MICROSTRUCTURE AND VOLUME RESISTIVITY OF COMPOSITES OF ISOTACTIC POLYPROPYLENE REINFORCED WITH ELECTRICALLY CONDUCTIVE FIBERS

The effect of processing method on the microstructure and volume resis tivity of polypropylene reinforced with nickel-coated graphite (NCG) f ibers or stainless steel (SS) fibers is presented. Samples were produc ed by compression molding, extrusion, and injection molding. The volum e resistivity of the composites was measured in three perpendicular te st directions to determine anisotropy. The stress and thermal fields e xperienced during processing determine the ultimate microstructure. Th e measured resistivity is dependent on this processing-induced microst ructure as reflected by the distribution of fiber orientation, length, and concentration. Composites in which the fiber orientation is aniso tropic also exhibit anisotropic resistivity. Volume resistivity is low est 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 concentra tion gradients are often produced, especially in the injection molded samples. High fiber concentrations generally resulted in poor dispersi on and wetting. The intertwining and bending of the SS fibers make pro cessing difficult. A comparison between the two types of fibers reveal s 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.