EP0366700B1

EP0366700B1 - Electrical wire

Info

Publication number: EP0366700B1
Application number: EP88905963A
Authority: EP
Inventors: Stephen Day; Richard John Penneck
Original assignee: Raychem Ltd
Current assignee: Raychem Ltd
Priority date: 1987-07-10
Filing date: 1988-07-08
Publication date: 1993-01-27
Anticipated expiration: 2008-07-08
Also published as: DE3877963T2; GB8716306D0; DE3877963D1; WO1989000761A1; EP0366700A1; ATE85146T1; JP3036753B2; CA1319738C; JPH02504201A

Abstract

An electrical wire comprises an elongate electrical conductor and electrical insulation that comprises: a) an inner insulating layer which comprises polyamide or polyester having aliphatic moieties; and b) an outer insulating layer which comprises an aromatic polymer. The wire exhibits good electrical and mechanical properties associated with highly aromatic polymers together with high resistance to electrical arcing.

Description

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 as aircraft wire, for both civil and military applications. 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 polyphenylene oxide, and polyimide amides. Highly aromatic 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 military 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 susceptible to breakdown due to arcing. 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, 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 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.
Another phenomenon that can be associated with tracking and arcing is erosion. In this case insulating material is removed by a vaporization process originated by an electrical discharge without the formation 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 provided an electrical wire which comprises an elongate electrical conductor and electrical insulation that comprises:

(a) an inner melt-extruded insulating layer which comprises a polyamide or polyester having aliphatic moieties and having a char residue of not more than 15% by weight; and
(b) an outer melt-extruded insulating layer which comprises an aromatic polymer.

