Biaxially oriented polyethylene films for thermoforming, method for their manufacture, their use, a method for thermoforming and its products

Description

Introduction

[0001] The present invention relates to a biaxially oriented film for thermoforming, characterized in that the film contains at least 85 percent by weight of polyethylene and at least 95 percent by weight of polyolefins, based on the total mass of the film. It further relates to a process for producing such films and their use. The invention also relates to a process for producing molded articles from such films and to the molded articles themselves. State of the art

[0002] Films made of biaxially oriented plastic films and their properties are known to those skilled in the art. In the production of such films, molten plastic granules are extruded, producing a cast film. The cast film is then oriented by mono- or biaxial stretching. Stretching can be performed in a sequential or simultaneous process. Examples of biaxially oriented plastic films used industrially include films made of biaxially oriented polypropylene (BOPP), films made of biaxially oriented polyethylene terephthalate (BOPET), films made of biaxially oriented polyamide (BOPA), and films made of biaxially oriented polystyrene (BOPS). Biaxial orientation results in a change in the morphology of the molecular structure, which provides the films with a variety of different advantageous properties while simultaneously reducing their overall thickness: Changes in physical properties (e.g. improved stiffness and tear resistance) Optical properties (e.g. increased transparency and increased gloss) Barrier properties (e.g. improved barrier properties for oxygen (or air) and water) Weight savings in packaging / material-efficient packaging More energy-efficient production

[0003] Stretching in the two stretching directions can be performed separately (sequentially) or in a single process step (simultaneously). In sequential stretching, stretching in the longitudinal direction (machine direction; "MD") and in the transverse direction (transverse direction; "TD") is performed separately. Typically, stretching in the longitudinal direction is performed first, followed by stretching in the transverse direction.

[0004] Known processes for producing biaxially oriented films include blown film processes (e.g., a double bubble process) and flat film processes (BO / LISIM ®< / MESIM ®<). The different processes are accordingly the simultaneous blown film process (double bubble) or the flat film process, which can be divided into simultaneous (LISIM ®< , MESIM ®<, etc.) and sequential processes.

[0005] Thermoforming is a primary polymer conversion process. In this process, a film is heated to its rubbery elasticity and then deformed, for example, through mechanical or pneumatic forming (e.g., vacuum drawing), whereby the surface of the film is shaped into a three-dimensional object. Major advantages of this technology include energy and manufacturing efficiency, inexpensive thermoforming tools and machines, and the ability to form multilayer films, cavitation, and pre-printed films.

[0006] This process typically involves the forming of amorphous and semi-crystalline polymers such as polyvinyl chloride (PVC), cast polypropylene (cast PP or CPP), cast polyethylene terephthalate (cast PET), polystyrene (PS), biaxially oriented polystyrene (BOPS), ethylene-vinyl alcohol copolymer (EVOH), acrylonitrile-butadiene-styrene copolymer (ABS), etc. Multilayer films containing polyethylene (PE) layers are also used, for example, coextruded multilayer blown films with a PS-EVOH-PE or PP-EVOH-PE three-layer structure. Depending on the combination, the polyethylene layers can be used to cost-effectively incorporate improved heat-sealing properties or barrier properties into a product.

[0007] The problem with thermoplastics such as polyethylene and polypropylene is that orientation causes a decrease in elongation at break. This is problematic in the thermoforming process. For this reason, few biaxially oriented films are suitable for the thermoforming process. This includes BOPS and, to a limited extent, biaxially oriented polypropylene (BOPP). One advantage of oriented polymers in thermoforming is that, due to the stretching, the film is less prone to sagging during heating. Film sagging leads to irregular deformation of the thermoformed product. The reason for film sagging is the linear thermal expansion of the raw material, which proves problematic for film types such as cast polypropylene film. This effect is not observable with biaxially oriented polystyrene.

[0008] Depending on the raw material, polyethylene cast film has very low modulus values ​​and very high shrinkage (and thus low dimensional stability) compared to cast films made of other polymers. Therefore, the use of polyethylene cast film in thermoforming processes is limited to specialized areas. This includes, for example, thermoforming with talc for the consolidation of large-sized fenders. Thus, polyethylene has only a very small market share in the thermoforming market. In contrast to the overall packaging market, which accounts for more than 50%.

[0009] One problem with thermoforming polyethylene is the uneven thinning of the film in more complex shapes at the edges during forming, due to uneven shrinkage and lower dimensional stability. If a film is thermoformed with too little thickness, insufficient material remains at the edges to reinforce them. Another problem is deflection under load, which can occur during the thermoforming process and lead to plastic deformation or material damage. The positive properties of polyethylene include the raw material price, availability, impact resistance, chemical resistance to many acids and alkalis, etc. Oriented polyethylene films suitable for practical use in thermoforming have not yet been produced in sufficient quality.

[0010] A major disadvantage of almost all thermoforming products is that they are not included in established recycling streams. In particular, current products that use biaxially oriented films with polystyrene or polypropylene copolymers (e.g., for low-temperature applications) are not included in recycling streams. As plastic waste is increasingly viewed critically by consumers and regulators around the world, and its disposal is costly, this represents a growing problem.

[0011] Examples of thermoformed parts made of various polymers include packaging (food, medicine), industrial applications (hoods, panels for mechanical and plant engineering, commercial vehicle construction, fittings), and consumer goods for the production of refrigeration equipment. Biaxially oriented polystyrene is used in thermoforming to achieve high rigidity with high transparency. It is frequently used in packaging where the packaged product must be clearly visible.

[0012] EP 3 152 128 B1 discloses a multilayer thermoplastic film comprising at least five layers, two of which inner layers each contain a high-density polyethylene (HDPE). In one embodiment, these two inner layers make up at least 35% of the total thickness of the multilayer thermoplastic film. Example 1 describes a film in which the thickness of the HDPE layers accounts for approximately two-thirds of the film thickness. The multilayer thermoplastic film according to Example 1 is produced by a single-bubble blown film process. The films are used for the production of blisters by thermoforming.

[0013] US 2016 / 020030 A1 discloses a process for producing a multilayer biaxially oriented film. The film is produced using a three-bubble blown film process. The film contains no more than 30% polyolefin, including LDPE and LLDPE.

[0014] German Patent Application DE 10 2005 041 134 A1 discloses a three-layer film comprising a printable thin film, an adhesive layer, and a thermoforming film (suitable for thermoforming). The thin film can be biaxially oriented. It can also contain HDPE. The thermoforming film can also contain polyethylene. However, it is not disclosed that the thermoforming film can be biaxially oriented.

[0015] Patent EP 1 395 415 B1 discloses a biaxially oriented film comprising up to 15 wt.% polyethylene. Published patent application EP 1 876 010 A1 discloses biaxially oriented polypropylene films for thermoforming, which may contain foamed polymers, such as polyethylene.

[0016] US Patent No. 10,752,740 B2 describes biaxially oriented films suitable for thermoforming.

[0017] It is disclosed that a polypropylene content of at least 20 percent by weight is necessary to achieve the required properties. The films may also contain polyethylene. Object of the invention

[0018] The object of the present invention was to provide a thermoformable film that is as pure as possible and therefore suitable for recycling. Furthermore, the film should be as inexpensive to produce as possible and preferably less expensive than conventional thermoforming films.

[0019] Furthermore, the aim is to provide a film that has the best possible barrier properties for oxygen, air, and water. The films should preferably also have the highest possible modulus of elasticity. The shapes produced therefrom should preferably have the highest possible dimensional stability. A further object of the present invention is to produce a uniformly thermoformable film which, after thermoforming, has a balanced balance of mechanical properties. The present invention further aims to provide polyethylene films in a form suitable for thermoforming. In addition, the object of the present invention was to enable more weight-efficient packaging. More energy-efficient packaging production is also sought.

