Multi-layer insulation system for electrical conductors

Abstract

A multi-layer insulation system for electrical conductors, an insulated electrical conductor, a process for preparing an insulated conductor, and an insulated conductor prepared by such a process are provided. The insulated electrical conductors are lightweight, qualify for temperature ratings of up to approximately 230° C., and demonstrate mechanical durability and hydrolysis resistance. As such, these insulated conductors are particularly useful for aircraft wire and cable.

Description

TECHNICAL FIELD OF THE INVENTION

[0002] The present invention basically relates to a multi-layer insulation system for electrical conductors, an insulated electrical conductor, a process for preparing an insulated conductor, and an insulated conductor prepared by such a process. The insulated electrical conductors of the present invention are lightweight, qualify for temperature ratings of up to approximately 230° C., and demonstrate mechanical durability, and hydrolysis resistance. As such, these insulated conductors are particularly useful for aircraft wire and cable.

BACKGROUND OF THE INVENTION

[0003] Electrical insulation must meet a variety of construction and performance requirements. These requirements are particularly severe for electrical cable which is to be used in aircraft and similar equipment. Electrical cable useful for such applications must demonstrate a balance of electrical, thermal, and mechanical properties, with overall performance being evaluated by assessing properties such as abrasion and cut-through resistance, chemical and fluid resistance, dry and wet arc tracking, and flammability and smoke generation. At the same time, such cables must adhere to rigid weight limitations.

[0004] Aircraft wire constructions comprising a polyimide inner layer, and a polytetrafluoroethylene (PTFE) outer layer, are known. In such constructions, the polyimide inner layer is formed by spiral-wrapping an adhesive (e.g., PTFE, fluorinated ethylene-propylene (FEP), or perfluoroalkoxy (PFA))-coated polyimide tape, in an overlapping fashion, about a conductor. The spiral-wrapped polyimide tape is heat-sealed at the spiral-wrapped tape joints. The PTFE outer layer is formed by spiral-wrapping unsintered PTFE tape about the heat-sealed polyimide inner layer. The unsintered PTFE tape outer layer is also heat-sealed at the spiral-wrapped joints by sintering the wrapped tape.

[0005] The above-referenced aircraft wire constructions have a temperature rating of approximately 260° C., and while demonstrating good mechanical durability, these wire constructions provide only low-to-moderate long-term humidity resistance and laser markability properties. In addition, the PTFE outer layer is easily scrapped off, thereby exposing the inner layer and rendering it susceptible to hydrolysis in humid environments.

[0006] As will be readily apparent to those skilled in the art, the aircraft wire constructions described above do not employ a radiation crosslinked outer layer, where exposing perfluorinated polymers such as PTFE, FEP, and PFA to radiation would serve to degrade these materials.

[0007] Aircraft wire constructions comprising one or more layers of extruded ethylene tetrafluoroethylene (ETFE) copolymer, are also known. In such constructions, the ETFE copolymer layer(s) is generally crosslinked by irradiation to achieve use-temperature ratings of greater than 150° to 200° C. The reduction in use-temperature ratings is partially offset by the fact that these wire constructions demonstrate mechanical durability, long-term humidity resistance, and laser markability properties which are superior to those noted above for polyimide/PTFE wire constructions.

[0008] A need therefore exists for an aircraft wire construction which qualifies for higher use-temperatures, while demonstrating improved mechanical durability, long-term humidity resistance, and laser markabilty properties.

[0009] It is therefore an object of the present invention to provide such an insulated wire construction.

[0010] It is a more particular object to provide a multi-layer insulation system for electrical conductors.

[0011] It is another more particular object of the present invention, to provide a lightweight insulated electrical conductor prepared using the above-referenced multi-layer insulation system, which qualifies for a temperature rating of up to approximately 230° C., and which demonstrates improved mechanical durability, and hydrolysis resistance.

[0012] It is yet another more particular object to provide an insulated electrical conductor that further demonstrates flame resistance and laser markability.

[0013] It is a further object of the present invention to provide a process for preparing such an insulated conductor, and an insulated conductor prepared by such a process.

SUMMARY

[0014] The present invention therefore provides a multi-layer insulation system for electrical conductors, which comprises:

[0015] (a) a polyimide or fluoropolymer inner layer,

[0016] wherein, when the inner layer is a polyimide inner layer, the layer is formed by wrapping a polyimide film, which has been coated with a sealable component, in an overlapping fashion, along a portion or length of an electrical conductor, wherein the polyimide film is substantially uniformly sealed to itself in overlapping regions along the length of the conductor, thereby forming an effective seal against moisture, wherein the sealable component comprises a perfluoropolymer, a crosslinked fluoropolymer, or a polyimide adhesive,

[0017] wherein, when the inner layer is a fluoropolymer inner layer, the layer is formed by either extruding a fluoropolymer material along a portion or length of the electrical conductor, or by wrapping a fluoropolymer film, in an overlapping fashion, along a portion or length of the conductor,

[0018] (b) optionally, a polyimide middle layer, wherein the polyimide middle layer is formed by wrapping an optionally coated polyimide film, in an overlapping fashion, along a portion or length of the inner layer formed on the electrical conductor, and

[0019] (c) an extruded, crosslinked fluoropolymer outer layer, wherein the fluoropolymer is selected from the group consisting of copolymers and terpolymers of ethylene-tetrafluoroethylene, and mixtures thereof,

[0020] wherein, when the inner layer is a fluoropolymer inner layer, the multi-layer insulation system includes a polyimide middle layer.

[0021] The present invention also provides an insulated electrical conductor that comprises an electrical conductor insulated with the multi-layer insulation system described above.

[0022] The present invention further provides a process for preparing an insulated electrical conductor, which comprises:

[0023] (a) forming a polyimide or fluoropolymer inner layer on an electrical conductor,

[0024] wherein, when the inner layer is a polyimide inner layer, the layer is formed by wrapping a polyimide film, which has been coated with a sealable component, in an overlapping fashion, along a portion or length of the electrical conductor, wherein the sealable component comprises a perfluoropolymer, a crosslinked fluoropolymer, or a polyimide adhesive,

[0025] wherein, when the inner layer is a fluoropolymer inner layer, the layer is formed by either: i) extruding a fluoropolymer material along a portion or length of the electrical conductor, or ii) wrapping a fluoropolymer film, in an overlapping fashion, along a portion or length of the electrical conductor,

[0026] (b) optionally, forming a polyimide middle layer on the polyimide or fluoropolymer inner layer by wrapping an optionally coated polyimide film, in an overlapping fashion, along a portion or length of the inner layer,

[0027] (c) when the inner layer is a polyimide inner layer or when a middle layer is formed using a coated polyimide film, heating the polyimide film or films to a temperature ranging from about 240° to about 350° C. to cause overlapping regions of the coated film or films to bond, thereby forming an effective seal against moisture along the length of the conductor,

[0028] (d) forming a fluoropolymer outer layer on either the inner or middle layer by extruding a fluoropolymer material along a portion or length of that layer; and

[0029] (e) crosslinking the fluoropolymer outer layer, wherein, when the inner layer or the sealable component comprises a perfluoropolymer (e.g., polytetrafluoroethylene, fluorinated ethylene propylene copolymers, perfluoroalkoxy resins), the fluoropolymer outer layer is crosslinked by exposing it to less than 60 megarads of radiation, with applied voltages ranging from about 50 to about 120 kilo volts,

[0030] wherein, when the inner layer is a fluoropolymer inner layer, the process for preparing an insulated electrical conductor includes forming a polyimide middle layer on the fluoropolymer inner layer.

[0031] The present invention also provides an insulated electrical conductor prepared by the process described above.

[0032] The foregoing and other features and advantages of the present invention will become more apparent from the following description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] FIG. 1 is an elevational side view of a stranded cable insulated with a preferred embodiment of the multi-layer insulation system of the present invention, having the outer insulating layer cut away for purposes of illustration;

[0034] FIG. 2 is an elevational side view of a stranded cable spiral-wrapped with a polyimide film or tape prior to undergoing a heat-sealing operation;

[0035] FIG. 3 is an elevational side view of a stranded cable axially-wrapped with a polyimide film or tape prior to undergoing a heat-sealing operation; and

[0036] FIG. 4 is an elevational side view of a stranded cable insulated with a more preferred embodiment of the multi-layer insulation system of the present invention, having middle and outer insulating layers cut away for purposes of illustration.

BEST MODE FOR CARRYING OUT THE INVENTION

[0037] The multi-layer insulation system of the present invention possesses or demonstrates a combination of characteristics or properties not found in conventional insulating materials. This unique combination of desirable properties make the inventive insulated conductor most valuable in applications such as aircraft, missiles, satellites, etc.

[0038] As will be described in more detail below, the high degree of high temperature adhesive bond strength demonstrated by the inner layer of a preferred embodiment of the present invention has been found to be particularly surprising.

[0039] Referring now to FIG. 1 in detail, reference numeral 10 has been used to generally designate a preferred embodiment of the insulated electrical conductor of the present invention. Insulated electrical conductor 10 basically comprises an electrical conductor 12, which is insulated with a multi-layer insulation system 14 comprising:

[0040] (1) a polyimide film inner layer 16;

[0041] wherein the polyimide film inner layer 16 is formed by wrapping the film, which has been coated with a sealable component, in an overlapping fashion, along a portion or length of the electrical conductor 12,

[0042] wherein the polyimide film is substantially uniformly sealed to itself in overlapping regions along the length of the conductor 12, thereby forming an effective seal against moisture, and

[0043] wherein the sealable component comprises a perfluoropolymer, a crosslinked fluoropolymer, or a polyimide adhesive; and

[0044] (2) an extruded, crosslinked fluoropolymer outer layer 18.

