The influence of covalently bound phosphorus-containing groups on the flammability of some styrenic and acrylic polymers

When subjected to heat, polystyrene (PSt), polymethyl methacrylate (PMMA) and polyacrylonitrile (PAN) can undergo extensive chain-scission/chain-stripping reactions to form relatively low-molecular weight volatile fragments, including their monomeric species. Given that PSt and PMMA are not typically char- forming polymers, they burn readily in air when subjected to a sufficient amount heat and in the presence of a suitable pilot. On the other hand, polyacrylonitrile (PAN) upon thermal decomposition can form a significant amount of char residue, owing to the condensation of the pendant nitrile units, which in turn depends on the rate of heating. In all cases, low molecular weight volatiles once formed can subsequently undergo combustion reactions, and hence PSt, PMMA and PAN are generally considered to be relatively flammable. With a view to improving the flame retardance of these polymers, we have chemically modified them (i.e. through an additive approach) with phosphorus-, or optionally, phosphorus/nitrogen-containing groups. The recovered polymers, after necessary purification, were subjected to a host of characterization techniques that included spectroscopic, thermal and calorimetric tests. We have also endeavored to formulate the modes of action of the flame-retardant groups, in the condensed- and gaseous-phase, occurring during the decomposition and combustion of these modified systems, by employing variety analytical instruments, that also included, optionally, some hyphenated techniques. The results revealed that phosphorus-containing moieties exert combustion inhibitory effects, in both condensed- and gaseous-phase, and the relative predominance of which in turn depend on the chemical environment of the phosphorus group and the nature of the polymer matrix in question.