Spherical filler‐promoting thermally conductive pathway in graphite‐containing polymer composites for high heat radiation

ABSTRACT Graphite is an efficient and affordable filler for polymer composites, allowing the control of thermal conductivity. In comparison to other thermally conductive fillers, graphite is lightweight and flexible but affords anisotropic thermal conductivity. Herein, the control of thermal conductivity of graphite‐containing polymer composite sheet using spherical polymer particles as additional fillers is described. The thermal conductivity in the through‐plane direction (λ t ) of the composite sheet is enhanced by varying the composition ratio of the two fillers (flaky graphite and spherical particles), and optimizing the forming temperature and pressure. Graphite‐containing (25 wt%) polymer composite sheet formed by compression at 150 °C and 10 MPa exhibits λ t value of 0.66 W/m K. Upon mixing of polystyrene microspheres, λ t is successfully increased. The maximum value of thermal conductivity for a composite sheet with 35 wt% of graphite and 50 wt% of spherical particles is 7.51 W/m K, at 180 °C and 10 MPa. The graphite‐containing polymer matrix forms a sequentially connected network‐like structure in the composite sheet. Excess polymer microspheres lead to the formation of void structures inside the composite sheet, reducing the thermal conductivity. Thermo‐camera observations proved that the composite sheets with higher λ t value showed comparably high heat radiations. © 2020 Wiley Periodicals, Inc. J. Polym. Sci. 2020 , 58 , 607–615