Demethylation of lignin with mild conditions and preparation of green adhesives to reduce formaldehyde emissions and health risks.

Demethylated lignin (DL) was prepared in a NaOH/urea solution at room temperature, and the DL solution was directly substituted for phenol to prepare demethylated lignin phenol formaldehyde (DLPF). The 1H NMR results showed that the benzene ring's -OCH3 content dropped from 0.32 mmol/g to 0.18 mmol/g, whereas the functional group content of the phenolic hydroxyl group increased by 176.67 %, increasing the reactivity of DL. The bonding strength of 1.24 MPa and formaldehyde emission of 0.059 mg/m3 met the Chinese national standard with a 60 % replacement of DL with phenol. The volatile organic compound (VOC) emissions of DLPF and PF were simulated, with 25 types of VOCs were found in PF plywood and 14 types found in DLPF plywood. Terpene and aldehyde emissions from DLPF plywood rose, but total VOC emissions were 28.48 % less than those from PF. For carcinogenic risks (CR), both PF and DLPF showed ethylbenzene and naphthalene as carcinogenic VOCs, whereas DLPF had a lower total CR of 6.50 × 10-5. Both plywood had a noncarcinogenic risks of <1, which was within the permissible range to harm humans. In this study, the mild modification conditions of DL benefit its large-scale production, and DLPF effectively reduces the VOCs released from plywood in indoor environments, diminishing the health risks to humans.