Structural and thermal behavior of a novel phenolic resin and its composite with excellent ablative performance.

• NaOH and trisilanolphenyl polyhedral oligomeric silsesquioxane (TPOSS) were used as catalysts to synthesize a novel boron-containing phenolic resin (TBPR). • NaOH/TPOSS results in more para -methylene linkage in TBPR structure, which is the key to the remarkable initial decomposition temperature. • The morphological transformation of TBPR at different temperatures was studied to verify the role of Si and B on the thermal pyrolysis stability. • CF/TBPR shows an outstanding ablative resistance due to the remakable thermal stability of TBPR. The heat resistance of existing phenolic resins cannot meet the requirements of the increasingly harsh spaceflight environment. Boron and silicon have been added to phenolic resins to improve the char yields. However, achieving both a high initial decomposition temperature and an outstanding char yield remains challenging. This study presented a strategy to regulate the methylene types of boron-containing phenolic resin (TBPR) by trisilanolphenyl polyhedral oligomeric silsesquioxane (TPOSS) and NaOH. Thermogravimetric analysis (TGA) in nitrogen of cured TBPR displayed remarkable thermal stability with an initial decomposition temperature of 450.0 °C and a char yield of 75.8 %. The effect of TPOSS and NaOH on the TBPR structure were further investigated using nuclear magnetic resonance (NMR) analysis and density functional theory (DFT) calculation. The results indicated that TPOSS could attract sodium phenate and formaldehyde through hydrogen bonds, and greatly reduced the energy barrier of the phenol–formaldehyde resin para -addition. The formation of more p-p' type methylene structures of phenolic resin profoundly contributed to the outstanding initial decomposition temperatures and char yield. TBPR matrix transformed into a stronger protective char layer, forming an effective thermal protection barrier. As a result, the mechanical strength and ablative resistance of the fiber-reinforced CF/TBPR composites were enhanced by the excellent thermal stability of TBPR matrix. [ABSTRACT FROM AUTHOR]