Reactive phosphonitrile-crosslinked EPDM insulation layer with enhanced ablation resistance via carbon crystal of the condensed-phase structure at elevated temperatures

Two tailored lightweight reactive phosphoronitrile with hexa/dodeca-allyl structure (HAA-PPCP/DAA-PPCP) were directional designed and synthesized to fabricate the crosslinked EPDM composites, which optimized the pyrolysis behavior of the insulation layer to resistance the thermochemical ablation through the homogeneous introduction of P-N heterocycles via covalent bonding, combined with the robust condensed-phase structure formed to improve its ablation resistance. 5phr HAA-PPCP and DAA-PPCP could increase the compressive strength of the carbon layer after ablation by 800 % and 400 %, resulting in a reduction of the linear ablation rate by 28.9 % and 26.7 %, as well as the mass ablation rate by 25.8 % and 28.0 % while maintaining the density. Their introduction into the insulation layer (in-service) resulted in a reduction of the linear ablation rate by 36.1 % and 46.3 %, and the mass ablation rate by 33.4 % and 36.0 %. TG-IR showed that reactive phosphonitrile can significantly reduce the release of carbon-containing pyrolysis gases, combined with the structure analysis of pyrolyzed carbides and ablated carbon layers of phosphoronitrile crosslinked co-polycondensation EPDM composites by small angle neutron scattering (SANS), XRD and SEM to reveal the condensed-phase structure transformation under pyrolysis and ablation, which indicated that the reactive phosphonitrile can modulate the carbon crystals size in the condensed-phase structure to strengthen the carbonization layer by promoting the CVD reaction and carbothermal reduction reaction. The strengthened carbonization layer can stick fast to the surface of the EPDM composites, resulting in an excellent resistance to ablation. This work provides a new approach to high temperature ablation resistance.