Renewable Epoxy Networks Derived from Lignin-Based Monomers: Effect of Cross-Linking Density

Three modification methods, which either improved molecular weight, orientation or the number of functional groups, were employed to increase the cross-linking density of biobased epoxy networks based on 2-methoxy-4-propylphenol (dihydroeugenol, DHE). The modifications were realized through o-demethylation and phenol–formaldehyde reactions. Structures of DHE-based monomers and cured networks were characterized by NMR and FTIR spectroscopy. Cross-linking densities of cured networks were calculated using rubber elasticity theory from dynamic mechanical analysis (DMA). Networks with higher cross-link density were found to exhibit greater mechanical and thermal performance as measured by DMA and thermogravimetric analysis. GEDHEO-NOVO, an epoxy monomer featuring all three modification processes, exhibited significant improvements in cross-linking density (0.39 to 9.77 mol/dm3), α-relaxation temperature (Tα, 40 to 139 °C) and statistic heat-resistant index temperature (Ts, 125 to 153 °C) compared to the unmodified DHEO-based networks.