Effect of Curing State on Time-Temperature Dependence of Mechanical Properties for Epoxy Composites

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In the research, we studied effect of curing state on mechanical properties of bisphenol A type epoxy resin and silica particulate-filled epoxy composite for long time and wide temperature ranges.Specimens were bisphenol A type epoxy resin cured under several combinations of temperature and time. Glass transition temperatures and fragilities were derived from the results of thermo-viscoelasticity. By Raman spectroscopic analysis, it was clear that the glass transition temperature is directly related to cross-linking degree reaction of epoxy resin. The fragility which denotes heterogeneous microstructure decreased when the glass transition temperature was saturated.Elastic moduli and fracture toughness at room temperature were measured for each epoxy resin. The elastic moduli were approximately constant for epoxy resin with saturated glass transition temperature. The fracture toughness was strongly dependent on curing conditions. The fracture toughness of the epoxy resin increased as th … More e fragility decreased when the glass transition temperature was saturated at approximately 400 K.Similar experiments were conducted for fused silica particle-filled bisphenol A type epoxy. As the results, the fracture toughness of composites increased for high glass transition temperature and low fragility. Therefore, it was confirmed that the glass transition temperature and the fragility are effective parameters for estimating mechanical properties of epoxy resin and its composite.Time-temperature dependences of fracture toughnesses for epoxy resin and silica particulate-filled epoxy composite were measured. The fracture toughnesses follow time-temperature equivalent principle determined by thermo-viscoelatic properties. The fracture toughness with a smaller fragility increased from lower temperature to the brittle-ductile transition temperature than that with a larger fragility when the glass transition temperature was approximately 400 K. Finally fracture of a cylinder with a crack was predicted mathematically based on time-temperature dependence of fracture toughness.Based on finding that glass transition temperature and fragility are very useful in estimating mechanical properties, these parameters were proposed as the parameters for setting the curing conditions of epoxy resin and its composite. Less