MICROWAVE EVALUATION OF THE CONDUCTIVE FILLER PARTICLES OF CARBON BLACK-RUBBER COMPOSITES

Samples of carbon black loaded nitrile rubber are measured at microwav e frequencies (waveguide transmission and reflection). The observed di electric relaxation is interpreted using a two phase effective medium model (Bottcher formula). The two phases are identified with free rubb er as insulating matrix and a conductive filler of a priori unknown pa rameters (volume fraction, permittivity, conductivity). Supported by S EM and dc measurements the results allow to associate the filler phase with adsorbate covered carbon black. In this way the bulk conductivit y of carbon black loaded rubber can be explained by (statistical) perc olation, which has been not possible taking pure carbon black particle s as the percolating elements. Because the effective conductivity of t he filler particles deduced from the microwave measurements characteri zes mainly the particles' surface it is supposed that the adsorbate ma y become conductive by the chemical and physical interactions of polym er and carbon black during the preparation of the composite. Reference is made to the doping of rubber resulting in conductivities of the sa me order of magnitude.