Optimization of crosslinked network structure of cured phenolic resin with high char yield

Phenolic resins (PRs) are one of the most important matrix resins for advanced aerospace thermal protection composites. However, improving the thermal stability and char yield of PRs while maintaining the good processability always remain a challenge. Herein, a resole-type PR (H-BPM), which contained no free phenol and exhibited good processability was synthesized from p, p’-dihydroxy diphenylmethane. The hydroxymethyl content of H-BPM was tuned to obtain resin networks with varied crosslinking density. The total number of crosslinking bridges, methylene bridges and dimethyl ether bridges were proposed to evaluate the different crosslinking bonding content in cured H-BPM, offering a quantitative description of the crosslinked structure. The cured resin with optimized crosslinked structure exhibited excellent thermal stability with a high thermal decomposition temperature of 376°C and a char yield (800°C, nitrogen) of 66%. Additionally, the carbon fabric composites based on optimized resin possesses high interlaminar shear strength of 37 MPa and superior ablative resistance with a linear ablation rate of 0.0195 mm/s and a mass ablation rate of 0.0460 g/s. This study provides new solutions to develop high char yield PRs matrix for ablative resistant composites without introducing any inorganic compounds.