[0020] Furthermore, it is the object of the present invention to produce a uniformly deep-drawable film which, after thermoforming, produces shaped bodies which have a balanced balance of mechanical properties. Description of the invention BOPE films

[0021] The present invention relates to a biaxially oriented film for thermoforming, characterized in that the film contains at least 85 weight percent polyethylene and at least 95 weight percent polyolefins, based on the total mass of the film. The films according to the invention can be successfully subjected to thermoforming. Effects

[0022] Biaxial orientation of the films according to the invention improves their physical properties. Stiffness, tear strength, and modulus of elasticity increase. Depending on the stretch ratio, the modulus of elasticity can be doubled or more by stretching. Density decreases as a result of stretching. Molded parts made from the films according to the invention exhibit better strength and tear resistance than molded articles made from the same non-oriented cast polyethylene films, at least to the extent that molded articles can be produced from the non-oriented cast polyethylene films. The molded articles can therefore be produced with less material and exhibit sufficiently high stiffness. The aforementioned changes in physical properties are advantageous for thermoforming.

[0023] Furthermore, the molded articles produced from the film according to the invention by the process according to the invention exhibit higher puncture resistance. Overall, biaxial stretching also leads to more homogeneous properties in the longitudinal and transverse directions of the film and thus also to more homogeneous properties of the molded articles produced from it. This is particularly true compared to a monoaxially stretched film. The thickness distribution is also significantly better than with non- or monoaxially oriented films. The same effect also leads to improved processability of the films.

[0024] Furthermore, the optical properties of the film are improved. Transparency and gloss increase through stretching, while haze decreases. The film according to the invention is therefore particularly suitable for packaging and the like where the contents must be clearly visible. The barrier properties against oxygen and air are improved through stretching. Furthermore, the use of the films according to the invention for thermoforming offers the possibility of reducing the weight of the products. This results, for example, in more weight-efficient packaging because the same amount of film material provides a larger film surface for the packaging after stretching. The film according to the invention also leads to more energy-efficient production, since significantly better properties are obtained through stretching with only minimal energy expenditure.Furthermore, compared to biaxially oriented films made of polypropylene or polystyrene, which are also suitable for thermoforming, the films according to the invention have all the advantages associated with the use of polyethylene, such as low raw material price, good availability, impact resistance, good chemical resistance to acids and bases, and so on.

[0025] A further advantage of the film according to the invention is that it does not tend to sag due to the stretching of the film during heating in the thermoforming process.

[0026] All weight percentages stated with reference to this invention are based on the total mass of the film, unless otherwise stated. During the thermoforming process, the film is deformed, and therefore, as with stretching, a type of stretching of the film occurs, albeit irregularly. To better distinguish between stretching during film stretching and stretching during thermoforming, the following nomenclature is used: With reference to stretching during film production, one of the verbs "stretch" or "orient" is used, which have the same meaning within the context of the present invention. With reference to forming or stretching during thermoforming, the verb "deform" (especially also in the form "formed" and the like) is used.

[0027] The high polyethylene content in the films used in this process ensures that the molded articles can be recycled into the polyethylene recycling cycle. This is a key advantage of the high polyethylene content. In this way, the film waste generated during the production of the film or molded articles can be disposed of cost-effectively, namely through recycling. Expensive waste disposal is no longer necessary. Furthermore, the molded articles produced from the films according to the invention are not subject to the restrictions for non-recyclable plastic goods, which significantly increases their acceptance in retail. Furthermore, consumer acceptance is significantly increased. composition

[0028] The films according to the invention can consist entirely of polyethylene, i.e., contain 100 percent by weight polyethylene. However, the films must contain at least 85 percent by weight polyethylene. If a polyethylene film containing less than 95 percent by weight polyethylene is used in the process according to the invention, the film must contain at least enough other polyolefins that the sum of the mass fraction of all polyolefins, including the polyethylene, is at least 95 percent by weight. For the sake of clarity, it should be emphasized here that, for the purposes of the present application, polyethylene is also a polyolefin. Films according to the invention that contain at least 90 percent by weight polyethylene are particularly preferred. Most preferred are films according to the invention that contain at least 95 percent by weight polyethylene. All films according to the invention comply with the regulations for most polyethylene recycling cycles.

[0029] The term polyethylene encompasses both homopolymers and copolymers which, in addition to ethylene, may contain up to 6 percent by weight, and preferably no more than 5 percent by weight, of other olefins as comonomers. The other olefins are very particularly preferably present in the copolymers in an amount of up to 3 percent by weight, and most preferably in an amount in the range of 0.1 to 3 percent by weight. These amounts are each based on the total mass of the copolymer. These olefins are preferably C3 to C20 olefins. Likewise, they are preferably alpha-olefins. C3 to C20 alpha-olefins are particularly preferred. The alpha-olefins are in turn preferably selected from the group consisting of propene, 1-butenes, 1-pentenes, 4-methyl-1-pentene, 1-hexene, 1-octene and 1-decene and particularly preferably selected from the group consisting of propene, 1-butene, 1-hexene and 1-octene.Other possible comonomers include cyclic olefins. Comonomers can also be olefins containing functional groups. Such olefins can be selected from the group consisting of dienes, aromatic olefins, olefins containing carboxylic acid groups, olefins containing ester groups, and olefins containing cyanide groups. An example of a diene would be butadiene. An example of a suitable aromatic olefin is styrene. Examples of olefins containing carboxylic acid groups and ester groups include acrylic acid and its esters, methacrylic acid and its esters, and maleic acid, the latter being preferred. Monomers containing heteroatoms other than oxygen atoms are not preferred.

[0030] For the purposes of specifying the polyethylene content of the films used in the process according to the invention, the polyethylene copolymers described herein are fully counted as the polyethylene content. For example, a polyethylene film containing 85 percent by weight of a polyethylene copolymer and no other polymers containing ethylene monomers is considered, for the purposes of this application, to be a polyethylene film containing 85 percent by weight of polyethylene.

[0031] Mixtures containing several different polyethylene homopolymers or mixtures of several different polyethylene copolymers may also be used. Furthermore, mixtures of one or more polyethylene homopolymers with one or more polyethylene copolymers may be used.

[0032] Preference is given to films according to the invention which are characterized in that they contain a polyethylene selected from the group consisting of high-density polyethylene (HDPE), medium-density polyethylene (MDPE) and linear low-density polyethylene (LLDPE) and mixtures thereof. The films according to the invention particularly preferably contain a polyethylene selected from the group consisting of high-density polyethylene (HDPE), linear low-density polyethylene (LLDPE) and mixtures thereof. The films according to the invention very particularly preferably contain high-density polyethylene (HDPE). The films according to the invention particularly preferably contain exclusively the aforementioned polyethylenes.

[0033] If the film according to the invention contains other polyolefins in addition to polyethylene, these can be any desired polyolefins. These are preferably polyolefins that contain alpha-olefins as monomers. They preferably contain exclusively alpha-olefins as monomers. The polyolefins are preferably selected from the group consisting of polypropylene, polybutylene, polyhexene and polyoctene. Furthermore, the polyolefins can be, for example, a polyolefin selected from the group consisting of polystyrene, polyacrylic acid, polymethacrylic acid and polyacrylic acid esters, polymethacrylic acid esters. The polyolefins used alongside polyethylene in the films of the process according to the invention can be present in mixtures (blends) with the polyethylene or the polyethylenes or used separately, for example in other layers. They can be used, for example, as a separate layer in the polyethylene film.

[0034] Polyethylenes produced by Ziegler-Natta polymerization are also preferred. Polyethylenes produced by suspension polymerization are also preferred. Polyethylenes produced by Ziegler-Natta polymerization in suspension are most preferred, especially HDPE produced using this method.

