EP1211696A1

EP1211696A1 - Insulated electrical conductor

Info

Publication number: EP1211696A1
Application number: EP00870285A
Authority: EP
Inventors: Noel Mortier; Alain Jacques; Eric D. Nyberg
Original assignee: Compagnie Royale Asturienne des Mines; Tyco Electronics Corp
Current assignee: Compagnie Royale Asturienne des Mines; TE Connectivity Corp
Priority date: 2000-12-01
Filing date: 2000-12-01
Publication date: 2002-06-05

Abstract

The present invention is related to an insulated electrical conductor comprising:

(a) an elongate electrical conductor (12), and

(b) electrical insulation (20) surrounding the conductor, said insulation comprising

(i) a coated mica tape inner layer (22) that surrounds the conductor, said coated mica tape containing from about 2 to about 30 weight percent of a first polymer impregnant, and being coated on at least one side with a fluoropolymer layer; and

(ii) a polymeric outer electrically insulating layer (24) that surrounds the coated mica tape layer (22).

Description

Field of the invention

This invention relates to insulated electrical conductors and the method of producing them.

Background of the invention

Polyimides, e.g. polypyromellitimides, and ethylene-co-tetrafluoroethylene copolymers such as those sold by E.I. du Pont de Nemours and Company, under the trademarks Kapton™ and Tefzel™ respectively, are known to have good thermal, mechanical, and flammability properties, as are certain polyetherimide polymers. These polymers, therefore, are used as insulation material for high performance wire and cable. Existing thin-wall, high performance wire and cable constructions utilize several different types of jacket construction. These include single or multi-layer cross-linked poly(ethylene-co-tetrafluoroethylene) (e.g. Specification 55™ wire), polyimide (e.g. Kapton), and a composite wire construction consisting of a Kapton tape polyimide inner layer covered with a PTFE (polytetrafluoroethylene) tape outer wrap. For example, Specification 55 wire is rated for a maximum of 200°C but has relatively low cut-through resistance at elevated temperatures. Most polyimides are not thermoplastic, and they therefore cannot readily be applied to a conductor by melt extrusion. Typically, they are formed into a tape, and the conductor is wrapped with that tape in an overlapping fashion. However, the polyimide tapes are not self-sealing. It has been known to coat a core layer of a polyimide with a fluoropolymer on one or, sometimes, both sides, thereby forming a laminated tape structure, which can be heated after wrapping to fuse the wrapped tape to itself. Tapes prepared from these laminates, and the insulating of electrical conductors by wrapping with such tapes, are known. For a description of prior art wire constructions see, for example, U.S. Patents Nos. 3,616,177; 4,628,003; 5,106,673; 5,220,133; 5,238,748; and 5,399,434. The disclosures of these and other patents, patent applications, and publications discussed in this application are incorporated herein by reference.
Because currently used polyimides are aromatic condensation polymers, wires insulated with such polyimides frequently show poor resistance to both hydrolysis and arc tracking. Hydrolysis is a chemical reaction which although occurring in the presence of water, is often promoted by other chemical species, and which generally reduces a polymer's mechanical strength by reducing its molecular weight. Arc tracking is a catastrophic failure in the presence of an electrical arc when a short circuit occurs between the conductor and ground or a conducting medium external to the insulation, such as another conductor (for example, when two wires rub together), a metallic structure (for example, when a wire rubs against a supporting metallic structure such as an airframe), or an even moderately conductive fluid. Such a failure causes mechanical damage to the insulation which rapidly propagates at the elevated temperature of the electric arc.
Because of the generally poor resistance of polyimides to both arc tracking and hydrolysis, wire for demanding applications such as, for example, the aerospace industry, is frequently fabricated by insulating electrical conductors with two wrapped layers: an inner layer of a polyimide or a polyimide-fluoropolymer laminate, and an outer layer of a fluoropolymer, particularly polytetrafluoroethylene. U.S. Patent No. 5,220,133 discloses the use of this multi-layer wrapped construction, which is asserted to be especially useful for insulated conductors having low diameter and weight, as is particularly desirable for use in aerospace applications. However, when a polytetrafluoroethylene tape is used as an outer layer, the high temperature needed to sinter and fuse the polytetrafluoroethylene may damage the tin plating of tin-plated copper conductors. Also, the resulting polytetrafluoroethylene outer layer is difficult to mark, and the outer surface of the insulated wire has a stepped contour which is readily damaged in handling. As indicated, composite Kapton/PTFE wire tends to have an uneven outer surface which may snag on corners, and may be subject to tearing during shop handling or installation. If the PTFE outer layer is damaged, the Kapton polyimide inner layer is thus subject to hydrolysis and arc tracking. This problem is, of course, exacerbated on Kapton wires having no PTFE tape covering over the Kapton jacket layer.
The most common current alternative to polyimide insulation for insulated conductors in aerospace and other high performance applications utilizes crosslinked fluoropolymers. A dual-layer insulated conductor, where the inner layer is an uncrosslinked or lightly crosslinked crystalline poly(ethylene-co-tetrafluoroethylene) and the outer layer is highly crosslinked crystalline poly(ethylene-co-tetrafluoroethylene), is described in U.S. Patent No. 5,059,483. While these materials and constructions are more resistant to arc-tracking and hydrolysis than polyimide based insulations, they are thermally stable to only about 200°C and exhibit inferior cut-through performance at temperatures greater than about 150°C. Cut-through performance, also referred to as "pinch", is measured as the force required for a blunt blade or edge to penetrate a wire insulation at a specified temperature.
