EP0258036A2

EP0258036A2 - Insulated conductor with multi-layer high temperature insulation

Info

Publication number: EP0258036A2
Application number: EP87307504A
Authority: EP
Inventors: Norman Ralph Harlow
Original assignee: Carlisle Corp
Current assignee: Carlisle Corp
Priority date: 1986-08-28
Filing date: 1987-08-25
Publication date: 1988-03-02
Anticipated expiration: 2007-08-25
Also published as: DE3786429D1; DE3786429T2; EP0258036B1; JPS6358709A; EP0258036A3

Abstract

An insulated conductor for high temperature use and methods of manufacture thereof. The conductor (1) is insulated by at least three layers of insulation, the inner layer (2) being made of a perfluoroalkoxy or a polytetrafluoroethylene resin compound, the second layer (3) being made of a polyimide resin compound and the third layer (4) being made of a perfluoroalkoxy, an ethylenetetrafluoroethylene copolymer or a polyvinylbidine fluoride resin compound. The layers are coated or extruded over the conductor and each other and preferably, the first layer is etched before the second layer is applied.

Description

This invention relates to an insulated conductor with a plurality of layers of insulation able to withstand relatively high temperatures and mechanical abuse and to the manufacture thereof.
Insulated conductors which have insulation which is satisfactory for use at relatively high temperatures, i.e. 200° and higher, are desirable for many applications and particularly for installation in aircraft and missiles. One such insulated conductor is described in Military Specification Sheet MIL-W-81381/7E and comprises a plurality of stranded wires plated with silver or nickel and surrounded by two helical layers of fluorocarbon/polyimide tape, the outer layer of tape being coated with an aromatic polyimide resin. The tape may be a film of the type sold by E.I. DuPont de Nemours & Co., Wilmington, Delaware, U.S.A. under the trademark KAPTON, such tape being a thermoset polyimide film combined with a fluorinated ethylene-propylene copolymer film for sealing purposes. The polyimide resin coating is used to provide color identification and to smooth and help protect the tapes with respect to the environment.
While the described insulation conductor will meet a 200°C temperature rating requirement, it has several disadvantages. The main disadvantage is the problem of proper adhesion of the coating to the tape because of its tendency to hydrolize. Other disadvantages are the necessity of winding two layers of opposite hand with proper overlap and the problem of maintaining dimensions at tape splices.
Another insulated conductor has two extruded layers of irradiated, and hence, cross-linked, ethylenetetrafluoroethylene copolymer (ETFE). However, such insulation has had limited success in meeting flammability requirements and is marginal in meeting even a 150°C temperature rating.
While polyimide, both thermoplastic and thermoset, has good high temperature properties, it is difficult to apply it directly to the conductors because of air in the interstices of the conductors and the polyimide has strong adhesion to the conductors making it difficult to strip the insulation. Polyimide layers are also subject to cracking and hydrolyzing unless the cross-linking thereof is substantially perfect, perfection being difficult to obtain under usual manufacturing conditions.
One object of the invention is to provide an insulated conductor which can be temperature rated at at least 200°C.
Another object of the invention is to overcome problems with insulated conductors described hereinbefore.
In accordance with the preferred embodiments of the invention, a first layer of perfluoroalkoxy resin (PFA) compound or a polytetrafluoroethylene resin (PTFE) compound is extruded over and in contact with the conductor or conductors which provides a layer of insulation which can withstand a temperature of at least 250°C and which makes the multi-layer insulation readily strippable from the conductor or conductors. The outer surface of the first layer is etched in a conventional manner, and thereafter, a layer of polyimide in a solvent having a relatively high polyimide content preferably is extruded over the first layer and the solvent is removed by heat to provide a solid second layer of polyimide. Then a third layer, a layer of a PFA compound, an ethylene-tetrafluoreothylene copolymer (ETFE) compound, an ethylene-tetrafluoroethylene copolymer (ETFE) compound or a polyvinylidenefluoride (PVDF) compound, is extruded over the second layer, such third layer protecting the polyimide, second layer from the environment.
Although in the preferred embodiment each of the three layers of insulation is extruded over the conductor, or the conductor with one or more layers of insulation thereon, in an alternative embodiment, one or more, or all, the layers may be applied by conventional coating techniques.
Other objects and advantages of the present invention will be apparent from the following detailed description of the presently preferred embodiments thereof, which description should be considered in conjunction with the accompanying drawings in which:

Figure 1 is a transverse cross-section of the insulated conductor of the invention;
Figure 2 is a partly schematic and partly cross-sectional diagram illustrating the manner in which a polyimide containing lacquer may be extruded over the first layer of the conductor;
Figure 3 is an end view of the die shown in Figure 2;
Figure 4 is a fragmentary, cross-sectional view of a modified form of the die shown in Figure 2; and
Figure 5 is a transverse cross-section of three of the insulated conductors of the invention twisted together and enclosed in a sheath of plastic material.

