Solvent Fractionation and Depolymerization Provide Liquid Lignin Fractions Exploited as Bio-based Aromatic Building Blocks in Epoxies

In the context of a green transition, alternatives to fossil-based aromatic compounds have to be implemented. In this study, the combination of sequential solvent fractionation followed by depolymerization via catalytic hydrogenolysis of lignin led to liquid oligomeric lignin structures with chemical properties governed by the choice of lignin soluble fraction. Lignin species of increasing molecular weight (MW) were obtained with MW reduction up to 72% compared to the parent lignin, with a corresponding increase in total hydroxyl content up to 18% obtained through cleavage of lignin ether bonds. Hydrogenolysis led to lignin depolymerization oils comprising macromolecular lignin fragments and app. 15–18 wt % lignin monomers, resulting in a liquid product mixture. The lignin species were epoxidized to show their potential as a bio-based aromatic substitute to fossil-based bisphenol A. Liquid epoxy resins were obtained with viscosities between 2 × 103 and 4 × 105 Pa·s, which after curing resulted in thermoset networks with Young's moduli between 0.9 and 1.4 GPa depending on the lignin fraction. Optimal viscosity to mechanical performance was obtained for ethyl acetate and ethanol solvent fractionated and depolymerized lignins as lower viscosity allows for reduced use of volatile organics.