The effect of ultrasonic process on the shear strength and the microstructure of the bonding interface of laminated bamboo lumber

The aim of this study was to understand the effect of ultrasonic treatment on the bonding shear strength and the microstructure of bonding interface and adhesive penetration of the laminated bamboo lumber. The ultrasonic cavitation treatment was used to increase the permeability of phenol formaldehyde (PF) adhesive in the surface of bamboo strips and the linkage between adhesive and bamboo. Bleached and carbonized bamboo strips were adopted to make two-ply laminated bamboo lumber with phenol formaldehyde adhesive. The orthogonal experiment (L9(3⁴)) was used for evaluating the effect of ultrasonic process parameters including time, temperature and power on the shear strength of laminated bamboo lumber. The variation of the microstructure and the bond line thickness of laminated bamboo lumber were studied by scanning electron microscopy. The average penetration and the effective penetration of PF adhesive in the bamboo surface were evaluated by fluorescent microscopy characterization. The orthogonal result showed that shear strength of the laminated bamboo lumber was affected by ultrasonic temperature, power and time in turn. Temperature had a more significant effect than the other two factors. Compared with the untreated group, the shear strength of laminated bamboo lumber with bleached bamboo strips was increased by 18% and that of laminated bamboo lumber with carbonized bamboo strips was increased by 12.5% after the optimal ultrasonic treatment of bamboo strips. Ultrasonic treatment can enhance the penetration of adhesive on the bamboo surface and the bondline thickness was correspondingly increased. The effect of ultrasonic process on bleached bamboo strips is more obvious than on carbonized bamboo strips, and correspondingly the increase in PF penetration in bamboo surface of laminated bleached bamboo lumbers is more. Ultrasonic treatment on bamboo strips can enhance the bonding strength of laminated bamboo lumber and PF adhesive penetration greatly, therefore, ultrasonic treatment should be an effective method to improve the adhesion between bamboo strips.