Patent Publication Number: US-2006008604-A1

Title: Multilayer structure having a layer based on polyamide and on HDPE

Description:
This application claims benefit, under U.S.C. § 119(a) of French National Applications Number 04.06635, filed Jun. 18, 2004; and 04.10391, filed Oct. 01, 2004 ; and also claims benefit, under U.S.C. § 119(e) of U.S. provisional applications 60/585494, filed Jul. 2, 2004 and 60/631933 filed Nov. 30, 2004. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates to a multilayer structure having a layer based on polyamide and on HDPE (high density polyethylene). It also relates to a tank composed of this structure having this layer in direct contact with the fluid contained in the tank. The layer based on polyamide and on HDPE of the structures of the invention constitutes one of the faces of the structure, that is to say that it is not inside the structure (sandwich). These structures are of use in the manufacture of devices for the transfer or storage of fluids and more particularly pipes, tanks, tank conduits, that is to say the pipe which is used to fill the tank, bottles and containers in which the layer based on polyamide and on HDPE is in contact with the fluid. It is of particular use for tanks.  
      The invention is of use for a fluid, such as motor vehicle petrol, by preventing losses through the structure, so as not to pollute the environment. It is also of use for liquids comprising volatile substances by preventing depletion of the liquid in this volatile substance. The invention is also of use for the liquid coolant of the engines, for the oil and for the fluid of the air-conditioning system.  
     BACKGROUND OF THE INVENTION  
      Patent EP 742 236 discloses petrol tanks composed of five layers, which are respectively:  
      high density polyethylene (HDPE);  
      a tie;  
      a polyamide (PA) or a copolymer having ethylene units and vinyl alcohol units (EVOH);  
      a tie;  
      HDPE. 
      A sixth layer can be added between one of the tie layers and one of the HDPE layers. This sixth layer is composed, for example, of the manufacturing scrap material resulting from the forming of the tanks or, in a much smaller amount, of tanks which have failed specification. This scrap material and these tanks which have failed specification are ground. This ground material is subsequently remelted and directly extruded on the plant for the coextrusion of the tanks. This ground material might also be melted and regranulated by an extrusion device, such as a twin-screw or single-screw extruder, before being reused.    

      According to one alternative form, the recycled product can be blended with the HDPE of the two outermost layers of the tank. It is possible, for example, to blend the granules of recycled product with the granules of virgin HDPE of these two layers. It is also possible to use any combination of these recycling operations. The level of recycled material can represent up to 50% of the total weight of the tank.  
      U.S. Pat. No. 6,177,162 discloses a tube comprising an inner layer comprising a blend of polyamide and of polyolefin with a polyamide matrix and an outer layer comprising a polyamide. These polyamide-based tubes are of use for the transportation of petrol and more particularly for conveying the petrol from the tank of the motor vehicle to the engine and also, but with a larger diameter, for the transportation of hydrocarbons in service stations between the distribution pumps and the underground storage reservoirs.  
      According to another form of the invention, it is possible to position, between the inner and outer layers, a layer of a polymer comprising ethylene units and vinyl alcohol units (EVOH). Use is advantageously made of the structure: inner layer/EVOH/tie/outer layer.  
      The tanks disclosed in EP 742 236, and which do not have the barrier layer in direct contact with the petrol, certainly have barrier properties but they are not sufficient when very low petrol losses are being looked for. EP 731 308 discloses pipes which have their outer layer made of polyamide and the barrier layer in direct contact with the petrol; the layer made of polyamide is necessary for the mechanical strength of the combined product.  
      Patent Application 2005089701 discloses a structure successively comprising:  
      a first layer of high density polyethylene (HDPE),  
      a tie layer,  
      a second layer of EVOH or of a blend based on EVOH,  
      optionally a third layer of polyamide or of a blend of polyamide and of polyolefin. Numerous blends of polyamide and of polyolefin which can constitute the third layer are described in this patent. It has now been found that this third layer is necessary and, furthermore, that it has to comprise HDPE in order to obtain good barrier properties. Furthermore, the conversion temperature of the polyamide of this third layer must not be too high in order not to be too far from that of the EVOH.  
     SUMMARY OF THE INVENTION  
      The present invention relates to a structure successively comprising: 
      a first layer of high density polyethylene (HDPE),     a tie layer,     a second layer of EVOH or of a blend based on EVOH,     optionally a tie layer,     a third layer of a blend comprising, by weight, the total being 100%:    

