Transforming interface properties of wood laminate composites functionalized by adhesive penetration

Quantitative analysis of interface mechanical properties of wood laminate composites has been an urgent task of the interface mechanics, however, which is hindered by the lack of knowledge of the correlation between physiochemical structure and micromechanics of the whole wood bonding interphase. This work aimed to evaluate the influence of adhesive distribution with different variables (i.e., adhesive types, grain directions, and cells from the glue line) on the transforming interface properties. The results showed that the physiochemical structure and micromechanics performance were significantly affected by adhesive types, grain directions, or wood cells with the interphase microstructure of earlywood laminates being more sensitive to adhesive penetration than that of latewood laminates. The Phenol-formaldehyde (PF)-laminated interphase possessed a more complicated resin behavior and distribution, and microstructure compared to the one component polyurethane (1C-PUR)-laminated interphase. The distribution of resins near the glue line could recover and enhance the loss of the Er caused by the pressing and machining process. Compared with the control cells (C1, C2), the improved range of Er and H are 9.2%–41.2% and 6.7%–22.2% for the PF-laminated interphase, respectively, and 1.7%–27.6% and 2.1%–8.5% for the 1C-PUR-laminated interphase, respectively. This improvement was mainly attributed to chemical fixing and physical filling into nanopores and physical supporting within cell walls, which could avoid the stress concentration to a different extent and protect the cell walls from the compressive deformation. The initial cracks probably resulting from the machining and/or shear slipping during the pressing process, could incur the global failure of wood laminates interphase.