Adhesion performance of demethylated hardwood kraft lignin phenol-formaldehyde adhesive for wood bonding

Hardwood kraft lignin (HKL) is a by-product of kraft pulping, and has potential application as a renewable and bio-based material, thereby reducing the reliance on fossil-based materials. However, the steric hindrance and low reactivity of the methoxy groups of HKL limit its potential for generating a thermosetting polymer network in lignin-based wood adhesives. Hence, the present study examines the adhesion performance of lignin phenol-formaldehyde (LPF) resins synthesized using demethylated hardwood kraft lignin (DHKL) for plywood bonding. By decreasing the methoxy content of HKL, the demethylation process enhances its reactivity with PF resins. The demethylation reaction is studied using various methods such as ATR-FTIR,1H NMR,31P NMR, DSC, and TGA. Both spectroscopic and thermal analyses confirm the successful demethylation of HKL by sodium sulfite. Subsequently, LPF adhesives with various DHKL substitution levels (10%, 20%, 30%, and 50% w/w phenol) are evaluated for their resin properties and adhesion performance with plywood. In particular, the LPF adhesive with 10% DHKL exhibits an improved tensile shear strength of 1.58 ± 0.22 MPa compared to 1.2 ± 0.2 MPa for the reference PF resin. Notably, both of these values exceed the requirements of the Korean National Standard (0.6 MPa) and the European Norm (EN) 314-2 (1 MPa). These results demonstrate that the incorporation of DHKL is a promising approach to the synthesis of LPF resins, offering eco-friendly adhesives for wood bonding with high adhesion performance.