INSULATED ELECTRICAL CONDUCTORS

Abstract: An insulated electrical conductor having improved flex life comprises an elongate electrical conductor and electrical insulation surrounding the conductor. The insulation has three layers: an inner layer which is a wrapped polymeric tape or an extruded polymeric layer; an intermediate layer which is a polymeric adhesive layer; and an outer layer which is an extruded polymeric layer. The adhesive layer adheres to both the inner layer and the outer layer.

Description: INSULATED ELECTRICAL CONDUCTORS This invention relates to insulated electrical conductors. Polyimides, e.g. polypyromellitimides and polyetherimides, are well known. Most polyimides are thermosetting, and cannot be extruded. Therefore, when a polyimide is used to insulate an electrical conductor, it is usually used in the form of a tape which is wrapped around the conductor. Often the tape is coated on both sides with a fluoropolymer, so that the tape can be fused to itself after it has been wrapped (see for example U.S. Patent Nos. 3,616, 177, 4,628,003, 5,106,673, 5,238,748 and 4,399,434). A disadvantage of polyimides is their poor resistance to arc tracking. It has been proposed, therefore, to cover a polyimide tape wrap insulation with an outer layer of a fluoropolymer, particularly polytetrafiuoroethylene (PTFE) (see U.S. Patent No. 5,220,133). However, conductors insulated in this way have poor flex life unless the polyimide tape is quite thick (e.g. greater than 200 μm) and, therefore, heavier and more expensive than is desirable. Other disadvantages are that the PTFE is difficult to mark and must be applied as a tape which is sintered in place, making use of temperatures which can damage the tin plating of tin-plated copper conductors. We have discovered, in accordance with the present invention, that an insulated conductor having improved flex life can be produced by placing an intermediate layer of a melt-extruded polymeric adhesive, preferably a fluoroelastomer, between (i) an inner layer comprising a wrapped polymeric tape, particularly a polyimide tape, and (ii) an outer layer of an extruded polymeric composition, preferably a fluoropolymer. We have further discovered, also in accordance with the present invention, that an insulated conductor having improved flex life can be produced by placing an intermediate adhesive layer between an inner insulating layer of a first melt extruded polymer and an outer insulating layer of a second melt extruded polymer. In a first preferred aspect, this invention provides an insulated electrical conductor which comprises: (A) an elongate electrical conductor; and (B) electrical insulation which surrounds the conductor and which comprises: ( 1 ) a first, inner layer which (a) surrounds the conductor, (b) is in direct contact with conductor, and (c) comprises a polymeric tape wrapped around the conductor, (2) a second, intermediate layer which (a) surrounds the first layer, (b) is composed of a polymeric adhesive, (c) has been melt extruded around the first layer, and (d) is in direct and adherent contact with the first layer, and (3) a third, outer layer which (a) surrounds the second layer, (b) is composed of a polymeric composition, (c) has been melt extruded around the second layer, and (d) is in direct and adherent contact with the second layer. In a second preferred aspect, this invention provides a method of making such lated conductor by (A) wrapping the polymeric tape around the conductor; (B) melt extruding the polymeric adhesive over the wrapped polymeric tape; and, (C) melt extruding the polymeric composition around the polymeric adhesive. In a third preferred aspect, this invention provides an insulated electrical conductor comprising: A. an elongate electrical conductor, and B. electrical insulation surrounding the conductor and comprising: 1. an inner electrically insulating layer that surrounds and is in direct physical contact with the conductor, the inner layer being an uncrosslinked extruded polymeric layer; 2. an extruded polymeric adhesive layer that surrounds and is in direct physical contact with the inner layer; and 3. an uncrosslinked extruded polymeric outer electrically insulating layer that surrounds and is in direct physical contact with the adhesive layer; the adhesive layer being adherent to both the inner layer and the outer layer. Conductors The electrical conductor is generally a metal wire, which may be solid or stranded. Stranded wires are preferred, particularly when vibration is a factor, e.g. in airplanes. The metal is generally copper, but may be a copper alloy or aluminum; copper is preferably plated with Sn, Ag or Ni to reduce oxidation and improve solderabilty. Stranded wires can be of the unilay, concentric stranding or other type. Tape-wrapped Inner Insulating Layers Insulating tapes used in this invention comprise a continuous self-supporting tape of a suitable polymer, preferably a polyimide, e.g.. a polypyromellitimide. Other polymers which can be used include fluoropolymers, e.g. ethylene/tetrafluoroethylene copolymers (ETFE), and polyesters, e.g. polyethylene terephthalate (PET), polybutylene terephthalate (PBT) and polyethylene naphthalate (PEN). The polymer is generally oriented. Particularly when the self-supporting tape is composed of a polyimide or other polymer which cannot be bonded to itself by heating, it is preferably coated on one or both sides with one or more layers of a suitable thermoplastic polymer or mixture of polymers, so that when the wrapped tape is heated, adjacent wraps