Mechanisms and kinetics of photostabilization of polyolefins with N-methylated HALS

Abstract

The present work concerns the mechanisms and kinetics of photostabilization with N-methyl HALS. It is found that the efficiency of this HALS type in polypropylene and polyethylene is often comparable to that of the corresponding unsubstituted HALS. This is attributed to very fast transformation of the N-methyl HALS into the formic acid salt of the corresponding NH-HALS. Such a transformation has been found previously and ascribed to free radical attack of the labile hydrogen atoms of the N-methyl group. Although free radical attack can account for corresponding findings on oven aging of polyethylene, it does not explain the findings on photooxidation of polyethylene. As a matter of fact, the rate of transformation of the N-methyl HALS is hardly affected by the presence of phenolic antioxidants. Furthermore, a benzoate-type UV stabilizer protecting mainly by free radical scavenging does not affect significantly the rate of transformation. To account for the effect observed with solid polyethylene, a photooxidative mechanism is envisaged. It is based on the formation of an exciplex oxygen-polymer. This exciplex is able to migrate in the polymer and is rather long lived. It is only when it involves a hydrogen atom that is sufficiently labile that there is reaction. The hydrogen atoms of the N-methyl group of HALS are reactive enough, so that reaction and transformation of the methyl group results, as found experimentally. Mechanism and kinetics of the reaction are discussed.