Effect of Dicarboxylic Acids’ Aliphatic Chain on the Curing of Epoxidized Soybean Oil (ESO) Resins

This work highlights the importance of renewable materials, specifically polymers derived from epoxidized oils, in promoting sustainable development and reducing the environmental impact of various industries. The use of carboxylic acids as hardeners for biobased resins from epoxidized oils is also discussed as a promising development in this field. However, there is a lack of research in this area, and finding the right combination of carboxylic acids and epoxidized oils remains a challenge. The present work aimed to address this gap by synthesizing biobased resins using epoxidized soybean oil (ESO) and dicarboxylic acids with specific aliphatic chain lengths, such as oxalic, succinic, adipic and sebacic acids with 0, 2, 4 and 8 carbons in aliphatic chain. The curing and properties of the resulting biopolymers were investigated using DSC, FTIR, TGA, and swelling ratio assessments. FTIR spectra evidenced that the reaction between epoxy and carboxylic groups occurred by the presence of ester groups, as verified after the disappearance of the epoxy bands at ~ 829 cm−1 and the appearance of related bands to hydroxyl (-OH) at ~ 3500 and ester at 1685–1742 cm−1, respectively. DSC scans presented exothermic peaks related to the curing reactions associated to an increasing enthalpy (ΔH) as the aliphatic chain size increases. The activation energy, Eac, estimated using Friedman and Vyazovkin approaches, is highly influenced by the chain size and pKa of the acids. Furthermore, the thermal and hydrocatalytic degradation increase with the decrease of aliphatic chain. The study also introduces oxalic acid as a biobased cross linker for ESO, which has not been so far combined without any synthetic modifications in its structure.