Effect of some processing and service conditions on the mechanical properties of an engine mount compound

Natural rubber is used for a variety of applications and especially in tyres and anti-vibration mounts, because of its superior mechanical properties. The useful (service) life of the rubber can be affected by the presence of flaws introduced during fabrication and by conditions experienced during service. In order to assess and predict the functional life of a natural rubber (NR) engine mount compound, the effect of ageing and the presence of deliberately introduced flaws on the physical and failure properties were determined. Accelerated ageing was carried out at temperatures ranging from 70°C to 110°C at times of between one hour and five weeks. Static and dynamic measurements were used to quantify the effects of ageing on properties and assess ageing mechanisms. Stiffness of the compound increased with ageing time while tensile strength and elongation at break decreased. Aerobic degradation mechanism of Ahagon Type I and III were predominant. The activation energy of ageing, in terms of its effect on tensile strength was found to be 93 kJ/mol for a cure temperature of 140°C and 86 kJ/mol for a cure temperature of 150°C when the Time Temperature Superposition method was applied. The tensile strength after ageing for 13 weeks at 50°C was predicted to be 18.7 MPa, which was very close to the experimental value of 19.0(± 2.25 SD) MPa. The presence of weld lines introduced before scorch did not significantly affect the tensile properties if sufficient time was allowed for healing, with the healing time reducing with increase· in cure temperature. The activation energy for healing was found to be 31 kJ/mol for rubber cured between 110°C and 150°C. The presence of flaws, such as pinholes of varying sizes had a significant effect on both tensile strength and fatigue life, with the pin hole equivalent intrinsic flaw size estimated to be about 200μm. [Continues.]