DE3786429T2 - Insulated cable with high-temperature resistant multi-layer insulation. - Google Patents

Description

This invention is directed to an insulated conductor having a plurality of insulation layers that can withstand relatively high temperatures and mechanical overload, and to the manufacture thereof.

Insulated conductors having insulation satisfactory for use at relatively high temperatures, i.e. 200ºC 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 spiral layers of a fluorocarbon/polyimide tape, the outer layer of the tape being coated with an aromatic polyimide resin. The tape may be a film of the type available from E.I. DuPont de Nemours & Co., Wilrnington, Delaware, U.S.A. under the trademark KAPTON, such tape being a thermosetting polyimide film combined with a fluorinated ethylene-propylene copolymer film for sealing purposes. The polyimide resin coating is used for color identification and to smooth the tapes and to help protect them from environmental influences.

While the insulated conductor described meets the requirements of a 200ºC temperature rating, it has several disadvantages. The main disadvantage is the problem of proper adhesion of the coating to the tape due to its tendency to hydrolyse. Other disadvantages are the need to wind two layers with opposite winding directions with precise overlap and the problem of maintaining dimensions at tape joints.

Another insulated conductor has two extruded layers of radiation-crosslinked ethylene tetrafluoroethylene copolymer (ETFE). However, insulation of this type has shown limited success in flammability requirements and are limited when required to meet a temperature design as low as 150ºC. EP-A-56510 is an example of a layered insulation that uses an EHE or polyimide tape as an inner insulating layer around a conductor.

While both thermoplastic and thermosetting polyimide have good high temperature properties, it is difficult to apply them directly to the conductors because of the presence of air in the spaces between the conductors and because the polyimide has a strong bond to the conductors, making it difficult to strip the insulation. Polyimide layers can also crack and hydrolyze if their cross-linking is not substantially perfect, and perfection is difficult to achieve under normal manufacturing conditions.

It is an object of the invention to provide an insulated conductor, the temperature design of which is at least 200ºC.

Another object of the invention is to solve the problems with the insulated conductors described above.

To this end, the invention provides 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 the first layer and is operatively connected to it, and a third layer which envelops the second layer and is operatively connected to it, and in which the insulation has a temperature rating of more than 150°C, characterized in that the first layer contains a perfluoroalkoxy resin, the second layer contains a polyimide resin, the third layer contains a plastic material selected from the group consisting of perfluoroalkoxy and ethylenetetrafluoroethylene copolymer resins, and the first layer does not contain a tape.

Although in the preferred embodiment each of the three insulation layers is provided over the conductor, or over the conductor with one or more insulation layers is extruded thereon, in an alternative embodiment one or more or all layers may be applied by conventional coating techniques.

The invention is described in more detail below using preferred embodiments in conjunction with the drawing.

Fig. 1 is a cross-section through the insulated conductor according to the invention;

Fig. 2 is a diagram, partly schematic and partly cross-sectional, showing the manner in which the polyimide can be extruded over the first layer of the conductor when contained in a varnish;

Fig. 3 is a side view of the nozzle shown in Fig. 2;

Fig. 4 is a broken-away cross-sectional view of a modified form of the nozzle shown in Fig. 2; and

Fig. 5 shows a cross section through three of the insulated conductors according to the invention, which are twisted together and surrounded by a sheath made of plastic material.

Fig. 1 shows an insulated conductor according to the invention having a central conductor 1 formed in this case by a plurality of copper wires. However, the conductor could also be a single wire. The wire or wires are plated with a metal such as tin, silver or nickel. Tin can be used if the design of the insulated conductor is to be at 150°C, silver if the design is to be at 200°C or nickel if the design is to be above 200°C, e.g. 250°C. Of course, nickel can be used for all designs and silver can be used for all designs up to 200°C. The temperature design of the insulated conductor according to the invention is determined by the tests described in the "Military Specification MIL-W-81381A" and by the tests referred to therein. The insulated conductor according to the invention can be used for above 200°C, eg 250ºC if the wire or wires are nickel plated. Of course, in some cases, particularly where no military specifications need to be met, the plating metal may be omitted.

