WIRE

Abstract: An electrical wire comprises an elongate electrical conductor and electrical insulation that comprises: (a) an inner insulating layer which comprises a polyester that has both aromatic and aliphatic moieties and has a molar carbon to hydrogen ratio of not more than 1.15 e.g. polybutylene terephthalate, (b) an outer insulating layer which comprises a fluorinated polymer. The wire exhibits a good balance of electrical and mechanical properties together with high resistance to wet tracking and dry arcing.

Description: WIRE This invention relates to electrical wires, and especially to wires that employ electrical insulation based on aromatic polymers. Electrical wire and cable that use aromatic polymer insulation have been used for many years in numerous applications. For example wires that employ polyimide wraps or tapes usually bonded with fluoro- polymer adhesive layers have been used extensively a- ~ aircraft wire, for both civil and military applica¬ tions. Other examples of aromatic insulation that have been used for equipment wire or "hook-up" wire, air frame wire and in wire harnesses include aromatic polyether ketones, polyether ether ketones, modified polyphenylβne oxide, and polyimide amides. Highly aro¬ matic polymers have been used successfully in many applications because they have a range of desirable properties especially high strength and toughness , abrasion resistance, temperature resistance, dielectric strength and are often inherently highly flame- retarded. The combination of these properties has enabled wire and cable fabricated from these polymers to be used in small lightweight constructions. Such constructions have been used increasingly in- both mili¬ tary and civil applications due to the high density and complexity of modern electrical systems. However, these highly aromatic polymers suffer from a major problem: they are particularly suscep¬ tible to tracking. Tracking is a phenomenon associated with the formation of permanent and progressive con¬ ducting 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 e.g. 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 e.g. 100V a.c. or less but becomes less likely as the voltage is reduced. A related phenomenon, to which these polymers are also highly susceptible, 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 con- ductors 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 d.c. or lower, and since, unlike tracking, no electro¬ lyte 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. Another phenomenon that can be associated with tracking and arcing is erosion. In this case insu¬ lating material is removed by a vaporization process originated by an electrical discharge without the for¬ mation of electrically conductive deposits so that failure of the insulation will not occur until complete puncture of the insulation occurs . According to the present invention, there is pro¬ vided an electrical wire which comprises an elongate electrical conductor and electrical insulation that comprises: (a) an inner insulating layer which comprises a poly¬ ester that has both aromatic and aliphatic moieties and has a molar carbon to hydrogen ratio of not more than 1.15; and (b) an outer insulating layer which comprises a fluorinated polymer. The invention has the advantage that it enables a wire to be formed that has a balance of properties such as solvent resistance, scrape abrasion resistance, toughness, weight and ability to strip in addition to very high resistance to tracking and arcing. The polyester preferably has a molar carbon to hydrogen ratio of not more than 1.1 and especially not more than 1.0. This will normally correspond to a car¬ bonaceous char residue of not more than 15%, preferably not more than 10%, most preferably not more than 5%, especially not more than 2% and most especially substantially 0% by weight. The char residue of the polymer components in the electrical wire according to the invention can be measured by the method known as thermogravimetric ana¬ lysis, or TGA, in which a sample of the polymer is heated in nitrogen or other inert atmosphere at a defined rate, e.g. 10 β C per minute to a defined tem¬ perature and the residual weight, which is composed of char, is recorded. The char residue is simply the quantity of this residual char expressed as a percen¬ tage of the initial polymer after having taken into account any non polymeric volatile or non-volatile com¬ ponents. The char residue values quoted herein are defined as having been measured at 850 β C. The polyesters that are used for layer (a) pre¬ f rably include those based on a polyalkylene diol, preferably having a least 3 carbon atoms, or a cyclo- aliphatic diol and an aromatic dicarboxylic acid. Preferred polyesters include polytetramethylene terephthalate, and cycloaliphatic diol/terephthalic acid