The Percolation Concept and the Electrical Conductivity of Carbon Black-Polymer Composites. 3. Crystalline Chloroprene Rubber Mixed with FEF Carbon Black

The filler concentration and temperature dependence of the electrical conductivity (a) of crystalline chloroprene rubber CR (Butaclor 41H) mixed with fast extrusion furnace (FEF) carbon black was investigated. It was found that the percolation concept held true for these composites, where σ shows a fairly sharp increase when the mass fraction of FEF carbon black in the mixture exceeds 20%. The estimated values of the filling factor (f) and the occupation number (Z) were found to be 0.82 and 8.2 respectively. The scaling law for percolation clusters [σ = σ_o (P - P_c)^β] was valid within the range P_c ≤ P ≤ 0.34 and the estimated value of the exponent β was 6.8, where P = concentration of filler and P_c = the critical concentration.

The conductivity was thermally activated below and at the percolation threshold, with the same activation energy. The decrease in c with temperature (followed by a subsequent increase above a certain temperature) was attributed to the phase transition of CR (41H) and to the change in size of the carbon black clusters from infinite to finite with temperature.

In the more highly loaded compounds, the temperature dependence of σ was controlled by the breakdown and reformation of carbon clusters with temperature.