Insulated Conductor With Multi-Layer, High Temperature Insulation

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:

FIG. 1 is a transverse cross-section of the insulated conductor of the invention;

FIG. 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;

FIG. 3 is an end view of the die shown in FIG. 2;

FIG. 4 is a fragmentary, cross-sectional view of a modified form of the die shown in FIG. 2; and

FIG. 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.

FIG. 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 a tin, silver or nickel. Tin may be used if the insulation conductor is to be rated at 150.degree. C., silver, if the insulated conductor is to be rated at 200.degree. C. or nickel, if the insulated conductor is to be rated at above 200.degree. C., e.g. 250.degree. C. Of course, nickel may be used for all such ratings, and silver may be used for all ratings up to 200.degree. 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.degree. C., e.g. 250.degree. 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, PTFE, ETFE or FEP 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, ETFE or FEP, 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 or polyimide lacquer.

FIGS. 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 is 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 FIG. 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 crosslinking 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.degree. C.-300.degree. C. and the temperature at the exit may be 600.degree. 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 FIGS. 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 crosslinking agent, such as an acetic anhydride and beta-picoline. Preferably, the cross-linking agent is supplied to the pump 19 as indicated in FIG. 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 FIG. 2.

FIG. 4 illustrates a modified form of the die insert 11 shown in FIGS. 2 and 3. Although the die of FIGS. 2 and 3 is preferred because it gives better control of the thickness and concentricity of the layer 3, a die with the insert lla shown in FIG. 4 may be found to be satisfactory.

In the embodiment shown in FIG. 4, the die body 13 is the same as the die body shown in FIGS. 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 lla, 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 a substantially non-hydrolizing polyamide which thermosets, but for a temperature rating of 200.degree. 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 a polyamide is used, it is extruded or coated over the layer 3 in a conventional manner, and if desired, when layer 4 comprises PVDF or a polyamide, the layer 4 may be cross-linked by radiation or with a cross-linking agent and heat.

If a polyamide is used for the layer 4, it is essential that it be a substantially non-hydrolizing polyamide which is thermosettable. Polyamides with such properties are known in the art and are commercially available, one such polyamide being known as Imitec extrudable polyamide available from Imitec, Inc., Schnectady, N. Y.

The radial thicknesses of the layer 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.degree. C., and if made with a layer 4 of PFA, will have a temperature rating in excess of 200.degree. 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, separation 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 in 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 completed 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 a 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.degree. 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.