Abrasion-resistant blown film with evoh outer layer and method for producing same

Abstract

In order to produce an abrasion-resistant plastic laminate (1) with an outer EVOH layer (2) in a blown film method, the invention proposes that, in order to bring about the abrasion resistance of the EVOH layer (2), the coextruded plastic tube (14) is cooled to below the glass transition temperature of the EVOH in the EVOH layer (2), preferably to at least 5°C below the glass transition temperature, before it is inserted into the roll gap (20) between the pinch rolls (17) and before it is compressed between the pinch rolls (17).

Description

[0001] Abrasion-resistant blown film with EVOH outer layer and process for its production

[0002] The present invention relates to a method for producing a plastic laminate with an outer, abrasion-resistant EVOH layer and a plastic layer adjacent thereto, wherein the EVOH layer and the plastic layer are co-extruded together in a blown film process, wherein the EVOH layer and the plastic layer are co-extruded concentrically in a co-extrusion die to form a plastic tube such that the EVOH layer lies inside the plastic tube, wherein the co-extruded plastic tube is drawn off in the extrusion direction after exiting the co-extrusion die and inflated with a pressurized gas to form a plastic bubble, wherein the plastic bubble is cooled and folded so that EVOH layers lie against one another in the folded plastic bubble, wherein the folded plastic bubble is passed through a roller gap between pull-off rollers and is compressed under pressure,and wherein the compressed plastic bubble is cut open in the extrusion direction after the peel rolls to form the plastic laminate and separated at the adjacent EVOH layers, so that the EVOH layer forms an outer layer in the plastic laminate. The invention also relates to a plastic laminate produced by this method.

[0003] A plastic laminate consists of several adjacent plastic layers. Plastic laminates are often used as packaging laminates for the production of packaging. The plastic layers in the plastic laminate can have different functions. A bonding layer, for example, primarily serves to create sufficient bonding between two plastic layers, between which the bonding layer is arranged and against which the bonding layer rests. A sealing layer is an outer layer in the plastic laminate and is used, for example, to produce a package with the plastic laminate by sealing the plastic laminate against itself or against another packaging part at the sealing layer. A substrate layer is usually an inner layer in the plastic laminate and gives the plastic laminate essential mechanical properties, such as strength, toughness, elongation at break, etc.A barrier layer is often used in a packaging laminate and serves to create a sufficient barrier effect against water vapor, oxygen, or aroma. In addition, the plastic laminate can of course contain other plastic layers. The structure of a plastic laminate naturally depends on its specific application, for example as a packaging laminate, and function. WO 2019 / 243456 A1 describes a packaging laminate with such layers as a plastic laminate, whereby this packaging laminate also includes an outer layer of ethylene-vinyl alcohol copolymer (EVOH) as a heat stability layer to improve the sealing properties. The heat stability layer can be used, in particular, to increase the sealing temperature. The other layers of the packaging laminate are primarily made of a polyethylene or a polyethylene copolymer material for reasons of recyclability.The outer EVOH layer, which is not a polyethylene or polyethylene copolymer material, is also very thin for recyclability reasons, with a maximum of 10% of the total thickness of the packaging laminate, but a maximum of 10 μm. Thus, the thin EVOH layer does not impair PE recycling.

[0004] A coextruded packaging laminate with an outer EVOH layer and additional plastic layers is also known from WO 98 / 49005 A1. EP 764 519 A1 and WO 2018 / 182712 A1 also describe a coextruded packaging laminate with an outer EVOH layer, each produced using the blown film process.

[0005] Plastic laminates are often manufactured using a coextrusion process, for example, blown film extrusion or cast film extrusion. The coextruded plastic laminate is usually wound onto long rolls and later further processed on converting machines, for example into packaging. In a converting machine, the plastic laminate is fed through the processing machine (very often at high speeds of up to 600 m / min, for example, when the plastic laminate is printed using gravure or flexographic printing). In the converting machine, the plastic laminate usually goes through various processing steps. During this process, the plastic laminate is also deflected within the converting machine, for example, around rollers, or guided past other parts of the converting machine in contact with each other.

[0006] Although plastic laminates produced using the blown film process by coextrusion with an outer EVOH layer are known from the state of the art, such plastic laminates have not yet established themselves on the market. This may be due to a problem related to blown film production.

[0007] It has been determined that a plastic laminate coextruded using a blown film process with an outer EVOH layer develops EVOH abrasion during further processing in a converting machine, even though the plastic laminate leaves the blown film line without any noticeable defects. This abrasion manifests itself in the form of fine fibers (typically ranging in thickness from 10 to 30 μm and lengths from a few millimeters to centimeters) or powder that adhere to the outer EVOH layer or adhere to and accumulate on parts of the converting machine. Such EVOH abrasion impairs the appearance of, for example, packaging made from the plastic laminate and is unacceptable.When using such a plastic laminate as a packaging laminate to produce packaging, there is also a risk of contamination of the contents due to EVOH abrasion, for example, during the packaging process or when opening the package. Such contamination is undesirable in any case, especially in the area of ​​food packaging. Accumulation of EVOH abrasion in the processing machine can impair the function of the processing machine and may result in costly maintenance work, such as cleaning or replacing parts of a processing machine. All of this is highly undesirable when processing a plastic laminate and should be avoided.

[0008] In a plastic laminate coextruded using a flat film process with an external EVOH layer, such EVOH abrasion was not observed.