      50 to 90% of polyamide (A) having a conversion temperature of at most 230° C.,  
      1 to 30% of high density polyethylene (HDPE),  
      5 to 30% of an impact modifier chosen from elastomers and very low density polyethylenes,  
      at least one of the HDPE and of the impact modifier being functionalized, in all or part, the layers being coextrudable.  
      Use may be made of a blend of different HDPEs. It can be a blend of different nonfunctionalized HDPEs, of a nonfunctionalized HDPE and of the same HDPE but functionalized, of a nonfunctionalized HDPE and of another HDPE but functionalized or of two different grafted HDPEs, or any combination of these possibilities.  
      Use may be made of a blend of different impact modifiers. It can be a blend of different nonfunctionalized impact modifiers, of a nonfunctionalized impact modifier and of the same impact modifier but functionalized, of a nonfunctionalized impact modifier and of another impact modifier but functionalized, of two different functionalized impact modifiers or of a functionalized impact modifier and of a functionalized HDPE with optionally a nonfunctionalized impact modifier and optionally a nonfunctionalized HDPE, or any combination of these possibilities.  
      The layers are “coextrudable”, meaning that they are in the same rheology range to form, for example, a parison which can be blow-moulded to form a hollow body or an extruded tube.  
      The proportion of functional groups of the HDPE and/or of the impact modifier must be sufficient for the layer based on polyamide (A) and on HDPE to have mechanical strength but not too great in order for the viscosity not to be so high that the layer is no longer coextrudable.  
      The present invention also relates to devices for the transfer or storage of fluids and more particularly to pipes, tanks, conduits, bottles and containers composed of the above structure in which the layer of the blend of polyamide (A) and of HDPE is in direct contact with the fluid present or transported. These devices can be manufactured by conventional techniques of the industry of thermoplastic polymers, such as coextrusion and coextrusion blow-moulding. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      As regards the first layer, high density polyethylene (HDPE) is described in Kirk-Othmer, 4th edition, Vol. 17, pages 704 and 724-725. It is, according to ASTM D 1248-84, an ethylene polymer having a density of at least 0.940. The name HDPE relates both to ethylene homopolymers and to copolymers of ethylene with small proportions of α-olefin. The density is advantageously between 0.940 and 0.965. In the present invention, the MFI of the HDPE is advantageously between 0.1 and 50. Mention may be made, as examples, of Eltex B 2008®, with a density of 0.958 and an MFI of 0.9 (in g/10 min at 190° C. under 2.16 kg), Finathene® MS201B from Fina and Lupolen® 4261 AQ from BASF. As regards the high density polyethylene of the first layer, its density is advantageously between 0.940 and 0.965 and the MFI is between 0.1 and 5 g/10 min. (190° C., 5 kg).  
      As regards the second layer, the EVOH copolymer is also known as saponified vinyl acetate/ethylene copolymer. The saponified vinyl acetate/ethylene copolymer to be employed according to the present invention is a copolymer having an ethylene content of 20 to 70 mol %, preferably of 25 to 70 mol %, the degree of saponification of its vinyl acetate component being not less than 95 mol %. With an ethylene content of less than 20 mol %, the barrier properties under conditions of high humidity are not as great as would be desired, while an ethylene content exceeding 70 mol % leads to declines in the barrier properties. When the degree of saponification or of hydrolysis is less than 95 mol %, the barrier properties are lost.  
      The term “barrier properties” is understood to mean impermeability to gases and to liquids and in particular to oxygen and to petrol for motor vehicles. The invention relates more particularly to the barrier to petrol for motor vehicles.  
      Among these saponified copolymers, those which have melt flow indices in the range from 0.5 to 100 g/10 minutes are of particular use. Advantageously, the MFI is chosen between 5 and 30 (g/10 min at 230° C. under 2.16 kg); “MFI” is the abbreviation for Melt Flow Index.  
      It is understood that this saponified copolymer can comprise small proportions of other comonomer ingredients, including α-olefins, such as propylene, isobutene, α-octene, α-dodecene, α-octadecene, and the like, unsaturated carboxylic acids or their salts, partial alkyl esters, complete alkyl esters, nitrites, amides and anhydrides of the said acids, and unsaturated sulphonic acids or their salts.  
      With regard to the blends based on EVOH, they are such that the EVOH forms the matrix, that is to say that it represents at least 40% by weight of the blend and preferably at least 50%. The other constituents of the blend are chosen from polyolefins, polyamides or impact modifiers which are optionally functionalized. The impact modifier can be chosen from elastomers, copolymers of ethylene and of an olefin having 4 to 10 carbon atoms (for example, ethylene-octene copolymers), and very low density polyethylenes. Mention may be made, as examples of elastomers, of EPR and EPDM. EPR (abbreviation for Ethylene-Propylene Rubber) denotes ethylene-propylene elastomers and EPDM denotes ethylene-propylene-diene monomer elastomers.  
      As first example of these blends based on EVOH of the second layer, mention may be made of the compositions comprising (by weight):  
      55 to 99.5 parts of EVOH copolymer,  
      0.5 to 45 parts of polypropylene and of compatibilizing agent, their proportions being such that the ratio of the amount of polypropylene to the amount of compatibilizing agent is between 1 and 5. 
      Advantageously, the ratio of the MFI of the EVOH to the MFI of the polypropylene is greater than 5 and preferably between 5 and 25. Advantageously, the MFI of the polypropylene is between 0.5 and 3 (in g/10 min at 230° C. under 2.16 kg). According to an advantageous form, the compatibilizing agent is a polyethylene carrying polyamide grafts and it results from the reaction (i) of a copolymer of ethylene and of a grafted or copolymerized unsaturated monomer X with (ii) a polyamide. The copolymer of ethylene and of a grafted or copolymerized unsaturated monomer X is such that X is copolymerized and it can be chosen from ethylene/maleic anhydride copolymers and ethylene/alkyl (meth)acrylate/maleic anhydride copolymers, these copolymers comprising from 0.2 to 10% by weight of maleic anhydride and from 0 to 40% by weight of alkyl (meth)acrylate.    

