Antioxidant effect of tannic acid on formation of formaldehyde and hexavalent chromium compounds in leather

The presence of formaldehyde and hexavalent chromium in leather products, results from the inclusion of formaldehyde in chemical manufacturing process of industrial chemicals, and the oxidation of Cr (III), which is used as tanning agent in leather tanning industry, to Cr (VI) under certain conditions. In this study, wet-blue sheep skins were treated with different concentrations of tannic acid such as 0.1, 0.5, 1, 2 and 3% at retanning process in order to investigate the antioxidant effect of tannic acid on chromium (VI) and free formaldehyde formation in leather. For this purpose following the retanning process, formaldehyde content of leathers were analyzed according to ISO TS 17226 method with HPLC device. Hexavalent chromium formation was forced by exposing leather samples to accelerated ageing conditions, such as high temperature and UV. The chromium (VI) content on leather samples were determined in accordance with TS EN ISO 17075 standard procedure. The physical properties of final leathers were also investigated. Inhibitory efficiency of tannic acid both on hexavalent chromium and formaldehyde was attained at concentrations above 1% tannic acid. The offer of 3% tannic acid fulfilled the Eco-label criteria for footwear and provided lower chromium (VI) values than 3 ppm. Besides, all four tested concentrations of tannic acid maintained the formaldehyde concentrations under restricted limits. The improvement effect on the physical characteristics of leathers was observed in direct proportion of increasing tannic acid concentrations. The results indicated that tannic acid has a remarkable antioxidant effect on chromium (VI) and free formaldehyde formation in leather