Influence du vieillissement thermo-oxydatif sur les comportements mécaniques du polychloroprène
Due to degradation during aging, most elastomers lose their particularly high extensibility as well as their ability to completely recover after deformation. In this study, the effect of thermo-oxidative aging on mechanical behaviors was investigated for neoprene (polychloroprene rubber). The results from tensile tests have shown that thermal aging resulted in an increase in crosslink density, tensile stress and modulus, as well as a decrease in ultimate elongation. The tensile stress-strain relationship at large strain obeys the eight-chains model. However, Mooney-Rivlin equation shows the best fit for the experimental data in the range of moderate strain and its parameters dependence on aging could be predicted using Arrhenius-type kinetic relation. With prolonged aging or/and at higher aging temperatures, the change in properties is more pronounced at the surface than in the bulk of the sample due to the effect of heterogeneous oxidation. Tearing energies measured at different tear rates and temperatures can be superimposed on a single master curve in accordance with the WLF (Williams-Landel-Ferry) rate-temperature relation, indicating that tearing in elastomers is governed by a viscoelastic process. During aging, the decrease in tearing energy can be associated with a decrease in the strain energy density in the crack tip region rather than with changes in the crack tip diameter. On the other hand, the fracture energy for cutting process is almost unchanged during aging due to a scale effect at the cut tip. The results from DMTA (Dynamic Mechanical Thermal Analysis) indicate an increase in Tg and a decrease in the damping value of neoprene after aging. In contrast, aging leads to a significant increase in the energy dissipation at high strain. This discrepancy can be attributed to the difference between the mechanisms of hysteresis of elastomers at low and high strain. A new theoretical model has been developed for predicting the hysteresis loss of elastomers under different aging and loading conditions. Finally, it has been found that the rate of thermo-oxidative aging in neoprene provides similar values of activation energy when measured either by the oxidative induction time (OIT), by the tearing energy or by the tensile ultimate elongation. For polychloroprene, the OIT corresponds to the moment when the elastomer reaches an optimized balance between strength enhancement from additional crosslink formation and the capability of the crosslinked network to dissipate deformation energy. The results show that thermal induction time tests at high temperatures can be used as a useful technique to predict the fracture performance of elastomers at lower temperatures.
- Génie – Thèses