Multilayer plastic composition having an electrically conductive inner layer

Abstract

A multilayer plastic composition having (I) an outer layer of a thermoplastic molding composition and (II) an inner layer of an electrically conductive thermoplastic molding composition containing graphite fibrils, which are useful for the transport of (petro)chemical materials and also in the motor vehicle sector for conveying brake, cooling and hydraulic fluid and also fuel.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a multilayer plastic composition pipe having an inner layer that has been made lastingly antistatic by addition of graphite fibrils.

2. Discussion of the Background

Multilayer pipes based on polyamides are known (DE-A 41 12 662, 41 12 668, 41 37 430, 41 37 431, 41 37 434, 42 07 125, 42 14 383, 42 15 608, 42 15 609, 42 40 658, 43 02 628, 43 10 884, 43 26 130, 43 36 289, 43 36 290, 43 36 291, 44 10 148, WO-A93/21466, EP-A-0 198 728 and EP-A-0 558 373). Making the inner layer conductive by addition of carbon black or carbon fibers is also know (see DE-A 40 25 301).

However, the use of such conductive inner layers is associated with a series of disadvantages:

a) They are insufficiently resistant to peroxide-containing fuels (sour gas). This shows up, for example, in a serious deterioration of the cold impact toughness occurring after only relatively short storage time in contact with sour-gas-containing fuels (e.g. according to the Ford specification FLTM AZ 105-01, PN 180 or according to the GM specification GM213M, PN 50). b) After bending, thermoforming or after prolonged contact with fuel, the layers have sharply reduced conductivity, which can lead to complete loss. c) The surface roughness leads to leakages at connections using conventional Quick Connectors. d) The high carbon black or fiber loading in the molding compositions results in a high pressure build-up during extrusion. This requires a lowering of the production rate to uneconomically low values. e) The use of carbon fibers causes an undesired increase in the flexural stiffness. f) Finally, the conventional amounts of added carbon black undesirably reduce the cold impact toughness.

SUMMARY OF THE PRESENT INVENTION

Accordingly, one object of the present invention is to provide a multilayer plastic composition having an electrically conductive inner layer, which avoids the above-noted disadvantages.

Another object of the present invention is to provide a plastic pipe prepared from the multilayer plastic composition which is useful for transport of (petro)chemical materials in either solid or liquid form.

Another object of the present invention is to provide filling port for tanks in the motor vehicle industry prepared from the multilayer plastic composition.

These and other objects of the present invention have been satisfied by the discovery of a multilayer plastic composition containing the following layers:

I. An outer layer comprising a thermoplastic molding composition and II. An inner layer comprising an electrically conductive thermoplastic molding composition, wherein the electrically conductive molding composition contains graphite fibrils.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a multilayer plastic composition comprising:

(I) an outer layer comprising a thermoplastic molding composition and (II) an inner layer comprising an electrically conductive thermoplastic molding composition and graphite fibrils.

The multilayer plastic composition can take any desired form, with multilayer pipes, hollow bodies (such as fuel tanks), filling ports for tanks, etc. being preferred.

In addition, further layers may be present, such as a conventional barrier layer for fuel components, as an intermediate layer. The barrier layer can, if desired, be adhesively bonded to the adjacent layers by means of a bonding agent.

The outer layer of the present invention comprises a polyamide molding composition, a polyolefin molding composition, a polyacetal molding composition, a polyketone molding composition, or molding compositions of thermoplastic polyesters or polyester elastomers. Molding compositions suitable as inner layers for the present multilayer pipe include those based on polyamides, polyolefins, polyacetals, polyketones, thermoplastic polyesters or fluoropolymers.

Polyamides useful in the present invention are primarily aliphatic homopolyamides and copolyamides. Suitable examples include the 4.6-, 6.6-, 6.12-, 8.10-, and 10.10-polyamides with 6-, 10.12-, 11-, 12- and 12.12-polyamides being preferred. (The naming of the polyamides corresponds to the international standard, where the first digit(s) indicates the number of carbon atoms in the starting diamine and the last digit(s) indicates the number of carbon atoms in the dicarboxylic acid. If only one number is given, this means that the starting material was an .alpha.,.omega.-aminocarboxylic acid or the lactam derived therefrom--H. Domininghaus, Die Kunst stoffe und ihre Eigenschaften, page 272, VDI-Verlag (1976).)

