WO2006005426A1

WO2006005426A1 - Fire resistant wire and cable constructions

Info

Publication number: WO2006005426A1
Application number: PCT/EP2005/006825
Authority: WO
Inventors: Philip James Hammond; David Barclay Tombs; John Wadeley
Original assignee: Tyco Electronics Uk Ltd.
Priority date: 2004-07-09
Filing date: 2005-06-23
Publication date: 2006-01-19
Also published as: GB0415389D0

Abstract

A fire-resistant insulated wire comprising a conductor wrapped with mica tape, a layer of sintered polytetrafluoroethylene (PTFE) overlying the mica layer(s), and a crosslinked layer of poly(ethylene-co-tetrafluoroethylene) (ETFE) overlying the PTFE layer.

Description

FIRE-RESISTANT WIRE AND CABLE CONSTRUCTIONS

This invention relates to fire-resistant wires capable of maintaining electrical circuit integrity when the wires, or cables containing them, are exposed to fire. These wires are designed for, but not limited to, use in the aerospace industry, where small diameter, lightweight and high performance are important requirements for wire harnesses.

Fire-resistant wires typically comprise a conductor wrapped with inorganic material in conjunction with one or more polymer layers. In a fire the inorganic material provides electrical insulation around the conductor once the usual layers of organic polymer insulation have been melted or burnt away. Of various known inorganic fire-resistant wrappings, mica tapes are generally preferred. This is due to this mineral's excellent thermal and dielectric properties which provide good fire resistance and high insulation values. Mica itself is also very stable to a wide range of chemicals, including those which promote hydrolysis.

The requirements of fire resistance, small size and light weight often have technical solutions which are not compatible and are not easily resolved. For example, fire resistance is often achieved by use of several layers of inflammable inorganic materials such as glass which makes the cables large and heavy whereas light weight and small size are achieved by thin layers of polymeric insulation. Large, heavy cables are not well suited for aerospace applications.

The present invention provides a unique wire construction capable of achieving a superior balance of characteristics such as small size, light weight, good fire-resistance, heat stability, ease of stripping, cut-through resistance, abrasion strength and solvent resistance. /Importantly, the construction provides these advantages without requiring a thick and/or heavy layer of insulation and can be readily manufactured using industry- standard wrapping and extrusion processes. It has been found according to the present invention that constructions offering a desirable combination of the required attributes can be made by first wrapping a conductor with mica tape, preferably with two layers of mica tape, then wrapping a layer of polytetrafluoroethylene (PTFE) tape over the mica layer(s), sintering the PTFE layer, and finally extruding and crosslinking (preferably by electron beam irradiation) a thin layer of poly(ethylene-co-tetrafluoroethylene) (ETFE) over the PTFE layer. The sintered PTFE intermediate layer (a) advantageously reduces or eliminates cracking and disintegration of the relatively brittle mica, which tends to occur when the ETFE outer layer is extruded directly onto the mica layer(s); and (b) unexpectedly facilitates stripping of the insulation from the conductor when the insulated wire is being connected to electrical equipment in use. The success of this fire wire insulation structure according to the present invention is surprising, since it is well known that PTFE is degraded by the electron beam irradiation which is preferred for cross-linking the ETFE outer layer.

Mica paper reinforced by a backing material of woven glass or of polyethylene film is produced as tape that can be spirally wrapped onto electrical conductors to give a degree of fire resistance. The mica wrapped conductor is then coated with a conventional polymer to impart the required electrical and mechanical properties. In a fire, the polymer is destroyed but the electrical integrity of the cable is maintained by the mica layer and the insulating char from the polymer. It is generally understood that the more mica the better from a fire performance point of view and tapes are available with different weights of mica (e.g. 80, 120,160g./sqm).