      According to another advantageous form, the compatibilizing agent is a polypropylene carrying polyamide grafts which results form the reaction (i) of a homopolymer or of a copolymer of propylene comprising a grafted or copolymerized unsaturated monomer X with (ii) a polyamide. Advantageously, X is grafted. The monomer X is advantageously an unsaturated carboxylic acid anhydride, such as, for example, maleic anhydride.  
      As second example of these blends based on EVOH of the second layer, mention may be made of the compositions comprising:  
      50 to 98% by weight of an EVOH copolymer,  
      1 to 50% by weight of a polyethylene,  
      1 to 15% by weight of a compatibilizing agent composed of a blend of an LLDPE polyethylene or metallocene polyethylene and of a polymer chosen from elastomers, very low density polyethylenes and metallocene polyethylenes, the blend being cografted by an unsaturated carboxylic acid or a functional derivative of this acid.  
      Advantageously, the compatibilizing agent is such that the MFI 10 /MFI 2  ratio is between 5 and 20, where MFI 2  is the melt flow index at 190° C. under a load of 2.16 kg, measured according to ASTM D1238, and MFI 10  is the melt flow index at 190° C. under a load of 10 kg, according to ASTM D1238.  
      As third example of these blends based on EVOH of the second layer, mention may be made of the compositions comprising:  
      50 to 98% by weight of an EVOH copolymer,  
      1 to 50% by weight of an ethylene/alkyl (meth)acrylate copolymer,  
      1 to 15% by weight of a compatibilizing agent resulting from the reaction (i) of a copolymer of ethylene and of a grafted or copolymerized unsaturated monomer X with (ii) a copolyamide.  
      Advantageously, the copolymer of ethylene and of a grafted or copolymerized unsaturated monomer X is such that X is copolymerized and this is a copolymer of ethylene and of maleic anhydride or a copolymer of ethylene, of an alkyl (meth)acrylate and of maleic anhydride.  
      Advantageously, these copolymers comprise from 0.2 to 10% by weight of maleic anhydride and from 0 to 40% by weight of alkyl (meth) acrylate.  
      As fourth example of these blends based on EVOH of the second layer, mention may be made of the compositions comprising:  
      50 to 98% by weight of an EVOH copolymer,  
      2 to 50% by weight of an elastomer which is optionally functionalized in all or part or of a blend of a functionalized elastomer and of another nonfunctionalized elastomer.  
      As regards the blend of polyamide (A) and of HDPE of the third layer, the term “conversion temperature” is understood to mean the temperature at which it is coextruded with the material of the other layers and/or coextruded and blow-moulded with the material of the other layers. For semicrystalline polyamides, this is a temperature above the melting point (usually denoted by M.p.) and, for amorphous polyamides, this is, of course, a temperature above the Tg (glass transition temperature). The term “above” is understood to mean generally a difference of 10 to 50° C.  
      This polyamide (A) is chosen from the products which comprise units originating:  
      from one or more amino acids, such as aminocaproic acid, 7-aminoheptanoic acid, 11-aminoundecanoic acid and 12-aminododecanoic acid, or from one or more lactams, such as caprolactam, enantholactam and lauryllactam;  
      from one or more salts or mixtures of diamines with diacids. Mention may be made, as examples of diacids, of isophthalic acid, terephthalic acid or dicarboxylic acids having from 6 to 18 carbon atoms, such as adipic acid, azelaic acid, suberic acid, sebacic acid and dodecanedicarboxylic acid. The diamine can be an aliphatic diamine having from 6 to 18 atoms, it can be an arylic and/or saturated cyclic diamine. Mention may be made, as examples, of hexamethylenediamine, piperazine, tetramethylenediamine, octamethylenediamine, decamethylenediamine, dodecamethylenediamine, 1,5-diaminohexane, 2,2,4-trimethyl-1,6-diaminohexane, polyoldiamines, isophoronediamine (IPD), methylpentamethylenediamine (MPDM), bis(aminocyclohexyl)methane (BACM), or bis(3-methyl-4aminocyclohexyl)methane (BMACM).  
      Use may also advantageously be made of copolyamides. Mention may be made of the copolyamides resulting from the condensation of at least two α,ω-aminocarboxylic acids or of two lactams or of a lactam and of an α,ω-aminocarboxylic acid. Mention may also be made of the copolyamides resulting from the condensation of at least one α-ω-aminocarboxylic acid (or one lactam), at least one diamine and at least one dicarboxylic acid.  
      Mention may be made, as examples of lactams, of those which have from 3 to 12 carbon atoms on the main ring and which can be substituted. Mention may be made, for example, of β,β-dimethylpropiolactam, α,α-dimethylpropiolactam, amylolactam, caprolactam, capryllactam and lauryllactam.  
      Mention may be made, as examples of α,ω-aminocarboxylic acids, of aminoundecanoic acid and aminododecanoic acid. Mention may be made, as examples of dicarboxylic acids, of adipic acid, sebacic acid, isophthalic acid, butanedioic acid, 1,4-cyclohexanedicarboxylic acid, terephthalic acid, the sodium or lithium salt of sulphoisophthalic acid, dimerized fatty acids (these dimerized fatty acids have a dimer content of at least 98% and are preferably hydrogenated) and dodecanedioic acid HOOC-(CH 2 ) 10 -COOH.  
      Mention may be made, as examples of copolyamides, of copolymers of caprolactam and of lauryllactam (PA 6/12), copolymers of caprolactam, of adipic acid and of hexamethylenediamine (PA 6/6-6), copolymers of caprolactam, of lauryllactam, of adipic acid and of hexamethylenediamine (PA 6/12/6-6), copolymers of caprolactam, of lauryllactam, of 11-aminoundecanoic acid, of azelaic acid and of hexamethylenediamine (PA 6/6-9/11/12), copolymers of caprolactam, of lauryllactam, of 11-aminoundecanoic acid, of adipic acid and of hexamethylenediamine (PA 6/6-6/11/12) or copolymers of lauryllactam, of azelaic acid and of hexamethylenediamine (PA 6-9/12). 
      All these polyamides (A) are known per se and are manufactured according to the usual processes for polyamides.    

