Effect and mechanism of hepta-phenyl vinyl polyhedral oligomeric silsesquioxane on the flame retardancy of silicone rubber

Abstract

It is challenging to simultaneously improve the flame retardancy and mechanical properties of silicone rubber. We herein proposed an efficient method to address this issue by incorporating hepta-phenyl vinyl polyhedral oligomeric silsesquioxane (HPVPOSS) into addition-cure liquid silicone rubber (ALSR). HPVPOSS meanwhile enhanced the flame retardancy and thermal stability of ALSR. With the addition of 10 phr HPVPOSS, ALSR passed the V-0 rating in the vertical burning test (UL-94) and had a limiting oxygen index (LOI) value as high as 31.0%. Moreover, compared with ALSR, the peak heat release rate and total heat release were decreased by 31.8% and 28.7%, respectively. Besides, the ALSR/HPVPOSS nanocomposites were demonstrated to have excellent mechanical properties. The possible flame retardant mechanism of HPVPOSS, which was attributed to the free-radical quenching effect and the catalysis effect on the crosslinking charring of ALSR, was further revealed. By generating the rigid core free-radicals and phenyl free-radicals, HPVPOSS suppressed the thermal degradation of ALSR in the condensed phase and quenched the active free-radicals in the gas phase. Moreover, HPVPOSS promoted the crosslinking charring of ALSR and the formation of a compact ceramic protection layer on the ALSR surface, effectively hindering the heat transfer and oxygen diffusion. Our findings provided a new way to fabricate silicone rubber with prominent flame retardancy and outstanding mechanical properties.