Mica tapes are applied by spiral wrapping with an overlap to maintain protection when the wrapped conductor is flexed, since the overlaps tend to open on flexing. To achieve a high level of fire protection either the weight of the mica tape can be increased or two or more layers of tape can be applied. The tapes become more difficult to wrap successfully on small diameter conductors which can lead to tape damage, wrinkling and a poor wrapped surface. The overlapped tape also tends to suffer from breakage of some of the glass weave reinforcement on small conductors due partly to the low elongation to break of the tapes. This causes glass fibre protrusions from the wrap. The quality of the mica-wrapped conductor can make subsequent extrusion processing more difficult and the final appearance and/or performance of the wire less satisfactory. In applications where size and weight are not important this problem can be overcome by using conductors of large diameter and/or thick polymer jackets. In other applications, such as aerospace, thick and heavy insulation layers are undesirable. The current invention provides a flame resistant, thin-wall, small size, lightweight wire that can be manufactured robustly using industry standard tape wrapping, extrusion and irradiation processes. Polyimides and crosslinked fluoropolymers are widely used as insulation materials for high performance wire and cable, especially airframe wiring. Wires based on fluoropolymers have important characteristics such as light weight and small diameter, good cut-through, arc-track and abrasion resistance and thermal stability, low flammability, and insensitivity to water and common solvents. A dual-layer insulated conductor, where the inner layer is an uncrosslinked or lightly crosslinked crystalline poly(ethylene-co-tetrafluoroethylene) and the outer layer is highly crosslinked crystalline poly(ethylene-co-tetrafluoroethylene), is described in U.S. Patent No. 5,059,483. Existing thin-wall, high performance wire insulation constructions include single or multi- layer cross-linked ETFE, as used for example in "Raychem Spec 55" (trade marks) wire. The polymeric composition can optionally contain suitable additives such as pigments, crosslinking agents, antioxidants, thermal stabilisers, acid acceptors and processing aids.

The dual fluoropolymer layer of the current invention preferably comprises a layer of PTFE covering the mica and another, separate outer layer of ETFE. Electron beam irradiation of the construction provides a crosslinked mechanically tough ETFE outer layer. The inner layer of PTFE does not crosslink. The finished construction therefore provides an outer crosslinked fluoropolymer layer and an inner uncrosslinked fluoropolymer layer. This type of dual layer construction is described in US 5059483 and can deliver increased resistance to cut through. In addition, the presence of an uncrosslinked layer between the mica and the crosslinked outer jacket may improve of stripping the mica-coated conductor.

This invention is related to an insulated electrical conductor comprising an elongate electrical conductor; an electrical insulation surrounding the conductor, said insulation comprising (a) an inner, electrically insulating layer that surrounds and is in direct physical contact with the conductor, the inner layer comprising a wrapped, coated mica tape layer as hereinafter described, (b) an intermediate tape-wrapped polymeric, electrically insulating layer that surrounds and is in direct physical contact with the inner micaceous layer, the intermediate layer comprising a wrapped polymer layer, preferably PTFE and (c) an outer, electrically insulating, extruded polymeric layer that surrounds and is in direct physical contact with the intermediate polymer layer, the outer layer comprising a fluoropolymer, especially a crosslinked fluoropolymer, more especially crosslinked ETFE. Therefore one aspect of the present invention provides an insulated electrical conductor comprising:

(a) an elongate electrical conductor, and

(b) electrical insulation surrounding the conductor, said insulation comprising (i) a mica tape inner layer that surrounds the conductor, said mica layer preferably comprising two wraps of mica tape. Preferably the first tape layer is applied with an overlap and the opposite lay direction to the strand lay, and the second tape layer applied with an overlap and the opposite lay direction to the first, (ii) a PTFE layer that surrounds the mica said PTFE preferably comprising a single wrap tape applied with an overlap and in the opposite direction to the second layer of mica, (iii) an extruded polymeric outer electrically insulating layer preferably comprising poly(ethylene-co-tetrafluoroethylene) (ETFE).

In accordance with the preferred embodiments of this invention, an electrical cable is provided with a primary insulation of micaceous material, a secondary insulation of a fluoropolymer and a third layer of a fluoropolymer. The cable itself may comprise one or more electrical conductors formed of any suitable metal, preferably copper or aluminum. In a preferred embodiment, the cable comprises one or more twisted electrical conductors or strands. The cable may be in any suitable size, such as 8 to 20 American Wire Gauge (AWG), preferably 14 to 26 AWG. The individual conductors may be individually provided with both primary, secondary and tertiary insulation layers before being formed into a composite cable or the individual conductors may be combined before or after wrapping with the primary insulation and prior to applying the secondary layer thereon.

