Thermal degradation study of vegetable tannins and vegetable tanned leathers

Abstract

In this study, hydrolyzable tannins (commercial chestnut, valonea and tara extracts), condensed tannins (commercial quebracho and mimosa extracts) as well as calf leathers produced using these vegetable tanning agents were characterized by thermal decomposition methods using slow and high heating rates. Calf gelatin obtained by heating calf pelt in water at 70 °C was chosen as a reference material. Thermogravimetry/mass spectrometry (TG/MS) and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) experiments were performed on the tannin and leather samples. The evolution profiles of the decomposition products as well as the thermal stability of tannins and leathers were studied by TG/MS. A net difference was observed in the thermal behavior of hydrolyzable and condensed tannins. The condensed tannins produced the highest char yield, while the hydrolyzable tara extract released the most volatile products. The tannins of higher reactivity produced more stable leathers as it results from their higher decomposition temperature. The composition of both vegetable tanned leathers and vegetable tanning agents was characterized by the pyrolysis product distribution measured by Py-GC/MS method. Resorcinol and its methylated derivative (orcinol) were found to be a characteristic decomposition product for both condensed tannins, i.e. mimosa and quebracho. They were identified among the pyrolysis products of the mimosa and quebracho tanned leathers, as well.