Waterborne sulfobutylation and demethylation of lignin for application in phenol–formaldehyde adhesives

Lignin is a renewable aromatic biopolymer that has been explored as a partial substitute for phenol in phenol–formaldehyde (PF) resins; however, its low reactivity often necessitates modification under harsh or solvent-intensive conditions. In this study, sulfobutylation (SB) was investigated as a pretreatment to render kraft lignin water-soluble, thereby enabling subsequent demethylation under milder, more environmentally compatible, and industrially scalable conditions for PF resin production. Following sulfobutylation, the methoxy and hydroxy contents of lignin decreased by 20% and 27%, respectively, while the sulfonate group content and charge density increased by 14% and 25%, respectively. Subsequent demethylation of sulfobutylated lignin in an aqueous system resulted in a 27% reduction in methoxy content, accompanied by a slight increase in hydroxy groups. Compared with one-step demethylation conducted in a solvent system, the two-step aqueous sulfobutylation–demethylation process enhanced lignin's effectiveness in improving resin performance, increasing dry and wet bonding strengths by 32% and 56%, respectively, and reducing flammability, as evidenced by lower smoke density and improved flame test results. These findings demonstrate that the two-step aqueous sulfobutylation–demethylation approach represents a viable alternative to one-step solvent-based systems for sustainable PF resin production. Further work is needed to integrate sulfobutylation and demethylation into a single-step process, replace thiol reagents with greener alternatives, mitigate the associated reduction in thermal stability and increase in water absorption, and conduct large-scale flame-retardancy evaluations to advance the development of aqueous lignin-incorporated resin fabrication systems. • Lignin was sulfobutylated and demethylated in a waterborne system. • The product was used in phenol-formaldehyde adhesive fabrication. • In addition to strength improvement, the adhesive improved the fire retardancy of wood. • A process was proposed to produce this lignin-incorporated adhesive.