CURRENT TRENDS IN MODE OF ACTION OF HINDERED AMINE LIGHT STABILIZERS

The stabilization mechanisms of Hindered Amine Light Stabilizers (HALS) involving mainly various oxidation products of HALS do not account for the most important experimental facts. Part of these facts pertains to the dependence of HALS performance on their concentration. In polypropylene the performance increases linearly with HALS concentration as long as this concentration,is not too high. In high density polyethylene the performance is a linear function of the square root of HALS concentration. Another part of the experimental facts concerns the decrease of the concentration of the light stabilizers on UV exposure of HALS stabilized polyolefins. The decrease is first-order in both PP and PE-HD. However, this decrease, independent of initial HALS concentration in PE-HD, decreases with increasing HALS concentration in PP. The role of the oxidation products of HALS in polyolefin stabilization is discussed in detail. It is found that there is a fundamental difference between the tertiary hydroxylamine ethers mainly formed in polypropylene and the secondary hydroxylamine ethers formed in polyethylene. The latter are postulated to be much more reactive than the former under photooxidative conditions because they contain an alpha-hydrogen atom prone to attack by peroxy radicals. Various mechanisms are proposed for this reaction. They account for the products found in polymers as well as in model systems. New stabilization mechanisms are proposed to explain the experimental facts. They are based on modified photooxidation mechanisms of unstabilized polymers. Initiation of photooxidation of polyolefins by excited polymer-oxygen complexes is of prime importance. This type of initiation remains essential throughout polyethylene photooxidation during which hydroperoxides do not build up. It usually becomes rapidly negligible in polypropylene in comparison with initiation by tertiary hydroperoxides. Initiation of photooxidation in HALS stabilized polyethylene involves HALS-oxygen complexes throughout the useful lifetime of the polymer. Initiation of photooxidation of HALS stabilized PP is attributed to photolysis of tertiary hydroperoxides as long as the concentration of HALS is not too high. The HALS stabilization mechanisms proposed involve quenching of excited polymer-oxygen complexes. Formation of complexes between peroxy radicals and secondary HALS is another important aspect of these mechanisms. The peroxy radical/HALS complex can either react photolytically on absorption of UV light or react with a second peroxy radical. The latter leads to a termination reaction between two peroxy radicals as it is observed in the absence of HALS. The formal kinetics based on the mechanisms envisaged explain the experimental results obtained with HALS stabilized PP and PE-HD. It is postulated that the mechanisms discussed are valid, at least in part, for many polymers other than polyolefins.