Incorporation of Lignin into Adhesives: Review

Abstract

Adhesives are essential in various industries; however, petroleum-based resins raise concerns about environmental and health impacts. Lignin, a renewable biopolymer, offers a sustainable alternative due to its functional groups, which enable its integration into adhesives. Lignin-based resins reduce dependence on fossil resources, lower costs, and improve sustainability. However, challenges such as low reactivity, structural heterogeneity, and performance limitations hinder its commercialization. This review comprehensively discusses the fabrication of lignin-derived adhesives. Modification strategies, such as hydroxymethylation, depolymerization, and phenolation, improve lignin’s reactivity, with the best performance observed when using enhancers, including urea, melamine, polyethyleneimine (PEI), and furfural. Lignin-phenol formaldehyde (LPF) and lignin-polyurethane (LPU) are closest to commercialization due to their lower production costs, which increase water resistance while maintaining bonding strength. Lignin-epoxy (LEP) and lignin-tannin adhesives exhibit strong mechanical properties but require further optimization. Lignin-phenol-formaldehyde (LPF) adhesives are among the most commercially available, with companies such as Latvijas Finieris incorporating bio-based lignin in plywood production while maintaining performance. Similarly, lignin-polyurethane (LPU) adhesives are advancing toward broader commercialization, with efforts focused on replacing 80-100% of phenol to improve sustainability and performance. Research on lignin-melamine, lignin-PEI, and lignin-furfural adhesives remains limited due to a lack of research, though they hold potential for improving durability and processing. The most promising approach to enhancing adhesive performance is to utilize urea, melamine, PEI, and furfural as additives to improve the adhesion and curing efficiency of lignin-based adhesives. Despite advancements, lignin-based adhesives continue to face challenges in terms of strength, durability, water resistance, and processing efficiency. Many require costly enhancers to achieve good performance, increasing production costs. Issues such as brittleness and high modification costs limit widespread adoption. Market reluctance and the lack of standardized formulations further complicate the commercialization process. Future research should focus on cost-effective processing, performance enhancement, and the development of formaldehyde-free formulations. Standardizing lignin modification techniques and expanding applications in construction, automotive, and packaging industries will be crucial to making lignin-based adhesives a viable commercial alternative.