HRP930115A2 - Laminated metal sheets - Google Patents

Description

This invention relates to a laminated sheet and to the process of manufacturing such laminar sheets.

The lamination of polymeric materials into sheets, such as metal strips, is a well-known and well-documented technique. The resulting laminates have many applications, including applications for making pull-out containers and containers with iron walls (designated as DWI containers).

It is known that iron or aluminum coated with polyolefin coatings are used as material for the production of DWI containers. Such materials are described, for example, in U.S. Pat. patent 4096815 and British patent 2003415 and, as far as we know, this material has not yet found commercial application.

We have found that polyolefin coatings cannot be molded as well as thermoplastic polyesters. Lamination of polyester coatings on steel and aluminum is described, for example, in British Patents 2123746 and 2164899. However, these patents emphasize that maintaining a certain biaxial orientation in the polyester coating contributes to adequate pot life. We have discovered that laminates such as those described in these patents cannot be subjected to the molding processes used to produce deep draw or DWI containers without seriously damaging the existing polyester coating. The retained orientation of the coatings, described in British Patents 2123746 or 2164899, limits the elongation at break value of the coating to relatively low values, which are exceeded by DWI vessel forming procedures. This kind of laminates are therefore unsuitable for forming deep drawing or DWI vessels.

Description of the solution to the technical problem with Implementation examples

We have now found that a thermoplastic polyester-bonded sheet laminate, which is generally in a non-crystalline (or amorphous) form, can withstand DWI procedures and retain an acceptable metal coating. These coatings outperform polyolefin coatings in forming DWI containers and retain better continuity and protection.

Accordingly, from one point of view, the present invention provides for the lamination of a sheet, on one or both sides of which a film of non-crystalline polyester is directly applied.

Non-crystalline polyester (also referred to as amorphous polyester) can be largely unoriented, as determined by X-ray diffraction or density measurement.

The method for measuring crystallinity using X-ray diffraction is described in GB 1566422. Crystallinity can also be measured by measuring density in the following way. Vc == (P – Pa) (Pc – Pa)-1 where

Vc is the crystallinity of the volume fraction,

P is the density of the sample,

So the density of the amorphous material is,

Pc is the density of the crystalline material.

Density can be measured in a solution of zinc chloride and water or n-heptane/carbon tetrachloride using a density column.

Non-crystalline polyester is usually polyethylene terephthalate (PET) or polybutylene terephthalate (PBT). PET materials typically have an intrinsic viscosity between 0.5 and 1.1 when measured in o-chlorophenol at 25°C and a concentration of 5 g/l.

Non-crystalline polyester, a film applied to a sheet, is obtained by laminating a film containing polyester to a sheet, under conditions in which the lamination is carried out so that during lamination the polyester film or films in the metal/polymer laminate are in a non-crystalline (amorphous) form, or if they are not, they need to be translated into an amorphous form.

It is best if each of the main sheet surfaces carries a film of non-crystalline polyester, as already described. However, the subject of the invention includes a sheet with non-crystalline polyester on one of the main surfaces and a layer of different thermoplastic polymer films on the other of the main sides of the sheet.

The metal substrate on which the polymer films are applied usually takes the form of a metal strip and is usually made of steel or aluminum or their alloys and is mainly used for steel or aluminum based containers in the packaging industry.

The measurement range is usually 0.05 to 0.4 mm for steel and 0.02 to 0.4 mm for aluminum; it is typically 0.25 to 0.35 mm for steel and aluminum DWI vessels.

The steel may be tin-plated and, if possible, also passivated with the usual chrome treatment, or alternatively, it may be in the form of steel with a galvanic layer of nickel or zinc, black iron sheet or phosphated black iron sheet, which must be rinsed with chromate after phosphating.

A better steel finish is electrolytically chrome coated steel (ECCS) with a double layer of metallic chromium and chromium oxide. In such steels, the contents of metallic chromium and chromium oxide can vary widely. Typically, chromium metal content ranges from 0.01 to 0.20 g/m2, while chromium oxide ranges from 0.005 to 0.05 g/m2. ECCS are usually made from precipitation systems with catalysts containing either sulfur or fluorine.

Aluminum, which is used, is best in the form of 3004 type alloy, machined or rolled ("milled"), or cleaned and oiled, or cleaned and then chromated, or chromate-phosphated and then oiled. For example, Alocrom A272 is a proprietary chromate-phosphate treatment system for aluminum strips.

