Controlling Cross-Linking Networks with Different Imidazole Accelerators toward High-Performance Epoxidized Soybean Oil-Based Thermosets

Epoxidized soybean oil (ESO)-based thermosets are attracting increasing attention because they are derived from renewable resources. To date, unfortunately, they have not been able to replace petroleum-based epoxy resins yet due to their poor mechanical properties and unsatisfactory thermal performances. To address the drawbacks, quercetin (QC) as a biobased polyphenol-type curing agent was chosen to increase the rigidity of ESO-QC thermosets. Meanwhile, three imidazole-type accelerators, including N-methylimidazole (N-MI), 4-methylimidazole (4-MI), and biobased histidine (His), were introduced to reduce the activation energy during curing and to tailor the network structures after curing. All ESO-QC thermosets possessed excellent thermal stabilities, thanks to the high rigidity and cross-linking density, among which the initial decomposition temperature of ESO-QC/His was up to 348 °C. Due to the different topological structures of ESO-QC thermosets resulted from different catalytic mechanisms of N-MI and 4-MI, superior tensile and flexural strengths of 44.7 and 81.8 MPa were obtained in ESO-QC/4-MI thermosets, respectively, while the ESO-QC/N-MI thermosets displayed outstanding tensile modulus of 1058 MPa, which could rarely be discovered in biobased systems. In addition, the Izod unnotched impact strength of ESO-QC/His thermosets reached 23.5 kJ·m–2, which was even higher than that of the commercial petroleum-based epoxy resins. These features provide great potential for the sustainable polymers for versatile applications.