The deterioration process of polymeric materials by the simultaneous action of UV-light and diffusive oxygen is analyzed theoretically based on an unsteady-state dynamics,taking account of the progress of deterioration inward from surface. It is supposed that the deterioration proceeds photochemically by depolymerization and photooxidation reactions of polymer molecules photoactivated by absorbing UV-light near the absorption band characteristic of polymeric materials.As a result it is shown under some assumptions that the depth of the deteriorated layer increases approximately in proportion to the square root of exposure time.

This law (parabolic law) has previously been observed in the cases of neutralization of concrete and oxidation of metals, but this theory suggests that the parabolic law should also apply to the case of the deterioration of polymeric materials.

Experimentally many polymeric materials show the power law of exposure time (tⁿ ; n=O.51.O). The difference between theory and experiment is considered to be due to the complicated deterioration mechanisms which involve depolymerization and photooxidation.

The reduction in flexural strength over time of polymeric materials is explained by the reduced contribution of the deteriorated surface layer to the strength.