AU2014369649A1

AU2014369649A1 - Binder for a multilayer structure

Info

Publication number: AU2014369649A1
Application number: AU2014369649A
Authority: AU
Inventors: Samuel Devisme; Claire MICHALOWICZ; Sebastien Quinebeche
Original assignee: Arkema France SA
Current assignee: SK Geo Centric Co Ltd
Priority date: 2013-12-18
Filing date: 2014-11-24
Publication date: 2016-07-07
Anticipated expiration: 2034-11-24
Also published as: CN105980504A; JP6543257B2; EP3083863A1; FR3014891B1; TR201906426T4; FR3014891A1; WO2015092184A1; CN105980504B; EP3083863B1; AU2014369649B2; JP2017509723A

Abstract

The present invention relates to a binder for multilayer structures, which includes: 60 % to 95 %, by weight of the composition, of a matrix made of a polyethylene selected from among the low-density radical polyethylenes (LDPE); 5 % to 40 %, by weight of the composition, of a mixture of two polyethylenes, the first polyethylene consisting of a metallocene polyethylene, co-grafted by an unsaturated carboxylic acid, grafting monomer, the grafts of unsaturated carboxylic acid being more than 0.5 %, by weight of the mixture, characterised in that the second polyethylene consists of a low-density radical polyethylene (LDPE) and in that the MFI or melt flow index (standard ASTM D1238, at 190 °C, under 2.16 kg) is 4 to 15 g/10 min. The invention also relates to a multilayer structure that includes the binder and to a method for manufacturing said multilayer structure.

Description

BINDER FOR A MULTILAYER STRUCTURE

Field of the invention

The present invention relates to a binder for multilayer structures conventionally comprising a metallic layer. This binder is more specifically an extrusion binder based on cografted polyethylenes and the invention relates to its use for making a multilayer structure and also to the structure obtained.

More particularly, the invention relates to a coating/lamination extrusion binder or a coating/lamination (co)extrusion binder based on polyethylene comprising a mixture of two types of polyethylene, one a metallocene polyethylene and the other a low-density radical polyethylene, said mixture being cografted with an unsaturated carboxylic acid and this mixture being placed in a low-density radical polyethylene matrix.

The invention relates to the field of packagings such as sachets, bags, sacks or packets made from these welded films. Nonlimiting examples that may be mentioned include crisp, biscuit, confectionery and coffee packets.

Prior art

Such a binder conventionally comprises a polyethylene matrix, conventionally of low-density radical polyethylene type (LDPE), for its processability properties, but this polymer alone does not adhere correctly to a metal surface, which is why an adhesion agent that is also capable of improving the mechanical properties of the assembly, known as a "binder", needs to be added thereto. EP 1 136 536, filed in the name of the Applicant, is known, which discloses an adhesion agent consisting of a polyethylene mixture, one consisting of a metallocene polyethylene of particular density and the other consisting of a non-metallocene linear low-density polyethylene (LLDPE), cografted with an unsaturated carboxylic acid and more particularly a maleic anhydride. This adhesion agent is placed in a matrix consisting of a polyethylene.

In particular, Examples 4 and 8 present polyethylene compositions comprising a linear low-density polyethylene and a metallocene polyethylene in a linear polyethylene matrix (Φ radical).

Binders of this type are satisfactory from the point of view of the mechanical properties and the adhesion to metal, but their processability in extrusion coating/lamination, i.e. their capacity to be melt-drawn in the air gap (distance in the air before encountering the solid substrate(s)), is very difficult and too often entails "neck-in" problems (reduction of the width of the film associated with the viscoelastic behavior of the polymer) and/or drawability problems which profoundly compromise the production of an industrially viable film. Examples 24 and 25 of the table below review, respectively, the examples of compositions 4 and 8 of said document EP 1 136 536. WO 2004/072 200 is also known, which discloses an adhesion agent consisting of a mixture of polyolefins, one consisting of a metallocene polyethylene of particular density and the other consisting either of a non-metallocene linear low-density polyethylene (LLDPE) or of a polypropylene, cografted with an unsaturated carboxylic acid or a functional derivative of this acid as a grafting monomer. This adhesion agent is placed in a matrix consisting of a polypropylene.

