Effect of the Resin/Hardener Ratio on Curing, Structure and Glass Transition Temperature of Nanofilled Epoxies

The curing reaction, structure, and glass transition behavior of epoxy-clay nanocomposites prepared using several different resin/hardener ratios were investigated. Nonisothermal DSC experiments evidenced that the incorporation of the organoclay did not induce appreciable changes in the curing enthalpy, but determined a slight acceleration of the curing reaction, without modifying the activation energy. TEM and SEM analyses of the nanocomposite resins showed the presence of micrometric aggregates for all the resin/hardener ratios investigated, even though these materials showed good optical clarity and their WAXD analyses did not evidence any organoclay peak. In addition, the higher the hardener content, the lower the tendency toward exfoliation and the broader the distribution of the interlamellar distances. The degree of cross-linking of cured resins was evaluated both from measurements of the elastic modulus in the rubbery plateau and from solvent sorption experiments. A maximum in cross-link density was observed near the stoichiometric composition. Both modulus and sorption experiments suggested that filler-matrix adhesion increased with increasing the resin/hardener ratio, a trend that was confirmed also by glass transition temperature data. An analysis of ethyl acetate sorption curves evidenced that a gradual transition from Fickian to Case II diffusion occurred as the resin/hardener ratio was raised and that the organoclay promoted deviation from the Fickian behavior.