The Effect of Foaming on the Electrical Conductivity of Thermoplastic/Carbon Composites Containing Nano and Micro Carbon Fillers in Compression and Injection Molding

The effect of foaming on the electrical conductivity of polystyrene/carbon composites with emphasis on the particle size of conductive filler and molding method has been studied. Carbon black and expanded graphite, as nano carbon fillers, and natural flake graphite, as a micro carbon filler, were used as conductive fillers. Compression and injection molding were employed to investigate the impact of molding method while foaming the composites. Polystyrene and the carbon fillers were mixed in a batch melt mixer and subsequently molded to rectangular sheets either by injection or compression molding. The electrical conductivity of the foam and solid composites were measured by two or four probe methods. The microstructure of the samples was studied by scanning electron microscopy and optical microscopy. The optical images showed a good dispersion and distribution of the filler particles in polystyrene with some degree of agglomeration. The results of electrical conductivity measurements showed that foaming can considerably enhance the electrical conductivity of the composites containing nano carbon fillers i.e. carbon black and expanded graphite but reduced that of the composites containing micro carbon fillers i.e. graphite. The enhancement was more significant in injection molding than compression molding. In the nanocomposites, the nano particles must have been re-localized, redistributed, reoriented or restructured via foaming to increase the electrical conductivity. Such phenomena are not likely to happen for large micro fillers so that the electrical conductivity of polystyrene/graphite composites reduced with foaming due to excluded volume of the cells.