SATOSHI MATSUDA

In this study the aim is to clear the effect of molecular weight distribution on the microstructure and the mechanical properties of the epoxy resins with same average molecular weight. Four types of epoxy blends with the different molecular weight distribution were formulated using commercial Bisphenol-A diglycidyl ethers. The microstructure and the mechanical properties of the blends were investigated. Dynamic mechanical analysis and microscopic observation revealed that difference of the crosslink density in the resin and the size of the repeated heterogeneous structure became larger as the molecular weight distribution was broader. In particular, the epoxy blend with the widest distribution had a co-continuous phase structure in submicron-scale as a result of reaction-induced phase separation. Whereas the flexural modulus and the flexural strength was insensitive to the molecular weight distribution and the morphology of the cured resin, the fracture toughness enhanced as the molecular weight distribution broadened and the heterogeneity of the crosslink density increased. Fracture mechanisms were discussed using proposed microstructure models.

Epoxy resins are utilized in many industrial areas as matrix of composite materials and adhesives because of high strength and stiffness. Fatigue properties are important in terms of the long time reliability. To enhance the fatigue life of the epoxy adhesives, the fracture mechanism of the resin between rigid substrates under cyclic loading should be clarified. In this study, the fatigue crack propagation behavior of toughened epoxy adhesive was investigated and was compared with that of bulk resin. The fatigue crack propagation behaviors of adhesives are similar to those of the bulk resins. As the results of fractographic observation, the fracture surface for the adhesive layer are very flat and was different from that of bulk resin where the plastic deformation was popular. The deformation around the crack tip was restrained by the rigid metal substrates and the tri-axial stress was kept in spite of the rubber particle cavitation.

Epoxy resins are utilized in various industrial areas as adhesives and matrix of composite materials. Epoxies are one of the network polymer and their crosslink density are mainly dominant of the mechanical behaviors. However the microstructures which is formed during curing process are not well known. In this study the effect of network structure of matrix resin on the fatigue crack propagation properties of rubber-toughened epoxy resin. Several DGEBA epoxy resins with different average molecular weight were used in this study. Core shell rubber (CSR) particles were well dispersed in the epoxy. Whereas the fracture toughness of CSR-modified epoxy monotonously enhanced with increasing molecular weight between crosslinks (Mc), the fatigue threshold had a peak against Mc. The size of the plastic deformation zone was well related to the fatigue resistance.