Electrical and Mechanical Properties of Some Butadiene Rubber Composites in the Vicinity of the Percolation Threshold

The electrical and mechanical properties of NBR/SBR blends with different compositions were studied before the addition of carbon black. The increase in permittivity ε′ and dielectric loss ε″ noticed by increasing NBR content is due to the increase in C ≡ N dipoles. The mechanical properties which include tensile and elongation at yield and rupture are also found to be increased. This work also includes the compatibility study, which was carried out using different tools and techniques (Heat of mixing, dielectric and scanning electron microscope). This study led to a conclusion that both blends are incompatible. The electrical as well as the mechanical properties were carried out on NBR, SBR and NBR/SBR blend (50/50) to be loaded with different concentrations of high abrasion furnace black (HAF) in order to find out the percolation thresholds in relation to the net work formation. The electrical conductivity of carbon-black-filled composites is increased from pure polymer to that of pure carbon, through the change in the different composites. Up till certain concentration of HAF (30 phr for both NBR and SBR) and 20 phr for NBR/SBR blends the conductivities of the composites are approximately the same and closed to that of the pure, electrically insulating polymer matrix. These concentrations are called percolation thresholds. Above such concentrations, the conductivity increases many orders of magnitude with very little increase in the filler amount. With this increase the tendency of conductivity chain formation increases through the aggregation of the carbon black particles network. The change in conductivity beyond the percolation threshold is expressed according to the percolation theory with straight line when plotted graphically versus P-Pc; Pc is the volume fraction of carbon black at the percolation threshold. In addition to the conductivity term, the data of permittivity ε′ and dielectric loss ε″ given at different frequencies from 100 Hz up to 100 kHz show an abrupt increase at 30 phr HAF for NBR & SBR and 20 phr HAF for NBR/SBR. More over, the relaxation times obtained from the analyses of these data using Fröhlich and Havriliak-Nagami functions, which ascribe the orientation of the large aggregates caused by the movement of the main chain also show an abrupt increase at the same concentration of HAF. The mechanical properties, which investigated through the measurements of tensile and elongation at yield and rupture indicate an abrupt increase at the same concentration of HAF found in the case of electrical measurements. This result gives evidence to the good applicability between both mechanical and electrical investigations through the network formations. Any how, the percolation threshold found in case of NBR/SBR blend is less than that for NBR itself. This result is attributed to the uneven distribution of the filler in the incompatible blend matrix.