CA1051191A - Hard foam material as well as its use with electrical equipment and apparatus - Google Patents
Hard foam material as well as its use with electrical equipment and apparatusInfo
- Publication number
- CA1051191A CA1051191A CA202,587A CA202587A CA1051191A CA 1051191 A CA1051191 A CA 1051191A CA 202587 A CA202587 A CA 202587A CA 1051191 A CA1051191 A CA 1051191A
- Authority
- CA
- Canada
- Prior art keywords
- foam
- constitute
- group
- electrical
- insulation
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
- C08J9/00—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof
- C08J9/04—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof using blowing gases generated by a previously added blowing agent
- C08J9/12—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof using blowing gases generated by a previously added blowing agent by a physical blowing agent
Landscapes
- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Materials Engineering (AREA)
- Health & Medical Sciences (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Medicinal Chemistry (AREA)
- Polymers & Plastics (AREA)
- Organic Chemistry (AREA)
- Manufacture Of Porous Articles, And Recovery And Treatment Of Waste Products (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
An electrical apparatus that comprises at least one part formed of a rigid polymeric foam which has an integral outer skin, and which is non-cellular. The cells of the foam contain at least one expanding agent which also acts as a dielectric and which has a dielectric strength greater than that of air. The foam also acts as a mechanical load-bearing high-voltage insulation.
An electrical apparatus that comprises at least one part formed of a rigid polymeric foam which has an integral outer skin, and which is non-cellular. The cells of the foam contain at least one expanding agent which also acts as a dielectric and which has a dielectric strength greater than that of air. The foam also acts as a mechanical load-bearing high-voltage insulation.
Description
-' ~ 5~L~91 B~C~Gl~Ol~ND OF T~I13 INV13NTJON
: . ' . : :' Field of the ~vention ~ ' __ , . .
'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 . ' ' ' ' ': ~ ,"
' ., . ' , ., .
... .. . .,. . .,.'.. . .' . '.
,.- ~,. .
:
~05~L191 ~ - ~
. 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.
~., . ''' , " -.
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~
_3_ -.
, . , . ., ~L~51~
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.
.~ .
~1 l . 1 ' ' ~105119~
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 _ ~5~
'.~ . .
- , ,,,, ., ,:,.,,.. .. ,,,.. . ,... -,. - - ~
`. !~` ' ` ~ I
iO5119~
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 _... . ....__ _..., .............
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 :~ _ -6 ,~, . I . .
. .
V, ' ,r ' ' ' '' ' ' ' ,, ,,~, , ,.p 1~5119~ ::~
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 .
,. , .,.'' ' ',' ,' , "''~'^'''. ~' . . ' ' . : -~
i '. . , ' ,' .','' ' . ~:
. , '' " ~ .
:
- . ' .
.i I
~ ~ i
: . ' . : :' Field of the ~vention ~ ' __ , . .
'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 . ' ' ' ' ': ~ ,"
' ., . ' , ., .
... .. . .,. . .,.'.. . .' . '.
,.- ~,. .
:
~05~L191 ~ - ~
. 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.
~., . ''' , " -.
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~
_3_ -.
, . , . ., ~L~51~
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.
.~ .
~1 l . 1 ' ' ~105119~
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 _ ~5~
'.~ . .
- , ,,,, ., ,:,.,,.. .. ,,,.. . ,... -,. - - ~
`. !~` ' ` ~ I
iO5119~
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 _... . ....__ _..., .............
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 :~ _ -6 ,~, . I . .
. .
V, ' ,r ' ' ' '' ' ' ' ,, ,,~, , ,.p 1~5119~ ::~
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 .
,. , .,.'' ' ',' ,' , "''~'^'''. ~' . . ' ' . : -~
i '. . , ' ,' .','' ' . ~:
. , '' " ~ .
:
- . ' .
.i I
~ ~ i
Claims (11)
1. Electrical apparatus comprising at least one part formed of a rigid polymeric foam selected from the group con-sisting of foamed polyurethane and foamed epoxy resin, said foam having an integral outer skin which is non-cellular, the cells of the foam containing at least one expanding agent selected from the group consisting of halocarbon, halohydrocarbon, sulfohalide and mixtures thereof, said agent also acting as a dielectric and having a dielectric strength greater than that of air, said foam acting as mechanical load-bearing high-voltage insulation therein.
2. Apparatus according to Claim 1, which is a switchgear installation in which said part(s) constitute stationary insul-ating component(s).
3. Apparatus according to Claim 2 in which said part(s) constitute electrical apparatus component(s) selected from the group consisting of a partition wall, a rod insulator, a busbar cladding, a supporting framework for a medium-voltage circuit breaker, a supporting framework for a high-voltage circuit breaker, and a load-bearing insulation for a cable termination.
4. Apparatus according to Claim 1, which is an electrical transformer in which said part(s) constitute insulation.
5. Apparatus according to Claim 1, which is an electrical machine in which said part(s) constitute end-winding or slot insulation.
6. Apparatus according to Claim 1, in which said part (s) constitute the stress-bearing parts of electrical equipment and apparatus.
7. Apparatus according to Claim 1, in which said part (s) constitute electrical apparatus component(s) selected from the group consisting of separators, stand-offs, and jackets of busbars, and supports for medium-voltage switched, supports fox high-voltage switches, and s-tress bearing insulation of cable terminations.
