Demethylation, phenolation, and depolymerization of lignin for the synthesis of lignin-based epoxy resin via a one-pot strategy

A one-pot method to synthesize demethylated-phenolated lignin (DPL) and demethylated-phenolated lignin-based epoxy resin (DPLINP) using phenol as a phenolic reagent and polymerization inhibitor was proposed in this study. This method avoids the occurrence of polycondensation caused by demethylation and simplifies the synthesis of lignin-based epoxy resins. Phenolated lignin (PL)-based epoxy resin (PLINP) and demethylated lignin (DL)-based epoxy resin (DLINP) were prepared to investigate the effects of different modification methods on the resulting lignin and epoxy resin. The results of 1H nuclear magnetic resonance (NMR), Fourier transform infrared, and gel permeation chromatography analyses indicated that the introduction of phenol could significantly reduce the occurrence of polycondensation while also decreasing the molecular weight and increasing the content of phenolic hydroxyl in DPL compared with DL and PL. 2D heteronuclear single quantum coherence NMR analysis was used to further investigate the structural changes of lignin during the modification process. The results obtained via dynamic mechanical analysis, flexural test, and impact test demonstrated that demethylated-phenolated lignin-based epoxy network (DPLIEN) possessed an improved cross-linking density, glass transition temperature, and mechanical performance compared with all other epoxy resins. Moreover, the polycondensation of DL caused by demethylation deteriorated the mechanical performance of demethylated lignin-based epoxy network (DLIEN). The thermal stability of DPLIEN was decreased compared with phenolated lignin-based epoxy network (PLIEN), as the removal of methoxy groups may have reduced the protective char layer formation in DPLIEN. This study provides a novel method for the modification of lignin and synthesis of lignin-based epoxy resins.