If copolyamides are used, these can contain coacids, such as adipic acid, sebacic acid, suberic acid, isophthalic acid or terephthalic acid or codiamine, such as bis(4-aminocyclohexyl)methane, trimethylhexamethylenediamine, or hexamethylenediamine.

The preparation of these polyamides is known (for example: D. B. Jacobs, J. Zimmermann, Polymerization Processes, pp. 424-467; Interscience Publishers, New York (1977); DE-B 21 52 194).

Likewise suitable as polyamides are mixed aliphatic/aromatic polycondensates as are described, for example, in US-A 2 071 250, 2 071 251, 2 130 523, 2 130 948, 2 241 322, 2 312 966, 2 512 606, 3 393 210 or in Kirk-Othmer; Encyclopedia of Chemical Technology, 3rd edition, Vol. 18, Wiley & Sons (1982), pp. 328 and 435. Further polycondensates which are suitable as polyamides are poly(ether esteramides) or poly(etheramides). Such products are described in DE-A 27 12 987, 25 23 991 and 30 06 961.

The number average molecular weight, Mn, of the polyamides is above 4000, preferably above 10,000. The relative viscosity (.eta..sub.rel) is preferably in the range from 1.65 to 2.4.

The polyamides can contain up to 40% by weight of other thermoplastics, as long as they do not interfere with the properties required in the multilayer composition of the present invention. Suitable other thermoplastics include polycarbonate (H. Schnell, Chemistry and Physics of Polycarbonates, Interscience Publishers, New York (1981)], acrylonitrile/styrene/butadiene copolymers [Houben-Weyl, Methoden der organischen Chemie, Vol. 14/1, Georg Thieme Verlag Stuttgart, pp. 393-406; Ullmanns Encyclopadie der technischen Chemie, 4th edition, Vol. 19, Verlag Chemie Weinheim (1981), pp. 279-284], acrylonitrile/styrene/acrylate copolymers [Ullmanns Encyclopadie der technischen Chemie, 4th edition, Vol. 19, Verlag Chemie Weinheim (1981), pp. 277-295], acrylonitrile/styrene copolymers [Ullmanns Encyclopadie der technischen Chemie], 4th edition, Vol. 19, Verlag Chemie Weinheim (1981), p. 273 ff.] or polyphenylene ethers (DE-A 32 24 691 and 32 24 692, U.S. Pat. No. 3 306 874, 3 306 875 and 4 028 341).

If required, the polyamides can be impact modified. Suitable modifiers include ethylene/propylene copolymers or ethylene/propylene/diene copolymers (EP-A-0 295 076), polypentenylene, polyoctenylene or random or block copolymers of alkenylaromatic compounds with aliphatic olefins or dienes (EP-A-0 261 748). Furthermore, core/shell rubbers having a viscoelastic core of (meth)acrylate, butadiene or styrene/butadiene rubber having glass transition temperatures T.sub.g <-10.degree. C. can be used as impact-modifying rubbers, with the core of the rubbers being crosslinkable. The shell can be made up of styrene and/or methyl methacrylate and/or further unsaturated monomers (DE-A 21 44 528, 37 28 685). The proportion of impact-modifying components is to be selected such that the desired properties are not impaired.

The polyamides can be used on their own or in mixtures.

Suitable polyolefins include homopolymers and copolymers of .alpha.-olefins having from 2 to 12 carbon atoms, such as ethylene, propene, 1-butene, 1-hexene or 1-octene. Also suitable are copolymers and terpolymers containing further ethylenically unsaturated monomers, in particular dienes such as ethylidenenorbornene, cyclopentadiene or butadiene.