Figure 1 illustrates an insulated conductor of the invention which comprises a central conductor 1 formed, in this case, by a plurality of copper wires. However, the conductor could be a single wire. The wire or wires are plated with a metal such as tin, silver or nickel. Tin may be used if the insulated conductor is to be rated at 150°C, silver, if the insulated conductor is to be rated at 200°C or nickel, if the insulated conductor is to be rated at above 200°C, e.g. 250°C. Of course, nickel may be used for all such ratings, and silver may be used for all ratings up to 200°C. The temperature rating of the insulated conductor of the invention is determined from the tests described in the Military Specification MIL-W-81381A and the tests referred to therein. The insulated conductor of the invention may be rated at above 200°C, e.g. 250°C, if the wire or wires are coated with nickel. Of course, in some cases, particularly, if it is not necessary to meet a military specification, the plating metal may be omitted.
The conductor 1 is covered by a first layer 2 extruded or coated thereover and in contact therewith. To provide ease of strippability, the layer 2 is a layer of PFA, a thermoplastic, which is extruded or coated over the conductor 1 by conventional wire covering equipment and techniques or is covered by a layer of PTFE which is coated over the conductor 1 or extruded over the conductor 1 in paste form by conventional wire covering equipment and techniques. If the layer 2 is an extruded layer of PFA, the layer 2 is merely cooled after application to the conductor 1, and if the layer 2 is an extruded layer of PTFE, it is heated to cause it to become sintered and thermoset.
After the first layer 2 is set, it is covered by a second layer 3 which is extruded or coated over the layer 2 and is in contact therewith. Preferably, the outer surface of the layer 2 is etched by conventional methods, such as by acid or plasma, before the layer 3 is extruded or coated thereover so as to improve the adhesion between layers 2 and 3.
When second layer 3 is formed by coating of the first layer 2, the coating is applied in a conventional manner, using a lacquer comprising thermosettable polyimide in a known solvent. The lacquer may include other known materials to prevent settling of the polyimide and for other purposes. If the polyimide is a thermoplastic, it can be extruded over the first layer 2 in a conventional manner.
However, in the preferred embodiment, a lacquer which contains at least 20% by weight of polyimide solids, so that the lacquer has a relatively high viscosity, is extruded over the layer 2. In this way, a relatively thick layer of the polyimide, e.g. 1-2 mils in thickness, can be applied to the layer 2 without requiring several passes of the conductor 1 with the layer 2 thereon through a bath of polyimide lacquer.
Figures 2 and 3 illustrate the novel process and apparatus of the invention for extruding a polyimide containing lacquer over an elongated article, such as the conductor 1 covered with the layer 2. However, the process and apparatus of the invention can also be used for covering other articles, such as tubes, rods, wires etc.
As illustrated in Figure 2, the conductor with the layer 2 thereon, designated as 6a, is fed from a reel 7 through an etching bath 8 where the exterior surface of the layer 2 is etched. The insulated conductor 6a is then passed through a bath 9 to remove the etchant and is thereafter dried. The conductor 6a is then passed around a pulley 10 to change its direction to a vertical path so that when the lacquer is applied thereto, it does not sag to one side of the conductor axis and make the layer 3 non-concentric with the axis of the conductor.
From the pulley 10, the conductor 6a passes through the bore of a die insert 11 having an extension 12 and received in a die body 13. The insert 11 has a groove 14 and the body 13 has a groove 15 for receiving a retaining clip (not shown) which retains the insert 11 in the body 13.
As the insulated conductor 6a leaves the extension 12, a layer of the viscous polyimide lacquer is extruded thereover to provide an insulated conductor, designated as 6b, insulated by the layers 2 and 3, and the insulated conductor 6b, is passed through an oven 16, which represents a series of ovens, where the lacquer solvent is driven off and the layer 3 becomes thermoset. The temperature in the ovens 16 is selected based on the boiling point of the solvent and the temperature required for the cross-linking of the polyimide, and the temperature normally increases from the entrance to the oven 16 to the exit from the oven 16. For example, the temperature at the entrance may be 250°C-300°C and the temperature at the exit may be 600°C. The time of transit of the insulated conductor 6b through the oven 16 is selected so as to both remove the solvent and cross-link, or thermoset, the polyimide.
The die, comprising the body 13 and the insert 11, has a cavity 17 around the extension 12 which receives, through the opening 18 in the body 13 (see Figures 2 and 3) the lacquer 23 which forms the layer 3. The lacquer 23 is extruded in tubular form around the insulated conductor 6a by reason of the extension 12, and contracts around the layer 2. By reason of the extension 12, the layer 3 is of uniform thickness around, and concentric with, the axis of the conductor.
The polyimide containing lacquer is supplied under pressure from the metering pump 19 by way of the line 21 and is supplied to the pump 19 from any conventional source by way of the line 22. For cross-linking accelerating purposes, the lacquer which is supplied to the opening 18 may contain a conventional cross-linking agent, such as a acetic anhydride and beta-picoline. Preferably, the cross-linking agent is supplied to the pump 19 as indicated in Figure 2 by another metering pump (not shown). The pressure at which the lacquer is supplied to the opening 18 depends, as is known to those skilled in the art, upon several factors including the viscosity of the lacquer and the speed at which the insulated conductor passes through the die. With a lacquer of the type described hereinafter, the pressure may be on the order of 150-200 psi.
