Factors of Polymer Weathering - Atmospheric Oxygen

Atmospheric Oxygen

Photooxidation accounts for most polymer failures that occur during outdoor exposure. It results from the effects of solar radiation in combination with oxygen. Oxygen can promote degradation in several ways. Free radicals, formed as a result of the cleavage of chemical bonds by solar radiation, react with oxygen to form peroxy radicals that initiate a series of radical chain reactions. The destructive effect of the radiation is multiplied manifold by propagation of bond breakage and the formation of hydroperoxides that further absorb solar ultraviolet radiation. This cascade effect results in an auto-acceleration of the weathering process, and may partially account for the general non-linearity of the weathering response to radiant exposure.

In addition to the reactions of oxygen in its normal ground state, some reactions of oxygen are due to the excited singlet state, a highly reactive form of the molecule. Singlet oxygen is responsible for the rapid deterioration of materials, particularly those with conjugated unsaturation such as natural rubber and synthetic elastomers. It is formed when triplet oxygen, the normal ground state, reacts with sensitizers, such as certain dyes and ketones, excited by radiation to their triplet states. Oxygen also increases the amount of solar radiation absorbed by conjugated unsaturated hydrocarbons through formation of a complex with these materials.

The extent of photochemical reactions involving oxygen differ in the inner and outer layers of both aromatic and aliphatic polymers due to their dependence on the diffusion of oxygen through the polymer. Photooxidation is significantly reduced at depths beyond which oxygen penetrates. Studies of the degradation profiles of low density polyethylene (LDPE), polymethylmethacrylate (PMMA) and polyvinylchloride (PVC) show that photooxidation was higher at the front and back surfaces than in the interior bulk of the material. Because ultraviolet radiation is not strongly absorbed by these materials, a considerable amount of the radiation incident on the front surface is transmitted to the back surface where it initiates photooxidation.