THE ROLE AND REACTIONS OF NITROXYL RADICALS IN HINDERED PIPERIDINE LIGHT STABILIZATION

The reactions of nitroxyl radicals play an important role in the inhibiting action of sterically-hindered piperidines as light stabilisers for polymers. These radicals participate in cyclic chain-termination reactions and so provide long induction periods in photo-oxidation of polymers. Two mechanisms of cyclic chain termination have been proposed and discussed in the literature. The role of any mechanism in polymer stabilisation depends on the conditions, especially on the temperature. A third mechanism of cyclic chain-termination by nitroxyls has been found in oxidation of preoxidised polypropylene. The decay of hydroperoxide groups in the polymer leads to hydrogen peroxide formation and this produces HO2. radicals, that reduce bond NO. to bond NOH. Thus the new cyclic chain termination appears: bond NO. --> HO2. bond NOH --> PO2. bond NO. The author and coworkers recently studied a new mechanism of catalytic chain termination by nitroxyls in oxidising ethylbenzene in which both hydrogen peroxide and acid are the necessary components of the inhibiting system.