Material properties of silica/epoxy nanocomposites

The use of composite materials in a variety of applications has increased considerably in the last couple of decades. The main reason for this is that composite materials are lightweight materials with high strength and stiffness, which makes them interesting as an alternative to other materials, such as metals. For this reason, there is also significant activity on the development of composite materials at FFI, with the main aim of utilising them in applications that are relevant for military purposes. This also includes research on polymer nanocomposites, where nano-sized reinforcements are employed. Epoxy polymers are widely used in composites and adhesives. For use in many applications, however, it is necessary to improve the material properties. One possible approach to improve the properties of the material is adding inorganic particles to the polymer. This reinforcement may modify mechanical properties, such as the stiffness, strength and toughness of the polymer material. In this study, the effect of adding 20 nm silica nanoparticles to two different epoxy polymers, one amine-cured and one anhydride-cured, has been investigated. A commercial product of predispersed silica nanoparticles in an epoxy resin, Nanopox F 400, containing as much as 40 wt% silica, was employed to reinforce the epoxies. The material properties of the neat epoxy polymers and the silica/epoxy nanocomposites were investigated by tensile testing, dynamic mechanical analysis (DMA), and indentation measurements. Particular emphasis was put on the measurement of the elastic modulus of the materials, and the elastic modulus obtained by the three different test methods was compared. The elastic modulus of both epoxy polymers was increased by the addition of the silica nanoparticles. The relative increase in the measured mechanical properties was higher for the anhydride-cured polymer. For the amine-cured epoxy system, there was very good agreement between the elastic modulus measured by tensile testing and DMA. The elastic modulus from the indentation measurements was consistently higher. For the anhydride-cured epoxy system, there was also, in general, good agreement between the elastic modulus obtained from tensile testing and DMA. The indentation measurements, on the other hand, gave very different values. The measurements also showed that part of the increase in the elastic modulus for the anhydride-cured silica/epoxy nanocomposite could be due to a change in the polymer network structure, as indicated by a change in the glass transition temperature.