Thermal Degradation of Condensed Tannins from Radiata Pine Bark

Abstract

In order to understand the influence of the inherent chemistry on the relative thermal stability of condensed tannins, the thermal degradation behaviors of various radiata pine bark extracts have been investigated using thermogravimetric analysis (TGA). Generally, results suggest pine bark extract fractions may be readily processed at temperatures below 200°C if co-extracted polysaccharides contents are minimized. Those extracts possessing greater carbohydrate content and lower tannin purity tend to have decreased thermal stability. The initial onset temperature for degradation of relatively crude extracts with high proportions of carbohydrate contents were relatively low (ca. 150°C), whereas extract purification to < 5% carbohydrate content gave increases in thermal stability of at least 50°C. The complicity of the carbohydrate content in the degradation of the tannin samples was also supported by calculated Ozawa activation energies and modulated TGA experiments. While no increase in the thermal stability was gained by acetylating the pendant hydroxyl groups of the pine bark tannin extract, chemical variations such as sulfonation have a large effect on thermal degradation, promoting lower degradation temperatures.