Enhancement of thermal conductivity in epoxy coatings through the combined addition of expanded graphite and boron nitride fillers

Expanded graphite (EG) and boron nitride (BN) were used as fillers to impart thermal conductivity (TC) while maintaining electrical insulation of a homopolymerized cycloaliphatic epoxy matrix. Even though EG leads to a higher increase of TC than BN (550% of enhancement with only a 7.5 wt.% of EG), EG is also electroconductive and its ratio in the formulation must be lower than the percolation threshold. Formulations with proportions between 2.5-7.5 wt.% of EG as the filler and mixtures with EG and a 40 wt.% of BN were thermally polymerized and composites with 70 wt.% of BN and 2.5/5.0 wt.% of EG were also prepared under pressure and then cured in the oven. Over 2 W/m K was achieved (i.e. more than 1500% of enhancement in reference to the neat epoxy). The composites containing a 40 wt. % of BN and 2.5 wt % of EG or 70% wt. % of BN and 5 wt % of EG were found to keep the insulation character. Mechanical and thermal characteristics of the prepared materials were also evaluated.