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Mineral oil has been widely used as liquid insulation for high voltage equipment such as a transformer. Due to environmental consideration and long-term availability, there are many efforts to find new liquid insulating materials that are environmentally friendly and renewable. Natural ester is among the candidates for those purposes. In order to be applicable, its interaction with the kraft paper as well as its degradation characteristics is essential to be studied. Accelerated thermal aging is a method that typically been used to assess the performance of oil and paper in the laboratory. In this paper, the result of the degree of polymerization (DP), tensile strength (TS), and Fourier transform infrared spectroscopy (FTIR) measurement were done to examine the degradation characteristic of kraft paper. FTIR which expected to be a new nondestructive diagnosis method of transformer insulation is used to determine the changes of the chemical bond of the kraft paper. The results show that moisture content affects the decrease of DP and TS characteristics. Hence, the FTIR measurement of kraft paper shows that the absorption intensity of C=O and C-H bond increase during the aging. Meanwhile, the absorbance intensity of the O-H bond decreases during aging time. Through regression method, it can be shown that the correlation between DP values to the peak absorbance is influenced by oil-paper moisture equilibrium. Exponential regression method is appropriate for high moisture condition and linear regression is suitable for low moisture condition.

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