WO2006045636A1

WO2006045636A1 - Tube based on a vulcanized elastomer and a modified fluoropolymer

Info

Publication number: WO2006045636A1
Application number: PCT/EP2005/011792
Authority: WO
Inventors: Anthony Bonnet
Original assignee: Arkema France
Priority date: 2004-10-19
Filing date: 2005-10-18
Publication date: 2006-05-04

Abstract

The present invention relates to a tube having, in its radial direction from the inside outwards: 1) a layer referred to as the inner layer intended to come into contact with a flowing fluid, the said inner layer comprising (i) a modified fluoropolymer (B1) modified by the radiation grafting of a graftable compound and optionally blended with a fluoropolymer (B2), and (ii) optionally an electrically conducting product; 2) optionally, a tie layer; and 3) a vulcanized elastomer layer. According to one particular embodiment of the invention, the inner layer comprises, apart from the modified fluoropolymer (B1) optionally blended with a fluoropolymer (B2) and optionally an electrically conducting product, a polyethylene carrying epoxy functional groups and an impact modifier chosen from elastomers and very low-density polyethylenes.

Description

TUBE BASED ON A VULCANIZED ELASTOMER AND A MODIFIED

FLUOROPOLYMER

Field of the invention The present invention relates to a tube based on a vulcanized elastomer and on a fluoropolymer that has been modified by radiation grafting, more particularly a tube having an inner layer comprising a modified fluoropolymer (B1) modified by radiation grafting and an outer layer made of a vulcanized elastomer. It also relates to the use of said tube in various applications.

The tube is useful for manufacturing fuel circuit elements, for example for fuel supply pipes, bringing petrol from the tank to the injection system, for the tubing between the petrol tank and the filling orifice and for the return tubings for petrol vapour. It may also be used for transporting a coolant like f luorohydrocarbons or chlorofluorohydrocarbons and for transferring fluids in a fuel cell. It may also be used for transferring the hydraulic liquid used in brakes.

The grafting of a graftable compound onto a polymer chain is a well-known operation that has already been employed widely to modify the physico- chemical properties of polymers. Thus, maleic anhydride is grafted onto a polyolefin (polyethylene or polypropylene) in the melt state in an extruder. To do this, a radical initiator, the decomposition temperature of which must be carefully chosen, is added to the molten compound. The grafting by means of a radical initiator onto a fluoropolymer that has hydrogen atoms in its structure is much less easy. This therefore explains the fact why there has been little description of maleic anhydride being grafted onto PVDF. In addition, the contents of maleic anhydride grafted onto PVDF are generally low and the grafting efficiency is therefore poor. Thus, a large portion of maleic anhydride that has not grafted therefore has to be removed in another step. Furthermore, the extrusion grafting technique uses high temperatures and high shear liable to degrade the fluoropolymer chains and to release hydrofluoric acid which, if it is not carefully removed, may be problematic. In the rest of the description, the expression "modified fluoropolymer", also denoted by "B1", is used to denote a fluoropolymer modified by radiation grafting.

It has now been found that radiation grafting makes it possible to achieve more effective grating than grafting using a radical initiator. This results in very good adhesion between the layer comprising the modified fluoropolymer and the vulcanized elastomer layer.

Prior art and the technical problem International application WO 99/48678 discloses a method of making a fluoropolymer adhere to an elastomer by incorporating antimony trioxide and adhesion promoters, such as organophosphonium salts, to the elastomer.

Patent EP 683 725 has disclosed tubes consisting, in succession, of an inner layer made of PVDF (polyvinylidene fluoride), a coextruded tie layer and an outer layer made of a vulcanized elastomer. They have the advantage of exhibiting good resistance to aggressive chemical fluids and of being a ba rrier to many fluids, in particular petrol and the fluids used in air-conditioning circuits.

However, they may be brittle at low temperature. It is known to improve the impact strength of PVDF, but this is to the detriment of its chemical resista nce and its barrier properties.

In the manufacture of multilayer tubes, it is necessary for there to be good interfacial adhesion between the layers so as to have good flexibility and to reduce crunching of the tubes (in French, the word "croquage" applies).

It has now been found that a tube having an outer layer made of a vulcani-zed elastomer and an inner layer comprising a modified fluoropolymer (F31), optionally blended with a fluoropolymer (B2), possesses a good compromise of properties, namely good chemical resistance of the inner layer against which the fluid flows and good adhesion between the two layers. In addition, it is sometimes necessary for the inner layer to be conducting. The friction of a solvent on the inner layer of a tube may in fact generate electrostatic charges, the accumu lation of which may result in an electrical discharge (spark) capable of igniting the solvent with catastrophic consequences (an explosion). Therefore these parts sometimes have to be conducting.