[0045] The electrical conductor 12 of the present invention may take various forms (e.g., metal wire, stranded cable), and may be prepared using any suitable conductive material including copper, copper alloys, nickel, nickel-clad copper, nickel-plated copper, tin, silver, and silver-plated copper. In a preferred embodiment, the electrical conductor is in the form of a stranded cable, and is prepared using copper or nickel-plated copper.

[0046] Any film-forming polyimide may be used in the practice of the present invention, with preferred polyimides being aromatic polyimide films. In a more preferred embodiment, the polyimide film is a polyimide copolymer film derived from the reaction of an aromatic tetracarboxylic acid dianhydride component comprising from 0 to 95 mole %, preferably from 10 to 95 mole %, of 3,3′,4,4′-biphenyltetracarboxylic dianhydride and from 5 to 100 mole %, preferably from 5 to 90 mole %, of pyromellitic dianhydride, and an aromatic diamine component comprising from 25 to 99 mole %, preferably from 40 to 98 mole %, of p-phenylene diamine and from 1 to 75 mole %, preferably from 2 to 60 mole %, of a diaminodiphenyl ether such as 4,4′-diaminodiphenyl ether, 3,3′-diaminodiphenyl ether or 3,4′-diaminodiphenyl ether. Such films are described in U.S. Pat. No. 5,731,088 to Philip R. La Court, which is incorporated herein by reference.

[0047] Polyimide films suitable for use in inner layer 16 of the present invention are films having a sealable component (i.e., a heat-sealable adhesive) coated or laminated on/to at least one surface. It is noted that such films are typically purchased with at least one surface coated with a heat-sealable adhesive, where the coating or lamination of such films constitutes a highly specialized area of practice undertaken by only a limited number of companies.

[0048] Heat-sealable adhesives which may be used in the present invention include perfluoropolymer, crosslinkable fluoropolymer, and polyimide adhesives.

[0049] Perfluoropolymer adhesives, suitable for use in the present invention, include PTFE, FEP, PFA, and copolymers of tetrafluoroethylene and perfluoromethylvinylether (MFA) adhesives, while suitable crosslinkable fluoropolymer adhesives include ETFE and chlorotrifluoroethylene (CTFE) copolymer and terpolymer adhesives which contain minor amounts of one or more fluorinated comonomers (e.g., HFP, HFIB, PFBE, VDF and VF).

[0050] Polyimide adhesives, suitable for use in the present invention, include thermoplastic polyimide adhesives, which soften and become fluid at or above 200° C.

[0051] Preferred heat-sealable films are polyimide films coated or laminated with a heat-sealable polyimide adhesive. Such materials are available from E.I. DuPont de Nemours and Company (“DuPont”), Wilmington, Del., under the trade designation KAPTON HKJ, KAPTON EKJ, and ELJ heat-sealable polyimide films.

[0052] The heat-sealable films are preferably applied to an electrical conductor 12 in tape form, by either spirally or axially wrapping the tape about the conductor 12.

[0053] For spiral-wrap applications, the tape preferably has a width ranging from about 0.30 to about 0.95 centimeters (cm), and a thickness ranging from about 0.01 to about 0.04 millimeters (mm). As best shown in FIG. 2, which depicts electrical conductor 12 spiral-wrapped with a polyimide tape 20 prior to undergoing a heat-sealing operation, the tape 20 is preferably wrapped so as to achieve a degree of overlap ranging from about 10 to about 70%.

[0054] In regard to axial-wrap applications for typical aircraft wire, the tape 20 preferably has a width ranging from about 0.15 to about 0.50 cm, and a thickness ranging from about 0.01 to about 0.04 mm. For much larger conductors, such as main power lines in aircraft, the tape 20 preferably has a width of from about 115 to about 150% of the conductor circumference, and a thickness ranging from about 0.01 to about 0.04 mm. As best shown in FIG. 3, which depicts the conductor 12 axially-wrapped with the polyimide tape 20 prior to undergoing a heat-sealing operation, the tape 20 is preferably wrapped so as to achieve a degree of overlap ranging from about 15 to about 50%.

[0055] After the tape 20 is applied to the conductor 12, the resulting assembly is heated to a temperature ranging from about 240 to about 350° C., preferably from about 260 to about 280° C. The purpose of the heating operation is to bond or fuse the overlapping regions of the polyimide tape 20, thereby forming an effective seal against moisture along the length of the conductor 12. As a result, the electrical integrity of the conductor 12 will be preserved.

[0056] The thickness of the inner layer 16 of the insulated electrical conductor 10 of the present invention preferably ranges from about 0.01 to about 0.08 mm, and more preferably ranges from about 0.02 to about 0.05 mm.

[0057] Inner layer 16 demonstrates a high temperature (i.e., 150° C.) adhesive bond strength ranging from about 100 to about 250 grams per inch-width (gm/inch-width). When inner layer 16 is prepared using a polyimide film coated or laminated with a heat-sealable polyimide adhesive, it demonstrates a high temperature (i.e., 150° C.) adhesive bond strength of greater than 1000 gm/inch-width, preferably greater than 1500 gm/inch-width. Such adhesive bond strengths are considerably higher than those demonstrated by prior art heat-sealed wire insulations. High temperature adhesive bond strength is measured in accordance with ASTM#1876-00—Standard Test Method for Peel Resistance of Adhesives (T-Peel Test).

[0058] As referenced above, the high degree of high temperature adhesive bond strength demonstrated by inner layer 16, when prepared using the preferred heat-sealable films, has been found to be particularly surprising.

[0059] Fluoropolymers which may advantageously be utilized in the outer layer 18 of the insulated electrical conductor 10 of the present invention include, for example, copolymers and terpolymers of ethylene-tetrafluoroethylene (ETFE), and mixtures thereof.

[0060] It is noted that extruded fluoropolymer outer layers change color as a result of thermal aging. Where polyimides demonstrate greater thermal stability than fluoropolymers, the noted color change in the outer layer can serve as an early warning signal that the insulated electrical conductor will need to be replaced. This feature is extremely valuable in aircraft wire and cable applications.

[0061] In a preferred embodiment, the fluoropolymer of outer layer 18 is an ETFE copolymer which comprises 35 to 60 mole % (preferably 40 to 50 mole %) of units derived from ethylene, 35 to 60 mole % (preferably 50 to 55 mole %) of units derived from tetrafluoroethylene and up to 10 mole % (preferably 2 mole %) of units derived from one or more fluorinated comonomers (e.g., HFP, HFIB, PFBE, VDF and VF). Such copolymers are available from DuPont under the trade designation TEFZEL HT 200, and from Daikin America, Inc. (“Daikin”), Orangeburg, N.Y., under the trade designation NEOFLON EP-541.

[0062] The fluoropolymer(s) preferably contains (as extruded) from about 4 to about 16% by weight of a crosslinking agent. Preferred crosslinking agents are radiation crosslinking agents that contain multiple carbon-carbon double bonds.

[0063] In a more preferred embodiment, crosslinking agents containing at least two allyl groups and more preferably, three or four allyl groups, are employed. Particularly preferred crosslinking agents are triallyl isocyanurate (TAIC), triallylcyanurate (TAC) and trimethallylisocyanurate (TMAIC).

[0064] In yet a more preferred embodiment, the fluoropolymer(s) contains a photosensitive substance (e.g., titanium dioxide), which renders the outer layer 18 receptive to laser marking. The term “laser marking,” as used herein, is intended to mean a method of marking an insulated conductor using an intense source of ultraviolet or visible radiation, preferably a laser source. In accordance with this method, exposure of the fluoropolymer outer layer 18 to such intense radiation will result in a darkening where the radiation was incident. By controlling the pattern of incidence, marks such as letters and numbers can be formed.

[0065] In yet a more preferred embodiment, the fluoropolymer(s) contains from about 1 to about 4% by weight, of titanium dioxide.

[0066] In addition to the above component(s), the fluoropolymer(s) may advantageously contain other additives such as pigments (e.g., titanium oxide), lubricants (e.g., PTFE powder), antioxidants, stabilizers, flame retardants (e.g., antimony oxide), fibers, mineral fibers, dyes, plasticizers and the like. However, some such additives may have an adverse effect on the desirable properties of the insulated electrical conductor of the present invention.

[0067] The components of the outer layer may be blended together by any conventional process until a uniform mix is obtained. In a preferred embodiment, a twin-screw extruder is used for compounding. The outer layer 18 is preferably formed by melt-extrusion, and then crosslinked using either known techniques, which include beta and gamma radiation crosslinking methods, or “skin irradiation” techniques. “Skin irradiation” techniques are described in more detail below.

[0068] The thickness of the outer layer 18 of the insulated electrical conductor 10 of the present invention preferably ranges from about 0.05 to about 0.25 mm, and more preferably ranges from about 0.10 to about 0.13 mm.

[0069] Referring now to FIG. 4 in detail, reference numeral 110 has been used to generally designate a more preferred embodiment of the insulated electrical conductor of the present invention. In this more preferred embodiment, insulated electrical conductor 110 demonstrates improved flexibility, and comprises an electrical conductor 112, which is insulated with a multi-layer insulation system 114 comprising:

[0070] (1) a fluoropolymer inner layer 116,

[0071] wherein the fluoropolymer inner layer 116 is formed by either extruding a fluoropolymer material along a portion or length of the electrical conductor 112, or wrapping a fluoropolymer film, in an overlapping fashion, along the length of the conductor 112,

[0072] (2) a polyimide film middle layer 117, wherein the polyimide middle layer 117 is formed by wrapping an optionally coated polyimide film, in an overlapping fashion, along a portion or length of the inner layer 116; and

[0073] (3) an extruded, crosslinked fluoropolymer outer layer 118.

