Preparation and photothermal characterization of nanocomposites based on high density polyethylene filled with expanded and unexpanded graphite: Particle size and shape effects

This work aimed at thermal transport characterization of high density polyethylene (HDPE) filled with 5 and 50 μm expanded graphite (EG) particles and with 0.4 μm unexpanded graphite (UG) particles. Sample platelets were produced by melt mixing followed by compression molding. Thermal conductivity k was determined by combining measurements of density, specific heat capacity and thermal diffusivity, the latter by modulated photothermal radiometry (PTR). Starting from an effective medium approximation model, we derived a linearized expression for the effective k of composites with low particle charge. It explains the unusually high experimental k values (up to four-fold increase) as the effect of strongly non-spherical EG particles (aspect ratio 1/p = 110–290). Larger particle sizes produce higher k enhancement, while the interfacial thermal resistance (Rbd = 2.1⋅10−7 m2⋅K/W) has an opposite effect. The same model is consistent with experimental k for low particle charge HDPE/UG composites. At higher particle charge the model fails due to particle interaction leading to validity break of the effective medium approximation.

► High density polyethylene filled with expanded and unexpanded graphite particles. ► Linearized expression for the effective thermal conductivity of composites. ► Thermal diffusivity measured by modulated laser photothermal radiometry. ► High thermal conductivity due to large and thin expanded graphite flakes. ► Maximum thermal conductivity enhancement if particle size is larger than Kapitza radius.