Biorenewable Epoxy Resins Derived from Plant-Based Phenolic Acids

Plant-derived phenolic acids are attractive substitutes for petroleum sources for the derivation of polymers, due to their rigid aromatic rings and chemical groups amenable to functionalization. Difunctional phenolic acids were investigated as replacements for the diglycidyl ether of bisphenol A (DGEBA) in anhydride-cured epoxy resins. Functionalization of each phenolic acid was carried out through allylation, followed by epoxidation. Epoxy resins were synthesized through reaction of either epoxidized salicylic acid (ESA) or epoxidized 4-hydroxybenzoic acid (E4HBA) with the curing agent methylhexahydrophthalic anhydride (MHHPA) (catalyzed by 1-methyl-imidazole, 1-MI). The MHHPA anhydride curing agent (catalyzed by 1-M1) was chosen due to the resulting high conversion and advantageous high polymer glass transition temperature. ESA and E4HBA had similar curing behavior to that of DGEBA when cured with MHHPA. A two-step protocol was developed to avoid monomer evaporation and polymer vitrification during curing. ESA- and E4HBA-based epoxy resins exhibited comparable tensile moduli and strengths relative to a conventional DGEBA-based epoxy resin. E4HBA- and DGEBA-based epoxy resins (with para placement of functional groups) fractured at comparable elongation at break values, which were higher than that of the ESA based epoxy resin (with ortho placement of functional groups). Epoxidized difunctional phenolic acids were found to be nontoxic and renewably sourced replacements for DGEBA in epoxy resins, producing epoxy resins of high modulus, high glass transition temperature, and elongation at break (in the case of E4HBA) comparable to a conventional DGEBA-based epoxy resin.