Multi-layer composite film

Abstract

NOT VISIBLE DUE TO STATUS OF PATENT

Description

[0001] Multi-layer composite film

[0002] The present invention relates to multi-layer composite films as well as to methods for producing multi-layer composite films. The invention particularly concerns films of this type, which have a cellulose-based substrate. The substrate may particularly be a paper substrate, wherein the films have at least some barrier properties against liquid and / or gaseous substances, such as water and / or oxygen.

[0003] Multi-layer films of this type are well known in the art in a large range of varieties. They are for example used for food packaging applications, where especially the barrier properties against oxygen are of utmost importance. Films with cellulose-based substrates are increasingly used to substitute full-plastics barrier films, in an attempt to reduce plastics waste and meet regulatory specifications in relation to the use of plastics.

[0004] Some known multi-layer compound materials containing cellulose-based substrates along with one or more plastics barrier layers offer good barrier properties, whilst being at least partially recyclable. Due to environmental and regulatory aspects, there is the need to further reduce the amount of plastics in such materials. However, a simple reduction of the amount of barrier material used typically leads to a reduction of barrier properties.

[0005] An object of the present invention may therefore be seen in providing a multi-layer composite film, which is low in plastics, but still offers excellent barrier properties. Another object may be seen in providing a multi-layer composite film with barrier properties against water and / or oxygen, which is recyclable.

[0006] As mentioned above, the multi-layer composite film may particularly comprise a cellulose-based substrate, on a first surface of which a coating is provided. The coating particularly comprises or consists of three layers, wherein a first and a third layer are polymer layers, such as heat sealable polymer layers, and wherein the second layer is a barrier layer, which comprises or consists of a barrier polymer. The second layer is typically arranged between the first layer and the third layer.

[0007] The second layer preferably is a dispersion coated layer.

[0008] In the course of the present invention it has been surprisingly found that provision of this feature may aid in solving at least one of the above-mentioned objects. In particular, the second layer being provided as a dispersion coated layer surprisingly may allow for a very thin barrier layer, whilst maintaining sufficient barrier properties against water and / or oxygen.

[0009] Additionally, it has been found that the provision of the third layer may increase oxygen barrier properties in a surprisingly high manner as compared to coatings that only have a first and second layer.

[0010] The feature “dispersion coated layer” may designate a layer, which has been formed by dispersion coating. The second layer may comprise or consist of any suitable barrier polymer, with specific examples being ethylene vinyl alcohol copolymer (EVOH) and / or polyvinyl alcohol (PVOH).

[0011] It is preferably provided that the second layer, i.e., the dispersion coated barrier layer, is an inner layer of a multi-layer stack, which in total comprises at least three layers. This multi-layer stack may also be referred to as the coating of the substrate.

[0012] The first and third layer, between which the barrier layer is sandwiched, may comprise or consist of one or more heat sealable polymers, such as polyethylene (PE), in particular low-density polyethylene (LDPE); ethylene-vinyl acetate (EVA) copolymer; ionomer; terpolymer; olefin-acrylic acid copolymer, in particular ethylene-acrylic acid copolymer; olefin-(meth)acrylic acid copolymer, in particular ethylene-(meth)acrylic acid copolymer. Also other polymers or mixtures of two or more polymers may be suitable for use in the first and / or third layer.

[0013] The material of first and third layer may be the same, partially the same, or different.

[0014] The first and third layer are typically extrusion coated layers. The first and third layer may be single layers, or they may be formed of two or more sub-layers, typically produced by co-extrusion. Sub-layers may be of the same, partially the same, or different material.

[0015] It may be provided that the first and third layer are in direct contact with the second layer. Alternately, it may be provided that a tie layer is provided in between, such as in between the second layer and the third layer. The tie layer may comprise or consist of typically used tie polymers, which enhance the adhesion of different polymer types, such as barrier polymers and heat sealable polymers. An example for a tie polymer may be an anhydride-modified polyolefin or a reactive acrylate terpolymer. Films with a tie layer are particularly advantageous if the films are intended to be processed by vertical form fill seal (VFFS) techniques. Thus, in a specific embodiment, the coating disclosed herein may consist of three layers in the following order: first layer; second layer; third layer. In another specific embodiment, the coating disclosed herein may consist of four layers in the following order: first layer; second layer; tie layer; third layer. In both of these embodiments, the first and third layer each may be a single layer or at least one of first and third layer may consist of several co-extruded sub-layers.

