Effects of curing conditions on adhesive and fatigue properties of hydrogenated epoxy resins in bulk state and single-lap-joint configuration

The adhesive and fatigue properties of single-lap-joint (SLJ) epoxy resins are strongly affected by not only the network structures of an epoxy resin adhesive, but also the adhesive strength between an adhesive and adherend. These properties are closely related to curing conditions, such as curing temperature, and curing time, and their combination. In this study, the effects of curing conditions on an adhesive and fatigue properties of an epoxy resin [hydrogenated diglycidyl ether of bisphenol A and cyclohexanebis(methylamine)] were investigated via a single lap-shear test, a fatigue test, and wide-angle X-ray scattering (WAXS). The tensile and fatigue properties of a bulk epoxy resin were also investigated for comparison. The SLJ and bulk samples were prepared with three different curing conditions to obtain high (H), middle (M), and low (L) cross-linking (CL) density of the network structure. They are denoted as “CL-H″, “CL-M″, and “CL-L″, respectively. SLJ-CL-L and Bulk-CL-L exhibited the lowest degree of cure, adhesive and tensile strengths of the three. Both Bulk-CL-H and Bulk-CL-M yielded in tensile testing and Bulk-CL-M was tougher than Bulk-CL-H. In contrast, neither SLJ-CL-H and SLJ-CL-M yielded, and SLJ-CL-H was tougher than SLJ-CL-M. This can be ascribed that the SLJ sample broke before yielding of epoxy resin adhesive and SLJ-CL-H exhibited a higher adhesive strength between the epoxy resin and adherend than SLJ-CL-M. This was confirmed by X-ray photoelectron spectroscopy. Concerning the fatigue properties, SLJ-CL-H exhibited enhanced properties than SLJ-CL-M, although Bulk-CL-M were better than Bulk-CL-H. This trend corresponds well with difference between single lap-shear and tensile tests. The WAXS measurements revealed that the strain values calculated using distance between molecular chains were almost identical for the bulk and SLJ samples immediately before yielding and breaking, respectively. This is because an increase in stress stopped at yielding of bulk and break of SLJ.