Accelerated Thermal Aging Behaviors of EPDM and NBR Vulcanizates

Abstract

The crosslink densities of rubber vulcanizates cured by a sulfur accelerator system are changed by thermal aging. Change of the crosslink density occurs by formation of new crosslinks and dissociation of existing crosslinks. Crosslink density change of a rubber vulcanizate with a high sulfur content after thermal aging was larger than that with a low sulfur content. Permanent deformation of a rubber vulcanizate results from the crosslink density change by thermal aging under the deformed state. Strain applied to a rubber vulcanizate affects the change of crosslink density. The crosslink density of the more strained vulcanizate was changed by thermal aging more than those of the less strained one. Ethylene-propylene diene rubber (EPDM) is a weatherresistant rubber and has slow cure characteristics of long induction time and slow cure rate. Acrylonitrile-butadiene rubber (NBR) is oil-resistant rubber and has fast cure characteristics of short induction time and fast cure rate. EPDM and NBR have different chemical properties each other. EPDM is nonpolar and has slow cure characteristics, whereas NBR is polar and has fast cure characteristics. In this work, we studied changes of crosslink densities of EPDM and NBR vulcanizates by thermal aging at high temperatures of 70-100 C for long time of 180 days. Levels of the crosslink density change and the activation energies of the vulcanizates were also compared. Thermal aging behaviors depending on the aging time were explained with the activation energy variation. Figures 1 and 2 show variations of the crosslink density changes of the EPDM and NBR vulcanizates with the aging time, respectively. The crosslink density change was obtained by dividing the difference in the crosslink densities after and before the thermal aging by the crosslink density of the initial sample. The crosslink density change increases as the aging time elapses. Level of the crosslink density change becomes larger by increasing the aging temperature. For the EPDM vulcanizate, the crosslink density change increases notably during the initial period and then the increment becomes smaller. The crosslink densities increase by 10.0, 17.6, and 22.6% after the thermal aging at 70, 85, and 100 C for 30 days, respectively, and increase by 17.3, 27.9, and 42.5% after the thermal aging for 180 days, respectively. The initial 30 day increments (0 to 30 days) are 10.0, 17.6, and 22.6% for 70, 85, and 100 C, respectively, while the final 30 day increments (150 to 180 days) are 0.2, 0.4, and 1.7%, respectively. For the NBR vulcanizate, the crosslink density change increases notably with increase of the aging time as shown in Figure 2. Especially, the crosslink density change at 100 Figure 1. Variation of the crosslink density change of the EPDM vulcanizate by the thermal aging as a function of the aging time. Squares, circles, and triangles indicate the aging temperatures of 70, 85, and 100 oC, respectively.