An additive for increasing the strength and modulus of amine-cured epoxy resins

The reaction product of 4‐hydroxyacetanilide and 1,2‐epoxy‐3‐phenoxypropane, when added at 19 wt% to a conventional epoxy‐resin‐curing agent mixture, increases the tensile strength of the cured system from 82 MPa to 123 MPa and increases the shear modulus (20°C, 1 Hz) from 970 MPa to 1560 MPa. As well as showing increased strength, the tensile‐test specimens also fail in a ductile fashion, i.e., the slope of the stress–strain curve is negative at failure, with appreciable localized deformation occurring during fracture. For notched samples (compact tension specimens), the fracture properties are strongly strain‐rate‐dependent. At low strain rates the additive‐containing sample has a fracture energy (G I c, critical strain energy release rate) about twice that of the additive‐free control, but at higher strain rates G I c falls to about 65° of the control value. The critical stress for crack propagation is also strain‐rate‐dependent and is about 50% higher than the control at low strain rates and about 10% less than the control at higher strain rates. Dynamic mechanical analysis and dielectric loss measurements indicate that the additive causes a decrease in the Tg and a suppression of the β‐relaxation. Chemically, the additive accelerates the cure process but does not significantly alter the final extent of reaction of the epoxy resin. After curing, the additive is almost totally extractable by solvent indicating that it is not chemicaly bound to the polymer. These observations are discussed in terms of the concept of antiplasticization.