Carbon Black-Polymer Composites The Insulator-Conductor Transition
Carbon black-filled conductive polymer compounds have been studied extensively since the 1970s, and the results are published in numerous technical articles and patents. Several reviews give an overview about the phenomenological aspects that have to be considered when creating conductive polymer compounds with the help of carbon black fillers (Probst 1993, 2009; Huang 2002; Davidson 2010). The incorporation of conductive particles into a polymer matrix fundamentally modifies the electrical and dielectrical behavior as well as the mechanical properties. The electrical resistivity decreases, typically abruptly by several orders of magnitude, at a critical conductive particle concentration at which the polymer composite becomes electrically conductive. Electrical resistivity curves show with increasing concentration of conductive particles three consecutive distinct zones, an insulation zone, a percolation zone, and a conduction zone, respectively (Fig. 6). Three parameters describe the percolation from the insulating to the conducting domain: the critical concentration or critical carbon volume fraction at which the percolation occurs (percolation threshold), the slope of the percolation, and the ultimate electrical resistivity level.
In carbon black-filled polymer compounds, the percolation threshold and slope from the insulating to the electrically conducting domain and the ultimate resistivity level are influenced by the carbon black type, the polymer type, as well as the mixing/compounding and finishing process. These variables will be elucidated in the following sections.
Fig. 6 Schematic representation of the electrical resistivity of a polymer compound as a function of the conductive particle filler concentration