[0074] Fluoropolymers which may advantageously be utilized in the inner layer 116 of the insulated electrical conductor 110 of the present invention include, for example, MFA, PFA, PTFE, ethylene-chlorotrifluoroethylene (ECTFE) copolymers, ethylene-tetrafluoroethylene (ETFE) copolymers, polyvinylidene fluoride (PVDF), tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride (THV), polyvinylfluoride (PVF) resins, and mixtures thereof.

[0075] In a preferred embodiment, inner layer 116 is extruded and the fluoropolymer comprises a copolymer or terpolymer of ETFE. In a more preferred embodiment, the polymer is an ETFE terpolymer that has been compounded with a TAIC crosslinking agent. Such polymers are available from DuPont and Daikin, under the product designations TEFZEL HT200 fluoropolymer resin and NEOFLON EP-541 fluoropolymer resin, respectively.

[0076] In yet a more preferred embodiment, inner layer 116 is extruded and crosslinked and the extruded fluoropolymer material of inner layer 116 is substantially the same as the material used to prepare outer layer 118, but contains less crosslinking agent.

[0077] In another preferred embodiment, inner layer 116 is wrapped and the fluoropolymer is PTFE tape. In a more preferred embodiment, the PTFE is in the form of a skived tape, with such tapes being available from Goodrich Corporation, Four Coliseum Centre, 2730 West Tyvola Road, Charlotte, N.C. 28217-4578, under the product designation PTFE Skived Tapes.

[0078] The fluoropolymer film inner layer 116 may be a heat-sealed or a non-heat-sealed fluoropolymer film inner layer. It is noted that wrapped fluoropolymer tapes or films will fuse or bond to themselves in overlapping regions at temperatures at or above the melting point of the fluoropolymer, thereby obviating the need to employ a heat-sealable adhesive with such films.

[0079] The polyimide film of middle layer 117 is preferably applied to inner layer 116 in tape form, by spirally wrapping the tape about inner layer 116, so as to achieve a degree of overlap ranging from about 10 to about 70%. In one embodiment, the polyimide film of middle layer 117 does not employ a heat-sealable adhesive and is not heat-sealed. In another embodiment, the polyimide film employs a heat-sealable adhesive and is substantially uniformly sealed to itself in over-lapping regions along the length of inner layer 116. In one such embodiment, inner layer 116 is formed using a fluoropolymer tape and the fluoropolymer tape is heated together with the coated polyimide film, but is not sealed.

[0080] Preferred non-heat-sealable polyimide films have a thickness ranging from about 0.01 to about 0.04 mm, and are available from DuPont, under the trade designation KAPTON H and KAPTON E polyimide films. Preferred heat-sealable polyimide films are the same as those noted above for inner layer 16.

[0081] The preferred insulated electrical conductor 110 described above, which employs a non-heat-sealed polyimide film middle layer, demonstrates a degree of flex which is substantially greater than prior art wire constructions. The degree of flex or wire flexibility is measured by: selecting a 0.9 meter section of insulated wire (i.e., an insulated stranded nickel plated copper conductor (20 American Wire Gage (AWG), 19 Strand, nickel plated copper) measuring 0.95 mm in diameter), which is substantially free of kinks and bends; attaching a ring connector to each end of the conductor; attaching a 100 gram weight to each ring connector; carefully suspending the insulated wire on a stationary mandrel having a diameter measuring 0.48 cm; waiting one minute; and measuring the width between parallel insulated wire segments at three different points along the length of the wire. The degree of flex or wire flexibility is an average of the three width measurements.

[0082] In a most preferred embodiment, insulated electrical conductor 110 comprises an electrical conductor 112, which is insulated with a multi-layer insulation system 114 comprising: (1) an extruded, crosslinked ETFE inner layer 116; (2) a non-heat-sealed polyimide film middle layer 117; and (3) an extruded, crosslinked ETFE outer layer 118.

[0083] In another most preferred embodiment, insulated electrical conductor 110 comprises an electrical conductor 112, which is insulated with a multi-layer insulation system 114 comprising: (1) a non-heat-sealed PTFE inner layer 116; (2) a heat-sealed polyimide film middle layer 117; and (3) an extruded, crosslinked ETFE outer layer 118.

[0084] It is noted that although the present inventive insulated electrical conductor 10, 110 has been described hereinabove as an insulated stranded cable, it is not so limited. The insulated conductor 10, 110 may comprise a single wire covered with the multi-layer insulation system 14, 114 of the present invention, or may comprise a plurality of bunched, twisted, or bundled wires, with each wire separately covered with the multi-layer insulation system 14, 114. The insulated conductor 10, 110 may also comprise a plurality of single or dual layer insulated wires which are coated with the polyimide or fluoropolymer inner layer 16, 116 and optionally, with the polyimide film middle layer 117. In this embodiment, the plurality of single or dual layer insulated wires are covered with a sheath consisting of the crosslinked fluoropolymer outer layer 18, 118.

[0085] The process for preparing the insulated electrical conductor 10, 110 of the present invention basically comprises:

[0086] (a) forming a polyimide or fluoropolymer inner layer 16, 116 on an electrical conductor 12, 112,

[0087] wherein, when the inner layer is a polyimide inner layer, the layer 16, 116 is formed by wrapping a polyimide film, which has been coated with a sealable component, in an overlapping fashion, along a portion or length of the electrical conductor 12, 112, wherein the sealable component comprises a perfluoropolymer, a crosslinked fluoropolymer, or a polyimide adhesive,

[0088] wherein, when the inner layer is a fluoropolymer inner layer, the layer 16, 116 is formed by either: i) extruding a fluoropolymer material along a portion or length of the electrical conductor 12, 112, or ii) wrapping a fluoropolymer film, in an overlapping fashion, along a portion or length of the electrical conductor 12, 112,

[0089] (b) optionally, forming a polyimide middle layer 117 on the polyimide or fluoropolymer inner layer 16, 116 by wrapping an optionally coated polyimide film, in an overlapping fashion, along a portion or length of the inner layer 16, 116,

[0090] (c) when the inner layer 16, 116 is a polyimide inner layer or when a middle layer 117 is formed using a coated polyimide film, heating the polyimide film or films to a temperature ranging from about 240° to about 350° C. to cause overlapping regions of the coated film or films to bond, thereby forming an effective seal against moisture along the length of the conductor 12, 112,

[0091] (d) forming a fluoropolymer outer layer 18, 118 on either the inner or middle layer 16, 116, 117 by extruding a fluoropolymer material along a portion or length of that layer; and

[0092] (e) crosslinking the fluoropolymer outer layer 18, 118, wherein, when the inner layer 16, 116 or the sealable component comprises a perfluoropolymer (e.g., polytetrafluoroethylene, fluorinated ethylene propylene copolymers, perfluoroalkoxy resins), the fluoropolymer outer layer 18, 118 is crosslinked by exposing it to less than 60 megarads of radiation, with applied voltages ranging from about 50 to about 120 kilo volts,

[0093] wherein, when the inner layer 16, 116 is a fluoropolymer inner layer, the process for preparing an insulated, electrical conductor includes forming a polyimide middle layer 117 on the polyimide or fluoropolymer inner layer 16, 116.

[0094] Insulated electrical conductors 10, 110 that do not employ perfluoropolymers are preferably subjected to an irradiation step to effect crosslinking in the fluoropolymer outer layer 18, 118. In a more preferred embodiment, the dosage of ionizing radiation (e.g., accelerated electrons or gamma rays) employed in the irradiation step is below 50 megarads (Mrads), more preferably, between 5 and 25 Mrads and, most preferably, between 15 and 25 Mrads, while applied voltages range from about 0.25 to about 3.0 mega volts (MV), and preferably range from about 0.5 to about 1.0 MV. The irradiation step is preferably carried out at ambient temperature.

[0095] Insulated electrical conductors 10, 110, which employ an inner layer or sealable component comprising a perfluoropolymer are subjected to a so-called “skin irradiation” process to effect crosslinking in the fluoropolymer outer layer 18, 118. The subject process employs ionizing radiation in the form of accelerated electrons, and basically comprises using an accelerated voltage such that the maximum attained distance of accelerated charged particles is less than or equal to the thickness of the outer layer 18, 118. More specifically, with an applied voltage of 120 KV, most electrons will penetrate outer layer 18, 118 to a maximum depth of approximately 0.13 mm.

[0096] Such a technique or process is briefly described in JP 4-52570 in regard to automotive low voltage wire coated with e.g. a soft vinyl chloride resin. JP 4-52570 is incorporated herein by reference.

[0097] In a preferred embodiment, the dosage of ionizing radiation (i.e., accelerated electrons) employed in the irradiation step is below 60 Mrads, more preferably, between 20 and 50 Mrads and, most preferably, between 30 and 40 Mrads, while applied voltages range from about 50 to about 120 kilo volts (KV), and preferably range from about 100 to about 120 KV. The “skin irradiation” technique or process is preferably carried out at ambient temperature.

[0098] It is noted that in the “skin irradiation” technique described above, where electrons do not reach the conductor during electron beam irradiation, electrons may accumulate in the insulation thereby increasing the possibility of flooding and/or channeling. As will be readily appreciated by those skilled in the art, electron flooding and channeling may damage the insulation by causing the formation of tiny pin-holes.

[0099] The present inventors have discovered that by exposing “skin irradiated” insulated electrical conductor 10, 110 to elevated temperatures ranging from about 150 to about 220° C., accumulated electrons may be more effectively drained off without damaging the insulation.

[0100] The insulated electrical conductor 10, 110 of the present invention is lightweight, and may be used in environments where temperatures may exceed 230° C. In addition, the inventive conductor 10, 110 demonstrates mechanical durability and resistance to hydrolysis.

