Thermal degradation and flame retardance in copolymers of methyl methacrylate with diethyl(methacryloyloxymethyl)phosphonate

Methyl methacrylate (MMA) has been free radically copolymerized, both in bu lk and in solution, with diethyl(methacryloyloxymethyl)phosphonate (DEMMP), to give polymers which are significantly flame retarded when compared with PMMA, as indicated by the results of limiting oxygen index (LOI) measureme nts, UL 94 tests, and the results of cone calorimetric experiments. The phy sical and mechanical properties of the copolymers are similar to those of P MMA, except that the bulk copolymers are slightly crosslinked, and are bett er than those of PMMA flame retarded to a similar extent by some phosphate and phosphonate additives. Examination of the some of the gaseous products of pyrolysis and combustion, and of chars produced on burning, show that fl ame retardation occurs in the copolymers by both a condensed-phase and a va pour-phase mechanism. The condensed-phase mechanism is shown to involve gen eration of phosphorus acid species followed by reaction of these with MMA u nits giving rise to methacrylic acid units. The methacrylic acid units subs equently form anhydride links, which probably impede depolymerization of th e remaining MMA sequences, resulting in evolution of less MMA (the major fu el when MMA-based polymers burn). By undergoing decarboxylation, leading to interchain cyclisation and, eventually, to aromaticisation, the anhydride units are probably also the principal precursors to char. (C) 2000 Elsevier Science Ltd. All rights reserved.