Efficient dissolution of lignin in novel ternary deep eutectic solvents and its application in polyurethane

Novel ternary deep eutectic solvents (DESs) were designed to efficiently dissolve the lignin for synthesizing the lignin-containing rigid polyurethane foam (LRPUF). These ternary DESs were constituted by choline chloride (ChCl), two hydrogen bond donors - glycerol (Gly) and polyethylene glycol (PEG-400). The maximum solubility of lignin in the sample DES-2 (ChCl:Gly:PEG-400 = 1:2:2) could up to 66.70 g/100 g solvent at 60 °C due to the hydrogen bonds are the main driving force, which was confirmed by the largest hydrogen bond basicity parameter β value and disappearance of crystalline peak. The main structure of the lignin did not change after dissolution in DES-2. The content of phenolic and total hydroxyl groups of the regenerated lignin increased, with reduction in their average molecular weights and polydispersity, indicating that the dissolution and regeneration of lignin in these ternary DESs can make it more homogeneous. The regenerated lignin was partially replacing for polyols in the LRPUF synthesis, the lignin replacement rate could up to 58.6% with good mechanical properties. The findings add to our understanding for efficient dissolution of lignin in DESs and for synthesizing the highly LRPUF.