INSULATED CONDUCTOR WITH MULTI-LAYER HIGH TEMPERATURE INSULATION

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°C 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. An example of a layered insulation using EFTE or polyimide tape as inner layer of insulation around a conductor, is EP-A-56510. 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. According to the invention there is provided an insulated conductor in which the insulation comprises a first layer which envelops and is operatively connected to the conductor, a second layer which envelops and is operatively connected to the first layer, and a third layer which envelops and is operatively connected to the second layer, and the insulation having a temperature rating greater than 150°C, characterised in that the first layer comprises a perfluoroalkoxy resin, the second layer comprises a polyimide resin, the third layer comprises a plastic selected from the group consisting of perfluoroalkoxy and ethylenetetrafluoroethylene copolymer resins, and the first layer not comprising tape. 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. Preferred embodiments of the invention will now be described with reference to 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 the polyimide may be extruded over the first layer of the conductor when in a lacquer; 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. The layer 2 is allowed to cool after application to the conductor 1. 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 a novel process and apparatus for use in producing the insulated conductor according to 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 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, such as PFA, EFTE, 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 is used, it is extruded or coated over the layer 3 in a conventional manner. 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 is 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.

Claims: Insulation of an insulated conductor in which the insulation comprises a first layer (2) which envelops and is operatively connected to the conductor (1), a second layer (3) which envelops and is operatively connected to the first layer (2), and a third layer (4) which envelops and is operatively connected to the second layer (3), and the insulation having a temperature rating greater than 150°C, characterised in that the first layer (2) comprises a perfluoroalkoxy resin, the second layer (3) comprises a polyimide resin, the third layer (4) comprises a plastic selected from the group consisting of perfluoroalkoxy and ethylenetetrafluoroethylene copolymer resins, and the first layer not comprising tape. Insulation as claimed in claim 1 wherein, said plastic insulation material of the first layer (2) and of the third layer (4) is perfluoroalkoxy resin, and insulation has a temperature rating of at least 200°C. Insulation as claimed in claim 1 or claim 2, wherein the second layer (3) is in contact with said first layer (2) and said third layer (4) is in contact with said second layer (3). Insulation as claimed in any of claims 1 to 3, wherein the radial thicknesses of the first layer (2) and the third layer (4) are greater than the radial thickness of said second layer (3). Insulation as claimed in any preceding claim, wherein the polyimide resin of said second layer is thermoset. Insulation as claimed in any preceding claim, wherein at least one of said layers is thermoplastic. Insulation as claimed in any preceding claim, wherein the second layer is an extruded layer. Insulation as claimed in any of claims 1 to 6, wherein at least one of the layers is an extruded layer. Insulation as claimed in any of claims 1 to 6, wherein each of the layers, is an extruded layer. Insulation as claimed in claim 1, wherein at least one of the layers is a coated layer. A method of making insulation for a conductor according to any one of claims 1 to 10, said method being characterised by: while advancing said conductor (1) lengthwise thereof, applying said first layer (2) of perfluoralkoxy resin therearound and in contact therewith; while advancing said conductor (1) lengthwise with said first layer (2) therearound, applying said second layer (3) of a polyimide resin around said first layer (2); and while advancing said conductor (1) lengthwise with said first layer (2) and said second layer (3) therearound, applying said third layer (4), of a plastic selected from the group of perfluoralkoxy and ethylenetetrafluoroethylene copolymer resins therearound. A method as claimed in claim 11, wherein the outer surface of the first layer (2) is etched prior to extruding the second layer (3) therearound. A method as claimed in claim 11, wherein the second layer (3) is applied by extruding a viscous lacquer around the first layer (2), the 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 the second layer with heating thereof. A method as claimed in claim 13, wherein the polyimide in the lacquer is thermoset during the heating of the second layer (3) to remove the solvent. A method as claimed in any one of claims 13 or 14, wherein the plastic material of the third layer is a perfluoroalkoxy compound. A method as claimed in claim 13, wherein the plastic material of the first layer and the third layer is a perfluoroalkoxy compound.

Inventors: HARLOW, NORMAN RALPH
Applicants: CARLISLE CORPORATION
Application year: 1987
Main classification: H01B7
IPC codes: H01B7, H01B13