CA2120513A1

CA2120513A1 - Electrical wire

Info

Publication number: CA2120513A1
Application number: CA002120513A
Authority: CA
Inventors: David John Durston; John Cambell Bovaird
Original assignee: Individual
Current assignee: Raychem Ltd
Priority date: 1991-10-01
Filing date: 1992-09-30
Publication date: 1993-04-15
Also published as: IL103308A0; IL103307A0; EP0606319A1; GB9120917D0; WO1993007628A1; WO1993007627A1; IL103307A

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 50µm and which contains a quantity of an additive that suppresses arc tracking for example titanium dioxide or silica.

Description

f~WO 93/07627 2 ~ ~ O ~ 1 3 PCr~GB92/01789 leçtrical Wire , This invention relates to electlical wire and cable, and especially 5 to electrical~ wire that is intended to be used in aircraft applications, or so-called~ airframe wire.
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For such wire weight considerations are of major importance which tend to reduce the thickness of the wire insulation as low as 10 possible. In addition, however, the wire should be resistant to arcing and tracking. Tracking is a phenomenon associated with the formation of pennanent and progressive conducting paths on the surface of the material by the combined effects of an electrical field and extern~
surface~pollution. Once commenced, the~ carbonaceous conducting 15 ~deposits often e~end quickly in dendritic fashion to give a characteIistic "tree ~pattern until failure occur~across t~e:surfa~e. Electrical tracking can occur~when a damaged energised bundle of wires become wet, eg.
~m-electrolytes orcondensation. This trackingmaylead to flashover and arcing that causes additional wires in the bundle to become 20 ~ damaged.~ A catsstrophic cascade failure can result from a fault to a single~ wire~ if adjacent wires that are at a dif3ferent electrical potential are also sus,ceptible to tracking or if the~ bundle is in contact with a grounded structure. Tracking can occur at low voItages eg. 100V ac or less but becomes less likely as the voltage is reduced.

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W093/07627 ~, 1205 ~ 2 - PCI/G1192/01789 ,~

A related phenomenon is that of breakdown due to arciing. 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 5 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 rapiidly, 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 10 voltages, for example 24V dc or lower, and since, unli~e tracking, no electrolyte or moisture is involved, it is a particularly hazardous phenomenon. Arcs may also be struck by drawing apart two conductors between whiich a current is passing as desc~ibed for example by J M
Bomerville ~rrhe Electric Arc", Methuen 1959.
One fDrm of airframe that has been proposed is described in French patent specification No. 2,617,325. This wire ha~ an insulation comprising a polyim~de tape, for example one sold under the trade name "Kapton" and a layer of a perfluoroalkoyl polymer or 20 polytetrafluoroethylene for protecting the polyi~ide layer against tracking. Ho~vever, 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 perormance by increasing the number of polytetrafluoroethylene layers, this will 25 increase the weight and diameter of the wire.
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Other forms of wire that employ fluoropolymers without any aromatic polymers have also been used as airframe wires. VVhile such wires e~hibit better arcing and tracking resistance than w~res that 30 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 35 abrasion resistance can be unacceptably reduced by such means. The ~ 0 93/U7627 2 12 0 ~ 1 3 P(~r/GB92/01789 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 ~uoropolymer layers, the wire including a top layer that has a thickness of not more than 50 llm 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 i~ 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 tracl~ng suppressing filler, does not suffer from a reduction in scrape abrasion resistance, 50 that the wire as a whole can have an abrasion resistance that is not significantly reduced from that of the wire before the traclcing suppressing additive was inForporated-The top layer is relatively thin and preferably has a thickness of not more than 40 and especially not more than 30 ~m, although it will normally have a thickness of at least 20~1m. The top layer may be applied in a number of ways, ~or example by pain~ng 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 u~derlying 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 WO 93/07627 PCI`/GB92/01789 ~! f : ~
'~ ~ 2 0 ~ 4 called the primary insulation arld the primary jacket. ~luoropolymers 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 5 copolymer (ETFE) and tetrafluoroethylene-propylene copolymer;
vinylidine ~uoride homo- and copolymers such as polyvinylidine fluoride and vinylidine fluoride he~afluoropropylene 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 p,olymer as is used for the insulation~ or at least for the primary jacket, in order to improve l~e adhesion between them.

Tbe fluoropolymer forming the insulation vill normally be crosslinked They may be crosslinked by any convenient method, for example by irradiation or, by chemical crosslinking using, for example, a peroxide. q~e peroxide may be absorbed on an inert carrier such as a 20 calcium carbonate, carbon black, or kiesleguhr.
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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 25 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 e~ec~ve.