The wire according to the invention has the advantage that it can combine the beneficial properties of highly aromatic polymers, e.g. their good electrical breakdown resistance, fire retardancy, temperature stability and mechanical toughness, with good arc-tracking resistance. In addition, it is possible according to the invention to employ polymers for the inner layer that are relatively inexpensive and light in weight as compared with fluorinated polymers that have been proposed, and which have greater toughness, e.g. greater resistance to cut- through and abrasion together with reduced tendency to wrinkle as compared with polyolefins.
Preferably the polyamide or polyester forming the inner layer has a carbonaceous char residue of not more than 10%, most preferably not more than 5%, especially not more than 2% and most especially substantially zero. The char residue of the polymer components in the electrical wire according to the invention can be measured by the method known as thermogravimetric analysis, or TGA, in which a sample of the polymer is heated in nitrogen or other inert atmosphere at a defined rate to a defined temperature 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 percentage of the initial polymer after having taken into account any non polymeric volatile or non-volatile components. The char residue values quoted herein are defined as having been measured at 850°C. This will normally be achieved by choosing a polyamide or polyester that has a relatively low molar carbon to hydrogen ratio. Preferably the polymer has a carbon to hydrogen ratio of not more than 1.1, more preferably not more than 1.0, especially not more than 0.75 and most especially not more than 0.65.
It is possible for the polyester or polyamide to include one or more aromatic moieties in addition to its aliphatic moieties, and indeed a number of preferred polymers do so. However the polymer should have sufficient aliphatic nature that the C:H ratio is not too high. Preferred polyamides include the nylons, e.g. nylon 46, nylon 6, nylon 7, nylon 66, nylon 610, nylon 611, nylon 612, nylon 11, nylon 12 and nylon 1212 and aliphatic/aromatic polyamides, e.g. those based on the condensation of an aromatic dicarboxylic acid and an aliphatic diamene such as polyamides based on the condensation of terephthalic acid with trimethylhexamethylene diamine (preferably containing a mixture of 2,2,4-and 2,4,4-trimethylhexamethylene diamine isomers), polyamides formed from the condensation of one or more bisaminomethylnorbornane isomers with one or more aliphatic, cycloaliphatic or aromatic dicarboxylic acids e.g. terephthalic acid and optionally including one or more amino acid or lactam e.g. s-caprolactam comonomers, polyamides based on units derived from laurinlactam, isophthalic acid and bis-(4-amino-3-methylcyclohexyl) methane, polyamides based on the condensation of 2,2-bis-(p-aminocyclohexyl) propane with adipic and azeleic acids, and polyamides based on the condensation of trans cyclohexane-1,4-dicarboxylic acid with the trimethylhexamethylene diamine isomers mentioned above.
Other preferred aliphatic polymers include those based on polyether and polyamide blocks, especially the so called a "polyether-ester amide block copolymers" of repeating unit:
wherein A represents a polyamide sequence of average molecular weight in the range of from 300 to 15,000, preferably from 800 to 5000; and B represents a linear or branched polyoxyalkylene sequence of average molecular weight in the range of from 200 to 6000, preferably from 400 to 3000.
Preferably the polyamide sequence is formed from alpha,omega-aminocarboxylic acids, lactams or dia- mine/dicarboxylic acid combinations that include C₄ to C₁₄ carbon chains, and the polyoxyalkylene sequence is based on ethylene glycol, propylene glycol and/or tetramethylene glycol, and the polyoxyalkylene sequence constitutes from 5 to 85%, especially from 10 to 50% of the total block copolymer by weight. These polymers and their preparation are described in UK Patent Specifications Nos. 1,473,972, 1,532,930, 1,555,644, 2,005,283Aand 2,011,450A.
The polyesters that are used in the inner layer preferably include those based on a polyalkylene diol, preferably having a least 3 carbon atoms, or cycloaliphatic 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:
and short-chain ester units of the formula
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 copolymers 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 polyamide or polyester may be blended with one or more other polymers. For example polyamides may be used as blends with the polyesters, polyolefins such as polyethylene, ethylene ethyl acrylate copolymers or styrene/diene block copolymers, and the polyesters may be used as blends with ionomers or the above polymers referred to in connection with polyamides.
The preferred aromatic polymers which are used in this invention are well known to those skilled in the art, and reference may be made for example to U.S. Patents Nos. 3,025,605, 3,306,874, 3,257,357, 3,354,129, 3,441,538, 3,442,538, 3,446,654, 3,658,938, 3,677,921, 3,838,097, 3,847,867, 3,953,400, 3,956,240, 4,107,147, 4,108,837, 4,111,908, 4,175,175, 4,293,670, 4,320,224, and 3,446,654, British Patents Nos. 971,227, 1,369,210 and 1,599,106 and European Patent Applications Nos. 170,065, 124,276 and 178,185. Such polymers include polyketones, polyether ketones, polyether ether ketones, polyether sulphones, polyether ketone/sulphone copolymers, polyether imides and polyphenylene oxides. Blends of different polymers can be used. Preferred aromatic polymers are polymers with a melting or softening point of at least 250°C, particularly at least 300°C and which may be crystalline or amorphous. Softening points of amorphous polymers may conveniently be measured by thermomechanical analysis (TMA), in which case the softening point refers to the temperature at which the probe has reached 60% penetration.
The polymers may be wholly aromatic or they may include one or more aliphatic moieties.
In one class of such polymers the polymer comprises, and preferably consists essentially of, units of the formula
the units being the same or different,
wherein Ar represents an unsubstituted or substituted divalent aromatic radical and Q represents -O-, -S-, - S0₂-, -CO-, -NH-CO- or -COO-, or Ar represents a trivalent radical and Q represents
each bond of the Q radical preferably being bonded directly to an aromatic carbon atom.
One preferred class of polymer comprises the polyphenylene oxides of the repeating unit
in which the groups R_1' which may be the same or different, each represents a hydrogen or halogen atom or a hydrocarbon atom having no tertiary alpha carbon atom.
In another class of aromatic polymers the aromatic polymer is a crystalline polyarylene ether comprising recurring units of the formula
where E is the residue of a dihydric phenol and E' is the residue of an aromatic compound having an electron withdrawing group in at least one of the positions ortho and para to the valence bonds, the E and E' radicals being linked to the -O- radicals through aromatic carbon atoms. In one preferred sub-class, E is a radical of the formula
wherein R₂ is a divalent radical; x is 0 or 1; Y is a radical selected from halogen atoms, alkyl radicals containing 1 to 4 carbon atoms and alkoxy radicals containing 1 to 4 carbon atoms; y is 0, 1, 2, 3 or4; Y' is a radical selected from halogen atoms, alkyl radicals containing 1 to 4 carbon atoms and alkoxy radicals containing 1 to 4 carbon atoms; z is 0, 1, 2, 3 or 4, and E' is a radical of the formula
wherein R₃ is a sulphone, carbonyl, vinyl, sulphoxide, azo, saturated fluorocarbon, organic phosphine oxide or ethylidene radical. In this class preferred poly-sulphones are those in which y and z are 0, x is 1, R₃ is a sulphone radical and R₂ is a radical of the formula
wherein each of R₄ is independently selected from hydrogen atoms; alkyl radicals containing 1 to 4,carbon atoms which may be unsubstituted or substituted by one or more halogen atoms; aryl, alkaryl and aralkyl radicals containing 6 to 10 carbon atoms which may be unsubstituted or substituted by one or more halogen atoms.
In another class of aromatic polymers, the polymer is a polyether imide or polysulphone imide which comprises recurring units of the formula
where Q is -O- or -SO₂-, Z is a trivalent aromatic radical, R₅ is a divalent aromatic radical and R₆ is a divalent organic radical. Preferably the aromatic polymer has the general repeat unit:
in which D represents a group of the formula:
and