This binder does not show satisfactory adhesion properties on metal and the processability still remains unacceptable.

Lastly, a final solution is also known, which consists in using terpolymers as adhesion agent, especially such as an ethylene-isobutyl acrylate-methacrylic acid. However, this solution requires very high installation and production costs since the production of such a terpolymer must be performed in a high-pressure reactor. Moreover, such terpolymers are liable to give off unpleasant odors due to the presence of residual acrylate monomers.

Thus, at the present time, the market is lacking an economic binder that can satisfy all the implementation and functionality conditions, namely, firstly, irreproachable processability in extrusion coating/lamination (in particular "neck-in" and drawability limit) and melt flow index (MFI), and, secondly, excellent mechanical characteristics and adhesion to a metal.

Brief description of the invention

The Applicant has discovered, surprisingly, in contradiction with the teachings of the prior art (processability and MFI criteria), that it is possible to prepare a mixture containing an adhesion agent consisting of two polyethylenes of radically different nature cografted with an unsaturated carboxylic acid and containing a low-density radical polyethylene. This mixture has properties in terms of processability and adhesion to metal that are superior to those of the compositions of the prior art.

The present invention thus relates to a binder for multilayer structures, comprising: - from 60% to 95%, by weight of the composition, of a polyethylene matrix chosen from low-density radical polyethylenes (LDPE), - from 5% to 40%, by weight of the composition, of a mixture of two polyethylenes, the first polyethylene consisting of a metallocene polyethylene, cografted with an unsaturated carboxylic acid, the grafting monomer, the unsaturated carboxylic acid grafts representing more than 0.5% by weight of the mixture, characterized in that the second polyethylene consists of a low-density radical polyethylene (LDPE) and in that the MFI or meltflow index (standard ASTM D 1238, at 190°C, under 2.16 kg) is between 4 and 15 g/10 min.

In the text hereinbelow, the term "multilayer" is, without preference, in the plural or in the singular and denotes a set formed from a plurality of layers.

Other characteristics or embodiments of the invention are presented below: - advantageously, the first polyethylene is present in the mixture in a proportion of from 50% to 70% by weight of the polyethylene mixture; - preferably, the MFI or melt flow index of the mixture of two polyethylenes is between 5 and 10 g/10 min; - according to a particularly advantageous aspect, the unsaturated carboxylic acid grafts of the mixture of two polyethylenes represent between 0.5% and 2%, preferably between 0.6% and 1.2%, by weight of the polyethylene mixture; - advantageously, the unsaturated carboxylic acid consists of maleic anhydride; - according to a possibility offered by the invention, the binder comprises between 0.1% and 5% of functional additives chosen from antiblocking agents, glidants, antioxidants, fillers, pigments, colorants and processing aids, to facilitate the implementation of this extrusion-coating or extrusion-lamination composition.

Some of these additives may be introduced into the composition in the form of masterbatches.

The invention especially has the advantages of being able to be performed easily, without risk of deterioration of the film produced.

The present invention also relates to a multilayer structure, comprising a plurality of adjacent layers including at least one metallic layer, characterized in that the binder as described above is attached directly to the metallic layer.

Preferably, the metallic layer is a layer of aluminum, iron, copper, tin, nickel, silver, chromium, gold, zinc or an alloy predominantly containing at least one of these metals (more than 60% by weight of the alloy). In the text hereinbelow, only a support made of aluminum was tested and presented herein, but the results collected on this support are identical, or virtually similar, on the other supports mentioned above.

Finally, the invention relates to a process for manufacturing the abovementioned multilayer structure, comprising a step of preparing the binder described above, said binder being arranged in the form of a film between a metallic layer and a layer of polymer, characterized in that the arrangement of said binder is produced by extrusion coating/lamination or by coextrusion coating/lamination.

