Microstructure and volume resistivity of composites of isotactic polypropylene reinforced with electrically conductive fibers

The effect of processing method on the microstructure and volume resistivity of polypropylene reinforced with nickel-coated graphite (NCG) fibers or stainless steel (SS) fibers is presented. Samples were produced by compression molding, extrusion, and injection molding. The volume resistivity of the composites was measured in three perpendicular test directions to determine anisotropy. The stress and thermal fields experienced during processing determine the ultimate microstructure. The measured resistivity is dependent on this processing-induced microstructure as reflected by the distribution of fiber orientation, length, and concentration. Composites in which the fiber orientation is anisotropic also exhibit anisotropic resistivity. Volume resistivity is lowest in the principal direction of fiber orientation. Samples with the greatest fiber length become conductive at the lowest fiber loadings. Resistivity decreases with an increase in fiber loading, but concentration gradients are often produced, especially in the injection molded samples. High fiber concentrations generally resulted in poor dispersion and wetting. The intertwining and bending of the SS fibers make processing difficult. A comparison between the two types of fibers reveals that, for the systems considered in this study, the SS fibers impart conductivity at lower loadings, but that the NCG fiber composites are ultimately more conductive.