Characteristics that are particularly desired in a high-performance (especially airframe) wire and cable are light weight and small diameter, good cut-through, arc-track and abrasion resistance and thermal stability, low flammability, insensitivity to water and common solvents, and a smooth outer surface contour. None of the currently available polymers including fluoropolymers either alone or in combination with other materials, for example polyimides, provides a wire insulation which meets all of these desired performance characteristics. It would be particularly desirable to produce an insulated conductor exhibiting this desired combination of performance characteristics and also comprising materials which are individually resistant to arc-tracking and hydrolysis. This would eliminate failure modes which may result from damage to an outer protective layer, thereby exposing a sub-layer to degradation (for example, tearing of a polytetrafluoroethylene outer layer protecting a polyimide layer beneath).
It is particularly difficult to obtain good cut-through performance at elevated temperatures in combination with the other required high performance wire properties. The most direct strategy for obtaining good cut-through resistance is to utilize thick walls, but this approach imposes the penalty of increased cost, diameter and weight. Second, one may employ thermoplastic polymers having melt or glass transition temperatures significantly above the wire's service temperature rating (but below the polymer's softening temperature). These materials, however, are either very expensive (e.g. polyarylether ketones), susceptible to hydrolysis (e.g. certain condensation polymers), prone to arc-tracking (e.g. many aromatic polymers), and/or have insufficient resistance to thermal degradation (e.g. polyolefins and polyesters). Above the polymer softening temperature, as measured for example by heat deflection temperature, even thick walls have little beneficial effect on cut-through performance. Third, one may employ tough, thin films or tapes as one component of the wire insulation, for example aromatic polyester or polyimide films. These tapes suffer from the limitations noted for many thermoplastic polymers: if they are aromatic polyimide condensation polymers they are expensive and are subject to both arc-tracking and hydrolysis; while polyesters exhibit poor thermal aging characteristics.
Attempts to improve the cut-through performance of melt processible polymers which otherwise approximately meet requirements by crosslinking or by the inclusion of reinforcing organic or inorganic fillers to form a composite material have also failed to meaningfully improve cut-through at elevated temperatures. In both cases, cut-through resistance drops to unacceptable levels as the polymer softens on heating. This is true even for polymers loaded with up to 30 volume percent of reinforcing fillers such as fumed silica, glass fiber or mica. Furthermore, filler loadings of 30 volume percent or sometimes even less may be impractical if they result in materials with melt viscosities so great that they cannot be conveniently extrusion processed, or afford tapes having insufficient tensile and/or elongation properties for tape-wrapping.
In the search for a wire insulation system possessing the necessary cut-through resistance at elevated temperatures along with the other required physical, chemical and electrical properties, wire constructions comprising continuous fibers have also been explored. Fiber can be incorporated in wire constructions by a number of methods, for example spiral wrapping continuous fibers directly, wrapping tape comprising densely packed fibers imbedded in a suitable polymer, wrapping dense fabric as a tape, with or without a polymer impregnation, or by braiding fiber on the wire. Wires comprising braided fibers underneath an extruded polymer outer layer for the purpose of improving cut-through at elevated temperatures suffer from two serious drawbacks, however. First, they are expensive to process because the fibers must be applied by braiding, an inherently slow process. Second, high cut-through forces are generally obtained with braided fibers only when the surrounding polymer is quite hard or rigid. U.S. Patent No. 5,171,635 reports values for cut-through at 150°C which possibly are only a small fraction of those observed at 23°C. Cut-through values are frequently not reported at higher temperatures because the materials which are thermally stable at these temperatures, for example ETFE or PFA (poly(perfluoropropylvinylethercotetrafluoroethylene)), are too soft to support the fibers as the cut-through blade or edge pushes into the insulation.
Mica tapes have long been used for the insulation of wire and cable in conjunction with one or more polymer layers. This is due to this mineral's excellent thermal and dielectric properties which provide good fire resistance and high insulation values. Mica itself is also very stable to a wide range of chemicals, including those which promote hydrolysis. However, existing mica tapes suffer from one or more of the following disadvantages
(a) excessive thickness and/or stiffness, particularly after exposure to elevated temperatures;
(b) irregular surfaces comprising materials to which it is difficult to bond a subsequent covering layer of tape or other material;
(c) insufficient mechanical strength to permit tape wrapping (especially of small gauge conductor) at practical processing speeds

