Submitted to: Journal of American Leather Chemists Association
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/1/2003
Publication Date: 10/1/2003
Citation: Brown, E.M., Taylor, M.M. 2003. Essential Chromium?. Journal of American Leather Chemists Association. 98:408-414. Interpretive Summary: Tanning or crosslinking of protein molecules in animal hides or skins produces leather, a high value co-product of the meat industry. Salts of the mineral chromium are the most commonly used tanning agents for the production of high quality leathers. Although the art of tanning is highly developed, the mechanisms are poorly understood. Because of environmental considerations, a primary focus of the tanning industry has been to limit the amount of chromium in their waste streams by increasing its affinity for the animal hide. The research reported here is an endeavor to assist the leather industry in determining whether or not leather now contains more chromium than essential. In this study we extracted leather waste, by several mild treatments, and examined the stability of the leather as chromium was progressively removed. The results suggest that up to 2% of the chromium is simply adsorbed, and has little effect on stability. An additional 7% to 10% of the chromium was removed easily, but nevertheless caused a significant drop in thermal stability, an important characteristic of well tanned leather. Beyond this level, chromium removal and decreased thermal stability proceeded smoothly. These results suggest that, indeed, most of the chromium present contributes to the stability of the leather.
Technical Abstract: During the production of chrome-tanned leather, chromium is incorporated into the leather with at least three types of interactions. Most important is the chromium that is complexed with collagen to give leather its characteristic properties. Other interactions include nonproductive binding of chrome to collagen and adsorption of chromium onto the collagen matrix. To evaluate the extent and possible effects of these other interactions, we extracted chrome shavings with water, HCl, or EDTA to remove adsorbed or loosely bound chromium. The effect of chromium removal on hydrothermal stability was followed by differential scanning calorimetry, and the chromium content of extracts was determined by atomic absorption spectrometry. Extraction of less than 2% of the chromium initially present had no effect on the denaturation temperature, but removal of 3% to 10% of the chromium resulted in a 20°C decrease in the denaturation temperature. As 10% to 60% of the chromium was extracted, the denaturation temperature decreased an additional 30°C, nearly reaching the temperature expected for hides that were not tanned. The results suggest that up to 2% of the chromium is simply adsorbed, and most easily removed. The next 7% to 10% is, however, important for the thermal stability of the shavings. Beyond this level, extraction with EDTA continually shifts the chromium-collagen equilibrium and the denaturation temperature to lower values. The final approximately 40% of the chromium appears to be bound to collagen, but no longer crosslinked.