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

Abstract

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 (TG) from 133 to 396°C at 0.48 pyrolytic conversion. To better understand this relationship, a non‐isothermal kinetic model was integrated, enabling the prediction of the TG onset, inflection, and endpoint based on the heat treatment protocol (process temperature (TP) vs. time) and the total volume of pyrolytic gases generated. By considering the TG behavior and anticipated gas effects, three distinct mechanisms were identified: (1) gas accumulation and matrix expansion leading to increased sample thickness (TG < TP), (2) crack network formation caused by gas‐pressure‐induced brittle failure (TG > TP), and (3) sample thickness reduction due to gas venting through the crack network (TG < TP). To validate this theory, thermal mechanical analysis was performed during pyrolysis, and changes in thickness (±ΔL/L0) were attributed to gas pressure‐related matrix deformation. Conversely, no change in ΔL/L0 indicated the formation of an interconnected crack network through which gases could vent without building a measurable internal pressure.