Leather Tanning

In leather production, it is imperative to stabilize the collagen fibers of the animal hides so that they are no longer subject to biodegradation. This is accomplished by the tanning of the leather with the use of stabilization/preservation agents. Chrome-based salts are the most common agents used in the so-called mineral tanning (chromium, zirconium or aluminium). Chrome salt is added after pickling, at low pH, to ensure that the chrome complexes formed, penetrate the pelt and fit in between the fibers and residues of the collagen. Subsequently, a basifying agent is added to slowly increase the pH which fixes the chromium complexes to the (carboxylic groups of the) collagen by a crosslinking process which stabilizes the pelt. Chrome-tanned skins are pale-blue, so they are referred to as wet blue. Chrome tanning produces a stretchable leather which is excellent for use in all leather products, from furniture and car seats to handbags and garments.

A gradual, gentle rise in the pH without overshoots is essential to ensure uniform stabilization and avoid staining or spotting of the pelt. This can only be achieved by the addition of a non-soluble agent such as magnesium oxide, typically added at 0,3% to 0,5% on limed hide weight. Performance is optimized when a DBM of balanced microstructure, reactivity and grain size is used, with low free lime, iron and manganese levels. In this way one can achieve:

• Reliable performance without pH initial peaks, leading to a uniform chrome distribution throughout the leather and a smoother, finer grain.
• Reduced labor/handling costs by eliminating the need for repeated additions.
• No risk of abrasion to the pelt and consistent reliable results.
• Great reduction of the risk of green stains caused by local precipitation of chrome
• Production of wet blue leather with a pale colour, suitable for the production of white and all dye shades.
• A higher exhaustion of chrome leading to lower offers of chrome, with consequent cost savings and effluent improvement.