Microwave evaluation of the conductive filler particles of carbon black-rubber composites

Abstract

Samples of carbon black loaded nitrile rubber are measured at microwave frequencies (waveguide transmission and reflection). The observed dielectric relaxation is interpreted using a two phase effective medium model (Böttcher formula). The two phases are identified with free rubber as insulating matrix and a conductive filler of a priori unknown parameters (volume fraction, permittivity, conductivity). Supported by SEM and de measurements the results allow to associate the filler phase with adsorbate covered carbon black. In this way the bulk conductivity of carbon black loaded rubber can be explained by (statistical) percolation, which has been not possible taking pure carbon black particles as the percolating elements. Because the effective conductivity of the filler particles deduced from the microwave measurements characterizes mainly the particles' surface it is supposed that the adsorbate may become conductive by the chemical and physical interactions of polymer and carbon black during the preparation of the composite. Reference is made to the doping of rubber resulting in conductivities of the same order of magnitude.