CA1037670A

CA1037670A - Electrical insulating material

Info

Publication number: CA1037670A
Application number: CA183,056A
Authority: CA
Inventors: Akio Tomago; Takashi Suzuki
Original assignee: Matsushita Electric Industrial Co Ltd
Current assignee: Panasonic Holdings Corp
Priority date: 1972-09-01
Filing date: 1973-10-10
Publication date: 1978-09-05
Also published as: JPS4959300A; NL7313991A; DE2350913B2; GB1421047A; DE2350913A1; FR2203143B1; FR2203143A1; DE2350913C3; JPS5132400B2

Abstract

ABSTRACT OF THE DISCLOSURE

Electrically insulating materials are provided comprising an insulating oil impregnated film of porous macro-molecular material which has an inter-filmplane orientation structure, a major part of the porous material consisting of a crystalline, synthetic macro-molecular material, said porous material having a pore volume of 5 to 50%, and a mean pore diameter of 0.01 to 1 micron, the volume of pores having a pore diameter of 10 microns or less being 90% of the volume of all the pores. It has been found that an extremely high break-down strength is attainable with the electrically insulating materials of the present invention, giving them very wide applicability in insulation technology.

Description

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The present invention relates to films of electrically insulating material having an extremely high breakdown strength.
Many insulating materials of low loss and high performance have been recently proposed for use instead of oil impregnated cellulose as oil impregnated electrical insulators in, for example, power cables, and capacitors. One example of insulating material which has been proposed is films of macro-molecular material. Since synthetic macro-molecular films show poor impregnability for the insulating oil, it has been proposed to - 10 laminate a kraft paper with the film. This, however, has an adverse effect on the low loss characteristic of the filmO
Various types of synthetic papers comprisiny synthetic macro-molecular materials have been prepared and kheir oil-impregnation aharacteristics have been examined, but none of them has been proved to have a breakdown characteristic superior to that of kraft paper. It has now been discovered, suprisingly; that when porous macro-molecular material of the type heretofore used in the manufacture of micro-filters or battery separators having a wide distribution of pore diameters is impregnated with insulating oil and the breakdown voltages thereof are measured, detailed analysis of the breakdown voltages and the distribution of the pore size of the porous material shows that a strong correlation exists therebetween. It has been discovered, according to the present invention, that if the mean pore diameter of previously used synthetic insulating paper, which is usually from about 10 microns to 100 microns, is reduced to 1 micron or less, then the breakdown voltage of such insulating paper is increased substantially above that of kraft paper.
.~ According to the present invention there is provided an electrically insulating material comprising an insulating oil ,. - 1 - ~

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-~037~70 impregnated film of porous macro-molecular material which has an inter-filmplane orientation structure, a major part of the porous material consisting of a crystalline, synthetic macro-molecular material, said porous material having a pore volume of 5 to 50%, and a mean pore diameter of 0.01 to 1 micron, the volume of pores having a pore diameter o 10 microns or less being 90% or more of the volume of all the pores.
The present invention will now be described in more detail with reference to the accompanying drawing, in which the single figure shows a typical pore size distribution of porous material employed according to the present invention.
When the crystalline macro-molecular material is placed under tension at or below its melting point, the molecules are generally oriented in the direction of the tension. This process is usually referred to as stretching, and imparts to the material an inter-filmplane oriented structure. The term "inter-filmplane oriented structure" as used herein means a structure which results from stretching the material to orient the crystals or molecules uni-axially or bi-axially along a plane of the film.
It has been found that such an inter-filmplane oriented structure is necessary to achieve a high breakdown strength in the porous macro-molecular material. The distribution of pore diameters in the porous material can be measured by a porosimeter of the mercury compression type, and all the pore diameters which appear in the present specification have been measured using such an apparatus. It has been found that material in which the inter-filmplane oriented structure has a mean pore diameter of 1 micron or less and 90% or more of the whole pore volume is constituted by pores having a diameter of 10 microns or less exhibits a high breakdown strength. It has also been found that ~0376~0 where the mean pore diameter is less than 0.01 micron, there is a risk of incomplete impregnation by the insulating oil, as encountered with the prior art films. It has been also found that a similar risk exists when the pore volume is less than 5~.
It has been found that a pore volume of 5% -to 50% gives optimum results. The breakdown characteristic is degraded when the pore volume is increased above 50~.
Preferred porous materials which can be used in the present invention comprise, as a major portion thereof, crystalline macro-molecular material such a polyethylene, isotactic polypropylene polyethylene terephthalate, polysulfone, polyphenylene oxide or a polycarbonate. Other macromolecular materials and anti-deteriora-tion agents can be mixed therewith as require~.
The insulating oil may comprise natural or synthetic oil such as mineral oil, an alkylbenzene, silicone or polyisobuty-lene. It will be appreciated however, that the term "insulating oil" can cover other materials which might be employed in special circumstances. For example liquid nitrogen may be used as an insulating oil when cryogenic insulation is required. In such a case it is desirable that the dielectric constant of the porous macro-molecular material is close to that of the insulating oil.
It will be appreciated that the insulating oil is applied to the porous macro-molecular material by any conventional technique provided that the resulting porous macro-molecular material is impregnated with the oil.
Examples of methods for manufacturing the films of porous macro-molecular material applicable to the present invention include:
(1) stretching a blend of two kinds of macro-molecular material and subsequently dissolving away one of the macro-molecular materials with a solvent, i~ ,i J,0;~76~

