Flame retardance in some polystyrenes and poly(methyl methacrylate)s with covalently bound phosphorus-containing groups: initial screening experiments and some laser pyrolysis mechanistic studies

Styrene (ST) and methyl methacrylate (MMA) have been copolymerized with a variety of comonomers containing covalently-bound phosphorus-containing groups, including vinyl phosphonic acid, several dialkyl vinyl phosphonates, and various vinyl and allyl phosphine oxides. The flame retardance of these polymers has been preliminarily assessed through thermogravimetric analysis and measurements of limiting oxygen index (LOI) and char yields. All the phosphorus-containing polymers produce char on burning land also on heating in air or nitrogen) and have LOIs higher than those of the parent homopolymers, indicating significant flame retardance involving condensed-phase mechanisms. However, despite there being general correlations between LOI, char yield and phosphorus-content, some copolymers have higher than expected LOI and/or char yield, whilst others have lower, indicating that phosphorus environment is important. Tn order to explore mechanisms of flame retardance in more detail, laser pyrolysis/time-of-flight mass spectrometry and mass spectrometric thermal analysis have been applied to study the decomposition behaviour of three of the MMA copolymers: those containing pyrocatecholvinylphosphonate (MMA-PCVP), diethyl-p-vinylbenzylphosphonate (MMA-DEpVBP) and di(2-phenylethyl)vinylphosphonate (MMA-PEVP) as comonomers. The laser pyrolysis experiments provide an insight into probable polymer behaviour behind the flame front in a polymer fire and show that copolymerization of MMA with the comonomers does not greatly change the pyrolysis mechanism compared with that of poly(methyl methacrylate) (PMMA). However, the amounts of MMA monomer evolved during pyrolysis are much reduced for the copolymerized samples. Since MMA is the major fuel evolved during the combustion of PMMA and its copolymers, this effect must be a major contributing factor to the reduced flammability shown by the copolymers. MMA-DEpVBP underwent the most extensive decomposition following laser pyrolysis. In particular, significant amounts of highly flammable methane and ethene were detected. Such increased amounts would occur also if the copolymer were to be exposed to high temperature conditions when burnt. Hence, its seems reasonable that the MMA-DEpVBP has a lower LOI value than expected, despite it giving a relatively high yield of char. Mass spectrometric thermal analysis studies of the MMA-PEVP provide evidence that the PEVP unit decomposes around 200 degrees C, eliminating styrene, with evolution of MMA reaching a maximum some 50 degrees C higher. Possible mechanisms for these processes are suggested. (C) 2000 Elsevier Science Ltd. All rights reserved.