An eco-friendly option for less-chrome and dye-free leather processing: in situ generation of natural colours in leathers tanned with Cr–Fe complex

Abstract

A chromium–iron tanning agent has been prepared and used for tanning with the objective of high exhaustion of chromium. Chromium–iron tanned leathers exhibit comparable shrinkage temperature and strength properties with those of conventional chromium tanned leathers. There is no change in strength or colour on ageing the leathers for 1 year. Spent tan liquor analysis reveals that the exhaustion of both the metals is more than 90%. Dyes based on synthetic formulations are being widely used in the leather industry. However, it is known that many aryl amine dyes are banned because of their carcinogenic nature. Environmental concern leads one to look for dyes based on natural resources. A novel approach has been taken to achieve an eco-friendly coloration process utilizing the presence of iron in the tanning salt, which gives rise to various colours on reacting with vegetable tannins. Skins have been tanned with Cr–Fe tanning salt and complexed with different vegetable tanning materials such as myrobalan and quebracho at various concentrations. Colours such as brown, black and grey have been developed. The colour shades obtained have been quantified by reflectance measurements. Thus, the study provides not only clean processes for a greener environment but also clean leather without harmful synthetic dyes.