The description that follows is given for purely illustrative purposes and without limitation.

Detailed description of the invention

As regards the polyethylene matrix, it is a low-density polyethylene (LDPE) radical-polymerized at very high pressure (1800 to 3000 bar). Radical low-density polyethylenes are by definition nonlinear polymers. Low-density polyethylenes are products that are well known to those skilled in the art and are commercially available. Low-density polyethylenes have densities of between 0.91 and 0.94.

As regards the first polyethylene of the polyethylene mixture, it is a metallocene polyethylene.

The term "metallocene polyethylene" denotes polymers obtained by copolymerization of ethylene and of alpha-olefin, for instance propylene, butene, hexene or octene in the presence of a single-site catalyst generally constituted of a metal atom that may be, for example, zirconium or titanium and of two cyclic alkyl molecules linked to the metal. More specifically, metallocene catalysts are usually composed of two cyclopentadiene rings linked to the metal. These catalysts are frequently used with aluminoxanes as cocatalysts or activators, preferably methylaluminoxane (MAO). Hafnium may also be used as metal to which the cyclopentadiene is attached. Other metallocenes may include transition metals from groups IV A, V A and VI A. Metals of the lanthanide series may also be used.

These metallocene polyethylenes may also be characterized by their ratio Mw/Mn < 3 and preferably < 2 in which Mw and Mn denote, respectively, the weight-average molar mass and the number-average molar mass. The term "metallocene polyethylene" also denotes those with an MFR (melt flow ratio) of less than 6.53, and an Mw / Mn ratio greater than MFR minus 4.63. MFR denotes the ratio of the MFI 10 (MFI under a 10 kg load) to the MFI 2 (MFI under a 2.16 kg load). Other metallocene polyethylenes are defined by an MFR greater than or equal to 6.13 and an Mw / Mn ratio of less than or equal to MFR minus 4.63.

Advantageously, the density of the first polyethylene of the mixture is between 0.900 and 0.930.

As regards the second polyethylene of the mixture, it is a low-density polyethylene (LDPE) radical-polymerized at very high pressure (1800 to 3000 bar). Radical low-density polyethylenes are by definition nonlinear polymers. Low-density polyethylenes are products that are well known to those skilled in the art and are commercially available. Low-density polyethylenes have densities of between 0.91 and 0.94.

The mixture of polyethylenes, the first and the second polyethylene described above, is grafted with an unsaturated carboxylic acid, i.e. these two polyethylenes are cografted. It would not constitute a departure from the context of the invention to use a functional derivative of this acid.

Examples of unsaturated carboxylic acids are those containing 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 the unsaturated carboxylic acids.

Unsaturated dicarboxylic acids containing 4 to 10 carbon atoms and functional derivatives thereof, particularly anhydrides thereof, are particularly preferred grafting monomers.

These grafting monomers comprise, for example, maleic acid, fumaric acid, itaconic acid, citraconic acid, allylsuccinic acid, cyclohex-4-ene-l,2-dicarboxylic acid, 4-methylcyclohex-4-ene-1,2-dicarboxylic acid, bicyclo(2,2,1 )hept-5-ene-2,3 -dicarboxylic acid and x-methylbicyclo(2,2,l)hept-5-ene-2,3-dicarboxylic acid, maleic anhydride, itaconic anhydride, citraconic anhydride, allylsuccinic anhydride, cyclohex -4-ene-1,2-dicarboxylic anhydride, 4-methylenecyclohex-4-ene-1,2-dicarboxylic anhydride, bicyclo(2,2,l)hept-5-ene-2,3-dicarboxylic anhydride and x-methylbicyclo(2,2,1 )hept-5-ene-2,2-dicarboxylic anhydride.

