WO1989000760A1

WO1989000760A1 - Wire

Info

Publication number: WO1989000760A1
Application number: PCT/GB1988/000553
Authority: WO
Inventors: Stephen Day; Richard John Penneck
Original assignee: Raychem Limited
Priority date: 1987-07-10
Filing date: 1988-07-08
Publication date: 1989-01-26
Also published as: IL87050A0; EP0376966A1; KR890702217A; FI900112A0; BR8807603A; AU606731B2; FI900112A; CA1319403C; JPH02504332A; AU2087788A

Abstract

An electrical wire comprises an elongate electrical conductor and electrical insulation that comprises: (a) an inner insulating layer which comprises a polyamide that has a glass transition temperature of at least 0°C; and (b) an outer insulating layer with comprises a fluorinated polymer, for example an ethylenetetrafluoroethylene copolymer. The wire exhibits a balance of properties, especially weight and cut through resistance combined with high resistance to wet and dry tracking, that make the wire useful inter alia for airframe wire.

Description

WIRE

This invention relates to electrical wires, especially although not exclusively, to wires intended for use in aircraft.

One phenomenon to which aircraft wires may be subjected is tracking. Tracking is associated with the formation of permanent and progressive conducting 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 wires may be subject, 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 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, 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 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 insulating layer which comprises a polyamide that has a glass transition temperature of at least 0°C; 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, arcing and erosion. For example the polymeric material (including any fillers) forming the inner, polyamide layer, and preferably the materials forming both layers will normally have an elongation to break of at least 50% and especially at least 100%, and, together, a cut through value at 150°C of at least 15, and preferably at least 20 N.

The polyamide preferably has a molar carbon to hydrogen ratio of not more than 1.0 and especially not more than 0.8. This will normally correspond to a carbonaceous 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 analysis, or TGA, in which a sample of the polymer is heated in nitrogen or other inert atmosphere at a defined rate, e.g. 10°C per minute 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 above are defined as having been measured at 850°C.

The polyamides employed in layer (a) may be entirely aliphatic or may have both aliphatic and aromatic moieties, and preferably have adjacent amide groups that are separated from each other by an average of at least 4 carbon atoms (excluding the amide car- bonyl group carbon atoms) in the polymer backbone, that is to say excluding any pendant groups but including all the carbon atoms in any monocyclic or fused aromatic rings in the polymer backbone. The most preferred polyamides have adjacent amide groups that are separated from one another by an average of at least 6 carbon atoms, but preferably by not more than 15 carbon atoms in the polymer backbone. The preferred polyamides may be defined by the following general formula I:

(I)

wherein each R and R' which can be the same or different, each represents an alkylene group having 1 to 30 carbon atoms, preferably 2 to 20 carbon atoms and most preferably 4 to 12 carbon atoms; or a divalent group having one or more alkylene, cycloalkylene or arylene moieties and from 3 to 30 carbon atoms, preferably 2 to 20 carbon atoms and most preferably 4 to 12 carbon atoms; or a mixed alkyl-cycloalkyl radical having from 4 to 30 carbon atoms, preferably four to 20 carbon atoms and most preferably 4 to 12 carbon atoms; or R can also be represented by the general structure (II):

(II)

in which R² and R³, which may be the same or different, each represents a hydrogen atom or an alkyl radical from one to 15 carbon atoms, preferably one to five carbon atoms; a cycloalkyl radical having from 3 to 16 carbon atoms, preferably five to 10 carbon atoms; a mixed alkylcycloalkyl radical having from 6 to 20 carbon atoms, preferably 6 to 10 or an aryl radical having from 6 to 20 carbon atoms, preferably 6 to 10 carbon atoms. In a preferred embodiment, each R² represents a methyl group and each R³ represents a hydrogen atom. n is an integer and, preferably, falls within the range of about 10 to about 500,000.

