In order to compare the characteristics of natural ester impregnated Kraft paper and thermally upgraded paper, sealed aging vessels containing copper, insulation paper (Kraft paper and thermally upgraded paper), and natural ester were aged at 90, 110, and 130?°C about 400 days. The results show that both degree of polymerization(DP) and leveling-off degree of polymerization (LODP) for thermally upgraded paper were higher than the ones of Kraft paper at the same aging condition. Furfural can be absorbed by thermally upgraded paper and can be used to monitor the aging condition of a certain natural ester impregnated paper insulation. However, furfural can not be used to compare the aging condition of different types of paper immersed in natural ester. The different disciplinarians of the water content in natural ester were analyzed and the suggestion is given that the thermally upgraded paper are more suitable for natural ester impregnated paper insulation. The infrared spectrum evidence was given that there was dicyandiamide in thermally upgraded paper, and the aging resistance mechanisms of thermally upgraded paper were analyzed.
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