Stabilizer packages and synergism

Frequently, different classes of light stabilizers are combined to optimize stabilizing efficiency [112]. For example, UVAs and HALSs, used in combination, often provide better photostability than either class alone. Light stabilizers are also used in combination with additives that protect the polymers against thermal degradation during processing, such as hindered phenols and phosphates [113]. Consequently, various bifunctional and trifunctional photostabilizers have been synthesized and some have been selected for use in commercial applications (see Chart 9.15). 

Chart 9.15 Chemical structures of typical bifunctional stabilizers [87]. 

In the context of multifunctionality, carbon black, a polycrystalline material, merits special mention. The surface layer of carbon black particles may contain quinones, phenols, carboxy phenols, lactones, etc. Therefore, apart from being a powerful UV absorber and a quencher of excited states (such as those of carbonyl groups), carbon black acts as a scavenger of free radicals in chain-breaking reactions and as a hydroperoxide decomposer [114, 115]. In polyethylene, carbon black forms a complex with macroradicals [115]. 

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