Impact of brominated flame retardants on the thermal degradation of high-impact polystyrene

The degradation of flame retarded high impact polystyrene (HIPS) was examined by thermogravimetry coupled with mass spectroscopy (TG-MS) and compared with that of polystyrene (PS). While the fate of the flame retardant draws a lot of attention, its impact on polymer degradation has been the focus of very little investigation. Temperature change was shown not to affect the product distribution of the thermal induced PS degradation. However, the presence of a brominated flame retardant resulted in changes in the HIPS degradation mechanism, with a larger variation of by-products formed and changes in the product distribution over the investigated temperature range. The early release of a large quantity of bromine radicals from the flame retardant caused the polymer backbone to break at various points (radical induced degradation). While the thermal degradation of PS was inhibited by the recombination of macro radicals (the cage-effect), the recombination of macro radicals induced by bromine radicals was prevented by the fast diffusion of HBr. Pure PS produced mainly styrene by the depolymerisation of the polymer chain after the formation of macro radicals and some oligomers from backbiting/β-scission. Flame retarded HIPS produced various oligomers, many of which were not produced from pure PS. It is assumed that after backbiting, the tertiary radical was terminated by recombination with other radicals, and dimers and trimers were formed from styrene during secondary reactions. One important source of hydrogen for this process was the formation of aromatic compounds, which kept the residual char small.