Properties of microinjection‐molded polypropylene/graphite composites

In this study, polypropylene (PP) composites filled with two different types of graphite particles, that is, flake‐shaped synthetic graphite (SG) and low‐temperature expandable graphite (LTEG), were prepared by melt blending, followed by microinjection molding (μIM). The microparts had three consecutive zones with decreasing thickness along the flow direction (FD). Results showed that, in addition to the larger particle size, the in situ exfoliation of LTEG during melt processing is crucial to the overall enhancement of electrical conductivity when compared with their SG‐containing counterparts, as corroborated by morphology observations. Moreover, the preferential alignment of conductive particles favors the construction of conductive pathways along the FD. The melting and crystallization behavior for PP, PP/LTEG, and PP/SG materials, and samples from each section of the corresponding microparts were evaluated by differential scanning calorimetry. Results indicated that both the addition of graphite particles and the typical thermomechanical history of μIM (i.e., high shearing and cooling rates) experienced in different sections of the three‐step microparts influence the melting and crystallization behavior of the composites. POLYM. ENG. SCI., 59:1560–1569 2019. © 2019 Society of Plastics Engineers