Carbohydrate elimination of alkaline-extracted lignin liquor by steam explosion and its methylolation for substitution of phenolic adhesive
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 FOR 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 C-5 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. (C) 2013 Elsevier B.V. All rights reserved.