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Free-radical processes underpin the thermo-oxidative degradation of polyolefins. Thus, to extend the lifetime of these polymers, stabilizers are generally added during processing to scavenge the free radicals formed as the polymer degrades. Nitroxide radical precursors, such as hindered amine stabilizers (HAS), are common polypropylene additives as the nitroxide moiety is a potent scavenger of polymer alkyl radicals (R•). Oxidation of HAS by radicals formed during polypropylene degradation yields nitroxide radicals (RR'NOD•), which rapidly trap the polymer degradation species to produce alkoxyamines, thus retarding oxidative polymer degradation. This increase in polymer stability is demonstrated by a lengthening of the "induction period" of the polymer (the time prior to a sharp rise in the oxidation of the polymer). Instrumental techniques such as chemiluminescence or infrared spectroscopy are somewhat limited in detecting changes in the polymer during the initial stages of degradation. Therefore, other methods for observing polymer degradation have been sought as the useful life of a polymer does not extend far beyond its "induction period". A sensitive technique for monitoring the alkyl radical precursors to polymer degradation involves spin-trapping by profluorescent nitroxides. |
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