copolymers e.g. copolymers of terephthalate and isophthalate units with 1,4-cyclohexanedimethyloxy units. The polyesters can include polyether esters, for example polyether polyester block copolymers having long chain units of the general formula: 0 0 it II • OGO-C-R-C- and short-chain ester units of the formula 0 0 II II -ODO-C-R-C- in which G is a divalent radical remaining after the removal of terminal hydroxyl groups from a polyalkylene oxide) glycol, preferably a poly (C2 to C4 alkylene oxide) having a molecular weight of about 600 to 6000; R is a divalent radical remaining after removal of carboxyl groups from at least one dicarboxylic acid having a molecular weight of less than about 300; and D is a divalent radical remaining after removal of hydroxyl groups from at least one diol having a molecular weight less than 250. Preferred examples of such copolyesters are the polyether ester polymers derived from terephthalic acid, polytetramethylene ether glycol and 1,4-butane diol. These are random block copoly¬ mers having crystalline hard blocks with the repeating unit: and amorphous, elastomeric polytetramethylene ether terephthalate soft blocks of repeating unit having a molecular weight of about 600 to 3000, i.e. n = 6 to 40. If desired the polyester may be blended with one or more other polymers. For example the polyesters may be used as blends with polyamides, polyolefins such as polyethylene, ethylene ethyl acrylate copolymers -styrene/diene block copolymers or ionomers. The fluorinated polymer used in layer (b) pre¬ ferably contains more than 10%, preferably more than 25%, by weight of fluorine. Thus the fluorocarbon polymer may be a single fluorine-containing polymer, a mixture of two or more fluorine-containing polymers, or a mixture of one or more fluorine-containing polymers with one or more polymers which do not contain fluorine. In one preferred class, the fluorocarbon polymer comprises at least 50%, particularly at least 75% espcially at least 85%, by weight of one or more thermoplastic crystalline polymers each containing at least 25% by weight of fluorine, a single such crystalline polymer being preferred. Such a fluoro- carbpn polymer may contain, for example, a fluorine- containing elastomer and/or a polyolefin, preferably a crystalline polyolefin, in addition to the crystalline fluorine-containing polymer or polymers. The fluorine- containing polymers are generally homo- or copolymers of one or more fluorine-containing olefinically unsa- turated monomers, or copolymers of one or more such monomers with one or more olefins. The fluorocarbon polymer usually has a melting point of at least 150°C, and will often have a melting point of at least 250°C, e.g. up to 350°C, the melting point being defined for crystalline polymers as the temperature above which no crystallinity exists in the polymer (or when a mixture of crystalline polymers is used, in the major crystalline component in the mixture). Preferably the polymeric composition, prior to cross-linking, has a viscosity of less than 10 4 Pa.s (10 5 poise) at a temp¬ erature not more than 60°C above its melting point. A preferred fluorocarbon polymer is a copolymer of ethy¬ lene and tetrafluoroethylene and optionally one or more other comonomers (known as ETFE polymers), especially a copolymer comprising 35 to 60 mole percent of ethylene, 35 to 60 mole percent of tetrafluoroethylene and up to 10 mole percent of one or more other comonomers. Other specific polymers which can be used include copolymers of ethylene and chlorotrifluoroethylene; polyvinylidene fluoride; copolymers of vinylidene fluoride with one or both of hexafluoropropylene and tetrafluoroethylene, or with hexafluoroisobutylene; and copolymers of tetra¬ fluoroethylene and hexafluoropropylene. Alternatively c l~ c 5 perfluoroalkoxy substituted perfluoroethylene homopolymers and copolymers with the above fluorinated polymers may be used. The wire insulation, or at least the outer layer, may be cross-linked, for example, by exposure to high energy radiation. Radiation cross-linking may be effected by expos¬ ure to high energy irradiation such as an electron beam or gamma-rays. Radiation dosages in the range 20 to 800 kGy, preferably 20 to 500 kGy, e.g. 20 to 200 kGy and particularly 40 to 120 kGy are in general appropriate depending on the characteristics of the polymer in question. For the purposes of promoting cross-linking during irradiation, preferably from 0.2 to 15 weight per cent of a prorad such as a poly- functional vinyl or allyl compound, for example, triallyl cyanurate, triallyl isocyanurate (TAIC), methylene bis acrylamide, metaphenylene diamine bis maleimide or other crosslinking agents, for example as described in U.S. patents Nos . 