ELECTRICAL WIRE

Abstract: An electrical wire comprises an electrical conductor (2) and electrical insulation which is based substantially solely one or more extruded fluoropolymer layers (3 and 4). The wire includes a top layer (5) that has a thickness of not more than 50 mu m and which contains a quantity of an additive that suppresses arc tracking for example titanium dioxide or silica.

Description: Electrical Wire This invention relates to electrical wire and cable, and especially to electrical wire that is intended to be used in aircraft applications, or so-called airframe wire. For such wire weight considerations are of major importance which tend to reduce the thickness of the wire insulation as low as possible. In addition, however, the wire should be resistant to arcing and tracking. Tracking is a phenomenon associated with the formation of permanent and progressive conducting paths on the surface of the material by the combined effects of an electrical field and external surface pollution. Once commenced, the carbonaceous conducting deposits often extend quickly in dendritic fashion to give a characteristic "tree" pattern until failure occurs across the surface. Electrical tracking can occur when a damaged energised bundle of wires become wet, eg. from electrolytes or condensation. This tracking may lead to flashover and arcing that causes additional wires in the bundle to become damaged. A catastrophic cascade failure can result from a fault to a single wire if adjacent wires that are at a different electrical potential are also susceptible to tracking or if the bundle is in contact with a grounded structure. Tracking can occur at low voltages eg. 100V ac or less but becomes less likely as the voltage is reduced. A related phenomenon is that of breakdown due to arcing. In this case a potential difference between two conductors, or between a conductor in which the insulation has been mechanically damaged, and ground, can result in the formation of an arc between the conductors or between the conductor and ground. The high temperature of the arc causes the polymer to degrade extremely rapidly and form an electrically conductive carbonaceous deposit which can extend rapidly, as with wet tracking, and lead to catastrophic failure in which many or all of the wires in a bundle are destroyed. Arcing can occur at very low voltages, for example 24V dc or lower, and since, unlike tracking, no electrolyte or moisture is involved, it is a particularly hazardous phenomenon. Arcs may also be struck by drawing apart two conductors between which a current is passing as described for example by J M Somerville "The Electric Arc", Methuen 1959. One for of airframe that has been proposed is described in French patent specification No. 2,617,325. This wire has an insulation comprising a polyimide tape, for example one sold under the trade name "Kapton" and a layer of a perfluoroalkoyl polymer or polytetrafluoroethylene for protecting the polyimide layer against tracking. However, such wires will still exhibit a significant degree of tracking in view of the presence of the aromatic polyimide layer. Although, it is possible to improve the tracking performance by increasing the number of polytetrafluoroethylene layers, this will increase the weight and diameter of the wire. Other forms of wire that employ fluoropolymers without any aromatic polymers have also been used as airframe wires. While such wires exhibit better arcing and tracking resistance than wires that include aromatic polymers, there is still room for improvement. In particular such wires can exhibit increased tracking after heat aging. Although it is possible to incorporate anti-tracking materials into the polymer in order to reduce the tendency of the insulation to track, we have found that this has the disadvantage that the wires' scrape abrasion resistance can be unacceptably reduced by such means. The reason for this appears to be that the filler acts as an abrasive in the insulation and so wears down the insulation rapidly when the wire is tested. According to the present invention there is provided an electrical wire which comprises an electrical conductor and electrical insulation which is based substantially solely on one or more extruded fiuoropolymer layers, the wire including a top layer that has a thickness of not more than 50 μm and which contains a quantity of an additive that suppresses arc tracking. The wire according to the invention has the advantage that it exhibits a very high arc tracking resistance without the resulting reduction in abrasion resistance due to the presence of the tracking suppressing filler. This appears to be due to the fact that, when the wire is subjected to scraping, the part of the top layer that is abraded soon rubs away exposing the underlying primary jacket. The underlying jacket, which