A new aqueous lignin adhesive has demonstrated high strength, water resistance and durability in plywood tests, pointing to a potential high-value use for lignin while highlighting challenges that remain before industrial scale-up.
A study published September 23 in the peer-reviewed journal Science Advances describes a formaldehyde-free adhesive made from lignin functionalized with cysteine. The approach is designed to address two recurring limitations of aqueous lignin adhesives: poor fluidity and the trade-off between bonding strength and toughness.
The formulation contained 20 wt% lignin solids and was reported to be sprayable and injectable. During hot pressing, the modified lignin forms a crosslinked network combining rigid and flexible molecular structures, which the researchers say improves bonding strength while reducing brittleness.
Strong results in plywood testing
In tests on poplar plywood, the adhesive reached a maximum dry bonding strength of 4.0 MPa. Under more intensive hot-pressing conditions, wet bonding strength reached 3.4 MPa.
Under the study’s comparison conditions, the lignin adhesive also outperformed the commercial phenol-formaldehyde resin used as a reference. The PF resin, originally supplied at 76.82% solids, was diluted to the same 20% solids level and tested under the same application conditions, including hot pressing at 190°C for eight minutes. The researchers reported dry and wet strengths one to two times higher for the lignin adhesive under those conditions.
The researchers also evaluated lignin derived from eucalyptus and pine. The eucalyptus-based adhesive achieved dry and wet bonding strengths of 3.1 and 2.3 MPa, respectively, while the pine-based formulation reached 3.7 and 3.0 MPa. Both wood-derived formulations exceeded the requirement of the Chinese plywood standard used in the study.
The authors caution, however, that extending the technology to other wood composites would require process optimization for individual substrates.
Durability was another focus. In laboratory tests, the bonded plywood retained substantial strength after exposure to water, high humidity, solvents and extreme temperatures. Tests included exposure to liquid-nitrogen temperatures, where bonding strength measured 3.78 MPa at −196°C, compared with 4.01 MPa at room temperature and 2.87 MPa at 150°C. Prolonged exposure to 190°C, however, resulted in a sharp loss of strength.
A potential route for lignin valorization
The work is notable because it treats lignin as a functional component of the adhesive rather than simply as a filler or partial substitute in a conventional resin system.
The researchers characterize the adhesive solids as fully bio-based and report that the formulation requires no additional crosslinking or curing agents.
The process could also fit within a broader biorefinery concept. After lignin extraction, a cellulose-rich solid fraction remains available for further conversion into value-added cellulose materials. The researchers said the approach could therefore support an integrated biorefinery model producing a lignin adhesive alongside cellulose-based products from the same feedstock.
Important barriers remain before industrial implementation. The authors identify solvent recovery efficiency and cost competitiveness as key issues, particularly because of cysteine consumption and separation. A fully closed-loop recycling process has not yet been demonstrated, and further work is needed on process integration and techno-economic optimization.
Storage stability is another practical consideration. The aqueous adhesive became more viscous and lost bonding performance during prolonged storage, leading the researchers to recommend storing the lignin as a powder and preparing the aqueous adhesive as needed.
Four of the study’s authors are named inventors on a patent application submitted by South China University of Technology covering the lignin-based adhesive, related plywood and associated preparation methods.
The results nevertheless point to a potential pathway for converting lignin into higher-value functional materials, while the remaining scale-up challenges underline the distance between laboratory performance and commercial deployment.