[0101] Preferably, insulated conductor 10, 110 weighs from about 1.9 to about 2.0 kilograms (kg) per 305 meters (m), which serves to satisfy the maximum weight limits set forth in the following Military Specifications—M22759/92-20, M22759/86-20, M22759/32-20, and M22759/34-20.

[0102] The 230° C. temperature rating of insulated electrical conductor 10, 110 was determined in accordance with Military Specification MIL-DTL-22759/87A—Accelerated Aging Test. This test, which requires aging wire samples for 500 hours in an air-circulating oven maintained at a temperature of 290° C., was modified to the extent that the oven temperature was reduced to 260° C.

[0103] Mechanical durability is evidenced by the ability of insulated electrical conductor 10, 110 to pass the following tests: (1) Wire-to-Wire Abrasion Resistance—Boeing Specification Support Standard BSS 7324 entitled “Procedure for Testing Electrical Wire and Cable” dated Dec. 2, 1998 (“Boeing BSS 7324); (2) Dynamic Cut-Through Resistance (at elevated temperatures of up to 260° C.)—ASTM D 3032, Section 22, and Military Specification MIL-DTL-22759/87A; and (3) Sandpaper Abrasion Resistance—Society of Automotive Engineers (SAE) test method J1128 Section 5.10.

[0104] The resistance to hydrolysis demonstrated by insulated electrical conductor 10, 110 was measured in accordance with SAE test method AS4373, Section 4.6.2, Method 602.

[0105] In a more preferred embodiment, the multi-layer insulation system and insulated electrical conductor 10, 100 of the present invention demonstrate other desirable properties including excellent resistance to flame, the ability to be marked using ultraviolet or visible radiation, electrical resistance, humidity resistance, low smoke generation, notch propagation resistance, weathering resistance, wet and dry arc track resistance, and resistance to common solvents and other fluids used in the aircraft industry.

[0106] The subject invention will now be described by reference to the following illustrative examples. The examples are not, however, intended to limit the generally broad scope of the present invention.

WORKING EXAMPLES
Components Used
In the Working Examples set forth below, the following components and
materials were used:
CONDUCTOR: a stranded nickel plated copper conductor (20
           American Wire Gage (AWG), 19 Strand, nickel
           plated copper) measuring 0.95 mm in diameter.
POLYIMIDE  heat-sealable polyimide film coated or laminated
FILM I:    on both sides with a heat-activated, high
           temperature polyimide adhesive, marketed under the
           trade designation KAPTON HKJ heat-sealable
           polyimide film, by DuPont.
POLYIMIDE  heat-sealable polyimide film coated or laminated on
FILM II:   both sides with a heat-activated, high temperature
           polyimide adhesive, marketed under the trade
           designation KAPTON EKJ heat-sealable polyimide
           film, by DuPont.
POLYIMIDE  heat-sealable polyimide film coated or laminated on
FILM III:  both sides with a heat-activated, medium
           temperature polyimide adhesive, marketed under the
           trade designation KAPTON ELJ heat-sealable
           polyimide film, by DuPont.
POLYIMIDE  heat-sealable polyimide film coated or laminated on
FILM IV:   both sides with a heat-activated perfluoropolymer
           adhesive, marketed under the trade designation
           KAPTON XP heat-sealable polyimide film, by
           DuPont.
POLYIMIDE  heat-sealable polyimide film coated or laminated on
FILM V:    both sides with a heat-activated perfluoropolymer
           adhesive, marketed under the trade designation
           OASIS TWT561 heat-sealable polyimide film, by
           DuPont.
ETFE:      a copolymer comprising 35 to 60 mole % of
           ethylene; 60 to 35 mole % of tetrafluoroethylene;
           and up to 10 mole % of a fluorinated termonomer,
           marketed under the trade designation TEFZEL HT
           200 fluoropolymer resin, by DuPont. Melting point
           of fluoropolymer resin is approximately 270° C.
ETFE(I):   a copolymer comprising 30 to 50 mole % of
           ethylene; 70 to 50 mole % of tetrafluoroethylene;
           and up to 10 mole % of a fluorinated termonomer,
           marketed under the trade designation TEFZEL
           HT 2127 fluoropolymer resin, by DuPont. Melting
           point of fluoropolymer resin is approximately
           243° C.
PTFE:      a skived polytetrafluoroethylene film, marketed
           under the trade designation TEFLON TFE fluoro-
           polymer resin, by DuPont.
TAIC:      a triallyl isocyanurate crosslinking agent, marketed
           under the designation TAIC triallyl isocyanurate, by
           Nippon Kasei Chemical Co., Ltd., Tokyo, Japan.
TiO2:      titanium dioxide pigment in powder form (≧96%
           in purity), marketed under the trade designation
           TIPURE titanium dioxide pigment, by DuPont.

Sample Preparation

EXAMPLES 1A TO 1E

[0107] A continuous strip of POLYIMIDE FILM I, measuring 0.64 cm in width and 0.03 mm in thickness, was spiral-wrapped, at a 53% overlap, about a CONDUCTOR. The spiral-wrapped CONDUCTOR was then heated in a continuous process to a temperature in excess of 300° C. for approximately 5 seconds to heat-seal the overlapping portions of the POLYIMIDE FILM I strip, and was then allowed to cool. The thickness of the heat-sealed, spiral-wrapped POLYIMIDE FILM I inner layer was 0.05 mm.

[0108] A quantity of ETFE was compounded with 8% by wt. TAIC and 2% by wt. TiO2 and was then extruded over the POLYIMIDE FILM I inner layer using a single-screw extruder having four heating zones which were set at 200°, 240°, 275°, and 290° C., respectively. The thickness of the extruded ETFE layer was 0.13 mm.

[0109] Test samples were then irradiated using electron-beam radiation, with air-cooling. Total beam dosages were 10, 15, 20, or 30 megarads, while applied voltages were either 120 KV, 150 KV, or 0.5 MEV.

[0110] The subject wire construction is described in Table 1, hereinbelow.

EXAMPLES 2, 3A TO 3C, 4A AND 4B

[0111] Four test samples of the wire construction labeled Example 2, ten test samples of Example 3, and six test samples of Example 4, were prepared substantially in accordance with the method identified above for Example 1, except that test samples for each Example were prepared using a different polyimide film. As above, total beam dosages were 10, 15, 20, or 30 megarads, while applied voltages were either 120 KV, 150 KV, or 0.5 MEV.

[0112] The subject wire constructions are more fully described in Table 1, hereinbelow.

EXAMPLE 5

[0113] One thousand feet of the wire construction labeled Example 5 were repared substantially in accordance with the method identified above for Examples 1A to 1E, except that total beam dosage was 18 megarads, while applied voltages were 0.5 mega electron volts.

[0114] The subject wire construction is more fully described in Table 1, hereinbelow.

EXAMPLES 6 TO 9

[0115] A continuous strip of PTFE, measuring 0.63 cm in width and 0.025 mm in thickness, was spiral-wrapped, at either a 54% overlap (Example 6) or a 15% overlap (Examples 7 to 9), about a CONDUCTOR. A continuous strip of either POLYIMIDE FILM III (Examples 6 and 7), measuring 0.63 cm in width and 0.025 mm in thickness or POLYIMIDE FILM II (Examples 8 and 9), measuring 0.63 cm in width and 0.018 mm in thickness, was then spiral-wrapped, at a 54% overlap, about the spiral-wrapped PTFE inner layer. The spiral-wrapped CONDUCTOR was then heated in a continuous process to a temperature in excess of 300° C. for approximately 5 seconds to heat-seal the overlapping portions of the POLYIMIDE FILM layer, and was then allowed to cool. The thickness of the inner and middle layers was 0.076 mm (Examples 6 and 7) and 0.061 mm (Examples 8 and 9).

[0116] A quantity of ETFE or ETFE(I) was compounded with 8% by wt. TAIC and 2% by wt. TiO2 and was then extruded over the POLYIMIDE FILM middle layer using a single-screw extruder having four heating zones which were set at 200°, 240°, 275°, and 290° C., respectively. The thickness of the extruded ETFE or ETFE(I) layers was 0.13 mm (Examples 6 and 7) and 0.14 mm (Examples 8 and 9).

[0117] Five hundred feet of each test sample wire construction were then irradiated using electron-beam radiation, with air-cooling. Total beam dosages were 18 megarads for Examples 6 and 7, and 36 megarads for Examples 8 and 9, while applied voltages were 0.5 MEV.

[0118] The subject wire constructions are more fully described in Table 1, hereinbelow.

EXAMPLES C-1 AND C-2

[0119] Four test samples each of prior art wire constructions C-1 and C-2 were prepared as set forth below.

[0120] C-1 was prepared substantially in accordance with the method identified above for Example 1, except that 0.06 mm thick PTFE tape was spiral-wrapped, with a 53% overlap, over a spiral-wrapped POLYIMIDE FILM IV inner layer prior to heat-sealing. The resulting wire construction was then exposed to a temperature in excess of 330° C. to effect heat-sealing in both layers.

[0121] C-2 was prepared by compounding ETFE with 1.5% by wt. TAIC, and then by extruding the compounded material over the CONDUCTOR using a single-screw extruder, as described above. A quantity of compounded ETFE material, which had been compounded with 8% by wt. TAIC, was then extruded over the ETFE inner layer, and the resulting wire construction irradiated using electron-beam radiation, with air cooling. Total beam dosage was 30 megarads, with an applied voltage of 0.5 MEV.

[0122] The subject prior art wire constructions are more fully described in Table 1, hereinbelow.

