The influence of water on dielectric behavior of silica-filled epoxy nano-composites and percolation phenomenon

The dielectric properties of epoxy resin were studied as a function of hydration by dielectric spectroscopy. The dielectric spectroscopy measurements show different conduction and quasi-DC behaviors at very low frequencies (< 10(-2) Hz) with activation energies dependent on the hydration. These observations lead to the development of a model in which a "water shell" is formed around the nanoparticles. The multiple shell model, originally proposed by Lewis and developed by Tanaka, has been further developed to explain low frequency dielectric spectroscopy results in which percolation of charge carriers through overlapping water shells was shown to occur. At 100% relative humidity, water is believed to surround the nanoparticles to a depth of approximately 10 monolayers as the first layer. A second layer of water is proposed that is dispersed by sufficiently concentrated to be conductive. If all the water had existed in a single layer surrounding a nanoparticle, this layer would have been approximately 5 nm thick at 100% RH. Filler particles that have surfaces that are functionalized to be hydrophobic considerably reduce the amount of water absorbed in nanocomposites under the same conditions of humidity. PEA results show that the wetted epoxy specimens have a higher threshold field of space charge accumulation than such dry specimens since water enhances charge decay.