The effects of phosphorus-based flame retardants and octaphenyl polyhedral oligomeric silsesquioxane on the ablative and flame-retardation properties of room temperature vulcanized silicone rubber insulating composites

Phosphorus-based flame retardants - ammonium polyphosphate (APP), aluminum diethylphosphinate (ADP) and octaphenyl polyhedral oligomeric silsesquioxane (OPS) - were incorporated into room temperature vulcanized (RTV) silicone rubber insulating composite, in order to study the influence of flame retardants (FRs) on the ablative and flame-retardation properties of the RTV silicone rubber composites. The results showed that the FRs remarkably improved the ablative and flame-retardation properties of the RTV silicone rubber composites. With the addition of APP, ADP or OPS alone, the linear rate of the RTV silicone rubber composite was reduced by about 16.4%, 26.0% and 17.1%, respectively. In addition, the FRs obviously improved the flame retardancy, smoke suppression and thermal stability, based on a cone calorimetry test. The peak heat release rate (p-HRR) for the pure RTV silicone rubber composite was 338 kW/m². This was reduced to 217 kW/m², 184 kW/m² and 196 kW/m² with the addition of APP, ADP and OPS, respectively. Moreover, scanning electron microscopy, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy and thermogravimetric analysis tests showed that the FRs are helpful in forming a denser and intact char layer, due to enhanced condensed phase flame retardancy action. Furthermore, APP or ADP combined with OPS exhibit a significant synergistic effect in improving the overall performance of RTV silicone rubber composite. Specifically, the linear ablation rate (0.079 mm/s) was reduced by 45.9% with 15 wt. % ADP and 10 wt. % OPS in the RTV silicone rubber composite.