High-performance flame-retarded natural rubber: Synergistic optimization of flame retardancy and mechanical properties via dual interfacial engineering

The catastrophic fire hazards associated with natural rubber (NR) present critical challenges for its deployment in emerging energy and transportation systems, particularly in rail transit applications. Conventional flame-retardant modifications often compromise mechanical integrity due to interfacial incompatibility and uncontrolled filler distribution. Herein, we propose a dual interfacial engineering strategy that reconciles flame resistance with mechanical enhancement through: (1) Molecular-level interfacial integration at the surfaces of flame retardants by encapsulating and (2) Mesoscale phase-structure control via a biphasic crosslinking system. The first interfacial hierarchy establishes flame-retardant-enriched high-crosslink rubber (MFRHNR) with interfacial bonding at the phase boundaries. The second interfacial hierarchy arises from precisely engineered polysulfide bridges between MFRHNR and low-crosslink rubber (LNR) phases. This strategy of dual-interfacial graded vulcanization engineering-induced gradient crosslinked structure achieved unprecedented performance harmonization: The optimized composite attains the UL-94 V0 rating with a limiting oxygen index of 26 %, while exhibiting exceptional tensile strength (23.4 MPa) and elongation at break (993.3 %) – surpassing state-of-the-art flame-retardant NR systems. Remarkably, the strategy enables breakthrough dynamic performance: 51.3 % reduction in hysteretic heat generation and 26.4 % enhancement in wear resistance compared with that of control sample with equal amount of flame retardant, and outstanding low-temperature flexibility with 98.5 % angular deflection retention at −40 °C. This work provides a new design paradigm for mechanical robustness in flame-retarded elastomers.