[0009] It is therefore an object of the present invention to provide a plastic laminate coextruded in the blown film process with an outer EVOH layer and a method for producing such a plastic laminate, with which the formation of EVOH abrasion during further processing of the plastic laminate is reliably prevented.

[0010] This object is achieved according to the invention in that the coextruded plastic tube, in order to create the abrasion resistance of the EVOH layer, is cooled to below the glass transition temperature of the EVOH in the EVOH layer, preferably to at least 5°C below the glass transition temperature, before entering the roller gap between the peel rolls and before being compressed between the peel rolls. The glass transition temperature is a known material parameter that can be assumed to be known for the EVOH material used in the outer EVOH layer, for example, from a data sheet for the EVOH material.

[0011] The inventors' experiments revealed that when the EVOH layers lying adjacent to one another in the folded plastic tube are compressed in the roller gap between the peel rollers at temperatures above the glass transition temperature, this leads to increased adhesion of the EVOH layers to one another. This would lead to mechanical weak spots on the surface of the EVOH layer during subsequent separation of the EVOH layers to form the plastic laminate. These mechanical weak spots on the surface of the plastic laminate are not visible to the naked eye, but they reduce the abrasion resistance of the EVOH and, as a result, increase the abrasion of the EVOH during subsequent processing, for example, in a processing machine, thus leading to the formation of undesirable EVOH abrasion.By ensuring according to the invention in the blown film process for producing the plastic laminate that the folded plastic tube is cooled to below the glass transition temperature of the EVOH, preferably to at least 5°C below the glass transition temperature, immediately before the pull-off rollers, i.e. before the flattened plastic tube enters the roller gap between the pull-off rollers, the formation of such mechanical weak points and subsequently the formation of EVOH abrasion during further processing of the plastic laminate can be reliably prevented.

[0012] If polyethylene is primarily used in the plastic layer, with a polyethylene content in the plastic layer of at least 60 vol%, preferably at least 70 vol%, and very particularly preferably at least 80 vol%, or if polypropylene is primarily used in the plastic layer, with a polypropylene content in the plastic layer of at least 60 vol%, preferably at least 70 vol%, and very particularly preferably at least 80 vol%, then a recycling-friendly plastic laminate can be produced that can be recycled using conventional, particularly mechanical, recycling processes. For this purpose, it is also advantageous if the polyethylene content in the sealing layer is at least 80 vol% if polyethylene is primarily used in the plastic layer, or if the polypropylene content in the sealing layer is at least 80 vol% if polypropylene is primarily used in the plastic layer.

[0013] It is also advantageous for the recyclability of the plastic laminate if the EVOH layer has a thickness of maximum 10%, preferably maximum 5%, of the total thickness of the plastic laminate but maximum 10 pm.

[0014] The present invention will be explained in more detail below with reference to Figures 1 to 6, which show exemplary, schematic and non-limiting advantageous embodiments of the invention.

[0015] Fig.1 the well-known basic principle of a blown film line,

[0016] Fig.2 the structure of a plastic laminate according to the invention,

[0017] Fig.3 and 4 embodiments of a structure of a plastic laminate according to the invention,

[0018] Fig.5 a test device for carrying out a test method for determining the abrasion resistance of the EVOH layer and

[0019] Fig.6 an EVOH layer with EVOH abrasion in the form of fibers.

[0020] Fig. 1 shows the well-known basic principle of a blown film line 10 for coextruding a plastic laminate 1 using the blown film process. According to the invention, the plastic laminate 1 consists of an outer EVOH layer 2 and a plastic layer 3, as shown, for example, in Fig. 2. Due to the coextrusion, the EVOH layer 2 lies directly against the plastic layer 3, i.e., the surface of the EVOH layer 2 facing the plastic layer 3 comes into contact with the surface of the plastic layer 3 facing the EVOH layer 2.

[0021] A known blown film line 10 comprises a coextrusion die 11 with a plurality of melt channels 12. Each melt channel 12 is connected to an extruder 13 (only one being shown for simplicity), in which plastic material is melted and with which the molten plastic material is conveyed into the associated melt channel 12. In the embodiment of Fig. 2, two melt channels 12 are provided, with which a two-layer plastic laminate 1, with an EVOH layer 2 and a plastic layer 3, can be coextruded. However, it is known that plastic laminates 1 with more than two layers, for example three layers, four layers, five layers or even more layers, can also be coextruded in a blown film line. For such plastic laminates 1, correspondingly more melt channels 12 and extruders 13 are provided.The individual layers of a plastic laminate 1 can also contain different plastic materials, but an outer layer forms an EVOH layer 2.

[0022] During coextrusion, the plastic melts are layered in the coextrusion die 11 in the desired sequence, so that the coextruded plastic melts exit the coextrusion die 11 in the form of a plastic tube 14 with concentrically arranged plastic layers. The thicknesses of the individual plastic layers are adjusted as required and do not have to be identical.

[0023] According to the blown film process according to the invention, the plastic layers of the plastic laminate 1 are coextruded in such a way that the EVOH layer 2 is arranged inside the plastic tube 14, thus forming the plastic layer located furthest inside the plastic tube 14, as indicated in Fig.1.