[0016] Typically, the barrier layer of the film disclosed herein is thin, namely thinner than 3.0 g / m2The total grammage of the coating may be at most 23 g / m2

[0017] It is particularly preferred if the grammage of the coating is at most 20% of the grammage of the multi-layer film. In other words, the ratio between the grammage of the substrate to the grammage of the coating is at least 80:20 or higher, such as 85:15 or 90:10.

[0018] In all features referring to or including the grammage of the substrate, this grammage may be determined in accordance with ISO 536:2019. The grammage of one or more polymer layers can be determined by any suitable method known in the art.

[0019] Generally, the substrate may be a cellulose-based substrate, i.e., a material, which contains cellulose in a substantial fraction. One example for such material is paper, with kraft paper, in particular sack kraft paper, being preferred materials. Another example is cardboard.

[0020] The coating is particularly provided on one side of the substrate. The other (or second) surface of the substrate may be free of a barrier coating and / or a plastics-containing coating. In some embodiments, the second surface may be provided with a second coating, for example a clay coating. The second surface may also be uncoated.

[0021] Generally, if the second surface of the substrate is provided with a second coating, this second coating is independent of the coating disclosed herein.

[0022] Further disclosed is a method for producing a multi-layer film, in particular a multi-layer film as disclosed herein. In such method, a cellulose based substrate may be coated with a multi-layer coating.

[0023] The method may comprise the following steps, in particular in the following order: extruding a first layer; dispersion coating a second layer; extruding a third layer.

[0024] The term “dispersion coating” particularly refers to a process, where a polymer or a polymer precursor is provided in the form of a dispersion, typically a water-based dispersion. The dispersion coating may be performed by any suitable process, such as by roll coating, blade coating, curtain coating, or bar coating.

[0025] The first layer may be extruded directly onto a first surface of the substrate. The second layer may be dispersion coated directly onto the first surface. The third layer may be extruded directly onto the second layer.

[0026] An aqueous dispersion containing the material of the second layer may be used for dispersion coating. The dispersion may have a solids content of between 5% and 12%. The viscosity of the dispersion may be between 100 mPa*s and 1 ,000 mPa*s, in particular between 200 mPa*s and 700 mPa*s, determined by a Brookfield synchronized-motor rotary type device at a temperature of 22°C after the dispersion had been heated to a temperature of between 90°C and 95°C for approx. 45 min under stirring.

[0027] It may be provided that the method contains the following additional step: extruding a tie layer. This step may particularly be performed between the steps of dispersion coating the second layer and extruding the third layer. In such embodiments, the tie layer may be extruded directly onto the second layer and the third layer may be extruded directly onto the tie layer.

[0028] The first and / or third layer may be co-extruded, so as to be formed by multiple, such as two, three, or more sub-layers.

[0029] The invention may relate to a multi-layer composite film, comprising a cellulosic fiberbased substrate provided with a coating on a first surface thereof, wherein the coating comprises or is composed of: a first layer, wherein the first layer comprises or is composed of a heat sealable polymer; a second layer, wherein the second layer comprises or is composed of a barrier polymer, wherein the barrier polymer is preferably ethylene vinyl alcohol copolymer (EVOH) and / or polyvinyl alcohol (PVOH); a third layer, wherein the third layer comprises or is composed of a heat sealable polymer.

[0030] It may be provided that the second layer is arranged between the first layer and the third layer.

[0031] It may be provided that the second layer is a dispersion coated layer.

[0032] It may be provided that the coating is provided in an amount of at most 20 wt.-% in relation to the total weight of the film.

[0033] It may be provided that the second layer is provided in an amount of between 0.8 g / m2and 3.0 g / m2, preferably of between 1 .0 and 2.5 g / m2.

[0034] It may be provided that the coating is provided in an amount of between 7 g / m2and 23 g / m2.

[0035] It may be provided that the second layer is in direct contact with the first layer and the third layer, and wherein the coating is optionally formed of three layers.

[0036] It may be provided that a tie layer is provided, which tie layer is in direct contact with the second layer and the third layer, and wherein the coating is optionally formed of four layers.

[0037] It may be provided that the first layer is in direct contact with the first surface of the substrate, and wherein the first layer is in direct contact with the second layer.

