Dual-functionalized lignin as a sustainable modifier for high-performance phenol-formaldehyde adhesives in plywood production.

Biopolymers derived from natural sources are sustainable, non-toxic, and biodegradable, making them attractive alternatives to fossil-based polymers. Among these, lignin has garnered significant attention due to its potential in adhesive applications. In this study, lignin was extracted from redwood (Pinus sylvestris L.) sawdust using an alkaline delignification process and subsequently modified through propargylation and silanization. Comprehensive characterization using FTIR, solubility tests, TGA/DTG, 1H NMR, SEM-EDX, and contact angle measurements confirmed significant improvements in physicochemical properties. Notably, silylated lignins exhibited enhanced solubility in low-polarity solvents and a marked increase in hydrophobicity, with water contact angles reaching 127.7°. Phenol-formaldehyde (PF) adhesives incorporating raw and modified lignins (5-15 wt%) were formulated and applied in plywood production. Mechanical testing revealed that plywood bonded with 10 wt% Prop-lignin-DH achieved a bond strength of 5.44 MPa and 100% wood failure, outperforming conventional lignin-based adhesives. Furthermore, compared to the control PF resin, the modulus of rupture (MOR) and modulus of elasticity (MOE) increased by 38% and 27.3%, respectively. Shear strength also improved significantly, with gains of 56.1% under dry conditions, 17.6% in cold water, and 74.1% after boiling. In addition, formaldehyde emissions were reduced by 20% compared to standard PF resins, highlighting the potential of silylated lignin as a sustainable adhesive modifier for wood-based composites.