Dyeing and finishing of fabrics is an important part of fabrics, any slight mistake may lead to the failure of dyeing and finishing of fabrics, causing defects in the whole batch of fabrics, affecting the quality and price. The following six steps will explain in detail the details of the fabric before dyeing and finishing.

⑴ Natural-color cotton design: There are three main points in natural-color cotton design: natural-color cotton inspection——turning cloth (batch, boxing, printing)—seaming.

⑵Singeing: The fluff on the surface of the fabric will affect the quality of dyeing and finishing and the wearability, so it must be singed to make the cloth surface clean. Singeing is to let the natural color cotton cloth pass through the flame of the singeing machine or rub the red hot metal surface quickly in an open width, so that the fluff on the cloth surface burns due to rapid heating, while the cloth itself has a relatively compact, thick and warm structure. Slower, before the temperature reaches the ignition point, it has already left the flame or red-hot metal surface, so as to achieve the purpose of burning the fluff without damaging the cloth.

(3) Desizing: Before weaving of woven fabrics, the warp yarns are generally subjected to sizing treatment to improve the warp yarn strength, wear resistance and smoothness, and facilitate weaving. However, the size on the natural color cotton cloth is not good for printing and dyeing, because the existence of the size will contaminate the finishing solution, consume dyeing materials, and even hinder the dyeing materials from dyeing fibers, affecting the quality of printing and dyeing products. Therefore, fabrics must be desized at the beginning of dyeing and finishing.

Desizing can be divided into enzyme desizing, alkali desizing, acid desizing and oxidant desizing. Desizing alkali is a common method used in printing and dyeing factories at present. It is used for pure cotton or blended chemical fiber fabrics, and it has a removal effect on most of the sizing. It can also decompose and remove the natural impurities on the cotton fiber to a certain extent, but because alkali desizing only reduces the adhesion between the size and the cloth, it cannot degrade the size, so it must be fully washed after desizing, and the lotion must be replaced constantly . Since the caustic soda used for desizing is generally spent soda, the cost of desizing is relatively low.

(4) Scouring: After the cotton fiber fabric is desized, most of the pulp and a small part of natural impurities have been removed, but most of the natural impurities in the cotton fiber, such as waxy substances, pectin, nitrogenous substances, cottonseed hulls, etc. And a small part of oil agent and a small amount of size remain on the cloth, making the cotton fiber fabric surface yellower, poor moisture absorption and permeability, and cannot meet the requirements of subsequent dyeing and finishing. In order to make the cotton fiber fabric have certain water absorption and permeability, which is beneficial to the adsorption and diffusion of dye auxiliaries during the dyeing and finishing process, it must be scoured after desizing to remove most of the residual impurities in the cotton fiber .

(5) Bleaching: After scouring the cotton fiber fabric, most of the impurities have been removed, and the water absorption has been greatly improved. However, due to the presence of natural pigments on the fiber, the appearance is not white enough. Except for a few varieties, it is generally bleached. Do not bleach, otherwise it will affect the color vividness of the coloring or printing. The purpose of bleaching is to destroy the natural pigment without visibly damaging the fibers, to give the fabric the necessary and stable whiteness, and to remove impurities remaining after scouring (especially cottonseed hulls). The bleaching agents currently used for cotton fiber fabrics mainly include sodium hypochlorite, hydrogen peroxide (commonly known as hydrogen peroxide) and sodium chlorite. These three bleaches each have their own characteristics and uses.

⑹Silk finish: Silk finish refers to the fact that the cotton fiber fabric is treated with concentrated caustic soda under a certain tension and maintains the required size. As a result, the fabric obtains a silky finish. In addition, the strength of the fabric , elongation and dimensional and morphological stability have also been improved, and the chemical reaction ability of the fiber and the adsorption ability of the dye have also been improved, so the silk finish of the fiber-containing fabric is one of the important tasks of dyeing and finishing.

The main factors affecting the effect of silk finish are the concentration of lye, temperature, action time and tension applied to the fabric. The commonly used method to test the effect of silk finish is the barium value method to measure the adsorption capacity of cotton fibers to chemicals. The higher the barium value, the better the effect of silk finish. Usually the barium value of the finished cotton fiber fabric is 100. The barium value of the thick silk fabric is usually between 130 and 150, and the barium value above 150 means that the cotton fiber is fully silked.

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