Preferred polyolefins are polyethylene and polypropylene. In principle, any commercial grade of these can be used. Thus, suitable examples are linear polyethylene of high, intermediate or low density, LDPE, ethylene copolymers with relatively small amounts (up to a maximum of about 40% by weight) of comonomers, including but not limited to n-butyl acrylate, methyl methacrylate, maleic anhydride, styrene, vinyl alcohol, acrylic acid, or glycidyl methacrylate, isotactic or atactic homopolypropylene, random copolymers of propene with ethene and/or 1-butene, and ethylene propylene block copolymers. Such polyolefins can also contain an impact-modifying component, such as EPM or EPDM rubber or SEBS. Furthermore, they can also contain functional monomers such as maleic anhydride, acrylic acid or vinyltrimethoxysilane in grafted form. Polyacetals, also known as polyoxymethylenes, are described in "Kunststoff-Handbuch" (Ed.: R. Vieweg, M. Reiher, H. Scheurlen), pp. 1-98, Vol. XI, Karl Hanser Verlag Munchen 1971.

Polyketones are alternating copolymers of carbon monoxide and olefins. They are described, for instance, in A. Wakker, G. Kormelink, P. Verbeke, J. C. M. Jordaan, Kunststoffe 85 (1995) 8, as well as in EP-A-0 121 965 and EP-A-0 181 014.

The thermoplastic polyesters have the following basic structure: ##STR1## where R is a divalent branched or unbranched aliphatic and/or cycloaliphatic radical having from 2 to 12, preferably from 2 to 6, carbon atoms in the carbon chain and R' is a divalent aromatic radical having from 6 to 20, preferably from 6 to 12, carbon atoms in the carbon skeleton.

Examples of suitable diols which can be used in the preparation of the polyesters are ethylene glycol, trimethylene glycol, tetramethylene glycol, hexamethylene glycol, neopentyl glycol, or cyclohexanedimethanol. Up to 25 mol % of the diol can be replaced by a diol having the following general formula ##STR2## where R" is a divalent radical having from 2 to 4 carbon atoms and x is from 2 to 50.

Preferred diols are ethylene glycol and tetramethylene glycol.

Suitable aromatic dicarboxylic acids for use in preparing the polyesters include terephthalic acid, isophthalic acid, 1,4-, 1,5-, 2,6- or 2,7-naphthalenedicarboxylic acid, diphenic acid, (diphenyl ether)-4,4'-dicarboxylic acid or their polyester-forming derivatives, such as their dimethyl esters.

Up to 20 mol % of these dicarboxylic acids can be replaced by aliphatic dicarboxylic acids such as succinic acid, maleic acid, fumaric acid, sebacic acid, or dodecanedioic acid.

The thermoplastic polyesters can be prepared by conventional processes (DE-A 24 07 155, 24 07 156; Ullmanns Encyclopadie der technischen Chemie, 4th edition, Volume 19, page 65 ff, Verlag Chemie GmbH, Weinheim 1980).

The polyesters used according to the present invention have a viscosity number (J value) in the range from 80 to 240 cm.sup.3 /g.

Preferred thermoplastic polyesters are polyethylene terephthalate, polybutylene terephthalate and polybutylene naphtalate.

If required, the polyesters can be impact modified.

Suitable fluoropolymers include ethylene-tetrafluoroethylene copolymers or terpolymers (ETFE; e.g. TEFZEL 200 from DuPont, HOSTAFLON ET 6235 from Hoechst or Neoflon EP 610/EP 620 from Daikin), tetrafluoroethylene-hexafluoropropene-vinylldene fluoride terpolymers (THV; e.g. HOSTAFLON TFB from Hoechst), ethylene-chlorotrifluoroethylene copolymers (ECTFE; e.g. HALAR from Ausimont) or polyvinylidene fluoride (PVDF). While these polymers can contain plasticizers, the use of plasticizer-free fluoropolymers is preferred. ETFE, THV and ECTFE are described in H. Domininghaus, Die Kunststoffe und ihre Eighenschaften, 4th edition, Chapter 2.1.7 (Fluor-Kunststoffe).

The preparation and structure of polyvinylidene fluoride are also known (see Hans R. Kricheldorf, Handbook of Polymer Synthesis, Part A, Marcel Dekker Inc. New York--Basle-Hong Kong, p. 191 ff.: Kunststoff-Handbuch, 1st edition, Volume XI, Carl Hanser Verlag Munich (1971), p. 403 ff.).

According to the present invention, polymers based on polyvinylidene fluoride containing up to 40% by weight of other monomers can also be present. Suitable examples of such additional monomers are: trifluoroethylene, ethylene, propene and hexafluoropropene.