Summary of the invention

It is readily apparent that the desired performance characteristics for a wire insulation are to a certain extent in conflict, for example, light weight and low thickness tend to be incompatible with high cut-through resistance. Thus, there is a need for a wire and cable construction which comprises multiple insulating layers and which is uniquely superior to current wire constructions in a number of performance characteristics. In particular, it is the object of the present invention to provide a unique wire construction having good strippability, flame resistance, heat stability, cut-through resistance, abrasion strength, solvent resistance, and which comprises materials which are resistant to both hydrolysis and arc-tracking. In addition, a wire construction providing these advantages without requiring a thick and/or heavy layer of insulation is highly desirable.
There is a further need for unique mica sheet and tape articles and methods for their fabrication which are thinner to reduce weight and size, more flexible to conform to a variety of substrates (especially small diameter wire), provide improved thermal performance. It would also be valuable to provide mica sheets or tapes and methods for their fabrication comprising materials which are self-sealing or otherwise allow bonding of the mica tapes to each other and to a variety of other materials employed in wire insulation, for example to a tape-wrapped or extruded outer polymer layer, in order to meet a variety of important handling and performance requirements.
This invention is related to an insulated electrical conductor comprising an elongate electrical conductor; an electrical insulation surrounding the conductor, said insulation comprising: an inner, electrically insulating layer that surrounds and is in direct physical contact with the conductor, the inner layer comprising a wrapped, coated mica tape layer as hereinafter described; and an extruded or tape-wrapped polymeric, outer electrically insulating layer that surrounds and is in direct physical contact with the inner micaceous layer. Multiple layer constructions of the aforementioned type are also contemplated by the present invention.
Therefore, in a first aspect, the present invention is related to an insulated electrical conductor comprising:
(a) an elongate electrical conductor, and
(b) electrical insulation surrounding the conductor, said insulation comprising
(i) a coated mica tape inner layer that surrounds the conductor, said coated mica tape containing from about 2 to about 30 weight percent of a first polymer impregnant, and being coated on at least one side with a fluoropolymer layer; and
(ii) a polymeric outer electrically insulating layer that surrounds the coated mica tape layer.
The invention is further more specifically related to an electrical conductor wherein said fluoropolymer layer is obtained by laminating a fluoropolymer film onto said mica tape.
According to a preferred embodiment, the layer thickness of said coated mica tape is lying between 15µm and 75µm.
According to a preferred embodiment, the conductor further comprises a layer of glass fibers between said coated mica tape and said fluoropolymer layer.
According to a preferred embodiment, the polymer impregnant is a silicone polymer comprising linear segments represented by the formula -Si(R₁) (R₂)-O-, wherein R₁ and R₂ may be methyl or phenyl groups.
In the insulated electrical conductor of the invention, said fluoropolymer may be selected from the group consisting of polytetrafluoroethylene, poly(hexafluoropropylene-co-tetrafluoroethylene), poly(perfluoropropylvinylether-co-tetrafluoroethylene), and poly(perfluoromethylvinylether-co-tetrafluoroethylene), and mixtures thereof.
In the insulated electrical conductor of the invention, said outer layer may have a thickness of between about 50 and 300 µm, preferably between about 75 and 200 µm. Said outer layer may comprise a perfluoropolymer, preferably poly(ethylene-tetrafluoroethylene).
Said outer layer may comprise a tape-wrapped polymer film, preferably a tape-wrapped polymer film which comprises a perfluoropolymer.
In the insulated electrical conductor of the invention, there may be interposed between the conductor and the mica tape layer, a separately applied polymer layer.
The present invention is equally related to a method of fabricating a coated mica tape or sheet comprising the steps of :
(a) treating a mica paper with from 2 to 30 weight% of a polymer impregnant in order to have a mica tape, and,
(b) providing a film made of fluoropolymer
(c) spraying a fluoropolymer onto at least one side of said mica tape, or onto at least one side of said film or onto both, mica tape and film,
(d) laminating said possibly sprayed side of said film onto said possibly sprayed side of said mica tape.
(e) applying a heat and/or pressure treatment to said end product
According to another embodiment, said method comprises the steps of :
(a) treating a mica paper with from 2 to 30 weight% of a polymer impregnant in order to have a mica tape, and,
(b) providing a film made of fluoropolymer,
(c) applying an adhesive layer onto at least one side of said mica tape, or onto at least one side of said film or onto both, mica tape and film,
(d) laminating said possibly coated side of said film onto said possibly coated side of said mica tape.
(e) applying a heat and/or pressure treatment to said end product.
According to another embodiment, said method of fabricating a coated mica tape or sheet comprising the steps of :
(a) treating a mica paper with from 2 to 30 weight% of a polymer impregnant in order to have a mica tape, and,
(b) providing a film made of fluoropolymer,
(c) applying an adhesive layer onto at least one side of said mica tape,
(d) spraying a fluoropolymer onto at least one side of said film,
(e) laminating said possibly coated side of said film onto said possibly coated side of said mica tape.
(f) applying a heat and/or pressure treatment to said end product
According to the invention, said laminating may be done in an oven, at a temperature of about 300°C. Said laminating may be done by a number of rolls.
According to the invention, one of said rolls is preferably at a temperature of about 300°C.
The invention further comprises a novel coated mica sheet or tape article comprising a mica paper core containing 2 to 30% by weight of a first polymer impregnant, and a second polymer layer either directly deposited from a liquid dispersion of the second polymer, preferably an aqueous dispersion or being laminated in the form of a coated film of the second polymer, on at least one side of the impregnated mica paper. In addition, the invention comprises a novel method of fabricating this mica sheet or tape article. As used herein, the term "impregnated mica tape or sheet" refers to the impregnated mica paper prior to application of the dispersion applied polymer layer. After application of the second polymer layer, the mica sheet or tape of the present invention will be referred to as "coated" mica sheet or tape.

Brief description of the drawings

The invention is illustrated by the drawings in which Figure 1 is a cross-section of a first embodiment of the insulated electrical conductor of this invention;
Figure 2 is a second embodiment of the insulated electrical conductor of this invention having a polymer layer interposed between the coated mica tape layer and the conductor;
Figure 3 is a third embodiment of the insulated electrical conductor of this invention comprising a polymer layer interposed between two layers of coated mica tape;
Figure 4 is a first embodiment of the coated mica sheet or tape of the invention having a polymer layer coating applied to one side only of the impregnated mica paper;
Figure 5 is a second embodiment of the coated mica sheet or tape of the invention comprising a polymer coating on both the top and bottom surfaces of the impregnated mica paper;
Figure 6 is a third embodiment of the coated mica sheet or tape of the invention comprising two polymer layers (which may be the same or different) applied to both the top and bottom surfaces of the impregnated mica paper;
Figure 7 is a fourth embodiment of the coated mica sheet or tape of the present invention comprising three polymer layers (the same or different polymer) applied to each surface of the impregnated mica paper; and
Figure 8 is another embodiment of the coated mica sheet or tape of the invention comprising a yarn layer interposed between two polymer layers.
Figure 9 is describing an embodiment of the method of production of an insulated electric conductor according to the present invention.
Figure 10 is describing another embodiment of the method of production of an insulated electric conductor according to the present invention.

