Time evolution of the concentration profiles of HALS stabilizers and of the corresponding oxidation forms across poly(propylene) plaques irradiated with UV-visible light

The determination of the concentration changes with time of hindered amine light stabilizers (HALS) and of their photoreaction products, that is nitroxides, R2NO°, hydroxylamines, R2NOH, and aminoethers, R2NOR′, observed in poly(propylene) plaques exposed to continuous irradiation for several months, is reported. The distribution of the additives across the irradiated plaques was also determined either by using the non-destructive EPR imaging techniques or by cutting the plaques in thin slices and analyzing each of them for the additive content. In HALS-stabilized samples the amine content showed a constant decrease upon irradiation, while the concentrations of the nitroxide and of the hydroxylamine initially increased reaching a maximum after about two months and then decreased. In samples initially containing the stabilizer in the form of nitroxide, irradiation produced a quick disappearance of the nitroxide concentration, while that of the hydroxylamine reached a maximum after one month of irradiation, and the amine concentration, surprisingly, increased steadily with time. After 2-3 months of irradiation the amine content was about 15% of the initial concentration of the nitroxide. This means that not only the nitroxide radicals can be recycled from the hydroxylamines and aminoethers, but also that the amine itself can be regenerated from its oxidation products. Concomitant with these changes was a redistribution of the nitroxide and hydroxylamine inside the polymer, which disappeared more rapidly in the interior of the plaque than in proximity of the external surface, while the amine content was uniform throughout the plaque. The interpretation of this effect, which may be of considerable relevance for a better understanding of the mechanism of polymer stabilization by HALS, is discussed in terms of two alternative models.