Construction of bio-based and halogen-/phosphorus-free carbon microsphere hybrids via hierarchical interfacial assembly strategy for balancing the single-component flame retardant performance of silicone rubber

High-performance silicone rubber (SR) was widely considered for engineering applications. However, its practical potential was constrained by inherent flammability. The prevalent reliance on halogen- and phosphorus-based flame retardants would be associated with long-term health and ecological concerns, while also raising the sustainability issues related to the progressive depletion of phosphate resources. In addition, the high loadings of inorganic fillers were often difficult to simultaneously maintain the strength and ductility of SR. To address the intrinsic limitation of SR with poor char-forming ability and the absence of effective flame-retardant pathway during combustion, a “single-component and multifunctional integration” strategy was adopted in place of conventional “multi-component stacking” method. A halogen-free, phosphorus-free, low-toxicity, and low-loading single-component flame retardant (Si-FMs@PANI@LDH) was constructed to enable the synergistic optimization of flame retardancy, mechanical performance, and corrosion-resistant durability. Specifically, guided by a brick-and-mortar-inspired hierarchical architecture, nitrogen-rich furfural-based microspheres (FMs) together with cobalt-iron layered double hydroxide (CoFe-LDH) nanosheets were employed as the principal structural domains, while polyaniline (PANI) and epoxy-modified silicone resin (EPSR) served as organic interfacial components. Through this design, thermal shielding, gas-phase dilution, residue reinforcement, and migration suppression were integrated into a single flame-retardant architecture. At a loading of only 13 phr, the resulting composites were upgraded to Underwriters Laboratories standards-94 (UL-94) V-0 rating, and the limiting oxygen index (LOI) was increased to 30.5%. Fire-hazard parameters were substantially reduced with total heat release reduction of 21.3% and total smoke release reduction of 21.9%. The production of CO and CO₂ was simultaneously suppressed. Moreover, the effective protection was maintained under extreme thermal shock above 1000 °C, and the glow-wire flammability index (GWFI) was higher than 960 °C, indicating high heat tolerance and self-extinguishing capability relevant to electrical safety scenarios. While the flame retardancy was enhanced, the mechanical reliability was also retained that was evidenced by the good self-supporting capability of SR-5 and its elongation at break of 253%. Under strongly alkaline corrosion condition, SR composites still maintained UL-94 V-0 rating after 120 days immersion tests, demonstrating excellent long-term durability and flame-retardancy retention. Overall, the hierarchical interfacial construction of Si-FMs@PANI@LDH provided a feasible route for achieving a multidimensional performance improvement without halogen or phosphorus elements. This work also offered a practical reference for developing flame-retardant SR with reduced resource dependence and improved service stability.