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Microcosmic Mechanism Analysis of Insulation Paper Thermally Aged in Mineral Oil and Ester Oils

Natural esters have been more and more widely used in recent years, while the analysis of the microcosmic mechanism is extremely important. In this paper, the accelerated thermal aging test for oil-paper samples are performed at 120°C, which composed of 25# mineral oil, soybean oil, palm oil, ordinary insulation pressboard and thermally-upgraded paper, to investigate the aging characteristics of oil - paper insulating system. The research on changing laws of degree polymerization (DP) of insulation pressboard is performed. The microcosmic mechanism analysis of insulation paper thermally aged in mineral oil and ester oils are studied by building different oil-paper composite models and simulated the diffusion of hydronium ions in insulating oilpaper system. The interaction energy and hydrogen bonds between cellulose, oil and hydronium ions are analyzed. Results show that the DP values of insulation pressboard immersed in soybean oil are the highest, while the ageing rate of insulation pressboard immersed in mineral oil is the highest, which is 2.59 times of that in soybean oil and 1.03 times of that in palm oil. The molecular simulation result of interaction energy and hydrogen bonds shows that soybean oil has the best ability of protecting cellulose, due to the high interaction energy with hydronium ions, which is 1.2 times higher than that of palm oil in the oil-paper composite model with 1 wt% hydronium ions. With the increment of the ratio of hydronium ions in oil-paper composite model, the interaction energy and the number of hydrogen bonds between cellulose and hydronium ions both show an increasing trend. In the oilpaper composite model of 5 wt% hydronium ions, the number of hydrogen bonds between cellulose and hydronium ions in mineral oil-paper composite model is 1.7 times of that in soybean oil-paper composite model and is 1.1 times of that in palm oil-paper composite model. It's the attraction of hydronium ions of esters which would prolong the lifetime of insulation paper.