Detailed description of the invention

Referring to the Figures, where like numerals denote like elements of the insulated electrical conductor of this invention, FIG. 1 is a front-view cross-section through the insulated electrical conductor of a first embodiment of this invention. The insulated wire comprises an elongated electrical conductor 10, shown as strands of metal wire 12 each with a metal plating 14. Electrical conductor 10 may alternatively be a solid wire rather than the stranded wire shown, but stranded wire is generally preferred in applications where vibration is a factor, such as in aerospace applications. Both solid and stranded wire of various metals may suitably be insulated using the coated mica tape of the present invention.
The electrical conductor 10 is typically of copper, but may be of copper alloy, aluminum, or other conductive metals. If the metal wire is of copper or a copper alloy, it is typically plated with a metal plating 14 to protect the copper from oxidation effects, and to improve the solderability of the conductor, although unplated copper or copper alloy wire is also suitable for use as the electrical conductor of this invention. Typical metal platings 14 are of tin, silver, nickel, or other commonly employed plating metals. Such platings are typically produced by electroplating a uniform thickness of high purity metal to the individual wires comprising the strand.
Stranded copper wire is available in several configurations. The wire may have a unilay construction, where a central core is surrounded by one or more layers of helically wound strands in the same lay direction and same lay length; may be constructed with concentric stranding where a central core is surrounded by one or more layers of helically wound strands in alternating lay directions and increasing lay length; or may be constructed with unidirectional concentric stranding where a central core is surrounded by one or more layers of helically wound strands in the same lay direction and increasing lay length.
Such stranded copper wires are readily available from numerous commercial sources. For example, Hudson International Conductors, Ossining, New York, supplies a unilay stranded copper wire consisting of nineteen strands of 200 µm diameter (32 AWG) copper, each individually coated with an electroplated layer of tin. Such a 19/32 AWG stranded wire has a nominal outside diameter of approximately 950 µm, and has an equivalent conductor diameter of 813 µm (20 AWG), i.e. it is regarded as the effective equivalent of an 813 µm diameter (20 AWG) solid copper wire.
The electrical conductor 10 is surrounded by a two-layer electrical insulation 20. The inner electrically insulating layer 22 of this first embodiment immediately surrounds the electrical conductor and comprises a wrapped coated mica tape in accordance with the present invention which will be described in greater detail later in this application. The coated mica tape layer 22 is itself surrounded by an extruded or tape-wrapped polymeric outer electrical insulating layer 24 which will also be hereinafter described in greater detail.
In the first embodiment, the coated mica tape forming the inner electrically insulating layer 22 is wrapped over the electrical conductor 10 by a process known to those of ordinary skill in the art. Standard tape-wrapping machines are commercially available, for example from companies such as United States Machinery Corporation, North Billerica, Massachusetts, or E.J.R. Engineering and Machine Company Incorporated, Lowell, Massachusetts; and the techniques of using these or like machines to wrap an electrical conductor with an insulating tape are well known.
The inner layer 22 may consist of one or a plurality of layers of coated mica tape. To provide a single tape layer, the tape is wrapped with approximately 0% overlay, i.e. "butt-wrapped". In butt-wrapping, however, small gaps between adjacent wraps of tape are inevitable in a production manufacturing environment. It is thus preferred to provide at least two coated mica tape layers which can be obtained using an overlap of approximately 50%, or alternatively, by applying two tapes, each of which is butt-wrapped. In the case of two layers of butt-wrapped tapes, the second butt-wrapped tape layer should be wrapped such that it largely covers the small gaps which may occur between the adjacent wraps of the innermost first tape layer. The two-layer butt-wrapped construction will provide a smoother finished wire surface contour, particularly in conjunction with an extruded outer layer, but the extent of overlap is not a critical feature of this invention. If desired, further layers of coated mica tape can also be applied to provide improved mechanical strength to the wire, e.g. greater cut-through resistance.
According to the invention, the thickness of the inner layer is preferably lying between 15µm and 75µm.
To provide a bond between wrapped layers of coated mica tape, at least one surface of the coated mica tape preferably comprises a thermoplastic polymer coating as thin as 1 µm; more preferably both surfaces of the tape are coated with a thermoplastic polymer coating with the mica paper layer being sandwiched therebetween. Thus when this tape polymer coating layer reaches its melt temperature it will function as an adhesive and the coated tape will bond or self-seal to an adjacent layer, for example another mica tape layer or to the outer extruded or tape-wrapped layer 24. To obtain satisfactory bonding between two coated mica tape layers, the tapes should have a relatively smooth surface and be in intimate contact. This is readily achieved by employing sufficient tension during the tape-wrapping step. It may also be useful to heat the inner layer 22 immediately prior to application of the outer layer 24, as an in-line process. Any suitable furnace which will provide the necessary temperature to the bond area will do (e.g. an induction, infrared or forced air oven). It is desirable that the coating polymer on the mica paper be compatible with the outer insulating layer 24, as hereinafter described, and also with the coating layer of a differently coated mica tape if a second such tape having a different polymer as a coating is also applied over the first tape layer.
