Stress relaxation of nitrile rubber composites filled with a hybrid metakaolin/carbon black filler under tensile and compressive forces

Stress relaxation of nitrile rubber composites filled with metakaolin and carbon black under tensile and compressive loading was assessed to determine the effect of the hybrid-system filler on the relaxation process. Nitrile rubber-filled formulations were designed with 50 parts of filler per hundred parts of rubber (phr), using carbon black-to-metakaolin ratios of 50:0, 35:15, 25:25, 15:35, and 0:50 phr. In addition, one unfilled vulcanized nitrile rubber (NBR) composite was prepared as reference. The results demonstrated that, although the presence of either filler system composition accelerated the relaxation process compared to the unfilled compound, the substitution of carbon black by metakaolin decreases the relaxation process for both the tensile and compressive tests. The lower stress relaxation caused by metakaolin in hybrid-system filler was ascribed to the reduction of filler-filler network and filler shape. Moreover, metakaolin imposed some constraints on the rubber chain movements when the load was applied in the tensile mode, but the opposite behavior was observed under compressive force.