EP2554487B1 - Tab seal for sealing a vessel to be closed by plug or cap and related manufacturing method - Google Patents

Description

The invention relates to a novel tongue seal for closing a closure container with a stopper or capsule and its method of manufacture.

The sealing of closure containers by cap or capsule, prior to their first opening, is performed by means of a seal which can be bondable or sealable, especially thermally on the rim.

The invention more particularly relates to a heat-sealable seal according to claim 1.

Such a seal consists of a lid placed around the entire upper rim of the neck of the container or drinking, thereby isolating the container from the outside, and on the other hand, a generally thicker support , inserted, without being fixed, at the bottom of the cap. Prior to the first opening of the container, the support and the lid may be secured by means of a temporary adhesive.

The role of the cap is first of all to confer a inviolability of the container before the first opening. In addition, it provides the primary sealing vis-à-vis the outside. In addition, it must develop certain properties of compatibility with the content that it is supposed to isolate and for example be of food quality as soon as the content itself is food.

The support, meanwhile, is intended to absorb the tolerances between the bottom of the capsule and / or the cap and the upper part of the neck of the container, and in addition, to ensure secondary sealing when the lid has been totally or partially removed. It is therefore likely to provide some compression with memory effect to precisely allow to absorb these tolerances.

In practice, the seal is inserted at the bottom of the capsule or cap. Once the container is filled, it comes to screw or clip the cap or the capsule with the seal on the container. The seal portion of the seal is then in contact with the rim. The sealing of the lid on the rim is then performed by induction heat sealing. This heat sealing is made possible by the combined action of a conductive material, inserted in the thickness of the seal, which, under the effect of an electric induction is heated, causing the softening of the sealing film on the rim. The conductive material is inserted into the thickness of the cap. When one proceeds for the first time to the opening of the container, the support inserted in the bottom of the cap disengages from the lid remained sealed on the rim. It is only then that the cap is removed by the consumer, sealing in the closed position of the container, then being assured only by the support part of the seal coming to press against the rim.

In order to allow efficient and easy tearing of the lid by the consumer, it has been proposed to add to said lid a tongue that is easily grippable, such as for example described in the document FR-A-2,716,407 .

Nevertheless, the presence of this prominent tongue makes it difficult to position the seal at the bottom of the cap or the capsule, because of the extra thickness created laterally. It is also difficult to securely position the tongue relative to the net or the clips arranged in the cap or the capsule to avoid any risk of damage to the tongue at the time of closure or opening of the cap or the capsule.

The document DE3920324 discloses an induction heat seal seal consisting of a support and a lid provided with a tongue, the support and the lid being at no time secured to one another. In three illustrated embodiments, only one of them ( figure 1 ) has a tongue turned over its entire surface. The tongue is constituted by the combination of a membrane and a sealing layer, the aluminum sheet being positioned in the support.

The document JP 2000-191021 discloses a joint consisting of a support and a seal associated with a temporary adhesive. The seal contains a tongue which is either turned over the support or hangs along the rim.

The document US 4,754,890 separately describes a cover and a support that are never associated temporarily.

The document GB 2,330,134 discloses a joint associating a support and a cover by means of a layer of wax. It has a tongue that is returned with the heat sealing layer between the support and the cover.

The document EP 2 045 194 A1 discloses a joint comprising a support and a lid provided with a tongue and associated by means of a temporary adhesive applied to the lid, once the tongue returned. In a first embodiment, the tongue consists solely of a reinforcing film devoid of heat-sealing layer. In this case, one is exposed to potential risks of delamination of the constituent layers of the seal at the time of tearing thereof. In a second embodiment, the tongue incorporates aluminum and has an S shape when folded. The presence of these two folds irremediably increases the thickness of the tongue which makes difficult the temporary assembly.

The document WO 2010/115811 describes a seal of the same type as before. When the tongue contains aluminum, it is always provided with a heat seal layer on its underside. To avoid sticking of the latter on the underside of the support at the time of induction, the heat-sealing layer is advantageously covered with an anti-adhesive film. Such a construction multiplies the number of layers and increases the cost of production. Meanwhile, in the preferred embodiment, the tab is made of a reinforcing film adjacent to the sealing layer, said tab being devoid of a heat seal layer and turned over an aluminum foil.

