Study of the thermal degradation of high performance fibrres -: application to polybenzazole and p-aramid fibres

This work investigates the thermal degradation of high performance fibres. Poly-p-phenylenediamine-terephthalamide fibres (PPTA) and poly(p-phenylene-2,6-benzobisoxazole) (PBO) fibres are examined when degrading in a furnace. Both gas and condensed phase are analysed. When degrading PPTA and PBO, CO, CO2 and H2O are detected by FTIR as major compounds composing the gas phase. As minor products, aromatic species, hydrocyanic acid (HCN) and ethylenic species are found (600 degreesC). Because of the higher thermal stability of PBO, their evolved amounts are always lower than those of PPTA and they are given off at much longer times. Moreover, no aromatic compound and a very low quantity of ethylenic species are detected in the gas phase suggesting they were involved in charring reactions. At 800 degreesC only nitric oxides are detected as minor products in the gas phase. A mechanism of formation of NOx from HCN is proposed. The investigation of the condensed phase by solid state NMR shows that PPTA and PBO are transformed into char when degrading. The char is composed of partially oxidised polyaromatic compounds, which can trap free radicals. It is suggested that the char structure of PBO may resist oxidation better than PPTA. (C) 2001 Elsevier Science Ltd. All rights reserved.