Tanning material

The exact nature of the tanning reaction is not known and a number of mechanisms have been proposed. It has been shown, however, that as the process progresses, the degree of unsaturation of the oil reduces, peroxy-derivatives are formed, hydroxyl functions appear and, more specifically, acrolein, CH2=CH CHO, is produced. It is thought that this and other aldehyde compounds are responsible for the chemical cross-linking and that coating the fibres with polymerised oils imparts the special physical characteristics to the leather. 

More is known of the mechanism of interaction between the simpler aldehydes, particularly formaldehyde, and collagen where a reaction first takes place at the free amino groups  

Crosslinking then occurs between the N-hydroxymethyl group and a free amino group from an adjacent polypeptide  

Formaldehyde was utilised to produce washable white leathers from the end of the nineteenth until the last third of the twentieth century when its use was phased out due to health and safety concerns. Other aldehydes have tanning properties but only glutaraldehyde has been employed successfully on a commercial scale. 

Mineral Tannages. The effects of chromium salts on skins were first investigated during the mid-nineteenth century and by the 1870s, leather was being produced commercially with these materials. Today over 90% of leather is manufactured using trivalent chromium compounds. 

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Tanning of hides by any of the primitive methods was a dirty job withHttle assurance of success. The tanner was a speciaUst generating little respect in the community. In some parts of the world, this ancient prejudice against tanners exists even as of this writing (ca 1994). The tanner in primitive societies was limited to making leather using the hides or skins available locally. Tanning materials were also limited to those in local supply. Techniques were dependent on the climate as well as avadabihty of materials. The leather made was for local needs. 

The apphcation of vegetable tanning materials has an additive effect on the leather. The more vegetable tannins appHed the more the leather becomes like vegetable-tanned leather. The color is changed, the fullness of feel increases, and the leather can be worked and embossed like vegetable leather. 

Modem chrome-tanning methods are weU controUed and employ an extensive knowledge of the chemistry of the system. The most common chromium-tanning material used is basic chromium sulfate [12336-95-7] Cr(0H)S04, made by the reduction of sodium bichromate with sulfur dioxide or by sulfuric acid and a sugar. 

Vegetable-tanning materials in commercial quantities come from many different countries. Quebracho is a principal tanning material from South America. Wattie or Mimosa is suppHed from several African sources. India and other Asian countries supply a variety of materials including Myrabolans, Gall Nuts, and Tara Pods (3). 

The vegetable-tanning materials are commercially extracted using hot water. The extraction is normally done in countercurrent extractors that permit the final removal of the extracts with fresh water. The dilute extracts are then evaporated to the desired concentration in multiple effect evaporators. Some extracts may be further dried by spray drying or any other means that proves effective without overheating the extract. Extract preparation depends on the type of extract, the si2e of the operation, and the desired concentration of the final product. 

There are three main uses for naphthalene sulfonic acid derivatives (75—79) as naphthalenic tanning material alkyl naphthalene sulfonates for industrial appHcations as nondetergent wetting agents and as dye intermediates. Consumption of naphthalene sulfonates as surfactants accounts for a large portion of usage. Naphthalene sulfonate—formaldehyde condensates are also used as concrete additives to enhance flow properties. Demand for naphthalene sulfonates in surfactants and dispersent appHcations, particularly in concrete, was expected to increase into the twenty-first century. Consumption as of 1995 was 16 x 10 kg/yr. 

Neooxygambirtannine (43) shown in Scheme 17 is an example of a heterocyclic mesomeric betaine belonging to class 4. This alkaloid, which is red in color, was found in Gambir and is a tanning material produced by... 

Gerbe-methode, /. tanning method, -mittel, n. tanning material, tan. 

Howes, F. N. (1953), Vegetable Tanning Materials, Butterworth, London. 

Howes, E., 1953, Sumac. In Vegetable Tanning Materials, Butterworths Scientific Publications, London, pp 209-218. 

Wolesensky, Edward, Investigation of synthetic tanning material, Bureau, of Standards Technologic Paper No. 302 (1925), pp. 6-7. 

Gambir or Gambier(Pale Catechu). An odorless brown powder prepd by drying an extract from a decoction of the leaves and twigs of an Indian shrub, Ourouparia or Uncaria gambir. It is insol in w and sol in ale. Used as an astringent and tanning material... 

Howes, F.N., "Vegetable Tanning Materials" Butterworths, London,1939. 

Statistics.—The following table expresses the imported quantities of the principal tanning materials in 1849 in hundredweights —... 

Figure 7. Tangential section of coalified Persea wood showing the yellow-tan material derived from the vessel walls and a red-brown granular material derived from alteration of the tylose wall. 113X... 

Yellow-tan material forming remnants of vessel walls... 

This family is almost cosmopolitan but is especially prominent in the north temperate zone. Members are used as food, timber, tanning materials, and cultivated ornamentals. 

The tanning materials required to be analysed may be the prime materials containing the tannin, i.e., bark, wood, roots, leaves, fruit and galls liquid or solid extracts obtained from these prime materials more or less pure commercial tannins. 

Analysis of tanning materials may comprise qualitative analysis for detecting and distinguishing them and quantitative analysis for the estimation of the proportions of the different components, especially of the tannin. 

The principal reactions and tests used for the identification of different tanning materials and for the detection of adulterations are described briefly below. 

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