It is known to lower the surface resistivity of polymeric resins or materials by incorporating conductive and/or semiconductive materials into them, such as carbon black, steel fibres, carbon fibres, and particles (fibres, platelets or spheres) metallized with gold, silver or nickel. Among these materials, carbon black is more particularly used, for economic and processability reasons. Apart from its particular electrically conductive properties, carbon black behaves as a filler such as, for example, talc, chalk or kaolin. Thus, those skilled in the art know that when the filler content increases, the viscosity of the polymer/filler blend increases. Likewise, when the filler content increases, the flexural modulus of the filled polymer increases and its impact strength decreases. These known and predictable phenomena are explained in "Handbook of Fillers and Reinforcements for Plastics", edited by H. S. Katz and J.V. Milewski - Van Nostrand Reinhold Company - ISBN 0-442-25372-9, see in particular Chapter 2, Section Il for fillers in general and Chapter 16, Section Vl for carbon black in particular. As regards the electrical properties of carbon black, the technical report "Ketjenblack EC - BLACK 94/01" by Akzo Nobel indicates that the resistivity of the formulation d rops very suddenly when a critical carbon black content, called the percolation threshold, is reached. When the carbon black content increases further, the resistivity rapidly decreases until reaching a stable level (plateau region). It is therefore preferred, for a given resin, to operate in the plateau region in which a metering error will have only a slight effect on the resistivity of the compound.

Under multiaxial impact, PVDF behaves in a brittle manner. The addition of an agent making it electrically conducting, such as carbon black, will make PVDF even more brittle. The various ways of improving the impact strength properties are usually based on the incorporation of soft elastomeric phases that may have morphologies of the "core-sheil" type in a PVDF matrix. The major drawback of such a combination is a large reduction in the chemical resistance, especially at high temperature.

It has now been found that it is possible to increase the impact strength of a tube having an inner layer made of a modified fluoropolymer and an outer layer made of a vulca nized elastomer using, for the inner layer, a particular composition comprising, apart from the modified fluoropolymer (B1) optionally blended with a fluoropolymer (B2), a polyethylene carrying epoxy functional groups and an impact modifier chosen from elastomers and very low-density polyethylenes.

Figures

Figures 1/2 and 2/2 are views of tubes according to the invention. In F igure 1/2, a tube 1 comprises an inner layer 2 and an outer layer 3. In Figure 2/2 , a tube 4 comprises an inner layer 5, a tie layer 6 and an outer layer 7.

Description of the invention

The present invention relates to a tube having, in its radial direction from the inside outwards:

1) a layer referred to as the inner layer intended to come into co ntact with a flowing fluid, the said inner layer comprising (i) a modified fluoropolymer (B1) modified by the radiation grafting of a graftable compound and optionally blended with a fluoropolymer (B2), and (ii) optionally an electrically conducting product;

2) optionally, a tie layer; and

3) a vulcanized elastomer layer.

The optional tie layer is therefore placed between the inner layer and the layer of the vulcanized elastomer. According to one particular embodiment of the invention, the inner layer includes, in addition to the modified fiuoropolymer (BI) optionally blended with a fluoropolymer (B2) and optionally an electrically conductive product, a polyethylene carrying epoxy functional groups and an impact modifier chosen from elastomers and very low-density polyethylenes.

The tubes of the invention have many advantages:

• they exhibit good cold impact strength (at - 40⁰C); • they exhibit excellent flexibility;

• they may be made antistatic;

• they have very good resistance to chemicals and therefore can be used to transport aggressive fluids;

• they act as a barrier to very many fluids, such as for example petrol for motor vehicles and air-conditioning fluids, even for a small thickness of the inner layer;

• they are clean, that is to say the inner layer essentially contains no substance that can migrate, such as oligomers or plasticizers, and there is therefore no risk of the fluid flowing in the tube entraining such substances, which could block devices placed in the circuit for this fluid; and

• the inner and outer layers do not delaminate when subjected to a mechanical and/or thermal stress.

The process used to obtain the modified fluoropolymer (B1) consists in: a) melt-blending a fluoropolymer and at least one graftable compound; b) the blend obtained is made in the form of films, sheets, granules or powder; c) irradiating this blend in the solid state by irradiation (which can be a γ or β radiation) with a dose of between 1 and 15 Mrad, optionally after having removed the residual oxygen; and d) removing the graftable compound that has not grafted and the residues liberated by the grafting, especially HF. Step d) can sometimes be optional if the amount of graftable compound that has not been grafted is low or not detrimental to the adhesion of other properties of the modified fluoropolymer.

The inner and outer layers may be coextruded or extruded sequentially. Using coextrusion, each layer is introduced in the melt state by an extruder in a coextrusion head that produces concentric streams forming the tube. Th is technique is known per SΘ. The tube is then passed through an oven or heating tunnel in order to vulcanize (crosslink) the elastomer. It is recommended during coextrusion to use a coextrusion head in which the elastomer stream remains at a low enough temperature (generally around 80 to 120°C) so as not to cause crosslinking before formation of the tube and especially so as not to block the extruder. It is also possible to manufacture by coextrusion a tube that does not comprise the elastomer layer, and then subsequently to pass this tube through a "sheathing" or "crosshead" device in order to cover it with the elastomer layer. All that is then required, as above, is to pass the tube through an oven or heating tunnel in order to vulcanize the elastomer.

As regards the fluorinated polymer, this denotes any fluoroplastic or fluoroelastomer containing units of general formula:

in which X and X' may be, independently of each other, a hydrogen atom, a halogen, especially fluorine or chlorine, or a perhaloalkyl, especially a perfluoroalkyl.

