Tannins

substances used in finishing (tanning) leather and fur. They are subdivided into mineral and organic types.

Mineral tannins include compounds of chromium, aluminum, zirconium, titanium, silicon, and iron. Most of the compounds are basic salts of sulfuric acid (for example, CrOHSO₄); they are used in the form of aqueous solutions. Tanning with compounds of chromium (so-called chrome tanning), which is used for all of the main types of leather and fur, especially for combination tanning methods, is the most common. In combination methods, preliminary chrome tanning (chroming) ensures high permeability of the leather semi-finished product, which sharply accelerates subsequent tanning (final tanning) with organic tannins and increases heat and wear resistance. Zirconium compounds which impart to leather increased wear resistance and a smooth, dense outer surface, are promising tannins.

Organic tannins are subdivided into animal and plant and artificial and synthetic categories. Animal tannins include polyunsaturated fats from various fish and marine animals (so-called blubbers), which are used in the production of suede. Tannins of plant origin, tannides, are contained in various plant organs. They are extracted from pulverized material by water or weak solutions of sulfite compounds at high temperatures. The solutions of tannides and accompanying substances are then concentrated and converted into liquid or solid extracts. The tannides extracted from various types of plants differ in chemical structure and properties and can be subdivided into hydrolyzing, condensed, and mixed types. In hydrolyzing tannides, the components of the molecule are linked by ether or ester bonds. Representative of this group are the tannins (esters of phenolcarbonates and simpler carbohydrates, mostly hexoses). Various polyoxy derivatives of 3-oxyflavan are the basis of most condensed tannins. In solid form, tannides are mostly amorphous substances that are completely soluble in water. Solutions of tannides can be used to tan any type of leather (sometimes sheepskins), both independently and in combination with other tannins.

Artificial tannins are macromolecular organic compounds with tanning properties. They are by-products of industries that process organic raw material. For convenience in using these products as tanning agents they are concentrated and sometimes purified. Typical representatives of artificial tannins are the sulfite-pulp extracts produced from factory wastes of cellulose processing. The main ingredient of any sulfite-pulp extract, which determines its leatherworking properties, is ligninsulfonic acids. Such tannins are very stable, and they penetrate the leather semifinished product more rapidly than tannides, but their other parameters (bonding temperature and filling and shaping the leather) are poorer than those of tannides. Concentrates of ligninsulfonic acids cleansed of calcium salts are widely used to produce high-quality synthetic tanning agents.

Synthetic tannins, or syntans, are produced from various organic compounds. The main raw materials for producing syntans are phenol and its derivatives, aromatic hydrocarbons (such as naphthalene), naphthols, sulfones, ligninsulfonic acids, Formalin, and urea. The syntans are subdivided into auxiliary syntans, which are used in small amounts together with plant tannins; tannide substitutes, which are used independently or in a mixture with tannides in amounts up to 60 percent by mass of the total tanning agent; and special-purpose syntans, which are used for tanning and simultaneous coloring, bleaching, or greasing. The creation of high-quality synthetic tannins makes it possible to shorten or completely eliminate the processing of valuable oak wood for tanning extracts, to expand the assortment and properties of the tanning agents used in various processes of leather production, and to improve the quality and intensify the process of leather production. The use of syntans is increasing from year to year; it accounts for about 40 percent of the USSR’s organic tannins.

Among the simplest organic substances, quinones and formaldehyde have tanning properties. Formaldehyde is used in combination tanning with tannides and chromium compounds.