Résumé : |
Liming/unhairing removes unwanted hair and globular proteins, opens the fibre structure and modifies the collagen. Traditional liming used either quicklime, CaO, or the slaked lime (hydrated), Ca(OH)2, to achieve this. Unhairing associated with traditional liming was slow, the alkaline Ca(OH)2 would break the keratin's disulphide links and with just lime present they could reform. Sharpening agents, such as sulphide, were added to prevent the reformation and to speed the breakdown of the keratin (hair and epidermis).
Alternative technology for the replacement of liming chemistry has focused on the substitution of sulphide, and amines (e.g., dimethylamine). Fellmongers, for generations, have always been reluctant to use lime because the Ca2+ ion can react with fat (common in sheepskins) to form calcium soaps. Calcium soaps can lose their Ca2+ in the presence of Cr3+ which will form pink-coloured stains on the sheepskin. A further reason why fellmongers choose not to use lime is that the saponification (alkaline reaction with fat) is lower than what can be achieved using other alkalis.
A more recent reason that tanners may avant to consider removing lime is because of the greenhouse gas implications of lime production from limestone, and the insolubility of slaked lime — leading to the pursuance of green chemistry ideals (e.g. an efficient atom economy and fewer intermediates of derivatives in the process).
This article will examine the technology that has been used to try and work with a lime-free industry. |