Crack Evaluation of Ozone-Aged Elastomers

Natural rubber (NR) is aged under a range of ozone concentrations under simultaneous static and cyclic mechanical load for different ageing times. In general, crack formation in the presence of deformation is a well-known result of ozone ageing. In this study, the crack appearance and evolution are analysed in detail. Within the artificial ageing process, thermo-oxidative ageing is considered to be neglectable at the chosen temperature of 40 °C for the ageing times applied up to 111 h. An acceleration in ageing is realized by an increase in ozone concentration in contrast to an often used temperature increase. The importance of the strain level during ageing for crack formation is judged by the surface crack pattern showing a clear correlation. Crack depths after static and cyclic load during ageing are analysed by two methods: Firstly, digital microscopy of the cross-section and secondly via the reduction in tensile stress due to the cracks. Comparing the methods for cyclic loading they agree well; however, the difference surpasses 100  \(\upmu \textrm{m}\) between measured versus evaluated crack depth regarding the samples aged under static strain. Ageing time, ozone concentration and strain level during ageing all affect nonlinearly the crack depth. However, it seems to settle on a maximum depth for sufficiently high values of these parameters. Fitting a basic kinetic approach to the detected crack data underlines the dependence of the maximum depth with time on the strain during ageing.