Phenol formaldehyde adhesives formulated for advanced X-ray imaging in wood-composite bondlines

A phenol formaldehyde (PF) adhesive was uniformly tagged with iodine such that it yielded sufficient X-ray computed tomography (XCT) gray-scale contrast for material segmentation in reconstructed wood-composite bondlines. Typically, untagged adhesives are organic and have a similar solid-state density as wood cell-walls, and therefore cannot be segmented quantitatively in XCT data. The iodinated PF development involved analysis and comparison of three trial adhesives containing rubidium, bromine, or iodine. Adhesive tag efficacy was measured in terms of X-ray absorption contrast enhancement and tag uniformity along the adhesive polymers. Cured adhesive density, tag element, and concentration were each found to significantly impact XCT contrast results, which in turn agreed with theoretical X-ray attenuation predictions for each resin. Ion chromatography confirmed the absence of free iodide in the liquid PF prior to bonding, and fluorescence microscopy and energy-dispersive spectroscopy (EDS) showed that iodine tags remained associated with the cured adhesive polymers. XCT and EDS results also demonstrated that when contrast agents are simply mixed into resins, rather than attached to the polymer chains, they are free to migrate independent of the penetrating adhesives during bonding. This then can cause complications with quantitative segmentation and analyses. The iodinated PF yielded consistent and uniform XCT gray-scale contrast; its formulation could be adjusted for other viscosity or molecular weight distribution, which would affect its penetration behavior.