The polyvinylidene fluoride used according to the present invention preferably has a melt flow index of <17 g/10 min, more preferably from 2 to 13 g/10 min (DIN 53 735), measured at 230.degree. and under a load of 5 kg.

Graphite fibrils are known. They are described, for example, in Plastics World, November 1993, page 10 ff. These are tiny fibers of crystalline graphite. In the material commercially available at present, their average diameter is in the order of 0.01 micron with an L/D ratio of the order of from 500:1 to 1000:1. Suitable graphite fibrils for use in the present invention also include those described in WO applications Nos. 8603455, 8707559, 8907163, 9007023 and 9014221 and in JP-A-03287821.

The content of graphite fibrils in the inner layer is generally from 1 to 30% by weight, preferably from 1.5 to 10% by weight and more preferably from 2 to 7% by weight.

The optionally present barrier layer for fuel components can comprise a molding composition based on thermoplastic polyester, polyvinylidene fluoride (PVDF), ETFE, THV, polyolefins, ethylene/vinyl alcohol copolymer (EVOH), polyketones or polyacetals. For suitable embodiments, reference may be made to the patent applications mentioned.

Preference is here given to using polybutylene terephthalate as thermoplastic polyester.

The multilayer compositions of the present invention are preferably produced by conventional coextrusion, preferably for the production of multilayer pipes. Such production of multilayer pipes by coextrusion is known.

The thickness of the inner layer is selected so that, on the one hand, an electrical potential formed can reliably be conducted away, but, on the other hand, as little material as possible is required for cost reasons. The inner layer can here be very thin, such as between 0.01 and 0.1 mm. However, for certain applications, thicknesses of from 0.2 to 0.3 mm can also be advantageous. In general, the ratio of the thickness of the inner layer to the thickness of the outer layer or to the sum of the thicknesses of the other layers is from 1:5 to 1:100.

The multilayer compositions of the present invention have a good sour-gas resistance and meet the demands of Ford specification WSL-M98D28-A towards a fuel according to Ford FLTM AZ 105-01 towards PN180 and also the specification GM213M towards PN50.

In addition, they effectively prevent the build-up of high potentials and meet GM specification GM213M (draft April 1993), point 4.19. Their surface resistance is preferably less than 10.sup.6 .OMEGA./sq. This also applies after bending a number of times and after storage in fuels.

The plastic pipes of the present invention are preferably used for the transport of petrochemical materials or in the motor vehicle sector for conveying brake, cooling and hydraulic fluids and also fuel, including, in particular, methanol-containing or ethanol-containing fuel. A further use of the multilayer compositions is in the production of hollow bodies such as tanks or filling ports, particularly for the motor vehicle sector. These hollow bodies are produced, for example, by a blow-molding process downstream of the coextrusion.

Furthermore, the plastic pipes of the present invention can be used as transport pipes for solids. Combustible, pulverulent or dust-like solids such as flour have to be conveyed in pipes having antistatic properties so as to avoid dust explosions.

Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified.

Examples denoted by letters are not according to the present invention. All percentages are by weight.

EXAMPLES

Materials Used: PA 1: Polyamide 12 (.eta..sub.rel :2.1; containing plasticizer, impact-modified; 60 mmol/kg of amino end groups; 10 mmol/kg of carboxyl end groups; VESTAMID.RTM. X 7297 from HULS AG). PA 2: Polyamide 12 (.eta..sub.rel :2.1; containing plasticizer, impact-modified; 15 mmol/kg of amino end groups; 40 mmol/kg of carboxyl end groups; VESTAMID.RTM. X 7293 from HULS AG). PES 1: VESTODUR.RTM. 1000 from HULS AG modified in accordance with DE-A-42 40 658 (homopolybutylene terephthalate). PVDF 1: Polyvinylidene fluoride (melt flow index 13 g/10 min; DYFLOR.RTM. LE from HULS AG). PVDF 2: PVDF 1 modified in accordance with DE-A- 43 26 130. GF 1: Graphite fibrils from Hyperion Catalysis International, grade BN. GF 2: Graphite fibrils from Hyperion Catalysis International, grade CC.

Conductivity black: KETJENBLACK EC 300 from Akzo Chemie.

The mixtures were prepared in a twin-screw compounder at a temperature of 260.degree. C.