• les risques de délamination des couches constitutives de l'opercule au moment de l'arrachage de celui-ci, lorsque la languette n'est pas formée dans toute l'épaisseur de l'opercule,
• la résistance mécanique de la languette qui peut être plus ou moins importante en fonction de la nature et du nombre de couches constitutives de l'opercule,
• lorsque la languette est pourvue de film thermoscellant, le collage de la languette sur le support au moment de l'induction,
• lorsque la feuille en matériau conducteur est comprise dans la languette, le risque d'un scellage trop faible sous la languette au moment de l'induction, nécessitant d'augmenter la puissance d'induction et donc de réduire la plage de température d'induction possible au moment du scellage,
• la rigidité de l'opercule du fait de son épaisseur, généralement supérieure à 200 micromètres,
• le procédé de fabrication qui requiert encore un nombre d'étapes relativement élevé, ayant une incidence sur le coût de production.

Although the joints proposed in the prior art provide interesting solutions, we still face a number of problems that include:

• the risk of delamination of the constituent layers of the cap at the time of tearing thereof, when the tongue is not formed in the entire thickness of the lid,
• the mechanical strength of the tongue which can be more or less important depending on the nature and the number of layers constituting the lid,
• when the tongue is provided with heat-sealing film, the gluing of the tongue on the support at the time of induction,
• when the sheet of conductive material is included in the tongue, the risk of too weak a seal under the tongue at the time of induction, requiring to increase the power of induction and thus reduce the induction temperature range possible at the time of sealing,
• the rigidity of the lid because of its thickness, generally greater than 200 micrometers,
• the manufacturing process that still requires a relatively high number of steps, affecting the cost of production.

To alleviate all these difficulties, the Applicant has developed a new seal comprising a lid provided with a tongue, the assembly having shape characteristics such that they allow the incorporation in said tongue of a sheet of conductive material including avoiding a too weak seal such as previously described. It follows that it becomes possible to return all the constituent layers of the lid, including the aluminum foil, with the exception of the heat seal layer. This has the consequence of avoiding the potential problems of delamination, strengthen the resistance of the tongue and facilitate the manufacturing process.

More specifically, the subject of the invention is a heat-sealable seal comprising a support made integral with a cap by means of a temporary adhesive, said cap being provided with a tongue whose entire surface is folded once on the face. the lid in contact with the corresponding face of the support, the lid comprising a complex comprising itself a reinforcing film and a sheet of conductive material, on the underside of which is applied a heat-sealing film, the temporary adhesive being applied over the entire surface of the lid once the tongue folded. A seal of this type is described in the document EP 2 045 194 A1 .

The seal is characterized in that the complex has throughout its thickness a notch from which emerges the tongue, the fold formed by the tongue once it folded having a length less than that of the notch.

In other words, the base of the tongue is shifted back from the periphery of the heat-sealing film, which means that the tongue-free surface of the reinforcing film / conductive material film is smaller than the surface of the film. heat-sealing film so that the complex does not cover the entire heat-sealing film.

More specifically, the combination of an indentation created in the reinforcing film / aluminum complex and a tongue whose length of the fold is narrow and less than that of the length of the indentation makes it possible to reduce the deflection as much as possible. That is the distance between the fold, once the tongue folded and the periphery of the heat sealing film. This has the advantage of less disturbing the induced field, thus ensuring a uniform seal of all layers on the rim as well as a wider induction temperature range.

As already said, the tongue emerges from the indentation created in the complex which means that the tongue emerges from the entire thickness of the complex. In other words, it consists of the same number of layers as the number of layers constituting the complex. In addition to the aforementioned advantages related to induction, this also makes it possible to avoid delamination phenomena of the constituent layers of the complex, to increase the resistance of the tongue and not to stick the tongue to the support (the lower face of the tongue is devoid of heat-sealing film).

In a preferred embodiment, the seal consists exclusively of the complex on the underside of which the heat-sealing film is applied.

When the lid is not only thus constituted, it may comprise, on the upper face of the complex additional layers, of the type for example reinforcing film which will therefore not be part of the tongue. This will not affect the seal.