[0035] The biaxially oriented film according to the invention is suitable for use in a thermoforming process. Orienting (also known as stretching or drawing) the film improves its properties and thus improves its suitability for thermoforming. Stretching films and thermoforming are similar processes. During stretching, the films are heated until they are easily deformable and stretched in length and / or width, i.e., perpendicular to the thickness. Due to the plastic properties of the film material, the films thereby change their shape (length, width, and height). The deformation occurs essentially regularly across the entire film. During thermoforming, the films are heated until they are easily deformable and then deformed by the application of a force perpendicular to one of their surfaces, i.e., in the thickness direction. Here, too, the film material is stretched, but irregularly and not in the plane of the film surface.Some parts of the film are significantly deformed (stretched), while others are only slightly deformed or not stretched at all. However, the ability of polyethylenes to be stretched (oriented) is limited. This is a material property that can be determined as elongation at break from the stress-strain diagram of the respective polyethylene. To prevent the films according to the invention from tearing at the highly stretched points during thermoforming, they should therefore exhibit high elongation at break. Since both orientation and thermoforming are carried out just below the melting point of the polyethylene, elongation at break is relevant at this temperature.Therefore, a film according to the invention is characterized in that the polyethylene has an elongation at break of at least 7 in the stress-strain diagram, wherein the stress-strain diagram is measured at a temperature 10°C below the melting point of the polyethylene and the melting point is determined by means of differential scanning calorimetry with a heating rate of 10°C per minute. The elongation at break is particularly preferably at least 8 and very particularly preferably at least 9. The elongation at break is given as a multiple of the original length of the sample. This is illustrated by . Figure 3This shows the stress-strain diagrams of three different polyethylenes. As can be seen, type C polyethylene breaks at a stretch ratio of approximately 3.7. This film therefore does not meet this characteristic. Polyethylenes A and B break at a stretch ratio of approximately 10.7 and approximately 9, respectively. These two polyethylenes therefore have an elongation at break of 9 or more.

[0036] The inventors' investigations have further shown that those polyethylenes are particularly suitable which exhibit the smallest possible change in stress in the plastic range of the stress-strain diagram with increasing strain. High stress changes lead to different wall thicknesses being formed when a force is applied during thermoforming, whereby very thin wall thicknesses can also occur or even cracks can occur if the stress changes are too great. It is therefore preferred that the stress-strain diagram of the polyethylene used exhibits an approximately straight line shape in the plastic range. This property is particularly found in tough-hard polyethylenes (see types A and B in Figure 3). These have a yield point, after which the stress decreases slightly. After that, these curves generally show at least partially an approximately straight course. Therefore, a film according to the invention is preferred which is characterized in that the stress-strain diagram of the polyethylene has an upper yield point and the yield stress is the highest stress in the stress-strain diagram, wherein the stress-strain diagram is measured as stated above. The yield stress is the stress at the yield point. This can also be determined from the Figure 3 illustrated. Type C has no yield strength. Types A and B do have a yield strength. However, at higher strains, Type B exhibits stresses above the yield stress. However, this is not the case with Type A, and this type is particularly suitable because its stress-strain diagram exhibits no stress above the yield stress.

[0037] It is also particularly advantageous if the stress in the stress-strain diagram of the polyethylene does not increase, or only slightly, at strain values ​​greater than the yield strain (strain at the yield point). If the stress increases at any point after the yield point, the curve forms a reversal point, the lower yield point, as shown in Figure 3 for polyethylenes of types A and B. Such an increase is called stress strain hardening. In this case, a film according to the invention is preferred which is characterized in that the stress-strain diagram of the polyethylene additionally has a lower yield point, and all stress values ​​at strain values ​​higher than the strain at the lower yield point are lower than the average of the yield stress and the stress at the lower yield point.

[0038] Even more preferred is a film according to the invention characterized in that the stress-strain diagram of the polyethylene exhibits no strain hardening at all. Such stress-strain diagrams have an upper yield point but no lower yield point. Thus, no increase in stress occurs at strains higher than the yield point. Characteristics

[0039] Preferred films according to the invention are those characterized by a modulus of elasticity of 800 MPa. More preferably, the modulus of elasticity is at least 1000 MPa, and most preferably at least 1100 MPa. A high modulus of elasticity is advantageous for most thermoforming applications. Films with a high modulus of elasticity impart not only high strength and rigidity to the produced molded part, but also high dimensional stability, which are important for producing and maintaining the desired shape of the molded article.

[0040] The films according to the invention preferably have a heat shrinkage in the longitudinal direction of at most 5%, more preferably of at most 3%, and most preferably of at most 2%. Furthermore, the films according to the invention have a heat shrinkage in the transverse direction (TD) of at most 5%, more preferably of at most 3%, and most preferably of at most 2%. It is also preferred that the films according to the invention have a heat shrinkage in both the longitudinal and transverse directions of at most 5%, more preferably of at most 3%, and most preferably of at most 2%. The heat shrinkage is measured in each case according to BMS TT 0.9 at 100°C and a duration of 5 minutes.

[0041] The films according to the invention preferably have a thickness in the range from 25 µm to 2000 µm. Particular preference is given to films according to the invention having a thickness in the range from 100 to 2000 µm. Very particular preference is given to films according to the invention having a thickness in the range from 100 to 1000 µm, and most preference is given to films having a thickness in the range from 150 to 500 µm. These are the film thicknesses as determined according to DIN 53370. Greater film thicknesses can also be obtained by laminating biaxially oriented, thinner films. Films with such thicknesses, after thermoforming, produce molded articles with thicknesses suitable for most industrial purposes. Furthermore, a film according to the invention is preferred which has a 2-sigma value of the film thickness distribution of 15% or less.The films according to the invention particularly preferably have a 2-sigma value of the film thickness distribution of 10% or less, and very particularly preferably a 2-sigma value of the film thickness distribution of 7% or less. Films according to the invention with a 2-sigma value of the film thickness distribution of 5% or less are most preferred.

[0042] The width of the films according to the invention is preferably in the range from 10 to 50 cm and particularly preferably in the range from 20 to 30 cm.

[0043] A further preferred subject of the present invention is a biaxially oriented film for thermoforming, which is characterized in that the film contains at least 85% by weight of polyethylene and at least 95% by weight of polyolefins, based on the total mass of the film, wherein the polyethylene has an elongation at break of at least 8 in the stress-strain diagram, wherein the stress-strain diagram of the polyethylene has an upper yield point and the yield stress is the highest stress in the stress-strain diagram and wherein the film has a thickness in the range of 25 µm to 2000 µm, a 2Sigma value of the thickness distribution of 10% or less, a shrinkage in the longitudinal and transverse directions of at most 5% and wherein the stretch ratio in the longitudinal and transverse directions is in the range of 1.2 to 5.5,where the stress-strain diagram is measured at a temperature 10°C below the melting point of the polyethylene and the melting point is determined by differential scanning calorimetry with a heating rate of 10°C per minute.

[0044] A further preferred subject of the present invention is a biaxially oriented film for thermoforming, which is characterized in that the film contains at least 80% by weight of polyethylene and at least 95% by weight of polyolefins, based on the total mass of the film, wherein the polyethylene has an elongation at break of at least 8 in the stress-strain diagram, wherein either (1) the stress-strain diagram of the polyethylene has an upper and a lower yield point and all stress values ​​at elongation values ​​higher than the elongation at the lower yield point are less than the average of the yield stress and the stress at the lower yield point or (2) the stress-strain diagram of the polyethylene has an upper yield point and does not show any strain hardening and wherein the film has a thickness in the range of 25 µm to 1000 µm, a 2Sigma value of the thickness distribution of 7% or less,has a shrinkage in the longitudinal and transverse directions of at most 3% and wherein the stretch ratio in the longitudinal and transverse directions is in the range of 1.2 to 5.0, wherein the stress-strain diagram is measured at a temperature 10 °C below the melting point of the polyethylene and the melting point is determined by means of dynamic differential calorimetry with a heating rate of 10 °C per minute.

[0045] A further preferred subject matter of the present invention is a biaxially oriented film for thermoforming, which is characterized in that the film contains at least 80% by weight of polyethylene and at least 95% by weight of polyolefins, based on the total mass of the film, wherein the polyethylene has an elongation at break of at least 9 in the stress-strain diagram, wherein either (1) the stress-strain diagram of the polyethylene has an upper and a lower yield point and all stress values ​​at elongation values ​​higher than the elongation at the lower yield point are less than the average of the yield stress and the stress at the lower yield point or (2) the stress-strain diagram of the polyethylene has an upper yield point and does not show any strain hardening and wherein the film has a thickness in the range of 150 µm to 500 µm, a 2Sigma value of the thickness distribution of 5% or less,has a shrinkage in the longitudinal and transverse directions of at most 2% and wherein the stretch ratio in the longitudinal and transverse directions is in the range of 1.2 to 3, wherein the stress-strain diagram is measured at a temperature 10°C below the melting point of the polyethylene and the melting point is determined by means of dynamic differential calorimetry with a heating rate of 10°C per minute.