The pipes were produced on a laboratory extrusion unit fitted with a five-layer die (in the production of the two-, three- and four-layer pipes, the channels not required remain closed).

Pipes having an external diameter of 8 mm and a total wall thickness of 1 mm were produced.

TABLE 1__________________________________________________________________________Properties of the pipes Cold impact test at -40.degree. C. in accordance Surface resistance according to with SAE J 2043 GM 213 M [.OMEGA./sq] 1000 h, 1000 h, storage storage PN 180, PN 50, for for Leakage 23.degree. C. 40.degree. C. 1000 h 1000 at the acc. to acc. after in PN in PN Quick Pressure as Ford to GM as bending 180, 50, Connector.sup.d)Example Layer structure required.sup.a) molded spec..sup.b) 213 M.sup.c) molded 5 times 23.degree. C. 40.degree. C. [g/day]__________________________________________________________________________1 0.4 mm PA 1 150 bar no no no 5 .multidot. 10.sup.3 5 .multidot. 10.sup.4 8 .multidot. 10.sup.3 2 .multidot. 10.sup.4 0.11 0.2 mm PES 1 fracture fracture fracture 0.3 mm PA 1 0.1 mm PA 2 with 5% GF 12 0.4 mm PA 1 135 bar no no no 10.sup.4 10.sup.5 5 .multidot. 10.sup.4 7 .multidot. 10.sup.4 0.09 0.2 mm PES 1 fracture fracture fracture 0.3 mm PA 1 0.1 mm PA 2 with 3% GF 23 0.4 mm PA 2 150 bar no no no 5 .multidot. 10.sup.3 3 .multidot. 10.sup.4 9 .multidot. 10.sup.3 2 .multidot. 10.sup.4 0.11 0.2 mm PVDF 2 fracture fracture fracture 0.3 mm PA 2 0.1 mm PA 2 with 5% GF 14 0.4 mm PA 2 135 bar no no no 2 .multidot. 10.sup.4 3 .multidot. 10.sup.5 5 .multidot. 10.sup.4 8 .multidot. 10.sup.4 0.10 0.2 mm PVDF 2 fracture fracture fracture 0.3 mm PA 2 0.1 mm PA 2 with 3% GF 25 0.9 mm PA 2 135 bar no no no 4 .multidot. 10.sup.3 3 .multidot. 10.sup.4 6 .multidot. 10.sup.4 10.sup.4 .sup.e) 0.1 mm PA 2 fracture fracture fracture with 5% GF 1A 0.4 mm PA 1 320 bar no not not 10.sup.8 10.sup.9 10.sup.9 10.sup.11 0.18 0.2 mm PES 1 fracture passed passed 0.3 mm PA 1 0.1 mm PA 2 with 20% con- ductivity blackB 0.4 mm PA 1 280 bar no not not 10.sup.10 10.sup.12 10.sup.11 10.sup.13 0.17 0.2 mm PES 1 fracture passed passed 0.3 mm PA 1 0.1 mm PA 2 with 15% con- ductivity blackC 0.4 mm PA 2 320 bar no not not 10.sup.8 --.sup.e) --.sup.e) --.sup.e) --.sup.e) 0.2 mm PVDF 2 fracture passed passed 0.3 mm PA 2 0.1 mm PA 2 with 20% con- ductivity blackD 0.9 mm PA 2 260 bar no no not 10.sup.13 --.sup.e) --.sup.e) --.sup.e) --.sup.e) 0.1 mm PA 2 fracture fracture passed with 10% con- ductivity blackE 0.9 mm PA 2 315 bar not --.sup.e) --.sup.e) 10.sup.8 --.sup.e) --.sup.e) --.sup.e) --.sup.e) 0.1 mm PA 2 passed with 20% con- ductivity black6 0.8 mm PA 2 210 bar no no no 10.sup.5 5 .multidot. 10.sup.5 4 .multidot. 10.sup.5 4 .multidot. 10.sup.5 0.13 0.1 mm PVDF 2 fracture fracture fracture 0.1 mm PVDF 1 with 4% GF 1F 0.8 mm PA 2 280 bar not --.sup.e) --.sup.e) 10.sup.6 .sup.e) .sup.e) .sup.e) 0.22 0.1 mm PVDF 2 passed 0.1 mm PVDF 1 with 15% con- ductivity black__________________________________________________________________________ a) for extrusion of the inner layer, 15 m/min b) conditioning of the pipes using sour-gas-containing fuel (PN 180 at 23.degree. C. flowing through for 1000 hours in accordance with Ford WSL-M98D28-A, point 3.4.10) c) conditioning of the pipes with Sour-gas-containing fuel (PN 50 at 40.degree. C. flowing through for 1000 hours in accordance with GM213M, point 4.10) d) test using FAM B, 60.degree. C., 4 bar gauge pressure in a permeation tester in accordance with the Opel specification GME 08 100, point 7.8 e) not tested.

