Analysis of vulcanization parameters' impact on cross-link density and aging performance of natural rubber

The purpose of this thesis is to investigate the effect of vulcanization parameters, vulcanization time specifically, on crosslink density and thus the performance and lifetime of natural rubber (NR) based auto parts. The natural rubber mixture prepared in this context was vulcanized with a sulfur curing system for different cure times (3, 5, 10 and 15 min.) at a constant temperature of 160°C. Mooney viscosity and rheometer tests were used for semi product assesment. Density, hardness, tensile stress-strain, thermal aging hardness & rupture change, ozone test and thermal gravimetric analysis were conducted for the final product to determine mechanical and physical properties. Compression set test (at 70°C and 100°C), swelling method and solid NMR analyses were used to examine cross-linking density in the vulcanized samples and the cross-link densities were calculated by the Flory Rehner equation. The ageing behavior of the samples were studied by stress relaxation tests. Stress relaxation tests are ideal for making lifetime estimations using an Arrhenius plot. Data showed that the cross-link density increases with vulcanization time reaching the maximum at 10 min. and then decreases. The samples vulcanized for 10 min. showed the best performance in mechanical tests. Stress relaxation analyses were performed at 3 different temperatures (85°C, 100°C and 120°C). The service lives of the samples in the range of 50°C and 120°C were estimated by extrapolation of Arrhenius plots. Natural rubber samples cured for 10 min. exhibited the maximum service life. It can be concluded that the parts that are fully cross-linked exhibit the best performance and service life under real operating conditions.