Textile Pretreatment Processes: Singeing, Desizing, Scouring, Bleaching, Mercerizing…

The fiber, yarn, or fabric without dyeing and finishing process contains a certain amount of impurities, common impurities include natural impurities and artificial impurities. Natural impurities are impurities that exist in natural textile fibers, such as waxes, pectin, pigments, and nitrogenous substances on cotton and linen fibers, silk gum on silk, sheep fat sheep sweat on wool, etc. Artificial impurities are additives that contaminate foreign impurities in the textile process, such as slurry, oil grease, etc.

The presence of these impurities not only makes textiles rough to the touch and of low whiteness but also has poor water absorption. The most important task of textile pretreatment is to remove these impurities by scouring and bleaching for their subsequent processing. In addition to scouring and bleaching, the pretreatment process also includes several processes that are aimed at improving the quality of textiles and must be completed before other dyeing and finishing processes, such as singeing, mercerizing, heat setting, wool washing, degumming, etc. There are many different kinds of textiles and different forms, so the pretreatment processes are also different.

The types of textiles covered in this article are cotton woven fabrics, cotton knitted fabrics, ramie fabrics, wool fabrics, silk fabrics, chemical fiber fabrics…

The pretreatment processes covered in this article are: singeing, desizing, scouring, bleaching, mercerizing, heat setting, wool washing, carbonizing, alkali decrement, degumming…

Pretreatment of cotton woven fabrics: singeing, de-sizing, scouring, bleaching, mercerizing

Cotton woven fabrics mainly refer to pure cotton fabrics and polyester-cotton blended, dimensional-cotton blended fabrics. These fabrics contain cotton fiber components, so more natural impurities, the pretreatment process is more complex, and the basic content mainly includes singeing, de-sizing, scouring, bleaching, mercerizing, etc.

1 Singeing

Cotton woven fabric is made of short fibers, and the surface will form nap of varying lengths, not only affecting the fabric’s finish but also the subsequent processing cause defects. Therefore, this layer of nap must be removed by singeing, singeing is carried out on the singeing machine, and the most commonly used machine is the gas singeing machine.

The degree of nap removal is used to determine the effect of singeing, the specific method is to put the fabric after singeing in a better light and visually inspect the rating. The general fabric should reach 3~4 grade, high-quality requirements should reach more than 4 grade, and thin fabric up to 3 grade.

• Grade 1: The gray cloth is not singeing
• Grade 2: Less long nap
• Grade 3: Basically no long nap
• Grade 4: Only short nap, and neater
• Grade 5: Singeing clean

2 Desizing

The purpose of designing is to remove the pulp and some of the natural impurities from the gray cloth for subsequent scouring and bleaching. The warp yarns of cotton woven fabrics are usually sized before weaving, and then appropriate de-sizing methods should be used according to the type of pulp and impurities. Commonly used desizing methods are enzyme desizing, alkali de-sizing, acid de-sizing, oxidizer de-sizing, and plasma de-sizing.

Desizing quality assessment: In production, it is generally required that the desizing rate is above 80% or the residual pulp is below 1% relative to the cloth weight, and the remaining pulp can be further removed in the scouring process.

3 Scouring

Fabric after de-sizing, has removed most of the pulp and part of the natural impurities, but most of the natural impurities such as cottonseed shells, waxy substances, pectin, etc. remain on the fabric, and must be completely removed after the scouring process. Caustic soda is mainly used for scouring, and the common additives include surfactants, sodium silicate, and sodium bisulfite.

Caustic soda has a strong ability to remove impurities. It can dissolve and disintegrate cottonseed hulls at high temperatures, it can hydrolyze nitrogenous substances and pectin into soluble substances, and it can saponify fatty acids on fibers and then emulsify and remove wax-like substances. The Surfactants can help wet the fabric and improve the effect of debridement. Sodium silicate is mainly used to adsorb iron and other impurities in the practice solution to prevent embroidery stains and deposition of impurities on the surface of the fabric and to improve the water absorption and whiteness of the fabric.

Effect assessment method: The purpose of scouring is to enhance the wetting performance and permeability of the fabric. Immerse one end of the fabric vertically into the water and measure the height of the water rising within 30 minutes, the general requirement is 8~10cm in 30min.

4 Bleaching

After scouring, a large number of impurities are removed, but pigments are still present and the appearance is not white enough. The purpose of bleaching is to remove pigments and give the fabric the necessary and stable whiteness without damaging the fiber.

The natural pigment on cotton fiber and its coloring system can be destroyed by the oxidizing agent to achieve the purpose of decolorization. Therefore, bleaching mainly uses oxidants, and the commonly used bleaching oxidants for cotton-type fabrics are sodium hypochlorite, hydrogen peroxide, and sodium chlorite.

5 Mercerizing

When natural cellulose fibers interact with highly concentrated (12.5%) caustic soda solution, irreversible swelling occurs, the molecular structure changes and the adsorption capacity for water and dyes increases. If proper tension is applied to the fiber during the swelling process, the fiber morphology changes and shows regular reflection to light, which can improve the luster of the fabric. This process is called mercerizing.

