Self-Adhered Bamboo via Fiber Defibration and Lignin Reconstitution in the Subsurface Layer: Mechanistic Insights into Continuous Interface Formation under Mild Conditions and Sustainable Applications

The widespread use of formaldehyde-based adhesives in bonding wood and bamboo materials raises significant environmental concerns and health risks. In response, self-adhesion technology has become a research hotspot in the field of wood processing. However, current self-adhesion techniques rely on harsh processing conditions (e.g., > 200 °C, > 20 MPa), resulting in conditions incompatible with daily use environments and causing irreversible changes in the natural properties of wood and bamboo, thus severely limiting their broader applications. In this study, we developed an innovative nondestructive self-adhesion strategy for wood and bamboo subsurface layers, employing mild processing conditions (≤90 °C, ≤ 0.6 MPa). First, NaOH was used to remove a portion of hemicellulose and lignin, exposing the fiber bundles in the subsurface layers of wood and bamboo. Next, the NaOH/urea mixed solution freeze–thaw technique was applied to controllably defibrate the fiber bundles into microfibrils, achieving a high-density distribution on the bamboo surface. During freeze–thaw, lignin underwent in situ enrichment and formed a layer on the surface of the microfibrils. Hot pressing resulted in the formation of a three-dimensional interpenetrated fiber network, with lignin playing a synergistic adhesive role. The combination of an interconnected fiber network and the adhesive properties of lignin created a stable multiscale bonding system for bamboo. Scanning electron microscopy (SEM) and optical microscopy (OM) confirmed the integrity of this self-adhered structure. Additionally, characterization results demonstrated that the continuity of the self-adhered interface transitioned into the substrate, achieving a self-healing bonding effect in bamboo. Under optimized conditions of 90 °C and 0.6 MPa for 5 h of hot pressing, bamboo lap joints exhibited a dry shear strength of 3.9 MPa. The same self-adhesion strategy was successfully applied to the production of three-layer plywood and bamboo particleboard, achieving a dry shear strength of 1.3 MPa and an internal bonding strength of 1.0 MPa, respectively. Notably, even under cold pressing conditions of 7 days at room temperature (30 °C) and a pressure of 0.1 MPa, bamboo lap joints still achieved a dry shear strength of 1.2 MPa. This study not only establishes a new paradigm for continuous interface self-adhesion in biomass materials but also enables efficient bonding under mild conditions, contributing to sustainable and environmentally friendly manufacturing. The findings show significant potential for application in the fields of premium furniture, lightweight construction, and other wood-based composite industries.