Examples of other grafting monomers comprise Ci-Cg alkyl esters or glycidyl ester derivatives of the unsaturated carboxylic acids such as methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, butyl methacrylate, glycidyl acrylate, glycidyl methacrylate, monoethyl maleate, diethyl maleate, monomethyl fumarate, dimethyl fumarate, monomethyl itaconate and diethyl itaconate; amide derivatives of the unsaturated carboxylic acids such as acrylamide, methacrylamide, maleic monoamide, maleic diamide, N-monoethylmaleamide, N,N-diethylmaleamide, N-monobutylmaleamide, Ν,Ν-dibutylmaleamide, fumaric monoamide, fumaric diamide, N-monoethylfumaramide, Ν,Ν-diethylfumaramide, N-monobutylfumaramide and Ν,Ν-dibutylfumaramide; imide derivatives of the unsaturated carboxylic acids such as maleimide, N-butylmaleimide and N-phenylmaleimide; and metal salts of the unsaturated carboxylic acids such as sodium acrylate, sodium methacrylate, potassium acrylate and potassium methacrylate. Maleic anhydride is preferred.

Various known processes may be used for grafting a grafting monomer onto the mixture of the two polyethylenes.

For example, this may be performed by heating the polyethylenes, metallocene polyethylene and radical polyethylene, to high temperature, about 150° to about 300°C, in the presence or absence of a solvent, with or without radical generator. Suitable solvents that may be used in this reaction are benzene, toluene, xylene, chlorobenzene and eumene, inter alia. Suitable radical generators that may be used comprise t-butyl hydroperoxide, cumene hydroperoxide, diisopropylbenzene hydroperoxide, di-t-butyl peroxide, di-t-amyl peroxide, t-butylcumyl peroxide, dicumyl peroxide, l,3-bis(t-butylperoxysopropyl)benzene, 2,5-dimethyl-2,5-bis(t-butylperoxy)hexane, t-butyl peroxybenzoate, t-butyl peroxy-2-ethylhexanoate, 0,0-t-butyl-0-(2-ethylhexyl) monoperoxycarbonate, 0,0-t-amyl-0-(2-ethylhexyl) monoperoxycarbonate, acetyl peroxide, dibenzoyl peroxide, isobutyryl peroxide, bis(3,5,5-trimethylhexanoyl) peroxide and methyl ethyl ketone peroxide.

Advantageously, the grafting process consists in extruding the polyethylene mixture in a corotating twin-screw extruder in the presence of a radical generator, maleic anhydride. The temperature is chosen so that the reaction takes place in the molten state of the polyethylene mixture and so that the radical generator decomposes totally in the time allotted to the extrusion. It should be noted that degassing is performed at the end of the extruder so as to remove from the polyethylene mixture the decomposition products of the radical generator and the unreacted monomers.

In the mixture of the first and second graft-modified polyethylenes, obtained in the abovementioned manner, the amount of grafting monomer may be chosen in an appropriate manner, but it is preferably from 0.5% to 2% and better still from 0.6% to 1.2% relative to the weight of grafted mixture.

The amount of grafted monomer is determined by assaying the succinic functions by FTIR (Fourier Transform Infra-Red) spectroscopy. The MFI of the mixture, i.e. of the first and second polyethylenes that have been cografted, is 4 to 15 g/10 min, advantageously between 5 and 10 g/10 min.

The combination between the polyethylene matrix and the abovementioned mixture of cografted polyethylenes is prepared in an entirely conventional manner that is well known to those skilled in the art, either by simple mixing of granules, or via a step of mixing in the molten state. The thickness of the binder according to the invention, in the form of a film, is generally between 5 and 30 microns (or micrometers pm), preferably between 5 and 25 microns, more preferably between 8 and 20 microns.

The binder according to the invention may advantageously comprise additives in a minor proportion, of the order of 0.1% to 5% by weight of the binder. These additives may be antiblocking agents, glidants, antioxidants, fillers, pigments, colorants and processing aids, to facilitate the implementation of this extrusion-coating or extrusion-lamination composition. Some of these additives may be introduced into the composition in the form of masterbatches.