In some cases it may be desirable to add to the crosslinkable 30 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 belieYed that they react with the initial radical formed on the polymer backbone to form a more stable radical, ~Yhich undergoes a 2~20~13 --~ 93/07627 PCr/GB92/0171~9 ~., 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 isocyanurat2, tetramethylene acrylate, or polyethylene oxide glycol dimethacrylate. The amount of the co-agent is prefèrably up to about 5 pa~ts by weight per 100 parts of the ~olymer composition and preferably from 1 to 3 parts by weight per 100 parts of t~e polymer c omposition.

The polymer forming the top layer, however, is preferably uncrosslinked, especially if it is a fluoropolymer, eg. ;
ethylene/tetrafluoroethylene copolymer, since crosslinked polymers 1~ often e~hibit a greater tendency to track than the uncrosslinked polymer. Th~lS the wire can enjoy relatively good mechanical properties due to the fact that the bulk of the insula1ion is crosElinked, while the absence of crosslinking in the external surface will improve the arc trackingproperties. ~;
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A number of materials may be employed as the tracking suppressing additives. Preferably the, tracking suppressing mate~al is an o~ide of a metal or metalloid having an atomic weight of not more than 180 a~d belo~gs to groups IIA, III A, IVA, IV B or VIB of the periodic table. Examples of such materials include silica, alumina, chromium o~de, calsium oxide, tin oxide, tit~ium dioxide, beryllium o~ide, magnesium onde and nickel oxide. The additive is preferably ~ ~
present in the top layer in a concentration of at least 5~o 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 t~e top layer material. As indicated above, the tracking suppressing addit*e is preferably absent from the underlying layers. The underlying layers may be e~tirely free of the addi~ves or small quantities of them may be present~ especially when they have another use. Normally the underlying layers will contain not more than ~% and especially not more wo 93~07627 2 ~ 2 0 ~ 13 6 - Pl`/GB92/01789 ,~-~

than 2% by weight of the additive based on the total weight of the layer material.
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'rhe composition may contain one or more additional materials, 5 for example fill~rs, flame retardants, anti-oxidants, ultraviolet stabilizers, processing aids, pigments, fi~ngicides or the like.

The insulation (including the top layer) preferably has an overall thickness of not more than 4011m and more preferably not more than 10 300 ~m in order to reduce the size and weight of the wire. Typically the wire vill have a total insulation thickness (including the top layer) in the range of from 200 to 280 ~

In addition to the improvement of the tracking performance of the 1~ wire that is provided by the top layer, the top layer may enable the wire to be mar~able by means of an infrared laser, eg. a C02 laser, if the top layer has a colour ~hat contrasts with that of the underlying insulation.
When subjected to a CO2 laser marking process, part of the top layer is removed by the laser to expose the underlying, colour-contrasting layer.
20 Such a wire is described in our copending British patent application No.
9120917, the disclosure of which is incorporated herein by reference.
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One form of wire in accordance with the present invention will now be described by way of example with reference to the accompanying 25 drawing which is a cross-section through the wire.
, Referring to the accompanying dra~ving a wire 1 comp~ises a conductor 2 for example a conventional nickel plated copper or nickel plated cadmium copper alloy, and an insulating layer comprising an 30 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 ,um thick while the pnmary jacket 4 will typically be 125 ~m thick.
The primaly jacket also includes a small quantity of carbon pigment to 35 colour it black. A 20,um thick top layer ~ also formed ~rom ETFE and ~V0 93/07627 2 ~ 13 P~/GB92/01789 containing 15% by weight titanium dioxide (based on the total weight of the top layer) is coextruded an the primary jacket. Af~er the extrusion step the wire is subjected to a crosslinking step in which the pri~ary insulation 3 and primary jacket 4 are crosslinked by high energy 5 electrons. The top layer remains substantially uncrosslinked since it contains no crosslinking promoter.

The wire has a white appearance due-tQ~the titanium dioxide in the top layer, and can be marked using a CO2 laser to remove a portion 10 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 significalltly improved arc tracking resistance over wires based on polyimide wrap/PTFE wrap combinations and wires based solely on crosslinked ETFE layers.
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Claims

Claims:

1. An electrical wire which comprises an electrical conductor and electrical insulation which is based substantially solely on one or more extruded fluoropolymer 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 fluoropolymer.

4. A wire as claimed in any one of claims 1 to 3, wherein the or each fluoropolymer 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.