Note: Descriptions are shown in the official language in which they were submitted.
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B~C~Gl~Ol~ND OF T~I13 INV13NTJON
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Field of the ~vention
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'rhis invention rela~es generally to foamed products which are
¦ useful as insulating material for electrical equipment.
¦ Description of the Prior Art
¦ Hard, foamed materials which can be used as electrical
¦ insulation materials should be capable of withstanding low temperature
¦ duty without significant loss of insulating characteristies. ~ey should
¦ have a high stress resista~ ce, and they should be capable of being
10 ¦ formed economically using relatively inexpensive materials.
Although many hard-foamed insulating materials are known
in the prior art, those heretofore disclosed are almost always charac-
terized by localized flaws and iaults. Moreover, the conventional molding
teehniques used in forming the prior art foams often result disadvantageously
in the production of foams which are void of thick skins. ~ the absence
, of a thick molding skirl, conventional molding processes usually result in
a rough surfaced material which, in dirty surroundings, can lead to
severe contamination and consequent formation of electrical leakage
paths across the insulation. A further disadvantage-of skinless foamed
materials is that the foamed material is generally more susceptible
to tho penetration of moisture into the interior since the only moisture
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. barricr is the cell walls, which are usually too thin to be sufficiently
.. impermcable. The presence of maisture in the interior of the foam,of course, .
. deteriorates the electrical properties.
; A still further disadvantage of skinless foamed materialæ is :
~, that the blowing agent gas which generally has a high insulating property
can be lost to the atmosphere and replaced wil:h air, which has a lower
. . electrical resistance~
-. Attempts at .~lleviating these prior art difficulties have been :
.( . generally disappointing. For instance, it has heen suggested to treat
the surface of the foamed product with a varnish, or with other coating
rnaterials, but the results have not proven`to be e~irely satisfactory.
A need continues to exist therefore ~or an improved foam ,
insulator of high electrical resistance.
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SUMMARY OF THE INVENTION .
Accordingly, one object of the present invention is to provide an :
electrically insulating foam material which is characteri~ed by a thick,
sealing typc skin which is generally impermeable to gas and liquid.
. ~ is a further object of this invention to provide a high
mechanical strength foamed electrically insulating material which is
20 ¦¦ charact izcd by a high g:ade surface which does not nced to be rGVO~I~C~
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or repxoces$ed ~nd ~h~c~ i~ c~pable of maint~ini~ng its ~ood
electr~cal propert~es over a rela~vely~wide tempeXature ~ange.
It ~s st~ll a ~lrt~er o~ect of this inven-t~on to provide
a method of pott~ng elect~c~l components-and e~u~pment, and
- 5 ~nsulat~ng high power equl'pment, wi~th a relia~le ~nsulatin~
material, wh~ch methbd ~s simple and results in a lo~ weight
~nsulation.
These and other ob~ects of th~s invention, as will here-
inafter become more read~ly apparent from the following description,
have been attained by providing a foamed polymeric material of an
open or closed cell structure, wherein the cells contain a gas
which has a higher dielectric strength than air. The gas thus
serves the dual purpose of functioning as a blowing agent or pro-
pellant for the formation of the foamed product, and to enhance
the electrical insulating value of the foam.
The objects are attained by the invention whi-ch contem-
plates an electrical apparatus that comprises at least one part
formed of a rigid polymeric foam selected from the group con- ;
sisting of foamed polyurethane and foamed epoxy resin which has
an integral outer skin which is non-cellular. The cells of the
foam contain at least one expanding agent selected from the
group consisting of halo~arbon, halohydrocarbon, sulfohalide
and mixtures thereof which also acts as a dielectric and which
has a dielectric strength greater than that of air. The foam
; 25 also acts as a mechanical load-bearing high-voltage insulation.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The foamed product of this invention is an integral foam
of non-uniform density. The boundaries of the foam are thick,
relatively dense surface layers. The density of the surface layer
may be the same as the non-foamed polymeric material from which
the foam had been formed.
The integral foam may be open or closed celled without
effecting the electrical properties of the foam.
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The foam of this invcntion may be preparcd from a varicty of
polymcric materials including polyurcthanc or epoxy resin. The blowing
~! agents which may be used in preparing the foam are any of those which
:,`'r, have a dielectric strength of greater than air, soch as halocarbons9 the
~} 5 halohydrocarbon~ or the sulfohalogens. ~or instance, suitable such
blowing agents include monofluorotrichloromethane (CCl3 F), tetra-
t fluoromethane (CF4), trifluorochloromethane (CF3Cl), hexafluoroethane
(C2F6~, trifluorobromomethane (CF3Br), difluorochloromethane (CHClF2),
pentafluorochloroethane ~C2ClF5), perfluoropropane (C'3F8), difluorodi-
chloromethane (CC12 F2) or sulfohexafluoride (SF6), or mixtures thereof.
The resulting foamed product will contain the blowlng agent
within the cells, which will be prevented îrom escaping from the product
due to the relatively impermeable outer skin at the boundaries of the
product ~
The hard integral foam material of this invention can withstand
greater mechanical stress than conventional hard foams, because the
strcngth of the outer skin of the hard integral fo~am is considerably greater
than that of the b~undary region of conventional hard foams. Ihis means
that the foam of this invention can be used in insulating applications in
smaller quantities since a smaller amount OI the foam will absorb an
equal amount of stress to larger quantities of conventional foamed
insulati
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The suitably tllicl; mold-s~in with prescribcd density acts for
the hard integral foam as a diffusion barrier wllich greatly hinders the
outward diffusion of the dielecl;ric gas in comparison with normal hard
foam. But in addition, the inward diffusion of liquids, as e.g., water,
as well as of gases, is effectively prevenled or at least made very
difficult for the same reason.
Beyond that, hard integral foam can be made, above all, even
- without further aftertreatment or finishing~with a surface smoothness
familiar in thermoplastic parts. Thus, in contrast to normal hard
foam, dirt will not collect on the surface of the hard integral foam and
so no leakage paths can form with application of high voltage.
A further significant advantage of the invention consists
particularly of the fact that ~e contents of the cells of the foam can
surprisingly be even high boiling point dielectric without possible
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condensation of such contents, because of the structure of the integral
, foam, affecting its electrical strength so greatly that the use of low boiling point fillers is necessary.
' ~ The foamed products of this invention can be used for the
manufacture of stress bearing parts of electrical equipment and apparatus
and for high voltage insulation. 1~ can be used even under low temperature
co nditio
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For instancc, the foarn of this invention can be used in the
manufacture of fixed, insulating elements for switch gears, or as
separators, stand-offs, or jackets for bus bars or supports for medium
and high voltage switches, or as stress bearing insulation ~or cable
termi natio ns .
The foam of this invention might also be used as insulation for
current or voltage convertersJ or as insulation for transformers.
Another potential use of the loamed product of this inv~tion is for
pottin~ of winding heads or slots OI electrical machines, or as protector
grounds for electrical equipment and apparatus.
~ laving now fully described the invention, it will be apparent to one
of c~rdinary skill in the art that many changes and modifications can be
made thereto without departing from the spirit or scope of the invention
as set r h hereln .
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