An investigation of thermal degradation and charring of larch lignin in the condensed phase: the effects of boric acid, guanyl urea phosphate, ammonium dihydrogen phosphate and ammonium polyphosphate

The thermal degradation and charring of larch lignin and larch lignin treated with boric acid, guanyl urea phosphate, ammonium phosphate and ammonium polyphosphate in the condensed phase, were investigated by a combination of thermogravimetry (TGA) and X-ray photoelectron spectroscopy (XPS). TGA data under nitrogen indicated that larch lignin had a very high char forming ability. Data from both TGA and XPS demonstrated that boric acid, guanyl urea phosphate and ammonium dihydrogen phosphate did not show an obvious char forming effect. This is because larch lignin is highly crosslinked by carbon-carbon bonds between guaiacyl propane units, which could be further proven by data showing low binding energy at room temperature compared with manchurian ash lignin reported in our previous work. In air, larch lignin completely decomposed at 500 degreesC, however, the flame retardants, especially ammonium polyphosphate, could promote char formation compared with the untreated larch lignin. The experimental results demonstrated that ammonium polyphosphate is a very effective char forming agent under both nitrogen and air for lignin, which is attributed to the catalytic crosslinking effect of polyphosphoric acid produced from ammonium polyphosphate decomposition. (C) 2001 Elsevier Science Ltd. All rights reserved.