USE OF MICRO-FTIR SPECTROPHOTOMETRY IN THE DETERMINATION OF POLYMER PHOTOAGING MECHANISMS

The mechanisms of artificial accelerated photoageing, in the meaning of the ''mechanistic'' approach recently developed, have been determined for many classes of polymeric materials. The mechanism accounts for the formation and conversion of the main intermediate photoproducts and for the accumulation of final photoproducts. Based on the observed kinetics of the chemical evolution, described through the variations of a single critical photoproduct in artificial conditions and in the early stages of weathering, the lifetimes of polymeric materials can be predicted in many circumstances. Moreover, in the mechanistic approach, the influence of each photostabilizer (anti-UV, conventional antioxidant, redox antioxidant, hydroperoxide reducer) is evaluated and rationalized from the rate of formation of the critical photoproducts. The recognition of the on-going mechanism which controls the weathering of unstabilized and stabilized polymers is therefore of major importance to allow the conversion of data collected in laboratory to data corresponding to the real use. Fourier Transform Infrared (FTIR) microspectrophotometric determinations of profiles of final critical photoproducts and of residual photostabilizers afford a direct control of the relevancy of laboratory experiments. This recent technique which couples the FTIR technique and IR microscopy allows a full chemical analysis of 100 mum2-microzones or of 5 mum-elementary layers at the surface of the sample. Profiles could be determined up to 200-300 mum from the surface. Examples of polymers whose photoageing is completely controlled by chemical defects like pigmented poly(vinyl chloride) (PVC) and poly(methyl methacrylate) (PMMA) will be presented. Photooxidation, thermooxidation and ozonization of dienic elastomers will be compared to actual weathering to demonstrate the irrelevancy of the ozone test for the prediction of longterm behaviour of elastomers in the presence of light.