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 v ariety of comonomers containing covalently-bound phosphorus-containing grou ps, including vinyl phosphonic acid, several dialkyl vinyl phosphonates, an d various vinyl and allyl phosphine oxides. The flame retardance of these p olymers has been preliminarily assessed through thermogravimetric analysis and measurements of limiting oxygen index (LOI) and char yields. All the ph osphorus-containing polymers produce char on burning land also on heating i n air or nitrogen) and have LOIs higher than those of the parent homopolyme rs, indicating significant flame retardance involving condensed-phase mecha nisms. 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 en vironment is important. Tn order to explore mechanisms of flame retardance in more detail, laser pyrolysis/time-of-flight mass spectrometry and mass s pectrometric thermal analysis have been applied to study the decomposition behaviour of three of the MMA copolymers: those containing pyrocatecholviny lphosphonate (MMA-PCVP), diethyl-p-vinylbenzylphosphonate (MMA-DEpVBP) and di(2-phenylethyl)vinylphosphonate (MMA-PEVP) as comonomers. The laser pyrol ysis experiments provide an insight into probable polymer behaviour behind the flame front in a polymer fire and show that copolymerization of MMA wit h the comonomers does not greatly change the pyrolysis mechanism compared w ith that of poly(methyl methacrylate) (PMMA). However, the amounts of MMA m onomer evolved during pyrolysis are much reduced for the copolymerized samp les. 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 redu ced flammability shown by the copolymers. MMA-DEpVBP underwent the most ext ensive decomposition following laser pyrolysis. In particular, significant amounts of highly flammable methane and ethene were detected. Such increase d amounts would occur also if the copolymer were to be exposed to high temp erature conditions when burnt. Hence, its seems reasonable that the MMA-DEp VBP has a lower LOI value than expected, despite it giving a relatively hig h yield of char. Mass spectrometric thermal analysis studies of the MMA-PEV P provide evidence that the PEVP unit decomposes around 200 degrees C, elim inating styrene, with evolution of MMA reaching a maximum some 50 degrees C higher. Possible mechanisms for these processes are suggested. (C) 2000 El sevier Science Ltd. All rights reserved.