As examples of monomers that can be used for the preparation of th e fluorinated polymer, mention may be made of vinyl fluoride; vinylidene fluorid e (VF2 of formula CH₂=C F₂); trifluoroethylene (VF3); chlorotrifluoroethylene (CTFE); 1 ,2-difluoroethylene; tetrafluoroethylene (TFE); hexafluoropropylene (HFP); perfluoro(alkyl vinyl) ethers, such as perfluoro(methyl vinyl) ether (PMVE), perfluoro(ethyl vinyl) ether (PEVE) and perfluoro(propyl vinyl) ether (PPVE); perfluoro(1 ,3-dioxole); perfluoro(2,2-dimethyl-1 ,3-dioxole) (PDD); the product of formula CF₂=CFOCF₂CF(CF₃)OCF₂CF₂X in which X is SO₂F, CO₂H, CH₂OH, CH₂OCN or CH₂OPO₃H; the product of formula CF₂=CFOCF₂CF₂SO₂F; the product of formula F(CF₂)_nCH₂OCF=CF₂ in which n is 1 , 2, 3, 4 or 5; the product of formula RiCH₂OCF=CF₂ in which Ri is hydrogen or F(CF₂)_Z and z is 1 , 2, 3 or 4; the product of formula R₃OCF=CH₂ in which R₃ is F(CF₂)_Z- and z is 1 , 2, 3 or 4; perfluorobutylethylene (PFBE); 3,3,3- trifluoropropene and 2-trif luoromethyl-3,3,3-trifluoro-1 -propene.

The fluoropolymer may be a homopolymer or a copolymer; it may also include non-fluorinated monomers such as ethylene. For instance, the fluoropolymer may be a VF2-HFP-TFE terpolymer.

Advantageously, the fluoropolymer is chosen from:

PVDF homo- or copolymers preferably containing, by weight, at least 50% VF2, the copolymer being chosen from chlorotrifluoroethylene (CTFE), hexafluoropropylene (HFP)₁ trifluoroethylene (VF3) and tetrafluoroethylene (TFE); homopolymers and copolymers of trifluoroethylene (VF3); and copolymers, and especially terpolymers, combining the residues of chlorotrifluoroethylene (CTFE), tetrafluoroethylene (TFE), hexafluoropropylene (HFP) and/or ethylene units and optionally VF2 and/or VF3 units.

Preferably, because of its good chemical resistance and its being extruded easily, the fluoropolymer is a PVDF homopolymer or a VF2/HFP copolymer containing at least 50 wt% VF2, advantageously at least 75 wt% VF2 and preferably at least 85 wt% VF2. Preferably, the fluoropolymer is a PVDF (polyvinylidene fluoride) homopolymer.

Advantageously, the PVDF has a viscosity ranging from 100 Pa. s to 2O00 Pa. s, the viscosity being measured at 230⁰C at a shear rate of 10O s^'1 using a capillary rheometer. Such PVDFs are in fact well suited to extrusio n and to injection moulding. Preferably, the PVDF has a viscosity ranging from 300 Pa. s to 1200 Pa.s, the viscosity being measured at 230⁰C at a shear rate of 100 s^"1 using a capillary rheometer. The MFI (melt flow index) of the fluoropolymer is advantageously between 5 and 30 g/10 min (at 23O°C under a load of 5 kg) in the case of a PVDF homopolymer and between 5 and 30 g/10 min (at 230⁰C under a load of 5 kg) for a VF2/HFP copolymer.

The PVDFs sold under the brand names KYNAR^® 710, KYNAR^® 720, KYNARFLEX^® 2850 and KYNAR SUPER FLEX^® 2500 are perfectly suited for this formulation.

With regard to the modified fluoropolymer (B1), this is obtained by the radiation grafting of a graftable compound onto a fluoropolymer. The fluoropolymer is modified by grafting the graftable compound by irradiation (using γ or β radiation) with an irradiation dose between 10 and 200 kGray, preferably between 10 and 150 kGray. The said fluoropolymer is preblended with the graftable compound by any melt blending techniques known in the prior art, preferably using an extruder.

The blend of the fluoropolymer and the graftable compound is then irradiated (γ or β radiation) allowing the reactive functional group to be grafted onto the fluoropolymer. This results in a graftable compound being grafted to an amount of 0.1 to 5% by weight (i.e. the grafted graftable compound corresponds to 0.1 to 5 parts per 99.9 to 95 parts of fluoropolymer), advantageously 0.5 to 5% and preferably 1 to 5%. The content of grafted graftable compound depends on the initial content of the graftable compound in the fluoropolymer/graftable compound blend to be irradiated. It also depends on the grafting efficiency, and therefore on the duration and the energy of the irradiation. Irradiation using a cobalt bomb is preferred.

Any graftable compound that has not been grafted and the residues liberated by the grafting, especially HF, may then be removed. This operation may be carried out using techniques known to those skilled in the art. Vacuum degassing may be applied, optionally heating at the same time. It is also possible to dissolve the modified fluoropolymer in a suitable solvent, such as for example N-methylpyrrolidone, and then to precipitate the polymer in a non- solvent, for example in water or in an alcohol.

