| 摘要 |
PROBLEM TO BE SOLVED: To reduce a leakage current and an electric power loss, restrain a propagation delay of an electric signal, and improve a characteristic of a propagating speed by setting a dielectric constant in a specific frequency of a polyester film in a specific range, and setting a rate of change of apparent density when soaked in ethyl alcohol not more than a specific value. SOLUTION: A dielectric constant of a polyester film at 1 kHz is set in a range of 1.5 to 3.0, and a rate of change of apparent density when soaked in ethyl alcohol is set not more than 10%. Therefor, a dielectric constant in the whole film is reduced by including air wrapping-in pores having extremely small dielectric constant and dielectric dissipation factor inside the film. In the respective pares, when the film is soaked in insulating oil, a refrigerant and a thermosetting resin, the respective ones do not permeate up to the inmost part inside of the film to restrain apparent density to a rate of change not more than a specific value. |
