Study on the green extraction of lignin and its crosslinking and solidification properties by geopolymer pretreatment

Different delignification processes lead to significant differences in the structure and activity of lignin. Consequently, complex modifications are necessary before lignin to be applied. In this paper, a green process for the selective catalytic extraction of lignin by geopolymer is proposed based on biomass refining. This process can obtain lignin with ideal performance on activity, crosslink ability and curability. Taking eucalyptus, fir and bagasse as examples, the optimal lignin yields reach 46.5 %, 34.8 % and 48.7 % respectively (mFiber/mGeopolymer = 3, 120 min, and 130 °C). Moreover, lignin isolated with geopolymer (GL) shows a similar narrow molecular weight distribution range to that of Milled Wood Lignin (MWL). Studies on crosslinking solidification mechanisms have demonstrated that the phenolic hydroxyl groups of GL participate in the formation of a multi-stage amine crosslinking and solidification network structure. GL does not rely on flexible chains in the crosslinking and solidification of wood adhesives. Since highly active lignin can condense with phenolic hydroxyl groups on the surface of wood, it provides the adhesive with higher bonding strength (3.8 MPa). This study presents a novel approach to fabricating lignin-based formaldehyde-free wood adhesives.