Effect of cure temperature on the thermal degradation, mechanical, microstructural and moisture absorption behavior of vacuum‐only, carbon‐fiber reinforced phenolic composites

Abstract Phenolic resin is the material of choice used for composite liners in solid rocket motor nozzles. We investigate the effect of the cure conditions (94, 116, and 155°C) on vacuum only (VO) processed composite mechanical, thermal, and microstructural changes. Although the T g 's correlated with the processing conditions, mechanical properties showed unexpected non‐linear degradation after 94°C processing. This difference is primarily due to the lower cure state of the resin, which results in poor fiber/matrix coupling and is further magnified by the reduced degree of consolidation observed in VO parts. Elevated thermal exposure was shown to generate larger tensile strains in the out of‐plane direction for the 94°C composites when compared to the other conditions. This resulted in significant increases in crack density, porosity, and delamination due to an increased volume of degradation by‐products, while the higher temperature cured composites maintained good consolidation. Even though all three cure states appeared to display equivalent moisture absorption saturation levels based on weight measurements, the 94°C cured sample was shown to partially dissolve in moisture during humidity exposure, which translated to even higher saturation levels. This larger net absorption resulted in a larger drop in T g for the 94°C cured material and thus resulted in a more severe thermal mechanical response than the other saturated specimens. This emphasizes how a reduction in cure state will not only affect thermal and mechanical performance but will also translate to a greater degree of degradation than more highly cured specimens, when exposed to humid environments.