Similarly, according to the invention, the heat-sealing film is applied to the underside of the complex. In the preferred embodiment, the film is applied directly to the underside of the complex, regardless of the possible presence of a binder layer applied between the complex and the heat sealing layer. As a result, there is no possible delamination of the constituent layers of the complex at the time of pulling. In a non-preferred alternative embodiment, there is applied between the complex and the heat-sealing film at least a separate intermediate layer of a binder, which may be in particular a reinforcing film. In this case, the additional layer will not be part of the tongue. There is a risk of delamination between the complex and the additional layer while solving the sealing problem.

To avoid the tearing primers of the tongue at the time of tearing of the lid, the indentation has at each of its ends a recessed area oriented towards the outside of the lid.

Advantageously, each hollow zone has a radius of curvature of between 0.3 and 5 mm, advantageously between 0.5 and 2 mm.

According to the invention, the fold is formed at the junction of the tongue itself and the notch.

It has in practice a length of between 3 and 15 mm, advantageously between 5 and 10 mm.

The notch has, for its part, advantageously a length of between 3 and 25 mm, preferably between 3.6 and 20 mm.

According to the invention, the tongue itself is formed of two parts respectively a base and a gripping zone. The base may have curvilinear or rectilinear lateral edges or a succession of curvilinear and / or rectilinear edges. Regardless chosen embodiment, the distance separating the lateral edges in this zone is less than the length of the fold.

In a preferred embodiment, the opposite side edges of the base have a hollow shape oriented towards the outside.

The gripping zone may be of any shape, in particular of generally circular shape. Advantageously, the distance separating the two opposite lateral edges of the tongue is greater than the length of the fold.

In general, the sheet of conductive material is in the form of an aluminum foil or equivalent.

In a preferred embodiment, the tongue comprises a complex consisting of an upper reinforcing film / sheet of conductive material / lower reinforcing film, the two reinforcing films being plastic films of the same chemical nature.

Of course, the upper and lower reinforcement films may themselves consist of several films. In this case, at least the two outermost films are of the same chemical nature.

The Applicant has found that such a construction makes it possible to bring into contact with the temporary adhesive, two surfaces of the same chemical nature, in this case, the upper reinforcing plastic film and the lower reinforcing plastic film once the folded tongue . The adhesive force between the temporary adhesive and the reinforcing films must be as small as possible to facilitate the separation between the cover and the support at the time of opening, without there being any risk of separation before placement of the seal on the rim, that is to say, especially during the transport of the seal. The fact that the two films are of the same chemical nature makes it possible moreover to have a homogeneous adhesion force between the temporary adhesive and the surface of the lid, including the surface of the tongue once folded. This force must also be less than the adhesion force of the cap on the rim after induction.

According to the invention, one or the other or both reinforcing films can be printed. In particular, the printing of the lower reinforcing film makes it possible to clearly distinguish the tongue on the upper face of the lid, once the tongue has folded.

In general, the constituent material of the reinforcing film is chosen so that the resistance of said material is greater than that of the tearing force of the cover.

In practice, the reinforcing films are, for example, biaxially oriented polyester films with a thickness of between 6 and 40 μm.

In the embodiment according to which the aluminum foil is sandwiched between two reinforcing films, said reinforcing films are advantageously made of PET (polyethylene terephthalate), which is generally resistant to temperature.

According to another characteristic, the heat-sealing film is made of a polyolefin-type material, chosen from the group comprising polyethylene, polypropylene or PET, of the holt-melt type, or else of the terpolymer or ionomer type. In general, the constituent material of the heat-sealing film is chosen according to the nature of the pot. The heat-sealing film may be a monolayer film applied to the underside of the complex by extrusion. It may also be a bilayer film (a heat-sealing lower layer proper and a binder upper layer) applied on the underside of the complex by coextrusion.

Depending on their nature and the manufacturing method used, the constituent layers of the cap may be associated by means of a binder, such as for example a two-component isocyanate-hydroxyl adhesive.

In practice, the cap has a thickness of between 20 and 200 μm, preferably between 30 and 120 μm.

The support may be made of any type of compressible material with recovery memory, in particular expanded polyethylene or expanded polypropylene, optionally provided on at least one of its two faces, with a polypropylene or polyethylene film to give it a certain rigidity . It can also be made of paper or cardboard. In practice, the supports envisaged have a thickness of between 0.5 and 4 mm, advantageously between 0.8 and 2 mm.

