Time controlled stabilization of polyolefins

Hydroperoxides are key initiators in both thermal and UV initiated oxidation processes in polymers. The control of hydroperoxide concentration and hence the rate of the oxidation process can be achieved either directly by means of antioxidants which destroy hydroperoxides in nonradical generating reactions (preventive antioxidants) or indirectly by means of compounds which trap free radicals (chain‐breaking antioxidants) leading to a slower rate of hydroperoxide buildup. Transition metal ions which catalyze the breakdown of hydroperoxides to free radicals lead to the rapid achievement of a stationary hydroperoxide concentration and a rapid and initially linear rate of thermal oxidation. Some metal complexing agents delay the achievement of this linear rate. The effect of UV light in photooxidation of polymers has a similar effect to metal ions in thermal oxidation and the combined effect of both transition metal ions and UV irradiation can be catastrophic in polyolefins. However, certain metal ion complexing agents behave both as thermal oxidative stabilizers and as UV stabilizers due to a combination of different kinds of activity in the same molecule. These include metal deactivation, UV absorption, peroxide decomposition, and radical trapping. The photolytic stability of these complexes ultimately determines the length of the induction period which precedes polymer degradation. In general, nickel and cobalt complexes are UV stable and behave as long term stabilizers, whereas iron complexes are photolytically unstable and although they give rise to a period of initial stabilization which is related to their concentration, they photolyze to become effective photoinitiators.