[0024] The plastic tube 14 is cooled after emerging from the coextrusion nozzle 11 so that the plastic melt begins to solidify. Fig. 1 shows a cooling unit 15 which, for example, blows cool air onto the plastic tube 14. At the same time, after emerging from the coextrusion nozzle 11, the plastic tube 14 is inflated into a plastic bubble 14a by means of a compressed gas 21, for example air, which is blown into the plastic tube 14. A blower for providing the compressed gas 16 is not shown in Fig. 1 for the sake of simplicity. The EVOH layer 2 is therefore arranged on the inside of the inflated plastic bubble 14a. The plastic bubble 14a is usually inflated in a known manner by regulating the supplied compressed gas 21 for inflation and regulating the compressed gas 21 discharged from the plastic bubble 14a (in Fig.1 not shown) so that the plastic bubble 14 neither collapses nor bursts, resulting in a stable blown film process. The compressed gas 21 also cools the coextruded plastic tube 14 from the inside.

[0025] By inflating the coextruded plastic tube 14, the plastic melts are stretched. During blown film extrusion, the plastic melts are typically stretched in the transverse direction by a factor of approximately 2 to 3 (the so-called blow-up ratio) and in the longitudinal direction by a factor of 1:10 to 1:100 (the so-called take-off ratio). The take-off ratio is also determined by the take-off rolls 17, as explained further below. If the coextruded plastic laminate 1 is not stretched after coextrusion, the coextruded plastic layers achieve their final thicknesses through coextrusion. If stretching is planned, the thicknesses in the plastic laminate 1 will still change due to the stretching.

[0026] The plastic bubble 14a is cooled below the melting temperatures of the plastic materials in the plastic tube 14 so that they solidify sufficiently, and then folded. For folding, a flattening device 16, for example an arrangement of rotatably mounted rollers or a guide surface, can be provided, past which the plastic tube 14a is guided and with which the plastic bubble 14a is folded. The optional flattening device 16 is arranged downstream of the melt nozzle 11 in the extrusion direction. By folding, the EVOH layer 2 inside the plastic tube 14 is placed on top of one another, so that two EVOH layers 2 lie next to one another in the folded plastic tube 14. However, the plastic bubble 14a can also be folded between the pull-off rollers 17 without a flattening device 16.However, in a blown film line 10, a flattening device 16 is usually provided in front of the pull-off rollers 17 in the extrusion direction.

[0027] The inflated and cooled, and possibly already collapsed, plastic bladder 14a is passed through a roller gap 20 between two pull-off rollers 17. The pull-off rollers 17 are driven to move the plastic tube 14 in the extrusion direction. The rotational speed of the pull-off rollers 17 essentially determines the draw-off ratio. In the roller gap 20 between the pull-off rollers 17, the collapsed plastic tube 14b is pressed together, even under pressure. This results from the roller gap 20 being less wide than the thickness of the collapsed plastic tube 14b. Downstream of the pull-off rollers 17 in the extrusion direction, the collapsed plastic tube 14b can be deflected as needed. The collapsed plastic tube 14b can then be wound onto a roll 18.To produce a plastic laminate 1, the folded plastic tube 14b must be cut open, which can also be done before winding onto a roll 18. This is typically done by cutting away the folded edges on both sides of the folded plastic tube 14b with a cutting unit 19 (indicated in Fig. 1), creating a pair of equally wide, flat plastic laminates 1. The plastic laminates 1 must then be separated from each other by pulling them apart and can be wound separately onto rolls 18. This separation process need not take place directly on the blown film line 10, but could also take place at a later time, for example, before a processing machine. Typically, however, the separation process takes place on the blown film line 10.

[0028] The plastic laminate 1 produced in this way can also be stretched before processing, either in the machine direction (in the extrusion direction) or in the transverse direction (transverse to the machine direction), or in both the machine direction and the transverse direction. The stretching of a plastic laminate is well known and requires no further explanation. Essentially, during stretching, the plastic laminate 1 is stretched in the stretching direction(s) at temperatures below the melting temperatures of the plastics in the plastic laminate 1 and is subjected to an orientation that can alter certain properties of the plastics in the plastic laminate 1.

[0029] To prevent mechanical weak points from forming on the surfaces of the adjoining EVOH layers 2 due to this separation process, the invention provides that the coextruded and inflated plastic tube 14 is cooled to below the glass transition temperature of the EVOH in the EVOH layer 2, preferably to at least 5°C below the glass transition temperature, before reaching the pull-off rollers 17. If the EVOH layer 2 contains different EVOH types, then cooling is carried out to at least 5°C below the glass transition temperature of the EVOH with the lowest glass transition temperature.This temperature is preferably reached before the EVOH layers 2 come into contact in the extrusion direction upstream of the peel-off rollers 17 by being laid together, for example in a flattening device 16 or by being inserted between the peel-off rollers 17, but in any case before the EVOH layers 2 are pressed against one another in the roller gap 20 between the peel-off rollers 17.

[0030] This cooling can be achieved by suitable cooling of the plastic tube 14 between the coextrusion die 11 and the pull-off rollers 17, or by reducing the extrusion speed of the plastic tube 14 (which increases the cooling time), or by extending the distance between the coextrusion die 11 and the pull-off rollers 17. A combination of these measures is also conceivable in principle, although a reduction in the extrusion speed is generally undesirable because it reduces the throughput in the blown film line 10. The temperature of the plastic tube 14 upstream of the roller gap 20 between the pull-off rollers 17 can thus be reliably monitored, controlled, and adjusted in the blown film line 10.

