Hygrothermal Aging Behavior of Epoxy Resin at Adhesive Interfaces

The adhesive properties of epoxy resins often deteriorate under combined exposure to moisture and heat, a phenomenon known as hygrothermal aging, which remains poorly understood due to the buried nature of adhesive interfaces. Since water molecules contribute to hygrothermal aging through both physical and chemical effects, understanding their distribution near the interface is an essential first step. To do so, we used back-incidence neutron reflectivity, a technique highly sensitive to buried interfaces and applicable to thick epoxy resins. Herein, we report the distribution of water in epoxy resin near the solid interface as a function of hygrothermal aging time and its correlation with interfacial adhesive properties. To address the effect of adherend surface chemistry on water distribution and adhesion, hydrogen-terminated silicon (SiH) and hydroxyl-terminated silicon (SiOH) substrates were used as model adherends. While the amount of water sorbed near the SiH interface remained almost unchanged during hygrothermal aging, that near the SiOH interface increased with aging time. Correspondingly, a marked reduction in adhesive energy was observed at the SiOH interface but not at the SiH interface. Such changes induced by hygrothermal aging at the SiOH interface could be associated with degradation reactions, leading to chain scission. This integrated approach provides molecular-level insights into the mechanisms of hygrothermal aging at buried interfaces, offering a general framework for correlating interfacial water behavior with adhesive degradation. The findings are expected to have far-reaching implications, not only for next-generation electronic devices, where epoxy resins are used as encapsulation materials, but also for mobility applications, such as automobiles and aircraft, as well as for infrastructure systems, where epoxy serves as a structural adhesive.