The influence of water absorption on dielectric properties of cycloaliphatic epoxy resin

The study was performed to investigate interfacial phenomena between fillers and resin in composite insulation materials. Relative permittivity ε_r and loss tangent tan δ of cycloaliphatic epoxy resin filled with silica powders (CEP) and non-filled resin (EP) were measured under dry and wet conditions. It is shown that for CEP ε_r and tan δ drastically increase with water absorption, but for EP they are hardly influenced by water absorption, even though EP absorbs water. Therefore, the ε _r and tan δ values of water-absorbed CEP are discussed assuming that a thin water layer formed around the filler as a shell. We can describe the complex permittivity of the water-absorbed CEP with the expanded Wagner´s equation and obtain the relationship between ε _r or tan δ and water-layer thickness from the equation. By the comparison of the calculated and experimental results, consequently, we concluded that water is absorbed in and near the interface between the fillers and resin and then the interfacial polarization in the area is emphasized and brings about the increases of ε_r and tan δ for water-absorbed CEP