[0031] According to the invention, it is important that the plastic tube 14 reaches the desired temperature of below the glass transition temperature of the EVOH in the EVOH layer 2, preferably at least 5°C below the glass transition temperature, before entering the roller gap 20 between the peel-off rollers 17, i.e., before being pressed together between the peel-off rollers 17. Although the peel-off rollers 17 are generally temperature-controlled in the blown film line 10, it would be too late if the plastic tube 14 were only cooled between the peel-off rollers 17 to the desired temperature of below the glass transition temperature of the EVOH in the EVOH layer 2, because the EVOH layers already lying against one another would then adhere too strongly due to the pressing together, which would lead to mechanical weak points on the surfaces of the EVOH layers 2 during subsequent separation.

[0032] It has been found that, without the measure according to the invention, the abrasion resistance of the EVOH layer 2 is reduced in every plastic laminate 1 with an outer EVOH layer 2 coextruded in the blown film process described above, which causes EVOH abrasion during further processing if the plastic bubble 14a is not cooled to below the glass transition temperature of the EVOH in the EVOH layer 2, preferably to at least 5°C below the glass transition temperature, before reaching the pull-off rollers 17. This is because, due to mechanical weak points on the surfaces of the EVOH layers 2 arising from the separation process on the adjacent EVOH layers 2, the EVOH layer 2 outer on the plastic laminate 1 is mechanically weakened, which leads to abrasion of the EVOH layer 2 during further processing. This reduces the abrasion resistance of the EVOH layer 2 and results in undesirable EVOH abrasion during further processing.In the case of plastic laminates 1 produced by the flat film process or by a lamination process with an external EVOH layer 2, this negative effect of the reduced abrasion resistance of the EVOH layer 2 could not be observed.

[0033] If the coextruded plastic laminate 1 is further stretched after coextrusion, this negative effect of reduced abrasion resistance is amplified by the stretching and the high orientation of the EVOH in the stretching direction. This can even increase EVOH abrasion without the measures according to the invention.

[0034] The above-described inventive manufacturing process improves the abrasion resistance of the outer EVOH layer 2 in a plastic laminate 1 in any case compared to plastic laminates 1 coextruded using the blown film process with an outer EVOH layer that were not produced according to the invention. This also applies to plastic laminates 1 that are additionally stretched. The properties of a plastic laminate 1 coextruded using the blown film process are therefore significantly improved by the inventive measure, which enables the further processing of such plastic laminates 1 without the undesirable EVOH abrasion.

[0035] Investigations by the inventors have demonstrated a strong correlation between the abrasion resistance of the EVOH layer 2 and the temperature of the plastic tube 14 before entering the roll gap 20 between the peel-off rolls 17. Variations in the EVOH types in the EVOH layer 2, the structure of the plastic layer 3 of the plastic laminate 1, or other process parameters of the blown film process, however, had no detectable influence on the abrasion resistance.

[0036] To measure the abrasion resistance of EVOH layer 2, a dedicated test method was developed. This test method is based on the well-known measurement of the coefficient of friction, which is a standard test and defined in many standards, for example, ASTM D 1894 or ISO 8295. In this test, a sled with the test object is pulled across a test surface, and the resulting tensile force is measured. The test is carried out under specified conditions, such as a certain temperature (e.g.,

[0037] room temperature of 23°C), with a test object of a certain size, a certain weight force and a certain pulling speed.

[0038] However, to measure abrasion resistance, the coefficient of friction is not determined. Instead, the tensile process is repeated a certain number of times. Then, the surface of the test object (EVOH layer 2) is assessed to determine whether abrasion debris in the form of fibers or powder has formed. If no abrasion debris in the form of fibers or powder is present, then EVOH layer 2 meets the required abrasion resistance; otherwise, it does not.

[0039] Fig.5 shows a test device 30 according to the test method for testing the abrasion resistance of the EVOH layer 2.

[0040] The test device 30 for determining abrasion resistance consists of a flat support table 31 and a carriage 32 of known mass. The carriage 32 rests on the support table 31. The carriage 32 and the support table 31 are movable relative to each other, whereby the support table 31 usually remains stationary and the carriage 32 is moved over the support table 31 (as in the embodiment according to Fig. 5). However, for the test, it is irrelevant whether the carriage 32 is moved over the support table 31 or whether the relative movement is performed by the support table 31. The support table 31 can be designed, for example, as a metal plate, glass plate, or plastic plate.

[0041] A piece of the plastic laminate 1 is arranged on each of the support table 31 and the carriage 32 in such a way that the EVOH layer 2 of the plastic laminate 1 is on the outside of the support table 31 and the carriage 32, as shown in Fig. 5. Thus, plastic laminate 1 is tested against plastic laminate 1. The EVOH layer 2 of the plastic laminate 1 on the carriage 32 therefore faces the EVOH layer 2 of the plastic laminate 1 on the support table 31. If the carriage 32 with plastic laminate 1 rests on the support table 31 with plastic laminate 1, the two facing EVOH layers 2 lie against one another. If there is a relative movement between the support table 31 and the carriage 32, the two adjacent EVOH layers 2 slide against one another and rub against one another. The pieces of the plastic laminates 1 are arranged such that the relative movement occurs in the direction of the extrusion direction of the plastic laminate 1.