Production of the formulations of the test compositions:

The grafted polyethylenes and the mixtures of cografted polyethylenes tested were prepared in a ZSK26MC corotating twin-screw extruder. The grafting monomer used is maleic anhydride supplied by the company CristalMan and the radical generator is Luperox® 101 from the company Arkema. The extruder is fed by means of weight meters. The extrusion conditions were: delivery rate = 46 kg/h, temperature = 250°C and screw speed = 700 rpm. The amounts of maleic anhydride and of Luperox 101 introduced are adjusted as a function of the targeted maleic anhydride content and MFI (melt flow index). The extruder is equipped with a degassing well which allowed devolatilization of the residuals at the end of the extruder by means of a liquid-ring pump (P = -0.95 bar in the degassing well). The grafted polyethylene or mixture of cografted polyethylenes leaving the extruder is cooled on contact with water and then granulated using a pelletizer.

The grafted polyethylenes and the mixtures of cografted polyethylenes tested were then combined with a low-density polyethylene by simple mixing of granules.

The tested binders thus obtained are the following:

Example 1: binder not falling within the context of the invention, comprising 100% of a low-density polyethylene LDPE2.

Example 2: binder not falling within the context of the invention, comprising 20% of an adhesion agent consisting of an ethylene-butyl acrylate-acrylic acid terpolymer combined with 80% of a low-density polyethylene LDPE2.

Example 3: binder falling within the context of the invention, comprising 10% of an adhesion agent consisting of a mixture of polyethylene (40% of a low-density polyethylene LDPE1 + 60% of a metallocene polyethylene mPE) cografted to a proportion of 0.68% with maleic anhydride and having an MFI of 7.1 g/10 min, combined with 90% of a low-density polyethylene LDPE2.

Example 4: binder falling within the context of the invention, comprising 20% of an adhesion agent consisting of a mixture of polyethylene (40% of a low-density polyethylene LDPE1 + 60% of a metallocene polyethylene mPE) cografted to a proportion of 0.68% with maleic anhydride and having an MFI of 7.1 g/10 min, combined with 80% of a low-density polyethylene LDPE2.

Example 5: binder falling within the context of the invention, comprising 35% of an adhesion agent consisting of a mixture of polyethylene (40% of a low-density polyethylene LDPE1 + 60% of a metallocene polyethylene mPE) cografted to a proportion of 0.68% with maleic anhydride and having an MFI of 7.1 g/10 min, combined with 65% of a low-density polyethylene LDPE2.

Example 6: binder not falling within the context of the invention, comprising 45% of an adhesion agent consisting of a mixture of polyethylene (40% of a low-density polyethylene LDPE1 + 60% of a metallocene polyethylene mPE) cografted to a proportion of 0.68% with maleic anhydride and having an MFI of 7.1 g/10 min, combined with 55% of a low-density polyethylene LDPE2.

Example 7: binder falling within the context of the invention, comprising 20% of an adhesion agent consisting of a mixture of polyethylene (60% of a low-density polyethylene LDPE1 + 40% of a metallocene polyethylene mPE) cografted to a proportion of 0.71% with maleic anhydride and having an MFI of 5.3 g/10 min, combined with 80% of a low-density polyethylene LDPE2.

Example 8: binder falling within the context of the invention, comprising 20% of an adhesion agent consisting of a mixture of polyethylene (20% of a low-density polyethylene LDPE1 + 80% of a metallocene polyethylene mPE) cografted to a proportion of 0.66% with maleic anhydride and having an MFI of 9.3 g/10 min, combined with 80% of a low-density polyethylene LDPE2.

Example 9: binder not falling within the context of the invention, comprising 20% of an adhesion agent consisting of a metallocene polyethylene mPE grafted to a proportion of 0.74% with maleic anhydride and having an MFI of 9.9 g/10 min, combined with 80% of a low-density polyethylene LDPE2.

Example 10: binder not falling within the context of the invention, comprising 20% of an adhesion agent consisting of a mixture of polyethylene (40% of a linear low-density polyethylene LLDPE + 60% of a metallocene polyethylene mPE) cografted to a proportion of 0.69% with maleic anhydride and having an MFI of 6.5 g/10 min, combined with 80% of a low-density polyethylene LDPE2.

