CLOSED-CELL RIGID FOAM PLASTIC

PLASTIQUE MOUSSE RIGIDE A ALVEOLES FERMEES

English Abstract

ABSTRACT OF THE DISCLOSURE
A closed-cell rigid foam for use as an
insulating element in conjunction with high-voltage
electrical equipment in general, the foam being made from
a polymer such as polyurethane which is expanded into
closed-cell form with halocarbon such as difluorodichloro-
methane. The advantages of the foam are the favorable
dielectric strength of the expanding agent which is greater
than that of air as well as its comparatively low boiling
point thus making the foam material particularly desirable
for use on electrical equipment operating at temperatures
down to about -20°C or less. Sulfur hexafluoride can also
be used as an expanding agent.

Claims

The embodiments of the invention in which an exclusive property
or privilege is claimed are defined as follows:
1. In an electrical apparatus encountering high electrical
voltages and subject to operation over a wide temperature range
having a lower limit of -20°C., the improvement which comprises
a high voltage electrical insulating component comprising a
rigid, closed-cell, polyurethane foam body, the closed cells
of which are filled with gaseous foaming agent having dielectric
properties and consisting essentially of a member selected from
the group consisting of sulphur hexafluoride and halogenated
hydrocarbons having boiling points not higher than -20°C and a
dielectric strength greater than that of air, said electrical
apparatus being further characterized by the absence of a casing
surrounding said insulating component.
2. The apparatus according to claim 1, wherein the
foaming agent consists of at least one member selected from the
group consisting of tetrafluoromethane (CF4), trifluoromethane
(CHF3), trifluorochloromethane (CF3Cl), hexafluoroethane (C2F6),
trifluorobromomethane (CF3Br), difluorochloromethane (CHClF2),
pentafluorochloroethane (C2ClF5), perfluoropropane (C3F8) and
difluorodichloromethane (CCl2F2).
3. The apparatus according to claim 1, in which the
gaseous foaming agent is difluorodichloromethane.
4. The apparatus according to claim 1, in which the
closed-cells are filled with a mixture of at least two halo-
genhydrocarbons at least one of which is a member of the group
consisting of tetrafluoromethane (CF4), trifluoromethane (CHF3),
trifluorochloromethane (CF3Cl), hexafluoroethane (C2F6), tri-
fluorobromomethane (CF3Br), difluorochloromethane (CHClF2),
pentafluorochloroethane (C2ClF5), perfluoropropane (C3F8),
and difluorcdichloromethane (CCl2F2), and at least one of which
is a member of the group consisting of octafluorocyclobutane

(C4F8), tetrafluorodichloroethane (C2Cl2F4), monofluorodichloro-
methane (CHCl2F), monofluorotrichloromethane (CCl3F), and hexa-
fluorodichloropropane (C3Cl2F6) and mixtures of these compounds.
5. The apparatus according to claim 1, in which said
body is a structural, mechanical load-bearing element.
6. The apparatus according to claim 1, 2 or 5, wherein
said cells are filled with said gaseous foaming agent.
7. In a switchgear apparatus according to claim 1, 2 or
4, wherein said component is a stationary insulating
component.
8. In a switch gear apparatus according to claim 1, 2 or
4, wherein said component is a partition wall,
a pin-type insulator, bus bar cladding, a supporting framework
for a medium or high voltage circuit breaker, or the load-
bearing insulation of a cable termination.
9. The apparatus according to claim 1, wherein said
gaseous foaming agent is sulphur hexafluoride.
10. Electrical apparatus encountering high voltages,
subject to operation over a wide temperature range having a
lower limit of -20°C, comprising at least a rigid, closed-cell
polyurethane foam body, the closed cells of which contain a
gaseous foaming agent having dielectric properties, and
selected from the group consisting of sulphur hexafluoride
and halogenated hydrocarbons having boiling points not higher
than -20°C and a dielectric strength greater than that of air,
as a mechanical load-bearing high voltage insulation therein,
said electrical apparatus being further characterized by the
absence of a casing surrounding said rigid, closed-cell poly-
urethane foam body.