      Advantageously, the copolyamide is chosen from PA 6/12 and PA 6/6-6.  
      Polyamide blends can be used. Advantageously, the relative viscosity, measured in 96% sulphuric acid, is between 2 and 5.  
      It would not be departing from the scope of the invention to replace a portion of the polyamide (A) with a copolymer comprising polyamide blocks and polyether blocks, that is to say to use a blend comprising at least one of the above polyamides and at least one copolymer comprising polyamide blocks and polyether blocks.  
      The copolymers comprising polyamide blocks and polyether blocks result from the copolycondensation of polyamide sequences comprising reactive ends with polyether sequences comprising reactive ends, such as, inter alia:  
      1) Polyamide sequences comprising diamine chain ends with polyoxyalkylene sequences comprising dicarboxyl chain ends.  
      2) Polyamide sequences comprising dicarboxyl chain ends with polyoxyalkylene sequences comprising diamine chain ends obtained by cyanoethylation and hydrogenation of aliphatic α,ω-dihydroxylated polyoxyalkylene sequences, known as polyetherdiols.  
      3) Polyamide sequences comprising dicarboxyl chain ends with polyetherdiols, the products obtained being, in this specific case, polyetheresteramides. Use is advantageously made of these copolymers.  
      The polyamide sequences comprising dicarboxyl chain ends originate, for example, from the condensation of α,ω-aminocarboxylic acids, of lactams or of dicarboxylic acids and diamines in the presence of a chain-limiting dicarboxylic acid.  
      The polyether can, for example, be a polyethylene glycol (PEG), a polypropylene glycol (PPG) or a polytetramethylene glycol (PTMG). The latter is also known as polytetrahydrofuran (PTHF).  
      The number-average molar mass {overscore (Mn)} of the polyamide sequences is between 300 and 15 000 and preferably between 600 and 5000. The mass {overscore (Mn)} of the polyether sequences is between 100 and 6000 and preferably between 200 and 3000.  
      The polymers comprising polyamide blocks and polyether blocks can also comprise randomly distributed units. These polymers can be prepared by the simultaneous reaction of the polyether and of the precursors of the polyamide blocks.  
      For example, polyetherdiol, a lactam (or an α,ω-amino acid) and a chain-limiting diacid can be reacted in the presence of a small amount of water. A polymer is obtained which has essentially polyether blocks and polyamide blocks, the latter being of highly variable length, but also the various reactants which have reacted randomly, which are distributed statistically along the polymer chain.  
      These polymers comprising polyamide blocks and polyether blocks, whether they originate from the copolycondensation of polyamide and polyether sequences prepared beforehand or from a one-stage reaction, exhibit, for example, Shore D hardnesses which can be between 20 and 75 and advantageously between 30 and 70 and an intrinsic viscosity of between 0.8 and 2.5, measured in meta-cresol at 250° C. for an initial concentration of 0.8 g/100 ml. The MFI values can be between 5 and 50 (235° C. under a load of 1 kg).  
      The polyetherdiol blocks are either used as is and copolycondensed with polyamide blocks comprising carboxyl ends or they are aminated, in order to be converted into polyetherdiamines, and condensed with polyamide blocks comprising carboxyl ends. They can also be blended with polyamide precursors and a chain-limiting agent in order to prepare polymers comprising polyamide blocks and polyether blocks having statistically distributed units.  
      Polymers comprising polyamide and polyether blocks are disclosed in U.S. Pat. No. 4,331,786, U.S. Pat. No. 4,115,475, U.S. Pat. No. 4,195,015, U.S. Pat. No. 4,839,441, U.S. Pat. No. 4,864,014, U.S. Pat. No. 4,230,838 and U.S. Pat. No. 4,332,920.  
      The ratio of the amount of copolymer comprising polyamide blocks and polyether blocks to the amount of polyamide is advantageously between 10/90 and 60/40, by weight.  
      As regards the HDPE of the third layer, its density is advantageously between 0.940 and 0.965 and the MFI between 1 and 10 g/10 min. (190° C., 5 kg).  
      As regards the impact modifier and first the elastomers, mention may be made of SBS, SIS and SEBS block polymers and ethylene/propylene (EPR) or ethylene/propylene/diene (EPDM) elastomers. With regard to the very low density polyethylenes, these are, for example, metallocenes with a density, for example, between 0.860 and 0.900.  
      Use is advantageously made of an ethylene/propylene (EPR) or ethylene/propylene/diene (EPDM) elastomer. The functionalization can be introduced by grafting or copolymerization with an unsaturated carboxylic acid. It would not be departing from the scope of the invention to use a functional derivative of this acid. Examples of unsaturated carboxylic acids are those having 2 to 20 carbon atoms, such as acrylic acid, methacrylic acid, maleic acid, fumaric acid and itaconic acid. The functional derivatives of these acids comprise, for example, the anhydrides, the ester derivatives, the amide derivatives, the imide derivatives and the metal salts (such as the alkali metal salts) of the unsaturated carboxylic acids.  
      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 acid, fumaric acid, itaconic acid, citraconic acid, allylsuccinic acid, cyclohex-4-ene-1,2-dicarboxylic acid, 4-methylcyclohex-4-ene-1,2-dicarboxylic acid, bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acid, x-methylbicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acid, maleic anhydride, itaconic anhydride, citraconic anhydride, allylsuccinic anhydride, cyclohex-4-ene-1,2-dicarboxylic anhydride, 4-methylcyclohex-4-ene-1,2-dicarboxylic anhydride, bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic anhydride and x-methylbicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic anhydride. Use is advantageously made of maleic anhydride.  
      Various known processes can be used to graft a grafting monomer to a polymer. For example, this can be carried out by heating the polymers at high temperature, approximately 150° to approximately 300° C., in the presence or in the absence of a solvent and with or without a radical generator. The amount of the grafting monomer can be appropriately chosen but it is preferably from 0.01 to 10%, better still from 600 ppm to 2%, with respect to the weight of the polymer to which the graft is attached.  
      It is possible to graft, in all or part, the impact modifier and to blend it with the HDPE. It is possible to graft the HDPE, in all or part, and to blend it with the impact modifier. It is also possible separately to graft, in all or part, the impact modifier, to graft the HDPE, in all or part, and then to blend the two grafted products. It is also possible to blend the impact modifier with the HDPE and to graft, in all or part, the blend.  
      The proportion of functionalized HDPE and/or of functionalized modifier with respect to the combined functionalized or nonfunctionalized HDPE and functionalized or nonfunctionalized impact modifier can be (by weight) between 0 and 70%, advantageously between 5 and 60% and preferably between 20 and 60%.  
      According to one form of the invention, the HDPE is not functionalized and the impact modifier is functionalized in all or part.  
      The proportions of the blend of the third layer are advantageously, the total being 100%: 
      55 to 80% of polyamide (A),     10 to 20% of high density polyethylene (HDPE),     10 to 30% of impact modifier.    