TABLE 1
Summary of Examples 1A to 1E, 2, 3A to 3C, 4A, 4B, 5 to 9, C-1 and C-2
	1A, 1B, 1C,
EXAMPLE	1D, 1E	2	3A, 3B, 3C	4A, 4B	5	6	7	8	9	C-1	C-2
Inner Layer	Polyimide	Polyimide	Polyimide	Polyimide	Polyimide	PTFE	PTFE	PTFE	PTFE	Polyimide	ETFE
	Film I	Film II	Film IV	Film V	Film I					Film V
Adhesive	PI1	PI	FP2	FP	PI	—	—	—	—	FP	N/A
Thickness of	0 05	0.08	0 08	0 06	0 05	0 03	0 03	0 03	0 03	0 06	0 09
Inner Layer
(mm)
Middle	—	—	—	—	—	Polyimide	Polyimide	Polyimide	Polyimide	—	—
Layer						Film III	Film III	Film II	Film II
Adhesive	—	—	—	—	—	PI	PI	PI	PI	—	—
Thickness of	—	—	—	—	—	0 05	0 05	0 04	0 04	—	—
Middle
Layer (mm)
Outer Layer	ETFE	ETFE	ETFE	ETFE	ETFE	ETFE	ETFE	ETFE (I)	ETFE	PTFE	ETFE
Thickness of	0.13	0 13	0 13	0 13	0 13	0.13	0 13	0 13	0.13	0 13	0 13
Outer Layer
(mm)
Total	0 20	0.20	0 20	0 20	0 20	0 21	0 20	0 20	0 21	0 20	0 20
Insulation
Thickness
(mm)
Total Weight	6 50	6 69	6 37	6 40	6 50	6 69	6 62	6 46	6 71	6 89	6 60
of Insulated
Wire
(gms/m)
1PI = polyimide adhesive
2FP = perfluoropolymer adhesive

[0123] The prepared test samples were then subjected to the test procedures identified below. Test procedures, with the exception of ease of peel, are fully described in the following publications: (1) Boeing Specification Support Standard BSS 7324 entitled “Procedure for Testing Electrical Wire and Cable” dated Dec. 2, 1998 (“Boeing BSS 7324”); (2) Military Specification MIL-DTL-22759/87A entitled “Wire, Electrical, Polytetrafluoroethylene/Polyimide Insulated, Normal Weight, Nickel Coated Copper Conductor, 260° C., 600 Volts,” and dated Feb. 23, 1998; (3) Military Specification MIL-STD-2223 entitled “Test Methods for Insulated Electrical Wire,” and dated Aug. 31, 1992; (4) Society of Automotive Engineers (SAE) test method AS4374 entitled “Test Methods for Insulated Electrical Wire,” and dated August, 1994; and (5) SAE test method J1128 entitled “Surface Vehicle Standard, Low Tension Primary Cable,” and dated May, 2000, all of which are incorporated herein by reference.

Test Methods
Accelerated Aging or           Boeing BSS 7324, paragraph no. 7.1a, pp. 12 to 14, conducted
Shrinkage Resistance (P, F):   at 280° C.
Current Overload               Boeing BSS 7324, paragraph no. 7.16, pp. 48 to 50, conducted
Capacity:                      at room temperature.
                               The insulated wire test samples were evaluated for current
                               overload capacity by removing 13 mm of insulation from wire
                               samples measuring 1.5 m in length. The samples were then
                               suspended horizontally in a test set-up with no visible sag.
                               Then, 33 amperes (amps) of current was applied to each test
                               sample for a period of 5 minutes and the samples cooled to
                               room temperature. Each test sample was visually inspected
                               during current application and after the samples were returned
                               to room temperature. The test samples were then subjected to
                               the dry dielectric test that is described in the Boeing BSS 7324
                               Specification. The test, which was repeated six times, was
                               deemed passed if at least five out of the six samples passed the
                               test.
Cut-Through                    MIL-DTL-22759/87
Resistance (lbs):              Boeing BSS 7324, paragraph no. 7.23, p. 58, Dynamic Cut-
                               Through
                               The insulated wire samples were tested for cut-through
                               resistance using the method described below. The cut-through
                               test measured the resistance of the wire insulation to the
                               penetration of a cutting surface and simulated the type of
                               damage that can occur when a wire is forced by mechanical
                               loading against a sharp edge. The test was performed at room
                               temperature (23° C.), at 150° C., at 200° C., and at 260° C., to
                               evaluate the effect of the elevated temperature on insulation
                               performance. The standard cutting edge used was stainless steel
                               and had a radius of 0.406 mm.
                               For each test, a 600 mm (in length) test sample was clamped in
                               place between a blade and a flat plate within an INSTRON
                               compression tester, and the ends of the conductor connected to
                               an 18 VDC electrical circuit. The cutting edge of the blade was
                               oriented perpendicularly to the axis of the sample. The cutting
                               edge was then forced through the insulation at a constant rate of
                               1.27 mm per minute until contact with the conductor occurred.
                               A detection circuit sensed contact of the cutting edge with the
                               conductor and recorded the maximum force, encountered during
                               the test. The test was then repeated four times rotating the
                               sample between tests to offset the effect of eccentric insulation.
                               The reported cut-through resistance was the arithmetic mean of
                               five tests performed on each sample.
Dry Arc Propagation            MIL-STD-2223 Method 3007.
Resistance (P, F, or           Boeing BSS 7324, paragraph no. 7.4, pp. 16 to 30, conducted at
number of wires passed):       room temperature.
                               The insulated wire samples were tested for dry arc propagation
                               resistance using the method described below. Each test sample
                               was cut into 7 pieces, with each piece measuring 35 cm in
                               length. The insulation from five of the seven pieces was
                               stripped from the ends of each piece exposing about 5 mm of
                               conductor and the pieces designated “active wires.” The
                               insulation from the remaining two wires was left intact and the
                               pieces designated “passive wires.”
                               The seven wire pieces were then bundled such that one active
                               wire was located in the center of the bundle while the
                               remaining six wire pieces surrounded the central active wire.
                               The two passive wires were located side-by-side within the
                               bundle. The seven-wire bundle was laced together at four
                               locations so as to keep all seven wires tightly held together
                               throughout the length of the bundle. The distance between the
                               two central laces was about 2.5 cm, while the distance between
                               the central two laces and the outer two laces was about 1.25
                               cm.
                               The wire bundle was then placed in a jig similar to that shown
                               in the Boeing BSS 7324 Specification. The two passive wires
                               were located at the bottom of the jig, while the stripped wires
                               were individually connected to an electrical circuit. More
                               specifically, the five active wires were connected to a three
                               phase 400 Hz power source. Then, a knife blade with a 250 gm
                               load was placed on top of the wire bundle perpendicular to
                               each wire and the blade movement initiated. The blade moved
                               back and forth at a speed of 0.75 cycles/second. When the top
                               two wires were shorted out, the system was de-energized. Each
                               wire was exposed to a 1000 volt wet dielectric withstand test to
                               check whether the remaining insulation could withstand such
                               voltage. When the insulation withstood 1000 volts, the voltage
                               was increased to 2500 volts. When the wire withstood 1000
                               volts, it is considered to have passed the test.
                               This test was deemed passed if: (1) a minimum of 64 wires
                               passed the dielectric test; (2) three wires or less failed the
                               dielectric test in any one bundle; and (3) actual damage to the
                               wire was not more than 3 inches in any test bundle.
Ease of Peel:                  Test samples employing a dual layer insulation system and
                               measuring 0.9 meter in length were tested for ease of peel by
                               (1) removing the outer insulation layer, (2) manually seizing a
                               leading edge of the inner insulation layer (i.e., polyimide tape),
                               and (3) slowly peeling the tape off of the conductor or wire.
                               The inner insulation layer was deemed “continuously peelable”
                               if the entire width of the tape could be continuously peeled
                               from at least five revolutions of the wire without tearing.
Hydrolysis Resistance (P, F):  MIL-DTL-22759/87A and SAE AS4373, Method 602 Test
                               (Unconditioned Wire: AS4373, Section 4.6.2.4.2)
                               Test samples having an insulation thickness of approximately
                               0.20 mm and measuring approximately 762 mm in length were
                               separately fixed and wound on an 8 mm mandrel and placed in
                               salt solution [5% (m/m) of NaCl in water] contained in a 2 liter
                               beaker. The ends of each wound test sample were positioned
                               outside or above the salt solution in the beaker. The test
                               samples were then allowed to age in the salt solution for from
                               672 to >10,000 hours at 70° C. ± 2° C. Starting at 672 hours, the
                               test samples were visually inspected and then periodically
                               subjected to the Withstand Voltage Test as described below.
                               The Hydrolysis Test was deemed “passed” if the sample, upon
                               being subjected to the Withstand Voltage Test, did not
                               demonstrate any electrical breakdown.
                               Withstand Voltage Test (P, F): For this test, the ends of each
                               test sample were twisted together to form a loop. The looped
                               test sample was then immersed in the salt solution contained in
                               the beaker. The ends of each test sample were located above
                               the solution. A test voltage of 2.5 kV (rms) was then applied
                               through an electrode between the conductor and the solution for
                               five (5) minutes.
Life Cycle (P, F):             MIL-DTL-22759/87A. Five (5) hours at 230 to 290° C. ± 2° C.
                               Dielectric test, 2.5 kV (rms) for five (5) minutes.
                               Test samples were tested for life cycle by aging the samples
                               and then by subjecting the aged samples to the Withstand
                               Voltage Test noted above. The samples were aged by
                               separately fixing the samples on a mandrel having a one-half
                               inch diameter and then placing the mandrel and test samples in
                               an air circulation oven set at 30° C. above the intended
                               temperature rating for the product, for a period of 500 hours.
Laser Markability:             Boeing BSS 7324, paragraph no. 7.36, pp. 82 to 83, conducted
                               at room temperature.
                               Test conducted by Spectrum Technologies PLC, Western
                               Avenue, Bridgend CF31 3RT, UK, using a CMS II Contrast
                               Meter.
Sandpaper Abrasion (mm):       SAE J1128, Section 6.10
                               Test samples having an insulation thickness of approximately
                               0.20 mm and measuring 1,000 mm in length were tested for
                               sandpaper abrasion resistance by removing 25 mm of insulation
                               from one end of each test sample and by horizontally mounting
                               each test sample (taut and without stretching) on a continuous
                               strip of abrasion tape in an apparatus that was built by Glowe-
                               Smith Industrial, Inc. (G.S.I. Model No. CAT-3) in accordance
                               with Military Specification MIL-T-5438 and that was capable
                               of exerting a force on the sample while drawing the abrasion
                               tape under the sample at a fixed rate. For each test, 150J garnet
                               sandpaper (with 10 mm conductive strips perpendicular to the
                               edge of the sandpaper spaced a maximum of every 75 mm) was
                               drawn under the sample at a rate of 1500 ± 75 mm/min while a
                               total force of 2.16 ± 0.05 N was exerted on the test sample.
                               The sandpaper approached and exited each test sample from
                               below at an angle of 29 ± 2° to the axis of the test sample and
                               was supported by a rod 6.9 mm in diameter. The length of
                               sandpaper necessary to expose the core or wire was recorded
                               and the test sample moved approximately 50 mm and rotated
                               clockwise 90°. The above-referenced procedure was repeated
                               for a total of four readings. The mean of the four readings
                               constituted the sandpaper abrasion resistance for the subject test
                               sample.
                               It is noted that since the test samples had very thin insulation,
                               this test had to be stopped frequently to observe failure points.
Strippability:                 ASTM D3032 Section 27.
                               Boeing BSS 7324, paragraph no. 7.48, pp. 96 to 97, conducted
                               at room temperature.
                               Test samples were tested for strippability by carefully removing
                               70 mm of insulation from test samples measuring 76 mm in
                               length. The bare conductor portion of the test specimen was
                               then threaded through a loosely fitted hole of a jig so that the
                               unstripped insulation stayed at one side of the jig and the
                               stripped wire at the other. Using an INSTRON Tensile Tester,
                               the bare conductor was pulled while the jig was fixed in place.
                               The force required to pull the remaining 6 mm slug of insulation
                               from the test sample was reported as strip force.
                               This test was deemed passed if the strip force fell within the
                               range of from ¼ to 6 pounds (lbs).
Wet Arc Propagation            MIL-STD-2223, Method 3006.
Resistance (P, F, or           Boeing BSS 7324, paragraph no. 7.4.6 & 7, pp. 26 to 29,
number of wires passed):       conducted at room temperature
                               Test samples were tested for wet arc propagation resistance by
                               preparing seven test samples measuring 35 cm in length from a
                               3 m long insulated wire sample. Five of the seven wire
                               segments were stripped at both ends exposing about 5 mm of
                               conductor. These stripped wire segments were designated
                               “active wires.” The remaining two wire segments that were not
                               stripped were called “passive wires.”
                               The seven wire pieces were then bundled such that one active
                               wire was located in the center of the bundle while the
                               remaining six wire pieces surrounded the central active wire.
                               The two passive wires were located side-by-side within the
                               bundle. The seven-wire bundle was laced together at four
                               locations so as to keep all seven wires tightly held together
                               throughout the length of the bundle. The distance between the
                               two central laces was about 2.5 cm, while the distance between
                               the central two laces and the outer two laces was about 1.25
                               cm.
                               Two wires located on top of the seven-wire bundle had slits
                               measuring 0.5 to 1.0 mm in width that were perpendicular to the
                               wire axis. The slits were positioned 6 mm apart. The stripped
                               wires were connected to a three phase power source according
                               to the scheme set forth in the Boeing BSS 73244 Specification.
                               The wire bundle was energized and a 5% aqueous salt solution
                               was dripped onto the wire bundle where the two exposed slits
                               were located. The rate of application of the salt solution was 8
                               to 10 drops per minute. This condition was continued for 8
                               hours unless the bundle failed by tripping a circuit breaker.
                               After an 8-hour exposure to the dripping salt solution under the
                               energized condition, the wire bundles were taken out. Each
                               wire was initially exposed to a 1000 volt wet dielectric
                               withstand test initially, then 2500 volts. When a wire
                               withstood a 1000 volt wet dielectric withstand test, it passed
                               the test.
                               This test was deemed passed if: (1) a minimum of 64 wires
                               passed the dielectric test; (2) three wires or less failed the
                               dielectric test in any one bundle; and (3) actual damage to the
                               wire was not more than 3 inches in any test bundle.
Wire-to-wire abrasion          Boeing BSS 7324, paragraph no. 7.57, p. 108.
resistance (cycles to failure, Test samples were tested for wire-to-wire abrasion resistance in
6,150,000 cycles minimum):     accordance with the following method. One wire test sample
                               measuring approximately 28 cm in length was crossed with
                               another wire sample measuring approximately 40 cm in length
                               at the center of the shorter wire as shown in the Boeing BSS
                               7324 Specification. One end of one wire specimen was fixed
                               on an upper plate while the other end of the same wire was
                               fixed on a lower plate. One end of the other wire was fixed on
                               the lower plate while the other end of the same wire was loaded
                               with a 1.13 Kg weight. The upper and lower plates were 45 mm
                               apart.
                               The lower plate moved back and forth with a 6.35 mm double
                               amplitude at 10 cycles per second. The fixed member of the
                               wire was connected to a power source so that the cycle counter
                               stopped when the two wire specimens made an electrical
                               contact by wearing out the insulation layer. If the cycle count at
                               the stopping point was greater than 6,150,000, the result was
                               considered passing.

