Flame and thermal resistance of phosphorus-functionalized poly(methyl methacrylate) and polystyrene

Abstract

A series of phosphorus functionalized poly(methyl methacrylate) (PMMA) and polystyrene (PS) samples where the phosphorus functions are located at the chain ends, as pendant groups and as blocks in the middle of the polymer chain were evaluated for their flame and thermal resistance. The limiting oxygen index values (LOI) of the polymers revealed that the flame retardant efficiency of the phosphorus was dependent on its nature and position in the polymer chain and the type of polymer rather than on its absolute concentration. On a comparative scale, polymers with phosphorus at chain ends and in the middle of the chain exhibited better flame retardant properties. The polymers were subjected to thermogravimetric analysis (TGA) in air and N 2 under dynamic conditions and the trends in thermal behaviour with change in structure and composition have been examined and compared with that of the homopolymers. Except when located at chain ends, the phosphorus functionalization adversely affected the initial thermal characteristics of the PMMA samples, but with an improvement at elevated temperatures. For PS, phosphorus functionalization as pendant groups reduced their thermal resistance whereas chain-end and mid chain functionalization either did not affect or slightly improved their overall thermal resistance. All phosphorus-containing polymers left some char residue at higher temperatures. The activation energies for the major step of thermal decomposition were calculated from the thermograms. As the phosphorus content increased, the activation energy for a given degradation step was found to decrease except when the degradation was principally due to the phosphorus containing polymer block where a reverse trend was observed. The flame resistance of the polymers could be correlated to some extent with their thermal behaviour.