As regards the temporary fastening between the support and the cover, this can be obtained by means of a temporary adhesive applied in the form of a film, preferably extruded or coextruded, each of the two faces of the layer having a different stickiness, the most tacky face being applied to the lower surface of the support, while the less tacky face is applied to the upper face of the cover. In In practice, such a film is based on a polymer, generally synthetic, and judiciously chosen by those skilled in the art, so as to obtain the desired properties of differential bonding. Such polymers are, for example, polymers or co-polymers based on acetate, acrylate and polyethylene. The temporary adhesive may also be in the form of a layer of wax.

The subject of the invention is also a method for manufacturing the heat-sealable seals described above.

• on prépare un complexe comprenant un film de renfort et une feuille en matériau conducteur,
• de manière repérée, on réalise dans le complexe une prédécoupe de languette se prolongeant à sa base par une échancrure de sorte à ce que la longueur du pli de la languette une fois repliée soit inférieure à celle de l'échancrure,
• on retourne mécaniquement une fois la totalité de la surface des languettes sur la face supérieure du complexe,
• on associe ensuite définitivement la face inférieure du complexe avec le film thermoscellant, l'ensemble constituant la bande opercule,
• parallèlement, on prépare une bande support,
• on solidarise ensuite temporairement la bande support à l'opercule au moyen d'un adhésif temporaire, pour former la bande joint,
• on découpe sur la totalité de l'épaisseur de la bande joint, les joints à la forme souhaitée de façon repérée par rapport aux languettes.

The method of the invention comprises the following steps:

• a complex comprising a reinforcing film and a sheet of conductive material is prepared,
• in a marked manner, a pre-cut of tongue is produced in the complex, extending at its base by an indentation so that the length of the fold of the tongue when folded is less than that of the notch,
• the entire surface of the tongues is mechanically returned once on the upper face of the complex,
• the lower face of the complex is then permanently associated with the heat-sealing film, the assembly constituting the operculum strip,
• in parallel, a support band is prepared,
• the support strip is then secured temporarily to the cover by means of a temporary adhesive, to form the joint strip,
• the joints of the desired shape are cut out over the entire thickness of the joined strip with regard to the tongues.

In known manner, the support strip is manufactured by association of films or foam materials constituting support, in particular those described above.

In a preferred embodiment, the complex consists successively of an upper reinforcing film, a sheet of conductive material and a lower reinforcing film.

By producing the tongue in the entire thickness of the complex, it eliminates additional steps, at the time of manufacture, including the preparation of a separate complex including for example aluminum / heat sealing film that would not be part of of the tongue.

According to the invention, the heat-sealing film is advantageously extruded or coextruded on the lower face of the complex.

• La figure 1 est une représentation vue de dessus de l'opercule de l'invention, en particulier sa languette en comparaison avec les opercules de l'art antérieur, la languette étant dépliée (la, 1c) ou repliée (1b, 1d).
• La figure 2 est un autre mode de réalisation de la languette de la figure 1c.
• La figure 3 est une représentation en coupe du joint de l'invention, avant solidarisation de la partie opercule et de la partie support selon un mode de réalisation préféré.

The invention and the advantages which result therefrom will become more apparent from the examples which follow, in support of the appended figures.

• The figure 1 is a top view of the lid of the invention, in particular its tab in comparison with the lids of the prior art, the tongue being unfolded (la, 1c) or folded (1b, 1d).
• The figure 2 is another embodiment of the tongue of the figure 1c .
• The figure 3 is a sectional representation of the seal of the invention, before securing the cap portion and the support portion according to a preferred embodiment.

The invention lies in the combination, in a seal (support + cap), a specific form of tongue, the presence of aluminum in said tongue and the absence of heat seal layer always in the tongue.

The figure 1 more particularly relates to the shape of the tabs of the cover. The Figures 1a and 1b illustrate the shape of the tabs known from the prior art whereas the Figures 1c and 1d relate to the shape of the tongue in a preferred embodiment of the invention.

The covers are each designated by the general reference 1. They each comprise the lid itself (2) of generally circular shape and the tongue (3). The lids are more precisely constituted by a heat-sealing film (4) and an aluminum complex / upper reinforcing film (5). The figures 1b, d distinguish between the two heat sealing film (4) and complex (5) elements after folding the tongue (3). On the other hand, the exact structure of the complex (upper reinforcement / aluminum) is not visible in these figures.

