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

Abstract

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. Polymer Journal (2011) 43, 930-936; doi:10.1038/pj.2011.95; published online 21 September 2011Keywords: CB particle/polymer composite; injection; resistivity INTRODUCTIONPolymers are often endowed with some outstanding properties when mixed with different fillers. Nanotechnology-stimulated research attempts to create multifunctional composites by integrating nanoparticles into polymer matrices. Nanoscale carbon black (CB) particles, a type of filler, have practical applications in manufacturing nanoparticle reinforced polymer composites because of their low cost and good moldability. In past decades, many researchers have focused on the electrical properties of CB-filled polymer composites. 1-3 For a given composite, its electrical conductivity is determined not only by the CB structure and concentration but also by the specific polymer matrix used and the morphology generated during processing. [4][5][6][7] There is extensive research on the conductive characteristics of polymer-containing CB particles with different morphologies [8][9][10] and the effect of the dispersion state of CB particles in the matrices on the electrical conductivity. 11,12 When CB particles fill a polymer matrix, it is beneficial to conductivity when the CB particles are located in the amorphous phase or in the interface between two phases. Both the conductive pathway of CB particles in the phase and the continuity of the phase in the composites are basic requirements for maintaining a conductive network through the composite. [13][14][15] When a given CB-filled polymer composite is injected into a molding chamber, the difference in conductivity levels m...

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