Polyimide lacquers containing at least 15% by weight of the lacquer of thermosettable polyimide solids are commercially available but are usually used for coating purposes. Such lacquers also usually contain suspensoids, anti-oxidants and other materials in minor amounts. The solvent used depends on various factors, but the polyimide solids content and the solvent employed for the lacquer are selected so that the lacquer has a relatively high viscosity. Preferably, the polyimide solids content is at least 25% and the lacquer has a viscosity of at least 200,000 centipoises. One suitable solvent is normal methyl pyrrolidone. A lacquer which has been found to be satisfactory has 25% by weight of the lacquer of thermosettable polyimide solids in such solvent and has a viscosity of about 240,000 centipoises. With such lacquer and the method and apparatus of the invention, a thermoset layer 3 of 1-2 mils in thickness can be obtained with a single pass through the die shown in Figure 2.
Figure 4 illustrates a modified form of the die insert 11 shown in Figures 2 and 3. Although the die of Figures 2 and 3 is preferred because it gives better control of the thickness and concentricity of the layer 3, a die with the insert 11a shown in Figure 4 may be found to be satisfactory.
In the embodiment shown in Figure 4, the die body 13 is the same as the die body shown in Figures 2 and 3, and the insert 11a is substantially the same as the insert 11, except for the extension 12a which is shorter than the extension 12 or which may be omitted entirely. With the insert 11a, the lacquer 23 impinges directly on the insulated conductor 6a within the cavity 17. With irregularities in the flow of the lacquer 23, the insulated conductor 6a may be displaced radially by the lacquer 23 causing the layer 3 to be not concentric with the axis of the conductor at various points in the direction of the axis of the conductor which, however, can be acceptable for some end uses for the insulated conductor.
Although it is preferred to cause the polyimide to become thermoset with heating and a cross-linking agent since the layer 3 is heated in the oven 16 to remove the solvent and heating in the oven 16 can cause both removal of the solvent and thermosetting of the polyimide, it will be apparent that the thermosetting of the polyimide can be accomplished by other conventional methods. Thus, the cross-linking agent can be omitted, and the thermosetting of the polyimide can be caused by heat alone or by irradiation of the layer 3.
After the layer 3 of polyimide lacquer is extruded or coated over the layer 2, the layer 3 is heated in a known manner to remove the solvent and to cause the material of the layer 3 to become thermoset.
After the layer 3 is set, a third layer 4 is extruded or coated over the layer 3. The third layer 4 can be a plastic material which will withstand the temperature to which the insulated conductor is subjected and which will protect the layer 3 from the environment. It is preferred that the third layer 4 comprise PFA, ETFE or PVDF, but for a temperature rating of 200°C and higher, it is preferred that the third layer 4 comprise PFA which is extruded or coated over the layer 3 in a conventional manner. When ETFE or PVDF are used, they are extruded or coated over the layer 3 in a conventional manner, and if desired, when layer 4 comprises PVDF, the layer 4 may be cross-linked by radiation.
The radial thicknesses of the layers 2, 3 and 4 may be selected so as to provide the desired radial thickness of insulation. For example, if the desired total thickness is 7 mils, the layers 2 and 4 may have thicknesses of 3 mils and the layer 3 may have a thickness of 1 mil. It is not necessary that the layers 2 and 4 have the same thickness, but normally the thicknesses of the layers 2 and 4 will be greater than the thickness of the layer 3, e.g., up to three times or more of the thickness of the layer 3.
An insulated conductor made as described hereinbefore will have a temperature rating of at least 150°C, and if made with a layer 4 of PFA, will have a temperature rating in excess of 200°C.
It will be observed that in the preferred embodiment, each layer of insulation is applied by extrusion which permits close control of the dimensions and concentricity of the insulation and eliminates multiple passes of the conductor through a liquid in obtaining any of the layers. Furthermore, no wrapping or splicing of tapes is required. In addition, the application of the polyimide resin directly to the conductor, and its attendant problems, is eliminated, and the problems of cracking, separating and hydrolizing of the polyimide layer are avoided or overcome.
The various layers of insulating material may be applied while the conductor is advanced lengthwise using conventional extrusion equipment which is used to extrude insulation over a conductor. Thus, while the conductor is advanced lengthwise, the first layer is extruded thereover. When the first layer is set and while the conductor with the first layer thereon is advanced lengthwise, the second layer is extruded thereover. When the second layer is set, the conductor with the first and second layers thereon are advanced lengthwise, and the third layer is extruded thereover. Each layer may be applied individually, i.e. with storage of the conductor on a reel after each layer is applied and subsequent feeding of the covered conductor from the reel to the apparatus for applying the next layer, but with proper spacing of extruders and the selection of appropriate treatment conditions intermediate the extruders, an insulated conductor of the invention may be manufactured in a continuous operation with the conductor being fed into one end of the production line and with the complete insulated conductor exiting from the other end of the line.
The insulated conductor of the invention may be used by itself or with a further insulating layer or a conductive sheath therearound. A plurality of such conductors may be stranded to form twisted pairs or may be assembled in a bundle covered by a protective sheath. Also, several insulated conductors may be placed in side-by-side relation and be bonded together at their contacting surfaces.
Fig. 5 illustrates three insulated conductors 6 of the invention stranded together and surrounded by a sheath 20 of plastic material. The plastic material for the sheath is selected to provide the desired temperature rating, as described hereinbefore, and for example, if the temperature rating is to be 200°C or higher, the plastic material can be PFA.
In the various embodiments, the plastic materials may have conventional fillers which do not materially affect the desired properties of the insulation of the cable in an adverse manner.
Although preferred embodiments of the present invention have been described and illustrated, it will be apparent to those skilled in the art that various modifications may be made without departing from the principles of the invention.

