Improving the fracture toughness and the cyclic-fatigue resistance of epoxy-polymer blends

Relatively tough epoxy-blend polymers are now commercially available for use as adhesives and as the matrices for fibre composites. Nevertheless, another failure property which may be of equal, or even of greater, importance in some applications is the resistance of the epoxy polymer to cyclic-fatigue loading. However, the cyclic-fatigue behaviour of epoxy polymers has not been studied in great detail, especially for epoxy polymers where the material has been modified by forming a polymer blend in order to increase its toughness under quasi-static test rates or impact test rates. Therefore, a major aim of the present work has been to undertake a novel investigation of a range of rubber and thermoplastic materials to modify an epoxy polymer to study whether both a relatively high toughness and a significantly improved cyclic-fatigue behaviour can be simultaneously achieved in a given formulation. The unmodified epoxy-polymer possessed a value of the fracture energy, G(Ic), of 495 J/m(2) and a value for the threshold value of the maximum strain-energy release rate in a fatigue cycle, G(th), (below which no significant crack growth occurs) of 155 J/m(2). Several epoxy-polymer blends have been identified which do show major increases in these values and probably the best combination of such properties were for the epoxy-polymers modified with a poly(polypropylene-glycol)-based polyurethane (PU) modifier: either when used by itself or as a 'hybrid' polymer-blend in combination with core shell rubber (CSii) particles, based upon a styrene-butadiene rubber core. For these PU-based epoxy polymers the values of G(Ic) and G(th) were found to increase to values of about 2475 J/m(2) and 445 J/m(2), respectively. The mechanisms of toughening that were induced by the addition of the polymer-blend modifier revealed that the presence of a multiphase in the epoxy-blend polymer was a critical requirement in achieving relatively high values of G(Ic) and G(th). This was due to the second-phase particles initiating plastic deformation of the epoxy-matrix phase, which was the major source of energy dissipation and toughening. In turn, the extent of energy dissipated by the plastic deformation of the epoxy-matrix phase is clearly greatly influenced by the degree of ductility exhibited by this phase of the epoxy-blend polymer. Thus, another important feature of the degree of toughening observed is the effect that the modifier has upon the yield stress and plastic failure strain of the epoxy-matrix phase. (C) 2014 The Authors. Published by Elsevier Ltd.