[0038] It may be provided that the heat sealable polymer of the first layer and / or the third layer is selected from one or more of the following group: polyethylene (PE), in particular low- density polyethylene (LDPE); ethylene-vinyl acetate (EVA) copolymer; ionomer; terpolymer; olefin-acrylic acid copolymer, in particular ethylene-acrylic acid copolymer; olefin-(meth)acrylic acid copolymer, in particular ethylene-(meth)acrylic acid copolymer.

[0039] It may be provided that the substrate is paper, in particular kraft paper, with a grammage of at least 40 g / m2determined in accordance with ISO 536:2019.

[0040] It may be provided that the film has an oxygen transmission rate determined in accordance with ASTM F1927 at a temperature of 23°C and 50% relative humidity of below 10 mL / m2 / d, in particular below 5.0 mL / m2 / d, or below 1.5 mL / m2 / d.

[0041] It may be provided that the substrate is paper, in particular kraft paper, with a grammage of between 40 g / m2and 120 g / m2determined in accordance with ISO 536:2019, wherein the coating has three layers, and wherein the first layer is a polyethylene (PE) layer with a grammage of between 3.0 g / m2and 9.0 g / m2; the second layer is a polyvinyl alcohol (PVOH) layer or an ethylene vinyl alcohol copolymer (EVOH) layer, with a grammage of between 0.8 g / m2and 3.0 g / m2; and the third layer is a polyethylene (PE) layer with a grammage of between 3.0 g / m2and 11 .0 g / m2.

[0042] It may be provided that the substrate is paper, in particular kraft paper, with a grammage of between 40 g / m2and 120 g / m2determined in accordance with ISO 536:2019, wherein the coating has four layers, and wherein the first layer is a polyethylene (PE) layer with a grammage of between 3.0 g / m2and 9.0 g / m2; the second layer is a polyvinyl alcohol (PVOH) layer or an ethylene vinyl alcohol copolymer (EVOH) layer, with a grammage of between 0.8 g / m2and 3.0 g / m2; and the third layer is a polyethylene (PE) layer with a grammage of between 2.0 g / m2and 10.0 g / m2; wherein a tie layer is arranged between the second layer and the third layer, which tie layer has a grammage of between 1 .0 g / m2and 3.5 g / m2.

[0043] It may be provided that a second surface of the substrate, which second surface is opposite to the first surface, is free of a barrier coating, wherein the second surface is in particular uncoated. It may be provided that the viscosity of the barrier polymer is between 2.0 mPa*s and 50 mPa*s, in particular between 20 mPa*s and 35 mPa*s, in a 4% aqueous solution of the barrier polymer at 20°C determined in accordance with DIN 53015:2019.

[0044] It may be provided that the barrier polymer has a degree of hydrolysis at least 95 mol- %, preferably of at least 97 mol-%.

[0045] It may be provided that the first layer is formed by one single layer or by at least two sub-layers.

[0046] It may be provided that the third layer is formed by one single layer or by at least two sub-layers.

[0047] The invention may also relate to a method for producing a multi-layer composite film, the method comprising the steps of providing a cellulosic fiber-based substrate; and providing a coating on a first surface of the substrate.

[0048] The coating may be formed by applying a first layer, wherein the first layer comprises or is composed of a heat sealable polymer; applying a second layer, wherein the second layer comprises or is composed of a barrier polymer, wherein the barrier polymer preferably is ethylene vinyl alcohol copolymer (EVOH) and / or polyvinyl alcohol (PVOH); applying a third layer, wherein the third layer comprises or is composed of a heat sealable polymer.

[0049] It may be provided that the second layer is arranged between the first layer and the third layer.

[0050] It may be provided that the second layer is formed by dispersion coating.

[0051] It may be provided that the first layer and / or the third layer are applied by extrusion coating. It may be provided that the first layer is directly applied, in particular extruded, onto the first surface of the substrate, and wherein the dispersion for the second layer is directly applied, in particular dispersion coated, onto the first layer.

[0052] It may be provided that the third layer is directly applied, in particular extruded, onto the second layer.

[0053] It may be provided that a tie layer is directly applied, in particular extruded, onto the second layer, and wherein the third layer is directly applied, in particular extruded, onto the tie layer.

[0054] It may be provided that the dispersion is applied by means of a roll coating unit, a blade coating unit, a curtain coating unit, or a bar coating unit.