Example 11: binder not falling within the context of the invention, comprising 20% of an adhesion agent consisting of a low-density polyethylene LDPE2 grafted to a proportion of 0.60% with maleic anhydride and having an MFI of 6.0 g/10 min, combined with 80% of a low-density polyethylene LDPE2.

Example 12: binder not falling within the context of the invention, comprising 20% of an adhesion agent consisting of a mixture of polyethylene (40% of a low-density polyethylene LDPE1 + 60% of a metallocene polyethylene mPE) cografted to a proportion of 0.68% with maleic anhydride and having an MFI of 7.1 g/10 min, combined with 80% of a polypropylene PP.

Example 13: binder not falling within the context of the invention, comprising 20% of an adhesion agent consisting of a mixture of polyethylene (40% of a low-density polyethylene LDPE1 + 60% of a metallocene polyethylene mPE) cografted to a proportion of 0.41% with maleic anhydride and having an MFI of 9.7 g/10 min, combined with 80% of a low-density polyethylene LDPE2.

Example 14: binder falling within the context of the invention, comprising 20% of an adhesion agent consisting of a mixture of polyethylene (40% of a low-density polyethylene LDPE1 + 60% of a metallocene polyethylene mPE) cografted to a proportion of 0.55% with maleic anhydride and having an MFI of 8.4 g/10 min, combined with 80% of a low-density polyethylene LDPE2.

Example 15: binder falling within the context of the invention, comprising 20% of an adhesion agent consisting of a mixture of polyethylene (40% of a low-density polyethylene LDPE1 + 60% of a metallocene polyethylene mPE) cografted to a proportion of 1.12% with maleic anhydride and having an MFI of 6.6 g/10 min, combined with 80% of a low-density polyethylene LDPE2.

Example 16: binder falling within the context of the invention, comprising 20% of an adhesion agent consisting of a mixture of polyethylene (40% of a low-density polyethylene LDPE1 + 60% of a metallocene polyethylene mPE) cografted to a proportion of 1.28% with maleic anhydride and having an MFI of 6.3 g/10 min, combined with 80% of a low-density polyethylene LDPE2.

Example 17: binder falling within the context of the invention, comprising 20% of an adhesion agent consisting of a mixture of polyethylene (40% of a low-density polyethylene LDPE1 + 60% of a metallocene polyethylene mPE) cografted to a proportion of 1.83% with maleic anhydride and having an MFI of 5.5 g/10 min, combined with 80% of a low-density polyethylene LDPE2.

Example 18: binder not falling within the context of the invention, comprising 20% of an adhesion agent consisting of a mixture of polyethylene (40% of a low-density polyethylene LDPE1 + 60% of a metallocene polyethylene mPE) cografted to a proportion of 2.18% with maleic anhydride and having an MFI of 5.1 g/10 min, combined with 80% of a low-density polyethylene LDPE2.

Example 19: binder not falling within the context of the invention, comprising 20% of an adhesion agent consisting of a mixture of polyethylene (40% of a low-density polyethylene LDPE1 + 60% of a metallocene polyethylene mPE) cografted to a proportion of 0.72% with maleic anhydride and having an MFI of 3.1 g/10 min, combined with 80% of a low-density polyethylene LDPE2.

Example 20: binder falling within the context of the invention, comprising 20% of an adhesion agent consisting of a mixture of polyethylene (40% of a low-density polyethylene LDPE1 + 60% of a metallocene polyethylene mPE) cografted to a proportion of 0.71% with maleic anhydride and having an MFI of 5.2 g/10 min, combined with 80% of a low-density polyethylene LDPE2.

Example 21: binder falling within the context of the invention, comprising 20% of an adhesion agent consisting of a mixture of polyethylene (40% of a low-density polyethylene LDPE1 + 60% of a metallocene polyethylene mPE) cografted to a proportion of 0.62% with maleic anhydride and having an MFI of 8.7 g/10 min, combined with 80% of a low-density polyethylene LDPE2.