[0046] The films according to the invention can have one or more layers and are therefore single-layer films or multilayer films. They preferably have up to nine layers. Some preferred films are described below: (i) Single-type, single-variety, similar-variety films

[0047] The polymers contained in the films according to the invention are preferably single-type, single-grade, or similar to grades according to DIN EN ISO 11469. The polymers can be selected from the group consisting of pure high-density polyethylene (HDPE), linear low-density polyethylene (LLDPE), and mixtures thereof. Particular preference is given to films according to the invention that comprise exclusively high-density polyethylene (HDPE) or exclusively linear low-density polyethylene (LLDPE). Films that contain exclusively single-type or single-grade polymers are particularly well-suited for recycling. (ii) Films for metallization and metallized films

[0048] In order to improve the barrier properties and / or the optical properties and / or other properties of the films according to the invention, the films according to the invention can be metallized. This refers to the application of a metal or metal oxide layer to the outside of at least one of the two outer layers of the film. To improve the adhesion of the metal or metal oxide layer, a layer with high surface energy is preferably applied to the outer polyethylene layer. The layer can be selected, for example, from the group consisting of polyamide layers, polyurethane layers, ethylene-vinyl alcohol copolymer layers (EVOH), terpolymer layers with surface treatment, polyethylene terephthalate layers (PET), and biaxially oriented polypropylene layers with surface treatment by corona, plasma, or flame. Other suitable layers are known to the person skilled in the art.A metal or metal oxide layer is then applied to the high surface energy layer. An exemplary structure for such a film contains an outer layer made of HDPE, a base layer made of HDPE, and another outer layer with a high surface energy, the latter preferably containing polyurethane. In a further embodiment of this film, a layer containing an adhesion promoter is inserted between the base layer made of HDPE and the outer layer with a high surface energy. The adhesion promoter can be selected from the group consisting of a polymer containing maleic acid as a monomer, chlorinated polyolefins (CPOs), acylated polyolefins (APOs), silane adhesion promoters, and organometallic adhesion promoters. The adhesion promoter is preferably a polymer containing maleic acid as a monomer. This is preferably an ethylene-maleic acid copolymer. (iii) Films with additives

[0049] The films according to the invention may contain auxiliaries. The auxiliaries may be selected from the group consisting of nucleating agents, in particular alpha-nucleating agents, antiblocking agents, adhesion promoters, dispersants, stabilizers, lubricants, antistatic agents, solid plasticizers, activating agents, promoters, anti-aging agents, burn mark preventers, binders, heat-resistant agents, initiators, polymerization catalysts, emulsifiers, plasticizers, heat stabilizers, light stabilizers, flame retardants, mold release agents, modifiers, clarifiers, and antifogging agents. Dispersants allow the stretching of films with a high filler content (for example, antiblocking agents) and result in a smooth surface. If these auxiliaries are present, the film according to the invention preferably contains auxiliaries in an amount of less than 5 percent by weight, based on the total mass of the film.A film according to the invention particularly preferably contains less than 3 percent by weight of excipients. The excipients are preferably present in an amount of at least 0.1 percent by weight, and more preferably in an amount of 1 percent by weight. If the film contains extenders, these are preferably selected from the group consisting of polyethylene waxes and polypropylene waxes. Polypropylene waxes are preferred. Suitable examples include commercially available polyethylene waxes and polypropylene waxes of the Licocene® type from Clariant in Frankfurt / Main, Germany.

[0050] An exemplary embodiment of such a film is a film with a three-layer structure comprising two outer layers containing HDPE and an inner base layer containing HDPE. Preferably, the three layers contain exclusively HDPE as the polymer. The polymers contained in these films are therefore single-type or single-variety, making the films easily recyclable. One or more of these layers can be provided with additives, as described above. (iv) Coloured films

[0051] The films according to the invention can also contain dyes. This allows the films to be customized for specific applications and adapted to specific requirements. For example, they can be white films, which have the advantage of eliminating the need for primer for printing. The films can also have different printing zones. This also leads to savings in ink when printing the films. Furthermore, such films are generally more easily recycled than corresponding printed films. Examples of structures include a single-layer film made of polyethylene containing a color masterbatch. Furthermore, a three-layer film in which the three layers contain HDPE or LLDPE (preferably HDPE) and one or both outer layers contain a masterbatch. The preferred color is white. (v) Opaque film

[0052] These are films according to the invention in which at least one layer is provided with cavities ("cavitated"). Such films are ideally suited for form-fill-seal (FFS) machines because of their soft texture. These films according to the invention also have high gloss, low water permeability, reduced weight, are directly printable, and can be produced with a smaller amount of polyethylene. The cavities can be created by adding particles ("cavitating agents"). These particles can contain, for example, materials selected from the group consisting of calcium carbonate, polybutylene terephthalate, polymethyl methacrylate, polyamide 6, and mixtures of such materials. The particles preferably consist of such materials. This layer preferably contains calcium carbonate particles. The particles can be present in an amount of up to 5% by weight.-% in the films according to the invention, based on the total mass of the film.

[0053] A suitable embodiment for this is a 5-layer structure with a base layer containing HDPE and voids. On each surface of the base layer there is a layer containing a blend of LLDPE and HDPE and a dye. On the outer surface of each of these layers there is an outer layer made of LLDPE, which may also contain a dye. The layers containing the blend of HDPE and LLDPE serve as adhesion promoters between the outer layer, which contains LLDPE, and the base layer, which contains HDPE. The dye is preferably white in all layers. Because of the outer layers made of LLDPE, such a film is sealable without the need for further modifications. (vi) Reduced density films

[0054] These are also films according to the invention in which at least one layer is provided with cavities ("cavitated"). Such films are ideally suited for form-fill-seal (FFS) machines. The films exhibit a reduced weight for the same dimensions compared to films without cavities. This leads to material savings and thus to lower costs. An exemplary structure for such a film consists of a base layer made of HDPE, which contains cavities and, depending on the desired density of the film, contains 5 to 25 weight percent of particles ("cavitating agents"), based on the total mass of the base layer. In this embodiment, the particles are not included in the total mass of the film. The materials can be selected from the above list. A layer of HDPE can be applied to each side of this base layer. Such films exhibit low water permeability and are directly printable. (vii) Films comprising layers containing ethyl-vinyl alcohol copolymers and / or polyamides

[0055] These films contain one or more layers containing a polymer selected from the group consisting of ethyl vinyl alcohol copolymers (EVOH), polyamides, and blends thereof. The polyamides can also be polyurethanes. The advantages of such films include a better barrier effect against the diffusion of water and oxygen. Additionally, puncture resistance is improved.

[0056] An example of such a film is a 7-layer film with a base layer made of HDPE. On one side of the film, a layer containing an adhesion promoter is applied, on which, in turn, a layer containing ethyl-vinyl alcohol copolymers, polyamides, or mixtures thereof is applied. On top of this layer, another layer containing an adhesion promoter is applied, and finally, an outer layer of the film containing LLDPE is applied. On the other side of the base layer, a layer containing a blend of HDPE and LLDPE is applied, on which, in turn, an outer layer of the film containing LLDPE is applied. (viii) Heat-sealable film which can be produced by co-extrusion

[0057] These films have a base layer that makes up at least 70 percent by weight based on the total mass of the film, preferably at least 80 percent by weight, and most preferably at least 85 percent by weight. This base layer contains HDPE, preferably at least 80 percent by weight HDPE, more preferably at least 90 percent by weight, and most preferably at least 95 percent by weight. Most preferably, this base layer consists of HDPE. On at least one side of this base layer, a layer consisting of a blend containing HDPE and LLDPE is applied. An outer layer of the film containing LLDPE is applied to this layer. Such a film thus contains at least three layers. The outer layer ensures that the film is heat-sealable. The layer containing the above-described blend serves as an adhesion promoter between the base layer and the outer layer.The blend-containing layer and the outer layer containing LLDPE can also be applied to both sides of the base layer. In this case, the film contains five layers.

