The effect of resin stoichiometry and nanoparticle addition on epoxy/silica nanodielectrics

Epoxy-based systems are used widely as dielectrics in electrical applications and, in such systems, the chosen stoichiometry is important in determining the nature of the molecular network that forms and, hence, the physical properties of the final system. However, the inclusion of nanoparticles with large interfacial areas into epoxy systems may introduce additional chemical reactions between moieties on the nanoparticle surfaces and the reactants, or may alter the rate and sequence of the various chemical pathways that occur during curing, with the consequence that the cross-linked network that forms within the matrix polymer will be altered. This study set out to investigate the effects of resin stoichiometry on material properties, including the glass transition temperature and electrical breakdown strength of an epoxy resin and its nanocomposites filled with silica particles, which were introduced using a Nanopox masterbatch system. The results indicate that the nanosilica affects network formation in complex ways and that the concept of an optimum stoichiometry is highly dependent upon the property of interest. From the perspective of breakdown strength, a stoichiometric excess of anhydride hardener is detrimental; the introduction of nanoparticles has a comparable effect to an excess of hardener.