Example 22: binder falling within the context of the invention, comprising 20% of an adhesion agent consisting of a mixture of polyethylene (40% of a low-density polyethylene LDPE2 + 60% of a metallocene polyethylene mPE) cografted to a proportion of 0.72% with maleic anhydride and having an MFI of 11.3 g/10 min, combined with 80% of a low-density polyethylene LDPE2.

Example 23: binder falling within the context of the invention, comprising 20% of an adhesion agent consisting of a mixture of polyethylene (40% of a low-density polyethylene LDPE2 + 60% of a metallocene polyethylene mPE) cografted to a proportion of 0.67% with maleic anhydride and having an MFI of 13.8 g/10 min, combined with 80% of a low-density polyethylene LDPE2.

Example 24: binder not falling within the context of the invention, comprising 20% of an adhesion agent consisting of a mixture of polyethylene (40% of a low-density polyethylene LDPE2 + 60% of a metallocene polyethylene mPE) cografted to a proportion of 0.61% with maleic anhydride and having an MFI of 17.4 g/10 min, combined with 80% of a low-density polyethylene LDPE2.

The binders tested were coextruded to give 37 pm/10 pm/50 pm/50 pm aluminum/binder/LDPE2/PE film structures so as to evaluate their properties of adhesion to aluminum. These structures were prepared on a Collin® brand extruder in extrusion lamination configuration. The extruder is equipped with a "feed block" (2-layer configuration) and a flat die with a 300 pm air gap and a width of 25 cm (Lo). These productions were made at a temperature of 290°C for a line speed of 25 m/min.

Starting materials for the test compositions: LDPE1: radical low-density polyethylene, with an MFI (190°C, 2.16 kg) of 7.5 g/10 min and a density of 0.920. LDPE2: radical low-density polyethylene, with an MFI (190°C, 2.16 kg) of 4 g/10 min and a density of 0.925. mPEl: ethylene-hexene metallocene copolymer, with an MFI (190°C, 2.16 kg) of 20 g/10 min and a density of 0.915. mPE2: ethylene-octene metallocene copolymer, with an MFI (190°C, 2.16 kg) of 1 g/10 min and a density of 0.870. mPE3: ethylene-octene metallocene copolymer, with an MFI (190°C, 2.16 kg) of 1 g/10 min and a density of 0.890. LLDPE1: ethylene-hexene non-metallocene copolymer, with an MFI (190°C, 2.16 kg) of 3.0 g/10 min and a density of 0.920. LLDPE2: ethylene-butene non-metallocene copolymer, with an MFI (190°C, 2.16 kg) of 3.0 g/10 min and a density of 0.920. FLDPE3: ethylene-butene non-metallocene copolymer, with an MFI (190°C, 2.16 kg) of 3.0 g/10 min and a density of 0.930. FFDPE4: ethylene-butene non-metallocene copolymer, with an MFI (190°C, 2.16 kg) of 3.0 g/10 min and a density of 0.910. PP: propylene-ethylene copolymer, with an MFI (230°C, 2.16 kg) of 7 g/10 min and a density of 0.905.

Terpolymer: ethylene-isobutyl acrylate-methacrylic acid terpolymer, with an MFI (190°C, 2.16 kg) of 11 g/10 min and a density of 0.920.

Tests performed:

Adhesion test

All the abovementioned compositions, considered to be of identical thickness, were tested according to exactly the same referential, namely on the same support and in the same standardized adhesion test (in accordance with standard ISO 11339).

Strips 15 mm wide were cut from the center of the width and in the direction of extrusion. The polymer coating was separated manually from the support over a distance of a few centimeters, and the two arms thus freed (aluminum and polymer coating, respectively) were then each placed in one of the two jaws of a Synergie 200 tensile testing machine from MTS. The peel strength was then evaluated with a peel speed of 200 mm/min. Five specimens were tested per adhesive reference. The tests were performed within 15 minutes of implementation (peeling at tO) and also after conditioning for 8 days at 23°C and 50% relative humidity (peeling at t8d). Thus, the adhesion is measured with regard to a peel force.

