Why is cotton cellulose resistant to alkali but not acid? What is its mechanism of action?
Answer The chemical structure of cotton cellulose macromolecules is composed of β-D glucose residues linked to each other by l, 4-glycoside bonds. The two terminal glucose residues of the macromolecule have different groups, and there are four free hydroxyl groups at one end. There are three free hydroxyl groups and one hemiacetal hydroxyl group at the other end. The glycoside bonds in the cellulose macromolecular chain have high stability to alkali, but are prone to hydrolysis to acids, which reduces the degree of polymerization of the macromolecules, weakens the intermolecular force, and thus reduces the strength.
At present, there are different opinions on the mechanism of action of cellulose on alkali, but it is generally believed that alkali can only act on the hydroxyl groups on the fiber macromolecules. Some people rinse the product after the action of concentrated alkali and cellulose with alcohol and other reagents for a long time, but some of the alkali cannot be washed away, indicating that the alkali actually reacts chemically with the cellulose. There are two theoretical explanations for its working principle: one view is that cellulose is a weak acid, which undergoes a similar neutralization reaction with alkali to form a sodium alcohol compound; the other view is that the alkali combines with the hydroxyl group of cellulose to form a molecular compound , that is, the theory of molecular combination. It is generally believed that the above two reactions may exist at the same time. But the product after the action of alkali and cellulose is called alkali cellulose. It is an unstable compound. It can still restore the original cellulose molecular structure after being washed with water, but the microstructure of the fiber has changed and the crystalline area has been reduced. The amorphous zone increases. The crystallinity of natural cotton fiber is 700. After the action of concentrated alkali, the crystallinity of silk-finished cotton fiber is reduced to 50%~60%. This shows that the alkali liquid puffs the fiber and destroys some of the crystallization areas. This effect is very effective. It has practical significance and is an important link in the dyeing and finishing of cotton fiber. Therefore, in dyeing and finishing, the effects of different concentrated alkali can be used to obtain silk finishing, alkali shrinkage and other effects, and can also produce products such as seersucker.
When cellulose fibers are exposed to acid, they often become hard to the touch, lose strength, or even become completely carbonized. This is because acid catalyzes the hydrolysis of glycoside bonds in cellulose molecules. After hydrolysis, a mixture with different degrees of polymerization is formed, which is called hydrolyzed cellulose. The chemical composition of hydrolyzed cellulose is no different from that of original cellulose, except that the degree of polymerization is lower and the relative molecular weight of cellulose is greater in polydispersity. If hydrolyzed cellulose continues to react with acid, the polymerization degree drops to about 50, which is called cellulose dextrin. If the cellulose glycoside bond is completely hydrolyzed and broken, the product is glucose. The factors that affect the hydrolysis of cellulose are mainly the properties of the acid, reaction temperature and action time. In actual production, if the acid process is used properly, serious fiber damage will not occur. The general rule of action between acid and cellulose is that the stronger the acidity, the stronger the catalytic ability and the faster the hydrolysis rate. Strong inorganic acids such as hydrochloric acid, sulfuric acid, and nitric acid are very strong in hydrolyzing cellulose; weak acids such as phosphoric acid and boric acid have weak catalytic activity; organic acids such as acetic acid are more gentle. Although acid is harmful to cellulose fibers, it still has many practical uses as long as it is used properly. For example, if fabrics are bleached with chlorine-containing bleach and then treated with dilute acid, the bleaching effect can be further enhanced; acid can be used to neutralize the remaining alkali on the fabric; cotton fabrics are treated with acid to produce organza; polyester/cotton fabrics are burnt-out sheaths, etc.
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