Enhanced Oxidation Stability of Natural Ester Transformer Oil by A Natural Antioxidant

Natural esters are eco-friendly and fireresistant alternatives to traditional transformer oils, but their inferior oxidation stability remains a challenge. Synthetic antioxidants like tert-butyl hydro-quinone (T B H Q) and butylated hydroxytoluene (BHT) can alleviate the issue, but they may pose health hazards. Therefore, a safer alternative is urgently in need. Here we propose to use carnosic acid, a naturally occurring antioxidant extracted from plants like rosemary, to mitigate the problem. Differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and cyclic voltammetry (C V), coupled with density functional theory (D F T) calculations, were utilized to assess and elucidate the antioxidant performance of carnosic acid. The oxidation onset temperature of refined soybean oil containing carnosic acid, as determined by DSC, is marginally lower than that of soybean oil with TBHQ but remains at least $20^{\circ} \mathrm{C}$ higher than that of neat refined soybean oil. CV analysis indicates that the oxidation potential follows the trend: carnosic acid $\lt TBHQ \lt BHT$, implying that carnosic acid exhibits remarkable efficacy in enhancing the oxidation stability of natural ester transformer oil. It is revealed that carnosic acid is oxidized prior to the oxidation of natural esters and the oxidation process is closely related to intramolecular hydrogen bonds within the carnosic acid. This study sheds light on the antioxidant design and provides an effective pathway to improve the oxidation stability of natural ester transformer oil.