Radical processes in polymer burning and its retardation. I. ESR methods for studying the thermal decomposition of polymers in the preflame and flame zones

Abstract

AbstractThe determination of the free radical distribution in the preflame and flame zones of a burning polymer (polypropylene) by ESR leads to the conclusion that in all phases of the burning polymer an exothermic reaction zone encloses an oxygen‐free pyrolytic zone. Whereas in the molten preflame zone (250–350°C) the polymer decomposes to oligomers, dimers, monomers, and the relevant free radicals or biradicals, in the gaseous flame zone the heat transport from the hot outer surface into the surrounding pyrolytic zone leads (with an increasing temperature gradient) to a progressive formation of thermodynamically more stable decomposition products. The CH. radicals generated at 400–800°C, after rapid cooling, yield polyaromaties with delocalized free electrons and the atomized carbon and its dimers at 800–1200°C, after cooling, yield graphite sheets with localized free electrons in its defects. Free radicals and paramagnetic species are trapped (a) in the gaseous pyrolytic products of heated polymers on the surface of a rotating cryostat, (b) in burning polymer drops by quenching in liquid nitrogen, and (c) in different zones of a burning diffuse flame. The superimposed ESR signals of the paramagnetic products are then qualitatively and quantitatively analyzed.