The primary layer is composed of a micaceous material. Preferably, this layer is in the form of a mica paper tape and is wrapped about the bare cable by conventional cable taping equipment or by direct feed into extrusion heads. The mica tape may, for example, have a thickness of about 12.7 μm (0.5 mils) to about 1.27 mm (50 mils), more preferably about 25.4 μm (1 mil) to about 101.6 μm (4 mils).

The fluoropolymer secondary insulation may be applied to the covered conductor by any suitable manner including tape wrapping. Especially preferred fluoropolymers include tetrafluoroethylene homopolymers (PTFE) and copolymers with hexafluoropropene, propylene or perfluorovinylpropyl ether, chlorotrifluoroethylene homopolymers. The tape width and thickness will be selected by those skilled in the art according to the conductor size and the degree of overlap required. Tapes can be approximately 5 mm - 25 mm, more preferably 10-20 mm, wide and between about 10 to 1000 μm, more preferably about 25 to about 100 μm, thick. The fluoropolymer wrapped wire is preferably sintered at high temperature, prefereably 350-450 ⁰C, to consolidate the insulation layer and reduce its thickness.

The tertiary insulation layer may be applied to the covered conductor by any suitable manner including extrusion coating, powder coating and the like. The extrusion of the fluoropolymer onto the secondary insulation is preferred since high rates of production can be obtained. Preferred fluoropolymers are copolymers of ethylene and tetrafluoroethylene (ETFE). Such copolymers may also contain minor amounts (e.g., up to about 15 mol %) of other comonomers; for example, a terpolymer of ethylene, chlorotrifluoroethylene and hexafluoroisobutylene may be used. Other fluoropolymers that may be employed include tetrafluoroethylene homopolymers and copolymers with hexafluoropropene, propylene or perfluorovinylpropyl ether, chlorotrifluoroethylene homopolymers and copolymers with various alkenes, vinylidene fluoride homopolymers and copolymers with hexafluoroisobutylene, and the like.

The fluoropolymer layer may also include conventional additives, such as stabilizers, fillers, crosslinking agents, pigments and the like. The thickness of the fluoropolymer layer may be in the range of about 127 μm (5 mils) to 2.54 mm (100 mils) or more, preferably about 254 μm (10 mils) to 508 μm (20 mils).

The product is preferably crosslinked by electron beam irradiation to further enhance the properties of the insulation.

Wires of the aforementioned construction may be employed either singularly, in bundles or in cable constructions employing additional components such as fillers, braids, tape wraps and outer jackets.

Examples of the current invention were provided as follows:

Nickel coated copper strand conductor (22AWG-19/34-NC) was wrapped with two layers of Cogebi 80P34A mica/glass tape 4mm wide. The first tape layer was applied at 30% overlap and the opposite lay direction to the strand lay, and the second tape layer applied at 20% overlap and the opposite lay direction to the first. Over the mica was wrapped a layer of PTFE. This tape was Plastomer RCL4 613A white 15/32" (11.9mm) wide with a thickness of 51 μm (2 mils). This was applied at 64% overlap and opposite direction to the second layer of mica. The PTFE was then sintered at 380 ⁰C. This product was then extrusion coated with a thin layer, approximately 0.2 mm, of Spec TM 55, a modified ETFE insulation. The finished wire was crosslinked using electron beam irradiation.

Nickel coated copper strand conductor (16AWG-19/29-NC) was wrapped with two layers of Cogebi 80P34A mica/glass tape 6mm wide. The first tape layer was applied at 30% overlap and the opposite lay direction to the strand lay, and the second tape layer applied at 20% overlap and the opposite lay direction to the first. Over the mica was wrapped a layer of PTFE. This tape was Plastomer RCL4 613A white 11/16" (17.5mm) wide with a thickness of 51 μm (2 mils). This was applied at 64% overlap and opposite direction to the second layer of mica. The PTFE was then sintered at 380 ⁰C. This product was then extrusion coated with a thin layer, approximately 0.2mm, of Spec TM 55, a modified ETFE insulation. The finished wire was crosslinked using electron beam irradiation.

