The temperature dependence of dielectric properties of carbon black-filled polyethylene matrix composites below the percolation threshold was studied based on the effect medium approximation theory (EMA theory). Although the literature on calculation based on the EMA theory by changing the parameters is vast, an overall discussion taking into account the EMA theory is required to explain the dielectric properties of the composites and their temperature dependence. Changes of the dielectric properties of carbon black-filled PMC and their temperature dependence below the percolation threshold for the volume fraction of carbon black have been discussed based on the EMA theory. The composites satisfy the universal law of conductivity above the percolation threshold, whereas it gives the critical exponent of s = 1 for dielectric constant below percolation threshold. The rates of the decrement of both % changes of dielectric constant and dielectric loss factor for temperature become larger with an increase in the volume fraction below percolation threshold. By incorporating the effect of thermal expansion into the EMA theory, the temperature dependence of dielectric properties is successfully explained.

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The electrical conductivity of carbon black-filled polymer matrix composites (PMC) with various matrices was studied as a function of carbon content to find the break point of the relationship between carbon content and conductivity. Conductivity jumps by as much as ten orders of magnitude at the break point. The critical carbon content corresponding to the break point varies depending on the matrix species and tends to increase with increase in the surface tension of the matrix. To explain the dependency of the critical carbon content on the matrix species, a simple equation was derived under some assumptions, the most important of which is that when the interfacial excess energy introduced by particles of carbon black into the matrix reaches a universal value ( ), the particles of carbon black begin to coagulate so as to avoid any further increase of the energy and to form networks which facilitate electrical conduction. The equation well explains the dependency through surface tension, as long as the difference of the surface tensions between the particles of carbon black and the matrix is not very small.

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