R' represents an arylene group.

Another class of polymers is the polyetherketones that have repeating groups comprising aromatic ether and aromatic ketone groups together with an imide, amide, ester, benzoxazole or benzothiazole group. Examples of such polymers are those having repeating units of the formula:
where R₇ represents an imide, amide or ester group.
Yet another class of aromatic polymer is the polyarylates. Polyarylates that may be used include those that are derived from dihydric phenols and at least one aromatic dicarboxylic acid. Examples of such polymers include those derived from a dihydric phenol of the general formula
in which the groups Y, which may be the same or different, each represent a hydrogen atom, a C₁ to C₄ alkyl group, or a chlorine or bromine atom; b is 0 or an integer from 1 to 4; R₈ represents a divalent saturated or unsaturated hydrocarbon group, e.g. an alkylene, alkylidine, cycloalkylene or cycloalkylidine group, an oxygen or sulphur atom or a carbonyl or sulphonyl group; and c is 0 or 1.
Preferred aromatic polymers consist essentially of repeating units having one of the following formulae

wherein each of x, m and n is 0 or 1, with n being 0 when x is 1, p is an integer from 1 to 4, with m being 1 and x being 0 when p is greater than 1, e.g.,
or

or
in which units derived wholly from isophthalic acid or terephthalic acid or a mixture of both are present.
Other polymers containing aromatic moieties e.g. poly 1,12-dodecamethylene pyromellitimide or 1,13-tri- decamethylene pyromellitimide, as described in U.S. patent No. 3,551,200, may be used.
Blends of any two or more of the above polymers may be employed as may copolymers based on any two or more of these polymers. In addition, blends of any of these aromatic polymers with aliphatic polymers, e.g. the aliphatic polymers referred to herein may be used.
Many aromatic polymers that are used in the wire insulation will have a char residue of at least 30%, some polymers having a char residue of at least 40% and even at least 50%. This does not mean to say that a high char value is desired for its own sake, but simply that good mechanical and physical properties of these aromatic polymers including temperature stability and fire retardancy, are usually associated with high char residues. The preferred aromatic polymers will usually have a molar C:H ratio of at least 1.0, preferably at least 1.2, more preferably at least 1.3 and especially at least 1.4. The toughest polymers such as the polyaryl ether ketones, which are associated with high char residues, will have C:H ratios greater than 1.5.
Although it is possible to employ the aromatic polymer in the form of a blend with one or more aliphatic polymers in addition to, or instead of, any other aromatic polymers for example as described in our copending British applications nos. 8716304 and 8716307 (from which International application nos. W089/00756 and W089/00758 claim priority), the outer layer will usually consist solely of the aromatic polymer as the polymeric component.
Preferably also, the wire insulation is substantially free of halogens, since the presence of significant quantities of halogens can cause corrosive and toxic gases to be emitted when the wire is subjected to a fire. Preferably the wire insulation contains not more than 10% by weight halogens, more preferably not more than 5% by weight halogens and especially substantially no halogens.
The wire insulation, or at least the inner layer may be cross-linked, for example, by exposure to high energy radiation.
Radiation cross-linking may be effected by exposure 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 irradiation.
The insulation may include additional additives, for example reinforcing or non-reinforcing fillers, stabilisers such as ultra-violet stabilisers, antioxidants, acid acceptors and anti-hydrolysis stabilisers, pigments, processing aids such as plasticizers, halogenated or non-halogenated 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 polyamide/polyester inner layer and the aromatic 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. Such a layer would enable the thickness of the inner insulating layer to be reduced. Alternatively or in addition a wet-tracking control layer, which will normally have a low carbonaceous char residue e.g. not more than 15% by weight and which may be formed, for example, from an aliphatic polymer, may be provided on top of the aromatic polymer in order to improve the resistance of the insulation to wet tracking (the phenomenon of wet tracking being described in our British patent application No. 8716304 (International Application W089/00756) mentioned above.
The wires and cables according to the invention may be formed by conventional techniques. For example the polymers may be blended with any additional components, in a mixer, pelletised, and then extruded onto a wire conductor.
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 interference, as well known in the art. In addition flat cables may be formed using the insulation materials according to the invention, either employing flat conductors or round conductors.
The invention will now be described by way of example with reference to the accompanying drawings, in which:

