Cross-linking light-induced degradation

One way in which most polymers do decay is by the action of oxygen in the air and of light You will be familiar with the way that some polymers go yellow after a time and some become brittle. Coloured plastics, in particular, absorb light and oxygen-induced radical reactions follow. The polymer becomes too cross-linked and loses flexibility. One ingenious application of this natural process helps to degrade the polythene rings that hold cans of beer in packs. These are often discarded and decay quite quickly because some carbon monoxide has been incorporated into the polyethylene to make it more sensitive to photolysis. 

Changes also occur as the polymer ages under the influence of time, air, light and heat. Cross-linking or oxidation of the chains may cause a polymer to become insoluble in solvents that can be safely used on an object (Sease, 1981). The polymer may also cause the object to degrade more rapidly than it otherwise would have done (Berger and Zeliger, 1975 Baer et al 1976). The cause of these deleterious effects is probably a combination of chemical reactions induced by the polymer and the physical restraint imposed by the polymer on a very different type of material. 

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