Optimizing Molecular Structure for Trimethylolpropane Ester-Insulating Oil: Achieving High Fluidity and Stability
Zijian Yang, Feipeng Wang, Chaofan Wang, Yingying Shu, Liangxuan Ouyang, Qiang Wang, Zhengyong Huang, Jian Li
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 (MTEs) were prepared by the esterification of a mixture of TMP blended with three saturated medium-chain fatty acids ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{C}_{{8}}$ </tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{C}_{{9}}$ </tex-math></inline-formula> , and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{C}_{{10}}$ </tex-math></inline-formula> ). 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{C}_{{9}}$ </tex-math></inline-formula> content, and the flash point slightly increases with increasing <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{C}_{{10}}$ </tex-math></inline-formula> content. The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{C}_{{8}}$ </tex-math></inline-formula> with a relatively short carbon chain and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{C}_{{9}}$ </tex-math></inline-formula> 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.