A number of different types of polyester film can be used to prepare metal-polymer laminates. Typical polyester materials for use in preparing the metal polymer laminates of this invention are:

i) cast thermoplastic polyester, such as polyethylene terephthalate or polybutylene terephthalate,

ii) biaxially oriented polyester films that have a semi-crystalline structure, usually biaxially oriented polyethylene terephthalate,

iii) cast coextruded composite polyester film,

iv) composite coextruded polyester film containing:

(A1) inner layer of mainly non-crystalline linear polyester, whose softening temperature is below 200°C and melting point below 250°C, but above 150°C and

(A2) outer layer of biaxially oriented linear polyester, whose crystallinity is greater than 30%.

When using a coextruded polyester film, it is better to use a film that has a thinner inner layer (A1) and a thicker outer layer (A2).

Usually the outer layer (A2) is PET homopolymer. It is good that its internal viscosity is between 0.5 and 1.1 or even better from 0.6 to 0.7, and for biaxially oriented film and greater than 0.9 for cast film.

The thinner inner layer (A1) is usually a mostly non-cryslinic linear copolyester of 80% ethylene terephthalate and 20% ethylene isophthalate. Alternatively, the inner layer is a largely non-crystalline copolyester produced from terephthalic acid and two dibasic alcohols, such as ethylene glycol and cyclohexane-dimethanol.

If necessary, the polyester layers can be colored, for example, with anti-blocking agents, such as synthetic silica or pigments that give a color or white appearance, for example, titanium dioxide. It is particularly suitable to paint layer A2 with titanium dioxide on the outer surface of the beverage container made of laminate.

It is preferable that the outer layer (A2) be made of co-extruded polyethylene terephthalate. Preferably, the inner amorphous layer (A1) is a linear copolyester, for example, an amorphous copolymer of approximately 80% ethylene terephthalate and approx. 20% ethylene isophthalate. Copolyesters of terephthalic acid and two alcohols, for example ethylene glycol and cyclohexane-dimethanol, are suitable for use as an internal amorphous layer (A1).

When the coextruded film is biaxially oriented, the crystallinity of the outer crystalline layer (A2) is usually 50%, but can be reduced to 40% or less if the biaxial orientation of the crystalline polymer is reduced.

A biaxially oriented film can be formed by stretching the amorphous extruded polymer in the direct direction at temperatures around the glass transition temperature of the polymer by a factor of 2.2 to 3.8 and, similarly, in the transverse direction by a factor of 2.2 to 4.2.

The laminar sheet of the present invention is prepared by a process which comprises bonding a film, containing polyester, to one or both main surfaces of the sheet, wherein the lamination conditions are such that during lamination the polyester film or films in the metal/polymer laminate are converted into a non-crystalline or amorphous form.

In a more convenient method for preparing metal/polymer laminates according to the present invention, a polyester monolayer film or films are glued to a sheet by heating the sheet to a temperature (T1) above the melting point of the polyester films, wherein the temperature (T1) is such that, during the lamination of the polyester films on the sheet the outer surfaces of the polyester films remain below the melting point by laminating the film or films onto the sheet, by reheating indirectly through the laminate to a temperature (T2) above the melting point of the polyester films and, after holding at these elevated temperatures, by rapidly cooling the polyester-coated metal to a temperature below glass transition of polyester resins as a coating.

In another, more favorable process, the polyester film or films are composite films (A), consisting of an inner layer (A1) and an outer layer (A2), and the composite polyester films are simultaneously applied to the sheet by a process consisting of:

i) heating the sheet to a temperature (T1) above the softening temperature of the polyester inner layer (A1), but below the melting point of the outer layer (A2),

ii) lamination of the film or films on the sheet,

iii) reheating through the laminate, so that the sheet reaches a temperature above the melting point of one or each polyester film and

iv) after keeping above this elevated temperature, sudden cooling of the metal coated with polyester to a temperature below the glass transition temperature of the polyester resins in the coating.

Preferably, said composite polyester films are coextruded polyester films, consisting of:

(A1) of the inner layer, mainly non-crystalline linear polyester, whose softening temperature is below 200°C and melting point below 250°C, but above 150°C and

(A2) of the outer layer of polyester, whose melting point is above 220°C, whereby polyesters have an internal viscosity of 0.5 to 1.1 measured in o-chlorophenol at 25°C and a concentration of 5 g/l.

