Recent Advances in Lignin-Based Wood Adhesives: Curing Behavior and Water Resistance

Lignin is a renewable aromatic feedstock for wood adhesives, but its structural heterogeneity complicates control over curing and water resistance. This review examines how lignin structure and functionalization govern curing reactions, kinetics, and crosslinked network development in lignin-based wood adhesives, and how these processes relate to bondline formation and wet bonding performance. Across condensation, epoxy, and polyurethane-based systems, molecular weight, aromatic condensation, and reactive-site accessibility influence lignin incorporation into polymer networks. However, chemical conversion, gelation, and effective network formation do not necessarily proceed in parallel; adhesive flow, penetration, and wood moisture further affect bondline development. Although crosslinking can reduce swelling and improve wet strength, faster curing or higher reactivity does not invariably yield better water resistance. The wet performance of wood joints also depends on network chemical stability, residual hydrophilic groups, interfacial bonding, and moisture-induced deformation and damage. Comparisons among studies remain limited by differences in lignin feedstocks, processing conditions, and water-exposure protocols. Future research should link curing kinetics, network architecture, moisture response, and interfacial failure under controlled testing conditions to establish verifiable structure–property relationships and guide formulation and processing.