Relating the microstructural and glass transition temperature evolution of a carbon-carbon composite precursor during pyrolysis using thermomechanical analysis

Polybenzoxazine thermoset resins exhibit significant potential as precursors for carbon-carbon composites, owing to their high char yield upon pyrolysis. During pyrolysis, the resin is converted to an amorphous carbon-rich material through a series of mechanisms, resulting in a linear increase of the glass transition temperature (T-G) from 133 to 396 degrees C at 0.48 pyrolytic conversion. To better understand this relationship, a non-isothermal kinetic model was integrated, enabling the prediction of the T-G onset, inflection, and endpoint based on the heat treatment protocol (process temperature (T-P) vs. time) and the total volume of pyrolytic gases generated. By considering the T-G behavior and anticipated gas effects, three distinct mechanisms were identified: (1) gas accumulation and matrix expansion leading to increased sample thickness (T-G < T-P), (2) crack network formation caused by gas-pressure-induced brittle failure (T-G > T-P), and (3) sample thickness reduction due to gas venting through the crack network (T-G < T-P). To validate this theory, thermal mechanical analysis was performed during pyrolysis, and changes in thickness (+/-Delta L/L-0) were attributed to gas pressure-related matrix deformation. Conversely, no change in Delta L/L-0 indicated the formation of an interconnected crack network through which gases could vent without building a measurable internal pressure.