High Performance Medium- and Long-Chain Diester Electrical Insulating Oil Biosynthesized via Lipase Catalyzation

Synthetic ester (SE) insulating oils have been more widely researched owing to their tailorable performance to satisfy the more rigorous oil requirements for power transformers. In this article, three biodegradable diesters [neopentyl glycol oleic acid diester (NPOE), neopentyl glycol caprylic-capric acid diester (NPCE), and neopentyl glycol caprylic-capric-oleic acid diester (NCOE)] based high performance electrical insulating oils were prepared by the esterification of neopentyl glycol (NPG) with C18, blends of C8 and C10, and blends of C8, C10, and C18, respectively. The high conversion yield of NPG is achieved by optimum conditions: e.g., on NPCE fatty acids to NPG molar ratio of 2:1, the initial water content of 1 wt%, water absorbent's adding at onset moment, three batches of adding NPG, the catalyst (IM 100) dosage of 1 wt%, and temperature of 50 °C. As compared to chemical catalyzation, enzymatic catalyzation displays milder reaction conditions, no toxicity, and no formation of side products. After the purification of vacuum distillation, adsorption, and filtration, the physicochemical properties of the three synthetic diesters were characterized. As a result, the optimized synthetic diester (NCOE) is recognized by the lowest pour point (-57 °C), sufficiently high flash point (closed cup 200 °C, open cup 214 °C), fire point (234 °C), and initial oxidative temperature (IOT) (196.6 °C). The high performance of the insulating oil (NCOE) is also identified by the excellent ac breakdown voltage (67.3 kV) and moisture resistance. This work offers a green and low energy consumption approach for producing a high-performance synthetic insulating oil, which is expected for scale-up production and application in power transformers.