Adhesion-related properties of silver birch (Betula Pendula Roth) wood as affected by hydrophilic extraction

For the utilization of silver birch (Betula pendula Roth) in load-bearing engineered wood products (EWPs), reliable bonding in production is a prerequisite. The current knowledge regarding the bonding of birch in EWP applications is limited. Extractives are considered a general factor of attention when securing bonding quality. Thus, in this study, the effects of hydrophilic extractives on several adhesion-related bulk and surface properties of silver birch wood were studied, e.g., vapor sorption, swelling behavior, microstructure, wettability, and mechanical properties. The extraction procedure slightly affected vapor sorption causing a reduction in swelling pressure. The extraction also led to a lower Young's modulus, as seen by compression tests. Control experiments with vapor-treated specimens, however, indicated that the effects were originating from the water imbibition and not due to the removal of extractives per se. This was supported by X-ray diffraction results, which were similarly affected by both vapor and extraction treatment. Therefore, the results indicate that the hygric history of the specimens was affecting the wood due to plasticization, increasing mobility, and thereby likely allowing biopolymer reconfiguration and subsequent quenching during re-drying, even though surface-free energy and wettability were not considerably affected. The extent to which these changes appear permanently or temporarily remains an open research question.