[0058] Such films have the advantage of being sealable without further modification and suitable for Form Fill Seal (FFS) machines. Unlike films with only one HDPE layer, this eliminates the need for a lamination step to make the films heat-sealable. Furthermore, the films can be easily manufactured by co-extrusion in a single process step. (ix) Films with outer layers containing polyolefins other than polyethylene

[0059] Films that contain a proportion of polyolefins are still recognized as pure in many recycling systems. At the same time, polyolefins that are different from polyethylene increase the thermal stability or can impart other properties to the films. Such films can, for example, have the following structure: On one side of a base layer made of polyethylene, a layer containing a blend of polyethylene and a copolymer of at least two different olefins is applied. On top of this layer containing the blend, an outer layer of the film is applied, which contains a copolymer of at least two different olefins. This is preferably the same copolymer that was used in the layer containing the blend. Any polymer that imparts the desired properties to the film can be considered as a copolymer. In particular, it can also be a terpolymer.The copolymers preferably contain at least one monomer selected from the group consisting of ethylene, propene, 1-butene, 1-hexene, 1-octene, and 1-decene. The copolymers preferably contain exclusively monomers selected from this group. However, the copolymers may also contain other monomers in amounts of up to 5 percent by weight. These other monomers are preferably selected from the group consisting of acrylic acid, acrylic acid esters, methacrylic acid, methacrylic acid esters, acrylonitrile, butadiene, styrene, and maleic acid.

[0060] The other side of the base layer can either be the outer side of the film, or the same layer structure can be applied to both sides. However, the other side of the base layer is preferably coated with a layer containing a blend of HDPE and LLDPE, to which an outer layer of LLDPE is applied. Such a film is also heat-sealable, which is ideal for packaging purposes. (x) HDPE-LLDPE blends

[0061] Both HDPE and LLDPE are particularly preferred for use in the films of the invention. Their combined use in a film allows for the adjustment of various film properties by varying the amounts of HDPE and LLDPE. A higher proportion of HDPE leads to improved mechanical properties, such as elastic modulus and shrinkage. A higher proportion of LLDPE leads to better properties such as appearance, puncture resistance, and tear resistance. At the same time, the use of blends can improve the adhesion of HDPE and LLDPE to each other, resulting in better bond strength between the layers when HDPE and LLDPE layers are used in a film.

[0062] These films have a base layer containing HDPE. A layer of a blend containing HDPE and LLDPE is applied to at least one side of this base layer. An outer layer of the film containing LLDPE is applied to this layer. Such a film thus contains at least three layers. The LLDPE outer layer can be heat-sealable. The layer containing the above-described blend serves as an adhesion promoter between the base layer and the outer layer. The layer containing the blend and the outer layer containing LLDPE can also be applied to both sides of the base layer. In this case, the film is preferably a five-layer film. This is particularly preferred. (xi) Coated film

[0063] This is a film in which at least one outer layer has been coated with a material. A coating in this sense means that the coating was not created during the extrusion of the film. The coating can be applied during the film production process after extrusion and cooling of the melt, or it can be applied to the finished film in a separate process. The coating can be applied, for example, by top coating or inline coating. Such coatings generally require very little material and can be very thin. The coated films are therefore generally still suitable for recycling. The coating can be used to give the film various properties, thus significantly improving its barrier properties against water and oxygen.This creates the possibility of producing a recyclable film with good barrier properties. The coating material is generally not polyolefins. It can be polyurethane, for example.

[0064] As an example of a suitable film structure, a film is given that has a base layer containing HDPE, to which an adhesion promoter (for example, Admer from Mitsui) is applied on at least one side, to which the coating is applied. Preferably, both surfaces of the base layer are covered by another layer containing HDPE. The adhesion promoter and coating are then applied to at least one surface of these layers. This 5-layer film is preferred. However, the adhesion promoter and coating can also be applied to the surfaces of both HDPE layers applied to the base layer. (xii) Laminates

[0065] There are various desired properties of films that cannot be achieved, or can only be achieved with difficulty, by extrusion. For this purpose, the films according to the invention can also be further processed by lamination to form further films according to the invention. Such laminates can be produced by adhesive lamination, extrusion lamination, hot-roll lamination, and other processes known to those skilled in the art. The laminates contain at least 75% by weight and preferably at least 95% by weight of the films according to the invention as layers and correspondingly less than 25% by weight, preferably less than 5% by weight, of layers containing other materials. The other laminated layers can be, for example, an LDPE blown film, which allows the sealing of the laminated film.These laminates may further contain 5% by weight of materials that are not polyolefins, and in particular not polyethylenes. Such materials can be used, in particular, to obtain a film that is easily thermoformable. These materials can be polymers selected from the group consisting of polyamides, ethylene-vinyl alcohol copolymer (EVOH), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), and acrylonitrile-butadiene-styrene copolymer (ABS). Such polymers can be used, for example, to modify the barrier properties or the surface properties of the films according to the invention. However, the laminates according to the invention must contain at least 75% by weight of biaxially oriented polymers. This favors thermoforming.

[0066] Such a film could, for example, be a base layer made of biaxially oriented HDPE to which a (non-oriented) LLDPE blown film is laminated. This film can achieve a hermetic seal after thermoforming. (xiii) Laminates of various films according to the invention

[0067] The films according to the invention, and in particular the films according to the invention mentioned under points (i) to (vii), can also be laminated together. This allows for the combination of various properties that cannot be imparted to a film through an extrusion process.

[0068] Such a film could, for example, be a base layer made of biaxially oriented HDPE, onto which a biaxially oriented LLDPE film is laminated. Even with such a film, a hermetic seal can be achieved after thermoforming.

[0069] The phrases "HDPE film," "LLDPE film," "HDPE layer," "LLDPE layer," and the like each describe films or layers that contain or consist of the polymers mentioned. In all cases (i) to (xiii) described above, in each case where it is described that a layer contains a specific polymer (e.g., HDPE, LLDPE, EVOH, PA, etc.), the case in which the layer consists of this polymer is also included, unless otherwise stated. Unless otherwise stated, this is also the preferred embodiment for all films according to the invention. Process for producing BOPE

[0070] Another object of the present invention is a process for producing a film according to the invention comprising the following steps: Extrusion of a film comprising at least one layer comprising polyethylene from at least one polymer melt, cooling the film to form a film, stretching the film in the longitudinal direction and in the transverse direction, wherein the stretch ratio in the longitudinal direction is in the range of 1.2 to 9 and the stretch ratio in the transverse direction is also in the range of 1.2 to 9.

[0071] The procedure is described in more detail below: (1) Recipe & Dosage

[0072] The raw material used to produce the films according to the invention can either be provided as such or it can be produced by mixing several ingredients during film production in an extruder or mixer. Plastic granules are preferably used as the raw material. If the film has multiple layers, a separate raw material can be provided for each layer. Depending on the desired thickness of each layer, the raw material is dosed separately for each layer.

[0073] The above statements regarding the film according to the invention apply to the polyethylene used in this process. A preferred process according to the invention is characterized in that the polyethylene used has a density in the range of 0.89 to 0.98 g / cm 3 . A density in the range of 0.91 to 0.97 is particularly preferred.

[0074] The polyethylene used in the process according to the invention further preferably has an MFI (190 °C / 2.16 kg) in the range of 0.3 to 8 g / 10 min, particularly preferably in the range of 0.7 to 5 g / 10 min and very particularly preferably in the range of 0.7 to 3 g / 10 min.

