Adhesion of Technical Lignin-Based Non-Isocyanate Polyurethane Adhesives for Wood Bonding

Lignin is the most abundant aromatic natural polymer, and receiving great attention in replacing various petro-leum-based polymers. The aim of this study is to investigate the feasibility of technical lignin as a polyol for the synthesis of non-isocyanate polyurethane (NIPU) adhesives to substitute current polyurethane (PU) adhesives that have been synthesized with toxic isocyanate and polyols. Crude hardwood kraft lignin (C-HKL) was extracted from black liquor from a pulp mill followed by acetone fractionation to obtain acetone soluble-HKL (AS-HKL). Then, C-HKL, AS-HKL, and softwood sodium lignosulfonate (LS) were used for the synthesis of technical lignin-based NIPU adhesives through carbonation and polyamination and silane as a cross-linker. Their adhesion performance was determined for plywood. FTIR spectra showed the formation of urethane bonds and the reaction between lignin and silane. The NIPU adhesives prepared with C-HKL showed the highest adhesion strength among the three lignin-based NIPU adhesives. As the silane addition level increased, the adhesion strength of NIPU adhesives increased whereas formaldehyde emission decreased for all NIPU adhesives prepared. These results indicate that NIPU adhesives based on technical kraft lignin have a great potential as polyol for the synthesis of bio-based NIPU adhesives for wood bonding.