Polypropylene film and capacitor made by using it as a dielectri

Details Polypropylene film and capacitor made by using it as a dielectri Polypropylene film and capacitor made by using it as a dielectri

1998-03-06

2000-07-25

Kincaid, Kristine

Electricity: electrical systems and devices

Electrostatic capacitors

Fixed capacitor

H01G 426

Patent

active

060943378

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

The present invention relates to a biaxially oriented polypropylene film and a capactor using it as a dielectric, particularly a biaxially oriented polypropylene film excellent in hear resistance and dielectric properties, lesser insulation defects, and excellent for impregnation of an insulating oil into the clearances between film layers, and in swelling resistance when immersed in insulating oil, and a capacitor having excellent dielectric properties, corona resistance, long-term thermal durability and electric current resistance, using the film as dielectric.


BACKGROUND ARTS

Biaxially oriented polypropylene films are widely used for packaging, tapes and capacitors, since they are excellent in optical properties such as clarity and gloss, mechanical properties such as tensile strength and elongation, moisture barrier properties and electrical properties. The biaxial orientation of film is conventionally achieved by drawing the film lengthwise (the "machine direction") and then, with a tenter, drawing the film in a direction normal to the machine direction, in the "transverse direction."Biaxially oriented polypropylene films are major materials used as dielectrics of film capacitors, but since they are lower in heat resistance as compared to polyester film, another major material, the capacitors obtained by using them are limited to about 85.degree. C. in maximum operating temperature. As for the cause, if the operating temperature becomes high, the dielectric strength peculiar to the polypropylene films drastically declines due to the influence of the amorphous domains of the film and impurities, particularly making them unable to withstand long-term use in some cases.
On the other hand, as electric apparatuses are being downsized, devices are assembled at higher densities and exposed to higher temperatures, and there is a strong demand to use conventional polypropylene film capacitors at higher operating temperatures. Especially for capacitors intended to be mounted on AC circuits, polypropylene films with a feature of small dielectric loss have been used as dielectrics for the necessity of inhibiting heat generation from inside the capacitor devices, but since the temperature of the environment surrounding AC circuits becomes higher, it is now difficult to use polypropylene films for such an application. This is one of the factors tending to inhibit the downsizing of electric apparatuses equipped with polypropylene film capacitors. As for the reason, even when a circuit has been positioned certain distance away from the heat source, or insulated from the heat source by a heat insulator lest the circuit should be heated by the heat source, there is an increasing demand to install such a circuit very close to a heat source without using any heat insulators, to meet the demand for downsizing. This demand is typically seen for the AC circuits used in the illumination stabilizers of street lighting, etc. and the control circuits of motors. To meet this demand, it is necessary that the polypropylene film maintains its performance even at temperatures higher than 85.degree. C., the maximum operating temperature of conventional polypropylene film capacitors, for a longer period of time.
So, polypropylene film used as a dielectric is demanded to be (1) moderately small in mechanical deformation, i.e., heat shrinkage when rapidly heated within a short time, (2) excellent in electrical properties at a high temperatures and (3) small in decline of electrical properties at high temperatures with the lapse of time.
As for the reason for requirement (1), when a capacitor device is produced, the polypropylene film wound with an electrode is generally annealed at a certain temperature, and thereby moderately thermally shrunken for tightening the winding to maintain its shape and to squeeze out the air between film layers, and in this case if the heat shrinkage is too large, the device may be deformed, lowering the capacity of the capacitor, or may be destroyed. If the heat shrinkage is too small, t

REFERENCES:
patent: 5724222 (1998-03-01), Hirane et al.

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