Wide Temperature Range Performance of Cyanobiphenyl Modified Natural Ester: High Flash Point and Low Pour Point

Abstract

Natural ester insulating (NEI) oil, which is nontoxic to the environment and fully biodegradable, emerges as a viable alternative to mineral insulating oil (MO). However, the poor low-temperature fluidity of NEI remains a critical challenge limiting its application to a wider operating temperature window, and traditional pour point depressants can hardly balance the low-temperature fluidity and high flash point of NEI simultaneously. In this work, three cyanobiphenyls (CBs: short-chain CB₁, long-chain CB₂, and branched-chain ${\mathrm{CB}}_{{3}}\text {)}$ with temperature induced phase transition characteristics are employed as additives, and the effects of their chain length and concentration (0–5 vol%) on the key temperature-dependent properties including the flash point and pour point of soybean NEI are systematically investigated, to broaden the operating temperature window of soybean NEI. Joint experimental and computational results show that the CB/soybean NEI systems possess a wider operating temperature window (higher flash point and lower pour point) compared with the pure soybean NEI system. Notably, the 0.5 vol% CB₃/soybean NEI system exhibits the lowest pour point of $- 26.1~^{\circ }$ C, while the 0.5 vol% CB₂/soybean NEI system exhibits the highest flash point of $273.5~^{\circ }$ C. Therefore, the optimal addition concentration of CBs is determined to be 0.5 vol%, and CBs with long chain or branched-chain molecular structures can more effectively improve the flash point and low-temperature fluidity of soybean NEI.