Exploration of balance between flame retardant and thermal insulation of ablative fillers in EPDM

EPDM rubber serves as an excellent matrix for ablation-resistant thermal protection materials. However, ablation resistance and flame retardancy are often mutually exclusive in such systems. Ablative fillers paradoxically compromise flame retardancy in certain configurations, contradicting conventional reinforcement expectations. Filler geometry and component serves as a critical factor governing condensed-phase flame-retardant mechanisms, representing a previously underexplored balance between flame retardant and thermal insulation in multifunctional material design. This study successfully developed an intumescent flame retardant (IFR) system that achieved a UL94 V-0 rating in ethylene-propylene-diene monomer EPDM rubber. Four geometrically distinct ablative fillers are then incorporated: spherical hollow glass microspheres, lamellar muscovite, irregular fumed silica, and networked silica aerogel. flame retardant and thermal insulation properties are systematically evaluated. At 20 phr (parts per hundred rubber) loading, only spherical filler HGM maintained the UL94 V-0 rating while enhancing self-extinguishing performance. Lamellar filler MS achieved the most significant reduction in peak heat release rate, with pHRR reaching 372.55 kW/m², and the failure time was significantly extended to 527 s. Irregular filler FS and networked filler SA substantially improved mechanical properties; particularly, SA incorporation increased tensile strength at break by 918 %. However, both fillers considerably compromised char formation during combustion.