More specifically, on the figure 1a , the heat-sealing film (4) and the complex (5) are superimposed and therefore have the same surface. In this case, the tongue originates at the periphery (6) of the lid and extends directly to the outside of the lid. The length (l ₁ ) of the fold (7) during folding is in this case equal to the distance (L ₁ ) separating the two ends (6) of the tongue. As shown in figure 1b the arrow (F ₁ ) is long which does not allow to benefit from a satisfactory sealing of the heat-sealing film without increasing the induction power.

On the figures 1c and Id, there is shown a preferred form of the lid according to the invention. The tongue itself represented on the figure 2 consists of a gripping zone (8) of length A and a base (9) of length B. The base (9) of the tongue emerges from a recess in rounded (10) recesses oriented towards the outside, made in the complex (5), by definition set back from the periphery of the heat-sealing film (4). The fold (7) is formed tangentially to the junction between the notch (10) and the base (9). As it appears in these figures, the fold (7) has a length (l ₂ ) which is less than the length (L ₂ ) of the notch (10). This allows, after flipping the tongue, to have an arrow F ₂ shorter length relative to the arrow F ₁ of the figure 1b illustrating the prior art. According to the embodiment of the figure 2 the opposite side edges of the base (9) have an outwardly oriented hollow shape so that the distance (l ₃ ) separating the two side edges is smaller than the length (1 ₂ ) of the fold. This configuration promotes the resistance of the tongue at the time of tearing of the lid. To enable a good grip, the distance l ₄ between two opposite edges of the gripping area is greater than the distance l ₂ of the fold (7).

We have shown on the figure 3 , a section of a seal according to the invention in a preferred embodiment.

The seal defined in this figure consists of a support (11) and a cover (12) provided with a tongue (13). The operculum consists of two elements respectively a complex (14) and a heat-sealing film (15). The complex (14) itself consists of an upper reinforcing film (16), an aluminum foil (17) and a lower reinforcing film (18). The two reinforcing films are made of PET, that is to say of the same chemical nature.

• de maitriser la force de collage de l'adhésif temporaire sur l'opercule de manière homogène entre la partie surface de l'opercule et la partie languette retournée,
• de disposer d'une force d'adhésion suffisante entre l'adhésif temporaire et la surface de l'opercule, y compris la partie languette retournée pour entrainer la séparation entre l'opercule et le support sans risquer de détériorer l'ensemble.

This thus makes it possible to have two chemically identical surfaces in contact with the temporary adhesive, which has the effect of:

• to control the adhesive force of the temporary adhesive on the lid in a homogeneous manner between the surface portion of the lid and the inverted tongue portion,
• to have a sufficient adhesive force between the temporary adhesive and the surface of the lid, including the tongue portion returned to cause separation between the lid and the support without the risk of damaging the assembly.

The heat-sealing film (15) is in turn made of polyethylene and applied on the underside of the complex (14) by extrusion or coextrusion. The total thickness of the lid is about 100 micrometers.

The support comprises a layer (19) made of expanded polypropylene or polyethylene and has a thickness of between 1.4 and 1.7 mm.

As shown in this figure, the tongue (13) is devoid of heat-sealing layer (15) and arranged recessed relative to the end of the heat-sealing layer, which corresponds to the depth of the notch (10) made in the complex (14).

Once folded, the tongue (13) comes into contact with the temporary adhesive layer (20) applied to the underside of the support (11).

The temporary adhesive is in the form of an extruded film of the polyethylene type, each of the two faces of the film having a different adhesive capacity, the most tacky face being applied to the lower surface of the support, while the lesser face sticky is applied on the upper face of the operculum.

In practice, these seals are positioned as such at the bottom of the cap, the cap is then screwed or clipped, as appropriate, on the neck of the container. Heat sealing of the seal is done by induction.

As regards the process, the cover is first manufactured. To do this, a strip of complex reinforcing film / aluminum foil / lower reinforcement film is prepared. Spotted manner, it carries out in this complex a precut tab extending at its base by a notch so that the length of the fold of the tongue when folded is less than that of the notch. The entire surface of the tongues is then mechanically returned once to the upper face of the complex. The heat-sealing film is then extruded on the lower face of the complex, the assembly constituting the sealing strip. In parallel, a support tape is prepared. The support strip is then temporarily secured to the lid by extrusion of the temporary adhesive to form the joint strip. The joints of the desired shape are cut out over the entire thickness of the joined strip with respect to the tongues.