8. Apparatus according to Claim 1, in which said part (s) constitute protector grounds for electrical equipment and apparatus.
9. Apparatus according to Claim 1, Claim 2 or Claim 3 wherein said expanding agent is selected from the group consisting of CC13F, CF4, CHF3, CF3C1, C2F6, CF3Br, CHC1F2, C2C1F5, C3F8, CC12F2, and SF6, and mixtures thereof.
10. Apparatus according to Claim 4, Claim 5 or Claim 6 wherein said expanding agent is selected from the group consisting of CC13F, CF4, CHF3, CF3C1, C2F6, CF3Br, CHC1F2, C2C1F5, C3F8, CC12F2, and SF6, and mixtures thereof.
11. Apparatus according to Claim 7 or Claim 8 wherein said expanding agent is selected from the group consisting of CC13F, CF4, CHF3, CF3Cl, C2F6, CF3Br, CHC1F2, C2C1F5, C3F8, CC12F2, and SF6, and mixtures thereof.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CH870773A CH579606A5 (en) | 1973-06-18 | 1973-06-18 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1051191A true CA1051191A (en) | 1979-03-27 |
Family
ID=4343787
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA202,587A Expired CA1051191A (en) | 1973-06-18 | 1974-06-17 | Hard foam material as well as its use with electrical equipment and apparatus |
Country Status (7)
| Country | Link |
|---|---|
| CA (1) | CA1051191A (en) |
| CH (1) | CH579606A5 (en) |
| DE (2) | DE7328613U (en) |
| ES (1) | ES427350A1 (en) |
| FR (1) | FR2233356B1 (en) |
| GB (1) | GB1476371A (en) |
| IT (1) | IT1017653B (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2018192990A1 (en) | 2017-04-19 | 2018-10-25 | Covestro Deutschland Ag | A rigid poiyu rethane foam, preparation method and application thereof |
Families Citing this family (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS5482977U (en) * | 1977-11-25 | 1979-06-12 | ||
| US4560829A (en) * | 1983-07-12 | 1985-12-24 | Reed Donald A | Foamed fluoropolymer articles having low loss at microwave frequencies and a process for their manufacture |
| US4894488A (en) * | 1988-03-21 | 1990-01-16 | Comm/Scope, Inc. | High frequency signal cable with improved electrical dissipation factor and method of producing same |
| DE3819630A1 (en) * | 1988-06-09 | 1989-12-14 | Bayer Ag | PROCESS FOR INTRODUCING DRAGENTS IN AT LEAST ONE OF THE FLOW -ABLE REACTION COMPONENTS FOR THE PREPARATION OF POLYURETHANE FOAMS |
| DE4013712A1 (en) * | 1990-04-28 | 1991-10-31 | Bayer Ag | Environmentally-friendly mfr. of foam-backed plastics parts - using polyurethane lacquer to protect plastic from partly-halogenated blowing agent used in polyurethane foam backing mixt. |
| DE19644483C1 (en) * | 1996-10-25 | 1997-11-20 | Siemens Ag | High voltage insulator for high voltage lines |
| DE102005000161A1 (en) * | 2005-11-21 | 2007-05-24 | Hilti Ag | Hand tool with foamed wiring |
| CN117903487B (en) * | 2024-03-20 | 2024-05-28 | 西安交通大学 | SF6Epoxy resin microporous foam insulating material and preparation method thereof |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE88126C (en) * | ||||
| CH523135A (en) * | 1970-04-24 | 1972-05-31 | Ite Imperial Corp | Foamed plastic bodies - esp insulators with sulphur hexafluoride filled pores |
| CH572269A5 (en) * | 1972-07-20 | 1976-01-30 | Bbc Brown Boveri & Cie |
-
1973
- 1973-06-18 CH CH870773A patent/CH579606A5/xx not_active IP Right Cessation
- 1973-08-04 DE DE19737328613 patent/DE7328613U/en not_active Expired
- 1973-08-04 DE DE19732339621 patent/DE2339621A1/en not_active Ceased
-
1974
- 1974-06-17 FR FR7420896A patent/FR2233356B1/fr not_active Expired
- 1974-06-17 GB GB2666874A patent/GB1476371A/en not_active Expired
- 1974-06-17 ES ES427350A patent/ES427350A1/en not_active Expired
- 1974-06-17 CA CA202,587A patent/CA1051191A/en not_active Expired
- 1974-06-18 IT IT2407474A patent/IT1017653B/en active
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2018192990A1 (en) | 2017-04-19 | 2018-10-25 | Covestro Deutschland Ag | A rigid poiyu rethane foam, preparation method and application thereof |
Also Published As
| Publication number | Publication date |
|---|---|
| IT1017653B (en) | 1977-08-10 |
| FR2233356B1 (en) | 1978-12-29 |
| ES427350A1 (en) | 1976-09-16 |
| CH579606A5 (en) | 1976-09-15 |
| DE7328613U (en) | 1975-08-28 |
| GB1476371A (en) | 1977-06-10 |
| FR2233356A1 (en) | 1975-01-10 |
| DE2339621A1 (en) | 1975-01-09 |
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