It is best to reheat the metal/polymer laminate behind the lamination bath using induction heaters, but infrared heating can also be applied.

The temperature to which the sheet should be heated before lamination depends on the thickness of the films to be laminated and on the chemical nature of said films. Uncoated metal can be processed directly or indirectly, for example by induction, infrared heating, hot air or hot rolling.

Temperatures of 140 to 350°C are suitable for coextrusion of biaxially oriented PET film, 130 to 250°C for cast coextruded polyester film, 260 to 350°C for biaxially oriented PET monofilm of high crystallinity or 200 to 300°C for PET film of low crystallinity. , and above 180°C for cast PET monofilm.

The temperatures applied to reheat the laminate behind the lamination bath are typically above 270°C for polyethylene terephthalate and 240°C for polybutylene terephthalate. Commercial procedures usually require a delay of about 2 seconds between reheating and cooling procedures. Cooling is uniform and rapid, and can be carried out with curtains of cold water directed at the strip. To prevent crystallization of the polyester, the laminate must be cooled to a temperature of approx. 190°C. To prevent blistering, the coating must be cooled below the melting point.

The laminates of this invention are particularly suitable for forming pull-out cans and iron-walled (DWI) cans. The usual processes for making DWI tin cans without organic coatings include the following stages:

1.- lubrication of tin or aluminum sheets,

2.- cutting a disc from sheet metal,

3.- placing the disc on the circular mold and fixing it with a cylindrical ring,

4.- pulling the punch through the mold while controlling the movement of the sheet with an elastic holder,

5.- forming a narrow cup from metal by inserting a punch through the mold until all the metal passes through the mold and removing the punch,

6.- transferring the glass to the punch whose radius is equal to the radius of the desired vessel,

7.- pulling out the cup again and pulling the punch and the cup through a series of concentric rings, each having a progressively smaller inner diameter and so that the space between the punch and the mold is smaller than the thickness of the cup material,

8.- the wall of the glass is strengthened and lengthened,

9.- stopping the shaped can and removing the punch,

10.- cutting off excess material from the top of the can wall,

11.- rinsing the can to remove the lubricant and, in the case of aluminum, removing the metal deposit,

12.- rinsing and drying the can.

After rinsing the aluminum beverage can, the following procedures are usually followed:

13.- Chemical treatment of the surface,

14.- rinsing and drying of the transport oven,

15.- applying the basic coating from the outside,

16.- processing of the basic coating,

17.- applying printed decorations,

18.- processing of printed decorations,

19.- applying the inner coating (by spraying),

20.- treatment of the inner coating,

21.- putting the sleeve and flange on the can, reducing the diameter of the sleeve to a value that coincides with the lid and putting the flange for double closing.

Alternatively, if a selective base coat is applied, the conventional printing process can be replaced by a dye sublimation printing process, as described in GB 2101530, 2145971, 2141382, 2010529 and 2147264. As the base coat is processed, the dye-impregnated paper label, it can be sublimated, wrapped around the can and held to it by a small amount of glue on the paper folds. The can passes through an oven, where the temperature is above the dye sublimation temperature, and printing is done without the use of solvents. The label can be removed with an air nozzle, leaving a printed can with outstanding print quality. This is a solvent-free process and is completely emission-free to the atmosphere.

Drawable and reinforced cans (DWI cans) made from the laminated material of this invention can be decorated and printed with conventional solvent based inks after forming the DWI can.

The heat-reactive polyester coating will quickly accept the sublimated dye and the dye transfer process from paper labels to DWI cans is commercially viable. We have discovered that thermoplastic polyester coatings on sheet metal receive sublimated dye. However, the high quality of the decoration from the paper label is only achieved if the orientation in the PET coating is maintained. If the coating is amorphous, either melted in the lamination process or made from a non-oriented film, the paper label sticks to the coating during the sublimation step and is a decoration.

Sublimation from a paper label is done by closely touching the paper and the coating and heating it to a temperature above 160°C, usually up to 220°C. Under these conditions, unoriented PET is above its glass transition temperature (Tg), relatively soft, and attaches to the paper. If the outer part retains the biaxially oriented coating, the paper will not stick to the polyester during dye sublimation. The outwardly oriented material in contact with the paper has a modified thermal behavior and its effective glass transition does not occur during dye sublimation.