WORKING EXAMPLE 1A

[0124] In this example, the prepared wire constructions or test samples were tested for shrinkage resistance, mechanical durability, hydrolysis resistance, and wet arc track resistance, while confirming the temperature rating of 230° C. The results are set forth in Table 2, hereinbelow.

TABLE 2
Summary of Example 1A
	TOTAL	ELECTRON				WET ARC	WIRE-TO-WIRE
	BEAM	BEAM	LIFE	ACCELERATED	HYDROLYSIS	PROPAGATION	ABRASION
	DOSAGE	VOLTAGE	CYCLE	AGING	RESISTANCE1	RESISTANCE	(6,150,000 cycles
EXAMPLE	(Mrad)	(MV)	(P, F)	(P, F)	(P, F)	(P, F)	minimum)
1A	30	0.5	P	P	P	P	42,885,600
12000 hour requirement met, test continuing.

[0125] As shown in Table 2, the insulated conductor of the present invention may be used at temperatures of up to 230° C., and demonstrates a balance of properties including shrinkage resistance, mechanical durability, hydrolysis resistance, and wet arc propagation resistance.

WORKING EXAMPLES 1B, 2, 3A, C-1 AND C-2

[0126] In these examples, the prepared wire constructions or test samples were tested for sandpaper abrasion resistance. The results are reported in Table 3, hereinbelow.

TABLE 3
Summary of Examples 1B, 2, 3A, C-1 and C-2
	TOTAL
	BEAM	ELECTRON
	DOSAGE	BEAM	SANDPAPER ABRASION (mm)
EXAMPLE	(Mrad)	VOLTAGE (MV)	OUTER LAYER	AVG	BOTH LAYERS	AVG
1B	30	0.5	40	42	117	124
			14		153
			41		151
			46		75
2	30	0.5	38	43	229	172
			41		158
			43		153
			48		146
3A	30	0.5	37	41	114	142
			40		148
			41		153
			46		151
C-1	N/A	N/A	9	12	117	109
			11		153
			13		79
			16		85
C-2	30	0.5	40	53	164	157
			53		151
			56		153
			62		158

[0127] As shown by Examples 1B, 2, and 3A in Table 3, the insulated conductor of the present invention demonstrated a resistance to sandpaper abrasion which was greatly improved over that demonstrated by the prior art wire construction Example C-1, which employed a PTFE outer layer.

WORKING EXAMPLES 1C, 1D, 1E, 3B, 3C, 4A AND 4B

[0128] In these examples, the prepared wire constructions or test samples were tested for ease of peel. The results are shown in Table 4, hereinbelow.

TABLE 4
Summary of Examples 1C, 1D, 1E, 3B, 3C, 4A and 4B
		BEAM
	TOTAL BEAM	VOLTAGE
EXAMPLE	DOSAGE (Mrad)	(KV)	EASE OF PEEL
1C	10	120	not continuously peelable
	15		not continuously peelable
	20		not continuously peelable
1D	10	150	not continuously peelable
	15		not continuously peelable
	20		not continuously peelable
1E	30	500	not continuously peelable
3B	10	120	not continuously peelable
	15		not continuously peelable
	20		not continuously peelable
3C	10	150	continuously peelable
	15		continuously peelable
	20		continuously peelable
4A	10	120	not continuously peelable
	15		not continuously peelable
	20		not continuously peelable
4B	10	150	continuously peelable
	15		continuously peelable
	20		continuously peelable

[0129] Examples 3B and 4A demonstrate that insulated conductors employing irradiation degradable perfluoropolymer adhesives may be successfully prepared using a “skin irradiation” technique which effects crosslinking of the outer layer using low electron beam voltages of less than or equal to 120 KV. As shown in Examples 3C and 4B, exposing these samples to electron voltages of 150 KV appears to degrade the adhesive resulting in a sample where the outer layer is continuously peelable along the length of the test sample.

[0130] Examples 1C, 1D and 1E, which employed a polyimide adhesive, were not easily peelable regardless of whether the sample was irradiated at 120, 150 or 500 KV, which indicated that higher electron beam voltages do not serve to degrade the polyimide adhesive.