The outer electrically insulating layer 24 may comprise any polymer which may be suitably applied by extrusion and/or tape-wrapping. Suitable materials for the outer electrically insulating layer 24 include for example polyethylenes, polyethylene copolymers, and fluoropolymers (e.g. poly(vinylidenefluoride) (PVDF), poly(ethylene-co-tetrafluoro-ethylene) (ETFE), poly(chlorotrifluoroethylene) (CTFE), poly ( hexafluoro-propylene-co-tetrafluoroethylene) (FEP), poly(perfluoropropylvinylether-co-tetrafluoroethylene) (PFA), poly(perfluoromethylvinylether-co-tetrafluoroethylene) (MFA), and polytetrafluoroethylene (PTFE)). For the purpose of providing a wire which is most resistant to hydrolysis and/or arc-tracking, the use of highly aromatic polymers such as polyesters (e.g. poly(ethylene-co-terepthalate) (PET), poly(butylene-co-terepthalate) (PBT) and poly(ethylene-co-napthalate) (PEN)), and polyimides, is not preferred for aerospace wire and cable, but is appropriate for some applications. In terms of providing a wire which is particularly advantageous for use in aerospace applications the most preferred polymers for use in outer layer 24 are PFA, MFA, or ETFE, especially radiation cross-linked ETFE.
The materials of the outer layer 24 have been described by reference to their primary polymeric constituents, and it should be appreciated that they may also contain other constituents such as are conventional in the polymer formulation art; for example, antioxidants, UV stabilizers, pigments or other coloring or opacifying agents (such as titanium dioxide), prorads (radiation enhancing agents) to promote radiation crosslinking, flame retardants, additives to promote marking, and the like. Outer layer 24 may also comprise more than one layer, for example a thin outermost polymer layer which contains additives to promote marking or provide a particular color, while the bulk of the underlying outer layer 24 will not contain some or all of these additives.
The coated mica tape and outer layers, layers 22 and 24, respectively, which together comprise the wire insulation may be applied in one or in separate operations depending upon the relative line speeds suitable for each. If the outer layer 24 is tape-wrapped, then a one-step operation to apply both layers 22 and 24 is feasible. If the outer layer is applied by extrusion, which typically runs at 10 to 100-fold the line speeds of tape-wrapping, then separate processing steps for applying layers 22 and 24 is generally preferred.
A second embodiment of the invention is shown in FIG. 2 in which an inner polymer layer 16 is located between the conductor 10 and the coated mica tape layer 22. This inner layer 16 may be applied by conventional methods such as powder coating, tape-wrapping or extrusion. Inner layer 16 may provide a greater degree of control over wire handling and installation properties such as insulation strip force and/or shrink-back. Suitable polymers include polyethylenes, polyethylene copolymers, and fluoropolymers (e.g. PVDF, ETFE, CTFE, FEP, PFA, MFA, and PTFE).
A third embodiment of the invention is presented in FIG. 3 in which a polymer layer 18 is located between two coated mica tape layers 22A and 22B. In this embodiment, it is necessary to apply the two layers 22A and 22B with a butt wrap. Polymer layer 18 generally provides an improved adhesive bond between layers 22A and 22B, thereby improving desired performance properties such as wrinkling and abrasion resistance. It is seen from this embodiment that the mica tape layer shown in the various embodiments may include a layer or layers which do not comprise coated mica paper. In this and other embodiments of the invention, layer 22 is intended to denote the layer which comprises at least one coated mica paper layer, and it is located nearer to the conductor than outer layer 24. According to the invention, the thickness of said outer layer is lying between 50µm and 300µm.
Other embodiments of the invention comprising multiple mica coated tape and/or polymer layers are of course conceivable, and are included within the scope of the present invention.
The coated mica tape per se, and also its method of fabrication, used in layer 22 as shown in the illustrative embodiments of the present invention, are both novel. This coated mica paper sheet is fabricated as follows: mica paper is first impregnated with an oligomer (polymer precursor) or first polymer solution such as a silicon resin in solvant, the solvent is evaporated and the oligomer impregnant is cured to provide sufficient structural integrity to permit further processing. The impregnated paper is then coated with a second polymer layer or with a mixture of polymers as hereinafter described. Commercial mica paper having a thickness of up to 75 µm, preferably less than 50 µm, most preferably less than 35 µm or even preferably less than 20 µm, is suitable. Suitable paper, for example, is sold under the trade name Cogemica™ by Cogebi, of Belgium. The paper may be formed from any of the known naturally occurring or synthetic micas as heretofore described, employing equipment and techniques known to those skilled in the art.
The mica paper is impregnated with a monomer, oligomer or polymer normally in a non-aqueous solvent to provide a paper which is dimensionally stable, can be handled in subsequent processing steps without tearing, and has the required performance characteristics, especially moisture resistance. The polymers currently used for mica paper impregnation are suitable, as are other polymers, provided they can be applied to the paper as a solution which itself does not damage the mica paper. Suitable solvents include relatively non-polar liquids such as higher alcohols, ketones, and aliphatic and aromatic hydrocarbons, and mixtures thereof, for example. Impregnation polymers may be thermosets (or precursors thereof) or thermoplastics. To obtain a thermoset impregnant, one applies a solution of the monomer or oligomer precursor of the thermoset polymer, for example polymer precursor solutions of polyimides, epoxies, or silicones. These polymer precursor solutions are well-known for mica paper impregnation, for example the polyimide precursor solutions sold under the tradename Kerimid™ (Ciba Geigy), epoxy precursor solutions sold as Epon 828™ (Shell), and silicone precursor solutions sold as Wacker K™ (Wacker GmbH). Also suitable are polymer impregnants such as silicone or hydrocarbon elastomers which form solutions in suitable solvents. For high temperature applications, polyimides and silicone precursor thermosets are the preferred impregnants. In particular, the methyl and phenyl substituted silicone polymers, -Si(R₁) (R₂)-O-, where R₁ and R₂ are methyl or phenyl groups, provide excellent thermal stability and arc track resistance.