[0075] The thickness of the cast film in the process according to the invention is preferably 300 to 5000 µm, particularly preferably 500 to 3000 µm. (2) Extrusion

[0076] The raw material or raw materials or their ingredients are fed into an extruder. They are mixed and melted in the extruder. If the film only has one layer, only one extruder is required. If the film has multiple layers, a separate extruder is usually used for each layer. However, multiple layers can also be produced from the same plastic granulate if two or more layers have the same composition. In this case, the material for two or more of these layers can be extruded from the same extruder. Furthermore, all layers can be extruded from one extruder if all layers have the same composition. Any type of extruder can be used. The extruder or extruders are preferably selected from the group consisting of single-screw extruders, cascade extruders and twin-screw extruders. Other mixing and processing units can also be used, such as:A bus kneader or a planetary roller extruder can be used. The extruders are preferably operated at a temperature in the range of 180 to 280 °C, more preferably at a temperature in the range of 200 to 260 °C, and most preferably at a temperature in the range of 230 to 250 °C. (3) Nozzle and cooling roller

[0077] The melt is discharged through a slot die and deposited on a chill roll. If the film has multiple layers, a multi-layer die is preferred. A device for applying the film to the chill roll (pinning system) is preferably used to improve the film's adhesion to the chill roll. A preferred device is an air knife, i.e., a directed air stream that exerts a force on the film in the direction of the chill roll. A pinning system leads to more uniform contact between the film and the chill roll, thus producing a smooth and even cast film.

[0078] The temperature of the chill roll is preferably in the range of 20 to 100 °C. If the layer applied to the chill roll contains a high proportion of HDPE, the temperature of the film should preferably be set to higher temperatures within this range. If the layer applied to the chill roll contains more than 60% by weight HDPE, the temperature of the chill roll is preferably in a range of 60 to 100 °C, more preferably in a range of 70 to 100 °C, and most preferably in a range of 75 to 90 °C. If the layer applied to the chill roll can be applied to the chill roll more easily than layers containing HDPE (for example LLDPE), the temperature of the chill roll can be lower. If there is a high proportion of LLDPE in the only layer of the film or in the layer of the film applied to the chill roll, the temperature of the film should preferably be set to lower temperatures within this range.If this layer contains more than 60% by weight of LLDPE, the temperature of the cooling roll is preferably in a range of 20 to 50 °C, more preferably in a range of 25 to 45 °C and most preferably in a range of 30 to 40 °C.

[0079] For more effective cooling, the film can also be passed through a cooling bath. The cooling bath is preferably installed near the cooling roller. The cooling bath is preferably a water bath. If a water bath is also used for cooling, the cooling roller can be rotatably mounted in a water bath, with the cooling roller typically immersed more than halfway into the water bath. A corresponding cooling roller is known, for example, from DE 197 05 796 C2. The cooling roller can also be cooled from the inside. (4) Stretching

[0080] The cast film produced in this way is subsequently stretched in the longitudinal direction (machine direction; MD) and in the transverse direction (TD). In the process according to the invention, the stretch ratio in the longitudinal direction and in the transverse direction is preferably in the range from 1.2 to 5.5. The films obtained by such a process allow the production of moldings with simple shapes, as are also possible with biaxially oriented polystyrene, from the films according to the invention, without the films tearing during thermoforming or resulting in a strongly non-uniform thickness distribution. The stretch ratio in the process according to the invention is preferably in the range from 1.2 to 5.0. Very particularly preferably, the stretch ratio in the process according to the invention is in the range from 1.2 to 3. Most preferably, the stretch ratio in the process according to the invention is in the range from 1.2 to 2. These details apply to the longitudinal and transverse directions.Films produced with such a stretch ratio also allow the production of molded articles with complex shapes through thermoforming. Such molded articles, for example, have not been available to date from biaxially oriented polystyrene.

[0081] With the stretched films according to the invention, problems can arise during thermoforming at critical points (thickness uniformity at corners, edges, etc.), as is also observed with other biaxially oriented films, such as films made of biaxially oriented polystyrene or biaxially oriented polypropylene, during the thermoforming process. The lower the stretch ratio in the manufacturing process of the films according to the invention, the better the critical points can be formed during the thermoforming process. Therefore, the goal is to achieve the lowest possible stretch ratio in the process according to the invention while maintaining high stiffness, thickness uniformity, and low shrinkage.

[0082] Biaxial orientation or stretching can be carried out both sequentially and simultaneously. Methods for sequential and simultaneous biaxial orientation or stretching are known to those skilled in the art. One method is described in published patent application DE 10 2019 119293 A1, in particular in paragraphs

[0045] to

[0055] . The method described therein is preferably also used correspondingly for the present invention. (5) Sequential stretching a) Stretching in the longitudinal direction

[0083] The sequential stretching is preferably carried out as follows. The cast film produced as described above is fed to a longitudinal stretching device and, after sufficient heating, is stretched in the machine direction. As stated above, the stretching in the machine direction can have a stretch ratio of 1.2 to 9. The film is normally stretched between pairs of rollers with different rotation speeds. The temperature of the film in the longitudinal stretching machine is preferably in the range of 30 to 140°C, more preferably in the range of 80 to 120°C. The speed of the film during longitudinal stretching is up to 600 m / min. b) Stretching in the transverse direction

[0084] Transverse stretching preferably occurs after longitudinal stretching. During transverse stretching, the film is fed into a transverse stretching device. As stated above, the film is stretched in the transverse direction at a stretch ratio of 1.2 to 9.

[0085] In this process, the film is first preheated. The temperature during transverse stretching is preferably in the range of 50 to 180 °C, particularly preferably in the range of 100 to 150 °C. After the film, already stretched in the longitudinal direction, enters the transverse stretching device, it is gripped laterally by holding clips guided on rails. By moving the holding clips on opposite sides of the film in opposite directions, the film is stretched in the transverse direction. (6) Simultaneous stretching

[0086] The films according to the invention can also be stretched in a simultaneous stretching process. The simultaneous stretching can be carried out by a method selected from the group consisting of the pantograph method, spindle method, LISIM method, and MESIM method. The temperature during simultaneous stretching is preferably in the range of 50 to 180°C, particularly preferably in the range of 80 to 150°C. The film speed is preferably 600 m / min or less. (7) Heat treatment

[0087] After stretching, the film is preferably subjected to heat treatment (also called stabilization or annealing). For this purpose, the film is held at an elevated temperature for a specific period of time. This preferably occurs immediately after stretching. In sequential stretching, the heat treatment is preferably performed immediately after the final stretching. During heat treatment, stresses within the film caused by stretching are relieved. This uniforms the properties across the entire surface of the film. (8) Relaxation

[0088] Preferably, the film is relaxed during heat treatment. This means that the tension exerted on the film by the stretching apparatus is reduced. This allows the film to contract again. This procedure reduces the stresses within the film. Relaxation can occur longitudinally, transversely, or in both directions. Relaxation is preferably performed in both directions.

[0089] It is further advantageous if the extent of relaxation in the process according to the invention is in the range of 2% to 12%, preferably in the range of 5 to 10%. This preferably applies to relaxation in both directions. Relaxation reduces the shrinkage of the film and the shrinkage of the molded article produced from such a film.

[0090] Accordingly, a process according to the invention is particularly preferred in which the stretching is followed by the following process step: Carrying out a heat treatment, wherein relaxation is carried out in the longitudinal direction and in the transverse direction during the heat treatment.

[0091] Stretching the film and performing a heat treatment (annealing) with relaxation results in the shrinkage of the film being reduced. (9) Inline coating

[0092] In the process according to the invention, the film can be coated during stretching. This is preferably done using inline coating technology (ILC). A coating is preferably applied to at least one outer layer of the film during the sequential stretching between the longitudinal and transverse stretching. This can be done, for example, by applying (in particular printing) an aqueous polyurethane suspension to the film surface. Upon heating prior to transverse stretching, the solvent of the suspension evaporates, and the coating is formed. (10) Further treatment

[0093] The biaxially oriented film produced in this way according to the present invention can be further processed by the following steps. The film can then be cut. This is preferably done at the edges to obtain smooth edges. The thickness profile of the film can be measured. If desired, the film can be surface-treated. Finally, the film is wound onto a winder. Furthermore, the film can be cut into slit rolls. Other possible treatment steps include metallization, lamination, coating (barrier, protection), printing, slitting, surface treatment, laser processing, painting, etc. use

[0094] A further object of the present invention is the use of the film according to the invention for thermoforming. Thermoforming process

[0095] A further subject of the present invention is a process for producing a shaped body, characterized in that a film according to the invention is shaped by thermoforming.

[0096] The process is preferably carried out in such a way that a process according to the invention for producing the film according to the invention is carried out and the film thus obtained is deformed by thermoforming.