[0042] How the relative movement between support table 31 and carriage 32 is generated is irrelevant for the test method. In the embodiment shown in Fig. 5, carriage 32 is connected to a cable 33. The cable 33 is pulled (indicated by force F), causing carriage 32 to move a predetermined distance s across support table 31 at a relative speed v.

[0043] The test method is carried out with the following test parameters at room temperature (23°C).

[0044] The weight of the carriage 32 is 200 g and the support surface A of the carriage 32 on the support table 31 is 60 mm x 66 mm, with the larger dimension in the direction of the relative velocity v. This weight results in a surface load on the EVOH of 495.4 N / m 2. The relative speed v is 100 mm / min. The distance s along which the adjacent EVOH layers 2 are moved with the relative speed v is 300 mm. The carriage 32 is moved relative to the support table 31 five times in succession using these test parameters. This means that the support table 31 and carriage 32 are returned to their starting position with each repetition and the relative movement is repeated using the test parameters. During this process, the EVOH layers 2 facing each other rub against each other. The EVOH surfaces of the EVOH layers 2 of the tested plastic laminates 1 are then examined. This examination is carried out with the naked eye, which can detect any EVOH abrasion. If EVOH abrasion in the form of fibers or powder is visible when examining according to the test method, then the EVOH layer 2 does not have the required abrasion resistance.If no EVOH abrasion is detectable, then EVOH layer 2 has the required abrasion resistance.

[0045] Fig. 6 shows an enlarged section of the EVOH layer 2 of a plastic laminate 1, on which EVOH abrasion 35 is visible in the form of EVOH fibers. This EVOH abrasion 35 is visible to the naked eye.

[0046] All tested plastic laminates 1 with an external EVOH layer 2 produced according to the invention using the blown film process exhibit no EVOH abrasion 35 when performing the above test method. However, plastic laminates 1 with an external EVOH layer 2 produced using the blown film process, in which the glass transition temperature is not exceeded before entering the roll gap 20 between the peel-off rolls 17, exhibit pronounced EVOH abrasion 35 and therefore do not have the required abrasion resistance for further processing.

[0047] The occurrence of the effect according to the invention is demonstrated using a specific exemplary embodiment. For this exemplary embodiment, a plastic laminate 1 with the structure EVOH / Tie / HDPE was produced using the blown film process and, after coextrusion, stretched in the machine direction with a factor of 4.5:1. The symbol " / " in a laminate structure denotes direct contact between two adjacent layers. The outer EVOH forms the EVOH layer 2, and Tie / HDPE forms the further plastic layer 3. The Tie layer is a connecting layer to ensure the composite adhesion in the plastic laminate 1 and consists of LLDPE-g-MAH (maleic anhydride (MAH) grafted linear low-density polyethylene). The Tie layer has a thickness of 2 μm. The EVOH layer 2 consists of 100% EVOH with 32 mol% ethylene and has a thickness of 2 μm.

[0048] The EVOH has a glass transition temperature (Tg) of 60°C. The HDPE layer of the additional plastic layer 3 is 16 μm thick and consists of HDPE with a density of 0.95 and an MFI of 1.0 (190°C / 2.16 kg). The HDPE may contain small amounts (up to 5 wt%) of common and known processing additives used in coextrusion.

[0049] This EVOH / Tie / HDPE construction of the plastic laminate 1 was produced using the blown film process, once (Case A) with a temperature of the plastic bubble 14a before entering the roll gap 20 between the peel-off rolls 17 of 55°C (5°C below the glass transition temperature Tg) and once (Case B) with a temperature of the plastic bubble 14a before entering the peel-off rolls 17 of 60°C (which corresponds to the glass transition temperature Tg). Both plastic laminates 1 were tested using the above test method. In Case A, no EVOH abrasion 35 was observed. In Case B, however, EVOH abrasion 35 was observed.

[0050] This result also occurs when different EVOH types, for example, different ethylene content, are used in the EVOH layer 2. This result is also independent of other process parameters of the blown film process, such as a melt temperature in the extruders 13, the peel ratio, or the blow-up ratio.

[0051] It was also determined that this result is independent of stretching parameters following coextrusion, such as the stretching temperature during the stretching of the plastic laminate 1. It was also determined that this result is independent of the blown film line. The tests were repeated with three different blown film lines 10, and the same result was always obtained.

[0052] The only decisive process parameter for achieving the abrasion resistance of the EVOH layer 2 is the temperature of the plastic bubble 14a coextruded in the blown film process before entering the roll gap 20 between the peel rolls 17, i.e., immediately before contact with the peel rolls 17. This temperature must be lower than the (possibly lowest) glass transition temperature Tg of the EVOH in the EVOH layer 2, preferably at least 5°C lower than the glass transition temperature Tg.

[0053] Different structures of the plastic laminate 1 with an external EVOH layer 2 are conceivable, as explained below. In principle, however, the structure of the plastic layer 3 is arbitrary, because the abrasion resistance of the EVOH layer 2 is crucial, and the structure of the plastic layer 3 essentially depends on the specific application of the plastic laminate 1.