      The blends of the third layer can be prepared by blending the various constituents in the molten state in conventional devices of the thermoplastic polymer industry.  
      The first layer can be composed of a layer of virgin HDPE and of a layer of recycled polymers (also referred to as regrind layer) originating from scrap material during the manufacture of the transfer or storage devices or from these devices which have failed specification, as is explained in the abovementioned prior art. This layer of recycled polymers is situated on the side of the tie layer. In the continuation of the text, these two layers will be denoted for simplicity by the term “first layer”. Functionalized polyolefins can be added to this layer of recycled polymers in a proportion which can, for example, be between 0.1 and 10% by weight. These functionalized polyolefins are advantageously chosen from the ties. Either HDPE or functionalized polyolefins or a blend of the two can be added to this layer of recycled polymers.  
      The thickness of the first layer can be between 2 and 10 mm, that of the second between 30 and 500 μm and that of the third between 500 μm and 4 mm. As regards the tanks, the overall thickness is usually between 3 and 10 mm.  
      As example of tie, mention may be made of functionalized polyolefins. The tie between the first and the second layer and that between the second and the third layer can be identical or different. In the following descriptions of ties, the term “polyethylene” denotes both homopolymers and copolymers.  
      As first tie example, mention may be made of a blend of a polyethylene (C1) and of a polymer (C2) chosen from elastomers, very low density polyethylenes and ethylene copolymers, the blend (C1)+(C2) being cografted by an unsaturated carboxylic acid.  
      According to an alternative form, mention may be made of a blend (i) of a polymer (C2) chosen from elastomers, very low density polyethylenes and ethylene copolymers, (C2) being grafted by an unsaturated carboxylic acid, and (ii) of a polymer (C2) chosen from elastomers, very low density polyethylenes and ethylene copolymers.  
      As second tie example, mention may be made of the blends comprising:  
      5 to 30 parts of a polymer (D) itself comprising a blend of a polyethylene (D1) with a density of between 0.910 and 0.940 and of a polymer (D2) chosen from elastomers, very low density polyethylenes and metallocene polyethylenes, the blend (D1)+(D2) being cografted by an unsaturated carboxylic acid,  
      95 to 70 parts of a polyethylene (E) with a density of between 0.910 and 0.930,  
      the blend of (D) and (E) being such that: 
          its density is between 0.910 and 0.930,     the content of grafted unsaturated carboxylic acid is between 30 and 10 000 ppm,     the MFI (ASTM D 1238, 190° C., 2.16 kg) is between 0.1 and 3 g/10 min, the MFI denoting the melt flow index.        