[0097] Furthermore, the process according to the invention for producing a shaped body according to the present invention is preferably characterized in that the film is formed with a degree of deformation in the range of 2 to 6.

[0098] Different shaped bodies can be formed using standard tools (e.g. positive / negative molds, ILLIG forming and punching tools, or a HYTAC-WFT (epoxy resin with hollow glass spheres and Teflon, mold air reduction, directly cooled hold-down device, and lateral mold air valves)) for deep drawing. For difficult-to-form molded bodies, edges, small radii, etc. of the molds must be specially designed, as is common with thermoforming. Temperature-controlled tools, for example, can be used. In the process according to the invention for thermoforming films, a film is preferably first heated until it becomes rubber-elastic and then deformed into a three-dimensional shaped body. The temperature at which the film assumes a rubber-elastic state depends heavily on the film used. In particular, the necessary temperature depends on the film material.In this process, the film according to the invention is preferably heated to a temperature in the range from 70 to 150 °C, particularly preferably to a temperature in the range from 100 to 140 °C and most preferably in the range from 120 to 130 °C.

[0099] The deformation is preferably carried out by mechanical or pneumatic forming. During pneumatic forming, different gas pressures are applied to the opposing surfaces of the film. These processes are well known to those skilled in the art. The thermoforming process can be carried out in a single step. The thermoforming process is preferably carried out by a process selected from the group consisting of vacuum thermoforming, drape thermoforming, free thermoforming, autoclave thermoforming, and diaphragm thermoforming. Alternatively, a multi-step thermoforming process can be used. This is useful if specific wall thicknesses are to be achieved at specific points on the mold. Among the two-step processes, pressure forming is preferred. The films according to the invention can be processed into various applications by the thermoforming process using thermoforming machines and suitable semi-finished products. molded parts made of polyethylene

[0100] A further subject matter of the present invention is a shaped body formed completely or partially by thermoforming from a biaxially oriented film according to the invention, characterized in that it comprises at least 85% by weight of polyethylene and at least 95% by weight of polyolefins, based on the total mass of the shaped body.

[0101] Such molded parts can be used, for example, in products selected from the group consisting of trays, coffee tabs, blister packs, refrigerator linings, low-temperature packaging, plastic cups and, in particular, yogurt cups, and packaging for, for example, fruit, vegetables, meat, or other foodstuffs. A molded article according to the invention is preferred, characterized in that the film is a film according to the invention. The film according to the invention, in all its embodiments, is particularly suitable for the production of molded articles.

[0102] Also preferred is a shaped body according to the invention which is characterized in that in the part of the shaped body formed by thermoforming, the ratio between the greatest thickness of the formed film and the smallest thickness of the formed film is not more than 2.

[0103] Likewise preferred is a molded article according to the invention which is characterized in that the water permeability (ASTM E 96) is not more than 8 g / m 2 / day (at 30 °C and 90% relative humidity) and particularly preferably not more than 5 g / (m 2 *day). Furthermore, preferred is a molded article according to the invention which is characterized in that the oxygen permeability (ISO 15105-2) is not more than 4000 cm 3 / m 2 / day (at 23 °C and 0% relative humidity), particularly preferably not more than 2700 cm 3 / m 2 / day. The films according to the invention contain a large proportion of polyethylene or consist thereof. Polyethylene films already have favorable barrier properties with regard to water and oxygen. These are further improved by stretching the film. The present films have very low 2-sigma values ​​of the thickness distribution.Molded articles made from these films therefore exhibit only a small number of defects and thin-walled sections. This also leads to improved barrier properties for the molded articles. These properties can be further improved by applying layers with good barrier properties to the films according to the invention, as described herein.

[0104] Short description of the images: Figures 1 and 2 show molded articles according to the invention which were produced from films according to the invention. Figure 3 shows the stress-strain diagrams of three different polyethylenes. List of abbreviations:

[0105] ABSAcrylonitrile butadiene styrene copolymer BObiaxially oriented BOPEbiaxially oriented polyethylene BOPPbiaxially oriented polypropylene BOPSbiaxially oriented polystyrene CPPcast polypropylene (cast PP) Cast PETcast polyethylene terephthalate EVOHEthyl vinyl alcohol copolymers HDPEHigh-density polyethylene LLDPELinear low-density polyethylene MDPEMedium-density polyethylene MDLongitudinal direction, machine direction PAPolyamide PEPolyethylene PETPolyethylene terephthalate PPPolypropylene PSPolystyrene PUPolyurethane PVCPolyvinyl chloride TDTransverse direction, across the machine direction Examples: Measurement methods:

[0106] Film thicknesses: DIN 53370; Layer thicknesses: DIN EN ISO 3146; Tensile strength: ASTM D 882; Elongation at break: ASTM D 882; Modulus of elasticity: ASTM D 882; Coefficient of friction: DIN EN ISO 8295, U / U; Heat shrinkage: BMS TT 0.9; 100 °C / 5 min and BMS TT 0.2; 120 °C / 5 min; Coefficient of friction: DIN EN ISO 8295; Haze: ASTM 1003; Gloss: ASTM 2457; Corona treatment: ASTM 2457; Puncture resistance: DIN EN 14477 (1.0 mm pin, round tip); Oxygen permeability ISO 15105-2; at 23 °C and 0% relative humidity; Water permeability: ASTM E 96, at 30 °C and 90% relative humidity. Example 1: Production of a film according to the invention

[0107] A three-layer film was produced. High-density polyethylene (HDPE) with a density of 0.953 g / cm³ and an MFI of 0.9 g / 10 min (190 °C; 16 kg), produced by suspension polymerization with a Ziegler-Natta catalyst ("Hostalen GD 9555" from Lyondell Basell), was fed into three extruders. A twin-screw extruder with vacuum and a 200 µm filter was used as the main extruder for the middle layer (base layer). The extruder was operated at a temperature of 200 to 260 °C in all zones. A single-screw extruder without vacuum and a 200 µm filter was used as the co-extruder for each of the two outer layers. Both layers were operated at a temperature of 230 to 250 °C in all zones. A three-layer die was used as the extrusion die. The melts were extruded through the die and applied to a chill roll. A standard high-pressure air knife was used to assist in pressing the film against the chill roll.The chill roll temperature was 70-100 °C. The chill roll surface speed was 2 m / min. A water bath was not used. The film was then wound up.

[0108] The cast film was produced with a thickness of 1850 µm and a width of 35 cm. The middle layer constituted 80% of the film thickness, and the other two layers each constituted 10% of the film thickness. The elastic modulus of the resulting cast film was 609 MPa in the longitudinal direction.

[0109] The film was then stretched on a test bench. A rectangle measuring 90 x 90 mm was cut out of the center of the film. This rectangle was clamped into the test bench. Stretching was carried out by simultaneous stretching in the longitudinal and transverse directions. The stretching temperature was 125°C for all tests. The preheating time was 300 seconds. The degrees of stretching and the extent of relaxation for the various films produced according to the invention can be found in Table 1 below. The films were stretched in the longitudinal and transverse directions at the same speed and with the same degree of stretching. Stretching lasted 2 seconds. Some of the films produced (see Table 1) underwent heat treatment. This took place for 5 seconds at 125°C. Finally, the films were relaxed. The extent of relaxation can be found in Table 1.The rate of relaxation was 12% per second. Seven films according to the invention were produced. Table 1 Slide No. 1 2 3 4 5 6 7 Degree of stretch MD x TD 3x3 3x3 3, 5x 3, 5* 3, 5x 3, 5** 4x4 6x6 6x6 Heat treatment Yes Yes Yes Yes Yes No Yes Relaxation [%] 15 10 8 8 - - - Tensile strength MD [MPa] 55 68 73 88 81 135 123 Elongation at break [%] 456 310 289 245 251 80 134 Elastic modulus [MPa] 1264 1212 1203 1209 1171 1523 1247 Film thickness [µm] 210 210 152 150 120 55 55 * Stretching speed 50% to 100% per second ** Stretching speed greater than 100% per second

[0110] As you can see, the elastic modulus roughly doubles when stretched. Example 2: Manufacturing of molded bodies

[0111] Molded bodies were produced by thermoforming from film 1 of Example 1. The film was heated to 125 °C and then molded into molded bodies in a thermoforming system. Thermoforming was carried out by vacuum thermoforming at a temperature of 125 °C. The die speed was 200 mm / s, and the die material was HYTAC-WFT (epoxy resin with hollow glass spheres and Teflon).