(2) stretching a blend of two or more kinds of macro-molecular material under particular conditions, or

(3) rapidly extruding a concentrated solution of macro-molecular material into a non-solvent for the macro-molecular material.
The following method may be included as a post-treatment.

(4) crosslinking the porous macro-m~olecular material by chemical or radioactive means. This post-treatment improves the heat and oil-resistance of the ~ilm.
Methods whiah have been employed extensively in the manufacture of prior porous macro-molecular materials include:

(5) dry or wet-spinning fibres and subsequently bondin~ the fibers by fusing or adhering them to each other to ~orm a porous material,

(6) blending two or more kinds of macro~molecular material or blending a macro-molecular material with a low molecular weight material and subsequently dissolving away part of the blend, without effecting orientation by stretching, and

(7) spreading a concentrated solution of macro-molecular material onto a roll and treating the macro-molecular material with a non-solvent, without effecting inter-plane orientation.
The mean pore diameter of the porous material manufac-tured by method (5) or a similar method, by which most of the prior art synthetic papers have been manufactured, is 10 microns or more, while the porous material manufactured by methods (6) and (7) have no inter-plane orientation structure. As a result, each of these materials exhibit a low break-down voltage when impregnated with oil.

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The following Table shows the porous materials manufactured by the above methods, wherein the materials described in Examples 1 to 7 are insulating materials in accordance with the present invention. These materials exhibit a high breakdown strength of 110 to 18~ KV/mm.
The Examples 8 and 12 represent materials having no inter-plane orientation structure. These materials exhibit a low break-down voltage of 60 to 70 KV/mm. Examples 9 and 11 represent materials wherein the mean pore diameter is larger than 1 micron, and Example 10 represents material wherein the mean pore diameter is 1 micron but the distribution of the pore diameter is broad and the total volume of the pores having a pore diameter oE 10 microns or more occupies 10% or more of the whole pore volume.
The attached drawing shows a typical pore diameter distribution, wherein shaded areas at both ends of the distribu-tion curve each occupies 10% of the whole pore volume. The width spanning over the area excluding these shaded areas is designated in the Table as 80% distribution width. The numbers given under manufacturing method in the Table correspond to those numbers used hereinabove to explain various manufacturing methods. The A.C. break-down voltage in the Table was measured by flat plate electrodes of 25mm diameter at room temperature with a voltage increase rate o~ S KV/mm.
As seen from the foregoing description, the electrically insulating materials according to the present invention have break-down strengths which are 1.5 to 2 times higher than that of the prior art kraft paper. Furthermore, when polyethylene or polypropylene is used as the macro-molecular material, an insulating material having an extremely low loss is provided, which is particularly suitable for power cables or capacitors.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An electrically insulating material comprising an insulating oil impregnated film of porous macro-molecular material which has an inter-filmplane orientation structure, a major part of the porous material consisting of a crystal-line, synthetic macro-molecular material, said porous material having a pore volume of 5 to 50%, and a mean pore diameter of 0.01 to 1 micron, the volume of pores having a pore diameter of 10 microns or less being 90% or more of the volume of all the pores.

2. An electrically insulating material according to Claim 1, wherein the crystalline, synthetic macro-molecular material is selected from the group comprising polyethylene, isotactic polypropylene, polythylene terephthalate, polysulphone, polyphenylene oxide or a polycarbonate.