Claims

1. An insulated conductor insulated by a plurality of superimposed layers of plastic insulation and having a temperature rating of at least 150°C, characterised by said conductor (1) having a first layer (2) of plastic insulation material therearound and in contact therewith, said plastic insulation material comprising a plastic selected from perfluoroalkoxy and polytetrafluoroethylene resins, having a second layer (3) comprising polyimide resin around said first layer (2) and having a third layer (4) of plastic insulation material around said second layer (3), the last mentioned said plastic insulation material comprising a plastic selected from perfluoroalkoxy, ehtylene-tetrafluoroethylene copolymer and polyvinylidine fluoride resins.

2. An insulated conductor as claimed in claim 1 characterised in that said plastic insulation material of said first layer (2) and of said third layer (4) is perfluoroalkoxy resin and said insulated conductor has a temperature rating of at least 200°C.

3. An insulated conductor as claimed in claim 1 or claim 2 characterised in that said second layer (3) is in contact with said first layer (2) and said third layer (4) is in contact with said second layer (3).

4. An insulated conductor as claimed in any of claims 1 to 3 characterised by the radial thicknesses of said first layer (2) and said third layer (4) being greater than the radial thickness of said second layer (3).

5. An insulated conductor as claimed in any preceding claim characterised by the polyimide resin of said second layer being thermoset.

6. An insulated conductor as claimed in any preceding claim characterised in that at least one of said layers is thermoplastic.

7. An insulated conductor as claimed in any preceding claim characterised in that said second layer is an extruded layer.