• éviter le risque de délamination des couches constitutives de l'opercule au moment de l'arrachage de celui-ci,
• favoriser l'adhésion en mettant en contact avantageusement des couches de même nature,
• faciliter le procédé.

The invention and the advantages derived therefrom are apparent from the foregoing description. In particular, the advantage of making a tongue seal of specific shape containing an aluminum foil and devoid of heat-sealing film, the tongue being folded over the entire thickness of the joint, so as to:

• to avoid the risk of delamination of the layers constituting the cap at the time of tearing it off,
• promote adhesion by advantageously bringing into contact layers of the same nature,
• facilitate the process.

Claims (12)

1. Heat-sealable seal (1) including a support (11) bonded to an inner seal (12) by means of a temporary adhesive (20), said inner seal (12) being fitted with a pull-tab (13) the entire surface of which is folded back once over the surface of the inner seal in contact with the corresponding surface of the support, the inner seal comprising a laminate (14) including a reinforcing layer (16) and a sheet of a conducting material (17), to the lower surface of which is applied a heat-sealing film (15), the temporary adhesive (20) being applied to the whole surface of the inner seal once the pull-tab (13) is folded back, characterised in that the laminate (14) has over its entire thickness a notch (10) from which the pull-tab (13) emerges, the fold (7) formed by the pull-tab (13) once it is folded back having a shorter length (l₂) than the length of the notch (L₂).
2. The seal as claimed in claim 1, wherein the inner seal consists exclusively of the laminate (14) and the heat sealing film (15).
3. Seal as claimed in claim 1 or 2, characterised in that the pull-tab (13) comprises an upper reinforcing layer (16), a sheet of a conducting material (17), and a lower reinforcing layer (18) laminate and in that the reinforcing layers are plastic films of the same chemical type.
4. Seal as claimed in claim 3, characterised in that the reinforcing layers are made of PET.
5. Seal as claimed in one of the previous claims, characterised in that the notch (10) has at each of its ends (6) a hollow area facing outward from the inner seal, each hollow area having a radius of curvature of between 0.3 and 5 mm, and advantageously of between 0.5 and 2 mm.
6. Seal as claimed in one of the previous claims, characterised in that the notch (10) has a length (L₂) of between 3 and 25 mm, and preferably between 3.6 and 20 mm.
7. Seal as claimed in one of the previous claims, characterised in that the fold (7) has a length (l₂) of between 3 and 15 mm, and advantageously between 5 and 10 mm.
8. Seal as claimed in one of the previous claims, characterised in that the pull-tab (13) is formed of two parts, a base (9) and a grasp area (8) respectively, the distance (l₃) separating the lateral edges of the base (9) being smaller than the length of the fold (l₂).
9. Seal as claimed in claim 8, characterised in that the distance (l₄) separating the two opposite lateral edges of the grasp area is greater than the length of the fold (l₂).
10. Seal as claimed in one of the previous claims, characterised in that the temporary adhesive (20) has two surfaces with different bonding power, the most adhesive surface being applied to the lower surface of the support, whereas the least adhesive surface is applied to the upper surface of the inner seal (12).
11. Method of producing a heat-sealable seal (1) including a support (11) bonded to an inner seal (12) by means of a temporary adhesive (20), the inner seal (12) comprising a laminate including a reinforcing layer (16) and a sheet of a conducting material (17), to the lower surface of which is applied a heat-sealing film (15), according to which:

    - the laminate is prepared,

    - in register, a pull-tab (13) pre-cut is made in the laminate extending at its base by means of a notch (10) in such a way that the length of the fold (7) of the pull-tab (13) once folded back is shorter than that of the notch (10),

    - the surface of the pull-tab (13) in its entirety is folded back once mechanically on the upper surface of the laminate,

    - the lower surface of the laminate is then associated permanently with the heat-sealing film (15), the assembly constituting the inner seal strip (12),

    - at the same time, a support strip is prepared (11),

    - the support strip (11) is then temporarily bonded to the inner seal (12) by means of a temporary adhesive (20), to form the seal strip,

    - the seals are cut out of the entire thickness of the seal strip to the required shape in register with the pull-tabs.
12. Method as claimed in claim 10, characterised in that the laminate comprises successively an upper reinforcing layer (16), a sheet of a conducting material (17) and a lower reinforcing layer (18).

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