One of the advantages of this radiation grafting process is that it is possible to obtain higher contents of grafted graftable compound than with conventional grafting processes using a radical initiator. Thus, typically, with the radiation grafting process it is possible to obtain contents of greater than 1 % (1 part of graftable compound per 99 parts of fluoropolymer), or even greater than 1.5%, whereas with a conventional grafting process carried out in an extruder the content is around 0.1 to 0.4% or may sometimes be unfeasible.

Moreover, the radiation grafting takes place "cold", typically at temperatures below 100°C, even below 70⁰C, sometimes at ambiant temperature, so that the fluoropolymer/graftable compound blend is not in the melt state, as in the case of a conventional grafting process carried out in an extruder. One essential difference is therefore that, in the case of a semicrystalline fluoropolymer (as is the case with PVDF for example), the grafting takes place in the amorphous phase and not in the crystalline phase, whereas homogeneous grafting is produced in the case of grafting carried out in an extruder. The graftable compound is therefore not distributed among the fluoropolymer chains in the same way in the case of radiation grafting and in the case of grafting carried out in an extruder. The modified fluoropolymer therefore has a different distribution of the graftable compound among the fluoropolymer chains compared with a product obtained by g rafting carried out in an extruder.

During this grafting step, it is preferable to prevent oxygen from being present. It is therefore possible to remove the oxygen by flushing the fluoropolymer/graftable compound blend with nitrogen or argon. With regard to the graftable compound, this possesses at least one double bond C=C, and at least one polar functional group that may be one of the following functional groups: a carboxylic acid; - a carboxylic acid salt; a carboxylic acid anhydride; an epoxide; a carboxylic acid ester; a silyl; - an alkoxysilane ; a carboxylic amide; a hydroxy I; an isocyanate.

It is also possible to envisage using mixtures of several graftable compounds.

Mention may be made by way of examples of graftable compounds of methacrylic acid, acrylic acid, undecylenic acid, zinc, calcium or sodium undecylenate, maleic anhydride, itaconic anhydride, crotonic anhydride, glycidyl acrylate, glycidyl methacrylate, allyl glycidyl ether and vinylsilanes, such as vinyltrimethoxysilane, vinyltriethoxysilane, vinyltriacetoxysilane a nd γ-meth- acryloxypropyltrimethoxysilane.

To obtain good adhesion between the inner and outer layers of the tube, it will be preferable to choose maleic anhydride or zinc, calcium or sodium undecylenates. These graftable compounds also have the advantage of being solids, which makes it easier for them to be introduced into an extruder. Maleic anhydride is most particularly preferred as it makes it possible to achieve excellent adhesion between the inner and outer layers of the tube.

Because of the presence of a C=C double bond in the graftable compound, polymerization of the graftable compound, to give polymer chains either grafted onto the fluoropolymer, or free chains, that is to say those not attached to the fluoropolymer, is not excluded. The term "polymer chain" is understood to mean a chain-linking of more than ten graftable compound units. Within the context of the invention, to promote the ahdesion, it is preferable to limit the presence of grafted or free polymer chains, and therefore to seek to obtain chains with fewer than ten graftable compound units. Chains limited to fewer than five graftable compound units will be preferred, and those having fewer than two graftable compound units will be even more preferred. Grafting only one compound unit is most preferred.

Likewise, it is not excluded for there to be more than one C=C double bond in the graftable compound. Thus, for example, graftable com pounds such as allyl methacrylate, trimethylolpropane trimethacrylate or ethylene glycol dimethacrylate may be used. However, the presence of more than one double bond in the graftable compound may result in crosslinking of the fluoropolymer, and therefore in a modification in the rheological properties, or even in the presence of gels, which is not desirable. It may then be difficult to obtain a high grafting efficiency, while still limiting crosslinking. Graftable compounds containing only a single C=C double bond are also preferred. The preferred graftable compounds are therefore those possessing a single C=C double bond and at least one polar functional group.

From this standpoint, maleic anhydride and also zinc, calcium or sodium undecylenates constitute good graftable compounds as they have little tendency to polymerize or even to give rise to crosslinking . Maleic anhydride is most particu larly preferred.

The modified fluoropolymer retains the very good chemical and oxidation resistance along with the thermomechanical behaviour of the fluoropolymer before grafting.

With regard to the electrically conducting products, these are all conductors of electricity. For example, metals and carbon-based products may be mentioned. Examples of carbon-based products that can be mentioned are graphite, carbon black, carbon nanotubes and carbon fibres. It would not be outside the scope of the invention to use several electrically conducting elements. Carbon-based products that can be used are described in Handbook of Fillers, 2nd edition, published by Chem Tec Publishing , 1999, page 62 § 2.1.22, page 92 § 2.1.33 and page 184 § 2.2.2.

