Unlocking the role of lignin for preparing the lignin-based wood adhesive: A review

Adhesives––the resins widely used in particleboards, oriented strength boards, and plywood––are traditionally produced from fossil-based polymers. Owing to the toxicity, environmental pollution, and non-renewable raw materials associated with these polymers, attempts to synthesize environmentally friendly and renewable adhesives from bio-based materials has attracted increasing attention. Lignin is an abundant natural and renewable phenolic polymer with multiple types of functional groups, such as hydroxyl, carboxyl, and aldehyde groups. This has drawn considerable attention to the exploitation of this resource based on these functional groups. Because of the advantages of lignin, continued research efforts have been focused on using lignin to partially or completely replace conventional raw materials for adhesives, such as phenol and melamine. In addition, lignin is also used with other biomass materials to produce formaldehyde-free biomass-based adhesives, such as lignin–furfural, lignin–soy protein, and lignin–tannin adhesives. Despite inspiring progress, the preparation of lignin-based adhesives with performances higher than that of fossil-based adhesives remains a serious challenge. This review summarizes the progress made in recent decades in research and commercial application of lignin-based adhesives for wood products, including lignin-based formaldehyde resins and lignin-based formaldehyde-free adhesives, and proposes perspectives on lignin-based adhesives.