Investigation of the effect of mixing schemes on cross-link distribution and tensile properties of natural–acrylic rubber blends

Abstract

This article reports a study on the effect of mixing schemes on cross-link distribution and tensile properties of natural–acrylic rubber blends. Sulfur, MBT, ZnO, and stearic acid were used to compound the natural rubber (NR), whereas sodium stearate (soap) and sulfur were used to compound the acrylic rubber (ACM). The overall content of sulfur in the blend was 2.5 phr and the weight ratios of sulfur/MBT and sulfur/soap were kept constant at 1/2 and 1/5, respectively. Three types of mixing schemes were conducted. In scheme 1, the compounding ingredients were added simultaneously to a mixture of the rubbers. In scheme 2, ingredients were added separately to each of the relevant rubbers in separate compounding operations and sulfur was added to the mixture of rubbers at the end. In scheme 3, sulfur was premixed with ACM compound prior to blending with the NR compound. From the tensile test, it was found that the changes in mixing scheme significantly affect tensile properties of the rubber blends. The tensile strengths of the blends prepared by mixing schemes 2 and 3 were higher (5 MPa) than that of the blend prepared by mixing scheme 1. In addition, the tensile elongation increased by ~300% and the modulus increased by ~500 kPa due to the same effect. By performing a differential swelling for various blends, it was found that volume fraction ( V r) of the ACM phase in the swollen gel increased when the mixing scheme was changed from scheme 1 to scheme 2 and 3. On the other hand, V r of NR phase in the swollen gel barely changed with the mixing method. Finally, swollen state 1H-NMR of various blends was carried out and the results were compared to those obtained from the differential swelling technique.