Lignin-Based Phenol-Formaldehyde Resins: Activation Strategies and Synergistic Pathways from Physical Pretreatment to Chemical Modification

Traditional phenol-formaldehyde (PF) resin adhesives rely on petroleum-based phenolic monomers, facing dual pressures from resource constraints and environmental concerns. Lignin, an abundant renewable aromatic polymer in wood cell walls with a molecular structure rich in phenolic hydroxyl groups, serves as an ideal bio-based precursor for producing green PF resins. Developing lignin-based phenol-formaldehyde (LPF) resins not only enables the high-value utilization of forest biomass but also aligns with sustainable development strategies. However, the large-scale industrial application of lignin remains challenging due to its inherent drawbacks, such as low reactivity. This review focuses on lignin-modified PF resins, systematically summarizing the main physicochemical modification methods—including phenolation, hydroxymethylation, demethylation, and depolymerization activation—along with their mechanisms of influence on resin properties. It compares and discusses the advantages and disadvantages of different modification routes, analyzes current key technical bottlenecks, and prospects future development directions, aiming to provide a reference for research and application of forest-based green adhesive materials. This review concludes that the combination of physical pretreatment and targeted chemical modification is the most promising approach for enhancing lignin reactivity and resin performance. Future research should prioritize developing green modification technologies, such as those based on deep eutectic solvents (DESs) and aqueous systems, and establish quantitative structure–property relationships for lignin-based resins to accelerate the transition of lignin-based phenolic resins from laboratory research to industrial-scale production.