Multispectroscopic Analysis in the Synthesis of Lignin-Based Biophenolic Resins

The production of biomass-based phenolic resins from phenolic oils derived from lignin can effectively improve the cost and environmental friendliness of these polymers. In this study, we focused on the multispectroscopic analysis in the synthesis of lignin-based biophenolic resins. In the first step, we hydrogenolyzed lignin to phenolic oil using a Ru/C catalyst along with boric acid and ethylene glycol. Nuclear magnetic resonance spectroscopy results revealed α-O-4 and β-O-4 ether bonds in the lignin structure to be broken, thereby increasing the phenolic hydroxyl group content of lignin from 0.0524 to 0.6299 mmol/L. The Mn value of the polymer was reduced from 2691 to 1042 g/mol, while the content of guaiacyl and syringyl monomers increased significantly. We found that the active phenolic oil preferentially polymerized with formaldehyde, rather than phenol, leading to a disordered lignin-based phenolic resin with a molecular chain structure, especially when 50 wt % of phenolic oil was used instead of phenol. The as-prepared biomass-based linear phenolic resin exhibited good thermal stability, softening point, free phenol content, polymerization time, and fluidity. Furthermore, the properties of the lignin-based phenolic molding compound (e.g., bending strength, no-notch impact strength, and heat distortion temperature) were superior to those of ordinary phenolic molding compounds. For example, the bending strength reached 101.5 MPa, while the heat distortion temperature reached 208.5 °C. Thus, we demonstrated that the introduction of lignin-phenolic oil resulted in phenolic molding plastic materials with higher rigidity and thermal stability, verifying the green and high performance of the bio-based phenolic molding plastic polymer material.