Advantageously, the electrically conducting product is chosen from carbon blacks. Carbon blacks may be semiconducting blacks or conducting blacks - these carbon blacks have a low BET surface area. Among carbon blacks that can be used, those from MMM Carbon are particularly satisfactory. Blacks having a nitrogen adsorption surface area of less than 500 m²/g will particularly be used. Advantageously, these carbon blacks have a nitrogen adsorption surface of less than 100 m²/g. Among these various types, ENSACO^® 250 is particularly suitable for the application, as it is easily dispersed and ensures good electrical conduction.

With regard to the tie layer, this refers to any product allowing adhesion between the inner layer and the vulcanized elastomer layer. As examples, mention may be made of blends of a fluoropolymer with PMMA and optionally with an acrylic elastomer, of the core/shell type; PMMA may include copolymerized acrylic or methacrylic acid. These ties are described in Patent US 5242976. Mention may also be made of blends based on poly(meth)acrylates modified by imidization and optional ly containing a fluoropolymer; they are described in Patents US 5939492, US 6040025, US 5795939 and EP 726926.

This layer is optional if the inner layer itself exhibits very good adhesion to the outer layer.

As regards the layer of vulcanized elastomer, the vulcaniz:able synthetic or natural elastomers which are suitable for carrying out the present invention are well known to those skilled in the art, in the definition of the present invention the term "elastomer" meaning that it may consist of blends of several elastomers.

These elastomers or blends of elastomers have a compression set (CS) at 100⁰C of less than 50%, generally between 5% and 40% and preferably less than 30°/ό. Among these elastomers, mention may be made of natural rubber, polyisoprene with a high content of cis double bonds, a polymerized emulsion based on styrene/butadiene copolymer, a polybutadiene with a high content of cis double bonds obtained by nickel, cobalt, titanium or neodymium catalysis, a halogenated ethylene/propylene/diene terpolymer, a halogenated butyl rubber, a styrene/butadiene block copolymer, a styrene/isopropene block copolymer, halogenated products of the above polymers, an acrylonitrile/butadiene copolymer, an acrylic elastomer, a fluoroelastomer, chloroprene, nitrile rubbers with incorporation of chlorinated paraffins, especially NBR rubbers, chlorinated polymers such as chlorosulphonated polyethylene (CSM), chlorinated polyethylene (CPE), nitrile-PVC rubbers (especially NBR-PVC). Also possible are epichlorohydrin rubber, an epichlorohydrin/ethylene oxide copolymer (in equimolar proportions or otherwise) or, preferably, an epichlorohydrin/ethylene oxide/allyl glycidyl ether terpolymer.

If the tube of the invention does not include a tie layer, it is recommended that the elastomer be chosen from functionalized elastomers, elastomers with acrylate units, halogenated elastomers and epichlorohydrin rubbers. As regards functionalized elastomers, the functional group is advantageously a carboxylic acid or carboxylic acid anhydride functional group. When the elastomers mentioned above comprise no carboxylic acid radicals or anhydride radicals deriving from the said acids (which is the case for most of them), the said radicals will be provided by grafting the abovementioned elastomers in a known manner or by blends of elastomers, for example with elastomers containing acrylic units such as acrylic acid. The abovementioned vulcanizable elastomers preferably have a weight content of carboxylic acid or dicarboxylic acid anhydride radicals of between 0.3 and 10% relative to the said elastomers.

Similarly, it is possible to blend elastomers which have no acrylate units or functions, which are not halogenated and which are not epichlorohydrin rubbers , with at least one elastomer chosen from fu nctionalized elastomers, elastomers containing acrylate units, halogenated elastomers and epichlorohydrin rubbers.

Among the elastomers mentioned above which may be selected are those included in the following group: carboxylated nitrile elastomers, acrylic elastomers, carboxylated polybutadienes, ethylene/propylene/diene terpolymers, these being grafted, or blends of these polymers with the same elastomers but which are not grafted, such as nitrile rubbers, polybutadienes and ethylene/propylene/diene terpolymers, alone or as a blend.

The vulcanizing systems that are suitable for the present invention are well known to those skilled in the art and, consequently, the invention is not limited to one particular type of system.

When the elastomer is based on unsaturated monomer (butadiene, isoprene, vinylidene norbornene, etc.), four types of vulcanizing system may be mentioned:

- sulphur systems consisting of sulphur combined with the usual accelerators such as metal salts of dithiocarbamates (zinc dimethyl- dithiocarbamate, tellurium dimethyldithiocarbamate etc.), sulphonamides, etc.; the systems may also contain zinc oxide combined with stearic acid;

- sulphur donor systems in which most of the sulphur used for the bridges is derived from sulphur-containing molecules such as the organosulphur compounds mentioned above; - p henolic resin systems consisting of dysfunctiona I phenol-formaldehyde resins which may be halogenated, combined with accelerators such as stannous chloride or zinc oxide;

- peroxide systems: any free-radical donor may be used (dicumyl peroxides, etc.) in combination with zinc oxide and stearic acid.

When the elastomer is acrylic (polybutyl acrylate with acid or epoxy functional groups or any other reactive functional group allowing crosslinking), the usual diamine-based crosslinking agents are used (orthotoluidyl guanidine, diphenylg uanidine, etc.) or blocked diamines (hexamethylenediamine carbamate, etc.) are used.