is substantially free of the tracking suppressing filler, does not suffer from a reduction in scrape abrasion resistance, so that the wire as a whole can have an abrasion resistance that is not significantly reduced from that of the wire before the tracking suppressing additive was incorporated. The top layer is relatively thin and preferably has a thickness of not more than 40 and especially not more than 30 μm, although it will normally have a thickness of at least 20μm. The top layer may be applied in a number of ways, for example by painting or by dip-coating, but preferably it is applied by extrusion, and most preferably by coextrusion with the other layers. Extruded top layers tend to have a better adhesion to the underlying layers than those applied by other means since the polymer of the top layer is intimitely bonded to that of the underlying layer. The electrical insulation may comprise a single layer only, but it is more usual for it to comprise more than one layer, usually two layers called the primary insulation and the primary jacket. Fluoropolymers that may be employed in the insulation are preferably thermoplastic, and more preferably those that include hydrogen, for example tetrafluoroethylene copolymers such as ethylene-tetrafluoroethylene copolymer (ETFE) and tetrafluoroethylene-propylene copolymer; vinylidine fluoride homo- and copolymers such as polyvinylidine fluoride and vinylidine fluoride hexafluoropropylene copolymer and the like. Preferred polymers are ethylene-tetrafluoroethylene copolymers in view of their balance of physical properties. The polymer forming the top layer may be chosen from the same polymers as those from which the insulation is formed and is preferably the same polymer as is used for the insulation, or at least for the primary jacket, in order to improve the adhesion between them. The fiuoropolymer forming the insulation will normally be crosslinked. They may be crosslinked by any convenient method, for example by irradiation or, by chemical crosslinking using, for example, a peroxide. The peroxide may be absorbed on an inert carrier such as a calciu carbonate, carbon black, or kiesleguhr. Preferably, however, the wires of the present invention are crosslinked using high energy radiation. Radiation dose levels to achieve crosslinking according to the present invention may range from about 20 to 800 kGy or more, but a dose of about 50 to 500 kGy is preferred. For most purposes a dose of about 80 to 200 kGy will be effective. In some cases it may be desirable to add to the crosslinkable polymer composition a co-agent to assist in the crosslinking reaction. Such co-agents usually contain multiple unsaturated groups such as alkyl or acrylic esters. While their mode of action is not known with certainty, it is believed that they react with the initial radical formed on the polymer backbone to form a more stable radical, which undergoes a coupling reactions to form crosslinks more readily than chain scission reactions. The co-agent can be for example, N,N'-m(phenylene)-dimaleimide, trimethylolpropane trimethylacrylate, tetraallyloxyethane, triallyl cyanurate, triallyl isocyanurate, tetramethylene acrylate, or polyethylene oxide glycol dimethacrylate. The amount of the co-agent is preferably up to about 5 parts by weight per 100 parts of the polymer composition and preferably from 1 to 3 parts by weight per 100 parts of the polymer composition. The polymer forming the top layer, however, is preferably uncrosslinked, especially if it is a fiuoropolymer, eg. ethylene/tetrafluoroethylene copolymer, since crosslinked polymers often exhibit a greater tendency to track than the uncrosslinked polymer. Thus the wire can enjoy relatively good mechanical properties due to the fact that the bulk of the insulation is crosslinked, while the absence of crosslinking in the external surface will improve the arc tracking properties. A number of materials may be employed as the tracking suppressing additives. Preferably the tracking suppressing material is an oxide of a metal or metalloid having an atomic weight of not more than 180 and belongs to groups IIA, III A, IVA, IV B or VIB of the periodic table. Examples of such materials include silica, alumina, chromium oxide, calcium oxide, tin oxide, titanium dioxide, beryllium oxide, magnesium oxide and nickel oxide. The additive is preferably present in the top layer in a concentration of at least 5% and especially at least 10% by weight but preferably not more than 30% and especially not more than 20% by weight, based on the total weight of the top layer material. As indicated above, the tracking suppressing additive is preferably absent from the underlying layers. The underlying layers may be entirely free of the additives or small quantities of them may be present, especially when they have another use. Normally