Toughening of dicyandiamide-cured DGEBA-based epoxy resins using flexible diamine

Abstract

This study has examined the toughening of dicyandiamide-cured diglycidyl ether of bisphenol-A (DGEBA)-based epoxy resin with a flexible diamine. Eight different formulations were prepared by mixing DGEBA resin with a mixture of two curing agents of dicyandiamide (Dicy) and flexible polyoxypropylene diamine (Jeffamine D-400) in the presence and the absence of Monuron as an accelerator. The effect of curing agents on the curing behavior of epoxy systems was studied using differential scanning calorimetry (DSC). The fracture surfaces of the tensile test samples were examined by scanning electron microscopy (SEM). The DSC results showed that the reaction between Jeffamine and epoxy resin occurred within a much wider range of temperatures than the reaction of Dicy in the presence of accelerator. The tensile strength of the modified epoxies was much higher than the tensile strength of samples cured by one curing agent alone, due to the synergistic effect of the former. The tensile modulus of all samples was approximately equal and the Izod notched impact strength remained fairly constant up to 60 % Jeffamine and then increased beyond this amount. The fracture toughness increased as the Jeffamine content increased. All samples containing Jeffamine showed greater elongation-at-break. This behavior was a confirmation that Jeffamine increased the flexibility and toughness of Dicy-cured epoxy resin, even at 20 % Jeffamine content. The glass transition temperature (T g) for epoxy resin containing 100 % Dicy (116 °C) was higher than that containing 100 % Jeffamine (73 °C). SEM observation showed a homogeneous phase with no phase separation and that the crack area increased as the content of the flexible curing agent increased.