Shear Strength and Microscopic Characterization of a Bamboo Bonding Interface with Phenol Formaldehyde Resins Modified with Larch Thanaka and Urea
The aim of this study was to understand the microscopic characteristics of the bamboo bonding interface with phenol formaldehyde resin modified with larch thanaka and urea (PTUF) and its effect on the shear strength of two-ply bamboo laminated lumber. Bleached and carbonized bamboo strips were used, and two assembly patterns (outer-to-outer and inner-to-inner) were adopted to make two-ply bamboo laminated lumber with PTUF. The microstructure of the bonding interface and the bond-line thickness were investigated using a scanning electron microscope. The average depth and effective depth of PTUF on bamboo surfaces were evaluated by fluorescent microscopy characterization. The shear strength of two-ply bamboo laminated lumber was also examined. The results revealed a shallow depth of penetration of PTUF into the bamboo surface that was distributed primarily in the broken cell cavities formed during preparation, as well as between the cell walls. When the assembly pattern was inner-to-inner, the depth of penetration and bond-line thickness were higher, but the shear strength was lower than that of the outer-to-outer pattern. The carbonized bamboo laminated lumber provided a greater resin penetration and bond-line thickness, but lower shear strength, than the bleached bamboo laminated lumber.
Effects of the drying process on the shear strength and morphology of the bonding interface of laminated wood
To investigate the influence of the drying process on the bonding properties of laminated wood, a poplar veneer was impregnated with low molecular-weight phenol formaldehyde (PF) resin and laminated wood was created and studied. The effects of drying temperature, drying time and solid content of the PF resin on the moisture content (MC) of the impregnated veneer after drying and the shear strength of the laminated wood were analyzed with an orthogonal experiment. The microscopic mechanism of the bonding performance of the laminated woods was investigated by fluorescence microscopy. The experimental results showed that the higher the PF resin solid content was, the larger the weight gain percentage of the impregnated veneer was. In addition to drying temperature and drying time, the solid content of PF also had a significant influence on the MC of the impregnated veneer after drying. The shear strength of the laminated wood affected by the drying temperature, solid content of PF and drying time and the drying temperature had a more significant effect than the other two factors. The best drying process was a drying temperature of 50 °C, a drying time of 2 h, and a PF solid content of 30%. The fluorescence analysis showed that the shear strength of the laminated wood was affected by the intensity of the PF resin on the bonding interface, and the average and effective penetration depth of the PF resin in the veneer increased with the increase in the drying temperature.
Comparative analysis of a wood: adhesive bondline
The wood–adhesive interface was analyzed using five methods with the objective of quantitatively assessing penetration of adhesive into the porous wood network. Methods included fluorescence microscopy, scanning electron microscopy, backscatter electron imaging, wavelength dispersive spectroscopy, and X-ray microtomography (XMT). Each method provided a visual inspection, and all of the analysis methods were applied to the same field of view. XMT was the primary technique of interest. Rubidium hydroxide was used in place of sodium hydroxide in the formulation of phenol–formaldehyde adhesive. Rubidium was found to increase the X-ray attenuation of the adhesive. However, rubidium migrated beyond the adhesive interphase during specimen preparation, thus reducing its effectiveness for image contrast enhancement. The wood species studied included red oak (Quercus rubra), Douglas-fir (Pseudotsuga menziesii), and hybrid poplar (Populus deltoides × Populus trichocarpa). All techniques used for this study were useful, but each presented some limitations for bondline analysis. Despite the problem with rubidium migration, XMT for this application was promising.