Improving ablation properties of liquid silicone rubber composites by in situ construction of rich-porous char layer

In this article, ammonium polyphosphate (APP) and ammonium pentaborate (APB) were introduced to liquid silicone rubber with an aim of building rich-porous char structure in situ, thereby improving thermal insulation properties. Thermogravimetric analyses indicated that the incorporation of APP greatly increased the char residue of the composites. Oxy-acetylene torch tests showed that the addition of either APP or APB powders effectively enhanced the ablation resistance of the composites, whereas Shore A hardness tests revealed that the APP-containing composites exhibited a higher hardness than APB-filled counterparts. The linear ablation rates of composites with 40 phr APP or APB were reduced by 34.16% and 36.19%, respectively, when compared with the control sample. The maximum back-face temperatures of composites with 40 phr APP or APB was reduced to as low as 73°C. The APP-containing composites exhibited superior ablation resistance, considering both the linear ablation rate and the mechanical properties of char layer. In addition to SiO₂, SiC, and C, B₂O₃ was produced in the APB composites, as characterized by XRD and Raman analysis. Combined with SEM, it was proven that the formation of a firm, continuous, rich-porous and thermal insulation char layer was advantageous to improve the ablation and insulation properties.