Impacts of Network Structure on Adhesion Strength of Epoxy Resins

Epoxy resins are one of the important classes of thermosets in which three-dimensional networks are formed through the curing reactions of epoxy and amine compounds. They exhibit excellent adhesion properties, thereby suggesting that their interior network structure is crucial for adhesion. However, the contribution of the network structure to the adhesion strength remains unclear, due to the difficulty in isolating the interfacial contribution to overall adhesion strength. In this study, we investigated the interfacial adhesion strengths for epoxy resins on silicon substrate with different cross-linking density. The cross-linking density of the epoxy films was controlled by systematically changing the molar fraction of monoamine to diamine (Φ). The interfacial adhesion strengths for various Φs were extracted by a surface and interfacial cutting analysis system (SAICAS). With increasing Φ, the interfacial adhesion strength of the epoxy film, as well as cross-linking density in the bulk, decreased. These results implied that interfacial strength depends on the cross-linking density at the adhesive interface. We will discuss the mechanism of these results in conjunction with the interfacial structures.