Nondestructive Evaluation of the Spatial Distribution of Nanofillers and Network Structures in Buried Epoxy Resins under Adhesion Conditions

A combined medium-angle X-ray scattering (MAXS) and computed tomography (CT) method was nondestructively applied to a buried epoxy resin sample. We successfully visualized the spatial distribution of the silica nanoparticle fillers and networks inside the filler-containing epoxy resin. Epoxy samples were prepared by gluing two aluminum plates together and creating an air-exposed area at the edge of the sample. When the mixing time of HDGEBA (epoxy agent) and CBMA (curing agent) containing silica nanoparticles with phenylsilane-modified surfaces is short, the silica nanoparticles tend to accumulate along the edge. This heterogeneous distribution is reduced by increasing the mixing time, regardless of the curing temperature. The spatial distribution of the network density, i.e., the cross-linked structures, also shows heterogeneity. An increase in the mixing time leads to suppression of heterogeneity. However, complete homogeneity is not obtained, and the network density distribution on the edge side becomes heterogeneous. This distribution becomes more pronounced at higher curing temperatures. CBMA diffuses to the edge, and the curing reaction rapidly proceeds because of factors such as the high molecular mobility at the free surface. These factors result in an increase in the network density. In contrast, when cured at a low temperature, CBMA forms crystalline structures. This crystallization reduces the network density. Thus, the high network density at the free surface is suppressed, resulting in a relatively homogeneous distribution of the network density throughout the sample. The findings from this study provide valuable insights into the design of epoxy curing agents and optimization of adhesive properties. Specifically, the silica nanoparticle distribution and the network density can be tuned by controlling the mixing time and curing temperature, which can lead to enhanced mechanical properties and improved adhesion performance in practical applications.