11. Apparatus according to claim 10, wherein said cells
are filled with said gaseous foaming agent.
12. In a method of manufacturing high voltage electrical
equipment subject to operation over a wide temperature range
having a lower limit of -20°C the improvement which comprises
assembling said equipment with at least one high voltage
electrical insulating component comprising a rigid, closed-
cell polyurethane foam body, the closed cells of which contain
a gaseous foaming agent having dielectric properties, and
selected from the group consisting of sulphur hexafluoride and
halogenated hydrocarbons having boiling points not higher than
-20°C and a dielectric strength greater than that of air,
said electrical equipment being further characterized by the
absence of a casing surrounding said rigid, closed-cell poly-
urethane foam body.
13. The method according to claim 12, wherein said
body is a structural, mechanical load-bearing element.
14. The method according to claim 12 or 13, wherein
the foaming agent consists of at least one member selected from
the group consisting of tetrafluoromethane (CF4), trifluoro-
methane (CHF3), trifluorochloromethane (CF3Cl), hexafluoro-
ethane (C2F6), trifluorobromomethane (CF3Br), difluorochloro-
methane (CHClF2), pentafluorochloroethane (C2ClF5), perfluoro-
propane (C3F8) and difluorodichloromethane (CCl2F2).
15. The method according to claim 12 or 13, in which
the gaseous foaming agent is difluorodichloromethane.

16. The method according to claim 12 or 13, in which
the closed-cells are filled with a mixture of at least two
halogenhydrocarbons at least one of which is a member of the
group consisting of tetrafluoromethane (CF4), trifluoromethane
(CHF3), trifluorochloromethane (CF3Cl), hexafluoroethane (C2F6),
trifluorobromomethane (CF3Br), difluorochloromethane (CHClF2),
pentafluorochloroethane (C2ClF5), perfluoropropane (C3F8),
and difluorodichloromethane (CCl2F2), and at least one of
which is a member of the group consisting of octafluorocyclo-
butane (C4F8), tetrafluorodichloroethane (C2Cl2F4), monofluoro-
dichloromethane (CHCl2F), monofluorotrichloromethane (CCl3F),
and hexafluorodichloropropane (C3Cl2F6) and mixtures of these
compounds.

Description

1045805
~ he present invention relates to foam type composition
for use as an electrical insulating component in conjunction with
high-voltage electrical equipment of various types, more parti-
cularly the invention is concerned with an electrical apparatus
comprising at least one part formed from a foamed composition.
The parts formed from the foam composition are useful
for stationary insulating components of switchgear installations,
for load-bearing parts of various types of electrical equipment
and apparatus designed to operate at low temperatures, for -
partition walls and pin-type insulators, for bus-bar cladding
or supporting frameworks for medium and high voltage circuit-
breakers, for load-bearing insulation of cable terminations,
for current and voltage transformers, for the end-turns of
electrical coils and for the slots of the stator element of a
high-voltage dynamo-electric machine. The improved foam can also
have conductive materials added thereto to establish earth pro-
tection for electrical apparatus.
The foam compositions employed in the invention com-
prise closed-cell rigid foams of expandable polymers.
A principal object of the invention is in general to
satisfy a long-standing need, resulting from the increasing
generation and distribution of electricity, to find simpler and
cheaper means of manufacturing electrical equipment and apparatus ~-
able to withstand very high voltage stresses so that, by dis-
pensing with old established construction methods and pursuing
development in a new direction, the products mentioned can be
; made with appreciable economies in materials, weight and space
requirements, compared with the past.
; The use of suitably designed dense plastic, such as
non-porous epoxide or polyester substances, for high-voltage
insulation is known. It is further known that hard or soft foams,
produced in various ways, have been proposed for the manufacture
~' ' '~
A

1~458~5
of high-voltage insulation. The expanding agents used for
making these foams are such that the cells of the foams are
filled with N2 or C02, for example, in the case of chemical
foaming processes, or with gaseous physical agents in the case
of physical foaming.
The dense plastics materials referred to are relatively
heavy and expensive. Also, foams produced by chemical foaming
processes, the cells of which then contain N2 or CO2, do not
exhibit good dielectric properties. On the other hand, foams
produced by physical expansion, e.g., with monofluorotrichloro-
methane (CC13F), have more favourable dielectric properties, but
the commonly used expanding agent mentioned unfortunately has a
relatively high boiling point of 24C. At comparatively low
temperatures, e.g. - 30C, therefore, the dielectric behaviour
of foams produced in this manner is very poor because the
expanding agent in the cells, otherwise in gaseous form, con- -;
denses at this temperature. The consequent reduction of pres-
sure in the cells impairs the dielectric properties, such as
breakdown strength, and therefore foams of this kind are unsuit- ~ -
able for high-voltage applications~
A more specific object of the invention is to provide
improved electrically insulating foams having a broad thermal
range of application which are able to withstand medium and high
voltages and high mechanical stress and which can be used to
make relatively light weight, reliable electrical components,
equipment or apparatus for high capacities both simply and with
appreciably smaller dimensions than hitherto.
This objective is achieved generally in that the cells
of the insulative rigid foam contain at least one expanding
agent which also acts as a dielectric, has a relatively low
boiling point, or at most -20C, and in addition has a dielectric
strength greater than that of air.
~ ''' ' .
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1g~45~0S
It is propo~ed that the cell walls should be made of
polyurethane.
According to the invention there is provided electrical
apparatus comprising at least one part formed of a high voltage
electrical insulating co~ponent of the invention, as a mechanical
load-bearing high voltage insulation therein.
The high voltage electrical insulating component of
the invention, which is subject to operation over a wide
temperature range having a lower limit of -20C., comprises a
rigid, closed-cell, polyurethane foam body the closed cells of
which contain a gaseous foaming agent having dielectric pro-
perties and selected from the group consisting of sulphur hexa-
fluoride and halogenhydrocarbons having boiling points not -
higher than -20C. and a dielectric strength greater than that
of air.
The invention provides improvements in an electrical
apparatus encountering high electrical voltages and subject to
operation over a wide temperature range having a lower limit
of -20C, in which the improvement comprises the above defined
electrical insulating component.
The invention further provides improvements in the
manufacture of high voltage electrical equipment subject to
operation over a wide temperature range having a lower limit of
-20C, wherein the improvement comprises assembling the equip-
ment with at least one high voltage electrical insulating com,
ponent as defined above.
The apparatus is further characterized in that there
is no casing surrounding the polyurethane foam body.
In one form of the invention the expanding agent can be
. ~" .

