Thermal behaviour of covalently bonded phosphate and phosphonate flame retardant polystyrene systems

Abstract

Pyrolyses of the reactively flame retarded polystyrene copolymers styrene/diethyl(acryloyloxyethyl)phosphate(S/DEAEP), styrene/diethyl(methacryloyloxyethyl)phosphate(S/DEMEP), styrene/diethyl(methacryloyloxymethyl)phosphonate(S/DEMMP) and styrene/diethyl(acryloyloxymethyl)phosphonate(S/DEAMP) have been investigated with a view to obtaining information pertinent to the mechanism of their flame retardant behaviour. Studies were also carried out on the additive polystyrene systems containing triethylphosphate (TEP) and diethylethylphosphonate (DEEP) for comparison. All the systems contained 3.5 wt% of phosphorus. A range of techniques were used, namely TG with EGA, DSC, SEM, laser and microfurnace pyrolysis mass spectrometry and isothermal pyrolysis/GC–MS, to study the decompositions under a range of conditions. In the case of the additive systems, the additive was shown to be evolved before polymer decomposition occurred. Very little, if any, char residues were observed. Thus the main mechanism of fire retardant action of the phosphorus incorporated into the polystyrene as an additive would occur in the vapour phase. This mechanism prevailed regardless of whether the additive was a phosphate (TEP) or a phosphonate (DEEP). The effectiveness of the fire retardant action would be limited as the fire retardant and fuel did not volatilise together. There was evidence that some interaction occurred in the condensed phase. In all the copolymers the phosphorus content of the char was substantial. This is characteristic of the condensed phase fire retardant action of phosphorus. SEM studies showed the interior of the char to be a network of channels which would give the char a sponge-like interior which would enhance thermal insulation. The surfaces were relatively dense thus providing a barrier to escape for any gaseous products formed in the interior. Char formation and cross-linking are assumed to be the result of the presence of the strong phosphoric and phosphonic acids resulting from initial pyrolysis. Since phosphonic is the weaker acid, the polymer degradation and release of volatile products may be less inhibited in the case of the phosphonate-containing copolymers compared to the phosphate-containing copolymers. This is consistent with their shorter times to ignition. There was also evidence for some potential phosphorus vapour phase fire retardant action as phosphorus-containing species were identified among the pyrolysis products for all samples. The rate of volatile evolution from the copolymers was reduced compared to that of the corresponding additive system.