Norbornene POSS crosslinked EPDM composites for thermal protection system: Significant boosting mechanical and ablation resistance

As a lightweight nanomaterial, Polyhedral Oligomeric Silsesquioxane (POSS) is widely applied to ablation resistance modification of Ethylene-Propylene-Dine Monomer (EPDM) insulation layer in aerospace propulsion thermal protection system. However, various structures of POSS can form different crosslinked structures within the EPDM, which can affect the insulation layer properties. Various functionality POSS, Mono-Norbornene POSS (MN-POSS) and Tri-Norbornene POSS (TN-POSS), were designed and synthesized to obtain crosslinked-modified EPDMs with enhanced mechanical properties and ablation resistance simultaneously, and the relationship between POSS functionality, the mechanical properties, ablation resistance, heat-shielding and thermal decomposition of EPDM/Aramid Fiber (AF) composites were explored comprehensively. MN-POSS and TN-POSS increased the tensile strength of EPDM composites by 25.3% and 75.2% respectively, reduced the linear ablation rate by 37.7% and 33.7% respectively, and reduced the back temperatures by 3.9 °C and 3.3 °C respectively. Under conditions of equal cage structure (T₈), the suspended crosslinked structure caused by MN-POSS exhibited better ablation resistance and heat-shielding performance as well as thermal decomposition, and the anchored crosslinked structure caused by TN-POSS exhibited better tensile strength. The structural transformation indicates that the POSS nanocages can be transformed into a ceramic structure in cruel environments to resist the erosion of heat flow and enhance the ablation resistance of insulation layer.