8. An insulated conductor as claimed in any of claims 1 to 6 characterised in that at least one of said layers is an extruded layer.

9. An insulated conductor as claimed in any of claims 1 to 6 characterised in that each of said first layer, said second layer and said third layer is an extruded layer.

10. An insulated conductor as claimed in claim 1 characterised in that at least one of said layers is a coated layer.

11. A method of making an insulated conductor having a temperature rating of at least 150°C, said method being characterised by:
while advancing said conductor (1) lengthwise thereof, applying a first layer (2) of plastic insulation material therearound and in contact therewith, said plastic insulation material being selected from compounds of perfluoroalkoxy and of polytetrafluoroethylene;
while advancing said conductor (1) lengthwise with said first layer (2) therearound, applying a second layer (3) of plastic insulation material around said first layer (2), the last-mentioned said insulation material being a polyimide compound; and
while advancing said conductor (1) lengthwise with said first layer (2) and said second layer (3) therearound, applying a third layer (4) of plastic insulation material therearound, the last-mentioned said plastic material being selected from compounds of perfluoroalkoxy, of ethylene-tetrafluoroethylene copolymer and of polyvinylidine fluoride resins.

12. A method as claimed in claim 11 characterised in that the outer surface of said first layer (2) is etched prior to extruding said second layer (3) therearound.

13. A method as claimed in claim 11 characterised in that said second layer (3) is applied by extruding a viscous lacquer around said first layer (2), said lacquer containing a solvent and polyimide, the weight content of the polyimide solids in the lacquer being at least 20% and wherein said solvent is removed from said second layer with heating thereof.

14. A method as claimed in claim 13 characterised in that said polyimide in said lacquer is thermoset during the heating of said second layer (3) to remove said solvent.

15. A method as claimed in claim 13 characterised in that the plastic material of said first layer is a polytetrafluoroethylene resin compound and is extruded over said conductor in paste form and wherein said first layer is heated to cause the polytetrafluoroethylene resin compound to become thermoset prior to extruding said second layer thereover.

16. A method as claimed in claim 15 characterised in that said lacquer is heated, during the heating of said second layer to remove said solvent, to a temperature and for a time which causes said polyimide to become thermoset.

17. A method as claimed in any of claims 13 to 16 characterised in that the plastic material of said third layer is a perfluoroalkoxy compound.

18. A method as claimed in claim 13 characterised in that the plastic material of said first layer and said third layer is a perfluoroalkoxy compound.

19. A method of applying a layer of polyimide around an elongated article (6a), said method being characterised by
advancing said article (6a) lengthwise through a die (13) having a passageway therethrough for the passage of said article through said die, said passageway having an inlet end and an outlet end and said die having a cavity (17) around said passageway and adjacent the outlet and thereof for receiving a polyimide containing lacquer under pressure;
while said article is advanced through said passageway, supplying a viscous lacquer comprising a solvent and at least 15% by weight of the lacquer of thermosettable polyimide solids to said cavity under a pressure sufficient to cause said lacquer to flow around said article from said cavity (17); and
as said article with said lacquer therearound is advanced, treating said article with said lacquer therearound to remove said solvent and so as to cause the polyimide to become thermoset.

20. A method as claimed in claim 19 characterised in that said article with said lacquer therearound is advanced along a vertical path.

21. A method as claimed in claim 19 or claim 20 wherein said die has a portion intermediate said cavity and said passageway which extends to said outlet end of said passageway and said die has a further passageway around said p[ortion and extending from said cavity to adjacent said outlet end of the first-mentioned said passageway and wherein said lacquer is caused to flow through said further passageway and around said article as it emerges from said outlet end of the first-mentioned said passageway.

22. A method as claimed in claim 19 characterised in that said passageway has its outlet end within said cavity and said cavity has an opening which is spaced from said outlet opening of said passageway in the direction of advance of said article and which opens outwardly of said die and wherein said lacquer is caused to flow around said article within said cavity.

23. A method as claimed in any of claims 19 to 22 characterised in that the content of polyimide solids in said lacquer is at least 25% and said lacquer has a vixcosity of at least 200,000 centipoises.

24. A method as claimed in any of claims 19 to 23 characterised in that said article with said lacquer therearound is heated at a temperature and for a time sufficient to remove said solvent and to cause the polyimide to become thermoset.

25. A method as claimed in claim 24 characterised in that said lacquer contains a cross-linking agent.