Effect of Mesophase Pitch Incorporation on the Ablation Behavior and Mechanism of Phenolic Composites

Phenolic resin plays a critical role in ablative thermal protection materials. However, its ablation and insulation properties need to be further improved to meet the growing requirements of the entire aerospace industry. In this work, composites are altered with various loadings of mesophase pitch (MP) to make mesophase pitch-modified boron-phenolic resin (BPR) composites (MPBPR). The compressive strengths of carbonized MPBPR composites are significantly improved (optimally by 2.7 times) via this modification, indicating the improved mechanical performance of MPBPR composites under high temperatures. Results reveal that the obtained MPBPR composites show better ablation resistance and thermal insulation performance under oxyacetylene flame. The linear ablation rates (4 MW/m2, lasting for 30 s) of the composites with 10 and 20 wt % MP are reduced by 41.5 and 55.4% compared to that of neat BPR. Meantime, MPBPR composites exhibit lower back-face temperature during the ablation process. Furthermore, results also show that the introduction of MP can effectively avoid the growth of large transversal microcracks, increase the proportion of micropores in the ablated char layer, and simultaneously improve the graphitization of resultant char. Both the microporous and graphitic structures of the residual char contribute to the improved ablation and insulation properties of MPBPR composites. The abovementioned results indicate that these MP-modified boron-phenolic resins are promising materials for application in a thermal protection system.