The conductor 1 is coated with a first layer 2 which is extruded onto it or coated in contact with it. To facilitate stripping, the layer 2 is a layer of PFA, a thermoplastic, which is extruded or coated onto the conductor 1 by conventional wire coating equipment and techniques. The layer 2 is allowed to cool after being applied to the conductor 1.

After the first layer 2 has been applied, it is covered by a second layer 3 which is extruded over or drawn into contact with the layer 2. Preferably, the outer surface of the layer 2 is etched by conventional methods, such as acid or plasma, before the layer 3 is extruded or drawn over it, so as to improve the adhesion between the layers 2 and 3.

If the second layer 3 is formed by coating the first layer 2, the coating is applied in a conventional manner using a varnish containing thermally curable polyimides in a known solvent. The varnish may also contain other known materials to prevent settling of the polyimide and for other purposes. If the polyimide is a thermoplastic, it may be extruded over the first layer 2 in a conventional manner.

In the preferred embodiment, however, a varnish containing at least 20% by weight of polyimide solids, so that the varnish has a relatively high viscosity, is extruded over layer 2. In this way, a relatively thick polyimide layer, e.g., 1-2 mils thick, can be applied to layer 2 without having to repeatedly pull conductor 1 with layer 2 thereon through a bath of polyimide varnish.

Figures 2 and 3 show a novel method and apparatus for use in the production of the insulated conductor of the invention for extruding a varnish containing polyimide over an elongated article such as the conductor 1 coated with layer 2. However, the method and apparatus can also be used for coating other articles such as pipes, rods, cables, etc.

As shown in Fig. 2, the conductor indicated at 6a with the layer 2 thereon is fed from a reel 7 through an etching bath 8 in which the outer surface of the layer 2 is etched. The insulated conductor 6a is then drawn through a bath 9 to remove the etching liquid and then dried. The conductor 6a then passes a deflection roller 10 so that its direction is changed to a vertical path so that when the varnish is applied to it it does not sag to one side of the conductor axis and cause the layer 3 to be concentric with the conductor axis.

From the pulley 10, the conductor 6a runs through the bore of a nozzle insert 11, which has an extension 12 and is received by a nozzle body 13. The insert 11 has a slot 14 and the body 13 has a slot 15 for receiving a retaining clip (not shown) which holds the insert 11 in the body 13.

As the insulated conductor 6a exits the extension 12, a layer of the viscous polyimide varnish is extruded over it to provide an insulated conductor designated 6b which is insulated by layers 2 and 3, and the insulated conductor 6b passes through an oven 16 which is a series of ovens where the varnish solvent is driven off and layer 3 is thermally cured. The temperature in the ovens 16 is selected based on the boiling point of the solvent and on the temperature required to crosslink the polyimide, and the temperature normally increases between the entrance to the oven 16 and 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 transit time of the insulated conductor 6b through the oven 16 is chosen so that both the solvent is removed and the polyimide is crosslinked or thermally cured.

The nozzle, which comprises the body 13 and the insert 11, has a cavity 17 around the extension 12 which, through the opening 18 in the body 13 (see Figs. 2 and 3), receives the lacquer 23 which forms the layer 3. The lacquer 23 is extruded through the extension 12 in a tubular manner around the insulated conductor 6a and contracts around the layer 2. Due to the extension 12, the layer 3 has a uniform thickness around the conductor axis and is concentric therewith.

The polyimide-containing paint is supplied under pressure from metering pump 19 via line 21 and is supplied to pump 19 from any conventional source via line 22. For purposes of accelerating the crosslinking process, the paint supplied to orifice 18 may contain a conventional crosslinking agent such as acetic anhydride and beta-picolin. Preferably, the crosslinking agent is supplied to pump 19 via another metering pump (not shown) as indicated in Figure 2. The pressure at which the paint is supplied to orifice 18 depends on various factors, including the viscosity of the paint and the speed at which the insulated conductor passes through the nozzle, as is known to those skilled in the art. For a paint as described below, the pressure may be on the order of 150-200 psi.

