Chrome tanning of leather

The primary function of a tanning agent is to stabilize the collagen fibers of the animal hide so that they are no longer subject to biodegradation. Over 95% of the leather produced in the United States is chrome tanned. 

The initial preparation of the raw hide involves the following steps  

After chrome tanning the leather then undergoes further tanning with synthetic resins and natural tannins and is then finally dyed and fat liquored  

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Miscellaneous. In ore flotation, sodium sulfite functions as a selective depressant. In textile processing, sodium sulfite is used as a bleach for wood (qv) and polyamide fibers and as an antichlor after the use of chlorine bleach. Synthetic appHcations of sodium sulfite include production of sodium thiosulfite by addition of sulfur and the introduction of sulfonate groups into dyestuffs and other organic products. Sodium sulfite is useful as a scavenger for formaldehyde in aminoplast—wood compositions, and as a buffer in chrome tanning of leather. 

PCMC has completely replaced pentachlorophenol in the leather industry, particularly as an antimold agent, during chrome tanning of leather. In fact, pentachlorophenol has... 

In connection with the chrome tanning of leather it has been found in technical practice that by the addition of neutral salts it is possible to use more basic chrome liquors The added salt prevents the precipitation of hydrated chromium oxide This phenomen has been regarded as due to an actual increase in hydrogen ion concentration It is much more probable, however, that we are dealing with an increase of activity rather than an increase of mere concentration Instead of dealing with the complex system presented in the tanning bath we shall refer briefly to the results obtained by Thomas and Miss Baldwin loc at) in the chemically better defined case of a solution of chromium chloride... 

The manufacture of leather foUows the same general steps for a great variety of leathers (Fig. 2). The largest category of hides taimed is catde hides. Of the cattle hides chrome tanning of unhaired hides is by far the dominant system used throughout the world. The tanning of other types of hides and skins requires variations in the systems used for cattle hides (3). 

Use Vegetable tanning, retanning of chrome-tanned upper leathers, dyeing, ore flotation, oil-well drilling fluids, flavoring. 

USE Insolubilization of gelatin in catalyst prepn as mordant in textile industry in tanning of leather in chrome plating in manuf of Cr, Cr03, and Cr alloys to improve disparsibility of vinyl polymers in water in manuf of green varnishes, paints, inks, glazes for porcelain. 

HISTORY. Chromium was discovered by the French chemist Vauquel in 1797, while he was studying the properties of crocoite, an ore which is rich in lead chromate, its common name of chrome was derived from the Creek word chroma, which means color, because the element is present in many different colored compounds. These compounds have long been used as pigments in dyeing, and in the tanning of leather. In the early 1900s, chromium became... 

Fig. 2. Process flow diagram for the production of chrome-tanned cattle hide leather. Adapted from Ref. 3.

The usual procedure in the manufacture of chrome-tanned leathers is the use of a continuous bate, pickle, and tan method. The hides or skins remain in the dmm from the lime washing through the chrome tanning stages. The entire process usually takes about 22—24 hours. The bate, pickle, and tan can be done faster, but most tanners find that a one-day cycle fits well into production scheduling and results in a quaUty leather. 

The chrome-tanned leather is removed from the dmm and wmng to remove the absorbed tanning solution. The leather is then inspected for quaUty of the grain and other characteristics of importance for the leather being made. In large tanneries where very uniform hides are worked, the leather may be trimmed and spHt to the desired thickness. Hides spHt before tanning need no splitting at this point. 

Chrome Tanning. The original chrome tanning was a two-bath process. The unhaired hides, delimed and bated, were treated with a solution of sodium bichromate [10588-01-9]. The amount of bichromate used was about 3—5% based on the weight of the hides. The bichromate was absorbed or adsorbed into the hide, the solution drained, and the hides refloated. Sodium bisulfite was added and two important reactions resulted in the formation of a basic chromium and coUoidal sulfur in the hide. This gave a chrome taimage and also helped to fiH the hide with the soHd sulfur. This cmde system, which continued in the industry in some types of leather for over 50 years, is obsolete. 

The chromium can be stabilized in a limited way to prevent surface fixation by addition of formate ions. The formate displaces the sulfate from the complex and masks the hydroxyl ions from forming the larger higher basicity complexes. This stabilization can then be reversed in the neutralization to a pH of about 4.0 and taimage becomes complete. This simple formate addition has decreased the time of chrome tanning by about 50% and has greatly increased the consistent quaHty of the leather produced. 

Chrome-tanned leather has chromium bonded to the leather fibers. This chromium can act as a mordant for acid dyes resulting in fast colors and intense shading at the surface of the leather. 

Around 1800, the attack of chromite [53293-42-8] ore by lime and alkaU carbonate oxidation was developed as an economic process for the production of chromate compounds, which were primarily used for the manufacture of pigments (qv). Other commercially developed uses were the development of mordant dyeing using chromates in 1820, chrome tanning in 1828 (2), and chromium plating in 1926 (3) (see Dyes and dye intermediates Electroplating Leather). In 1824, the first chromyl compounds were synthesized followed by the discovery of chromous compounds 20 years later. Organochromium compounds were produced in 1919, and chromium carbonyl was made in 1927 (1,2). 

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. 

The Oeko-Tex label tests for and restricts a high number of chemicals in the product. It should be noted that chrome-tanned leather cannot receive the Oeko-Tex Standard 100 certificate due to stringent demands on chromium. Since around 85-90% of all leather in the world is tanned with chromium, the certificate is only valid for a minor part of the leather production. 

Chromates and dichromates are used in industry as oxidising agents, for example in the coal tar industry, in the leather industry (chrome tanning), and in the dye industry as mordants. Some chromates are used as pigments, for example those of zinc and lead. Chromates and dichromates are poisonous. 

Chromium(lll) sulfate is used as the electrolyte for obtaining pure chromium metal. It is used for chrome plating of other metals for protective and decorative purposes. Other important applications of this compound are as a mordant in the textile industry in tanning leather to dissolve gelatin to impart green color to paints, varnishes, inks, and ceramic glazes and as a catalyst. 

The complete and systematic examination of leather, including physical and mechanical tests 1 and chemical analysis, is carried out principally for tannin- or chrome-tanned leathers. The methods to be used in this examination arc given below and are preceded by the procedure to be followed in sampling. 

Figure 7.11 Chrome tanned leather from Overton Down experimental earthwork after 32 years of burial in a chalk environment. (Photo Experimental Earthworks Committee/R. C. Janaway. With permission.) (See color insert following p. 178.)...

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