WORKING EXAMPLES 5 TO 9, C-1 AND C-2

[0131] In these examples, the prepared wire constructions or test samples were tested for hydrolysis, sandpaper abrasion, cut-through, wet and dry arc propagation and wire-to-wire abrasion resistance, laser markability, strippability, life cycle and current overload capability. The results are set forth in Table 5, hereinbelow.

TABLE 5
Summary of Examples 5 to 9, C-1 and C-2
			Sandpaper Abrasion
	Total		Resistance (mm)
	Insulation	Hydrolysis	Outer
	Thickness	Resistance	Layer	Whole	Cut-Through Resistance (lbs)
Example	(mm)	(P, F)	Only	Insulation	23° C.	150° C.	200° C.	260° C.
5	0 200	P	40	172	89.0	73 9	53 9	66 2
6	0 210	P	45	182	95 7	64 0	54.0	51 3
7	0 198	P	41	192	89 6	52.7	50 2	46 7
8	0 198	P1	28	77	80 0	75 0	64 0	54 0
9	0 210	P1	22	74	79 0	67 0	60 0	52 0
C-1	0 203	P	5	116	45	54	42	30
C-2	0 203	P	55	156	35	5 5	—	—
		Wet Arc Propagation Resistance	Dry Arc Propagation Resistance
	Total	(# of bundles, # of wires passed)	(# of bundles, # of wires passed)
	Insulation	# of	# of	# of	# of	# of	# of	# of	# of
	Thickness	bundles	bundles	wires	wires	bundles	bundles	wires	wires
Example	(mm)	tested	passed	tested	passed	tested	passed	tested	passed
5	0 200	15	12	75	60	—	—	25	21
6	0 210	15	15	75	75	15	15	75	71
7	0 198	15	15	75	75	15	15	75	69
8	0 198	—	—	—	—	15	15	75	68
9	0 210	15	15	75	75	—	—	—	—
C-1	0 203	15	15	75	73	15	15	75	74
C-2	0 203	15	15	75	74	15	15	71	71
	Laser
	Mark-
	ability		Wire-To-
	or	Strip-	Wire	Life Cycle (# of wires passed)
	Total	Mark-	pability	Abrasion	Aging			Current
Ex-	Insulation	ing	(lbs. of	Resistance	Temper-	# of	# of	Overload
am-	Thickness	Contrast	strip	(6,150,000	ature	Wires	Wires	Capacity
ple	(mm)	(%)	force)	minimum)	° C.	Tested	Passed	(P, F)
5	0.200	—	3.76	42,885,600	230	3	3	P
					260	3	3
6	0.210	—	0.63	—	230	3	3	P
					260	3	0
7	0.198	—	1.15	23,600,000	230	3	3	P
					260	3	0
8	0.198	74	0.38	22,775,000	230	3	3	P
					260	3	2
9	0.210	74	0.39	—	230	3	3	P
					260	3	0
C-1	0.203	63	—	>6,150,000	230	3	3	P
C-2	0.203	—	—	>6,150,000	290	3	3	P
1test continuing, expect to pass

[0132] As shown in Table 5, the insulated conductors of the present invention demonstrate a balance of properties including mechanical durability and hydrolysis resistance. More specifically, Examples 5 to 7 demonstrated good hydrolysis resistance, with Examples 8 and 9 noted as currently being tested but expected to demonstrate the same level of resistance. With regard to sandpaper abrasion resistance, Examples 5 to 7 performed similar to Comparative Example C-2. Examples 8 to 9 showed a slight drop-off in this property, while Comparative Example C-1 performed poorly presumably due to the nature of the PTFE outer layer. In terms of cut-through and wire-to-wire abrasion resistance properties, the insulated conductors of the present invention demonstrated greatly improved cut-through resistance over Comparative Examples C-1 and C-2, at all of the temperatures tested, while Examples 5, 7 and 8 demonstrated remarkable levels of wire-to-wire abrasion resistance. With regard to wet arc propagation resistance, Examples 6, 7 and 9 passed each test, while Example 5 passed a majority of the tests. Similar results were obtained for dry arc propagation resistance, with each Example passing all, or a majority of, the tests. In addition, Examples 8 and 9 both demonstrated improved laser markability over Comparative Example C-1, while all of the inventive insulated conductors successfully passed the industry standard for strippability, namely—a strip force of from {fraction (1/4)} to 6 lbs. With regard to life cycle and temperature ratings, Example 8 qualified for a temperature rating of 230° C. Finally, all of the test samples satisfied the requirements for threshold current overload capacity.

[0133] Although the present invention has been shown and described with respect to detailed embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail thereof may be made without departing from the spirit and scope of the claimed invention.

Claims

1. A multi-layer insulation system for electrical conductors, which comprises: (a) an inner layer selected from the group of polyimide inner layers and fluoropolymer inner layers, wherein, when the inner layer is a polyimide inner layer, the layer is formed by wrapping a polyimide film, which has been coated with a sealable component, in an overlapping fashion, along a portion or length of an electrical conductor, wherein, the polyimide film is substantially uniformly sealed to itself in overlapping regions along the length of the conductor, thereby forming an effective seal against moisture, wherein, the sealable component is selected from the group of perfluoropolymer, crosslinked fluoropolymer and polyimide adhesives, wherein, when the inner layer is a fluoropolymer inner layer, the layer is formed by either extruding a fluoropolymer material along a portion or length of the electrical conductor, or by wrapping a fluoropolymer film, in an overlapping fashion, along a portion or length of the conductor, (b) optionally, a polyimide middle layer, wherein the polyimide middle layer is formed by wrapping an optionally coated polyimide film, in an overlapping fashion, along a portion or length of the inner layer formed on the electrical conductor, and (c) an extruded, crosslinked fluoropolymer outer layer, wherein the fluoropolymer is selected from the group of copolymers and terpolymers of ethylene-tetrafluoroethylene, and mixtures thereof, wherein, when the inner layer is a fluoropolymer inner layer, the multi-layer insulation system includes a polyimide middle layer.

2. The multi-layer insulation system of claim 1, wherein the inner layer is a polyimide inner layer.

3. The multi-layer insulation system of claim 2, wherein the polyimide inner layer demonstrates a high temperature (150° C.) adhesive bond strength (ASTM#1876-00) ranging from about 100 to about 250 grams per inch-width.

4. The multi-layer insulation system of claim 2, wherein the sealable component coated onto the polyimide film is a perfluoropolymer sealable component selected from the group of polytetrafluoroethylene, fluorinated ethylene-propylene, perfluoroalkoxy, copolymers of tetrafluoroethylene and perfluoromethylvinylether, and mixtures thereof.

5. The multi-layer insulation system of claim 2, wherein the sealable component coated onto the polyimide film is a crosslinked fluoropolymer sealable component selected from the group of ethylene-tetrafluoroethylene copolymers, chlorotrifluoroethylene copolymers and terpolymers containing minor amounts of one or more fluorinated comonomers, and mixtures thereof.

6. The multi-layer insulation system of claim 2, wherein the sealable component coated onto the polyimide film is a polyimide sealable component selected from the group of thermoplastic polyimides which soften and become fluid at greater than or equal to 200° C.

7. The multi-layer insulation system of claim 6, wherein the polyimide inner layer demonstrates a high temperature (150° C.) adhesive bond strength (ASTM#1876-00) of greater than 1000 grams per inch-width.

8. The multi-layer insulation system of claim 1, wherein the inner layer is a fluoropolymer inner layer and wherein the fluoropolymer is selected from the group of copolymers of tetrafluoroethylene and perfluoromethylvinylether, perfluoroalkoxy, polytetrafluoroethylene, ethylene-chlorotrifluoroethylene copolymers, ethylene tetrafluoroethylene copolymers, polyvinylidene fluoride, tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride, polyvinylfluoride resins, and mixtures thereof.

9. The multi-layer insulation system of claim 1, wherein the inner layer is a non-heat-sealed fluoropolymer film inner layer.

10. The multi-layer insulation system of claim 1, wherein the inner layer is a heat-sealed fluoropolymer film inner layer, wherein the fluoropolymer film is substantially uniformly sealed to itself in overlapping regions along the length of the conductor, thereby forming an effective seal against moisture.

11. The multi-layer insulation system of claim 1, wherein the system comprises a non-heat-sealed polyimide middle layer.

12. The multi-layer insulation system of claim 1, wherein the system comprises a polyimide middle layer formed by a polyimide film coated with a sealable component, wherein the polyimide film is substantially uniformly sealed to itself in overlapping regions along the length of the inner layer, thereby forming an effective seal against moisture and wherein the sealable component is selected from the group of perfluoropolymer, crosslinked fluoropolymer and polyimide adhesives.

13. The multi-layer insulation system of claim 1, wherein the fluoropolymer of the fluoropolymer outer layer is an ethylene-tetrafluoroethylene copolymer which comprises 35 to 60 mole % of units derived from ethylene, 35 to 60 mole % of units derived from tetrafluoroethylene and up to 10 mole % of units derived from one or more fluorinated comonomers.

14. The multi-layer insulation system of claim 1, wherein the fluoropolymer of the fluoropolymer outer layer contains a photosensitive substance rendering the outer layer receptive to laser marking.

15. A multi-layer insulation system for electrical conductors, which comprises: (a) a polyimide inner layer, wherein the polyimide inner layer is formed by wrapping a polyimide film, which has been coated with a sealable component, in an overlapping fashion, along a portion or length of an electrical conductor, wherein, the polyimide film is substantially uniformly sealed to itself in overlapping regions along the length of the conductor, thereby forming an effective seal against moisture, and wherein, the sealable component is selected from the group of perfluoropolymer, crosslinked fluoropolymer and polyimide adhesives; and (b) an extruded, crosslinked fluoropolymer outer layer, wherein the fluoropolymer is selected from the group of copolymers and terpolymers of ethylene-tetrafluoroethylene, and mixtures thereof.