The difficulties associated with dye transfer to amorphous coatings make dye sublimation in DWI cans coated with amorphous polyester look awkward. Unexpectedly, we have found that DWI cans made from amorphous polyester coated laminates in accordance with the present invention can be successfully dye sublimated using standard labels and sublimation conditions.

The decal application must be slightly modified to avoid contact of the decal with approx. 2 mm of the can wall, which is marked with "d" in Figure 7a of the attached drawings. If this procedure is followed, the paper will not stick to the coating. Generally speaking, an amorphous polyester coating will stick to paper if they come into contact above its Tg. However, can forming processes introduce orientation into the polyester laminate coatings of this invention and therefore raise the actual Tg. The amount of orientation introduced is relatively small at the top of the can wall and is very different for the inner and outer coatings, so it is surprising that it produces the beneficial effect of preventing paper sticking.

The laminates described in this invention can be used to make DWI cans while maintaining excellent integrity and adhesiveness. In addition, the coated containers can be decorated by conventional printing or dye sublimation process.

The laminates of this invention can also be used to make other packaging parts, especially non-retorted parts. Typical examples of these other parts are:

- Re-draw drawing of beverage cans 54 mm diameter and 70 mm high made of 0.21 mm ECCS, 350 N/mm2.

- Notched lids for beverage cans that are easy to open, made of 65 mm steel or aluminum.

- Integral neck of rectangular ends for rectangular vessels.

- Colored parts of cans, such as rings, ends and lids.

- Parts for aerosols, such as cones and domes.

The main advantages of this invention are:

- The elimination of all solvent emissions was practically achieved and environmental protection was improved.

- Large can washers can be replaced by smaller washers, and this saves on the costs of washing chemicals.

- Energy consumption is reduced by reducing the number of passes through the furnace, which are required to complete the can.

- The external protection of the base has been improved.

- The internal protection of the complex base profile has been improved.

- The use of lubricants can be omitted when making cans.

- The size and installation costs of the factory can be reduced, as well as labor costs.

- Excellent quality of external printing.

During this specification, internal viscosities were measured in o-chlorophenol solutions at a concentration of 5 g/l.

This invention will now be described in detail, exclusively by means of examples and attached drawings, of which:

figures 1 and 2 show schematics of devices suitable for carrying out the process according to the present invention,

Fig. 3 shows a cross-section of a laminate according to the present invention, comprising a monolayer polymer film (A) applied to a metal strip (M),

Fig. 4 shows a cross-section of a laminate similar to the laminate in Fig. 3, but having a complex multi-layer polymer film (A) applied to a metal strip (M),

figure 5 represents a section of a laminate similar to the laminate of figure 4, but containing an additional film (B) of thermoplastic polymer applied to the opposite sides of the metal strip (M),

Figure 6 shows a can finish formed from a laminate in accordance with the present invention and

Figures 7a and 7b show the drawing and strengthening of the can and the drawing of a redrawn can made of laminate in accordance with the present invention.

Examples 1 to 13

Polymer/metal/polymer laminates are prepared by the lamination process, which is carried out in the device shown schematically in Figures 1 or 2 of the attached drawings. The sheet metal is pre-heated by infrared or induction heating to the appropriate temperature. T1 using heater 1. The temperature T1 is usually between 140 and 350°C. Polyester films A and B are fed with feed rollers 2 and 4 and laminated on opposite sides of the preheated sheet between laminating rollers 6 and 8, the diameter of which is usually 100 - 400 mm. Lamination is usually done by applying a tension force of 200 - 400 N/m between the laminating rollers.

With lamination tightening, a uniform and tight contact is established, without wrinkling, between the sheet and the polymer films. Behind the laminating rollers, the resulting laminate is reheated, preferably using an induction heater 10 or infrared, to the lamination temperature T2 at which the polymer films (A) react and form a solid bond with the sheet. The T2 temperature is usually in the range of 220 to 270°C for PBT and 260 to 300°C for PET. The metal/polymer laminate is held at or below T2 temperature for a short time, typically no more than 2 seconds, and then rapidly and uniformly cooled with water to a temperature below the glass transition temperature of the polyester in the films, for example at approx. 80°C for PET. Cooling can be carried out in the usual way, but is usually carried out by passing the laminate through a tank 12 with water, shown in Figure 1, or by passing the laminate through a curtain 14 of cooling water, as shown in Figures 1 and 2.