Polyimide varnishes containing at least 15% by weight of the varnish of thermally curable polyimide solids are commercially available but are usually used for coating purposes. Such varnishes usually also contain suspensing agents, antioxidants and other materials in small amounts. The solvent used depends on various factors, but the solids content of the polyimide and the solvent used for the varnish are selected so that the varnish has a relatively high viscosity. Preferably the solids content of the polyimide is at least 25% and the varnish has a viscosity of at least 200,000 centipoise. A suitable solvent is normal methylpyrrolidone. A varnish which has been found to be satisfactory contains 25% by weight of the varnish of thermally curable polyimide solids in such a solvent and has a viscosity of about 240,000 centipoise. With such a varnish and the method and apparatus of the present invention, a thermosetting layer having a thickness of 1-2 mils can be achieved with a single pass through the die shown in Fig. 2.

Fig. 4 shows a modified form of the nozzle insert 11 shown in Figures 2 and 3. Although the nozzle according to Figures 2 and 3 is preferred because it allows better control of the thickness and concentricity of the layer 3, a nozzle with the insert 11a shown in Figure 4 can be considered satisfactory.

In the embodiment shown in Fig. 4, the nozzle body 13 is identical to the nozzle body shown in Figs. 2 and 3, and the insert 11a is essentially identical to the insert 11, with the exception of the extension 12a, which is shorter than the extension 12, but which can also be omitted entirely. Through the insert 11a, the paint 23 directly impinges on the insulated conductor 6a within the cavity 17. If the paint 23 flows irregularly, the insulated conductor 6a can be radially displaced by the paint 23, which leads to the layer 3 not being concentric with the conductor axis at various points in the direction of the conductor axis, which can, however, be accepted for some end inserts for the insulated conductor.

Although it is preferred that the polyimide be thermally cured by heating and a cross-linking agent, since the layer 3 is heated in the oven 16 to remove the solvent and the heating in the oven 16 can both remove the solvent and thermally cure the polyimide, it is obvious that the curing of the polyimide can be carried out by other conventional methods. Thus, the cross-linking agent can be omitted and the thermal curing of the polyimide can be accomplished by heat alone or by irradiating the layer 3.

After layer 3 of the polyimide lacquer is extruded or drawn over layer 2, layer 3 is heated in a known manner to remove the solvent and to thermally cure the material of layer 3.

After layer 3 has cured, a fourth layer 4 is extruded or drawn over layer 3. The fourth layer 4 may be a plastic material which can withstand the temperature to which the insulated conductor is exposed and which protects layer 3 from environmental influences, such as PFA, EHE, but for a temperature rating of 200ºC and higher it is preferred that the third layer 4 contains PFA which is extruded or drawn over layer 3 in a conventional manner. If ETFE is used it is extruded or drawn over layer 3 in a conventional manner.

The radial thicknesses of layers 2, 3 and 4 can be selected to provide the desired radial thickness of insulation. For example, if the desired total thickness is 7 mils, layers 2 and 4 can have thicknesses of 3 mils and layer 3 can have a thickness of 1 mil. It is not necessary that layers 2 and 4 be the same thickness, but typically the thicknesses of layers 2 and 4 are greater than the thickness of layer 3, e.g. up to three times or more the thickness of layer 3.

An insulated conductor manufactured as described above will have a temperature rating of at least 150ºC and when manufactured with a layer 4 of PFA will have a temperature rating of well over 200ºC.

It will be noted that in the preferred embodiment, each insulation layer is applied by extrusion, which allows fine control of the dimensions and concentricity of the insulation and eliminates the need to pass the conductor through a liquid several times to obtain each of the layers. In addition, the tape does not need to be wrapped or glued. Furthermore, the application of the polyimide resin directly on the conductor and its accompanying problems are eliminated, and the problems of cracking, peeling and hydrolysis of the polyimide varnish are avoided or solved.