This application is based on German patent application No. 195 07 025.9 filed Mar. 1, 1995, incorporated herein by reference. Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

Claims

1. A multilayer plastic composition comprising:

(I) an outer layer comprising a thermoplastic molding composition and

(II) an inner layer consisting essentially of an electrically conductive thermoplastic molding composition made conductive by adding graphite fibrils, wherein the graphite fibrils have an average diameter on the order of 0.01 .mu.m and a length/diameter ratio on the order of from 500:1 to 1000:1.

2. The multilayer plastic composition according to claim 1, wherein the graphite fibrils are present in the inner layer (II) in an amount of from 1 to 30% by weight based on the amount of the inner layer (II).

3. The multilayer plastic composition according to claim 1, wherein the graphite fibrils are present in the inner layer (II) in an amount of from 1.5 to 10% by weight based on the amount of the inner layer (II).

4. The multilayer plastic composition according to claim 1, wherein the graphite fibrils are present in the inner layer (II) in an amount of from 2 to 7% by weight based on the amount of the inner layer (II).

5. The multilayer plastic composition according to claim 1, wherein the outer layer (I) comprises a polyamide molding composition, a polyolefin molding composition, a polyester molding composition or a polyester elastomer molding composition.

6. The multilayer plastic composition according to claim 1, wherein the inner layer (II) comprises a polyamide molding composition, a polyolefin molding composition, a polyester molding composition or a fluoropolymer molding composition.

7. The multilayer plastic composition according to claim 6, wherein the inner layer (II) is a fluoropolymer molding composition comprising a fluoropolymer selected from the group consisting of an ethylene-tetrafluoro-ethylene copolymer or terpolymer (ETFE), a tetrafluoroethylene-hexafluoropropene-vinylidene fluoride terpolymer (THV), an ethylene-chlorotrifluoroethylene copolymer (ECTFE) and a polyvinylidene fluoride (PVDF).

8. The multilayer plastic composition according to claim 7, wherein the fluoropolymer is a PVDF homopolymer or a copolymer of vinylidene fluoride containing up to 40% by weight of other monomers.

9. The multilayer plastic composition as claimed in claim 1, further comprising a barrier layer for fuel components as an intermediate layer between said inner layer and said outer layer.

10. The multilayer plastic composition according to claim 9, wherein the barrier layer comprises a polyester molding composition, a PVDF molding composition, an EVOH molding composition, a polyketone molding composition or a polyacetal molding composition.

11. The multilayer plastic composition according to claim 10, wherein the barrier layer is polybutylene terephthalate.

12. The multilayer plastic composition according to claim 1, wherein said inner layer and said outer layer each have a thickness, and wherein one or more additional layers each having a thickness are optionally present, wherein a ratio of the thickness of the inner layer to the thickness of the outer layer or to a sum of thicknesses of all other layers is from 1:5 to 1:100.

13. A plastic pipe for the transport of materials prepared from the multilayer plastic composition of claim 1.

14. A hollow body prepared from the multilayer plastic composition of claim 1.

15. A filling port for a tank used in a motor vehicle prepared from the multilayer plastic composition of claim 1.

16. The multilayer plastic composition according to claim 1, wherein the outer layer (I) comprises a polyacetal molding composition or a polyketone molding composition.

17. The multilayer plastic composition according to claim 1, wherein the inner layer (II) comprises a polyacetal molding composition or a polyketone molding composition.

18. The multilayer plastic composition according to claim 1, wherein the outer layer (I) comprises a polyamide molding composition and the inner layer (II) comprises a polyamide molding composition.