Optimisation of tannin extraction from Coriaria nepalensis bark as a renewable resource for use in tanning

Abstract

Tannin as a renewable resource with the potential to play an increased role in the tanning of leather by the virtue of its low pollution and biodegradability. In this work, a valuable tannin resource was obtained from the Coriaria nepalensis bark (CNB), the extraction of tannin from CNB with sodium hydroxide was optimized by the Box-Behnken design response surface. The effects of extraction time, extraction temperature, liquid-solid ratio and sodium hydroxide concentration on the yield of CNB tannin were examined. Extracting the bark at 82 °C over 63 min with 0.22 % sodium hydroxide at a 23 mL g−1 liquid-solid ratio, the CNB tannin extraction yield was 16.02 ± 0.08 %. Nuclear magnetic resonance spectroscopy revealed that CNB tannin is a hydrolysable tannin that contains carbonyl, phenolic hydroxyl and glycosyl, with relative molecular weights between 424 Da and 1464 Da, as determined by gel permeation chromatography. CNB tannins were used to tan sheep skin, and the shrinkage temperature of the tanned leather was found to be 76.4 °C. Other physical properties, including elongation at break (75.8 %), tear strength (38.2 N mm-1) and tensile strength (6.1 MPa), were comparable to those achieved using commercial valonia tannin reagents. Scanning electron microscope and atomic force microscope revealed that CNB tannin penetrates into the collagen fibers of leather to provide stable cross-linking, resulting in firm and delicately grained surfaces.