Delignification treatment of birch veneer for manufacturing of highly curved laminated veneer lumber (LVL)

Wood is an eco-friendly and renewable natural material. However, it cannot be easily shaped for curved products with mold-pressing technology while maintaining its intrinsic properties. In this study, different delignification treatments (acid, alkali, and combined acid–alkali) were applied to birch veneer for the manufacturing of highly curved laminated veneer lumber (LVL). The physical properties, including color, shrinkage, density, and mass loss, of treated birch veneers changed significantly after the delignification treatment. Characterization of lignin content and FTIR showed partial removal of lignin and hemicellulose. The crystallinity of treated birch veneer, calculated from XRD data, increased due to the reduction of the non-crystalline regions. SEM demonstrated that birch veneer and acid-treated veneer contain many hollow vessel structures and fibers, while alkali-treated and acid–alkali-treated veneers observed completely closed fibers. The magnified examination of alkali-treated birch veneer reveals a completed and smoothed crease, whereas acid–alkali-treated veneers exhibit rough creases with loose fibers. In contrast, untreated birch veneer and acid-treated veneers tend to fracture upon folding. Finally, highly curved LVL with minimal angles of 30° was prepared with alkali-treated veneers. Compressive performance increases from 8.7 MPa for curved LVL with angles of 120° to 19.8 MPa for curved LVL with angles of 30°. These findings could be used to manufacture shaped furniture, irregular building formwork, reconstituted decorative lumber and automobile interiors in the future.