Processability test

Tests were performed on a Dr Collin brand extruder in coating configuration, in which the test binder is coated onto an aluminum foil (37 pm). All of these tests were performed with a temperature profile of 280°C.

An identical screw speed and air gap are chosen for all the tests, filming is then started and a line speed of 5 m/min is set. Under these conditions, the width (Lf) and thickness (tf) of the coated film are noted.

Once these parameters have been noted, the line speed is increased until the first instabilities are observed. These instabilities may be in several forms: "melt" rupture or "draw" resonance, which are well known to those skilled in the art (visible instability on the edges of the film which leads to a variation on the width). These observations are performed three times in succession, so as to confirm the results, and a mean value of this series is taken for exploitation of the results.

Drawability limit (m/min) = line speed at which the start of the instabilities (draw resonance or melt rupture) appears.

Neck-in (%) = (width of the die - width of the coated film) / width of the die x 100 = (Lo-Lf)/Lo x 100

Test results:

In general, the following results for the various tests must be obtained in order for a composition to be satisfactory in the envisaged application:

The drawability limit must be greater than 25 m/min and more preferentially greater than 30 m/min (meters per minute).

The neck-in must be less than 15% and more preferentially less than 10%.

The peel force at to must be greater than 2 N/15 mm (newtons per millimeter) and more preferentially greater than 2.2 N/15 mm.

The peel force at t 8 days (after eight days) must be greater than 1.8 N/15 mm and more preferentially greater than 2.3 N/15 mm.

Claims

1. A binder for multilayer structures, comprising: - from 60% to 95%, by weight of the composition, of a polyethylene matrix chosen from low-density radical polyethylenes (LDPE), - from 5% to 40%, by weight of the composition, of a mixture of two polyethylenes, the first polyethylene consisting of a metallocene polyethylene, cografted with an unsaturated carboxylic acid, the grafting monomer, the unsaturated carboxylic acid grafts representing more than 0.5% by weight of the mixture, characterized in that the second polyethylene consists of a low-density radical polyethylene (LDPE) and in that the MFI or meltflow index (standard ASTM D 1238, at 190°C, under 2.16 kg) is between 4 and 15 g/10 min.

2. The binder as claimed in claim 1, characterized in that the first polyethylene is present in the mixture in a proportion of from 50% to 70% by weight of the polyethylene mixture.

3. The binder as claimed in claim 1 or 2, characterized in that the MFI or melt flow index of the mixture of two polyethylenes is between 5 and 10 g/10 min.

4. The binder as claimed in any one of the preceding claims, characterized in that the unsaturated carboxylic acid grafts of the mixture of two polyethylenes represent between 0.5% and 2%, preferably between 0.6% and 1.2%, by weight of the polyethylene mixture.

5. The binder as claimed in any one of the preceding claims, characterized in that the unsaturated carboxylic acid consists of maleic anhydride.

6. The binder as claimed in any one of the preceding claims, characterized in that it comprises between 0.1% and 5% of functional additives chosen from antiblocking agents, glidants, antioxidants, fillers, pigments, colorants and processing aids, to facilitate the implementation of this extrusion-coating or extrusion-lamination composition. Some of these additives may be introduced into the composition in the form of masterbatches.

7. A multilayer structure, comprising a plurality of adjacent layers including at least one metallic layer, characterized in that the binder as claimed in any one of the preceding claims is attached directly to the metallic layer.

8. The multilayer structure as claimed in the preceding claim, characterized in that the metallic layer is a layer of aluminum, iron, copper, tin, nickel, silver, chromium, gold, zinc or an alloy containing at least one of these metals.

9. A process for manufacturing a multilayer structure as claimed in claim 7 or 8, comprising a step of preparing the binder as claimed in any one of claims 1 to 6, said binder then being arranged in the form of a film between a metallic layer and a layer of polymer, characterized in that the arrangement of said binder is produced by extrusion coating/lamination or by coextrusion coating/lamination.