Examples of preferred polyamides include, but are not limited to: a 50:50 mole ratio copolymer of 2,2'-(bis(4-aminocyclohexyl) propane and a 60/40 weight percent mixture of azelaic acid and adipic acid, (for example as prepared in accordance with U.S. Pat. No. 3,840,501 which disclosure is hereby incorporated by reference), an amorphous polyamide derived from a 50:50 ratio of hexamethylene diamine and an equal mixture of terephthalic acid and isophthalic acid, an amorphous polyamide derived from dimethylterephthalate and a 50:50 mixture of 2,2,4- and 2,4,4-trimethylhexamethylene diamine, a polyamide of m-xylylenediamine and adipic acid, a polyamide 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 comonomers, e.g. ε-caprolactam comonomers, and especially polyamides based on units derived from laurinlactam, isophthalic acid and bis-(4-amino-3-methylcyclohexyl) methane. Amorphous polyamides preferably has a crystallinity of not more than 2% as measured by differential scanning calorimetry (DSC) e.g. as described in U.S. patent No. 4,528,335, the disclosure of which is incorporated herein by reference. Crystalline or semi-crystalline polyamides that may be employed according to the invention 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.

If desired the polyamide may be blended with one another or with other polymers. For example the polyamides may be used as blends with polyesters,polyolefins such as polyethylene, ethylene ethyl acrylate copolymers or styrene/diene block copolymers or phenylene ether homo- or copolymers. In many cases it is highly desirable for the polyamide to be blended with an aromatic polymer, preferably a wholly aromatic polymer or one that includes aliphatic moieties e.g. pendant alkyl groups or alkylene groups in the polymer backbone, such alkyl or alkylene groups preferably having no more than 5, especially no more than 3 carbon atoms e.g. methyl groups or, as alkylene groups, methy- lene or isopropylidine groups. Such compositions, and wires that employ such compositions, are disclosed in our International Applications entitled "Electrical Wire and Cable" and "Electrical Wire" which are filed on even date herewith and claim priority from British applications Nos. 8716304 and 8716307, the disclosures of which are incorporated herein by reference.

The fluorinated polymer used in layer (b) preferably 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% especially 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 fluorocarbon 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 10⁴ Pa.s (10⁵ poise) at a temperature not more than 60°C above its melting point. A preferred fluorocarbon polymer is a copolymer of ethylene 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 tetrafluoroethylene and hexafluoropropylene. Alternatively C₁-C₅ perfluoroalkoxy substituted perfluoroethylene homopolymers and copolymers with the above fluorinated polymers may be used.

Either or both layers of the wire insulation may be, and preferably are, 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 polyfunctional vinyl or allyl compound, for example, triallyl cyanurate, triallyl isoσyanurate (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, antioxi- dants, acid acceptors and anti-hydrolysis stabilisers, pigments, processing aids such as plasticizers, haloge- nated or non-halogenated flame retardants e.g. hydrated metal oxides such as alumina trihydrate or magnesium hydroxide if the processing conditions allow, zinc borate or decabromodiphenyl ether, fungicides and the like.

In many cases the wire insulation will consist solely of the polyamide inner layer and the fluoropolymer outer layer. However, if desired one or more other layers may be present. For example an additional inorganic layer may be provided directly on the conduc tor, formed for example by deposition of an inorganic material on the conductor. Alternatively or in addition a highly aromatic polymer layer may be provided 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 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 components, in a mixer, pelletised, and then extruded onto a wire conductor. Other, non-preferred, wires may be formed by a tape-wrapping method although it is preferred for both the fluoropolymer and the polyamide 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 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 be described by way of example with reference to the accompanying drawings in which:

Figure 1 is an isometric view of part of an electrical wire according to 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. On the conductor an inner insulating layer 12 or primary insulation has been extruded. The insulation is formed from nylon 12 or a blend of nylon 12 with a polyaryl ether imide which contains about 5% by weight triallyl isocyanurate crosslinking promotor. After the inner layer 12 has been formed an outer layer 13 or primary jacket formed from an ethylene-tetrafluoroethylene copolymer, containing about 7% by weight triallyl isocyanurate cross- linking promotor, is extruded on the inner layer 12. Each layer has a wall thickness of about 100 pm. 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, hydrolysis of the insulation, ageing, etc. Current flow through the electrolyte results in heating and evaporation of the solution. This causes one or more dry bands to appear across which the test voltage is dropped, resulting in small, often intense, scintillations which damage the insulation.

Figure 2 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 adjacent wires are notched circumferentially to expose 0.5mm 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% ammonium perfluoroalkylcarboxylate surfactant in distilled or deionised water. The pump is set to deliver this solution at a rate of approximately 100mg 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.

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 3 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 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±0.1mm. 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 positive 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 0.4mm 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. The condition of the final bundle 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. 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.