The elastomeric compositions may be modified for certain particular properties (for example improvement in the mechanical properties) by adding fillers such as carbon black, silica, kaolin, alumina, clay, talc, chalk, etc. These fillers may be surface-treated with silanes, polyethylene glycols or any other coupling molecule. In general, the content of fillers in parts by weight is between 5 and 100 per 1 00 parts of elastomers.

In addition, the compositions may be flexibilized with plasticizers such as mineral oils derived from petroleum, phthalic acid esters or sebacic acid esters, liquid polymeric plasticizers such as low-mass polybutadiene optionally carboxylated, and other plasticizers that are well known to those skilled in the art.

The vulcanization agent combinations used are such that they must allow the elastomer to be completely or almost completely crosslinked at a rate resulting in good properties as regards resistance to separation of the elastomer layer and the inner layer or the tie layer. With regard to the inner layer, this comprises (i) a modified fluoropolymer (B1) optionally blended with a fluoropolymer (B2) and (ii) optionally an electrically conducting product.

This inner layer comprises from 100 to 5 parts, preferably from 100 to 50 parts, of modified fluoropolymer (B1) per 0 to 95 parts, preferably 0 to 50 parts, of fluoropolymer (B2).

The fluoropolymer (B2) is any fluoropolymer described above. It is possible to obtain (B1 ) from a fluoropolymer that is not necessarily of the same type as the fluoropolymer (B2). For example, (B1) may be a modified PVDF homopolymer or copolymer and (B2) may be a polytetrafluoroethylene. It is also possible to envisage (B1) being a modified fluoropolymer obtained from a fluoroelastomer and (B2) being a thermoplastic fluoropolymer. Conversely, (B1) may be a modified fluoropolymer obtained from a fluorinated thermoplastic and (B2) an elastomeric fluoropolymer.

The proportion of electrically conducting product is between 0 and 25% by weight relative to the modified fluoropolymer optionally blended with a fluoropolymer.

According to one particular embodiment of the invention, the inner layer comprises, apart from the modified fluoropolymer (B 1) optionally blended with a fluoropolymer (B2), and optionally an electrically conducting product, a polyethylene carrying epoxy functional groups and an impact modifier chosen from elastomers and very low-density polyethylenes.

More precisely, the inner layer therefore comprises: 5 to 30% by weight of a blend (A) comprising: a polyethylene carrying epoxy functional groups, - an impact modifier chosen from elastomers and very low-density polyethylenes, the said impact modifier being completely or partially functionalized; 95 to 70% by weight of a blend (B) comprising:

- a modified fluoropolymer (B1),

- a fluoropolymer (B2).

It may also include an electrically conducting product.

With regard to the blend (A) and firstly the polyethylene carrying epoxy functional groups, this may be a polyethylene onto which epoxy functional groups have been grafted, or an ethylene/unsaturated epoxide copolymer.

As examples of ethylene/unsaturated epoxide copolymers, mention may be made of copolymers of ethylene with an alkyl (meth)acrylate and an unsaturated epoxide, or copolymers of ethylene with a vinyl ester of a saturated carboxylic acid and an unsaturated epoxide. The amount of epoxide may be up to 1 5% by weight of the copolymer and the amount of ethylene at least 50% by weight. Advantageously, the proportion of epoxide is between 2 and 12% by weight. Advantageously, the proportion of alkyl (meth)acrylate is between 0 and 40% by weight and preferably between 5 and 35% by weight.

Advantageously, this is an ethylene/alkyl (meth)acrylate/unsatu rated epoxide copolymer.

Preferably, the alkyl (meth)acrylate is such that the alkyl has 1 to 10 carbon atoms.

The MFI (melt flow index) may for example be between 0.1 and 50 (g/10 min at 19O°C/2.16 kg).

Examples of alkyl acrylates and methacrylates that can be used are especially methyl methacrylate, methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate and 2-ethylhexyl acrylate. Examples of unsaturated epoxides that can be used are, in particular: - aliphatic glycidyl esters and ethers, such as allyl glycidyl ether, vinyl glycidyl ether, glycidyl maleate, glycidyl itaconate, glycidyl acrylate and glycidyl methacrylate; and

- alicyclic glycidyl esters and ethers, such as 2-cyclohexen-l-yl glycidyl ether, diglycidyl cyclohexene-Aδ-carboxylate, glycidyl cyclohexene-4- carboxylate, glycidyl 2-methyl-5-norbornene-2- carboxylate and diglycidyl endo- cis-bicyclo[2.2.1]hept-5-ene-2,3- dicarboxylate.

With regard to the blend (A) and now the impact modifier, and firstly the elastomers, mention may be made of block polymers of the SBS, SIS and

SEBS type and ethylene-propylene (EPR) and ethylene-propylene-diene m onomer (EPDM) elastomers. As regards very low-density polyethylenes, these are for example polyethylenes obtained by metallocene catalysis, having a density for example between 0.860 and 0.900. In the examples, mention may be made of the polyethylenes sold by Dow Chemical under the brand name

Affinity^®, especially Affinity EG 8100 G, Affinity EG 8150 G and Affinity KC

8852 G.

Acrylic elastomers are unsuitable as they result in permeability to petrol. By acrylic elastomers it is meant elastomers based on at least one monomer chosen from acrylonitrile, alkyl (meth)acrylates and core/shell copolymers. As regards core/shell copolymers, these are in the form of fine particles having an elastomer core and at least one thermoplastic shell (usually of PMMA), the size of the particles generally being less than 1 μm and advantageously between 50 and 300 nm.

