Tannage

After about 4 to 12 hours the hides are drained, then floated with lime without additional sulfides. The dmm or hide processor is mn at slow speeds intermittently during the unhairing process. The continued action of the lime solution on the hides brings the pH to 12—13. At this pH the hides swell to about twice the original thickness. This swelling has a beneficial opening of the fibers to permit better tannage (Fig. 3a). 

Control of chromium penetration, essential to permit tannage of the center of the hide, is accompHshed by pH adjustment. At a pH > 3.0 the reactivity of the hide to the chromium complex is greatiy increased. The pH is therefore raised gradually to the desired point by addition of a mild alkah, usually sodium bicarbonate. The chemistry of chrome tanning involves competing reactions that must be controlled for satisfactory results. 

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. 

AUein-gerbstoff, m. self-tannin, -gerbung, /. self-tannage, -handel, m. monopoly, -her-staller, m. sole producer, aileinig, a. sole, unique. 

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. 

Schwcfel-gerbung, /. sulfur tannage, -germanium, n. germanium sulfide, -geruch, m. sul fur odor. 

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

It is noteworthy that several of fhe above-mentioned mediators of discoloration have been found fo inhibif sfrepfococcal proliferation. In addition, collagen cross-linking caused by enzymatic and non-enzymat-ic browning or by tannage with tannins might render dentin less susceptible to proteolytic degradation. 

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. 

In the tanning process hides are first washed or soaked, hair and keratinous debris are removed, bated (enzymes are used to break down non-collagenous components, which are washed out) and the hide is acid-pickled to prepare for the addition of the chromium salt. Contemporary processes are exclusively based on one-bath procedures and utilize chromium(III). The older two-bath process is now obsolete, mainly because it involved the in situ reduction of chromate, a major environmental and toxicological hazard (cf. chromate toxicity p. 947) to chromium(III) on the hide. A useful review of the history of chromium tannage processes is available.1205... 

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. 

The degree of tannage indicates the tannin combined with 100 parts of hide substance if / is the percentage of combined tannin, the degree of tannage is expressed by 100 t d. 

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

These facts play an important factor in the dyeing behavior. The isoelectric point of hair is at a pH of ca. 5, whereas untreated collagen has its isoeletric point at a pH of ca. 6, which varies with the kind of tannage. For pure chrome suede it is close to pH 7 or about pH 6 after slightly anionic retanning. Consequently, for good dye fixation the acidification at the end of a dyeing is around pH 3 for fur hair and ca. pH 4 for chrome-tanned suede. 

Experiments carried out in pilot-scale plants onliming, degreasing, and chromium tannage show the possibility to realize more efficient operations for recovering by-products (e.g., proteins and fats) and chemicals (e.g., chromium) [22],... 

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. 

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 ... 

Formaldehyde is a main component in synthetic tannages. The concentration of formaldehyde can be tested mainly by two methods titration and colorimetric analyses (Meites, 1963). 

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