[0055] It may be provided that the dispersion has a solids content of between 4.0 wt.-% and 30 wt.-%.

[0056] Further features of the invention can be derived from the appended claims, the drawings, and the description of the exemplary embodiments.

[0057] In the following, specific embodiments will be described in detail, in order to illustrate beneficial aspects of the present invention. It is to be understood that these embodiments shall not limit the scope of the patent claims.

[0058] In the drawings:

[0059] Fig. 1 shows a schematic cross-sectional view of a first embodiment of a multi-layer film; and

[0060] Fig. 2 shows a schematic cross-sectional view of a second embodiment of a multi-layer film.

[0061] Unless otherwise specified, the following features I elements are shown in the drawings: substrate 1 ; first surface 2 (of substrate 1 ); coating 3; first layer 4; second layer 5; third layer 6; tie layer 7; second surface 8 (of substrate 1 ). It shall be noted that the drawings are not to scale, which in particular means that the apparent thickness of the layers is not indicative of the actual amount of material.

[0062] Fig. 1 shows a schematic cross-sectional view of a first embodiment of a multi-layer film according to the invention.

[0063] This film is composed of a substrate 1 made of kraft paper with a grammage of approx. 80 g / m2determined in accordance with ISO 536:2019, in combination with a three-layer coating 3, provided on a first surface 2 of the substrate 1 .

[0064] The coating 3 is composed of a first layer 4, a second layer 5, and a third layer 6. The first layer 4 and the third layer 6 are made of extruded low density polyethylene (LDPE) material, wherein the first layer 4 is provided in an amount of approx. 4.0 g / m2and the third layer 6 is provided in an amount of approx. 8.0 g / m2.

[0065] The second layer 5 is a dispersion coated layer made of ethylene vinyl acetate copolymer (EVOH) with a degree of hydrolysis of between 97.5 and 99.0 mol-%. The viscosity of the material of the second layer 5 is between 25.0 and 30.0 mPa*s, measured in a 4% aqueous solution at 20°C determined by a Brookfield synchronized- motor rotary type device ( / .e., in accordance with DIN 53015:2019).

[0066] The second layer 5 is provided in an amount of approx. 2.4 g / m2.

[0067] Thus, the grammage of the coating 3 is approx. 14.4 g / m2, which amounts to around 15% of the grammage of the complete film.

[0068] The film has an oxygen transmission rate of approx. 0.22 mL / m2 / d determined in accordance with ASTM F1927 at a temperature of 23°C and 50% relative humidity.

[0069] In this embodiment, the second surface 8 of the substrate 1 , i.e., the surface of the substrate, which is opposite of the first surface 2, is uncoated. However, in other embodiments, also the second surface 8 may be provided with a coating, namely a second coating. The film shown in Fig. 1 can be prepared by providing the substrate 1 and extruding the material of the first layer 4 on the first surface 2. In the next step, the dispersion for the second layer 5 is coated onto the first layer 4. Thereafter, the dispersion is dried in order to form the second layer 5. Coating may be performed in a conventional manner for example by roll coating the dispersion. Drying of the dispersion, i.e., evaporating the water contained in the dispersion, may be done in a drying tower. In the last step, the material for the third layer 6 is extruded onto the second layer 5.

[0070] For the extrusion of first layer 4 and third layer 6 standard temperature settings and extrusion speeds of between 150 to 200 m / min were used.

[0071] The aqueous dispersion for the second layer 5 had a solids content of approx. 8.5% with a viscosity determined by a Brookfield synchronized-motor rotary type device of approx. 530 mPa*s at a temperature of 22°C. Viscosity was determined after the dispersion had been heated to a temperature of between 90°C and 95°C for approx. 45 min under stirring.

[0072] Other film thicknesses than the one described above have been tested in further embodiments of the invention. Here, the same polymers as described in connection with the first embodiment have been used, and only the thicknesses have been varied.

[0073] These embodiments are summarized in Table 1 below:

[0074] Table 1 : Further embodiments

[0075] Fig. 2 shows a schematic cross-sectional view of a second embodiment of a multi-layer film according to the invention. In contrast to the first embodiment, the coating 3 is composed of four layers, with a tie layer 7 being arranged between second layer 5 and third layer 6.

