Radical processes in polymer burning and its retardation. II. An ESR study of flame retardation of polypropylene

Qualitative and quantitative free radical transformations between polypropylene alkyl radicals, oxygen, and brominated flame retardants in the molten and gaseous phases of thermally treated samples are described. By ESR technique it has been proved that, depending on the applied pressure during pyrolysis, highly reactive peroxy radicals or less reactive radicals of the retardant are formed. For flame-retarded samples, compared with polypropylene without additives, the concentration of the primary alkyl radicals transferred from the molten to the gaseous phase is reduced by two orders of magnitude (from 7 × 10¹⁴ to 7 × 10¹² spins/0.02 g), whereas the limiting oxygen index LOI] is raised from 17–18% to 25–26%. The great variety of physical and chemical processes proceeding in molten polymer in the preflame zone and burning gaseous phase calls for use of different retardant types with a programmed release of Br and HBr with the temperature increase. The chain oxidation of the “fuel,” a product of the endothermic decomposition of polymer, determines the temperature of self-ignition according to the number of initiating alkyl and allyl radicals formed per unit time; the LOI] index depends more on the length of the kinetic chain of propagation reactions in the stationary process of oxidation at a given pressure.