In recent decades, elastomer ageing behaviour become enormously important due to their use in increasingly complex and high load applications. In particular, the influence of oxygen and elevated temperature limits the long service life behaviour of elastomer components and increase interest in predicting a material’s properties as well as the mechanism behind it. In this work, elastomers thermo-oxidative ageing was analysed in detail by approaching the topic via several disciplines including: engineering, material science and chemistry. After providing a broad fundamental understanding of polymer ageing, oxygen ingress and diffusion into the elastomer is described and forms the basis of experimental investigation and modelling.
Experimental investigation started with the absorption of oxygen by the elastomer sample. A test setup using a respirometer was constructed to measure the quantity of oxygen molecules are taken up by the elastomer during exposure. Test results showed the time dependent absorption rate and clarified the behaviour of certain elastomer types, as well as, the influence of antioxidants. Oxygen adsorption was found as a temperature mediated process with a variable absorption rate during ageing. In order to include information about the spatial distribution in the material, the behaviour of oxygen in the elastomer is also explored. Since diffusion was thought to be very important for the overall oxidation and Diffusion-Limited-Oxidation (DLO) represents a key process when dealing with thermo-oxidative ageing, investigations were expanded to include test methods aimed at understanding nonhomogeneous oxidation. Computer tomography was introduced as a novel approach to give an insight to local changes in density but serves as an indicator for heterogenous ageing only. A closer view on the chemical changes occurring was given by infrared spectroscopy (FT-IR) which provides spatial changes in the adsorption behaviour of functional groups, like carbonyl. The results showed surface oxidation occurring while the interior is only slightly affected. Furthermore, the effects of chemical changes were shown by mechanical testing of aged elastomer samples.
Analogously to the experimental approach, modelling was conducted along the chain of causation. Oxygen absorption was described by Henry’s law, with solubility influenced by the progress of ageing. In order to include nonhomogeneous effects, a modular model was developed which is based on a diffusion-reaction equation. Reactivity was coupled with the presence of oxygen and the level of oxidation. The latter represents saturation effects which occur when oxidative reactions proceed. Furthermore, reaction rate decreases due to decreasing number of reaction partners. Diffusivity was described as a function of the level of oxidation. As a measure of oxidation, ageing parameters for network degradation and reformation were introduced. The model was able to describe experimental testing results for which ageing parameter for network degradation was theoretically equal to the carbonyl nitrile ratio obtained by FT-IR analysis. In the course of this work the model was critically reviewed steadily and continuously adjusted to optimise performance and reduce the degree of phenomenology.
Nonhomogeneous oxidation is visualised by plotting ageing parameters over the sample’s cross-section which clearly shows the effect of diffusion-limited oxidation. The outcome was used to describe spatial mechanical properties and clarified the dualism of network degradation and reformation. Moreover, the ageing profiles were coupled with the amount of the absorbed oxygen providing further understanding of thermo-oxidative ageing. «
In recent decades, elastomer ageing behaviour become enormously important due to their use in increasingly complex and high load applications. In particular, the influence of oxygen and elevated temperature limits the long service life behaviour of elastomer components and increase interest in predicting a material’s properties as well as the mechanism behind it. In this work, elastomers thermo-oxidative ageing was analysed in detail by approaching the topic via several disciplines including: en... »