Facile preparation of diformylxylose-derived adhesive for wood bonding

Wood-based panels like plywood alleviate the rapidly-growing global demand for timber, yet their production remains highly dependent on fossil-derived adhesives (e.g., urea-formaldehyde and phenol-formaldehyde resins) that impose considerable environmental and ecological concerns. As such, various renewable macromolecules such as lignin, starch, proteins, and hemicelluloses have been developed as bio-adhesives. However, the structural variability of these biomacromolecules, influenced by plant source and isolation methods, often leads to inconsistent adhesive performance during industrial scale-up. While adhesives synthesized from biomass-derived monomers can ensure better batch-to-batch uniformity, conventional refining systems (e.g., acidic and alkaline aqueous systems) still suffer from low efficiencies in isolating biomacromolecules from lignocelluloses and converting biomacromolecules into platform monomers (e.g., monosaccharides and furanics). Based on diformylxylose (DFX), a by-product derived from the acetal-protected lignocellulosic refining process, we herein report a novel DFX-derived bio-adhesive. This adhesive is prepared through two-step acid-catalyzed pre-reactions, i.e., pre-reaction I and II, followed by blending with a small amount of defatted soybean flour (DSF). After hot-pressing, it achieves both dry and wet bonding strengths of 1.4 MPa, significantly exceeding those of urea-formaldehyde resin as well as other sugar- (e.g., sucrose, starch and hemicelluloses) and protein-based (e.g., DSF) bio-adhesives. Additionally, residual formaldehyde molecules of pre-reaction I can be effectively captured by urea and additive (melamine or polyvinyl alcohol) in pre-reaction II, reducing the formaldehyde emission of three-layer plywood to as low as 0.58 mg/L. Characterizations indicate that the furanic prepolymers derived from pre-reaction I can undergo further covalent and non-covalent (e.g., hydrogen) cross-linking with additive (polyamine, or polyol) and DSF during pre-reaction II and hot-pressing, enhancing the cohesion and interfacial adhesion of DFX-derived bio-adhesives.