Figure 1 is an isometric view of a wire in accordance with the invention;
Figure 2 is a schematic view of the test arrangement for wet tracking; and
Figure 3 is a schematic view of the test arrangement for dry arcing.

Referring initially to figure 1 of the accompanying drawings, an electrical wire comprises a conductor 11 which may be solid or stranded as shown and is optionally tinned. A 100 micrometre thick inner layer 12 (primary insulation) formed from polybutylene terephthalate or a butylene oxide-butylene terephthalate block copolymer is extruded onto the conductors followed by a 100 micrometre thick layer 13 of polyetherketone, polyether ether ketone or a polyaryletherimide. After the insulating layers have been extruded, or even before layer 13 has been extruded, layer 12 may be crosslinked by irradiating the wire with high energy electrons to a dose of about 120 kGy.
The following Examples illustrate the invention. In the Examples the following test procedure was used:

Dry Arc Test

This test is designed to simulate what happens when a fault in a wire bundle causes arcing under dry conditions. Agraphite 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 1 shows the sample set-up. A wire bundle 21 is prepared from seven 10cm lengths 22 of 22AWG tinned-copper or nickel-plated 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 spectrographically 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 conventional pencil sharpener of European design to give an angle of 10 degrees off vertical with a tip diameter of 0.4±0.1 mm. A 100g weight 24 is clamped onto the top of the rod 23 to maintain contact during the arc initiation and also acts as a device to limit the depth of penetration of the rod by restricting its downward travel. The rod passes through a PTFE bush which allows it to slide freely up and down.
The arrangement of levers enables precise positioning 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 7.5A 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 terminal, 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 100g weight and the PTFE bush is adjusted to the diameter of the insulated wire under test 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 phases, 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.
The following wire constructions were prepared by extruding onto 22 AWG nickel plated copper wire unless otherwise stated using a 20mm Baughan extruder. In the cases where a blend has been used, it has been prepared using a Baker Perkins twin-screw extruder, and in all cases the inner layer contained 5% TAIC and was crosslinked by high energy electron irradiation to a dose of 120 kGy. Examples 1 to 5 were tested for dry tracking with a 115 V 50 Hz, single phase power source, and the results are given in Table I. Examples 6 to 12 were tested using 115 V, 400 Hz three phase supply, and the results are given in Table II.
In the Examples the following polymers were used:
Polyaryletheretherketone: A polymer having the repeat unit of the formula:
Polyetherimide: A polymer having a repeat unit of formula:

Example 1

100 f..lm of an aromatic-aliphatic polyamide (polymer formed from a mixture of 2,2,4- and 2,4,4-trimethylhexamethylenediamine and terephthalic acid) was extruded as the inner insulating layer with 100 µm of polyaryletheretherketone as the outer insulating layer.

Example 2

100 µm of a blend of polytetramethylene terephthalate, an ionomer resin (Surlyn 9090 from Dupont) and a crosslinking agent (Diacryl 101) in the ratio of 77.5: 17.5: 5 as the inner insulating layer with 100 um of polyaryletheretherketone as the outer insulating layer.

Example 3

125 um of a blend of polytetramethylene terephthalate and a poly(ether-ester) block copolymer comprising approximately 57% by weight polybutylene terephthalate hard blocks and approximately 43% by weight poly(butylene glycol polyether terephthalate) soft blocks in the ratio of 70:30 as the inner insulating layer with 125 µm of polyaryletheretherketone as the other insulating layer.

Example 4

As Example 3 with the exception that the inner insulating layer also contains 20% by weight hydrated zinc borate.

Example 5 (Control)

250 µm polyaryletheretherketone as a single insulating layer.

Example 6

100 µm of a polyether block amide as the inner layer, and 100 µm of a polyetherimide as the outer layer.

Example 7

100 µm of polyethylene terephthalate as the inner layer and 100 µm of a polyetherimide as the outer layer.