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In addition to mercerizing with caustic soda, cotton fabrics can also be treated with liquid ammonia for the mercerizing process. The temperature of liquid ammonia is -33℃, which can penetrate the fiber quickly and the treatment effect is uniform, especially suitable for high twist and compact fabrics. At present, the research and application of this kind of technology are still relatively small, Germany Monfort already has the relevant equipment and process.

Pretreatment of cotton knitted fabrics: scouring, bleaching, alkali decrement

The voids between the yarns of knitted fabrics are larger and the yarns are not sized before weaving, so the pretreatment is easier. The requirements for scouring and bleaching of knitted fabrics vary with the varieties and uses. For example, cotton sweaters are mostly natural or dark in color, so they simply need scouring, not bleaching.

In addition to scouring and bleaching, cotton knitted fabrics often need to do alkali decrement treatment, which refers to treating the fabric with concentrated caustic soda solution in a relaxed state to cause free shrinkage of fiber yarns to increase the density and elasticity of the fabric.

After scouring and bleaching, the grease and wax on the knitted fabric fibers are removed, and the fibers do not slide easily when sewing and are easily broken by the needle, resulting in the dispersion of the loops, so it is often necessary to carry out soft treatment, i.e. dewatering and then dipping and rolling paraffin emulsion and drying.

Pretreatment of ramie textiles: degumming, singeing, scouring and bleaching

Compared with cotton textiles, in addition to pectin and oil wax, ramie has a large amount of sugar gum and a small amount of lignin, which makes pretreatment relatively difficult.

Ramie contains a large amount of sugar gum, which is removed before spinning, commonly known as degumming. After degumming, ramie is made into fabric, due to the rigidity of ramie fibers, there is much hairiness after yarn formation, and there is a prickly feeling when wearing ramie, so it is necessary to singeing, in addition, the ramie fabric should have proper scouring and bleaching.

Pretreatment of wool textiles: wool washing, carbonization, bleaching

Wool contains a large amount of impurities, mainly sheep fat and sweat, as well as some foreign dust and debris, so wool washing is the main process to remove the sheep fat, sweat, and dust.

The most common process of wool washing is the soap alkaline method, using a washing solution containing soap and soda ash to treat the wool. The wool-washing process requires strict control of temperature, pH, and time to avoid damage to the wool.

For impurities such as branches, leaves, and grass seed fragments in wool, hydrolysis in strong inorganic acids, i.e. carbonization, is required. For example, if wool is treated with sulfuric acid, the impurities in the wool become fragile hydrolyzed cellulose at high temperatures. Wool is more resistant to acid than cellulose and will not be significantly damaged as long as the carbonization conditions are right. In addition, for wool fabrics with high whiteness requirements, bleaching is required.

Pretreatment of silk textiles: degumming, bleaching

Raw silk is mainly composed of fibroin and sericin, as well as impurities such as fat waxes, inorganic substances, and pigments, which can affect the feel and luster of silk. In addition, oil, pulp, and coloring dyes have adhered during the weaving process, these natural and foreign impurities must be removed by pretreatment.

The pretreatment of silk includes two aspects: degumming and bleaching. The purpose of degumming is to remove sericin. For textile products with high whiteness requirements, a small amount of reducing or oxidizing agent can be added to the refining for bleaching without a separate bleaching process.

Pretreatment of chemical fiber textiles

Chemical fibers have very few impurities and the purpose of pretreatment is mainly to remove the sizing and oil stains added during the weaving process, so the process is not complicated.

1 Pretreatment of artificial fiber: singeing, de-sizing, scouring, bleaching

The pretreatment of viscose fiber is usually composed of several processes of singeing, de-sizing, and bleaching. Desizing commonly used synthetic detergent, the scouring process can be omitted, and the bleaching process is not necessary, if necessary, an available oxidation bleaching process.

It should be noted that viscose fibers have a relaxed structure, high chemical sensitivity, and low wet strength, so strong chemical action and mechanical tension should be avoided during processing, and milder process conditions and non-tension equipment should be used for processing.

2 Pretreatment of synthetic fiber pretreatment: alkali decrement, heat setting

The scouring and bleaching of synthetic fiber are relatively simple, desizing can use hot water, scouring commonly used soda ash, and surfactants for medium and high-temperature treatment, generally do not need bleaching.

Most synthetic fibers are thermoplastic fibers, and they produce wrinkles that are difficult to eliminate under hot and humid conditions and cause dimensional changes in the fabric due to shrinkage, so the heat setting process must be used to improve this defect.

Heat-setting is the process of making the fabric under proper tension, maintaining a certain size, treating it with high temperature for a certain time, and then cooling it at a proper speed. Heat setting can eliminate wrinkles, improve heat stability performance, and improve the strength, hand feel, and dyeability of the fabric to a certain extent.

The heat setting process varies with fabric variety, structure, cleanliness, dyeing method, and factory conditions, and commonly includes gray cloth setting, pre-dyeing setting, and post-dyeing setting.

The above is the content of the pretreatment process of common textiles, about each step, there is no detailed introduction here, if you want to know the textile industry-related information, please save or subscribe to the ChiuVention Blog, here will continue to update for you.