Leather Tannage

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. 

Approximately 200000 tons of quebracho Schinopsis sp.) and 100000 tons of wattle (Acacia mearnsii) tannins are produced world-wide annually. Although statistics are not so accurate, about 200000 tons of vegetable tannins derived from other plants such as hemlock (T uga sp.), chestnut (Castanea sp.), oak (Quercus sp.) and myrobylan (Terminalia sp.) extracts add to this for a total world production thought to be about 500000 tons per year, (see also Chap. 1.1). 

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). 

For comparison, about 400000 tons of phenol used in phenol-formaldehyde resins were produced in the United States annually in the period 1983-1985 (30). While demand for condensed tannins does not compare with basic commodity chemicals such as phenolic resins, they can be considered to be specialty polymers with sizable markets. 

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. 

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Another characteristic of the condensed tannins was usually observed during leather tannage with these materials. Aqueous suspensions of tannin that were acidic from the tanning process gradually precipitated insoluble materials known as tanner s reds or phlobaphenes. These substances, derived from the tannin, were no longer soluble in water, but they could be dissolved in polar solvents such as ethanol or acetone or in aqueous base. Since most species of bark contain an extractive fraction that physically resembles the tanner s reds, they are referred to as phlobaphenes in the literature. Very little characterization work has been done on this fraction, and there is substantial reason to believe that the bark phlobaphene fraction contains a variety of water-insoluble polymers, some of them totally unrelated to the condensed tannin family. 

The chrome tanning is one step in a compHcated series of leather operations leading from the raw hide to the finished products. Chrome tanning is the most important tannage for all hides except heavy catde hides, which are usually vegetable tanned. In heavy shoe uppers and soles, a chrome tanned leather is frequently given a vegetable retan to produce chrome retan leather. 

Sackfflter, n. bag filter, sack filter, sackfbrmig, a. sack-like, bag-like, pouch-like. Sack-gerbung,/. (Leather) bag tannage, -lein-wand, /., -leinen, n. sackcloth, sacking, bagging. 

Tran-flillung, /. Leather) train stuffing, -ger-bung, /, fish oil tannage, -geruch, m, odor of train oil. 

Knapp discovered in 1858 that chromium chloride converted raw skins into leather, but he failed to realize the commercial significance of his discovery. Others developed his ideas. A two-bath process due to Schultz (1894) and an adaptation of Knapp s original one-bath process by Dennis (1893), meant that by the early twentieth century chromium tannage was commercially important. At present it accounts for the vast majority of leather production. 

The principal determinations to be made on leather are those of moisture, ash, fatty substances, substances soluble in water (tannins and non-tannins) and nitrogenous substances (hide substances) from the results it is then possible to calculate the combined tanning substances and the true leather substances, also the leather yield and the degree of tannage. Other determinations which may be required are those of the sugars, free sulphuric add and lime (in the ash). 

Calculation of the Leather Yield and of the Degree of Tannage. —The leather yield, is the weight of leather (with normal moisture content) derived from 100 parts of hide substance. If d is the percentage of the latter (referred to the leather with normal moisture content), the leather yield is given by 10000 -r d. 

Mois- ture. leather Yield Degree of Tannage ... 

For accounts of the various methods of chrome tannage, see Procter, Leather Trades Review, 1897, p. 390 1898, p. 400 Cobb Lecture to Society of Arts, 1918 Stiasny, Collegium, 1912, p. 293 Wintgen and Lowenthal, Kolloid. Zeitsch., 1924, 34, 289. 

Use Tannage of white kid suede, glove leathers retannage of chrome leather. 

No matter what the source of chromium tannages, either from raw materials or from recycled solutions, care must be taken to maintain the quality of the leather. This can be only achieved by constant chemical analysis and chemical control. The chromium content in the solution can be tested by the following two methods, titration and atomic absorption analysis ... 

Resistance to Microbiological Attack. Under normal ambient temperatures, a wet raw skin will decay rapidly due, primarily, to the action of bacterial proteolytic enzymes. Leather, on the other hand, resists such microbiological attack even if it remains wet. This fundamental difference has been used to define tannage. 

Physical Properties. It is generally considered that if a raw skin is allowed to dry it will become hard, horny, translucent and relatively inflexible. If the hair has been removed first, these characteristics have been exploited to produce such diverse objects as rawhide mallet heads and dog chews. Leather on the other hand is expected to dry to give a soft, flexible, opaque product with a characteristic feel. This has been cited as evidence that tannage has occurred. 

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. 

Tannage process. Tannage consists in injection of tanning substances into the dermis structure and their interaction with functional groups of protein molecular chains, which results in formation of additional stable cross bonds. Tannage is one of the most important processes for leather manufacture. This stage in the leather industry radically changes dermis... 

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