Carboxylic Acid-Cured Epoxidised Soybean Oil under Catalyst-Free Conditions: Kinetics and Sustainable Packaging Potential

Abstract

This work investigates the curing behavior of epoxidised soybean oil (ESO) with citric (CA), maleic (MA), and salicylic (SA) acids. The curing process was monitored by Fourier-transform infrared (FTIR) and dynamic differential scanning calorimetry (DSC) at heating rates of 5–20 °C min–1. FTIR analysis showed a progressive decrease of the epoxide band (∼950 cm–1) for ESO/CA and ESO/SA systems, while ESO/MA retained oxirane signals after thermal treatment, indicating incomplete curing. ESO/CA and ESO/SA exhibited gel contents above 80% and low swelling, with cross-link densities of ∼1.7–2.1 mmol cm–3, whereas ESO/MA presented a much lower value (∼0.3 mmol cm–3). DSC analysis revealed two exothermic curing stages for ESO/CA and ESO/SA that shifted to higher temperatures, consistent with primary cross-linking followed by secondary reactions. Cure kinetics were evaluated using the Friedman, Vyazovkin, Ozawa–Flynn–Wall (OFW), and Kissinger–Akahira–Sunose (KAS) models. Model-free Friedman and Vyazovkin approaches provided the best description of the experimental data (R2 > 0.99), revealing conversion-dependent activation energies that increased with the degree of conversion, reaching ∼85 kJ mol–1 for CA and ∼87 kJ mol–1 for SA systems. In contrast, OFW and KAS models showed lower fitting quality (R2 < 0.88), indicating that single-activation-energy approaches are insufficient to describe the curing process. Tensile tests confirmed structure–property differences between the networks: ESO/CA exhibited higher modulus and strength (∼0.85 and ∼0.55 MPa), while ESO/SA showed greater elongation (∼147%). Overall, catalyst-free ESO/CA and ESO/SA systems form cross-linked biobased thermosets with tunable mechanical behavior and well-defined curing kinetics, demonstrating their potential for the development of sustainable epoxy materials.