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 mu m expanded graphite (EG) particles and with 0.4 mu m 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 (R-bd = 21.10(-7) m(2).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. (C) 2012 Elsevier Masson SAS. All rights reserved.