Thermal stability, pore structure characteristics of char layer and ablation resistance mechanism of new thermal insulation composites based on polyphosphazene polymer

•Polyphosphazene polymers and composites have excellent thermal stability and oxidation resistance.•Polyphosphazene composites have excellent ablation resistance, but poor thermal insulation.•The ablation resistance of polyphosphazene composite is due to the high strength of the char layer, and the poor thermal insulation is due to the high thermal conductivity of the material and the char layer.•The high strength of the char layer of polyphosphazene composite is due to the high carbon residue rate of the material and the bonding effect of silicon phosphate produced at high temperature, which greatly enhances its strength and toughness. Polyphosphazene (PDPP) composites have emerged as a hot research topic in the field of low-density composite insulation materials because of their good thermal stability and flame retardancy. In this study, the ablative properties and mechanism of PDPP composites reinforced by fiber and functional fillers are studied. The test results show that PDPP composites have good thermal stability and oxidation resistance. The results of the oxyacetylene ablation test show that PDPP composites has strong ablation resistance, and its char layer has high structural strength, as well as high bonding strength with the material matrix. However, the thermal insulation performance of PDPP composites is weak. Based on the physical and chemical analysis of the composite material and the char layer, it is concluded that the excellent ablation resistance is mainly due to the high strength of the char layer, which bnefits from high char yield of polyphosphazene polymer and the formation of silicon phosphate. However, the high thermal conductivity of composite and char layer lead to poor thermal insulation performance.