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The quality of wood composites is dependent on the bondlines' performance and their moisture resistance. The effect of moisture intrusion into bondlines is the focus of this study. The strain profile in the bondline during loading was investigated and the effects of weathering on the bondline were quantified by digital image correlation (DIC), while load deflection data were collected from lap-shear tests (LShTs). A total of 75 LShTs were evaluated including four different adhesive types with Douglas-fir wood, out of which 39 served as controls and 36 as substrates for weathering. The effect of weathering on failure load was not statistically significant. Only the bonded surface and adhesive type have significant effects on failure load, as well as the three-way interaction between the bonded surface, adhesive, and weathering, as revealed by three-way analysis of variance (ANOVA). The average effective shear moduli for the weathered samples were significantly lower than those for the control samples. [ABSTRACT FROM AUTHOR]

Microscale X-ray computed tomography (XCT) is discussed as a technique for identifying 3D adhesive distribution in wood-adhesive bondlines. Visualization and material segmentation of the adhesives from the surrounding cellular structures require sufficient gray-scale contrast in the reconstructed XCT data. Commercial wood-adhesive polymers have similar chemical characteristics and density to wood cell wall polymers and therefore do not provide good XCT attenuation contrast in their native form. Here, three different adhesive types, namely phenol formaldehyde, polymeric diphenylmethane diisocyanate, and a hybrid polyvinyl acetate, are tagged with iodine such that they yield sufficient X-ray attenuation contrast. However, phase-contrast effects at material edges complicate image quality and segmentation in XCT data reconstructed with conventional filtered backprojection absorption contrast algorithms. A quantitative phase retrieval algorithm, which isolates and removes the phase-contrast effect, was demonstrated. The article discusses and illustrates the balance between material X-ray attenuation and phase-contrast effects in all quantitative XCT analyses of wood-adhesive bondlines. [ABSTRACT FROM AUTHOR]

Micro X-ray computed tomography (XCT) was used to analyze the 3D adhesive penetration behavior of different wood–adhesive bondlines. Three adhesives, a phenol formaldehyde (PF), a polymeric diphenylmethane diisocyanate (pMDI), and a hybrid polyvinyl acetate (PVA), all tagged with iodine for enhanced X-ray attenuation, were used to prepare single-bondline laminates in two softwoods, Douglas-fir and loblolly pine, and one hardwood, a hybrid polar. Adhesive penetration depth was measured with two separate calculations, and results were compared with 2D fluorescent micrographs. A total of 54 XCT scans were collected, representing six replicates of each treatment type; each replicate, however, consisted of approximately 1500 individual, cross-section slices stacked along the specimen length. As these adhesives were highly modified, the presented results do not indicate typical behavior for their broader adhesive classes. Still, clear penetration differences were observed between each adhesive type, and between wood species bonded with both the PF and pMDI adhesives. Furthermore, penetration results depended on the calculation method used. Two adhesive types with noticeably different resin distributions in the cured bondline, showed relatively similar penetration depths when calculated with a traditional effective penetration equation. However, when the same data was calculated with a weighted penetration calculation, which accounts for both adhesive area and depth, the results appeared to better represent the different distributions depicted in the photomicrographs and tomograms. Additionally, individual replicate comparisons showed variation due to specimen anatomy, not easily observed or interpreted from 2D images. Finally, 3D views of segmented 3D adhesive phases offered unique, in-situ views of the cured adhesive structures. In particular, voids formed by CO 2 bubbles generated during pMDI cure were clearly visible in penetrated columns of the solidified adhesive. [ABSTRACT FROM AUTHOR]

The importance of adhesion in production of wood-based composites is well-known. Adhesive penetration and interphase morphology play an important role in bond strength and product performance. Many studies offered different methods for interphase characterization and analysis. Both qualitative and quantitative methods have been used to analyze the bondline. Most of the quantitative methods were developed for bonded assemblies of veneer or wood blocks. Adhesive penetration measurement in particulate wood-based composites is the main scope of this study. For this purpose, uniform oriented strand board with veneer strands was produced by the use of phenol formaldehyde resin with 2, 6, and 10 % strand moisture content (MC). The strands were formed into well-organized mats to achieve a uniform structure for the composite. Cross-section specimens were prepared from each board and observed by epifluorescence microscopy. Six positions for growth-ring angle in adhesive joints were denoted to evaluate penetration in different directions of the wood. Four parameters were also determined to quantify resin penetration patterns by counting the quantity of the main anatomical elements of wood, which were filled with resin. Finally, it was determined that this method could recognize the differences between adhesive penetration in boards produced with different level of strand MC. Resin penetration in strands with 6 and 10 % MC was the greatest. Growth-ring angle of the strands influenced penetration as it was increased when both strands meet each other in an oblique position, although this effect was significant only in maximum penetration depth in fibers. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: Dielectric analysis (DEA) was conducted for continuous in-process monitoring of the curing of phenol formaldehyde (PF) adhesive. The influence of wood moisture content (MC) on the dielectric signal obtained in the PF adhesive bond was examined. The cure kinetics of the PF adhesive was evaluated at five different press temperatures, ranging between 120 and 200°C. The results showed that dielectric changes monitored by a LCR meter (impedance analyzer) and fringe-field sensor were related to the PF adhesives cure—which involves chemical reaction and water movement/diffusion from the adhesive bond line—but not related to the changes of the moisture in the wood. The rate of cure was strongly affected by the press temperature. The higher press temperatures induced a faster temperature rise in the PF adhesive bond, and thus accelerated the curing process. An nth-order kinetic equation was fit to the DEA degree of cure data to demonstrate the utility of DEA to characterize PF adhesive cure. [Copyright &y& Elsevier]

Bond performance of hygro-thermally compression densified wood was studied using hygro-thermally treated and control yellow-poplar wood (Liriodendron tulipifera). Opening mode double cantilever beam fracture testing and cyclic boiling were used to evaluate bond performance. Phenol–formaldehyde (PF) film and polymeric diphenylmethane diisocyanate (pMDI) adhesives were used to bond specimens for fracture testing. Fracture toughness of hygro-thermal samples bonded with PF film was significantly higher than control samples, while no difference was found for densified samples. Fracture toughness of densified samples bonded with pMDI was significantly higher than control samples, however no change was seen for hygro-thermal samples. Cyclic boiling reduced the fracture toughness of hygro-thermal fracture samples only, irrespective of adhesive type. [ABSTRACT FROM AUTHOR]