[0112] Figure 1shows two molded articles that were produced in this way. As can be seen, they are regularly shaped. The round shape has a diameter of 150 mm at the upper vertex and a height of 20 mm. The film thickness in the center of the molded article is 100 µm. The film thicknesses of the rectangular shapes are in the range of 190 to 210 µm. The stretching is therefore uniform and the formed molded articles have sufficient strength to be used as packaging materials. The lattice of the rectangular molded article also shows that the stretching is very uniform and without any major irregularities. This leads to very regular wall thicknesses of the molded articles, which is a prerequisite for their dimensional stability and barrier effect. The water permeability is less than 6 g / m 2 / day and the oxygen permeability is less than 3500 cm 3 / m 2 / day.

[0113] Figure 2shows a molded part in which the side walls are perpendicular to the original plane of the film. With such a mold, the deformation (stretching) at the side walls is very high. Nevertheless, the mold was successfully manufactured. The side walls have a film thickness of 120 µm, and the bottom (or lid) in the center of the molded part has a film thickness of 210 µm. The side walls are noticeably more transparent than the bottom of the molded part. Example 3:

[0114] Producing a film according to the invention.

[0115] The material, layer structure, and layer thickness of the cast film correspond to the specifications in Example 1. A BT-55-32D main extruder with double vacuum and a 400 µm filter was used as the extruder for the middle base layer. The extruded melt temperature was 258 °C. For each of the two outer layers, a BT-43-30D co-extruder with single vacuum and a 200 µm filter at a temperature of 263 °C was used. An EDI 3-layer die was used. The chill roll had a temperature of 80 °C, and a high-pressure air knife was used to assist in the film's adhesion to the chill roll. A water bath was not used.

[0116] The resulting film was biaxially stretched in a simultaneous process. The film first passes through three preheating zones in the stretching oven, with temperatures ranging from 136 to 144 °C. The film then passes through two stretching zones, with temperatures ranging from 116 to 130 °C. This is followed by two further heating zones, where heat treatment takes place. These further heating zones are operated at a temperature of 105 to 110 °C. Finally, the film passes through a cooling zone, with a temperature of 80 °C.

[0117] The resulting film has a thickness of 51.99 µm and a 2-sigma film thickness of 8.32%. The 2-sigma value is important for the film of the invention because films with a large thickness variation result in defective products during thermoforming. If very thin areas of the film are located in areas subject to strong mechanical stress during thermoforming, the thermoformed products will become leaky and / or exhibit mechanical weaknesses that impair their properties.

[0118] A total of three films were produced. The following table provides further data on this experiment. Table 2 Characteristics Unit Slide No. 8 9 10 Degree of stretch MD x TD 3,2x2,9 4,2x3,8 5x4,2 Thickness - nominal value µm 52.0 30.0 25.0 thickness MD Center µm 51,99 30,47 24,54 TD µm 54,21 31,27 25,16 Tensile strength MD MPa 76 116 130 TD MPa 122 144 149 Elongation at break MD % 330 192 152 TD % 242 148 131 Elastic modulus MD MPa 1340 1221 1296 TD MPa 1399 1178 1194 Friction coefficient r / r* detention 0,20 0,19 0,14 r / r Glide 0,21 0,20 0,16 r / ru* detention 0,26 0,22 0,18 r / ru Glide 0,27 0,24 0,20 ru / ru detention 0,19 0,21 0,19 ru / ru Glide 0,21 0,21 0,18 turbidity % 20,60 10,70 8,09 Shine 45° 40 65 72 Heat shrink 100 / 5 MD % 3,17 3,73 3,43 TD % 2,33 3,87 0,93 Heat shrink 120 / 5 MD % 12,50 14,67 14,20 TD % 13,67 16,43 12,83 Puncture resistance (1.0 mm pen round tip) Power N 9,49 9,31 7,81 Power g 968,2 949,5 796,2 Force / Area N / mm 175,4 280,8 311,8 Work until break mJ 10,35 8,31 6,44 Elongation at break mm 2,12 2,00 1,91

[0119] Notes: Thickness, tensile strength, elongation at break, elastic modulus, haze, and heat shrinkage were measured immediately after production. All other measurements were taken one day later. Abbreviations: r: up; ru: down.

Claims

1. Biaxially-oriented film for thermoforming, wherein the film contains at least 85% by weight polyethylene and at least 95% by weight polyolefins, based on the total mass of the film, characterised in that the polyethylene has an elongation at break of at least 7 in the stress-strain diagram, wherein the stress-strain diagram is measured at a temperature of 10°C below the melting point of the polyethylene and the melting point is determined by differential scanning calorimetry using a heating rate of 10°C per minute.

2. Film according to claim 1, characterised in that the stress-strain diagram of the polyethylene has an upper yield point and the yield stress is the highest stress in the stress-strain diagram, wherein the stress-strain diagram is measured as specified in claim 1, in particular wherein the stress-strain diagram of the polyethylene additionally has a lower yield point and all stress values in the case of strain values higher than the strain at the lower yield point are lower than the mean value of the yield stress and the stress at the lower yield point, in particular wherein the stress-strain diagram of the polyethylene exhibits no stress-strain hardening.

3. Film according to one of the preceding claims, characterised in that the polyethylene is a polyethylene chosen from the group consisting of HDPE, MDPE and LLDPE, and / or in that the modulus of elasticity of the film is at least 800 MPa, and / or in that the shrinkage of the film is at most 5% in the longitudinal direction and at most 5% in the transverse direction.

4. Film according to one of the preceding claims, characterised in that the film has a thickness in the range from 25 µm to 2000 µm, and / or in that the film has a 2-sigma thickness distribution value of 15% or less.

5. Film according to one of the preceding claims, characterised in that the stretch ratio in the longitudinal and in the transverse direction lies in the range from 1.2 to 5.5.

6. Process for producing a film according to one of claims 1 to 4, characterised in that the film is produced according to a process comprising the following steps: - extruding a film comprising at least one layer comprising polyethylene composed of at least one polymer melt, - cooling the film to form a film, - elongating the film in the longitudinal direction and in the transverse direction, wherein the stretch ratio in the longitudinal and in the transverse direction lies in the range from 1.2 to 9.

7. Process according to claim 6, characterised in that a heat treatment is subsequently carried out, wherein during the heat treatment relaxation is performed in the longitudinal direction and in the transverse direction.

8. Process according to claim 6 or 7, characterised in that the polyethylene used has a density in the range from 0.89 - 0.98 g / cm3, and / or in that the polyethylene used has an MFI (190°C / 2.16 kg) of 0.3-8 g / 10 min.

9. Process according to one of claims 7 or 8, characterised in that the extent of the relaxation is in the range from 2% to 12%.

10. Use of a film according to one of claims 1 to 5 for thermoforming.

11. Process for producing a shaped body, characterised in that a film according to one of claims 1 to 5 is deformed by thermoforming.

12. Process according to claim 11, characterised in that a process according to one of claims 6 to 9 is carried out and the film thus obtained is deformed by thermoforming.

13. Process according to either claim 11 or 12, characterised in that the film is formed with a degree of deformation in the range from 2 to 6.

14. Shaped body formed fully or partly by thermoforming from a biaxially-oriented film, characterised in that it comprises at least 85 wt.% polyethylene and at least 95 wt.% polyolefins, based on the total mass of the shaped body, wherein the film is a film according to any of the claims 1 to 5.

15. Shaped body according to claim 14 characterised in that in the part of the shaped body that was formed by thermoforming, the ratio of the greatest thickness of the formed film to the smallest thickness of the formed film is not more than two, and / or in that the water permeability measured to ASTM E 96 at 30°C and 90% relative humidity is not more than 8 g / m2 / day, and / or in that the oxygen permeability measured to ISO 15105-2 at 23°C and 0% relative humidity is not more than 4000 cm3 / m2 / day.