Photooxidation of fire retarded polypropylene II. Photooxidation mechanism

In the preceding article, we reported on the kinetic aspects of the photoox idation of fire retarded polypropylene. The mechanisms are now considered. The photoproducts generated throughout irradiation of fire retarded polypro pylene have been identified by FTIR analysis. The comparison with those obt ained in the case of the pure polymer showed that the same carbonylated pho toproducts were formed for both the fire retarded and the pure polypropylen e in the same relative concentrations. Our results showed the formation of the same hydroxylated photoproducts, but it was found that the relative amo unt of hydroxylated/carbonylated photoproducts was lower in the case of the fire retarded polymer. Investigating the formation of the various photopro ducts as a function of the oxygen consumption confirmed these results. It w as postulated that the mechanism of Formation of the carbonylated compounds remained mainly unchanged whereas modification in the hydroperoxide format ion could be expected. The influence of the irradiation wavelengths has als o been studied. It was shown that irradiation in a domain where no oxidatio n of pure polypropylene can be observed within a reasonable time (lambda > 360 nm) led to an important degradation of the fire retarded sample. Moreov er, FTIR analysis showed that the fire retardant (DBDPE) degraded quickly u nder irradiation. The increase of the photooxidation rate was attributed to radicalar attack by the free radicals generated by photolysis of the DBDPE . This effect was confirmed on the basis of the results obtained when study ing the influence of a free radical scavenger which led to a notable decrea se of the photooxidation rate. (C) 1999 Elsevier Science Ltd. All rights re served.