Interfacial Water Sorption Behavior in Hygrothermally Aged Epoxy Resins

The adhesive properties of cured epoxy resins often deteriorate under moisture and heat, known as hygrothermal aging, which remains poorly understood due to the buried adhesive interfaces. Since water molecules affect hygrothermal aging through both physical and chemical effects, the water distribution along the depth direction from the interface in cured epoxy resins should be studied as a first benchmark. In this study, the water distribution near interfaces with different surface chemistries was examined by back-incidence neutron reflectivity (NR), which was a technique that is highly sensitive to buried interfaces and applicable to thick epoxy resins. Hygrothermal treatment at 373 K under water vapor at 85% RH and NR measurements at 85% RH were repeated for cured epoxy films on OH- and H-terminated Si wafers, representing hydrophilic and hydrophobic surfaces, respectively. As a result, while the amount of water sorbed near the SiH interface remained almost unchanged, it near the SiOH interface increased with hygrothermal aging. Correspondingly, a significant reduction in adhesive energy was observed at the SiOH interface, whereas no decrease at the SiH interface. In our presentation, these changes will be discussed in relation to the mechanism of interfacial degradation.