4,121,001 and 4,176,027, -are incorporated into the composition prior to irra¬ diation. The insulation may include additional additives, for example reinforcing or non-reinforcing fillers, stabilisers such as ultra-violet stabilisers, antioxi- dants, acid acceptors and anti-hydrolysis stabilisers, pigments, processing aids such as plasticizers, haloge- nated or non-ha ogenated flame retardants e.g. hydrated metal oxides such as alumina trihydrate or magnesium hydroxide, or decabromodiphenyl ether, fungicides and the like. In many cases the wire insulation will consist solely of the polyester inner layer and the fluoropo- ly er outer layer. However, if desired one or more other layers may be present. For example an additional inorganic arc-control layer may be provided directly on the conductor, formed for example by deposition of an inorganic material on the conductor. Alternatively or in addition a highly aromatic polymer layer may be pro¬ vided between layers (a) and (b) in order to improve for example the high temperature properties of the insulation. Examples of such aromatic polymers are disclosed in our copending European application entitled "Electrical Wire" which is filed on even date herewith and claims priority from British application No. 8716306. The wires and cables according to the invention may be formed by conventional techniques . For example the polymers may be blended with any additional com¬ ponents, in a mixer, pelletised, and then extruded onto a wire conductor. Other wires may be formed by a tape- wrapping method although it is preferred for both the fluoropolymer and the polyester layers to be melt extruded. The wires may be used individually as equipment or "hook-up" wires, or airframe wires, or in bundles and harnesses, both jacketted and unjacketted, and may be used in multiconductor cables. The wires, harnesses or cables may be unscreened or they may be provided with a screen to protect them from electromagnetic inter¬ ference, as well known in the art. In addition flat cables may be formed using the insulation materials according to the invention, either employing flat con¬ ductors or round conductors . The invention will be described by way of example with reference to the accompanying .drawings in which: Figure 1 is an isometric view of part of an elec¬ trical wire according to the invention ? Figure 2 is a schematic view of the test arrange¬ ment for wet tracking; and Figure 3 is a schematic view of the test arrange¬ ment for dry arcing. Referring initially to figure 1 of the accom¬ panying drawings an electrical wire comprises a conduc¬ tor 11 which may be solid or stranded as shown and is optionally tinned. On the conductor an inner insu¬ lating layer 12 (primary insulation) has been extruded. The insulation is formed from polybutylene terephtha¬ late which contains about 5% by weight triallyl isocyanurate crosslinking promoter. After the inner layer 12 has been formed an outer layer 13 (primary jacket) formed from an ethylene-tetrafluoroethylene copolymer, containing about 7% by weight triallyl iso¬ cyanurate crosslinking promotor, is extruded on the inner layer 12. Each layer has a wall thickness of about 100 m. After both layers have been extruded the insulation is irradiated by high energy electrons to a dose of about 120 kGy. The following Examples illustrate the invention. In the Examples the following test procedures were used: WET TRACKING TEST This test is designed to simulate the condition occuring when a damaged wire bundle comes into contact with an electrolyte. Under actual conditions, the electrolyte may be moisture containing dust particles or other ionic contaminant. Damage to the bundle may occur through a number of reasons e.g. abrasion, hydro¬ lysis of the insulation, ageing, etc. Current flow through the electrolyte results in heating and evapora¬ tion of the solution. This causes one or more dry bands to appear across which the test voltage is dropped, resulting in small, often intense, scin¬ tillations which damage the insulation. Figure 1 shows the sample set-up. A wire bundle 1 is prepared from seven 18cm lengths 2 of 20AWG tinned- copper conductor coated with a layer of the material -under test. The bundle 1 is arranged with six wires around one central wire and is held together using tie wraps 3 so that the wires are not twisted. Two adja¬ cent wires are notched circumferentially to expose 0o5mm bare conductor on each wire. The notches 4 are arranged such that they are 5mm apart with the tie wraps 5mm either side of them. One end of each wire is stripped to enable connections to be made to the power supply via insulated crocodile clips . The sample is held at an angle of 30 degrees to the horizontal using a simple clamp made of an electrically insulating resin so that the damaged wires are uppermost and the stripped ends are at the upper end of the bundle. A piece of filter paper 5 20 x 10mm wide is wrapped around the bundle