[0076] As compared to the material without tie layer 7, these films generally allow for a higher seal strength due to an enhanced adhesion between second layer 5 and third layer 6. The same materials as for the first embodiment have been used for the multi-layer film of the second embodiment and will therefore not be described in detail here. The tie layer 7 consists of a reactive acrylate terpolymer. The substrate 1 of the film according to the second embodiment is the same as in the first embodiment.

[0077] In the second embodiment, the first layer 4 is made of LDPE provided in an amount of approx. 4.0 g / m2. The second layer 5 is made of EVOH, as described in connection with the first embodiment, provided in an amount of approx. 2.4 g / m2. The third layer 6 is provided in an amount of approx. 5.0 g / m2.

[0078] The tie layer 7, disposed between the second layer 5 and the third layer 6 and in direct contact with these layers 5, 6, is provided in an amount of approx. 3,0 g / m2.

[0079] Thus, the grammage of the coating 3 is approx. 14.4 g / m2, as in the first embodiment, which amounts to approx. 15% of the grammage of the complete film.

[0080] The film has an oxygen transmission rate of approx. 0.22 mL / m2 / d determined in accordance with ASTM F1927 at a temperature of 23°C and 50% relative humidity. Therefore, the film according to the second embodiment provides essentially the same oxygen transmission barrier as the film according to the first embodiment, but with higher seal strength.

[0081] Other film thicknesses than the one described above have been tested in further embodiments of the invention. Here, the same polymers as described in connection with the second embodiment have been used, and only the thicknesses have been varied. These embodiments are summarized in Table 2 below: Table 2: Further embodiments

[0082] When converted in a packaging process, the oxygen transmission rate of the film according to the second embodiment (and the further embodiments thereof), generally increases by a certain amount, but no full breakdown of the barrier properties could be observed.

Claims

Claims1. A multi-layer composite film, comprising a cellulosic fiber-based substrate (1 ) provided with a coating (3) on a first surface (2) thereof, wherein the coating (3) comprises or is composed of: a first layer (4), wherein the first layer (4) comprises or is composed of a heat sealable polymer, a second layer (5), wherein the second layer (5) comprises or is composed of a barrier polymer, wherein the barrier polymer is preferably ethylene vinyl alcohol copolymer (EVOH) and / or polyvinyl alcohol (PVOH), a third layer (6), wherein the third layer (6) comprises or is composed of a heat sealable polymer, wherein the second layer (5) is arranged between the first layer (4) and the third layer (6), characterized in that the second layer (5) is a dispersion coated layer.

2. Multi-layer composite film according claim 1 , wherein the coating (3) is provided in an amount of between 7 g / m2and 23 g / m2.

3. Multi-layer composite film according to claim 1 or 2, wherein the coating (3) is provided in an amount of at most 20 wt.-% in relation to the total weight of the film.

4. Multi-layer composite film according to any of claims 1 to 3, wherein the second layer (5) is provided in an amount of between 0.8 g / m2and 3.0 g / m2, preferably of between 1.0 and 2.5 g / m2.

5. Multi-layer composite film according to any of claims 1 to 4,- wherein the second layer (5) is in direct contact with the first layer (4) and the third layer (6), and wherein the coating (3) is optionally formed of three layers, and / or- wherein a tie layer (7) is provided, which tie layer (7) is in direct contact with the second layer (5) and the third layer (6), and wherein the coating (3) is optionally formed of four layers.

6. Multi-layer composite film according to any of claims 1 to 5, wherein the first layer (4) is in direct contact with the first surface (2) of the substrate (1 ), and wherein the first layer (4) is in direct contact with the second layer (5).

7. Multi-layer composite film according to any of claims 1 to 6, wherein the heat sealable polymer of the first layer (4) and / or the third layer (6) is selected from one or more of the following group: polyethylene (PE), in particular low-density polyethylene (LDPE); ethylene-vinyl acetate (EVA) copolymer; ionomer; terpolymer; olefin-acrylic acid copolymer, in particular ethylene-acrylic acid copolymer; olefin-(meth)acrylic acid copolymer, in particular ethylene-(meth)acrylic acid copolymer.

8. Multi-layer composite film according to any of claims 1 to 7, wherein the substrate (1 ) is paper, in particular kraft paper, with a grammage of at least 40 g / m2determined in accordance with ISO 536:2019.

9. Multi-layer composite film according to any of claims 1 to 8, wherein the film has an oxygen transmission rate determined in accordance with ASTM F1927 at a temperature of 23°C and 50% relative humidity of below 10 mL / m2 / d, in particular below 5.0 mL / m2 / d, or below 1 .5 mL / m2 / d.

