Why the fracture resistance of natural rubber compound reduced when subjected to thermal ageing?

Elastomeric products in service condition undergoe thermo-oxidative degradation under the influence of temperature, oxygen, ozone etc. Tyre is one such product where ageing phenomena affect its performance properties as it is subjected to various temperature and environmental conditions during its service life. However, ageing is a complex process and depends on many factors that include rubber type, temperature, stress level, etc. To study the underlying mechanism and kinetics of such intriguing phenomenon, a Natural Rubber based compound was chosen in the present work. The rubber compound was subjected to an aerobic ageing at a fixed temperature of 70 degrees C and the effect of prolonged ageing time (1 week, 2 weeks, 4 weeks, and 8 weeks) on key performance properties such as, mechanical, dynamic mechanical and fatigue crack growth resistance was evaluated. Fatigue crack growth (FCG) studies were conducted to understand the degradation of crack growth resistance with respect to ageing. The morphology of the fractured surface was also captured by Field Emission Scanning Electron Microscope to reveal the nature of fracture failure. The static as well as dynamic modulus of aged compound was found to be significantly increased with increase in ageing time which was an indication of enhanced cross-linking density. This enhanced stiffness leads to deterioration of the fatigue resistance property and as a result FCG rate increased with ageing time. The samples were characterized by Fourier transform infrared spectroscopy to understand the changes in chemical structure during ageing which revealed that intensity of double bond content was reduced by increasing ageing time. Apparent cross-linking density and other relevant chemical analysis were also performed to establish the proposed mechanism.