approximately 2mm above the upper notch; this is best held in place with the upper tie wrap. A peristaltic pump conveys the electrolyte from the reservoir to the sample via a dropping pipette 6, and a power supply is provided to energise the bundle. The electrolyte used is 2% sodium chloride and optionally 0.02% in ammonium perfluoroalkyl carboxylate surfactant in distilled or deionised water. The pump is set to deliver this solution at a rate of approxima¬ tely lOOmg per minute through the pipette 6 which is positioned 10mm vertically above the filter paper 5. The power is supplied by a 3-phase 400Hz 115/200V generator of at least 5kVA capacity or a single phase 50Hz 115V transformer of at least 3kVA capacity. A device for recording time to failure is provided which records the time when either a wire goes open circuit, or when a circuit breaker comes out. Leakage currents can be followed with the use of current clamps surrounding the wires and connected to a suitable oscilloscope. In the case of the three- phase supply, adjacent wires of the bundle are connected to alternate phases of the power supply via 7.5A aircraft-type circuit breakers e.g. Klixon with the central wire connected directly to neutral. In the case of the single phase supply, alternate wires are connected to neutral with the remaining wires including the central conductor to live. A few drops of electrolyte are allowed to fall onto the filter paper to ensure saturation prior to starting the test. The power is switched on and the timer started. The test is allowed to continue until: a) one or more circuit breakers come out; b) a wire becomes open circuit; or c) 8 hours have elapsed. The condition of the final bundle, and the time to failure is noted in all cases . Where failure has occurred due to breakers coming out, the power is then reapplied and each breaker is reclosed in turn until there is no further activity. . The condition of the bundle is again noted. Failure due to the wire becoming open circuit (result (b)) is indicative of erosion. If failure occurs due to one or more circuit breakers coming out (result (a)) then the absence of further crepitation on resetting of the circuit breakers indicates failure due to erosion, while further crepitation indicates tracking failure. Dry Arc Test This test is designed to simulate what happens when a fault in a wire bundle causes arcing under dry conditions. A graphite rod is used to initiate the arc which causes thermal degradation of the insulation. Continuation of the fault current can only occur through the wire bundle under test due to shorting across adjacent phases through a conductive char, or direct conductor-conductor contact such as might occur if the insulation is totally removed by the duration of the arc. Figure 2 shows the sample set-up. A wire bundle 21 is prepared from seven 10cm lengths 22 of 20AWG tinned-copper conductor coated with a layer of the wire- insulation under test. The bundle 22 is arranged with six wires around one central wire and held together with tie wraps spaced about 5cm apart. One of the outer wires is notched circumferentially between the tie wraps to expose 0.5mm bare conductor and one end of each wire is stripped to enable connections to be made via insulating crocodile clips. A rod 23 is provided which is made of a spectro- graphically pure graphite, diameter 4.6mm, with an impurity level not more than 20ppm. It is prepared before each test by sharpening one end using a conven¬ tional pencil sharpener of European design to give an angle of 10 degrees off vertical with a tip diameter of 0.4±0.1mm. A lOOg weight 24 is clamped onto the top of the rod 23 to maintain contact during the arc ini¬ tiation and also acts as a device to limit the depth of penetration of the rod by restricting its downward tra¬ vel. The rod passes through a PTFE bush which allows it to slide freely up and down. The arrangement of levers enables precise posi¬ tioning of the rod 23 on the wire bundle 21 which is held securely in place by means of a simple clamp 25 made of an electrically insulating resin and mounted on a block 26 made of the same material. The power source can be either: a) a 3-phase 400Hz 115/200V generator of at least 5kVA capacity b) a single phase 50Hz 115V transformer, at least 3kVA capacity c) 24V d.c. supplied by two 12V accumulators. The fault current is detected by means of current clamps surrounding the connecting leads and the voltage at failure is measured using a 10:1 voltage probe. The transducer signals are fed into a multi-channel digital storage oscilloscope where they can be displayed and manipulated to obtain power curves (voltage x current) and energy (integration of power curve). The wire bundle 21 is positioned in the clamp 25 so that the notched wire is uppermost. Adjacent wires of the bundle are connected to different