Fundamental processes in the u.v. degradation and stabilization of polymers
The phenomenon of 'weathering' of polymeric materials is usually caused by a complex series of chemical reactions initiated by the absorption of ultra-violet light which ultimately result in the deterioration of the physical properlies of the polymer. One of the most likely initiating sites for ultraviolet absorption in polymers is the ketone carbonyl group which is often introduced into the polymer by oxidation during its processing or preparation. Studies have been made of the photochemistry of polymers containing various types of ketone groups to determine the effects of polymer structure, external and internal viscosity, and temperature on the quantum yields of the chemical and physical processes which dissipate the energy of the absorbed light. The application of this information to the stabilization of polymers against ultra-violet degradation is discussed in this paper. The term 'weathering' refers to a wide variety of chemical reactions and physical processes which occur when macromolecules, either natural or synthetic, are exposed for extended periods of time to outdoor conditions. Extensiveresearch over the past thirty or forty years has shown that for the great majority of synthetic macromolecules, used as plastics and synthetic fibres, the most important degradative mechanisms are associated with the absorption of ultra-violet (u.v.) light. Natural polymers, on the other hand, although they also degrade by photochemical mechanisms usually are more rapidly degraded biologically, that is by the attack of micro-organisms. The protection of polymers against the effect of u.v. radiation, thus becomes of particular importance to the plastics and synthetic fibre industries. The problern of stabilizing synthetic polymers against u.v. light is not a simple one because polymers break down by a variety of mechanisms. Furthermore the reactions frequently involve the presence of moisture, oxygen or pollutants such as sulphur dioxide, hydrochloric acid, ozone, etc. This makes it very difficult to predict the effective lifetime of plastics in * Visiting Professor, Centrede Recherehes sur les Macromolecules, Strasbourg, France.