The various layers of insulation material can be applied as the conductor is advanced lengthwise using conventional extrusion equipment used to extrude insulation onto a conductor. Thus, the first layer is extruded onto the conductor as it is advanced lengthwise. When the first layer has cured, and as the conductor is advanced lengthwise with the first layer thereon, the second layer is extruded onto it. When the second layer has cured, the conductor is advanced lengthwise with the first and second layers thereon, and the third layer is extruded onto it. Each layer can be applied individually, i.e. by storing the conductor on a reel after each layer is applied and then feeding the coated conductor from the reel to the device for applying the next layer, however, by maintaining accurate spacing between the extruders and selecting suitable processing conditions between the extruders, an insulated conductor according to the invention can be manufactured in a continuous process, w6with the conductor being fed into one end of the production line and with the finished insulated conductor being fed out of the other end of the line.

The insulated conductor of the invention can be used alone or with a further insulating layer or with a conductive sheath around it. A plurality of such conductors can be stranded to form twisted pairs or they can be assembled into a bundle covered with a protective sheath. Also, several insulated conductors can be placed next to each other and connected to each other at their contact surfaces.

Fig. 5 shows three insulated conductors 6 according to the invention, which are connected and surrounded by a sheath 20 of plastics material. The plastics material for the sheath is selected to provide the desired temperature rating as described above and, for example, if the temperature rating is to be for 200ºC or more, the plastics material may be PFA.

In the various designs, the plastic materials can contain conventional fillers that do not adversely affect the desired properties of the cable insulation.

Claims (16)

1. Insulation of an insulated conductor, the insulation comprising a first layer (2) enveloping and operatively connected to the conductor (1), a second layer (3) enveloping and operatively connected to the first layer (2), and a third layer (4) enveloping and operatively connected to the second layer (3), and the insulation having a temperature rating of more than 150°C, characterized in that the first layer (2) contains a perfluoroalkoxy resin, the second layer (3) contains a polyimide resin, the third layer (4) contains a plastic selected from the group consisting of perfluoroalkoxy and ethylenetetrafluoroethylene copolymer resins, and the first layer does not contain a tape. 2. Insulation according to claim 1, wherein the insulating plastic material of the first layer (2) and the third layer (4) is perfluoroalkoxy resin, and the insulation has a temperature rating for at least 200°C. 3. Insulation according to claim 1 or 2, wherein the second layer (3) is in contact with the first layer (2) and the third layer (4) is in contact with the second layer (3). 4. Insulation according to 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 the second layer (3). 5. Insulation according to one of the preceding claims, wherein the polyimide resin of the second layer is thermally cured. 6. Insulation according to one of the preceding claims, wherein at least one of the layers is thermoplastic. 7. Insulation according to one of the preceding claims, wherein the second layer is an extruded layer. 8. Insulation according to one of claims 1 to 6, wherein at least one of the layers is an extruded layer. 9. Insulation according to one of claims 1 to 6, wherein each of the layers is an extruded layer. 10. Insulation according to claim 1, wherein at least one of the layers is a coated layer. 11. A method for producing insulation for a conductor according to one of claims 1 to 10, the method being characterized by the following features: while the conductor (1) is advanced lengthwise, the first layer (2) of perfluoroalkoxy resin is applied around it and in contact with it; while the conductor (1) with the first layer (2) around it is moved lengthwise forward, the second layer (3) of polyimide resin is applied around the first layer (2); and while the conductor (1) with the first layer (2) and the second layer (3) is moved lengthwise forward, the third layer (4) made of a plastic selected from the group consisting of perfluoroalkoxy and ethylenetetrafluoroethylene copolymer resins is applied. 12. A method according to claim 11, wherein the outer surface of the first layer (2) is etched before extruding the second layer (3). 13. A method according to claim 11, wherein the second layer (3) is applied by extruding a viscous varnish around the first layer (2), the varnish containing a solvent and polyimide, the weight fraction of polyimide solids in the varnish being at least 20% and wherein the solvent is removed from the second layer by heating it. 14. A method according to claim 13, wherein the polyimide in the lacquer is thermally cured during heating of the second layer (3) to remove the solvent. 15. A method according to any one of claims 13 or 14, wherein the plastic material of the third layer is a perfluoroalkoxy mixture. 16. A method according to claim 13, wherein the plastic material of the first layer and the third layer is a perfluoroalkoxy mixture.