Example 1

A 20mm Baughan extruder was used to produce a wire construction comprising a 20 AWG tin-plated copper conductor with a 100 urn inner insulating layer formed from an aromatic/aliphatic polyamide (Polyamide 1) based on laurinlactam, isophthalic acid and bis-(4-amino-3-methylcyclohexyl) methane containing 5% by weight triallyl isocynaurate and a 100 um outer insulating layer formed from ETFE containing about 7% by weight triallyl isocyanurate. After formation the wire was irradiated with high energy electrons to a dose of1 120 kGy to crosslink the insulation. The physical and mechanical properties of the wires are shown in Table I together with those of a conventional airframe wire having a 200 μm thick ETFE insulation, from which it can be seen that the wire according to the invention is lighter and has superior cut through and scrape resistance than the ETFE wire.

Notes ( i ) measured according to BS G230 : 1984 test 26 with crosshead speed of 5mm minute^-1.

( ii ) measured according to BS G230 : 1984 test 30 using a tungsten carbide blade with a 0 .005 inch radius blade edge, an 800g weight and conducted at 20 °C.

While the physical and mechanical properties of the wire are considerably improved by replacing a proportion of the ETFE with a polyamide , the ETFE cannot be completely replaced with the polyamide in view of the very poor erosion performance of the polyamide wires when subjected to the wet tracking test and dry arcing test as described above. The results of these tests for wire construction (a) and for a wire having a 200 um thick insulation formed from the same polyamide as used in construction (a) are shown in Table II.

Example 2

Example 1 was repeated with the exception that the following polyamides were used: nylon 12 (polyamide II), an amorphous polyamide derived from dimethylterephthalate and a 50:50 mixture of 2,2,4- and 2,4,4-trimethylhexamethylene diamine (ex Dynamit Nobel) (polyamide III), an amorphous polyamide based on hexamethylene diamine and isophthalic acid (polyamide IV), and an amorphous polymer based on xylylene diamine and adipic acid (polyamide V). In addition tests were performed on single wall polyetherimide and polyetherketone wires by way of comparison. The results for the wet tracking and dry arcing tests are shown in Tables III and IV.

Example 3

Example 1 was repeated with the exception that the fluorocarbon layer comprised perfluoroalkoxy polymer -Teflon PFA (Trademark) and a fluorinated ethylenepropylene polymer - Teflon FEP (Trademark) and that neither layer contained any crosslinking promotor nor was irradiated. The results are shown in Tables V and VI.

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 polyamide that has a glass transition temperature of at least 0°C; and

(b) an outer insulating layer which comprises a fluorinated polymer.

2. A wire as claimed in claim 1, wherein the polyamide has adjacent amide groups that are separated from each other by an average of at least 4 carbon atoms in the polymer backbone.

3. A wire as claimed in claim 2, wherein the polyamide is a nylon.

4. A wire as claimed in claim 1 or claim 2, wherein the polymer of layer (a) comprises a polyamide based on terephthalic acid and trimethylhexamethylene diamine, a polyamide based on laurinlactam, isophthalic acid and bis-(4-amino-3-methylcyclohexyl)methane, a polyamide based on 2,2'-bis(4-aminocyclohexyl) propane, azeleic acid and adipic acid, or a polyamide based on m- xylylene diamine and adipic acid.

5. A wire as claimed in any one of claims 1 to 4, wherein the polyamide is crosslinked.

6. A wire as claimed in any one of claims 1 to 5, wherein the fluoropolymer is crosslinked.

7. A wire as claimed in any one of claims 1 to 6, wherein the fluorinated polymer is a fluorinated addition polymer.

8. A wire as claimed in claim 7, wherein the fluorinated polymer comprises a homopolymer of hexafluoropropylene, tetrafluoroethylene, vinylidine fluoride or a C₁-C₅ perfluoroalkoxy substituted perfluoroethylene or a copolymer thereof with one or more fluorinated or non-fluorinated comonomers.

9. A wire as claim in claim 8, wherein the fluorinated polymer is a polymer based on ethylene, tetrafluoroethylene and optionally up to 10% by mole of one or more other comonomers.

10. A wire as claimed in any one of claims 1 to 9, wherein the inner and outer layer have both been formed on the conductor by extrusion.

11. A wire as claimed in any one of claims 1 to 10, wherein the inner and outer layer each has a wall thickness in the range of from 50 to 150 micrometres.

12. A wire as claim in any one of claims 1 to 11, which includes an additional layer between the inner and outer insulating layer.

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