Study on extraction of lignin and synthesis of lignin-based epoxy resins using ionic liquid

Lignin molecules contain alcohol hydroxyl and phenolic hydroxyl, so lignin can be used instead of toxic bisphenol A (BPA) to synthesize epoxy resin, which can not only reduce the amount of formaldehyde emission but also achieve the purpose of waste utilization and environmental protection. In this study, the lignin of Eucalyptus urophylla was extracted with ionic liquid as the extraction agent, and subsequently phenolated by methylphenol as phenolic agent with concentrated sulfuric acid as catalyst to prepare phenolated lignin (PLG). Then, using NaOH as catalyst, the PLG was epoxidized with epoxy chloropropane (ECH) to synthesize the lignin-based epoxy resin (LGEP). The extraction of lignin and the synthesis of LGEP were investigated by the single-factor and orthogonal test, and the Eucalyptus urophylla raw materials, lignin, PLG, and LGEP were characterized by UV, FTIR, XRD, SEM, and TGA. The results demonstrated that when the ionic liquid was [ChCl][Gly], the temperature was 90 °C, the time was 4 h, and the liquid-solid ratio was 20:1; the extraction rate of lignin reached 93.73%, and the purity of the regenerated lignin was 96.3%. The mass ratio of PLG to ECH was 1:2.5, the mass fraction of NaOH was 20% of the mass of PLG, the reaction time was 3 h, and the reaction temperature was 95 °C. Under this condition, the epoxy value of LGEP was 0.364.