Carbohydrate elimination of alkaline-extracted lignin liquor by steam explosion and its methylolation for substitution of phenolic adhesive

• Steam explosion decreased carbohydrate content in alkali-extracted lignin solution.
• Lignin liquor without purification was methylolated under optimized conditions.
• Methylolated lignin liquor substituted 50% phenolic adhesive with approved properties.
• FTIR indicated formaldehyde was incorporated to aromatic C5 of G unit by methylolation.
• Phenolic substitution by lignin from steam-explosion-derived biorefinery is feasible.

Steam explosion pretreatment was proposed to eliminate carbohydrates in the alkali lignin liquor isolated from corn stalk and the obtained lignin liquor was concentrated and methylolated without further purification process for substitution of phenolic resin as plywood adhesive. Moreover, FTIR and GPC analysis technology was applied to characterize the lignin variation during the steam explosion treatment and the methylolation modification. The results demonstrated that steam explosion could effectively decrease the carbohydrate content of lignin liquor and the operational parameters of steam pressure 1.8 MPa and pressure maintaining time 5 min were obtained as preferable conditions for less carbohydrate content and higher lignin yield. The lignin methylolated under optimized conditions was found to successfully substitute 50% of phenolic resin as wood adhesive with approved bonding strength and lower free formaldehyde and phenol contents. The FTIR and GPC analysis suggested that lignin showed higher content of carbonyl group and narrower molecular weight distribution due to depolymerization effect during steam explosion and formaldehyde was mainly incorporated into the aromatic C5 position of G unit in the methylolation process. Overall, the study indicates that steam explosion is an effective treatment to improve the purity of lignin liquor and the direct substitution of phenolic adhesive by methylolated lignin liquor significantly reduces the costs for lignin recovery, which offers a promising approach for enhancing the commercial operation of a steam-explosion-derived biorefinery.