Enhancing Oxidation Stability of Natural Esters: A Multi-Response Statistical Analysis Approach

The exploration of natural esters as green insulating liquids has gained significant attention over the past two decades due to their exceptional qualities for power transformers. However, due to their chemical compositions, the fatty acids make them more susceptible to oxidation compared to mineral oils, highlighting the need for improvements. This study explores the thermo-oxidative enhancement of two natural esters, NE1 and NE2, through the incorporation of tert-butylhydroquinone (TBHQ) and citric acid (CA) antioxidants. The research proceeds in two phases. In the first phase, oxidation stability is evaluated using a full factorial design to analyze the target responses, dissipation factor, acidity, and viscosity through standard methods. The results, assessed through analysis of variance (ANOVA), identify optimal antioxidant concentrations of 0.2 wt.% TBHQ and CA for NE1, and 0.2 wt.% TBHQ with 0.15 wt.% CA for NE2. TBHQ primarily influences viscosity and acidity, while CA notably impacts the dissipation factor. In the second phase, accelerated thermal aging tests are conducted to evaluate the performance of the oils under thermal stress. The antioxidant-treated samples demonstrate significant improvements in oxidative stability, with NE1 showing only an 11.45% increase in viscosity, compared to a 20% increase in NE2 after 1500 hours of aging. These results highlight the importance of tailoring antioxidant formulations to the chemical structure of natural esters, supporting their broader application in transformer insulation.