Thermal behavior of covalently bonded phosphonate flame-retarded poly(methyl methacrylate) systems

Abstract

Pyrolysis studies of reactively flame-retarded methyl methacrylate copolymers MMA/diethyl(acryloyloxymethyl)phosphonate (MMA/DEAMP) and MMA/diethyl(methacryloyloxymethyl)phosphonate (MMA/DEMMP) have been undertaken with a view to gaining information pertinent to the mechanisms of their flame-retardant behavior. The homopolymer with and without the additive diethylethylphosphonate was also investigated for comparison purposes. Each system contained 3.5 wt% of phosphorus. A range of techniques, namely TG with EGA, DSC, SEM, laser and microfurnace pyrolysis mass spectrometry, and isothermal pyrolysis-GC-MS, enabled the study to be carried out under a range of conditions. In the case of the additive system, the additive was evolved prior to the polymer decomposition. Little or no char residue remained at the end of the decomposition. Thus, the additive acted almost completely in the vapor phase. In the case of the MMA/DEMMP and MMA/DEAMP copolymers, substantial char residues were formed with 1.0 and 2.5 wt% phosphorus contents, respectively. This is evidence that predominantly condensed phase flame retardation occurs in these systems. SEM studies showed the chars from both copolymers to have flaky and smooth surfaces. Both copolymer samples showed slower decomposition rates compared to PMMA, the MMA/DEAMP being the slowest. Decomposition of MMA/DEMMP occurred mainly via end-chain scission with both MMA and DEMMP appearing concurrently, which would facilitate condensed-phase flame-retardant action. On the other hand, in the case of MMA/DEAMP, end-chain scission is to some extent is inhibited so that both end and the slower random-chain scission occur. As a result, the decomposition of the acrylate system was slower than that of the methacrylate copolymer. The former system yields more char which contains the higher percentage of phosphorus than does the methacrylate copolymer. Thus, MMA/DEAMP showed the greater condensed-phase flame-retardant action. This is consistent with the results of our previous flammability studies of these copolymers. The flame-retardant abilities of these two phosphonate copolymers are assessed with reference to previous studies of their equivalent phosphate copolymers. Copyright © 2008 John Wiley & Sons, Ltd.