Synthesis of crosslinkers based on octa vinyl polyhedral oligomeric silsesquioxane and their flame retardant applications in silicone rubber

It is challenging to improve the flame retardancy and thermal stability of silicone rubber. An effective method was proposed here to solve this problem by incorporating crosslinker made of octa vinyl polyhedral oligomeric silsesquioxane (OV-POSS) into condensed room temperature curing silicone rubber (PDMS). Meanwhile, OV-POSS with different numbers of crosslinking groups were also applied to PDMS and effectively improved its flame retardancy and thermal stability. With the addition of 10 phr nC-POSS (an OV-POSS has an average of n participating click response groups, including 2,4,6,8C-POSS), silicone rubber (SR-nCPOSS) passed the V-0 rating in the vertical combustion test (UL-94) with a high limiting oxygen index (LOI) value of 31–32 %. Moreover, compared with silicone rubber (SR) using tetraethyl orthosilicate, the peak heat release rate was reduced by 46 % and the initial decomposition temperature under nitrogen was increased from 460 °C to 568 °C. In addition, the possible flame-retardant mechanism of nC-POSS, the flame-retardant and catalytic charring effects and free radical quenching effects of PDMS multi-crosslinked networks, were further revealed. The multi-crosslinked network formed by nC-POSS could inhibit the main chain decomposition caused by the terminal hydroxyl group at low temperatures and promote the crosslinking charring of SR, which effectively increased the service temperature of silicone rubber. Moreover, nC-POSS inhibited thermal degradation of SR in the condensed phase and quenches reactive radicals in the gas phase by generating rigid core radicals and phenyl radicals. Our results provide a simple new method for the fabrication of silicone rubber with excellent thermal stability and flame retardancy.