Renewable Phenolic Oligomers from Self-Acid Condensation of Vanillyl Alcohol and Vanillyl Alcohol/Lignosulfonate Mixtures for Use in Epoxy/Amine Thermosets

Epoxy resins derived from bisphenol are sourced from fossil fuels and are linked to health concerns due to their endocrine-disrupting properties, highlighting the need for renewable and less toxic alternatives. Lignin and its derivatives offer potential replacements for bisphenol, although the current solutions face limitations. Here, acid condensation reactions using lignin-derived precursors provided potential substitutes for bisphenol in epoxy/amine thermosets. Under reflux, the selected precursors, vanillyl alcohol and sugarcane lignosulfonate, dissolved in water, ensuring a homogeneous aqueous phase and rapid reaction. The self-condensation of vanillyl alcohol produced insoluble phenolic oligomers with a degree of polymerization of 2-6. Incorporating lignosulfonate into the reaction resulted in higher-molar-mass products due to 21% (w/w) lignosulfonate integration. 2D-HSQC NMR analysis provided insights into the oligomerization process that occurred via benzylic alcohol dehydration followed by predominant condensation at the C5 and C6 positions of the aromatic ring. The resulting phenolic oligomers produced oily textured epoxidated resins, even with lignosulfonate incorporation, and were suitable for curing with polyamines. Produced thermosets presented relatively high thermal stability and efficient wood adhesive properties. The newly developed biomass-derived epoxy resins could serve as viable substitutes for bisphenol with potential environmental benefits.