Synergistic Enhancement of the Flame Retardancy of Silicone Rubber via Vinyl-Functionalized Modified Expandable Graphite and SiC/ZrO₂

Through molecular structure design, divinyltetramethyldisilazane (DVTMDZ) was hydrolyzed to produce vinyl(dimethyl)silanol, which subsequently underwent a dehydration condensation reaction with hydroxyl groups on the expandable graphite (EG) surface. This process enabled the chemical grafting of vinyl functional groups onto EG, yielding vinyl-functionalized expandable graphite (Vi-EG). The introduced vinyl groups served as crosslinking sites, allowing Vi-EG to chemically bond with silicone rubber (SR) via block copolymerization, thereby enhancing interfacial adhesion and dispersion within the SR matrix. The successful grafting modification was confirmed by Fourier-transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM/EDS). Vi-EG was then physically blended with silicon carbide (SiC) and zirconia (ZrO₂) as flame-retardant additives into a room-temperature vulcanized (RTV) SR matrix to construct an efficient multiphase flame-retardant system. SiC and ZrO₂, as high-melting-point ceramic fillers, synergistically worked with Vi-EG to promote the formation of thermally stable and compact char layers, which effectively suppressed heat transfer and oxygen ingress. In addition, SiC contributed to the mechanical reinforcement of the composite due to its inherent rigidity and good compatibility with the SR matrix. Meanwhile, ZrO₂ also played a positive role in improving the thermal stability of the composite system. The optimized formulation (SR: Vi-EG: SiC: ZrO₂ = 100:8:3:2.5) exhibited a high limiting oxygen index (LOI) of 37.1 vol.%, representing a 67.11% increase compared to pristine SR, and achieved a UL-94 V-0 rating. The composite demonstrated excellent thermal stability, with a char yield of 59.53 wt.% at 900°C (R₉₀₀), an 18.32% improvement over pristine SR. Cone calorimetry results revealed a peak heat release rate (PHRR) of 138.3 kW/m², total heat release (THR) of 13.0 MJ/m², and total smoke production (TSP) of 3.2 m², all significantly reduced compared to the pristine matrix. Importantly, while maintaining high flame retardancy, the composite retained favorable mechanical properties, including a tensile strength of 1.863 MPa, an elongation at break of 349.4%, a tear strength of 8.917 kN/m, and a compressive modulus of 6.158 MPa under 50% compressive strain. Therefore, this study focuses on constructing a multiphase synergistic flame-retardant SR system incorporating Vi-EG, SiC, and ZrO₂ to identify an optimized formulation through systematic evaluation of thermal, mechanical, and flame-retardant properties.