Mechanisms and kinetics of photostabilization of polyolefins with hals

Abstract

AbstractThe HALS stabilization mechanisms usually envisaged do not explain the experimental facts. New mechanisms are proposed instead. They include quenching of excited charge‐transfer complexes (CTC) polymer‐oxygen by CTC HALS‐oxygen. In addition, the formation of CTC between peroxy radicals and NH‐HALS is of prime importance. These CTC HALS‐peroxy radicals can either react photolytically after absorption of UV radiation or with a second peroxy radical, thus promoting termination between two peroxy radicals.Initiation of photooxidation in HALS stabilized polyethylene involves usually CTC HALS‐oxygen throughout the useful lifetime of the polymer. In contrast to that, initiation of photooxidation in HALS stabilized polypropylene is classical and due to photolysis of tertiary hydroperoxides.The formal kinetics based on the mechanisms envisaged explain the experimental results observed with HALS stabilized polyethylene and polypropylene. On the one hand, they concern, the performance of HALS: in polyethylene it is a linear function of the square root of HALS concentration whereas in polypropylene it increases linearly with HALS concentration as long as the last does not become too high. On the other hand, the experimental facts explained, concern the decrease of the HALS concentration during W exposure of HALS‐stabilized polyethylene and polypropylene. The decrease is first order in both polymers but is independent of the initial HALS concentration in polyethylene, whereas it decreases with increasing initial HALS concentration in polypropylene. This is in agreement with the corresponding initiation mechanisms envisaged for polyethylene and polypropylene.