Inhibition of secondary photochemical processes

The use of ultraviolet absorbers, in principle, inhibits photophysical as well as photochemical processes in polymers. However, there are some limitations to the usefulness of this method of stabilization. In fact, the fraction of light absorbed by the additive and dissipated as heat can be calculated, if the Beer—Lambert s law is valid, to be 

On the other hand the residual absorption of the polymer in the presence of a given concentration of ultraviolet absorber can be calculated as a function of the thickness of the sample [135]. As an example, Fig. 30 

It can thus be concluded that the efficiency of ultraviolet absorbers as photostabilizers is severely reduced when the polymer itself has a high absorption in the solar ultraviolet, or in thin samples such as fibres. In these cases search for another method of stabilization is recommended. 

Deactivation of excited states by transfer of energy to a suitable acceptor can efficiently inhibit secondary photochemical processes. The reaction can be represented schematically as 

The transfer of energy can be practically quantitative and result in a complete inhibition of secondary photochemical processes when excited triplet states only are involved. This is illustrated by the photo-reticulation 

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