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We investigated the mode II fracture toughness (KIIc) with time and temperature dependence of the bisphenol A type of epoxy resin. We performed an asymmetric four-point bending test under various conditions of temperature and displacement rate. We found that KIIc strongly depended on the displacement rate and the temperature, even at room temperature. Moreover, it was governed by the time–temperature equivalence principle in regard to the fracture time. The time–temperature dependency of KIIc was similar to that of the loss modulus (E″), and the transition of brittle to ductile fractures occurred nearly simultaneously when E″ peaked. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 51–55, 2005

The relationship between the postcuring conditions and fracture toughness on three silica particulate-filled epoxy composites was investigated. The glass transition temperature, Tg, and the fragility parameter, m, derived from the thermo-viscoelasticity, were used to characterize the composites, which were postcured under various conditions. The glass transition temperature and fragility both depended on both of the curing conditions and the volume fraction of silica particles. The glass transition temperature increased with the postcuring time and temperature, while the fragility generally decreased as the volume fraction increased. There was no direct correlation between the glass transition temperature and fragility. The fracture toughness depended on both the glass transition temperature and fragility. The composites with a high glass transition temperature and low fragility had high fracture toughness. These results indicate that the glass transition temperature and fragility are useful parameters for estimating the fracture toughness of the silica particulate-filled epoxy composites. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2261–2265, 2002

The relationship between the postcuring conditions and the fracture toughness of a bisphenol A-type epoxy resin cured with acid anhydride was investigated. The glass transition temperature and fragility parameter, derived from the thermo-viscoelasticity, were used to characterize the epoxy resin postcured under various conditions. Relationship between these two parameters and the fracture toughness was then investigated, based on the fractography results of a microscopic roughness examination of a fractured surface. The values of the glass transition temperature and fragility greatly depended on the postcuring conditions. The glass transition temperature was approximately 400 K when the crosslinking reaction was saturated. The fragility was independent of the saturation of the reaction and varied between 50 and 180. The results of the fracture test and fractography examination showed that there was no direct correlation between the glass transition temperature, the fracture toughness, and the roughness. On the other hand, there was a correlation between the fragility, fracture toughness, and roughness when the glass transition temperature saturated (at 400 K). As the fragility decreased from 180 to 50, the fracture toughness increased from 0.6 to 1.1 MPa · m1/2 at the same glass transition temperature. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 10: 2266–2271, 2002