Improved dispersion of aluminum nitride particles in epoxy resin by adsorption of two-layer surfactants

The purpose of this research is to enhance the thermal conductivity of polymeric protective coatings on electronic components. Thermally conductive but electrically insulating solid powders of aluminum oxide or aluminum nitride (AlN) are commonly dispersed into liquid epoxy resin, and this dispersion is applied as a coating, before the complete polymerization that occurs afterwards. The viscosity cannot be high initially, or damage to the delicate component might result during application. In the present study, the dispersibility of aluminum oxide powder in epoxy monomer resin was found to be better than that of aluminum nitride powder, because of the weaker basicity of the oxide surface compared with the nitride. To improve the dispersibility of higher conductivity AlN in liquid epoxy monomer, the solid AlN surface was modified by pretreatment with silane coupling agents. Methylsilane gave lower viscosities than chloro- or methoxysilane, while pretreatments using organic acids increased the viscosity of the AlN dispersion. The viscosity changes and FTIR peak intensity trends suggested that the silane molecules could be adsorbed on AlN surfaces in the form of a monolayer during optimization experiments, and the best silane monolayer coverage on the AlN powder surfaces was achieved with 2 wt.% amounts in a carrier solvent during a 3 h pretreatment. In addition, a particular phosphate ester was a good second layer dispersant for the AlN-plus-epoxy system. When that additional dispersant was added onto the silane-treated filler surfaces, the degree of viscosity reduction was dependent on the types of silane coupling agent functional groups. The conclusion was that silane pretreatment followed by the second dispersant was better than either dispersant alone.