Do you know the pretreatment technology of these knits?

Knitted fabric has a soft feel, good air permeability, and is flexible and comfortable to wear. It is often used to make sweaters, sweaters, sweatshirts, gloves and socks. Knitted fabrics are composed of loops, so they are loose in structure, easy to deform under external force, and cannot withstand large tension. Therefore, the fabric must be kept loose during processing, and low tension processing equipment shall be used. At the same time, the processing process shall be shortened as much as possible.

Pretreatment of cotton knits~

The main products of cotton knitwear are undershirt, cotton wool cloth, etc. Knitting yarn is not sized before weaving, so there is no size on the fabric. In the pre-treatment process, singeing and desizing are generally not required, and only scouring, bleaching and softening are usually carried out. Some varieties (such as undershirt) also need alkali shrinking to increase the density and elasticity of the fabric. The pretreatment process of cotton knitted fabric mainly takes the following forms.

Process flow of bleached undershirt:

Grey cloth → alkali shrinkage → scouring → sodium hypochlorite bleaching → hydrogen peroxide bleaching → whitening → softening treatment → dehydration → drying

Process flow of dyeing (printing) undershirt:

Grey cloth → alkali shrinkage → scouring → sodium hypochlorite bleaching → dyeing (printing) → finishing

Process flow of dyeing (printing) cotton wool cloth:

Grey cloth → scouring → sodium hypochlorite bleaching → dyeing (printing) → finishing

(1) Alkali condensation

Alkali shrinkage is a process in which cotton knitted fabric is treated with concentrated alkali solution in a relaxed state.

① Purpose and steps of alkali condensation

The purpose of alkali shrinkage is to increase the density and elasticity of the fabric, improve the strength and luster of the fabric, and reduce the shrinkage of the fabric. The completion of alkali shrinking process involves three steps: pickling and rolling alkali liquor, stacking and shrinking, and washing and removing alkali.

② Alkali shrinking equipment for knitted fabrics

The cylinder knitted fabric alkali shrinking machine is a combination machine consisting of cloth feeding plate, alkali soaking tank, rolling car, alkali washing tank, cloth storage box, cloth dropping device, etc., as shown in Figure I-10.

③ Alkali shrinking process

Process conditions of alkali condensation: caustic soda 140-200g/L, normal temperature, time IO-20min.

Technological process of alkali shrinking: sewing head → expanding → dipping and rolling concentrated alkali liquor → stacking (or dipping in concentrated alkali liquor) → alkali removal → hot water washing → cold water washing

There are two methods for alkali shrinkage of knitted undershirt: alkali shrinkage of dry cloth (dry shrinkage) and alkali shrinkage of wet cloth (wet shrinkage). Dry shrinkage refers to alkali shrinkage of knitted grey fabric before scouring, and wet shrinkage refers to alkali shrinkage of knitted grey fabric after scouring. The former process is simple, but the alkali absorption of the fabric is uneven; The latter has uniform alkali absorption, good elasticity and luster, but has a long process flow. In actual production, dry cloth alkali shrinkage is often used.

(2) Scouring

① Purposes of scouring

The purpose of knitted fabric scouring is to remove the cottonseed hull, cellulose symbiosis and greasy impurities on the grey fabric during weaving.

Knitted fabrics can be boiled in cloth boiling pot or rope steam. No matter which method is used, the scouring conditions of cotton cloth should be more relaxed than those of ordinary cotton cloth, so as to keep more baked substances on the cloth, so as not to affect the fabric feel and cause sewing holes.

② Scouring process

The process flow of continuous steaming and boiling of knitted fabric is as follows:

Knitted fabric after alkali shrinkage → dip rolling and scouring solution → steaming → 60-70 ℃ hot water washing → cold water washing → pickling → cold water washing → neutralization → cold water washing

(3) Bleaching

Sodium hypochlorite, hydrogen peroxide and sodium chlorite can be used as bleaching agents for knitted fabrics.

① The bleaching effect of sodium hypochlorite is general, and the fiber is easy to be damaged, which has a great impact on the environment. However, its cost is low and the equipment is simple. Therefore, it is still widely used in the bleaching of cotton knits, and has been gradually replaced by hydrogen peroxide.

② Hydrogen peroxide has good bleaching effect, little damage to fibers, and certain ability to remove impurities. It is not only suitable for cotton, but also can make other fibers obtain good bleaching effect, so it is especially suitable for bleaching of cotton and synthetic fiber blended fabrics.

③ Sodium chlorite is an ideal bleaching agent with good bleaching effect, less damage to fibers, strong ability to remove impurities and wide adaptability to fibers. However, due to its high cost, strong corrosivity to equipment, and toxic gas released during bleaching, it has a great impact on the environment, so its use is greatly limited.

The technological process and conditions for bleaching knitted fabrics are similar to those for ordinary cotton fabrics. For products with high whiteness requirements, re bleaching and fluorescent whitening treatment are also required.

(4) Soft treatment

In order to avoid sewing holes in knitted fabrics when they are ready to wear, in addition to taking appropriate measures during scouring, they also need to be softened. The most commonly used softener for knitted fabrics is self-made paraffin lotion, so the softening treatment is also called waxing. Softener is sometimes used for convenience.

Pretreatment of polyester knitted fabric~

The purpose of the pretreatment of polyester knitted fabric is to remove the oil agent, antistatic agent applied on the fiber during spinning and the oil stain stained during weaving. All fabrics set before dyeing shall be pretreated to avoid defects caused by oil stains and impurities on the fibers during heat setting.

For the pretreatment of polyester knitted fabric, 0.5%~1% (weight of fabric) soap or synthetic detergent solution can be used to add a small amount of soda ash, and then it can be treated at 80~90 ℃ for about 30min, followed by hot water washing and cold water washing.

Pure polyester knitted fabric does not need to be bleached, because the polyester itself is very white, even for the extra white varieties, it can meet the whiteness requirements only by fluorescent whitening.

Pretreatment of medium length chemical fiber fabrics~

The medium length chemical fiber fabrics are mainly polyester viscose fabrics, and there are also polyester nitrile blended medium length fiber fabrics and polyester nitrile viscose blended medium length fiber fabrics. The medium and long chemical fiber fabrics are all wool like fabrics, and the thick fabrics are required to feel full, fluffy, elastic and have a thick wool style. The quality of these styles depends on the quality of dyeing and finishing.

The medium and long chemical fiber fabrics do not contain natural impurities, and the scouring and bleaching process is relatively simple. Only singeing, desizing and heat setting are required. In order to improve wool like effect, the pretreatment of medium and long chemical fiber fabrics must adopt loose processing technology.

1. Singeing. The singeing process with less flame, strong flame and fast turning speed is adopted. The arrangement sequence of singeing process can be determined according to the factory conditions and variety requirements. For varieties dyed under high temperature and pressure, singeing should be carried out after dyeing to avoid affecting the dyeing uniformity.

2. Desizing. Desizing is carried out in loose, damp and hot processing, so that the fabric can fully shrink while desizing, so as to obtain fluffy and plump wool like style. PVA is the main size for medium and long chemical fiber fabrics, which can be desized by alkali or oxidant. After alkali desizing, the handle of the fabric is slightly rough and hard, and the dyed color is slightly dark under the same hot melt conditions. It can also be desized with detergent. The fabric after desizing has soft handle, good elasticity and easy stretching. After the desizing water is washed, it is washed on the loose washer, and then dried on the loose dryer.

3. Heat setting. The setting temperature should be 190 ℃. Setting before dyeing can ensure dimensional stability during dyeing, reduce wrinkles and improve dyeing quality.