The antioxidant role of u.v. stabilisers

Abstract

The mechantsrns of action of the more important classes of uv stabtltsers for polyoleftns are considered in the light of present views on the mechanisms of antioxidant behaviour. The metal thiolate catalytic peroxide decomposers (PD-C) are seen to be effective uv stabilisers in their own right and as synergtsts with 'uv absorbers' . Chain—breaking antioxidants (CB—A), of which the hindered nttroxyl radicals are the most Important, are very effective as slngle component uv stabilisers but their precursors, the hindered amines, are thermal pro—oxidants . Donor chain—breaking antioxidants (CB—D), as ttfted by the hindered phenols, are weak photostabilisers with only a small concentration gradient . Their radical scavenging effect, Is augmented when they are used In conjunction with the 'uv absorbers ' but they antagonise with the peroxide decomposers. The 'uv absorbers' appear to function at least in part by a CB—D mechanism and their effectiveness is due mainly to their uv stability. They are destroyed by both hydroperoxides and by excited carbonyl chromophores and their synergism with the peroxide decomposers (PD—C) Is to a large extent due to their protection by this class of uv stabiliser. INTRODUCTION Uv stabilisers have always been categorised as a sub—group within the general antioxidant class of polymer additives (1). However, the ability of many photostabiltsers to absorb uv light without forming chemically reactive species has led to attention being focussed on light screening or filtering processes. Very adequate explanations for both the excellent screening behaviour and the high photostability of typical 'uv absorbers' have been given in recent reviews (2,3). Many effective photostabilisers do not however function primarily by absorbing uv light but are nevertheless preventive antioxidants. One of the earliest classes of uv stabiliser to be distinguished from the uv absorbers was the metal dtthtocarbamates (4) whose function as catalytic peroxide decomposers es already known (5). The contribution of uv screening to the function of these compounds has been shown to be relatively unimportant (6,7). Other types of peroxldolyttc agents have since been shown to be effective as uv stabtlisers, sometimes alone (6—8) but, more effectively, in combination with the 'uv absorbers' (9,10). More recently the effectiveness of many nickel complexes as uv stabilisers has been ascribed to the quenching of photo—excited states of impurities present in polymers. This antioxidant mechanism can also be classified as preventive (11). in that It interferes with the initiation of autoxidatton by removing potential free radical initiators. Although there is little doubt that many metal complexes can deactivate excited states of potential chromophores in polymers or singlet oxygen formed by their quenching with ground state oxygen, the importance of this kind of process in the stabilisation of polymers under practical conditions is still very much in question. (12). The function of some of the more effective uv stabilising metal complexes ts generally explained on the basis of other known photo—initiation processes occurring in the polymer; for example, many are either kinetic chain—breaking antioxidants or peroxide decomposers (6,13,14). Classification of antioxidant mechanisms Since the sequence of reactions occurring during photo—oxidation is the same as that