Robust and dimensionally stable birch wood laminates with increased relative cellulose content and phenol formaldehyde resin impregnation

The beneficial effects of delignification, densification, and resin impregnation on wood properties are well studied. However, only few studies address all aspects mentioned in a single series of experiments, and rather focus on delignification or resin impregnation separately. To close this gap, a comprehensive series of experiments using birch was performed. Laminates from alkaline-extracted veneers were impregnated with phenol formaldehyde (PF) resin in various concentrations and characterized by three-point bending and water immersion tests. For reference, non-extracted specimens were produced and characterized. Several significant effects were observed. Firstly, densification leads to improved mechanical performance, which is more pronounced after extraction of non-cellulosic cell wall components. Secondly, PF-resin impregnation improves mechanical performance for both variants. Once impregnated, further increases in PF-resin concentration improve mechanics for non-extracted specimens, whereas mechanical performance is reduced in extracted variants. Maximum values of bending properties with an elastic modulus of 48 GPa and a bending strength of 470 MPa were observed at 5 % resin concentration in the impregnation solution. As for dimensional stability, PF-resin impregnation significantly reduced the swelling of densified specimens in liquid water. At a PF-resin concentration of 10 % in the impregnation solution, dimensional stability equal to native wood was achieved.