A dense packing structure constructed by flake and spherical graphite: Simultaneously enhanced in-plane and through-plane thermal conductivity of polypropylene/graphite composites

Flake graphite (FG) has been widely used in enhancing the thermal conductivity of polymer composites. However, flake graphite has high aspect ratio and shows anisotropic thermal conductivity. In this work, spherical graphite (SG) was used to adjust the packing and orientation of FG to enhance the thermal conductivity. It was found that the addition of 5 wt% SG simultaneously enhances both in-plane and through-plane thermal conductivity of polypropylene (PP)/FG/SG composites when the total filler content is 50 wt%. Moreover, the mechanical properties of the composite at this SG loading are also in its maximum. It is considered that small amount of SG fills the vacancies of FG and induces the denser packing of the fillers in the matrix. At the same time, the addition of SG leads to the anisotropic orientation of the FG in the plane during the hot press molding.