Thermal and Thermo-Oxidative Degradation of PBO: Determination of Kinetics and Reaction Products

Abstract

Poly(para-phenylene- cis-benzobisoxazole), PBO, is a promising thermal and fire resistant material because of its excellent thermal stability, virtual nonflammability (limiting oxygen index = 56), high char yield, and negligible smoke generation [1]. Decomposition reaction products and kinetics were studied to further characterize this material. Chemical changes in the solid film were studied by FTIR (Fourier transform infrared spectroscopy) before and after isothermal heating in vacuum. Decom position was detected at 600°C while little, if any, was observed at 500°C. In other studies using a constant heating rate, released volatiles were identified by FTIR and an electrochemical selective ion probe. Degradation rates in nitrogen (thermal) and air (thermo-oxidative) were de termined by isothermal thermogravimetric analysis (ITGA). Activation ener gies for the thermal and thermo-oxidative degradations were calculated. These results are compared to other thermally resistant polymers. For example, the PBO film decomposition rate is over one hundred times slower than a high- temperature-resistant polyimide (Kapton® H film) in nitrogen, and about two times slower than the polyimide film in air. These findings suggest possible lower rates of combustion, especially in situations where the polymer is decom posed in a locally oxygen starved environment. Finally, an empirical model for the thermo-oxidation of PBO was obtained from the TGA data. Reliable predictions of weight percent remaining are made as a function of time and temperature.