(734j) Effect of Flow Induced Orientation on the Thermal Conductivity of Polyethylene-Hexagonal Boron Nitride Composites

Composites filled sheet-like filler, such as hexagonal boron nitride (hBN) and graphite (Gr), are subject to process-induced filler orientations, resulting in composites with anisotropic thermal conductivities (k) that is low in the through-plane direction. In this presentation, the effects of induced filler orientations during extrusion and compression molding of polyethylene composites with 40 wt.% hBN or Gr on the thermal conductivity are discussed. The extrusion-induced filler orientation was studied using various circular, triangular, square, and rectangular dies. The shaped extrudates exhibited hBN orientations parallel to the extrusion flow direction, which prompted other hBN orientations when pressed. For example, k varied from 0.945 in the R composite to 1.67 W m^-1 K^-1 in the Sq composite. To exploit the extrusion hBN orientations, the C, T, and Sq extrudates were pelletized, and the pelletized C, T, and Sq composites exhibited 53, 59, and 6.2 % improvement in k over their original composites, respectively. The compression molding-induced hBN orientations were studied by varying the composite sheet thickness of powdered extrudates with random hBN orientation. Increasing the compression-molded sheet thickness from 0.5 to 3 mm enhanced k by 150%, demonstrating the induced orientation's impact on orientation compression molding process. Gr filled composites showed better k values than those filled with hBN due to the higher intrinsic k and bigger particle size of the Gr filler. For instance, k of hBN and Gr filled composites fabricated by pelletizing the circular strands were 2.1 and 3.5 W m^-1 K^-1, respectively. Finally, composites with highly aligned fillers were synthesized using a thin rectangular die and subsequently stacking the extrudates by compression molding. These oriented composites exhibited the maximum k of 5.14 and 11.8 W m^-1 K^-1 for the hBN and Gr filled composites, which is 1200 and 2900% k of HDPE, respectively.