Example 8 (Control)

100 µm of polyaryletheretherketone as the sole layer.

Example 9 (Control)

100 µm of polyetherimide as the sole layer.

Example 10

100 µm of an amorphous polyamide based on laurinlactam, isophthalic acid bis-(4-amino-3-methylcyclohexyl) methane, as the inner layer and 100 µm of polyaryletheretherketone as the outer layer.

Example 11

100 µm of polytetramethylene terephthalate as the inner layer and 100 µm of polyaryletheretherketone as the outer layer.

Example 12

100 µm of the same polyamide as in Example 1 for the inner layer, and 100 µm of polyaryletheretherketone as the outer layer.

Claims

1. An electrical wire which comprises an elongate electrical conductor and electrical insulation that comprises:

(a) an inner melt-extruded insulating layer which comprises polyamide or polyester having aliphatic moieties and having a char residue of not more than 15% by weight; and

(b) an outer melt-extruded insulating layer which comprises an aromatic polymer.

2. A wire as claimed in claim 1 wherein the polyamide or polyester has a molar carbon-to-hydrogen ratio of not more than 1.1.

3. A wire as claimed in claim 1 or claim 2, wherein the inner layer comprises a nylon or a polyamide based on an aromatic dicarboxylic acid and an aliphatic or cycloaliphatic diamine.

4. A wire as claimed in any one of claims 1 to 3, wherein the inner layer comprises nylon 46, nylon 6, nylon 7, nylon 66, nylon 610, nylon 611, nylon 612, nylon 11, nylon 12, a polyamide based on terephthalic acid and trimethylhexamethylene diamine or bisaminomethylnorbornane, a polyamide based on laurinlactam, isophthalic acid and bis-(4-amino-3-methyleyclohexyl), methane, a polyamide based on 2,2-bis-(p-amino- cyclo-hexyl)propane, adipic acid and azeleic acid, or a polyamide based on transcyclohexane-1,4-dicarboxylic acid and trimethylhexamethylene diamine.

5. A wire as claimed in claim 1 or claim 2, wherein the inner layer comprises a polyester based on a polyalkylene diol or a cycloaliphatic diol and a aromatic dicarboxylic acid.

6. A wire as claimed in claim 5, wherein the inner layer comprises polytetramethylene terephthalate, a polyester based on a copolymer of terephthalic acid and isophthalic acid units with 1,4-cyclohexanedimethyloxy units or a segmented polyether polyester block copolymer having long-chain ester units of the general formula:

and short-chain ester units of the formula

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.

7. A wire as claimed in any one of claims 1 to 6, wherein the aromatic polymer has a char residue of at least 25% by weight.

8. Awire as claimed in any one of claims 1 to 7, wherein the aromatic polymer has a molar carbon to hydrogen ratio of at least 1.0.

9. A wire as claimed in claim 8, wherein the aromatic polymer has a molar carbon to hydrogen ratio of at least 1.3.

10. A wire as claimed in any one of claims 1 to 9, wherein the aromatic polymer comprises a polyaryl ether ketone, a polyarylether sulphone, a polyarylether, a polyarylate and/or a polyether imide. 11. A wire as claimed in claim 10, wherein the aromatic polymer comprises:

i) units of the general formula:

the units being the same or different,
wherein Ar represents an unsubstituted or substituted divalent aromatic radical and Q represents -O-, -S-, -S0₂-, -CO-, -NH-CO- or -COO-, or Ar represents a trivalent radical and Q represents

the Q radical preferably being bonded directly to an aromatic carbon atoms;
ii) a crystalline polyarylene ether comprising recurring units of the formula: -0-E-O-E'- wherein E is the residue of a dihydric phenol and E' is the residue of an aromatic compound having an electron withdrawing group in at least one of the positions ortho and para to the valence bonds, the E and E' radicals being linked to the -O- radicals through aromatic carbon atoms; or

iii) polyether imide or polysulphone imide which comprises recurring units of the formula

where Q is -O- or-SO₂, z is a trivalent aromatic radical, R is a divalent aromatic radical and R' is a divalent organic radical.

12. A wire as claimed in claim 11, wherein the aromatic polymer comprises a polymer having a repeating unit of the formula:

wherein each of x, m and n is 0 or 1, with n being 0 when x is 1, p is an integer from 1 to 4, with m being 1 and x being 0 when p is greater than 1,

in which the units may be derived wholly from terephthalic acid or isophthalic acid or both;