1~458Q5
a h~locarbon or a halocarbon mixture. It is thus appropriate
that the expanding agent should be o~ tetra~luoromethan~ (CF4)
or trifluoromethane (CHF3) or tri~luorochloromethane (C~3Cl)
or hexa~luoroethane (C2F6) or trifluorobromomethane (CF3Br) or
difluorochloromethan~ (CHClF2) or penta~luorochloroethane
(C2ClF5) or perfluoropropane (C3F8) or difluorodichloromethane
(CC12F2) or o~ mixtures o~ these compounds. It is also appro-
priate to use sulphur hexafluoride (SF6) as the expanding agent.
In another ~orm o~ the invention at least one expanding
agent can be added which has a boiling point greater than - 20C.
Such additions can be octafluorocyclobutane (C4F8) or tetra~
fluorodichlorethane (C2C1~4) or mono~luorodichloromethane
(CHCl ~) or mono~luorotrichloromethane (CC13F) or hexafluoro-
dichloropropane (C3C1~6) or a mixture of these compouhds.
The rigid foams are produced by means o~ the expanding
agent~ stated above in such a manner that the ~oam incorporates
cells or pores which are at least largely, but in general
completely, closed and remain ~illed with expanding gas or
mixture o~ expanding gases employed.
It is proposed that polyurethane foams should be
expanded with di~luorodichloromethane (CC12F2).
On~ particular advantage o~ the invention is that -
dielectrically ~avourable gases or gas mixtures having a low
boiling point are used for the expanding process, and that these
remain in ~he closed cells o~ the rigid ~oam.
By using expanding agents which also serve as a
dielectric and remain gaseous down to very low temperatures, or
by using mixtures of such dielectrics having a low boiling point
with additions o~ other gases which con~nse at higher temper-
atures, rigid foams made with these dielectrics are particularly
suitable ~or use as high-voltage insulation.
': - . ~ , - -:

1~3458~5
Rigid foams of this kind can be employed in the
manufacture of load-bearing parts o~ electrical equipment and
apparatus which can be used at low temperatures and at the same
tiEe serve as high-~oltage insulation, particularly in the
manu~acture o~ stationary insulating components of switchgear
installations. Deserving special mention are partition walls
or pin-type insulators, busbar cladding or supporting frameworks
~or medium and high-voltage circuit-breakers, or the load-
bearing insulation of cable terminations. The subject o~ the
invention can also be used in the manufacture of insulation ~or
currentand voltage transformers~ and in particular as ~oam
insulation ~or the end-windings and slots of electrical machines.
By incorporating conductive materials in the ~oam this can also
be used to create protective earthing for electrical e~uipment
and apparatus.
Further advantages o~fered by the invention consist
especially in simplifying the design and construction of
existing or ~uture electrical equipment and apparatus. Thus
these can be made smaller and considerably cheaper than conven-
tional steel constructions, ~or service down to low temperatures,
i.e. at least over the whole specified range. Owing to the
functional arrangement o~ indi~idual components or assemblies -~
made in this manner for equipment and apparatus, the space
required is approximately hal~ that required for equivalent
installations at present in existence. Because the weight o~
the product is lower, transport is easier and erection costs are
greatly reduced.
A further important advantage is that with products
incorporating rigid foams o~ the kind described there is no
danger of cracking, which is possible with dense cast plastics,
and hence corona discharge and electrical breakdown.
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