Methyl carnosate as a sustainable antioxidant for enhancing oxidation stability in soybean oil-based dielectric fluids

Natural ester dielectric fluids are eco-friendly alternatives to petroleum-based dielectrics, but oxidative instability from unsaturated bonds limits operational lifespan in transformers. While carnosic acid (CA) enhances soybean oil (SO) oxidation resistance, it introduces unacceptable dielectric loss and acidity. This challenge was addressed through the development of methyl carnosate (MC), a CA derivative that reduces acidity and dielectric loss while retaining the antioxidant catechol moiety. Thermal analysis and electron paramagnetic resonance (EPR) spectroscopy show MC significantly improves SO oxidation stability without compromising dielectric properties. MC-doped SO exhibits > 10 °C higher oxidation onset temperature and delayed thermo-oxidative decomposition due to potent radical scavenging. Conformational analysis and bond dissociation enthalpy calculations reveal that MC’s dominant conformation, stabilized by intramolecular hydrogen bonding, facilitates preferential H-donation from the hydroxyl group ortho to the aliphatic ring, confirming sacrificial antioxidant behavior. MC is a promising eco-friendly antioxidant for natural ester dielectrics, enhancing longevity and performance