Basic principles and purpose of cooking and bleaching cotton fabrics
The principle and purpose of cotton fabric refining is to use chemical and physical methods to remove natural impurities from cotton fabrics in order to achieve the purpose of refining and purifying cellulose. Cooking is a very important part of the pre-treatment process. It especially emphasizes the repeatability of the process and the consistency of product quality.
Cotton fibers with high maturity have a cellulose content of more than 94%, and the content of impurities that need to be removed during refining of the fabric is about 6%.
The closer the outer layer is to the surface, the higher the content of impurities; these impurities are mainly located on the surface of the primary cell wall and form a hydrophobic network layer with pectin as the backbone through the pectin main chain and branch chain adhesion. The main purpose of refining is to destroy and remove this hydrophobic network layer, especially to remove wax.
During the growth of natural fibers, a pigment associated with acid dyes is produced. This pigment cannot be completely removed during the cooking process, and can only be removed through the bleaching process.
The purpose of bleaching is mainly to remove natural pigments and improve the whiteness of the fabric; at the same time, other impurities remaining on cotton fabrics, such as cottonseed hulls, waxy substances, and nitrogen-containing substances, are also removed to further improve the wettability of cotton fabrics .
Oxidants and reducing agents can bleach natural fibers, but the reducing agents cannot completely destroy the pigment in natural cellulose. One of the reduced bleached fabrics has limited whiteness, and the other is that the fabrics will still yellow after a long time. Therefore, the bleaching effect is ideal; and the oxidant can destroy the pigment molecules in natural fibers, and the bleaching is durable.
Commonly used reducing bleaching agents are: sodium sulfite, sodium hyposulfite, insurance powder, etc. Commonly used oxidizing bleaches are: hypochlorite, sodium chlorite, hydrogen peroxide, potassium permanganate, etc.
At present, the main method for bleaching cellulose fibers in China is hydrogen peroxide bleaching. Hypochlorite bleaching is only used in hemp fabrics. Other bleaching methods are basically replaced by the above methods for various reasons. There is only dialogue in the production process. For fabrics with special requirements, other bleaching methods will be used.