It is advantageous to use an ethylene-propylene (EPR) or ethylene/propylene/diene monomer (EPDM) elastomer. The functionalization may be provided by grafting or copolymerizing with an unsaturated carboxylic acid. It would not be outside the scope of the invention to use a functional derivative of this acid. Examples of unsaturated carboxylic acid are those having from 2 to 20 carbon atoms, such as acrylic, methacrylic, maleic, fumaric and itaconic acids. The functional derivatives of these acids comprise, for example, anhydrides, ester derivatives, amide derivatives, imide derivatives and metal salts (such as alkali metal salts) of unsaturated carboxylic acids. Among these functional derivatives, it is preferred to use acid anhydrides, especially maleic anhydride.

Products from the EXXELOR® VA, especially EXXELOR® VA 1803, are preferred.

Unsaturated dicarboxylic acids having 4 to 10 carbon atoms and their functional derivatives, particularly their anhydrides, are particularly preferred grafting monomers. These grafting monomers comprise, for example, maleic, fumaric, itaconic, citraconic, allylsuccinic, cyclohex-4-ene-1 ,2-dicarboxylic, 4- methylcyclohex-4-ene-1 ,2-dicarboxylic, bicyclo[2.2.1]hept-5-ene-2,3- dicarboxylic and x-methylbicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acids and maleic, itaconic, citraconic, allylsuccinic, cyclohex-4-ene-1 ,2-dicarboxylic, 4- methylenecyclohex-4-ene-1 ,2-dicarboxylic, bicyclo[2.2.1]hept-5-ene-2,3- dicarboxylic and x-methyl-bicyclo[2.2.1]hept-5-ene-2,2-dicarboxylic anhydrides. Advantageously maleic anhydride is used.

Various known processes can be used to graft a grafting monomer onto a polymer. For example, this may be achieved by heating the polymers to a high temperature, from about 150⁰C to about 300⁰C, in the presence or absence of a solvent and with or without a radical initiator. The amount of grafting monomer may be chosen appropriately, but it is preferably from 0.01 to 10%, better still from 600 ppm to 2%, with respect to the weight of the polymer onto which the graft is attached.

With regard to the proportions for the particular embodiment of the invention, those of the blend (A) are from 5 to 30%, advantageously from 5 to 10%, per 95 to 70%, advantageously 95 to 90%, of (B) respectively. The proportion of polyethylene carrying epoxy functional groups may be from 1 to 2 parts per 5 parts of impact modifier. With regard to the blend (B), there are 100 to 5 parts, preferably 100 to 50 parts, of modified fluoropolymer (B 1) per 0 to 95 parts, preferably 0 to 50 parts, of fluoropolymer (B2). The proportion of electrically conducting p roduct is between 0 and 25% by weight relative to the total weight of blend (A) and blend (B).

With regard to the preparation of the compositions of the invention, these may be prepared by melt blending the constituents using standard techniques for thermoplastics, eg an extruder.

The inner and outer layers may also contain additives, which may be:

- dyes;

- pigments; - antioxidants;

- fire retardants;

- UV stabilizers;

- nanofillers; and

- nucleating agents.

The tubes of the invention may have an outside diameter between 8 mm and 50 cm, preferably between 8 and 25 cm. The thickness of the inner layer may be between 15 and 400 μm, preferably between 15 and 200 μm, while that of the optional binder is between 5 and 100 μm .

Applications

The tube of the invention may be used in any motorized vehicle as a fuel pipe. This can be a fuel supply pipe bringing petrol from the tank to the injection system, a tubing between the petrol tank and the filling orifice or the return tubing for petrol vapour. It may also be used for transferring the hydraulic liquid used in brakes. It may also be used for transporting a coolant, like fluorohydrocarbons or chlorofluorohydrocarbons and for transferring a fluid in a fuel cell.

Examples The petrol permeability of the tubes is measured by a static method at 23°C with fuel C containing 15% methanol (static method).

PRODUCTS USED

KYNAR® 720: a PVDF homopolymer sold by Arkema, having an MFI of 14 g/10 min at 230°C/5 kg and a melting point of 169°C.

HYDRIN® 2000: an epichlorohydrin/ethylene oxide copolymer from Zeon

Chemical.

KYNAR® ADX 120: a modified PVDF containing 1 % grafted maleic anhydride obtained using the operating method described below, based on a KYNAR® 720.

EXXELOR® VA 1803: an EPR elastomer grafted by maleic anhydride, having an MFI of 3 g/10 min (230°C/2.16 kg).

LOTADER® 8840: an ethylene/glycidyl methacrylate copolymer from Arkema, having an MVI (Melt Volume Index) of 5 cm³/10 min (190°C/2.16 kg). It contains 92% ethylene and 8% glycidyl methacrylate by weight.

