Comparative study on the effect of partial replacement of silica or calcium carbonate by bentonite on the properties of EPDM composites

The effects of the partial replacement of silica or calcium carbonate (CaCO3) by bentonite (Bt) on the curing behaviour, tensile and dynamic mechanical properties and morphological characteristics of ethylene propylene diene monomer (EPDM) composites were studied. EPDM/silica/Bt and EPDM/CaCO3/Bt composites containing five different EPDM/filler/Bt loadings (i.e., 100/30/0, 100/25/5, 100/15/15, 100/5/25 and 100/0/30 parts per hundred rubber (phr)) were prepared using a laboratory scale two-roll mill. Results show that the optimum cure (t90) and scorch (tS2) time decreased, while the cure rate index (CRI) increased for both composites with increasing Bt loading. The tensile properties of EPDM/CaCO3/Bt composites increased with the replacement of CaCO3 by Bt from 0 to 30 phr of Bt. For EPDM/silica/Bt composites, the maximum tensile strength and Eb were obtained at a Bt loading of 15 phr, with enhanced tensile modulus on further increase of Bt loading. The dynamic mechanical studies revealed a strong rubber-filler interaction with increasing Bt loading in both composites, which is manifested by the lowering of tan δ at the glass transition temperature (Tg) for EPDM/CaCO3/Bt composites and tan δ at 40 °C for EPDM/silica/Bt composites. Scanning electron microscopy (SEM) micrographs proved that incorporation of 15 phr Bt improves the dispersion of silica and enhances the interaction between silica and the EPDM matrix.