16. A multi-layer insulation system for electrical conductors, which comprises: (a) a polyimide inner layer, wherein, the polyimide inner layer is formed by wrapping a polyimide film, which has been coated with a sealable component, in an overlapping fashion, along a portion or length of an electrical conductor, wherein, the polyimide film is substantially uniformly sealed to itself in overlapping regions along the length of the conductor, thereby forming an effective seal against moisture, wherein, the sealable component is selected from the group of perfluoropolymer, crosslinked fluoropolymer and polyimide adhesives, (b) a polyimide middle layer, wherein the polyimide middle layer is formed by wrapping a polyimide film, in an overlapping fashion, along a portion or length of the inner layer formed on the electrical conductor, and (c) an extruded, crosslinked fluoropolymer outer layer, wherein the fluoropolymer is selected from the group of copolymers and terpolymers of ethylene-tetrafluoroethylene, and mixtures thereof.

17. A multi-layer insulation system for electrical conductors, which comprises: (a) a fluoropolymer inner layer, wherein, the inner layer is formed by wrapping a fluoropolymer film, in an overlapping fashion, along a portion or length of an electrical conductor, (b) a polyimide middle layer, wherein the polyimide middle layer is formed by wrapping a polyimide film, in an overlapping fashion, along a portion or length of the inner layer formed on the electrical conductor, and (c) an extruded, crosslinked fluoropolymer outer layer, wherein the fluoropolymer is selected from the group of copolymers and terpolymers of ethylene-tetrafluoroethylene, and mixtures thereof.

18. The multi-layer insulation system of claim 17, wherein the fluoropolymer inner layer is a non-heat-sealed fluoropolymer film inner layer.

19. The multi-layer insulation system of claim 18, wherein the fluoropolymer film is a polytetrafluoroethylene film.

20. The multi-layer insulation system of claim 17, wherein the fluoropolymer inner layer is a heat-sealed fluoropolymer film inner layer, wherein the fluoropolymer film is substantially uniformly sealed to itself in overlapping regions along the length of the conductor, thereby forming an effective seal against moisture.

21. The multi-layer insulation system of claim 17, wherein the polyimide film of the polyimide film middle layer is coated with a sealable component and is substantially uniformly sealed to itself in overlapping regions along the length of the inner layer, thereby forming an effective seal against moisture and wherein the sealable component is selected from the group of perfluoropolymer, crosslinked fluoropolymer and polyimide adhesives.

22. A multi-layer insulation system for electrical conductors, which comprises: (a) a fluoropolymer inner layer, wherein, the inner layer is formed by extruding a fluoropolymer material along a portion or length of an electrical conductor, (b) a polyimide middle layer, wherein the polyimide middle layer is formed by wrapping a polyimide film, in an overlapping fashion, along a portion or length of the inner layer formed on the electrical conductor, and (c) an extruded, crosslinked fluoropolymer outer layer, wherein the fluoropolymer is selected from the group of copolymers and terpolymers of ethylene-tetrafluoroethylene, and mixtures thereof.

23. The multi-layer insulation system of claim 22, wherein the extruded fluoropolymer inner layer is a crosslinked extruded fluoropolymer inner layer.

24. An insulated electrical conductor that comprises an electrical conductor and a multi-layer insulation system, wherein the multi-layer insulation system comprises: (a) an inner layer selected from the group of polyimide inner layers and fluoropolymer inner layers, wherein, when the inner layer is a polyimide inner layer, the layer is formed by wrapping a polyimide film, which has been coated with a sealable component, in an overlapping fashion, along a portion or length of the electrical conductor, wherein, the polyimide film is substantially uniformly sealed to itself in overlapping regions along the length of the conductor, thereby forming an effective seal against moisture, wherein, the sealable component is selected from the group of perfluoropolymer, crosslinked fluoropolymer and polyimide adhesives, wherein, when the inner layer is a fluoropolymer inner layer, the layer is formed by either extruding a fluoropolymer material along a portion or length of the electrical conductor, or by wrapping a fluoropolymer film, in an overlapping fashion, along a portion or length of the conductor, (b) optionally, a polyimide middle layer, wherein the polyimide middle layer is formed by wrapping an optionally coated polyimide film, in an overlapping fashion, along a portion or length of the inner layer formed on the electrical conductor, and (c) an extruded, crosslinked fluoropolymer outer layer, wherein the fluoropolymer is selected from the group of copolymers and terpolymers of ethylene-tetrafluoroethylene, and mixtures thereof, wherein, when the inner layer is a fluoropolymer inner layer, the multi-layer insulation system includes a polyimide middle layer.

25. An insulated electrical conductor that comprises an electrical conductor and a multi-layer insulation system, wherein the multi-layer insulation system comprises: (a) a fluoropolymer inner layer, wherein, the inner layer is formed by wrapping a fluoropolymer film, in an overlapping fashion, along a portion or length of the conductor, (b) a polyimide middle layer, wherein the polyimide middle layer is formed by wrapping an optionally coated polyimide film, in an overlapping fashion, along a portion or length of the inner layer formed on the electrical conductor, and (c) an extruded, crosslinked fluoropolymer outer layer, wherein the fluoropolymer is selected from the group of copolymers and terpolymers of ethylene-tetrafluoroethylene, and mixtures thereof.

26. A process for preparing an insulated electrical conductor, which comprises: (a) forming an inner layer on an electrical conductor, wherein the inner layer is selected from the group of polyimide inner layers and fluoropolymer inner layers, wherein, when the inner layer is a polyimide inner layer, the layer is formed by wrapping a polyimide film, which has been coated with a sealable component, in an overlapping fashion, along a portion or length of the electrical conductor, wherein the sealable component is selected from the group of perfluoropolymer, crosslinkable fluoropolymer and polyimide adhesives, wherein, when the inner layer is a fluoropolymer inner layer, the layer is formed by either: i) extruding a fluoropolymer material along a portion or length of the electrical conductor, or ii) wrapping a fluoropolymer film, in an overlapping fashion, along a portion or length of the electrical conductor, (b) optionally, forming a polyimide middle layer on the inner layer by wrapping an optionally coated polyimide film, in an overlapping fashion, along a portion or length of the inner layer, (c) when the inner layer is a polyimide inner layer or when a middle layer is formed using a coated polyimide film, heating the polyimide film or films to a temperature ranging from about 240° to about 350° C. to cause overlapping regions of the coated film or films to bond, thereby forming an effective seal against moisture along the length of the conductor, (d) forming a fluoropolymer outer layer on either the inner or middle layer by extruding a fluoropolymer material along a portion or length of that layer; and (e) crosslinking the fluoropolymer outer layer, wherein, when the inner layer or the sealable component comprises a perfluoropolymer, the fluoropolymer outer layer is crosslinked by exposing it to less than 60 megarads of radiation, with applied voltages ranging from about 50 to about 120 kilo volts, wherein, when the inner layer is a fluoropolymer inner layer, the process for preparing an insulated electrical conductor includes forming a polyimide middle layer on the inner layer.

27. A process for preparing an insulated electrical conductor, which comprises: (a) forming a fluoropolymer inner layer on an electrical conductor by wrapping a fluoropolymer film, in an overlapping fashion, along a portion or length of the electrical conductor, (b) forming a polyimide middle layer on the fluoropolymer inner layer by wrapping a polyimide film, which has been coated with a sealable component, in an overlapping fashion, along a portion or length of the fluoropolymer inner layer, wherein the sealable component is selected from the group of perfluoropolymer, crosslinked fluoropolymer and polyimide adhesives, (c) heating the polyimide film to a temperature ranging from about 240° to about 350° C. to cause overlapping regions of the film to bond, thereby forming an effective seal against moisture along the length of the conductor, (d) forming a fluoropolymer outer layer on the polyimide middle layer by extruding a fluoropolymer material along a portion or length of that layer, and (e) crosslinking the fluoropolymer outer layer, wherein, when the inner layer or the sealable component comprises a perfluoropolymer, the fluoropolymer outer layer is crosslinked by exposing it to less than 60 megarads of radiation, with applied voltages ranging from about 50 to about 120 kilo volts.

28. An insulated electrical conductor that comprises an electrical conductor and a multi-layer insulation system, wherein the multi-layer insulation system comprises: (a) a fluoropolymer inner layer, (b) a polyimide middle layer, and (c) an extruded, crosslinked fluoropolymer outer layer, wherein the fluoropolymer is selected from the group of copolymers and terpolymers of ethylene-tetrafluoroethylene, and mixtures thereof, wherein, the insulated electrical conductor is prepared by a process that comprises: (i) forming a fluoropolymer inner layer on an electrical conductor by wrapping a fluoropolymer film, in an overlapping fashion, along a portion or length of the conductor, (ii) forming a polyimide middle layer on the fluoropolymer inner layer by wrapping a polyimide film, which has been coated with a sealable component, in an overlapping fashion, along a portion or length of the fluoropolymer inner layer, wherein the sealable component is selected from the group of perfluoropolymer, crosslinked fluoropolymer and polyimide adhesives, (iii) heating the polyimide film to a temperature ranging from about 240° to about 350° C. to cause overlapping regions of the coated film to bond, thereby forming an effective seal against moisture along the length of the conductor, (iv) forming a fluoropolymer outer layer on the polyimide middle layer by extruding a fluoropolymer material along a portion or length of the middle layer, and (v) crosslinking the fluoropolymer outer layer, wherein, when the inner layer or the sealable component comprises a perfluoropolymer, the fluoropolymer outer layer is crosslinked by exposing it to less than 60 megarads of radiation, with applied voltages ranging from about 50 to about 120 kilo volts.