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

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 (MTEs) were prepared by the esterification of a mixture of TMP blended with three saturated medium-chain fatty acids ( $\text{C}_{{8}}$ , $\text{C}_{{9}}$ , and $\text{C}_{{10}}$ ). The MTEs were synthesized via a vacuum esterification method with the following optimized reaction conditions: 1 wt.% p-toluenesulfonic acid (TsOH) as a catalyst, reaction temperature of 120 °C, the 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 nine MTEs, it is found that the viscosity of MTEs increases with increasing mean molecular weight, the pour point decreases with increasing $\text{C}_{{9}}$ content, and the flash point slightly increases with increasing $\text{C}_{{10}}$ content. The $\text{C}_{{8}}$ with a relatively short carbon chain and $\text{C}_{{9}}$ 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.