Assessment of oxidative stability and physical properties of high oleic natural esters

Natural esters (triglycerides) possess desirable attributes such as the higher fire points and better environmental compatibility relative to petroleum-derived insulating mineral oil, as a result, the use of such natural esters is on the rise as insulating and heat transfer fluids in electrical devices such as transformers. However, natural esters differ significantly in chemical composition from mineral oils and are more prone to oxidation than mineral oils. High susceptibility of natural esters can be addressed in part through the use of natural esters with higher monounsaturated fatty acid-high oleic natural esters. Recently several high oleic acid soybeans have been introduced commercially. These include Plenish® (Pioneer / Dupont) Vistive Gold® (Monsanto / Bayer) and Soyleic® (Missouri Soybean Association). Both Plenish® and Vistive Gold® are obtained from GMO soybeans, while Soyleic® has been developed through more traditional plant breeding. The oleic acid content of the three natural esters was found to be 79%, 71%, and 81% respectively. While polyunsaturated fatty acid (linoleic and linolenic) contents of the three natural esters were found to be 10.3%, 22.5%, and 7.8% respectively. High oleic soybean oils along with canola oil with oleic content of 75% and polyunsaturated fatty acid content of 17.2% and generic soybean with oleic content of 24.5% and polyunsaturated fatty acid content of 60.0% were subjected to accelerated aging under conditions outlined in IEC 62770 2013. Test results showed that natural esters with higher oleic and lower polyunsaturated acid contents were more stable than the natural esters with lower oleic and higher polyunsaturated content. This stability was reflected in little or no increase in kinematic viscosity and little increase in dissolved acidity. These natural esters are therefore more suitable for dielectric applications where long-term stability is of paramount concern.