In principle, the procedure shown in Figure 1 with vertical lamination is a better solution. The vertical movement of the metal strip through the lamination stage contributes to a faster cooling rate and provides better and more even cooling.

Figure 1 also schematically shows a diagram of a typical temperature profile that may be encountered in the process and with the device shown in Figure 1.

Thus, laminates are obtained from the materials listed in Table I by preheating the metal strip using infrared or induction heating, passing the metal strip and polymer films through a pair of tensioning rollers and laminating both main metal surfaces simultaneously with the polymer films. The resulting laminate is reheated using infrared or induction heaters, held above 200°C for two seconds, and rapidly and evenly cooled with water.

Table II shows a number of examples with results obtained when these laminates are produced using different metal temperatures (T1) in the pre-lamination stage and different reheat temperatures (T2) in the post-lamination stage.

Table I. Laminate types.

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Markings in table I

PET composition - type I: Co-extruded cast PET composite film that has:

i) the inner layer, which is a copolyester of terephthalic acid with ethylene glycol and cyclohexane-dimethanol i

ii) the outer layer, which is a PET homopolymer with an internal viscosity greater than 0.9.

PET composition - type II: Same as PET composition - type I, but TiO2 pigment was additionally introduced into the outer layer.

PET composition - type III: Coextruded biaxially oriented PET composite film that has:

i) the inner layer which is a copolyester of terephthalic acid and isophthalic acid with ethylene glycol i

ii) the outer layer, which is a PET homopolymer with an intrinsic viscosity of approximately 0.6 to 0.7.

PET monofilm: A monofilm of co-extruded biaxially oriented PET having an intrinsic viscosity of approx. 0.6 to 0.7.

PBT monofilm: Monofilm of cast polybutylene terephthalate (PBT).

PP composition - type I: Cast co-extruded polypropylene composite film that has:

i) inner layer of polypropylene modified by grafting with maleic anhydride i

ii) outer layer of polypropylene.

PP composition - type II: Same as PP composition - type I with the addition that it has an outer layer that is colored with TiO2 and with synthetic silicon dioxide.

Al 3004 alloy: Aluminum alloy 3004, whose surface is treated with chromate-phosphate (Alocrom A272).

Al 3004 alloy

milled: Aluminum alloy 3004 untreated and untreated after cold rolling.

Table II.

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The molding ability of the laminate is assessed by covering it with a coating after drawing and strengthening the laminate wall in two stages:

Stage 1: The glass (height 35 mm and diameter 86 mm) is pulled out of the laminate, lubricated in a suitable way.

Stage 2: The body of the can (diameter 65 mm and height 130 mm) is shaped by drawing again and strengthening the wall.

After molding, the cans were washed in water and dried. The coating was checked by immersion in an acidified copper sulfate solution for 2 minutes and visual inspection of copper deposition or the "enamel evaluation" procedure using a sodium chloride solution, voltage of 6.3 V and current measurement in milliamps.

The influence of lamination temperatures on the structure of the polyester coating and the ability to shape the laminate was verified using X-ray diffraction. According to this procedure, the film or laminate is placed in an X-ray diffractometer. The count rate was measured when a flat sample was placed in a beam of monochromatic X-rays using a suitable detector. The sample and detector rotate together relative to the beam, maintaining a geometry such that the sample-to-beam (0) and beam-to-detector angles remain 1:2, as in normal powder diffraction imaging. These data provide information on planes parallel to the surface of the sample.

In the biaxially oriented film, the plane (1,0,0) gives a large count-off at 0 = 13°, but in amorphous PET the peak does not appear.

The relationship 0 = 13°, the height of the peak for the laminate and the film depends on the amount of retained orientation in the laminate. Our results are presented as the ratio of the peak height to the peak height of the laminated PET.

Laminate material B, laminated in accordance with GB 2123746, to retain orientation (see example 4) has little formability and results in making cans without cracking the metal or seriously damaging the coating. However, when laminate B is made to remove orientation and crystallinity, as in Example 3, then it has excellent formability and still good protection after forming.

Similarly, biaxially oriented coextruded laminates C and J also give good formability if they are amorphous and little formability if they retain their orientation in the laminate (compare Example 5 with Example 6 and Example 13 with Example 14).