of the tape bond to each other and preferably also to the conductor. Suitable thermoplastic polymers include fluoropolymers, e.g. polytetrafluoroethylene; tetrafluoroethylene copolymers, e.g.. ETFE, tetrafluoroethylene-hexafluoropropylene copolymers (also known as fluorinated ethylene-propylene copolymers, FEP), tetrafiuoroethylene- perfluoroalkylvinylether homopolymer and copolymers, such as tetrafluoroethylene- perfluoromethylvinylether copolymer (MFA) and tetrafluoroethylene- perfluoropropylvinylether copolymer (PFA); polyvinylidene fluoride; vinylidene fluoride copolymers; polychlorotrifluoroethylene; and chlorotrifluoroethylene homopolymer and copolymers. Melt-extruded Inner Insulating Layers When the inner insulating layer is a melt-extruded layer, rather than a wrapped tape, suitable melt-extrudable polymers include fluoropolymers, e.g. ETFE, ethylene- chlorotrifluoroethylene copolymers (ECTFE), tetrafluoroethylene- perfluoroalkylvinylether copolymers, e.g. MFA and PFA; polyesters e.g. PET, PBT and PEN; polyethers, e.g. poly(ether ketones) and poly(ether ether ketones) (PEEK); and poly(ethersulfones). Preferred are the fluoropolymers, especially ETFE, ECTFE, MFA and PFA. Preferably, the layer is not crosslinked and contains no crosslinking agents. Intermediate Adhesive Insulating Layers The polymeric adhesive for the intermediate layer should be chosen by reference to the inner and outer layers. The adhesive layer is adherent to both the inner layer and the outer layer, but it need not bond strongly to either layer; even a relatively slight tack between layers helps to enhance the flex life of the insulated conductor. The polymeric adhesive is preferably elastomeric, i.e. it will readily recover its original shape after reasonable deformation. Preferably the 1% secant modulus [as measured by the method described in Standard D 882-88 of the American Society of Testing Materials (ASTM)] of the material of the adhesive layer is desirably lower than the 1% secant modulus of the material of each of the inner and outer layers, preferably less than 50%, particularly less than 33%, of the 1% secant modulus of the material of each layer. When the inner layer is a wrapped tape, and the tape is a base tape coated with a fusible polymer the modulus to be compared for this purpose is the modulus of the material of the base tape. When the inner layer has a fluoropolymer on its outer surface and the outer layer has a fluoropolymer on its inner surface, the polymeric adhesive is preferably also a fluoropolymer, especially a fluoroelastomer, e.g. one of the THV polymers containing units derived from tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride and available from 3M Co., one of the Viton fluoroelastomers available from du Pont Co. (block copolymers containing the same monomers), one of the "Aflas" fluoroelastomers available from 3M Co., or one of the T-530 and T-630 fluoroelastomers available from Daikin. When the inner and outer layers are based on polyesters, such as PET, then the adhesive is preferably a lower melting copolyester or an acrylic resin. A person of ordinary skill in the art, having regard to his own knowledge and the disclosure of this specification, will have no difficulty in choosing a suitable material for the adhesive. Outer Melt-extruded Insulating Layers The outer insulating layer is composed of a melt-extruded polymer, preferably a fluoropolymer, e.g.. ETFE, CTFE. MFA, or PFA, or a polyester, e.g.. PET, PBT or PEN. When the inner layer comprises a wrapped tape, the outer layer generally is not crosslinked (and does not, therefore, contain a crosslinking agent). However, it can be crosslinked; irradiation crosslinking should be avoided if it would degrade one of the polymers e.g.. a PTFE coating on the tape. When the inner layer is a melt-extruded layer, the outer layer is not crosslinked (and does not, therefore, contain a crosslinking agent). When the inner layer is a melt- extruded layer, the inner and outer layers will generally not be composed of the same materials, because a single layer of that material of the same total thickness would be expected to be of a comparable performance and easier to make. However, particularly where the single material has low flexibility (a high 1% secant modulus), the three layer insulation of this invention may retain the desirable notch propagation characteristics of a dual layer insulation while also obtaining improved flex life characteristics. Thus, a weakly bonded, low strength adhesive layer can be chosen so that a notch made in the outer layer does not propagate through the adhesive layer upon sharp bending of the insulated conductor at the notch. Other Ingredients in the Polymeric Compositions The material of the insulating layers have been described above by reference to their primary polymeric constituents. However, they may also contain other conventional constituents, e.g.. antioxidants, UV stabilizers, pigments or other coloring or opacifying agents (e.g.. titanium dioxide) and flame retardants. Application of the Insulation When the inner layer of insulation is a wrapped tape, the tape can be applied by standard techniques, e.g. as described in the U.S. Patents referred to above. The overlap is preferably about 50% (i.e. so that there are two thickness of tape on the conductor). To minimize ridging, an overlap of 45 to 50%), e.g. 46 to 49%, may be preferred. When the tape comprises a fusible material, so that overlapping tapes can be bonded to each other, the tape-wrapped conductor is heated by conventional methods