      The density of the tie is advantageously between 0.915 and 0.920. Advantageously, (D1) and (E) are LLDPEs; preferably, they have the same comonomer. This comonomer can be chosen from 1-hexene, 1-octene and 1-butene.  
      As third tie example, mention may be made of the blends comprising:  
      5 to 30 parts of a polymer (F) itself comprising a blend of a polyethylene (F1) with a density of between 0.935 and 0.980 and of a polymer (F2) chosen from elastomers, very low density polyethylenes and ethylene copolymers, the blend (F1)+(F2) being cografted by an unsaturated carboxylic acid,  
      95 to 70 parts of a polyethylene (G) with a density of between 0.930 and 0.950,  
      the blend of (F) and (G) being such that: 
          its density is between 0.930 and 0.950 and advantageously between 0.930 and 0.940,     the content of grafted unsaturated carboxylic acid is between 30 and 10 000 ppm,     the MFI (melt flow index), measured according to ASTM D 1238 at 190° C. and 2.16 kg, is between 5 and 100.        

      As fourth tie example, mention may be made of polyethylene grafted by maleic anhydride having an MFI of 0.1 to 3 and a density of between 0.920 and 0.930 and which comprises 2 to 40% by weight of materials which are insoluble in n-decane at 90° C. In order to determine the materials which are insoluble in n-decane, the grafted polyethylene is dissolved in n-decane at 140° C., is cooled to 90° C. and products precipitate; it is then filtered and the level of insoluble materials is the percentage by weight which precipitates and is collected by filtration at 90° C. If the level is between 2 and 40%, the tie has good resistance to petrol.  
      Advantageously, the grafted polyethylene is diluted in an ungrafted polyethylene such that the tie is a blend of 2 to 30 parts of a grafted polyethylene with a density of between 0.930 and 0.980 and of 70 to 98 parts of an ungrafted polyethylene with a density of between 0.910 and 0.940, preferably 0.915 and 0.935.  
      As fifth tie example, mention may be made of the blends comprising: 
          50 to 100 parts of a polyethylene (J) homo- or copolymer with a density of greater than or equal to 0.9,      0  to 50 parts of a polymer (K) chosen from (K1) polypropylene homo- or copolymer, (K2) poly(1-butene) homo- or copolymer and (K3) polystyrene homo- or copolymer,     the amount of (J)+(K) being 100 parts,     the blend of (J) and (K) being grafted by at least 0.5% by weight of a functional monomer,     this grafted blend being itself diluted in at least one polyethylene homo- or copolymer (L) or in at least one polymer with an elastomeric nature (M) or in a blend of (L) and (M).        

      According to one form of the invention, (J) is an LLDPE with a density of 0.910 to 0.930, the comonomer having from 4 to 8 carbon atoms. According to another form of the invention, (K) is an HDPE, advantageously with a density of at least 0.945 and preferably of 0.950 to 0.980.  
      Advantageously, the functional monomer is maleic anhydride and its content is from 1 to 5% by weight of (J)+(K).  
      Advantageously, (L) is an LLDPE, the comonomer of which has from 4 to 8 carbon atoms, and its density is preferably at least 0.9 and preferably 0.910 to 0.930.  
      Advantageously, the amount of (L) or (M) or (L)+(M) is from 97 to 75 parts per 3 to 25 parts of (J)+(K), the amount of (J)+(K)+(L)+(M) being 100 parts.  
      As sixth tie example, mention may be made of the blends composed of a polyethylene of HDPE, LLDPE, VLDPE or LDPE type, 5 to 35% of a grafted metallocene polyethylene and 0 to 35% of an elastomer, the total being 100%.  
      As seventh tie example, mention may be made of the blends comprising:  
      at least one polyethylene or one ethylene copolymer,  
      at least one polymer chosen from polypropylene or a propylene copolymer, poly(1-butene) homo- or copolymer, or polystyrene homo- or copolymer, and preferably polypropylene,  
      this blend being grafted by a functional monomer and this grafted blend being itself optionally diluted in at least one polyolefin or in at least one polymer with an elastomeric nature or in their blend. In the preceding blend which is grafted, the polyethylene advantageously represents at least 50% of this blend and preferably 60 to 90% by weight.  
      Advantageously, the functional monomer is chosen from carboxylic acids and their derivatives, acid chlorides, isocyanates, oxazolines, epoxides, amines or hydroxides and preferably unsaturated dicarboxylic acid anhydrides.  
      As eighth tie example, mention may be made of the blends comprising:  
      at least one LLDPE or VLDPE polyethylene,  
      at least one ethylene-based elastomer chosen from ethylene/propylene copolymers and ethylene/butene copolymers,  
      this blend of polyethylene and of elastomer being grafted by an unsaturated carboxylic acid or a functional derivative of this acid,  
      this cografted blend being optionally diluted in a polymer chosen from polyethylene homo- or copolymers and styrene block copolymers,  
      the tie having 
      (a) an ethylene content which is not less than 70 mol %,     (b) a content of carboxylic acid or of its derivative of 0.01 to 10% by weight of the tie, and     (c) an MFI 10 /MFI 2  ratio of 5 to 20, where MFI 2  is the melt flow index at 190° C. under a load of 2.16 kg, measured according to ASTM D1238, and MFI 10  is the melt flow index at 190° C. under a load of 10 kg, according to ASTM D1238.    