The impregnant polymer solution may be applied by any of a variety of methods, for example by dip, kiss, contact, or spray coating or vacuum impregnation . After applying the polymer solution, the solvent is evaporated, normally at a temperature greater than its boiling point, and for thermosetting impregnants, a higher temperature heat treatment may be necessary to complete the crosslinking reaction and obtain optimal mechanical strength. Using dimethylsilicone polymer impregnant as an example, from 2 to 30% by weight of impregnant may be applied to the mica paper (as percent initial mica paper weight) to provide the required dimensional stability and strength to the impregnated paper to prepare it for the subsequent step of coating. More preferably the mica paper is impregnated with from 3 to 15% by weight of dimethylsilicone polymer containing functional groups suitable for catalytic and/or heat initiated cross-linking. Relatively low levels of impregnation polymer are surprisingly effective at providing the mica paper with the solvent resistance necessary for the subsequent step of coating. The use of lower levels of polymer impregnation provides several advantages. First, the resultant impregnated mica tape is less prone to stiffening due to oxidation of the impregnant when exposed to high use temperatures; hence it is less prone to fraying or cracking, which may expose the conductor. Second, the reduced level of impregnant affords a mica tape that, although solvent stable, is still sufficiently porous to provide a remarkably strong bond to the subsequently applied coating and/or adhesive which are applied. The polymer particles of the coating or of the adhesive are able to interpenetrate the surface of the mica paper to form a strongly adherent bond.
The optimum range of impregnation loading using polymers other than dimethylsilicone may be different but will normally be within the above-indicated 2 to 30% by weight loading range.
One or more polymer layers may be applied sequentially to the impregnated mica paper. In FIG. 4 is shown a first embodiment of the coated mica sheet or tape 28 of the present invention consisting of two layers: an impregnated mica paper 30 and a single polymer layer 40 obtained by the application of a first polymer film. Polymer layer 40 may be obtained in a single laminating step, or from multiple steps using several films.
In FIG. 5 is presented a second embodiment of coated mica sheet or tape 28 consisting of three layers: impregnated mica paper 30, polymer film 40 on one side of the mica paper, and a second polymer film 50 on the other side of the paper. Polymer films 40 and 50 may be of the same polymer or different, and may be applied in one coating step or in multiple steps.
A third embodiment of this coated mica sheet or tape is the five-layer construction illustrated in FIG. 6. In this case an additional thin polymer film 44 and 54 is applied to polymer layers 40 and 50, respectively. Polymer layers 44 and 54 are distinguished from layers 40 and 50, respectively, by chemical composition and/or molecular weight. Layers 44 and 54 may be alternatively an adhesive layer which promotes adhesion between two sheets or tapes, or to another material to which the sheet or tape is to be bonded.
A fourth embodiment of the present sheet or tape invention is presented in FIG. 7. This is a seven-layer construction having thin polymer layers 46 and 56 cast directly onto the two faces of the impregnated mica paper. On polymer layer 46 is cast a further thin polymer layer 40, and on polymer layer 56 is cast a layer 50 of greater thickness than polymer layer 40. Finally, additional thin layers 44 and 54 are cast on top of layers 40 and 50. Layers 46 and 56 may, for example, promote adhesion between mica layer 30 and layers 40 and 50. A variety of other embodiments are possible, all of which are included within the scope of the present invention, for example a seven-layer construction with layers 40 and 50 being of equal thickness.
Fillers, additives and reinforcements may be included in one or more of the polymer layers. Fillers include infusible polymer particles and/or inorganic particles such as clays, glass spheres, glass fibers, and fumed silica, among others. Additives which may be present in the polymer layers include antioxidants, UV stabilizers, pigments or other coloring or opacifying agents (such as titanium dioxide), prorads (additives to promote radiation crosslinking), and flame retardants. One may also include in one or more of the polymer layers continuous fibers, for example glass fibers or yarns, oriented polymer fibers, or glass fabrics.
FIG. 8 shows another embodiment of the invention in which a yarn layer 50' is positioned between two fluoropolymer layers of nonsymmetric thickness 54, 56.
The present invention is equally related to the method of producing an insulated electrical conductor, wherein a coated mica tape inner layer is surrounding said conductor, said coated mica tape containing from about 2 to about 30 weight percent of a first polymer impregnant, and being coated on at least one side by laminating a fluoropolymer film onto said tape.
According to a first embodiment, the mica tape is sprayed with a fluoropolymer, possibly the same fluoropolymer used for the reinforcing fluoropolymer film. The sprayed layer is then dryed until the solvent has disappeared and the fluoropolymer is in powder form on the tape. After that, it is brought into contact with the film in an oven at +-300°C to allow the bonding of the fluoropolymer to the mica tape.
According to another embodiment, both the mica tape and the fluoropolymer film are sprayed in this way, prior to the bonding. Alternatively, it is possible that only the film is sprayed with said fluoropolymer prior to bonding of said mica tape and said film.
According to still another embodiment, an adhesive polymer layer, e.g. a silicon adhesive layer having preferably a similar composition than the impregnant of the mica tape is applied to the mica tape or to the fluoropolymer film ,or to both, prior to the bonding of the two. In this last case, an adhesive layer is always present in the resulting tape.
In case a fluoropolymer layer is sprayed onto the mica tape and/or the film, the bonding of the mica tape and the reinforcing fluoropolymer layer takes place in a furnace at about 300°C. It can also be done by rolling the tape 101 and the film 102 together between a number of rolls, as is shown in figure 9. All of these rolls can be put in a furnace or one of these rolls (100) can be heated to +-300°C, in order to acquire the optimal bonding between the mica tape and the reinforcing film.
In case an adhesive is applied to the mica tape and/or the reinforcing film, a curing of said adhesive is necessary before or after the joining of the two parts. Said curing may take place at a temperature of +/- 120 °C.
According to a further embodiment, an adhesive, e.g. silicone based, may be applied on the mica tape and a fluoropolymer sprayed onto the film, or vice versa, after which the film is laminated onto the mica tape, by one of the previous methods, e.g. by use of rolls.