Preparation of KYNAR® ADX 120

A blend of Kynar^® 720 PVDF from Arkema and of 2 wt% maleic anhydride was prepared. This blend was prepared using a twin-screw extruder operating at 230⁰C and 150 rpm with a throughput of 10 kg/h. The granulated product thus prepared was bagged, in impermeable aluminium-lined bags and then oxygen was removed by flushing with a stream of argon. These bags were then irradiated by γ irradiation (Co⁶⁰ bomb) at 3 Mrad (10 MeV acceleration) for 17 hours. A 50% grafting level was determined, this level being checked after a step of dissolving the material in N-methylpyrrolidone and then precipitation in a water/THF mixture (50/50 by weight). The product obtained after the grafting operation was then placed under vacuum overnight at 130⁰C in order to remove the residual maleic anhydride and the hydrofluoric acid liberated during the irradiation.

The final grafted maleic anhydride content was 1 % (infrared spectroscopic analysis of the C=O band at around 1780 cm^'1).

Preparation of the elastomer:

A composition was produced in an internal mixer, comprising in parts by weight:

HYDRIN 2000 100 stearic acid 1

FEF N550 carbon black 30

EXTRUSIL (a silica from Degussa) 20

MgO 3

CaCO₃ 5

This composition was blended on a roll mill with 1 part of ZISNET F (triazine from Zeon Chemical).

Example 1 : A tube based on 70% KVNAR® 720 by weight and 30% ADX 120 by weight was extruded (in a Werner 40 extruder) of 6/7 mm diameter (internal layer) and then taken to an elastomer jacketing unit that extruded the HYDRl N-based composition at 90⁰C through a jacketing die (external layer). The inner and the outer layers are in close contact.

The tube was placed in an oven at 170⁰C for 20 minutes.

Example 2 (comparative example):

The conditions of Example 1 were repeated, but ADX 120 was not used. The inner layer was therefore composed only of KYNAR® 720. Example 3:

The conditions of Example 1 were repeated, but using, for the internal layer, a compound comprising a blend of KYNAR® 720 (64 wt%), with 30% Kynar ADX 120, 1% LOTADER 8840 and 5% EXXELOR VA 1803. This blend, once produced, had a nodular morphology, the mean size of the dispersed phase being less than 5 μm. The inner and the outer layers are in close contact.

Claims

1. Tube having, in its radial direction from the inside outwards:

1) a layer referred to as the inner layer intended to come into contact with a flowing fluid, the said inner layer comprising (i) a fluoropolymer (B1) modified by the radiation grafting of a graftable compound and optionally blended with a fluoropolymer (B2), and (ii) optionally an electrically conducting product; 2) optionally, a tie layer; and 3) a vulcanized elastomer layer.

2. Tube according to Claim 1 , in which the fluoropolymer that is modified is a fluoroplastic or a fluoroelastomer containing units of general formula: X X¹

U U ( I)

F H in which X and X' may be, independently of each other, a hydrogen atom, a halogen, especially fluorine or chlorine, or a perhaloalkyl, especially a perfluoroalkyl.

3. Tube according to Claim 2, in which the fluoropolymer that is modified is a PVDF homo- or copolymer.

4. Tube according to Claim 3, in which the fluoropolymer that is modified is a VF2/HFP copolymer containing at least 50wt% VF2.

5. Tube according to one of Claims 1 to 4, in which the graftable compound possesses at least one C=C double bond and at least one polar functional group that may be a carboxylic acid functional group, a carboxylic acid salt, a carboxylic acid anhydride, an epoxide, a carboxylic acid ester, a silyl, an alkoxysi lane, a carboxylic amide, a hydroxyl or an isocyanate.

6. Tube according to Claim 5, in which the graftable compound contains only a single C=C double bond.

7. Tube according to Claim 6, in which the graftable compound is maleic anhydride or zinc, calci urn or sodium undecylenate.

8. Tube according to one of Claims 1 to 7, in which the electrically conducting product is a carbon black.

9. Tube according to Claim 8, in which the carbon black has a nitrogen adsorption surface area of less than 100 m²/g.

10. Tube according to Claim 9, in which the carbon black is ENSACO^® 250.

11. Tube according to any one of the preceding claims, in which the inner layer comprises a polyethylene carrying epoxy functional groups and an impact modifier chosen from elastomers and very low-density polyethylenes.

12. Tube according to Claim 11 , in which the inner layer comprises: 5 to 30% by weight of a blend (A) comprising:

- a polyethylene carrying epoxy functional groups,

- an impact modifier chosen from elastomers and very low- density polyethylenes, the said impact modifier being completely or partly functionalized; 95 to 70% by weight of a blend (B) comprising:

- a modified fluoropolyrner (B1),

- a fluoropolymer (B2); and, optionally an electrically conducting product.

13. Use of the tube according to any one of Ciaims 1 to IZ as fuei pipe, or for transferring the hydraulic liquid used in brakes.

14. Use of the tube according to any one of Claims 1 to 12 for transporting a coolant or a fluid in a fue l cell.