Integrating multi-scale studies of adhesive penetration into wood

The development of new and improved wood adhesives is hindered by an incomplete mechanistic understanding of what makes a wood-adhesive bond able to withstand changes in moisture levels. Although common methods are used to test and report the bulk level properties of bondlines, such as bondline shear strength and wood failure analysis, sufficient experimental tools to study wood-adhesive bondlines and the effects of moisture at the nm- to mm-length scales are lacking. Here we review recently developed tools that, when combined, are capable of studying bondlines and the effects of moisture across these length scales. The tools include X-ray computed tomography (XCT), X-ray fluorescence microscopy (XFM), nanoindentation, and small angle neutron scattering (SANS). Their utility has been demonstrated by studying model bondline systems using phenol formaldehyde adhesives. The combined results lead to new insights towards understanding moisture durability in wood-adhesive bondlines and a new model to facilitate the discussion of adhesive penetration into the wood nanostructure.