Tanning processes

K. H. Gustavson, The Chemistry of Tanning Processes, Academic Press, Inc., New York, 1956. 

Leather. Natural leather is made from hides, which are salted and cured, then taimed. Through the preparing process, useless matter which caimot be taimed, such as outerskin (epidermis) and flesh, are removed, leaving the tme skin (corium). In the tanning process, the fluid matter which maintains the skin in a flexible and moist condition is removed, and there remains nothing but the fibrous portion to be acted on by the tanning chemicals (1-3). 

Chromium is also important in converting animal hides into leather. In the tanning process, hides are treated with basic solutions of Cr(HI) salts, which causes cross-linking of collagen proteins. The hides toughen and become pliable and resistant to biological decay. 

In conventional chrome tanning processes 20 to 40% of the chrome used is discharged into wastewaters. In the new process 95 to 98% of the waste Cr3+ can be recycled. 

Leather is the material made from animal skin by the process of tanning, which entails chemically altering the composition of the skin so as to make it durable and resistant to decay. Leather is therefore not a protein but a protein derivative. Although the tanning process alters the composition of skin, leather retains the fibrous structure and utilitarian functionality that make skin suitable for multifarious applications. Shelter, clothing, and decorative objects made from leather are, unlike skin or hide, stable to physical, chemical, and biological decay under dry or wet conditions (O Flaherty et al. 1965 ... 

Mineral tanning was probably first practiced in ancient Mesopotamia and then spread to Egypt, the Middle East, and the Mediterranean Sea area (Levey 1958). Mineral-tanned leather is soft to handle, has a velvety texture, and is almost white, a color practically impossible to achieve by other tanning processes. It is, however, very sensitive to humidity and water under wet conditions the alum in the leather is hydrolyzed (decomposed by water), forming sulfuric acid, a very strong acid that attacks the leather and causes its rapid decay. Mineral-tanned leather that has been humid or wet for a more or less extended period of time loses some of its characteristic properties, such as softness, pliability, and strength, and becomes hard, horny, and brittle. 

Liritan A leather tanning process, introduced in 1960 and now widely used worldwide. The leather is first pickled in a solution of sodium hexametaphosphate and then soaked in baths of various vegetable products. 

Some Protein-Chemical Aspects of Tanning Processes... 

Cellophane is, after chemical modification, obtained from the cellulose in wood, just as paper (from cellulose and lignin), cellulose fibres ( rayon ), and cellulose plastics. Leather is made from animal hides in a tanning process. 

Tannins are amorphous substances, which produce colloidal acidic aqueous solutions with astringent taste. With iron salts (FeCls) they form dark blue or greenish black water-soluble compounds. Tannins form insoluble and indigestible compounds with proteins, and this is the basis of their extensive use in the leather industry (tanning process), and for the treatment of diarrhoea, bleeding gums and skin injuries. 

C. Chlorophenols. Chlorophenols have been extensively used since the 1930 s as insecticides, fungicides, mould inhibitors, antiseptics and disinfectants. The annual production volume is estimated to be in the order of 150 000 tons. In the US penta-chlorophenol is the second heaviest in use of all pesticides (38). The most important use of 2,U,6-tri, 2,3,, 6-tetra- and pentachlorophenol (or their salts) is for wood protection. Penta-chlorophenol is also used as a fungicide for slime control in the manufacture of paper pulp and for a variety of other purposes such as in the tanning process of leather and an additive in cutting oils and fluids, paint, glues and out-door textiles. 

A similar study was performed in the textile industry where the workers were exposed during the fabrics impregnation, and in the leather industry where the workers were exposed during the tanning process. In these two branches pentachlorophenol or pen-tachlorophenol laurate were used, products highly contaminated by PCDDs, the level of PCDFs being much lower, see Table III. The values of the blood and urine analyses are given in Table IX. 

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