Optimizing Molecular Structure for Trimethylolpropane Ester-Insulating Oil: Achieving High Fluidity and Stability

Abstract

Synthetic ester exhibits a unique combination of advantages, including remarkable oxidation stability similar to mineral oil, as well as high flash point and biodegradability of natural ester, which makes it a promising insulating fluid alternative for transformers. In this work, a series of medium-chain fatty acid trimethylolpropane (TMP) esters (MTE) were prepared by esterification of a mixture of TMP blended with 3 saturated medium-chain fatty acids (C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sub> , C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sub> , and C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub> ). The MTEs were synthesized via a vacuum esterification method with the following optimized reaction conditions: 1 wt.% p-toluenesulfonic acid (TsOH) as catalyst, reaction temperature of 120 °C, internal air pressure of 5 kPa, and the acid:TMP molar ratio of 3.2:1. The MTEs were obtained through a rationally designed purification procedure. By characterizing the physical and electrical properties of the 9 MTEs, it is found that the viscosity of MTEs increases with increasing mean molecular weight, the pour point decreases with increasing C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sub> content, and the flash point slightly increases with inscreasing C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub> content. The C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sub> with a relatively short carbon chain and C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sub> with an odd carbon chain may be beneficial for enhancing the oxidation stability of MTEs, whereas the AC breakdown voltage of the MTEs shows a negligible correlation with the fatty acid ratios.