing machine.
The singeing effect is judged by the degree of fuzz removal. The specific method is to put the single fabric under better light, refer to the standard, and visually evaluate it. The singeing quality of general fabrics should reach grade 3~4, those with high-quality requirements should reach grade 4 or above, and thin fabrics should reach grade 3.
2- Desizing
The purpose of desizing is to remove the size and some natural impurities on the original cloth for subsequent scouring and bleaching. The warp yarns of cotton and cotton-type woven fabrics are usually sized before weaving. When desizing, it is necessary to adopt an appropriate method according to the type of sizing and the impurity content. The commonly used desizing methods are enzyme desizing, alkali desizing, acid desizing, oxidant desizing, and plasma desizing.
Desizing quality assessment: 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 scouring process.
3- Scouring
After desizing, the fabric has removed most of the sizing and some natural impurities, but most of the natural impurities, such as cottonseed hulls, waxy substances, pectin, etc., remain on the fabric and must be completely removed through the scouring process. Caustic soda is mainly used for scouring, and commonly used additives include surfactants, sodium silicate, and sodium bisulfite.
Caustic soda has a strong ability to remove impurities. It can swell and disintegrate cottonseed hulls at high temperatures, hydrolyze nitrogen-containing substances and pectin into soluble substances, saponify fatty acids on fibers, and emulsify and remove waxy substances. Surfactants can help wet fabrics and improve the effect of impurity removal. Sodium silicate, commonly known as sodium silicate, is mainly used to absorb iron and other impurities in the training liquid, prevent embroidery stains and deposit impurities on the surface of the fabric and improve the water absorption and whiteness of the fabric.
Effect evaluation method: The purpose of scouring is to enhance the wettability and permeability of the fabric. One end of the fabric is vertically immersed in water, and the height of the water rising within 30 minutes is measured. It is generally required to reach 8~10cm within 30 minutes.
4- Bleach
The fabric has been scoured to remove a lot of impurities, but there are still pigments, and the appearance is not white enough. The purpose of bleaching is to remove the pigment under the premise of ensuring that the fibers are not damaged and to give the fabric the necessary and stable whiteness.
The natural pigment on cotton fiber and its color system can be destroyed by oxidants to achieve the purpose of decolorization. Therefore, oxidants are mainly used for bleaching, and the commonly used bleaching oxidants for cotton fabrics are sodium hypochlorite, hydrogen peroxide, and sodium chlorite.
5-mercerized
When natural cellulose fibers such as cotton and hemp interact with high-concentration (12.5%) caustic soda solution, they will swell irreversibly, the molecular structure will change, and the adsorption capacity of water and dyes will be improved. If proper tension is applied to the fiber during the swelling process, the shape of the fiber will change and reflect light regularly, which can improve the luster of the fabric. This process is called mercerizing.
In addition to mercerizing with caustic soda, cotton cloth can also be mercerized with liquid ammonia. The temperature of liquid ammonia is -33℃, it can quickly penetrate into the fiber, the degree of swelling of the fiber is small, and the treatment effect is uniform, especially suitable for high twist and tight fabrics.
Pretreatment of cotton fiber knitted fabrics: scouring, bleaching, alkali shrinkage, softening
The gaps between the yarns of knitted fabrics are large, and the yarns are not sized before weaving, so the pretreatment is easier. Scouring and bleaching requirements for knitted fabrics vary by variety and use. Undershirts, etc. require good water absorption and whiteness, soft hand feeling, and higher requirements for cooking and bleaching. Cotton sweaters are mostly natural or dark, so they must only be scoured, not bleached.
In addition to scouring and bleaching, cotton-type knitted fabrics often need to be treated with alkali shrinkage. Alkali shrinkage is also called alkali reduction. Density and elasticity.