EXAMPLE 1

Commercial mica paper (Cogemica™, Cogebi), 15 µm thick, 30 cm width and 100 m length, and having an initial tensile strength of 2.3 N/cm, was impregnated with either a dimethyl or phenylmethyl silicone oligomer solution in toluene (both 50 wt% solutions as purchased; Wacker K). The silicone oligomer solutions were applied by kiss-coating to one side of the mica paper after dilution with toluene to the desired concentration as indicated in Table 1. The toluene was evaporated at 150°C/30 seconds, which also induces polymerization. The weight percent silicone polymer present in the impregnated mica paper was determined by extraction with refluxing KOH solution. Also in Table 1 are summarized the tensile strengths and water absorption characteristics for several samples prepared using two different concentrations of both dimethyl and phenylmethyl silicone oligomer solutions. It is evident that small amounts of silicone resin impregnation, as little as 4% by weight for Sample 3, impart very good water resistance. In contrast, the untreated mica paper disintegrates virtually immediately upon contact with water. A further important and surprising result is the remarkable increase in tensile strength for impregnated mica paper at all loadings using silicone oligomer materials. For example, only 4% dimethyl silicone (Sample 3) provides an 8-fold increase in this property. Increased tensile strength provides a mica tape which can, for example, be wrapped at greater tensions. Tape tension is important for a robust tape-wrapping process and control of important wire properties such as strip force and tape-tape adhesion.

Sample	Silicone Type	Silicone solution concentration	Measured silicone in impregnated paper	Tensile Strength (N/cm)	Water Absorption
1	n.a.	no treatment	n.a.	2.3	Disintegr ation
2	dimethyl	25%	15%	24.0	0.4%
3	dimethyl	10%	4%	19.0	1.2%
4	methylph enyl	15%	20%	18.7	---
5	methylph enyl	6%	8%	13.8	---

EXAMPLE 2

A mica tape according to the invention is impregnated by spraying fluoropolymer on the mica paper in two steps. In a first step, said spraying is done on one side of the mica, using FEP as spray product. In a second step, the other side of the mica paper is sprayed with the same product, FEP.
A PTFE film is laminated onto this mica tape to form the reinforcing layer (e.g. a DF1700 film, Chemfab corporation).
Prior to laminating, said film is sprayed with a fluoropolymer, namely FEP on one side of the film. Then the film passes through a drying oven at 100°C. After that, the FEP is in powder form on the film, in a density of 4gr/m2.
The assembling of the mica tape 101 to the chemfab film 102 is done in an oven at 300°C. Three metal rolls are placed in this oven (figure 10). The product is going under and above these rolls to provide a good contact between the mica and the film. A pressure is exerted on the tape and film by the two top rolls 103 and 100. The speed of the rolls is 1m/minute.
An other assembling method is by calendering at 310 °C .
The same experiment was done with different combinations of products sprayed on the mica tape and on the film.

EXAMPLE 3

Mica sheets reinforced with glass yarn, 30 cm wide and 100 m long, were prepared as illustrated in Figure 8 from commercial mica papers with two thicknesses: 15 µm and 20 µm. The mica papers were impregnated as for Sample 3 in Example 1, then coated on each surface with 2 µm of a 1:1 mixture of PTFE 30B/FEP 120A. Glass yarn (Owens-Corning; D1800 1/0 1.OZ 620-1 7636) was applied to one surface of mica sheets during the application of a 10 µm layer of PTFE 30B layer. The glass yarns were continuously fed onto the PTFE aqueous dispersion layer (wet) then immediately dried and sintered as above. Mica sheet samples having yarn densities of 1 and 2 yarns/mm width (1.0 and 0.5 mm spacings) were produced. A fluoropolymer film of 13µm was applied over the layer comprising the glass yarn. The tensile strengths of these four tapes are presented in Table 2. Oasis™ tape, a Dupont Corporation product, is used to construct commercial composite airframe wire. This tape comprises a 16 µm polyimide layer coated with PTFE and FEP on both surfaces. The inclusion of 1 yarn/mm width triples the tensile strength; and 2 yarns/mm provides a greater strength than provided by commercial Oasis tape.