      The various layers of the structure of the invention, including the tie layers, can additionally comprise at least one additive chosen from: 
          fillers (inorganic, flame-retardant, conductive, and the like),     nanofillers, such as, for example, nanoclays,     nanocomposites,     fibres,     dyes,     pigments,     optical brighteners,     antioxidants,     nucleating agents,     UV stabilizers.        

     EXAMPLES  
      Polymers used:  
      PA A1: Terpolymer of caprolactam (L6), adipic acid (AA) and hexamethylenediamine (HMDA) possessing an L6/[AA+HMDA] ratio by mass of 85/15 and a “viscosity number” of 186 according to Standard ISO 307.  
      PA A2: Copolymer of caprolactam and of lauryllactam possessing a monomer composition by weight of 70/30 and an intrinsic viscosity (measured at 20° C. for a concentration of 0.5 g per 100 ml of meta-cresol) of 1.3 dl/g.  
      PA A3: Copolymer of caprolactam and of lauryllactam possessing a melting point of 190° C. and a melt flow index of 120 according to Standard ISO 1133, measured under the conditions: 275° C. under a load of 5 kg.  
      PA A4: Lauryllactam homopolymer possessing an intrinsic viscosity (measured at 20° C. for a concentration of 0.5 g per 100 ml of meta-cresol) of 1.55 to 1.74 dl/g.  
      PA A5: 11-Aminoundecanoic acid homopolymer possessing an intrinsic viscosity (measured at 20° C. for a concentration of 0.5 g per 100 ml of meta-cresol) of 1.35 to 1.52 dl/g.  
      PA A6 (10.10): Equimolar copolymer of sebacic acid (SA) and of decanediamine (DA) possessing an intrinsic viscosity (measured at 20° C. for a concentration of 0.5 g per 100 ml of meta-cresol) of 1.4 dl/g.  
      PA A7 (MXD.10): Equimolar copolymer of meta-xylylenediamine (MXD) and of sebacic acid (SA) possessing an intrinsic viscosity (measured at 20° C. for a concentration of 0.5 g per 100 ml of meta-cresol) of 1.4 dl/g.  
      PA A8 (MXD.12): Equimolar copolymer of meta-xylylenediamine (MXD) and of dodecanedioic acid (DDA) possessing an intrinsic viscosity (measured at 20° C. for a concentration of 0.5 g per 100 ml of meta-cresol) of 1.4 dl/g.  
      PE 1: Polyethylene possessing a density of 0.952 according to Standard ISO 1183 and a melt flow index of 23 according to Standard ISO 1133, measured under the conditions: 190° C. under a load of 2.16 kg.  
      PE 2: Polyethylene having a density of 0.949 according to Standard ISO 1183 and a melt flow index of 8 g/10 min according to Standard ISO 1133, measured under the conditions: 190° C. under a load of 2.16 kg.  
      P1: Terpolymer of ethylene, of propylene and of diene monomer possessing a density of 0.89 and a Mooney viscosity (ML, 1+4, 125° C.) of 30 and grafted by maleic anhydride at a level of 1%.  
      P2: Terpolymer of ethylene, of propylene and of diene monomer possessing a Mooney viscosity of 30 under the conditions ML (1+4) 100° C.  
      EVOH: Copolymer of ethylene and of p2yl alcohol possessing an ethylene fraction by weight of 29% and a melt flow index of 3.2, measured according to Standard ISO 1133 under the following conditions: 210° C. under a load of 2.16 kg.  
      T1 (Orevac): Polyethylene grafted by 3000 ppm of maleic anhydride and possessing a melt flow index of 1, measured according to Standard ASTM 1238 under the following conditions: 190° C. under a load of 2.16 kg.  
      Alloy 1: Compatibilized blend of PA and of PP possessing an M.p. of 255° C. and a melt flow index of 15, measured according to Standard ISO 1133 under the following conditions: 275° C. under a load of 2.16 kg, sold by the Applicant Company under the reference Orgalloy® RS6600.  
      Alloy 2: Compatibilized blend of PA and of PE possessing an M.p. of 225° C. but a conversion temperature of 250° C. and a melt flow index of 2, measured according to Standard ISO 1133 under the following conditions: 235° C. under a load of 2.16 kg, sold by the Applicant Company under the reference Orgalloy® LE 6000.  
      Alloy 3: Compatibilized blend of PA and of PE possessing an M.p. of 195° C. and a melt flow index of 3, measured according to Standard ISO 1133 under the following conditions: 235° C. under a load of 2.16 kg, sold by the Applicant Company under reference Orgalloy® LEC601.  
      Preparation of the alloys of polyamide and of polyolefin:  
      The alloys of polyamide and of polyolefin are prepared using a corotating twin-screw extruder of Werner &amp; Pfleiderer ZSK 40 type (diameter=40 mm, L=40D).  
      Preparation of multilayer hollow bodies by coextrusion blow-moulding:  
      Multilayer bottles are prepared using a Bekum coextrusion blow-moulding line equipped with 5 extruders, the barrels of which are regulated at 220° C., unless otherwise mentioned. The blow-moulded structures are of two types: 
      four-layer structures described as follows, from the inside outwards:     1. Alloy of polyamide and of polyolefin: Thickness: 30% of the overall thickness (extruder 1)     2. EVOH: Thickness: 5% of the overall thickness (extruder 2)     3. T1: Thickness: 5% of the overall thickness (extruder 3)     4. PE2: Thickness: 60% of the overall thickness (extruder 4)     The overall thickness is 3 mm on average. 
 
