One-Step Synthesis of Waterborne Epoxidized Lignin Nanoparticles with High Epoxy Value and Stability for High-Strength Adhesives

Lignin, being the most abundant renewable aromatic polymer, exhibits immense potential in sustainable biobased epoxy adhesives. However, the tedious preparation process and uncontrolled homopolymerization of epoxidized lignin hinder their large-scale production. Herein, we propose a facile one-step strategy to synthesize a high-performance waterborne lignin-based epoxy adhesive based on the high selectivity of the phenol-epoxy reaction. Owing to the acidity difference between phenol and aliphatic hydroxyl, the phenolic hydroxyl groups in lignin can be preferentially initiated with epoxy groups, which inhibit the reaction on polyglycidol chain propagation. The resulting epoxidized lignin nanoparticles (ELNPs) feature nanoscale (150–250 nm), low molecular weight, and high epoxy value (∼3.84 mmol g–1). The ELNP colloidal dispersions possess high zeta potentials (−34.20 mV) and impressive long-term storage stability (temperature = 4–60 °C, pH = 2–12). The use of cost-effective ELNPs as a wood adhesive has shown a competitive shear strength of ∼14.89 MPa, comparable to those of commercial bisphenol A epoxy resin and phenol-formaldehyde resin. The thermo-curing ELNPs demonstrate excellent chemical solvent resistance and thermal stability (∼395 °C). Additionally, the one-step fabrication of ELNPs demonstrates good scalability (≥15 kg) and commercial viability (472 €/ton) in wood adhesives. This one-step synthetic strategy provides a promising pathway for producing high-performance and cost-efficient lignin-based epoxy resins that can be used as sustainable adhesives.