10. Multi-layer composite film according to any of claims 1 to 9, wherein the substrate is paper, in particular kraft paper, with a grammage of between 40 g / m2and 120 g / m2determined in accordance with ISO 536:2019, wherein the coating (3) has three layers, and wherein- the first layer (4) is a polyethylene (PE) layer with a grammage of between 3.0 g / m2and 9.0 g / m2,- the second layer (5) is a polyvinyl alcohol (PVOH) layer or an ethylene vinyl alcohol copolymer (EVOH) layer, with a grammage of between 0.8 g / m2and 3.0 g / m2, and- the third layer (6) is a polyethylene (PE) layer with a grammage of between 3.0 g / m2and 11.0 g / m2.11 . Multi-layer composite film according to any of claims 1 to 9, wherein the substrate is paper, in particular kraft paper, with a grammage of between 40 g / m2and 120 g / m2determined in accordance with ISO 536:2019, wherein the coating (3) has four layers, and wherein- the first layer (4) is a polyethylene (PE) layer with a grammage of between 3.0 g / m2and 9.0 g / m2,- the second layer (5) is a polyvinyl alcohol (PVOH) layer or an ethylene vinyl alcohol copolymer (EVOH) layer, with a grammage of between 0.8 g / m2and 3.0 g / m2, and- the third layer (6) is a polyethylene (PE) layer with a grammage of between 2.0 g / m2and 10.0 g / m2, wherein a tie layer (7) is arranged between the second layer (5) and the third layer (6), which tie layer (7) has a grammage of between 1 .0 g / m2and 3.5 g / m2.

12. Multi-layer composite film according to any of claims 1 to 11 , wherein a second surface (8) of the substrate (1 ), which second surface (8) is opposite to the first surface (2), is free of a barrier coating, wherein the second surface is in particular uncoated.

13. Multi-layer composite film according to any of claims 1 to 12, wherein the viscosity of the barrier polymer is between 2.0 mPa*s and 50 mPa*s, in particular between 20 mPa*s and 35 mPa*s, in a 4% aqueous solution of the barrier polymer at 20°C determined in accordance with DIN 53015:2019.

14. Multi-layer composite film according to any of claims 1 to 13, wherein the barrier polymer has a degree of hydrolysis at least 95 mol-%, preferably of at least 97 mol-%.

15. Multi-layer composite film according to any of claims 1 to 14,- wherein the first layer (4) is formed by one single layer or by at least two sublayers, and / or- the third layer (6) is formed by one single layer or by at least two sub-layers.

16. A method for producing a multi-layer composite film, the method comprising the steps of:- providing a cellulosic fiber-based substrate (1 ) and- providing a coating (3) on a first surface (2) of the substrate, wherein the coating (3) is formed by applying a first layer (4), wherein the first layer (4) comprises or is composed of a heat sealable polymer, applying a second layer (5), wherein the second layer (5) comprises or is composed of a barrier polymer, wherein the barrier polymer preferably is ethylene vinyl alcohol copolymer (EVOH) and / or polyvinyl alcohol (PVOH), applying a third layer (6), wherein the third layer (6) comprises or is composed of a heat sealable polymer, wherein the second layer (5) is arranged between the first layer (4) and the third layer (6), characterized in that the second layer (5) is formed by dispersion coating.

17. Method according to claim 16, wherein the first layer (4) and / or the third layer (6) are applied by extrusion coating.

18. Method according to claim 16 or 17, wherein the first layer (4) is directly applied, in particular extruded, onto the first surface (2) of the substrate, and wherein the dispersion for the second layer (5) is directly applied, in particular dispersion coated, onto the first layer (4).

19. Method according to any of claims 16 to 18,- wherein the third layer (6) is directly applied, in particular extruded, onto the second layer (5), or- wherein a tie layer (7) is directly applied, in particular extruded, onto the second layer (5), and wherein the third layer (6) is directly applied, in particular extruded, onto the tie layer (7).

20. Method according to any of claims 16 to 19, wherein the dispersion is applied by means of a roll coating unit, a blade coating unit, a curtain coating unit, or a bar coating unit.21 . Method according to any of claims 16 to 20, wherein the dispersion has a solids content of between 4.0 wt.-% and 30 wt.-%.