Toughening mechanisms of silica nanoparticle-modified epoxy polymers

The present work investigates the ability of several different epoxies to be toughened with the addition with 20 nm silica nanoparticles (nanosilica). The formation of ‘hybrid’ epoxy polymers, containing both silica nanoparticles and carboxyl-terminated butadiene-acrylonitrile (CTBN) rubber micro-particles, is also discussed. The structure/property relationships are considered, with an emphasis on the toughness and the toughening mechanisms. Particular attention was given to an anhydride cured diglycidyl ether of bisphenol-A (DGEBA) system where the fracture energy increased from 83 to 204 J/m2 with the addition of 20 wt. % of silica nanoparticles. Plastic shear bands followed by debonding of the matrix from the silica nanoparticles, and subsequently plastic void growth of the epoxy were found to be the operative toughening mechanisms. The largest increases in toughness observed were for the ‘hybrid’ materials where a synergistic behaviour on the fracture energy. A maximum fracture energy of 1051 J/m2 was measured for a ‘hybrid’ epoxy polymer containing 10 wt. % silica nanoparticles and 9 wt. % rubber micro-particles. The toughening mechanisms for such systems were postulated to be rubber-particle cavitation, shear band yielding and void growth and debonding and plastic void growth of the nanosilica necklaces. Ultimately, these polymers are intended to be used as matrices in fibre-reinforced composites. Therefore, resistance to delamination as fibre-composites has been examined for such modified epoxies. The interlaminar fracture energies for the fibrecomposite materials were found to increase even further by a fibre bridging toughening mechanism. However, the fibre-matrix adhesion is shown to be an important parameter. The present work has extended an existing model to predict the toughening effect of the nanoparticles in the epoxy polymer. There was excellent agreement between the predictions and the experimental data for epoxy containing the silica nanoparticles, and for epoxy polymers containing rubber or coreshell particles. Inferences have been made about the toughenability of the epoxy being sensitive to particle-matrix adhesion and the ability for the matrix to shear yield