the underlying layers will contain not more than 5% and especially not more than 2% by weight of the additive based on the total weight of the layer material. The composition may contain one or more additional materials, for example fillers, flame retardants, anti-oxidants, ultraviolet stabilizers, processing aids, pigments, fungicides or the like. The insulation (including the top layer) preferably has an overall thickness of not more than 40μm and more preferably not more than 300 μm in order to reduce the size and weight of the wire. Typically the wire will have a total insulation thickness (including the top layer) in the range of from 200 to 280 μm. In addition to the improvement of the tracking performance of the wire that is provided by the top layer, the top layer may enable the wire to be markable by means of an infrared laser, eg. a CO 2 laser, if the top layer has a colour that contrasts with that of the underlying insulation. When subjected to a CO 2 laser marking process, part of the top layer is removed by the laser to expose the underlying, colour-contrasting layer. Such, a wire is described in our copending British patent application No. 9120917, the disclosure of which is incorporated herein by reference. One form of wire in accordance with the present invention will now be described by way of example with reference to the accompanying drawing which is a cross-section through the wire. Referring to the accompanying drawing a wire 1 comprises a conductor 2 for example a conventional nickel plated copper or nickel plated cadmium copper alloy, and an insulating layer comprising an extruded primary insulation layer 3 formed from ethylene tetrafluoroethylene copolymer (ETFE) and an extruded primary jacket 4 also formed from ETFE. The primary insulation 3 will typically be 100 μm. thick while the primary jacket 4 will typically be 125 μm thick. The primary jacket also includes a small quantity of carbon pigment to colour it black. A 20μm thick top layer 5 also formed from ETFE and containing 15% by weight titanium dioxide (based on the total weight of the top layer) is coextruded an the primary jacket. After the extrusion step the wire is subjected to a crosslinking step in which the primary insulation 3 and primary jacket 4 are crosslinked by high energy electrons. The top layer remains substantially uncrosslinked since it contains no crosslinking promoter. The wire has a white appearance due to the titanium dioxide in the top layer, and can be marked using a CO 2 laser to remove a portion 6 of the top layer corresponding to the mark and thereby expose a part of the underlying primary jacket 4. In addition, the wire exhibits significantly improved arc tracking resistance over wires based on polyimide wrap PTFE wrap combinations and wires based solely on crosslinked ETFE layers.

Claims: Claims: 1. An electrical wire which comprises an electrical conductor and electrical insulation which is based substantially solely on one or more extruded fiuoropolymer layers, the wire including a top layer that has a thickness of not more than 50 μm and which contains a quantity of an additive that suppresses arc tracking. 2. A wire as claimed in claim 1, wherein the thickness of the top layer has a thickness of not more than 30μm. 3. A wire as claimed in any claim 1 or claim 2, wherein the top layer is based on a fiuoropolymer. 4. A wire as claimed in any one of claims 1 to 3, wherein the or each fiuoropolymer is an ethylene tetrafluoroethylene copolymer. 5. A wire as claimed in any one of claims 1 to 4, wherein the top layer is extruded. 6. A wire as claimed in any one of claims 1 to 5, wherein the quantity of tracking suppressing filler in the top layer is in the range of from 10 to 20% by weight, based on the total weight of the top layer. 7. A wire as claimed in any one of claims 1 to 6, wherein the additive comprises titanium dioxide or silicon dioxide. 8. A wire as claimed in any one of claims 1 to 7, wherein the electrical insulation comprises primary insulation layer, and, located on the primary insulation layer, a primary jacket layer. 9. A wire as claimed in any one of claims 1 to 8, wherein the electrical insulation is cross-linked. 10. A wire as claimed in any one of claims 1 to 9, wherein the top layer is substantially un-crosslinked. 11. A wire as claimed in any one of claims 1 to 10, wherein the tracking suppressing additive is substantially absent from the layer(s) of electrical insulation. 12. A wire as claimed in any one of claims 1 to 11, wherein the total thickness of the wire insulation and top layer is in the range of from 200 to 300 μm. :$: $ $ * --. 4.

Inventors: BOVAIRD JOHN CAMPBELL, DURSTON DAVID JOHN, BOVAIRD, JOHN, CAMPBELL, DURSTON, DAVID, JOHN
Applicants: RAYCHEM LTD, RAYCHEM LIMITED
Application year: 1992
Main classification: H01B7
IPC codes: H01B7