15 May Leather and Textile
Leather making refers to the process of tanning raw leather into leather. Removal of hair and non-collagen fibers, so that the dermis collagen fibers moderately loose, fixed and strengthened, plus finishing (physical) and a series of chemical (including biochemical) and mechanical treatment. The tanning process is usually divided into three stages: preparation, tanning and finishing. The most used equipment in the tannery process is the drum, the process of soaking, soaking, hair removal, softening, acid leaching, tanning, dyeing, and emulsion filling are all completed in the drum. Through the mechanical action of the drum, it promotes the uniform penetration of various chemical materials and completes the chemical effect of the preparation to the skin.
The process of leather industry mainly includes the following three aspects: (1) Preparatory engineering removes the useless leather products (such as wool, skin, fat, fibrous interstitium, subcutaneous tissue, etc.) from raw leather, and loosely collagen fibers to prepare for tanning. (2) Tanning engineering is the deterioration process from raw leather to leather, and it is the key of the whole leather processing process. Tanned leather can obtain a series of tanning effects. (3) Finishing engineering can obtain a variety of use value through a series of leather chemicals and various mechanical processing.
The preparation process for light and heavy leather is roughly the same. The raw hide with the same specifications and production areas was formed into production batches according to the requirements of the products. After a series of chemical treatments, the structure of raw hide fibers was moderately loose. Remove unnecessary substances in raw skin, remove all kinds of non-collagen components in dermis as far as possible without damaging collagen, and create necessary conditions for tanning.
Because the thickness of light leather is much thinner than the natural thickness of raw material leather, a series of treatments for the natural thickness of raw material leather must be carried out by various machines starting from the preparation section. For example, the raw skin after soaking should be removed by a meat remover, and the semi-finished products after soaking or tanning should be divided into the required thickness according to the thickness of the finished leather.
Dehairing and softening
The process of proper separation of collagen fibers by removing the fibrous interstitium between fur and epidermis on raw skin and collagen fibers. The common depilation methods are alkali and enzymatic methods. In addition, oxidative depilation and dimethylamine depilation require strict protective measures, which are harmful to the health of operators, so they are seldom used in production.
1. Alkali depilation. There are lime-alkali method, saline-alkali method, alkali-alkali method and so on. Sodium sulfide is mainly used to hydrolyze hard keratin on raw skin and epidermis in alkaline solution, desulfurize disulfide bond, weaken the attachment of hair and epidermis to dermis, and achieve the purpose of depilation. At the same time, the fibrous interstitium was partially removed, which resulted in the separation of collagen fibers. The harmful sulfides in the waste liquor of alkaline depilation must be treated effectively to prevent environmental pollution.
2. Enzymatic depilation. The fur and epidermis of raw skin are removed by biochemical action. Protease is used to digest the basic substances around hair follicles and linking the epidermis with dermis, destroy the structure of soft keratin, weaken the close attachment of hair and epidermis to dermis, and make the hair and epidermis peel off under mechanical action, so as to achieve the purpose of peeling. Enzymatic depilation is accompanied by enzymatic softening. The process control conditions of enzymatic depilation are stricter than that of alkali depilation, but its waste liquid contains no sulfide, which is beneficial to environmental protection.
The use of enzymes to remove scales, digest some non-collagen proteins and all degradable substances, make the leather soft and smooth. Trypsin is usually used for softening, and other microbial proteases also have softening effect. The softening effect of various enzymes is better, and the softening effect of trypsin combined with microbial protease is better.
The process of transforming naked leather into leather. After depilation and a series of preparatory processing of raw skin is called naked skin. Only such bare skin can be tanned.
Tanning of light leather
The naked leather of light leather is neutralized, dyed and filled with emulsion after tanning. Most varieties need retanning after neutralization. Generally speaking, these are tanning stages.
1. Tanning methods: Light leather is mainly tanned with mineral tanning materials, the most important of which is chrome tanning. For light leather, it can be tanned with aluminum and zirconium, etc.
Chrome tanning is the main tanning method of modern light leather. The conventional one-bath chrome tanning method is to use the acid-leaching waste liquor from the original drum to add chrome tanning liquor with alkalinity of 33% and 45% successively. The drum rotates continuously for 3-4 hours. After the chrome tanning liquor penetrates into the inner layer of raw skin, a certain amount of alkali extractant (such as sodium bicarbonate) is added to improve the alkalinity of the tanning liquor and enhance the tanning effect in order to increase Cr3+ and collagen. The combination of carboxyl groups on chrome tanned leather is completed until the chrome tanned leather does not shrink in hot water not less than 95 C. With the continuous improvement of chrome tanning process, oil pretreatment one bath chrome tanning, alkyl sulfonyl chloride pretreatment one bath chrome tanning, variant two bath chrome tanning, concealed chrome tanning and less chrome tanning have appeared, which have certain effects on increasing the absorption and combination of raw leather to chrome, reducing the chrome content in chrome tanning waste liquor, and reducing the pollution of chrome in tanning wastewater.
Aluminum tanning is an ancient tanning method. Aluminum tanned leather is characterized by white, soft, compact and meticulous grain. But after washing, the leather will be untanned, and the wet leather will become flat, thin and hard after drying. So this method is seldom used alone. Aluminum salts are mostly used in combination tanning.
Zirconium tanning began in the early 1930s. Zirconium tanned leather is white, plump and firm, but its elasticity and moisture-heat stability are not as good as chrome tanned leather, and its shrinkage temperature is about 90 C. Zirconium tanning is carried out under strong acidity (pH less than 3). If improperly controlled, it is easy to produce surface overtanning and grain shrinkage. If adding organic acid or