Tanning with Aluminum-Gallic Acid Complex A New Way to Improve the Tanning Effect of Aluminum Salts

Abstract

To figure out how polyphenol compounds with a low molecular weight helped in aluminum tannage, a complex of gallic acid and aluminum as well as its application to the tanning of hide powder was studied in this paper. Precipitation points of aluminum chloride solution with different concentrations of gallic acid were measured to guide the basification in hide powder tannage. UV-Visible spectra showed that gallic acid and aluminum would form soluble complexes in aqueous solution and the complexation reaction was pH reliable. Through stoichiometry studies, generations of the 1:2 complex under pH 4.0 and 2:3 complex under pH 4.5 were discovered. Results of stoichiometry studies and Fourier Transform Infrared Spectroscopy (FTIR) spectra confirmed that aluminum-gallic acid complex played the role of links which connected collagen molecules rather than gallic acid or aluminum forming individual bridges with collagen. According to the results of differential scanning calorimetry (DSC) and thermal gravity analysis (TGA), denaturation temperature (T_d) of hide powder tanned with different methods, such as aluminum, gallic acid-aluminum combination or aluminum-gallic acid complex, was 76.1°, 82.8° and 85.5°C respectively, and the initial decomposition temperature for the aluminum-gallic acid complex tannage was 300.7°C, also higher than those for another two methods. Furthermore, the results of inductive coupled plasma emission spectroscopy (ICP) showed that after washing for 12 h, the Al₂O₃ content in hide powder tanned with complex could be maintained at 93.42% of that in unwashed samples, which was the highest among three tanning methods.