to cause such bonding, preferably before the additional layers of insulation are applied. The melt-extruded layers of insulation are applied by standard techniques. The intermediate and outer layers are preferably applied in a single coextrusion step, but this may not be possible if their melting points differ by more than 50°C. Drawings The invention is illustrated in the accompanying drawings, in which FIG. 1 is a cross-section through an insulated electrical conductor of this invention; FIG. 2 is a perspective view of a insulated electric conductor according to the first preferred aspect of the invention, with partial removal of the layers; and FIG. 3 is a perspective view of an insulated conductor according to the third preferred aspect of the invention, with partial removal of the layers. In each of the Figures, an insulated conductor comprises a stranded metal wire conductor 10, an inner insulating layer 22, an intermediate adhesive insulating layer 24 and an outer insulating layer 26. Each of the strands of the conductor comprises a copper core 12 and an electroplated tin coating 14. In Figure 2, the inner layer is a tape wrap 2A. In Figure 3, the inner layer is a melt- extruded layer 22B. Examples The invention is illustrated by the following Examples, some of which are comparative examples. The conductor used in the Examples was a stranded conductor, containing 19 strands of 200 μm diameter (32 AWG) tin-plated copper with unilay stranding, having a nominal outside diameter of 950 μm (37.5 mils) and an equivalent conductor diameter of 813 μm (20 AWG). The insulated conductors produced in the Examples were tested by the method of Boeing Specification Support Standard 7324 (1992) Section 7.24. In this test, a length of wire loaded with a 900 g (2 lb) weight is repeatedly bent through a 180° total arc (± 90°) between an opposed pair of 12.7 mm (0.5 inch) diameter mandrels spaced 1.8 mm (.07 inch) apart at 18 cycles/minute until the conductor breaks. The insulated conductors were also tested by a method which was the same except that the load was 450 g (1 lb). The number of bending cycles (one cycle is +90° → -90° → +90°) to break the conductor is recorded. Nine samples were tested in each Example. The Table below shows the average of the results obtained. The bare conductor, tested at the 900 g. load, broke after 380 cycles (average of three samples). Example 1 The conductor was wrapped, at an overlap of 46-49%, with a polyimide tape 19 μm (0.75 mil) thick and 5.5 mm (0.218 inch) wide, and having a coating of a fluoropolymer 13 μm (0.5 mil) thick on each surface thereof. This tape is available under the tradename Chemfilm DF2909 from Chemfab Corporation, Merimak, New Hampshire. The wrapped conductor was fed at 8.5 m/minute through a 90 cm long horizontal furnace, using a Lindberg type radiant heater set at 800° C, to bond the wraps of the tape layer to each other and to the conductor; and was then quenched by passage through a cold water quench placed 40 mm from the heater exit. Under these conditions, discoloration of the tin plating of the conductor was minimized, while the tape wraps could not be peeled apart without physical damage. The nominal outside diameter of the wrapped conductor was 1100 μm (43.5 mil). After drying the wrapped conductor at 60°C for 24 hours or more, the adhesive layer and the outer layer were tube extruded over the wrapped conductor, using a coextrusion apparatus. The wrapped conductor was preheated with a Lindberg type heater so that the outer surface temperature of the inner layer as it entered the extruder was about 260°C. The adhesive was an elastomeric copolymer of tetrafluoroethylene, hexafluoropropylene, and vinylidene fluoride (available under the tradename THV-500G from 3M Inc.), and was extruded at about 280°C to a nominal thickness of 13 μm (0.5 mil.). The outer layer was ETFE (available under the tradename Tefzel-280 from du Pont) to which had been added 4% by weight of a white color concentrate of ETFE and titanium dioxide (available under the tradename Tefzel 210 from ICI), and was extruded at about 310°C to a nominal thickness of 87 μm (3.5 mil), so that the nominal outer diameter of the insulated electrical conductor was 1300 μm (51.5 mil). Example 2 (comparative) The procedure of Example 1 was followed except that no adhesive layer was used and the outer layer was extruded to a nominal thickness of 100 μm (4 mil) to give an insulated electrical conductor of the same outer diameter. Example 3 The conductor was extrusion coated with ETFE (Tefzel 280) at about 330°C to a nominal thickness of 75 μm (3 mil). The adhesive layer and the outer layer were then tube extruded over the extrusion-coated conductor, using a coextrusion apparatus. The ETFE-coated conductor was preheated so that the outer surface temperature of the ETFE as it entered the extruder was about 200°C. The adhesive was the same fluoroelastomer as in Examples 1 and 2, and was extruded at about 280°C to a nominal thickness of 25 μm (1 mil). The outer layer was ETFE (Tefzel 280) and was extruded at about 310 ° C to a nominal thickness of 75 μm (3 mils), so that the nominal outer diameter of the insulated electrical conductor was 1300 μm (51.5 mil). Example 4 (comparative) The procedure of Example 3 was followed except that (a) no adhesive layer was used, (b) the nominal thickness of the inner layer was 88 μm(3.5 mils), and (c) the nominal thickness of the outer layer was 88 μm(3.5 mil), so that the insulated electrical conductor was 1300 μm (51.5 mil). TABLE * comparative example.

