Adhesive-free wood strand composites via alkaline-hydrothermal lignin activation: Correlating chemical structure with interfacial strength

Adhesive-free joining of lignocellulosic substrates requires the formation of a strong interphase capable of stress transfer under shear. Here, we demonstrate an intrinsic bonding strategy for wood strands enabled by a synergistic alkaline-hydrothermal treatment that activates native lignin at the interface. Process parameters (NaOH soaking time, temperature, and autoclaving conditions) were systematically optimized, yielding a maximum lap-shear strength of 1.91 ± 0.11 MPa after 5 h of NaOH soaking followed by 30 min of autoclaving at 120 °C, comparable to a phenol-formaldehyde (PF) bonded control. X-ray photoelectron spectroscopy (XPS) and FTIR indicated substantial surface chemical restructuring, showing depletion of carbohydrate-associated functionalities and an increase in lignin-associated carbon (C-C/C-H) from ∼7% to ∼80%, consistent with enrichment of an aromatic lignin-rich interphase. In contrast, extending the alkaline treatment to 7 h produced spectral signatures consistent with excessive lignin condensation, reduced thermoplastic mobility during consolidation, and diminished bond strength. These results establish a direct correlation between lignin exposure and interfacial shear performance, providing an adhesive-free pathway for engineered wood bonding without added resin.