Role of Acid Structure in Structure–Property Relationships of Reprocessable Epoxidized Soybean Oil Thermosetting Networks

Thermosetting polymers offer excellent thermal stability, chemical resistance, and mechanical integrity, but their permanent covalent crosslinks limit recyclability and reprocessability. In this work, ESO-based vitrimer-like polyester networks were synthesized from epoxidized soybean oil (ESO) using tartaric, maleic, succinic, and tannic acids as catalyst-free curing agents. The influence of curing-agent structure on epoxy conversion, network homogeneity, thermal behavior, mechanical properties, chemical resistance, and reprocessing performance was systematically investigated. FTIR analysis indicated extensive epoxide ring opening and polyester network formation in all formulations. Tartaric and maleic acid systems exhibited stronger ester absorptions, higher gel content, and improved film uniformity, indicating more efficient network formation. The resulting materials showed good thermal stability, with degradation onset temperatures of 265–280 °C and maximum decomposition temperatures up to 400 °C. Mechanical performance strongly depended on acid structure: tannic acid produced the stiffest and strongest films, tartaric acid provided the best balance between strength and ductility, and succinic acid yielded less-uniform networks with reduced structural integrity. Reprocessing experiments demonstrated thermo-mechanical reprocessability consistent with vitrimer-like behavior in the tartaric- and maleic-acid-cured systems. These findings highlight curing-agent architecture as a key parameter for designing catalyst-free, renewable, and reprocessable ESO-based thermosets with tunable structure–property relationships.