Coatings of cast non-oriented PET or PBT were effective, provided they were laminated to give an amorphous rather than crystalline structure. The crystalline structure is achieved, for example, by a slight cooling after the reheating phase.

Examples 11 and 12 show that laminates formed from polypropylene materials of the type described in GB 2003415 exhibit poor formability. Such materials have been found to cause damage to the metal when the can is being formed.

The degree of reorientation of polyester coatings was checked by testing tensile and reinforced cans made of laminate under the conditions of example 5. The following results were obtained:

Sample location XRD (0 =13°) peak Can base - inner coating <50

Base of the can - outer coating <50

Top of can wall - inner coating 100

Top of can wall - outer coating 450

The XRD data confirm that the laminated film has an amorphous coating and shows that the upper wall of the coating is oriented, more so at the outer coating of the can.

The bottom 2 mm of the coating on the outer wall of the can was not significantly attacked during the forming of the can and remained mostly amorphous.

The outer walls of the DWI cans, formed from the laminates of examples 1 to 13, are decorated with the usual dye sublimation process. The quality of the transferred decoration was excellent, provided that the label was not in contact with the bottom 2 mm of the can wall, area "d" of Figure 7a.

Claims (13)

1. A process for preparing a laminated sheet according to any of the preceding claims, characterized in that it is a process that includes bonding directly to one or both main surfaces of a sheet of film containing polyester, wherein the lamination conditions are such that during lamination the polyester film or the films in the laminate transform the metal/polymer into a non-crystalline or amorphous form. 2. The method according to claim 1, characterized in that the polyester film or films are monofilms, which are applied to the sheet by heating the sheet to a temperature (T1) that is above the melting point of the polyester films, wherein the temperature (T1) is such that during lamination of polyester films on the sheet, the outer surfaces of the polyester films remain below their melting point with the lamination of the film or films on the sheet, by reheating, indirectly, the laminate to a temperature (T2) above the melting point of the polyester films and, after holding at these elevated temperatures, by rapidly cooling the polyester-coated metal to temperatures below the glass transition temperature of polyester resins as coatings. 3. The method according to claim 1, indicated by the fact that the polyester film or films are actually composite films of the inner layer (A1) and the outer layer (A2) and the composite polyester films are simultaneously applied to the sheet by a process consisting of: i) heating the sheet to a temperature (T1) above the softening temperature of the polyester inner layer (A1), but below the melting point of the outer layer (A2), ii) lamination of the film or films on the sheet, iii) reheating through the laminate, so that the sheet reaches a temperature above the melting point of one or each polyester film and iv) after keeping above this elevated temperature, sudden cooling of the metal coated with polyester to a temperature below the glass transition temperature of the polyester resins in the coating. 4. The method according to claims 2 or 3, characterized in that the temperature to which the laminate is reheated after lamination is from 220 to 300°C. 5. The method according to claims 2, 3 or 4, characterized in that the means for indirectly heating the laminate is actually an induction heater. 6. The method according to any one of claims 1, 2, 4 and 5, characterized in that one or each polyester film is actually cast thermoplastic polyester. 7. The method according to claim 6, characterized in that the molded thermoplastic polyester is actually polyethylene terephthalate or polybutylene terephthalate. 8. The method according to any one of claims 1, 2, 4 or 5, characterized in that one or each polyester film is actually a biaxially oriented polyester having a semi-crystalline structure. 9. The method according to claim 8, characterized in that the biaxially oriented polyester is polyethylene terephthalate. 10. The method according to any one of claims 3 to 5, characterized in that one or each of the polyester films is actually a coextruded cast polyester film composition. 11. The method according to any one of claims 3 to 5, characterized in that one or each of the polyester films is a complex co-extruded cast polyester film consisting of: (A1) inner layer, mainly e-crystalline linear polyester, whose softening temperature is below 200°C and melting point below 250°C, but above 150°C and (A2) of the outer layer of biaxially oriented linear polyester having a crystallinity greater than 30%. 12. The method according to claim 11, characterized in that the linear polyester of the inner layer (A1) is actually an amorphous copolymer of ethylene terephthalate and ethylene isophthalate or a copolymer obtained from terephthalic acid and two divalent alcohols. 13. The method according to claims 11 or 12, characterized in that the outer layer (A2) is biaxially oriented polyethylene terephthalate.