Inherently flame retardant vinyl bearing hyperbranched polysiloxanes having improved thermal stability-Ceramization and analysis of associated thermal properties

This study describes the synthesis and characterization of novel hyperbranched silicone polymers having high thermal stability and inherent flame retardancy achieved purely by molecular architecture, without the incorporation of any specific additive or filler. A series of hyper-branched terpolymers (MeDPhDViT) containing vinyl, phenyl and methyl moieties were synthesized by the acid catalysed copolymerization of vinyltriethoxysilane (VTES), diphenyldimethoxysilane (DPDMS) and dimethyldiethoxysilane (DMDES). Curing of these polymers is effected through condensation of the hydroxyl groups triggered by dibutyltindiluarate (DBTDL) catalyst. Glass to rubber transition temperature (Tg)of the cured polymers vary systematically from −110 °C to +50 °C with the increase in the content of vinyl T (ViT) units in the polymer backbone. The terpolymer with maximum ViT-unit content shows an onset of thermal decomposition above 540 °C and leaves 86% char residue at 900 °C, attributed to the retardation of depolymerisation by ViT unit. The limiting oxygen index (LOI) of the terpolymers increases from 26 to 37% with increase in ViT unit content alone, from 13.4 to 53.3 mol% and the terpolymer with 53.3 mol % ViT unit rates as V-0, the best in the UL-94 flame test. The ceramic residue from the cured elastomer comprises of free carbon and silica/Si-O-C ceramics. A probable mechanism has been proposed for the rearrangement of vinyl groups to carbonaceous ceramics under high temperature environment. The vinyl groups promote the formation of a carbonized, in particular graphitized, layer and SiOC phases during pyrolysis, as evidenced by TG-MS, pyrolysis GC-MS, Raman spectroscopy and FESEM analysis. Comparative study with a non-phenylated silicone elastomer confirms the mechanism of formation of carbon rich residue by the thermal rearrangement of vinyl groups.