Flame retarding poly(methyl methacrylate) with phosphorus-containing compounds: comparison of an additive with a reactive approach

Abstract

The flame retardance and thermal stability of a methyl methacrylate (MMA) copolymer reactively modified by copolymerisation of the MMA with diethyl (methacryloyloxymethyl) phosphonate (DEMMP) have been compared with those of poly(methyl methacrylate) (PMMA) containing equivalent amounts of the additive diethyl ethyl phosphonate (DEEP). DEEP can be regarded as having a structure similar to that of a DEMMP comonomer unit and therefore the two compounds might be expected to confer about the same levels of flame retardance to PMMA when used at similar concentrations. The incorporation of 3.5 wt.% phosphorus in both cases raises the limiting oxygen index of PMMA from 17.2 to over 22. However, cone calorimetry shows that the MMA/DEMMP copolymer is inherently more flame retardant than PMMA containing DEEP: the former has a significantly lower peak rate of heat release than the latter (449 and 583 kW m −2, respectively) and gives rise to a greater amount of char. Thermogravimetric analysis (TGA) of the polymers indicates also that the MMA/DEMMP copolymer is more thermally stable than PMMA whilst PMMA containing DEEP is less thermally stable. Dynamic mechanical thermal analysis (DMTA) shows that the MMA/DEMMP copolymer has physical and mechanical properties similar to those of PMMA, whilst the low molecular weight DEEP plasticises PMMA, resulting in a significantly reduced glass transition temperature, T g. A condensed phase mechanism of flame retardance in MMA/DEMMP has been identified.