Effect of the processing of injection-molded, carbon black-filled polymer composites on resistivity

To improve the application of carbon black (CB)-filled polymer composites, we investigated the relationship between the processing parameters, microstructure and electrical properties of injection moldings made from the material. Standard tensile specimens were fabricated under different injection pressures and packing pressures. Up to five layers were removed from the surfaces of the molded specimens to observe the microstructure at different positions within the moldings. Microstructures were observed with a scanning electron microscope, and electrical properties were measured at room temperature with a standard two-terminal direct current (DC) resistor. The results showed that CB particles form the best conductive path at high packing pressures combined with high injection pressures. If the packing pressure is low, the resistivity in the skin zone when loaded by high injection pressures is less than when loaded by low injection pressures, but resistivity increases in the sub-skin zone. We found that the sub-skin zone is a high-resistivity area that can be expanded under the action of higher injection pressures along with lower packing pressures. In contrast to an injection-molded single polymer, an injection–molded, CB-filled polymer composite develops a highly oriented microstructure in the core zone rather than in the skin or sub-skin zones because of the migration of CB particles. Our study explored microstructures at skin, sub-skin and core zones of injection-molded carbon black particles/polypropylene composite. Figure 3 in the paper showed the schematic map on definition of each zone. We found that strong oriented microstructure was formed at core instead of at sub-skin, especially under lower packing pressure. A band with non-continuous conductive path was found under the action of higher injection process, and it will expand with lowering packing pressure.