phases of the supply through 7.5 aircraft type circuit breakers, and the central wire is connected directly to neutral. In the case of single phase or d.c. supplies, alternate wires are connected to neutral or the negative ter¬ minal, with the remaining wires, including the central wire, connected through circuit breakers to live or the postive terminal. The carbon rod is also connected to neutral or the negative terminal and positioned so that the point is in contact with the exposed conductor . The gap between the lOOg weight and the PTFE bush is adjusted to 0.4 mm using a suitable spacer to limit the penetration of the rod into the sample. A voltage probe is connected across the damaged wire and the rod, and current clamps positioned on each of the three pha¬ ses, or on the wires connected to the live side of the supply. A protective screen is placed in front of the test set-up and the power switched on. A material is deemed to pass this test if: a) no circuit breakers come out and the activity is relatively non-eventful, or b) there is no further activity on resetting the breakers after a non-eventful test. In addition, non-tracking materials will have relatively few spikes in the current trace with a correspondingly low total energy consumed. Tracking materials, on the other hand, show many spikes usually on all three phases, which are accompanied by violent crepitation and large energy consumption. Examples 20 AWG tinned copper conductors were provided with an extruded dual-wall insulation of approximately 100 micrometres wall thickness for each layer by means of a 20 mm Baughan extruder. The inner layer contained approximately 5% by weight triallyl isocyanurate cross¬ linking promotor while the outer layer contained approximately 7% triallyl isocyanurate. After extru¬ sion the wire was irradiated with high energy electrons to a dose of approximately 120 kGy in order to cross¬ link the insulation. The ultimate elongation, tensile strength, 125 β C cut through resistance, wet tracking and dry arcing were measured, and the results are shown in the Table. Blends of polybutylene terephthalate with the ionomer (Surlyn 9020) contained 80% PBT, 20% ionomer, and blends with the b ' utylene ether/butylene terephtha¬ late copolymer (BEST) contained 70% PBT, 30% BEBT. All percentages given are by weight. TORTR Wat Tracking tty arcing ESartpLe liner Outer Break- Tine to Crepi- Break- Kb. Ospit- Tb al IrBulaticn 125°C ULt±iabe Tensile layer layer ers 1st 0/C tatiαn ers Q/C taticn wall waiφt out elcn^ticn strength (S) en reset αi reset thickness fl fa) (un) (N) U) * o

Claims: CLAIMS : 1. An electrical wire which comprises an elongate electrical conductor and electrical insulation that comprises: (a) an inner insulating layer which comprises a poly¬ ester that has both aromatic and aliphatic moieties and has a molar carbon to hydrogen ratio of not more than 1.15; and (b) an outer insulating layer which comprises a fluorinated polyme . 2. A wire as claimed in claim 1, wherein the polymer of layer (a) has -a molar carbon-to-hydrogen ratio of not more than 1.0. 3. A wire as claimed in claim 1 or claim 2, wherein the polymer of layer (a) has a carbonaceous char resi¬ due of not more than 10% by weight. 4. A wire as claimed in claim 3, wherein the polymer of layer (a) has a carbonaceous char residue of not more than 5% by weight. 5. A wire as claimed in any one of claims 1 to 4, wherein the polymer of layer (a) comprises a polyester based on polybutylene terephthalate and/or a segmented polyether polyester block copolymer having long-chain ester units of the general formula: 0 0 -OGO-C-R-C- and short-chain ester units of the formula 0 0 -ODO-C-R-C- in which G is a divalent radical remaining after the removal of terminal hydroxyl groups from a polyalkylene oxide) glycol, preferably a poly (C2 to C4 alkylene oxide) having a molecular weight of about 600 to 6000; R is a divalent radical remaining after removal of carboxyl groups from at least one dicarboxylic acid having a molecular weight of less than about 300; and D is a divalent radical remaining after removal of hydroxyl groups from at least one diol having a molecular weight less than 250. 6. A wire as claimed in any one of claims 1 to 5, wherein the fluorinated polymer is a fluorinated addi¬ tion polymer. 7. A wire as claimed in claim 6, wherein the fluorinated polymer comprises a homo- or copolymer of hexafluoropropylene, tetrafluoroethylene, vinylidine fluoride or a C1-C5 perfluoroalkoxy substituted per- fluoroethylene. 8. A wire as claimed in any one of claims 1 to 7, wherein at least layer (a) is crosslinked.

Inventors: DAY STEPHEN, PENNECK RICHARD JOHN, DAY, STEPHEN, PENNECK, RICHARD, JOHN
Applicants: RAYCHEM LTD, RAYCHEM LIMITED
Application year: 1988
Main classification: H01B7
IPC codes: C08L27, C08L67, C09D5, C09D167, H01B3, H01B7