[0054] The EVOH layer 2 in the plastic laminate 1 consists of at least 80% by volume ethylene-vinyl alcohol copolymer (EVOH), preferably 100% by volume EVOH. EVOH is available in various versions. For the EVOH layer 2, EVOH with an ethylene content of a maximum of 50 mol%, preferably between 24 mol% and 50 mol%, is used. Depending on the ethylene content in the EVOH, the melting temperature of the EVOH can range, for example, between 155°C at 48 mol% ethylene content and 190°C at 27 mol% ethylene content. The glass transition temperature Tg of the EVOH in the EVOH layer 2 is typically between 50°C at 48 mol% ethylene content and 63°C at 27 mol% ethylene content.These physical parameters, in particular the glass transition temperature Tg, and other physical parameters such as the melt flow index (MFI), density, water vapor transmission rate (WVTR), and oxygen transmission rate (OTR) of an EVOH are derived from the data sheets of the EVOH material manufacturers and can be assumed to be known for each EVOH material. Preferably, only EVOH is used in EVOH layer 2, i.e., 100 vol% EVOH. However, a mixture of EVOH with a small proportion, maximum 20 vol%, of an ethylene (co)polymer could also be used. Polyethylene is used as an ethylene (co)polymer in the form of a homopolymer such as high-density polyethylene (HDPE) with a density between 0.94 and 0.97 g / cm³. 3 , Low-density polyethylene (LDPE) with a density between 0.915- 0.935 g / cm 3or other PE homopolymers, or in the form of a copolymer such as linear low-density polyethylene (LLDPE), linear metallocene low-density polyethylene (mLLDPE), ethylene-vinyl acetate copolymer (EVA), ethyl methacrylate (EMA), ethylene / acrylic acid copolymer (EAA) or ethylene-butyl acrylate copolymer (EBA).

[0055] The plastic layer 3 of the plastic laminate 1 consists of at least one plastic layer.

[0056] Due to the coextrusion in the blown film process, the polymers of all plastic layers of the plastic laminate 1 according to the invention have an MFI at 190°C and 2.16 kg of a maximum of 4, preferably a maximum of 3, and thus also differ from polymers which are used in the flat film extrusion process (with typical MFIs of up to a maximum of 6 to 8). Even if individual polymeric raw materials in an MFI range of 1.5 to 4 can certainly be used in both processes, typical blown films have an average MFI of 0.3 to 1.5 for all plastics used in the structure of the plastic laminate 1, whereas films and BOPE films produced in the flat film extrusion process have an MFI above 1.5. This also makes it possible to draw conclusions about the manufacturing process based solely on the MFI of the polymers used in a plastic laminate 1.

[0057] The plastic layer 3 is preferably designed in several layers, ie with several plastic layers that are coextruded together with the outer EVOH layer 2.

[0058] In one possible embodiment according to Fig. 3, the plastic layer 3 is designed with a connecting layer 4 (tie layer), a substrate layer 5 and a sealing layer 6, resulting in a structure of EVOH / tie / substrate layer / sealing layer. Due to the coextrusion, the connecting layer 4 lies directly on the EVOH layer 2 and the substrate layer 5 directly on the connecting layer 4. In the case of a coextruded sealing layer 6, the sealing layer 6 would lie directly on the substrate layer 5. In the case of an additional connecting layer between the substrate layer 5 and the sealing layer 6 to increase the bond adhesion, this additional connecting layer 4a (tie layer) would lie directly on the substrate layer 5 and the sealing layer 6, resulting in a structure of EVOH / tie / substrate layer / tie / sealing layer.In this context, “directly” means that the surfaces of the respective plastic layers facing each other are in contact.

[0059] A substrate layer 5 or a sealing layer 6 can, however, in turn be designed in several layers, ie they can themselves comprise several plastic layers lying directly next to one another which are coextruded.

[0060] A bonding layer in the plastic laminate 1, such as the bonding layer 4 or 4a, primarily serves to achieve sufficient bond adhesion in the plastic laminate 1. Suitable bonding layers preferably consist of polymers with increased polarity, for example based on polymers compatible with polyethylenes in terms of recycling properties, such as maleic anhydride-modified polyolefins (such as PE or PP), ethylene-vinyl acetate copolymers (EVA), ethylene / acrylic acid copolymers (EAA), ethylene-butyl acrylate copolymers (EBA), or similar polyolefin copolymers, in particular PE copolymers.

[0061] However, the sealing layer 6 can also be bonded to the plastic layer 3 after the coextrusion of the EVOH layer 2 and the plastic layer 3, for example by extrusion lamination, extrusion coating, or by adhesive lamination with a suitable bonding layer 4a, as shown in Fig. 4. During lamination, the sealing layer 6 is bonded to the plastic layer 3 using a suitable laminating adhesive, for example based on polyurethane adhesives in extrusion coating or polyolefin copolymers, in particular PE copolymers, in extrusion lamination. The thickness of the laminating adhesive is preferably 1 to 5 g / m 2 , or preferably 2 to 5 g / m 2 , for conventional polyurethane-based adhesives or 5 to 20 g / m 2 during extrusion lamination.

[0062] In a preferred embodiment, the plastic layer 3, and optionally also the sealing layer 6, consists primarily of polyethylene (PE) material and small proportions of compatible materials with regard to recyclability. "Primarily" means that the PE content in the plastic layer 3, and optionally also the sealing layer 6, is at least 60 vol%, preferably at least 70 vol% PE content, and most preferably at least 80 vol%. Thus, the high polyethylene content allows for the production of a recyclable plastic laminate 1.

