Neutralisation: the key to successful retannage, dyeing and fatliquoring

摘要

The main purpose of the neutralisation process in the production of chrome-tanned leathers is to reduce the acidity throughout the hide or skin structure. This is to enable the retanning, dyeing and fatliquoring processes to be carried out successfully. In chrome leathers, pH values usually range from 3.0 to 4.0, and this is far too acidic to allow penetration of anionic products. Neutralisation increases the leather pH to levels that are generally between pH 4.2 and 6.5. These values depend upon the type of leather being produced, and it is only rarely that higher pH values are employed. The acidity of wet blue leathers originates from both the acids used in the pickling process, and from the fixation of chrome salts to the collagen. And, because of this acidity, chrome-tanned leathers have a very strong cationic character. In order to carry out retanning, dyeing and fatliquoring, the leather must be de-acidified to reduce the reactivity with process products. As most of these are anionic, this reduced reactivity allows their penetration and diffusion into the deepest layers within the leather structure.