 five-layer structures described as follows, from the inside outwards: 
    1. Alloy of polyamide and of polyolefin: Thickness: 30% of the overall thickness (extruder 1)     2. Ti: Thickness: 5% of the overall thickness (extruder 5)     3. EVOH: Thickness: 5% of the overall thickness (extruder 2)     4. T1: Thickness: 5% of the overall thickness (extruder 3)     5. PE2: Thickness: 55% of the overall thickness (extruder 4)     The overall thickness is 3 mm on average.    

      Impact strength of the bottles:  
      The blow-moulded bottles, conditioned beforehand at −40° C., are tested on one of their flat surfaces with regard to impact strength under the following conditions: T=−40° C. and impact speed=4.3 m/s.  
      The force-displacement curve resulting from this test makes it possible to calculate the impact strength of the multilayer bottle.  
      Results:  
     Examples 1 to 3  
      3 four-layer bottles, the structures of which are collated in the table below, were extruded blow-moulded on the Bekum extrusion line.  
                                               Example 1*   Example 2**               (comparative)   (comparative)   Example 3           Alloy 1   Alloy 2   Alloy 3           EVOH   EVOH   EVOH           T1   T1   T1       Structure   PE2   PE2   PE2                  Quality of the   Lack of coextrusion   Lack of coextrusion   Correct       coextrusion                 *Extruder 1 is regulated at 280° C.            ** Extruder 1 is regulated at 250° C.             
 
      These experiments demonstrate that it is advisable to use a polyamide possessing a conversion temperature of less than 230° C. in order to provide correct processing by coextrusion blow-moulding.  
     Examples 4 to 7  
      The alloys of polyamide and of polyolefin collated in the tables below were prepared:  
                                                               Composition:   Alloy 4   Alloy 5   Alloy 6   Alloy 7                                                                PA A1       50   50               PA A2   71           60           PEl   25   15   15   15           P1   4   35   29   19           P2           6   6                      
 
     Examples 8 to 11  
      4 five-layer bottles, the structures of which are collated in the table below, were extruded blow-moulded on the Bekum extrusion line.  
                                                   Example 8   Example 9   Example 10   Example 11           Alloy 4   Alloy 5   Alloy 6   Alloy 7           T1   T1   T1   T1           EVOH   EVOH   EVOH   EVOH           T1   T1   T1   T1       Structure   PE2   PE2   PE2   PE2                  Quality of the   Correct   Lack of   Lack of   Correct       coextrusion       coextrusion   coextrusion       Impact strength   No*   Yes**   Yes   Yes                 *”No” means that the value of the impact strength measured is less than 50 J            **“Yes” means that the value of the impact strength measured exceeds 50 J             
 
     Examples 12 to 17  
      The alloys of polyamide and of polyolefin collated in the tables below were prepared:  
                                                                           Alloy   Alloy   Alloy   Alloy   Alloy   Alloy           Comp.   12   13   14   15   16   17                                                                        PA A3                                   PA A4   60           PA A5       60           PA A6           60           PA A7               60           PA A8                   60           PE 1                       60           P 1   15   15   15   15   15   15           P 2   19   19   19   19   19   19               6   6   6   6   6   6                      
 
     Examples 18 to 22  
      6 five-layer bottles, the structures of which are collated in the table below, were extruded blow-moulded on the Bekum extrusion line.  
                                                           Example 17   Example 18   Example 19   Example 20   Example 21   Example 22           Alloy 12   Alloy 13   Alloy 14   Alloy 15   Alloy 16   Alloy 17           T1   T1   T1   T1   T1   T1           EVOH   EVOH   EVOH   EVOH   EVOH   EVOH           T1   T1   T1   T1   T1   T1       Structure   PE2   PE2   PE2   PE2   PE2   PE2                  Quality of the   Correct   Correct   Correct   Correct   Correct   Correct       coextrusion       Impact   Yes*   Yes   Yes   Yes   Yes   Yes       strength                 *“Yes” means that the value of the impact strength measured exceeds 50 J