[0063] In plastic layer 3, a specific PE type can be used, such as LDPE, LLDPE, MDPE, HDPE, or PE copolymers, or a mixture of different PE types or even multiple layers of different PE types can be used to achieve the desired PE content. The PE content can approach 100 vol%, although due to common additives in a plastic laminate 1 (such as slip additives, antiblocking additives, dyes, fillers, etc.), 100 vol% PE content is generally never achieved. Additives are added in small quantities if necessary (maximum 5 vol%). However, due to the small amount, additives do not impair the recyclability of the packaging laminate 1.

[0064] The sealing layer 6 consists predominantly of a PE material, whereby the PE content of the total polymer content of the sealing layer 6, without any added mineral or other fillers or additives, should preferably be at least 80 vol%. Various PE types, such as LDPE, LLDPE, MDPE, HDPE, or PE copolymers, can be used here, either in pure form or as a blend or in multiple layers.

[0065] Any residual plastic material in the plastic laminate 1, in addition to the PE content (and possible additives), is preferably a compatible polyolefin material that does not impair recyclability. In principle, any type of polyethylene can be considered as a compatible polyolefin material, in particular ethylene copolymers, such as ethylene-vinyl acetate copolymer (EVA), ethyl methacrylate (EMA), ethylene / acrylic acid copolymer (EAA), or ethylene-butyl acrylate copolymer (EBA). Polypropylene (PP) or a cycloolefin copolymer (COC) can also be used as compatible polyolefin materials, up to a maximum of 20 vol%.In the case of PP, a polypropylene random copolymer with ethylene as comonomer (usually 5 to 15 mol%), a polypropylene copolymer with ethylene or a polypropylene homopolymer that is sufficiently compatible with linear PE types such as mLLDPE, LLDPE or HDPE is preferably used in order to achieve at least limited recyclability.

[0066] The PE and the compatible polyolefin material can be present in the plastic layer 3, and optionally also in the sealing layer 6, as a mixture or in the form of several layers, with one (or more) PE layer and one (or more) layer of the compatible polyolefin material.

[0067] In a particularly advantageous embodiment, HDPE is primarily used in the substrate layer 5 of the plastic layer 3, with at least 60 vol%, preferably at least 70 vol% and most preferably at least 80 vol% HDPE content.

[0068] In another embodiment, polypropylene (PP) is primarily used in the substrate layer 5. In such an embodiment, the sealing layer 6 is also made of PP.

[0069] The coextruded plastic laminate 1 can also be stretched unidirectionally (usually in the machine direction) or bidirectionally before being bonded to the sealing layer 6. This has the particular advantage that the asymmetric structure with the EVOH in the outer layer is less likely to curl (so-called "curling"). However, even in this case, the sealing layer 6 is preferably not stretched.

[0070] The degree of stretching is preferably at least 4:1 in the machine direction. Unidirectional stretching can be carried out much more easily and cost-effectively than bidirectional stretching and is therefore preferred.

[0071] It should be noted here that in blown film extrusion, the extrusion gap (typically 1.5 to 2.5 mm for blown film) is significantly larger than the final thickness of the extruded film (typically between 10 and 200 μm). The extruded melt is stretched at temperatures significantly above the melting point of the extruded polymer, thereby reducing the thickness. In blown film extrusion, for example, the melt is typically stretched in the transverse direction by a factor of approximately 2 to 3 (the so-called blow-up ratio) and in the longitudinal direction by a factor of 1:10 to 1:100 (the so-called draw-off ratio). However, the effect of this stretching during extrusion cannot be compared to the stretching of a plastic film, since stretching usually takes place at temperatures just below the melting point of the polymer in order to permanently align the disordered polymers and the semi-crystalline regions by stretching in the stretching direction.

[0072] Although EVOH is also made from ethylene, like polyethylene, it is not a polyethylene or polyethylene copolymer, but rather forms a separate polymer class. Therefore, EVOH is particularly problematic with regard to the recycling of a plastic laminate 1. To avoid negatively impacting the recyclability of the plastic laminate 1, the EVOH layer 2 preferably has a thickness of no more than 10%, preferably no more than 5%, of the total thickness of the plastic laminate 1 (including a sealing layer 6 and any bonding layer between the sealing layer and the substrate layer 5), but in absolute terms (regardless of the total thickness) no more than 10 μm.If the plastic laminate 1 contains several EVOH layers, for example in the form of a barrier layer, then the EVOH layers together preferably have a thickness of a maximum of 10%, preferably a maximum of 5%, of the total thickness of the plastic laminate 1 (including a sealing layer 6 and any bonding layer between the sealing layer and the substrate layer 5), but in absolute terms (regardless of the total thickness) a maximum of 10 μm. Due to the small thickness of the EVOH layer 2, the recyclability of the plastic laminate 1 is not impaired. This is particularly advantageous in combination with a plastic layer 3 consisting primarily of a PE material or PP material.

[0073] Other plastic layers can also be provided in the plastic layer 3. In particular, an additional barrier layer, preferably made of EVOH, can be provided, for example, between the connecting layer 4 and the substrate layer 5 or between the substrate layer 5 and the sealing layer 6. For reasons of composite adhesion, it can be advantageous if such an EVOH barrier layer is integrated into the plastic laminate on both sides by means of a connecting layer (tie layer). A possible structure then results, for example, as EVOH / Tie / EVOH / Tie / Substrate layer (e.g. HDPE) / Sealing layer or EVOH / Tie / Substrate layer (e.g. HDPE) / Tie / EVOH /

[0074] Tie / Sealing layer or EVOH / Tie / EVOH / Tie / Substrate layer (e.g., HDPE) / Tie / Sealing layer. The symbol " / " indicates that two layers are directly adjacent to one another.

