Effects of Chemistry of Silicon Surfaces on the Curing Process and Adhesive Strength for Epoxy Resin

The adhesive strength of epoxy resins is generally dependent on the surface chemistry of an adherend. Although the free space, or the nanoscopic void space, formed at the adhered interface due to the curing shrinkage is expected to have a significant impact on the adhesive strength, the molecular picture is not yet well understood. In this study, all-atom molecular dynamics simulations were used to investigate how the curing reaction and thereby adhesive strength of an epoxy resin differed on hydrophilic and hydrophobic silicon substrates. Before the reaction, a hardener of amine with a smaller molecular size was segregated at the silicon surface, and the extent became more remarkable on the hydrophilic surface with hydroxy groups that formed hydrogen bonds with amine. The epoxy resin shrank as the curing reaction proceeded, forming the overall 5-10% free space. The resin remained attached to the hydrophilic substrate, but was partly separated from the hydrophobic surface, resulting in the 15% free space in the 0.2 nm adhered interfacial region and thus a lesser contact area. Reflecting this, under tensile deformation, cohesive failure and interfacial delamination occurred for the hydrophilic and hydrophobic surfaces, respectively, under a yield stress of 200 MPa and a strain of 0.1. Our findings make it clear that the surface chemistry of an adherend was crucial for the adhesive strength of the epoxy resins via the microstructure formation at the interface.