The Mechanism of Action of Hindered Amine Stabilizers. Production of Nitroxy Radicals and Inhibition of the Photo‐Oxidation in Different Polymers

Oxidation of bis[2,2,6,6‐tetramethyl‐4‐piperidinyl] sebacate (HALS) to itroxy radicals has been monitored by ESR spectroscopy in three polymers: an ethylene‐propylene random copolymer (EPM), a styrene‐butadiene‐styrene block copolymer (SBS) and the same block copolymer previously hydroperoxidized by reaction with singlet oxygen. Nitroxy radicals are produced in all three polymers on irradiation in the presence of oxygen and, in the last one also in the dark, at room temperature. The mechanism of the reaction is discussed: it involves direct interaction of a peroxy radical with the piperidine nitrogen. The role of clustered hydroperoxides as a source of peroxy radicals is emphasized. Inhibition of the photo‐oxidation of all three polymers by HALS has been studied by oxygen absorption measurements and by infra‐red spectroscopy in the carbonyl region. The results indicate that HALS not only inhibits the propagation of the photo‐oxidation but also catalyses a new type of termination that, in some polymers, delays the production of ketones. It is suggested that the practical efficiency of HALS in keeping the mechanical properties of polyolefins exposed to sunlight is mainly due to that last effect. Copyright © 1990 Wiley‐VCH Verlag GmbH & Co. KGaA, WeinheimSCOPUS: ar.j