[0075] The plastic laminate 1 is typically between 10 and 40 μm thick, meaning the thickness of the EVOH layer 2 is preferably between 1 μm and 4 μm. The thickness of the substrate layer 5 is preferably between 5 and 35 μm. The thickness of a bonding layer 4 is typically between 1 and 5 μm. If a sealing layer 6 is present, the plastic laminate 1 is typically between 10 and 120 μm thick. The thickness of the sealing layer 6 is typically between 20 and 100 μm.

Claims

Patent claims 1. A method for producing a plastic laminate (1) with an outer, abrasion-resistant EVOH layer (2) and a plastic layer (3) adjacent thereto, wherein the EVOH layer (2) and the plastic layer (3) are co-extruded together in a blown film process, wherein the EVOH layer (2) and the plastic layer (3) are co-extruded concentrically in a co-extrusion die (11) to form a plastic tube (14) such that the EVOH layer (2) lies inside the plastic tube (14), wherein the co-extruded plastic tube (14) is drawn off in the extrusion direction after emerging from the co-extrusion die (11) and is inflated with a pressurized gas (21) to form a plastic bubble (14a), wherein the plastic bubble (14a) is cooled and collapsed such that EVOH layers (2) lie against one another in the collapsed plastic bubble (14a),wherein the collapsed plastic bubble (14a) is passed through a roller gap (20) between pull-off rollers (17) and is compressed under pressure, and wherein the compressed plastic bubble (14b) is cut open in the extrusion direction after the pull-off rollers (17) to form the plastic laminate (1) and is separated at the adjacent EVOH layers (2), so that the EVOH layer (2) forms an outer layer in the plastic laminate (1), characterized in that the coextruded plastic tube (14) is cooled to below the glass transition temperature of the EVOH in the EVOH layer (2), preferably to at least 5°C below the glass transition temperature, before entering the roller gap (20) between the pull-off rollers (17) and before being compressed between the pull-off rollers (17), in order to produce the abrasion resistance of the EVOH layer (2).

2. Method according to claim 1, characterized in that the coextruded plastic layer (3) is formed from a connecting layer (4) and a substrate layer (5) directly adjacent thereto, wherein the connecting layer (4) in the plastic laminate (1) is directly adjacent to the EVOH layer (2).

3. Method according to claim 1 or 2, characterized in that the coextruded plastic laminate (1) is stretched in the extrusion direction after the take-off rollers (17), preferably only in the machine direction.

4. Method according to one of claims 1 to 3, characterized in that the plastic laminate (1) is connected after coextrusion, and optionally after stretching, with a, preferably an unstretched, sealing layer (6), wherein the sealing layer (6) is connected to the plastic layer (3) of the plastic laminate (1), preferably with a connecting layer (4a) between the plastic layer (3) and the sealing layer (6).

5. Method according to claim 1, characterized in that the coextruded plastic layer (3) is formed from a connecting layer (4), a substrate layer (5) directly adjacent to the connecting layer (4) and a sealing layer (6) directly adjacent to the substrate layer (5), wherein the connecting layer (4) in the plastic laminate also directly adjoins the EVOH layer (2).

6. The method according to claim 1, characterized in that the coextruded plastic layer (3) is formed from a connecting layer (4), a substrate layer (5) directly adjacent to the connecting layer (4), a further connecting layer (4a) directly adjacent to the substrate layer (5) and a sealing layer (6) directly adjacent to the further connecting layer (4a), wherein the connecting layer (4) in the plastic laminate also directly adjoins the EVOH layer (2).

7. Method according to one of claims 1 to 6, characterized in that polyethylene is primarily used in the plastic layer (3), with a polyethylene content in the plastic layer (3) of at least 60 vol%, preferably at least 70 vol% and very particularly preferably at least 80 vol%, or polypropylene is primarily used in the plastic layer (3), with a polypropylene content in the plastic layer (3) of at least 60 vol%, preferably at least 70 vol% and very particularly preferably at least 80 vol%.

8. Method according to one of claims 4 to 7, characterized in that the polyethylene content in the sealing layer (6) is at least 80 vol% if polyethylene is predominantly used in the plastic layer (3) or the polypropylene content in the sealing layer (6) is at least 80 vol% if polypropylene is predominantly used in the plastic layer (3).

9. Method according to one of claims 1 to 8, characterized in that the EVOH layer (2) has a thickness of at most 10%, preferably at most 5%, of the total thickness of the plastic laminate (1) but at most 10 pm.

10. Plastic laminate with an outer, abrasion-resistant EVOH layer (2) produced by a method according to one of claims 1 to 9.

11. Plastic laminate according to claim 10, characterized in that the abrasion resistance of the EVOH layer (2) is established in the form of a lack of EVOH abrasion on a surface of the EVOH layer (2) when a first piece of the plastic laminate (1) which is a carriage (32) with a weight of 200 g and with a support surface of 60x66 mm, is placed on a second piece of the plastic laminate (1) which is fastened on a support table (31), so that the EVOH layers (2) of the first and second pieces of the plastic laminate (1) lie against one another and the first and second pieces of the plastic laminate (1) are moved one after the other five times relative to one another and under the effect of the weight of the carriage (32) at a relative speed (v) of 100 mm / min along a distance (s) of 300 mm.

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