Modelling and experimental study of toughening mechanisms of core-shell rubber particle- and nanosilica particle-modified epoxy polymers

Abstract

Epoxies are high performance thermosetting polymers widely used in many engineering applications as structural adhesives, coatings and matrices of fibre reinforced polymer composites. They possess exceptional mechanical properties, chemical resistance and high temperature properties. However, the highly cross-linked epoxy networks, are brittle, and epoxies need toughening for engineering applications. This work presents the mechanical properties, fracture performance and toughening mechanisms of an anhydride cured epoxy polymer modified by different combinations of preformed core-shell rubber particles (CSR) and hybrid CSR-nanosilica particles. Two types of CSR particles, with nominal diameters of 100 nm and 300 nm, and nanosilica particles with a diameter of 20 nm were used. The Young's modulus of the epoxy decreased with the increasing weight percentage of CSR particles, but increased with the addition of the nanosilica particles. Both the compressive modulus and yield stress decreased with the increasing CSR particle content; however, the addition of nanosilica led to an increase of the compressive modulus, while the yield stress remained unchanged. The glass transition temperature of the unmodified epoxy was 159 °C and this was unchanged by the addition of the CSR particles or the nanosilica particles. The fracture energy increased from 78 J/m2 for the unmodified epoxy to 530 J/m2 with an addition of 9 wt% of the small CSR particles, this was further enhanced by the addition of the nanosilica particles. The hybrid toughening effect observed was additive, and no synergy was observed. A reactive diluent was added to further enhance the fracture toughness of the particle modified epoxies, and a synergistic toughening effect was observed. The toughening mechanisms identified were the cavitation of the CSR particles followed by plastic void growth in the epoxy matrix and shear band yielding of the epoxy induced by both the CSR and the nanosilica particles. The increases in toughness were successfully predicted using an analytical model. The fracture performances of carbon fibre reinforced polymer composites with matrices of the toughened epoxies were investigated. A complete transfer of toughness from the bulk epoxies to the fibre composites was observed. The toughening mechanisms of the modified epoxies were also investigated through non-linear finite element analyses. A two-dimensional plane strain elastic plastic model was developed to study the shear band yielding mechanism of the CSR modified and the hybrid modified epoxies. The model predicted that the CSR particle size has no significant effect on the shear band yielding mechanism and verified the contribution of the nanosilica particles to the shear band yielding of the hybrid epoxy. The toughening mechanism of the nanosilica particles was studied using a two-dimensional axisymmetric unit-cell model of nanosilica-embedded epoxy under triaxial loading, considering an elastic-plastic response for the epoxy and cohesive interaction between the nanosilica and the epoxy matrix. The model predicted that there is significant plastic deformation of the epoxy before it debonds from the nanosilica.