Toughening of epoxy resin systems using low-viscosity additives

This study has evaluated three low-viscosity epoxy additives as potential tougheners for two epoxy resin systems. The systems used were a lower-reactive resin based upon the diglycidyl ether of bisphenol A (DGEBA) and the amine hardener diethyltoluene diamine, while the second epoxy resin was based upon tetraglycidyl methylene dianiline (TGDDM) and a cycloaliphatic diamine hardener. The additives evaluated as potential tougheners were an epoxy-terminated aliphatic polyester hyperbranched polymer, a carboxy-terminated butadiene rubber and an aminopropyl-terminated siloxane. This work has shown that epoxy-terminated hyperbranched polyesters can be used effectively to toughen the lower cross-linked epoxy resins, i.e. the DGEBA-based systems, with the main advantage being that they have minimal effect upon processing parameters such as viscosity and the gel time, while improving the fracture properties by about 54 % at a level of 15 wt% of additive and little effect upon the T g . This result was attributed to the phase-separation process producing a multi-phase particulate morphology able to initiate particle cavitation with little residual epoxy resin dissolved in the continuous epoxy matrix remaining after cure. The rubber additive was found to impart similar levels of toughness improvement but was achieved with a 10-20°C decrease in the T g and a 30 % increase in initial viscosity. The siloxane additive was found not to improve toughness at all for the DGEBA-based resin system due to the poor dispersion within the epoxy matrix. The TGDDM-based resin systems were found not to be toughened by any of the additives due to the lack of plastic deformation of the highly cross-linked epoxy network