Knitted fabrics after scouring and bleaching, because the grease and wax on the fibers are removed, the fibers are not easy to slip during sewing and are easily broken by needles, causing the coils to fall apart. Therefore, softening treatment is often required, that is, dehydration and dipping in paraffin wax Lotion and tumble drying.
Pretreatment of ramie textiles: degumming, singeing, scouring, bleaching
Compared with cotton textiles, in addition to pectin and oil wax, the impurities in ramie also have a large amount of sugar gum and a small amount of lignin, so the pretreatment is relatively difficult.
Ramie contains a lot of sugar gum, which should be removed before spinning, commonly known as degumming. After the degummed ramie is made into fabric, due to the high rigidity of the ramie fiber, there will be a lot of hairiness after yarn formation, and it will feel itchy when worn. Therefore, it needs to be signed.
Pretreatment of wool textiles: scouring, carbonization, bleaching
Wool contains a lot of impurities, mainly sheep fat and sheep sweat, as well as some foreign dust and debris. Wool washing is the main process to remove sheep fat, sheep sweat, and dust.
The most commonly used process for wool washing is the soap-alkali method. The wool is treated with a washing solution containing soap and soda ash. The wool washing process must strictly control the temperature, PH value, and time to prevent the wool from being damaged.
For impurities such as branches, leaves, and grass seed fragments in wool, carbonization treatment is required, that is, hydrolysis in strong mineral acid. For example, wool is treated with sulfuric acid to make the impurities in it turn into fragile hydrolyzed cellulose at high temperatures. Wool is more acid-resistant than cellulose. As long as the carbonization conditions are appropriate, it will not be significantly damaged. In addition, wool fabrics with high whiteness requirements must be bleached. The bleaching of wool usually adopts the process of oxidative bleaching and then reduction bleaching.
Pretreatment of silk textiles: degumming, bleaching
aw silk is mainly composed of fibroin and sericin, as well as impurities such as fat wax, inorganic substances, and pigments, which will affect the feel and luster of silk. In addition, oil, size, and coloring dyes also adhere during the weaving process. , these natural and foreign impurities must be removed by pretreatment.
The pretreatment of silk includes two aspects of degumming and bleaching. The purpose of degumming is to remove sericin. For textile products requiring high whiteness, a small amount of reducing or oxidizing agent can be added for bleaching during refining without a separate bleaching process.
Pretreatment of chemical fiber textiles
There are few impurities in chemical fibers, and the purpose of pretreatment is mainly to remove the added size and oil stains during weaving, so the process is not complicated.
1. Pretreatment of man-made fibers: singeing, desizing, scouring, bleaching
The pretreatment of viscose fiber usually involves singeing, desizing, and bleaching. Synthetic detergents are commonly used for desizing, the scouring process can be omitted, and the bleaching process is not necessary. If necessary, an oxidation bleaching process can be used.
It should be noted that viscose fiber has a loose structure, high chemical sensitivity, and low wet strength. Strong chemical action and mechanical tension should be avoided during processing, and mild process conditions and tension-free equipment should be used for processing.
2.Synthetic fiber pretreatment: alkali shrinkage, heat setting
The scouring and bleaching of synthetic fibers are relatively simple. Hot water can be used for desizing, and soda ash and surfactants are commonly used for refining at medium and high temperatures. Generally, bleaching is not required.
Synthetic fibers are mostly thermoplastic fibers. They will produce wrinkles that are difficult to eliminate under hot and humid conditions, and the size of the fabric will change due to shrinkage. Therefore, this defect must be improved through the heat-setting process.
The so-called heat setting is to make the fabric maintain a certain size under appropriate tension, treat it with a high temperature for a certain period of time, and then cool it at an appropriate speed. Heat setting can eliminate wrinkles, improve thermal stability, and improve the strength, feel, and dyeability of fabrics to a certain extent.
The heat setting process varies with the fabric type, structure, cleanliness, dyeing method, and factory conditions. The common ones are gray cloth setting, setting before dyeing and setting after dyeing.