Tape Sample	Yarns/m m	Mica Paper thickness (µm)	Tape Thickness (µm)	Tensile Strength (N/cm)
Example 2	0	15	43	11
Example 3a	1	15	59	33
Example 3b	2	15	59	67
Example 3c	1	20	65	27
Example 3d	2	20	65	71
Oasis™ Tape	n.a.	16 (polyimide)	30	54

EXAMPLE 4

A mica tape was constructed by lamination of a thin polymer film to a silicone impregnated mica paper. This allowed the determination that such a construction, once slit into tape, can be tape-wrapped onto conductor in long lengths at resonable speeds. One side of the impregnated mica paper prepared as for Sample 2 of Example 1 (20 µm thick after impregnation) was kiss coated with a 5 µm thick layer of phenylmethyl silicone resin (25% solution in toluene) which was then dried at 150°C/30 sec to provide a tacky surface. To this was laminated a 19 µm thick PTFE film (DF1700 film; Chemfab Corporation) using a two-roll calender, followed by an oven heat cure (300°C/ 2 min). The PTFE surface of this DF1700 film in contact with the silicone layer on the impregnated mica paper was coated with 1 µm FEP which promotes adhesion. This was slit using circular knives to a width of 4 mm, providing a coated mica tape having a thickness of only 49 µm, well within the target for use in thin-wall insulated conductors.
Wire samples (0.35 mm² (20 AWG), nickel plated 19-strand copper conductor) were prepared for measurement of robustness during tape-wrapping. This tape was wrapped at 10 ft/min in three continuous lengths of 300 ft without a break. Tensions were sufficient to provide a wrinkle-free wrapped surface.

Claims

An insulated electrical conductor comprising:

(a) an elongate electrical conductor, and

(b) electrical insulation surrounding the conductor, said insulation comprising

(i) a coated mica tape inner layer that surrounds the conductor, said coated mica tape containing from about 2 to about 30 weight percent of a first polymer impregnant, and being coated on at least one side with a fluoropolymer layer; and

(ii) a polymeric outer electrically insulating layer that surrounds the coated mica tape layer.
The insulated electrical conductor according to claim 1 wherein said fluoropolymer layer is obtained by laminating a fluoropolymer film onto said mica tape.
The insulated electrical conductor according to any of the preceding claims, wherein the layer thickness of said inner layer is lying between 15µm and 75µm.
The insulated electrical conductor of any of the preceding claims, further comprising a layer comprising glass fibers between said coated mica tape and said fluoropolymer layer.
The insulated electrical conductor of any of the preceding claims wherein the polymer impregnant is a silicone polymer comprising linear segments represented by the formula -Si(R₁) (R₂)-O-.
The insulated electrical conductor of Claim 5 wherein R₁ and R₂ are methyl or phenyl groups.
The insulated electrical conductor of any of the preceding claims wherein said fluoropolymer is selected from the group consisting of polytetrafluoroethylene, poly(hexafluoropropylene-co-tetrafluoroethylene), poly(perfluoropropylvinylether-co-tetrafluoroethylene), and poly(perfluoromethylvinylether-co-tetrafluoroethylene), and mixtures thereof.
The insulated electrical conductor of any of the preceding claims wherein said outer layer has a thickness of between about 50 and 300 µm, preferably between about 75 and 200 µm.
The insulated electrical conductor of any of the preceding claims wherein said outer layer comprises a perfluoropolymer, preferably poly(ethylene-tetrafluoroethylene).
The insulated electrical conductor of any of the preceding claims wherein said outer layer comprises a tape-wrapped polymer film, preferably a tape-wrapped polymer film which comprises a perfluoropolymer.
The insulated electrical conductor of any of the preceding claims, wherein there is interposed between the conductor and the mica tape layer, a separately applied polymer layer.
A method of fabricating a coated mica tape or sheet comprising the steps of :

(a) treating a mica paper with from 2 to 30 weight% of a polymer impregnant in order to have a mica tape, and,

(b) providing a film made of fluoropolymer

(c) spraying a fluoropolymer onto at least one side of said mica tape, or onto at least one side of said film or onto both, mica tape and film,

(d) laminating said possibly sprayed side of said film onto said possibly sprayed side of said mica tape.

(e) applying a heat and/or pressure treatment to said end product
A method of fabricating a coated mica tape or sheet comprising the steps of :

(a) treating a mica paper with from 2 to 30 weight% of a polymer impregnant in order to have a mica tape, and,

(b) providing a film made of fluoropolymer,

(c) applying an adhesive layer onto at least one side of said mica tape, or onto at least one side of said film or onto both, mica tape and film,

(d) laminating said possibly coated side of said film onto said possibly coated side of said mica tape.

(e) applying a heat and/or pressure treatment to said end product.
A method of fabricating a coated mica tape or sheet comprising the steps of :

(a) treating a mica paper with from 2 to 30 weight% of a polymer impregnant in order to have a mica tape, and,

(b) providing a film made of fluoropolymer,

(c) applying an adhesive layer onto at least one side of said mica tape,

(d) spraying a fluoropolymer onto at least one side of said film,

(e) laminating said possibly coated side of said film onto said possibly coated side of said mica tape,

(f) applying a heat and/or pressure treatment to said end product.
The method of any one of claims 12 to 14 wherein said laminating is done in an oven, at a temperature of about 300°C.
The method of one of claims 12 to 15, wherein said laminating is done by a number of rolls.
The method of claim 16, wherein one of said rolls is at a temperature of about 300°C.