Claims: CLAIMS 1. An insulated electrical conductor which comprises: (A) an elongate electrical conductor; and (B) electrical insulation which surrounds the conductor and which comprises: (1) a first, inner layer which (a) surrounds the conductor, (b) is in direct contact with conductor, and (c) comprises a polymeric tape wrapped around the conductor, (2) a second, intermediate layer which (a) surrounds the first layer, (b) is composed of a polymeric adhesive (c) has been melt extruded around the first layer, and (d) is in direct and adherent contact with the first layer, and (3) a third, outer layer which (a) surrounds the second layer, (b) is composed of a polymeric composition, (c) has been melt extruded around the second layer, and (d) is in direct and adherent contact with the second layer. 2. An insulated conductor according to claim 1 wherein the inner layer comprises a polyimide tape. 3. An insulated conductor according to claim 2 wherein the inner layer is composed of a polyimide tape which has a coating of a fluoropolymer on each surface thereof, and overlapping portions of the polyimide tape are fused to each other through the fluoropolymer coatings. 4. An insulated conductor according to claim 2 or 3 wherein the polyimide is a polypyromellitimide. 5. An insulated conductor according to any one of the preceding claims wherein the inner layer has thickness of 40 to 150, preferably 50 to 125, μm. 6. An insulated conductor according to any one of the preceding claims wherein the adhesive layer comprises a fluoroelastomer, preferably a copolymer of tetrafluoroethylene, hexafluoropropylene, and vinylidene fluoride. 7. An insulated conductor according to any one of the preceding claims wherein the adhesive layer has a thickness of 5 to 50, preferably 12 to 40, μm. 8. An insulated conductor according to anyone of the preceding claims wherein the polymeric tape is composed of a material having a 1% secant modulus Mi, the second layer is composed of a material having a 1% secant modulus M 2 , and the third layer is composed of a material having a l% secant modulus M 3 ; and M 2 is (a) less than Mi, preferably less than 0.5Mj, and (b) less than M 3 , preferably less than 0.5 M 3 . 9. An insulated conductor according to anyone of the preceding claims wherein the outer layer comprises a fluoropolymer, preferably an ethylene-tetrafluoroethylene copolymer. 10. An insulated conductor according to anyone of the preceding claims wherein the outer layer has a thickness of 40 to 150, preferably 50 to 125, μm. 11. An insulated conductor according to any one of the preceding claims wherein the conductor is a stranded plated copper wire having an effective conductor diameter of 400 to 1000, preferably 500 to 800, μm 12. A method of making an insulated conductor as claimed in any one of the preceding claims, the method comprising: (A) wrapping the polymeric tape around the conductor; (B) melt extruding the polymeric adhesive over the wrapped polymeric tape; and (C) melt extruding the polymeric composition around the polymeric adhesive. 13. An insulated electrical conductor comprising: A. an elongated electrical conductor, and B. electrical insulation surrounding the conductor and comprising: 1. an inner electrically insulating layer that surrounds and is in direct physical contact with the conductor, the inner layer being an uncrosslinked extruded polymeric layer; 2. an extruded polymeric adhesive layer that surrounds and is in direct physical contact with the inner layer; and 3. an uncrosslinked extruded polymeric outer electrically insulating layer that surrounds and is in direct physical contact with the adhesive layer; the adhesive layer being adherent to both the inner layer and the outer layer.

Inventors: MEHAN ASHOK K, MEHAN, ASHOK, K.
Applicants: RAYCHEM CORP, RAYCHEM CORPORATION
Application year: 1998
Main classification: H01B7
IPC codes: H01B7