Polyphosphazene-Functionalized Boron Nitride Nanosheets as a Multifunctional Synergist for Enhanced Thermal-Oxidative Stability and Fire Safety of EPDM Composites

Ethylene propylene diene monomer (EPDM) is widely used in automotive and HVAC sealing systems, where its long-term thermal-oxidative stability and fire safety are critical, yet challenged by high-temperature operation and flammability risks. To address these limitations, this work develops a surface-engineered boron nitride synergist (h-BN@PDT) through NaOH-assisted hydroxylation and subsequent functionalization with hexachlorocyclotriphosphazene and tannic acid. The modified h-BN@PDT integrates phosphorus/nitrogen flame-retardant groups and radical-scavenging phenolic hydroxyls, achieving multifunctional enhancements in EPDM composites. Thermal-oxidative aging tests (125 °C, 168 h) demonstrated exceptional stability, with 11.12 MPa of tensile strength for aged h-BN@PDT/EPDM, compared to 2.78 MPa of traditional flame-retardant EPDM without BN fillers. Doubling oxidation induction time was attributed to enhanced thermal conductivity and pyrolysis activation energy, which contributed to BN’s oriented heat-transfer pathways and phenolic hydroxyl-mediated radical quenching. Flame retardancy evaluations revealed that h-BN@PDT can effectively replace partial conventional flame retardants while enabling the composite to achieve a UL-94 V-0 rating with only 25 phr of flame-retardant loading. Cone calorimetry tests demonstrated a significantly reduced heat and smoke release rate. Mechanistic studies identified that h-BN@PDT enhances flame retardancy through the synergistic effects of the lamellar barrier effect and promoted catalytic charring capability. These mechanisms effectively inhibit heat transfer, block the diffusion of pyrolysis products, and restrict oxygen access, thereby substantially improving the flame-retardant efficiency of conventional systems. This work resolves the inherent trade-offs among flame retardancy, aging resistance, and mechanical integrity in EPDM, offering a scalable strategy for next-generation sealing materials in electric vehicles and high-temperature engineering applications.