These two component wires were assembled into the following braided cable constructions :

09A) Two screened and jacketed twisted pairs in 16AWG with an overall screen and jacket 10A) Five screened and jacketed twisted pairs in 22 AWG plus one screened and jacketed twisted triple in 22 AWG with an overall screen and jacket. 11A) One screened and jacketed twisted triple in 22AWG

The approximate weights per metre of 9A, 10A and 11A were 210, 323 and 41 grams respectively.

In fire testing at 950 ⁰C using the burner described in BS 6387 all three cables retained circuit integrity at operating voltages of 200 volts, 50 volts and 50 volts respectively for greater than 15 minutes. The accompanying drawing shows by way of example in schematic end view the structure of a fire wire according to the present invention. A metallic wire conductor 1 , which may be of various solid or stranded constructions as known per se, is enclosed by two layers 2, 3 of mica tape of wire-wrapping structure known per se applied by wire- wrapping equipment and techniques generally known per se. A layer 4 of PTFE tape, wrapped (by methods known per se) over the mica layers 2, 3, has been sintered to produce a sintered PTFE intermediate layer according to the present invention, and a layer 5 of ETFE has been extruded over the sintered PTFE layer 4 and cross-linked by electron beam irradiation of the wire using equipment and conditions known per se to administer a radiation dose producing the desired degree of cross-linking in the ETFE layer.

Claims

1. A fire-resistant insulated wire comprising a conductor wrapped with mica tape, a layer of sintered polytetrafluoroethylene (PTFE) overlying the mica layer(s), and a crosslinked layer of poly(ethylene-co-tetrafluoroethylene) (ETFE) overlying the PTFE layer.

2. The fire-resistant insulated wire according to claim 1 wherein the mica tape is in direct physical contact with the conductor.

3. The fire-resistant insulated wire according to claim 1 or 2 wherein the sintered PTFE layer is in direct physical contact with the mica tape.

4. The fire-resistant insulated wire according to any of the proceeding claims wherein the outer ETFE layer is in direct physical contact with the intermediate PTFE layer.

5. The fire-resistant insulated wire according to any of the proceeding claims wherein the mica tape is a mica type paper reinforced by a baking material of woven glass or polyethylene film.

6. The fire-resistant insulated wire according to claim 5 wherein the baking material is of woven glass.

7. The fire-resistant insulated wire according to any of the proceeding claims wherein the conductor comprises twisted strands.

8. The fire-resistant insulated wire according to claim 7 wherein the mica tape is spirally wrapped with an overlap and an opposite lay direction to the strand lay.

9. The fire-resistant insulated wire according to any of the proceeding claims wherein the conductor comprises two wraps of mica tape.

10. The fire-resistant insulated wire according to claim 9 wherein the second mica tape is spirally wrapped with an overlap and an opposite lay direction to the first mica tape.

11. A method of making a fire-resistant wire, comprising the steps of (1) wrapping a bare conductor with mica tape, (2) wrapping a layer of polytetrafluoroethylene (PTFE) over the mica layer(s), (3) sintering the PTFE layer, and (4) extruding and crosslinking a layer of poly(ethylene-co-tetrafluoroethylene) (ETFE) over the PTFE layer.

12. The method according to claim 11 wherein the outer ETFE layer is crosslinked by electron beam irradiation.

13. The method according to claim 11 or 12 wherein the bare conductor comprises twisted strands and the mica tape is spirally wrapped with an overlap and in an opposite lay direction to the twisted strands.

14. The fire-resistant insulated wire according to any one of claims 11 to 13 wherein two layers of mica tape are wrapped onto the bare conductor and the second mica tape is spirally wrapped with an overlap and an opposite lay direction to the first mica tape.

15. An electrical cable containing at least one wire according to claim 1.