The effect of expanded graphite on the thermal stability, latent heat, and flammability properties of EVA/wax phase change blends

Abstract

This article reports on the morphology, melting and crystallization behavior, thermal stability, flammability and thermal conductivity of shape-stabilized phase-change materials (PCM) for thermal energy storage, based on a soft Fischer–Tropsch paraffin wax, the PCM, blended with ethylene vinyl acetate (EVA). These immiscible blends were melt-mixed with expanded graphite (EG) (up to 9 wt%) to improve the thermal conductivity and flame resistance of the material. It was observed that the EG particles agglomerate in the absence of wax, but disperse much better in the EVA/wax blend, probably because the wax penetrates in between the EG layers (there seems to be a better interaction between wax and EG than between EVA and EG) and separates the layers, giving rise to smaller and better dispersed EG particles. This gives rise to better thermal conductivity and flame resistance. There were no significant changes in the melting temperature of EVA in the EVA/EG composites, while the crystallinities of EVA were observably lower in the presence of EG. The thermal stability and flammability results show an increase in thermal stability and flame resistance of EVA, which further improved in the presence of wax because of the smaller and better dispersed EG particles in these systems.