Disordered graphite platelets in polypropylene (PP) matrix by spherical alumina particles: Increased thermal conductivity of the PP/flake graphite composites

The strategy of filler hybriding has been widely used to enhance the thermal or electrical conductivities of polymer composites. Flake graphite (FG) has a very high thermal conductivity, but the flaky structure makes it prone to generate planar orientation in polymer matrix during melt processing. This inevitably leads to significant difference in thermal conductivities along the in-plane and through-plane directions of molded sheet samples. The arrangement of FG in parallel induces the limited thermal conductivity along the vertical direction and this poses a great limitation for the real application. In this work, spherical alumina (Al2O3) was introduced into the highly filled (>= 30 wt%) polypropylene (PP)/FG composites. It was found that a small content of alumina replacing the FG significantly increases the thermal conductivity on the through-plane direction of the sheet samples. The analysis indicated that spherical alumina particles effectively destroyed the orderly alignment of graphite platelets in the matrix, especially in the composites with high FG loadings. More interconnection between the FG platelets was therefore achieved on the thickness direction of the composites and the composites exhibited enhanced thermal conductivity along the through-plane direction. This work endows a new strategy for composites with two-dimensional functional fillers to improve the vertical thermal conductivity.