Hydrodeoxygenation of condensed lignins followed by acid-mediated methylolation enables preparation of lignin-based wood adhesives

Lignin is a phenolic polymer, making it a sustainable alternative to phenol for synthesizing lignin–phenol–formaldehyde (LPF) adhesives. However, technical lignins produced in pulping or biorefining processes are generally highly condensed and have low reactivity, limiting their uses in synthesizing phenolic-based adhesives. Herein, we report a two-step strategy to produce lignin-based wood adhesives with industrially available technical lignins through hydrodeoxygenation followed by acid-mediated methylolation of aromatic C2/6 positions to improve the flowability, water resistance, and crosslinking ability of lignins. We demonstrated that this biobased lignin–formaldehyde adhesive had a lighter color and far better adhesion performance compared to traditional LPF resins prepared from base-mediated methylolation of lignins. Mechanistic investigation suggests that the chemical crosslinking of methylolated lignin to form diarylmethane structures and the physical interlocking interaction at the interfaces of wood veneers contribute to the strength of plywood.