Tannins leather manufacture

This review of literature on tannin degradation shows that our knowledge of this topic is only very slowly improving. Only a handful of laboratories are currently involved in this area. Of these, the Indian laboratories have made several interesting investigations recently, presumably because they are very active in leather manufacture and need to control the toxicity of their tannery effluents. 

Of these, leather manufacture will only be briefly discussed as it is beyond the scope of this review and because extensive technical and scientific literature is available on the subject [1, 5, 7, 14, 15-19, 27]. Suffice it to say that it is the interaction between the phenolic hydroxy groups of the tannins and the polar groups of proteins that give... 

Condensed tannins have been used to make leather goods since before recorded history - at least, earlier than 10000 B.C. Despite falling markets for leather products in general and a declining proportion of leather production using natural vegetable tannins, the condensed and hydrolyzable tannins remain important products of world commerce. The use of tannins in leather manufacture remains one of the most important commercial applications of all extractives of woody plants. 

The leather-tanning industry of India used about 120000 tons of assorted vegetable tannins in 1970 (203). India imported about 25000 tons of wattle extracts to supplement domestically available tannins such as myrobalan nut tannins (3500 tons/year) and wattle tannins (2000 tons per year). Vegetable tannins (primarily mixtures of chestnut, wattle, and quebracho tannins) are also used widely in Italy (173). In the Peoples Republic of China, much of the leather manufactured is made using vegetable tannins. Research efforts are being accelerated on the use of tannins from indigenous plants such as Larix and Firms spp. (Sun and Foo, personal communication 1986). 

Research on the use of condensed tannins in leather manufacture has come to a near standstill in western industrialized countries. Sparrow s fundamental work on condensed tannin-collagen interactions (228) and studies on the combination vegetable-aluminum (110, 226, 234, 241, 242) tannage are important thrusts of current efforts on the use of condensed tannins in leather manufacture. 

The following is a list of the materials which contain tannin in the largest quantities, and which may lie employed in the manufacture of leather. 

Leayes.—Of the foliage of trees containing tannin, very few, if any, are now employed in the manufacture of leather. The leaves of the heath were once extensively used in tins country but this material has long been abandoned, preference being given to oak barks and other substances of native and foreign growth,... 

The barks of some species of acacias are sometimes used as sources of tannins, chemicals that are mostly used to manufacture leather from animal skins. Species used for this purpose mcXudQ Acacia dealbata, A. decur-rens, and A pycnantha, all native to Australia but also cultivated elsewhere. 

Mangrove forests are commercially important in some places. Lumber can be manufactured from all of the mangrove trees, but the most durable wood is that of Ceriops. Where it is abundant, Rhizophora may be harvested to manufacture lumber or pulp. In some places, mangroves trees are harvested and used to manufacture charcoal. The bark of mangroves is rich in tannins, and has been used for the commercial production of these chemicals, which are utilized to tan animal skins into leather. Mangrove forests are also commonly harvested for local use as firewood. 

Iron, titanium, and zirconium salts can be substituted for the chromium ones.146 Vegetable tannins, such as 4.20 can also be used, but they slow down the tanning process. Similar phenols can be found in the residue from tea leaves left after the manufacture of instant tea. Perhaps, they could be used in making leather to eliminate or reduce the amount of another waste product. Getting reagents to penetrate the hide is a problem. Newer methods, such as ul-trasonication and supercritical fluid extraction, may help reduce the time required to make leather, so that these alternative-tanning agents can be used instead of the chromium. 

OTHER COMMENTS used as a solvent for old oil colors, for tannins, lacquers, celluloid, nitrocellulose, and camphor used as a pear flavoring in mineral waters and syrups useful in masking undesirable odors also used in the manufacture of artificial silk, photographic films, leather or pearls, waterproof varnishes, metallic paints, celluloid cements, and dyeing and finishing textiles. 

Among the polyphenols present in the tree barks, tannins are by far the most interesting oligomers (molecular weights of 1000-4000) in terms of their utilization as macromonomers for the crosslinking of proteins in leather (tanning) and for macromolecular syntheses. The two representative structures of the flavonoid units in tannins are shown in Fig. 1.8. The most salient aspects related to the sources, structures and production of tannins and to their exploitation in polymer modification and manufacture are given in Chapter 8. 

Although these tannins can be reacted with formaldehyde and other aldehydes, the rates of these interactions are low, and they are therefore not favoured for the preparation of resins. They have, however, been used successfully as partial substitutes (up to 50 per cent) of phenol in the manufacture of phenol-formaldehyde resins [9, 10]. Their chemical behaviour towards formaldehyde is analogous to that of simple phenols of low reactivity and their moderate use as phenol substitutes in the above-mentioned resins does not present difficulties. Their lack of macro-molecular stmcture, the low level of phenol substitution they allow, their low nucleophilicity, limited worldwide production and relatively high price, somewhat decrease their chemical and economical interest for resin production. Consequently, their main use is for leather tanning where their performance, especially in terms of clarity of colour and light resistance, is truly excellent. 

Condensed tannins constitute more than 90 per cent of the total world production of commercial tannins (200000 tons per year) [11]. Their high reactivity towards aldehydes and other reagents renders them both chemically and economically more interesting for the preparation of adhesives, resins and other applications apart from leather tanning. The main commercial species, such as mimosa and quebracho, also yield excellent heavy duty leather. Condensed tannins and their flavonoid precursors are known for their wide distribution in nature and particularly for their substantial concentration in the wood and bark of various trees. These include various Acacia (wattle or mimosa bark extract), Schinopsis (quebracho wood extract), Tsuga (hemlock bark extract), Rhus (sumach extract) species, and various Pinus bark extract species, from which commercial tannin extracts are manufactured. 

Saayman H M 1967 Manufacture of experimental particleboards using black wattle tannin adhesives. Leather Ind Res Inst Bull 446 1-8... 

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