Biaxially oriented foil comprising cycloolefin polymer and alpha-olefin polymer, method for its preparation and use in capacitors

Abstract

A biaxially oriented foil comprising a) 1 to 25 weight percent of at least two cycloolefin polymers having a glass transition temperature in the range of 120 to 180° C., and b) 75 to 99 weight percent of at least one semicrystalline alpha-olefin polymer having a crystalline melting temperature in the range of 150 and 170° C., wherein the foil comprises at least a first cycloolefin polymer and a second cycloolefin polymer, wherein the second cycloolefin polymer has a higher glass transition temperature than the first cycloolefin polymer, and wherein the weighted arithmetic mean of the glass transition temperatures of all cycloolefin polymers contained in the foil is less than or equal to the weighted arithmetic mean of the crystalline melting temperatures of all semicrystalline alpha-olefin polymers contained in the foil.

Description

Field of the Invention

[0001] A biaxially oriented foil comprising a) 1 to 25 weight percent of at least two cycloolefin polymers having glass transition temperatures in the range of 120 to 180° C., and b) 75 to 99 weight percent of at least one semicrystalline alpha-olefin polymer having a crystalline melting temperature in the range of 150 and 170° C., wherein the foil comprises at least a first cycloolefin polymer and a second cycloolefin polymer, wherein the second cycloolefin polymer has a higher glass transition temperature than the first cycloolefin polymer, and wherein the weighted arithmetic mean of the glass transition temperatures of all cycloolefin polymers contained in the foil is less than or equal to the weighted arithmetic mean of the crystalline melting temperatures of all semicrystalline alpha-olefin polymers contained in the foil. Background Art

[0002] Thermoplastics play an important role in electronics as insulators and dielectrics, especially in the form of foils. Alpha-polyolefins and especially polypropylene (PP) are the most widely used. Patent documents WO 2015 / 091829 A1, US 5,724,222 A and EP 2 481 767 A2 describe biaxially oriented polypropylene foils as dielectrics in capacitors. Polyethylene foils are generally well suited for use as dielectrics, but they show high shrinkage at high temperatures. This makes these foils unsuitable for use at high temperatures. Biaxially oriented polypropylene foils are mainly used in foil capacitors. The reason for this is especially the smaller loss factor and high voltage tolerance of polypropylene. However, there are limitations in terms of temperature stability (application range of 85 to 105°C). Voltage derating is required at temperatures above 105°C, that is, the maximum permissible voltage must be reduced. Only under greater restrictions can it or cannot be used at temperatures above 115°C.

[0003] Cycloolefin polymers have a significantly better thermoforming tolerance than conventional polyolefins. It is known to use cycloolefin polymers (COC) as capacitor foils. These polymers have the high brittleness caused by their high glass transition temperature, making it difficult or impossible to carry out biaxial stretching. Its application in capacitors is known from files EP 0 992 531 A1, WO 00 / 63013 A2 and CA 2 115 196 C. The foil formed by cycloolefin polymers can only be prepared on a machine specially manufactured for this purpose and does not allow high draw ratios, which is disadvantageous for preparing thin foils (as preferably used in capacitors).

[0004] In order to be able to use the temperature resistance of polyolefin polymers for applications in capacitors, blends of polypropylene and cycloolefin polymers can be used. Blends of polypropylene and cycloolefin polymers are known from JP H05-262989, DE 10 2010 034643 A1 and DE 195 36 043. Foils for use in foil capacitors are specifically described in WO 2018 / 197034 A1 and DE 10 2017 118 202 A1.

[0005] WO 2018 / 197034 A1 describes a foil having a cycloolefin polymer with a glass transition temperature of 120 to 170° C. and a semicrystalline α-olefin polymer (generally polypropylene) with a crystalline melting temperature in the range of 150 to 170° C. in a content of 10 to 45 weight percent, wherein the glass transition temperature of the cycloolefin polymer is less than or equal to the crystalline melting temperature of the α-olefin polymer. The prepared foil has a favorable surface roughness and a small thermal shrinkage. Here, at a high cycloolefin polymer ratio, the properties of the cycloolefin polymer enter the given value range to a greater extent, that is, at a high cycloolefin polymer ratio, the foil has good thermal properties, including a lower shrinkage, but is more brittle and therefore less processable, and in particular reduces the maximum possible elongation when preparing a thin foil, which makes it impossible to prepare a thin foil at a high elongation. If a high elongation is used at a high cycloolefin polymer ratio, the foil is easy to tear and the required tape length may no longer be produced. Figure 3 This problem is demonstrated. Other things being equal, the elongation, in particular in the longitudinal direction, is reduced in order to still obtain a foil which can be processed on conventional conveyor belts at a high proportion of cycloolefin polymer. Figure 3 These points in give the maximum possible elongation in the longitudinal direction, wherein the foil can still be produced continuously at a given cycloolefin polymer content. On modern conveyor belts, the foil is usually produced in lengths of up to 30,000 meters. If the foil tears easily, it is no longer possible to produce foil strips with a defined length. This prevents the economical production of the foil and its economical use.

[0006] The same problem occurs when using cycloolefin polymers with a high glass transition temperature. When using cycloolefin polymers with a high glass transition temperature, the brittleness of the COC / PP mixture increases significantly, so that even by adapting the process parameters, stable foil production can only be achieved to a limited extent or not at all. Overall, there is a loss of operating stability, that is, multiplied cracks occur during the stretching during foil production, making stable production impossible. The required running lengths of up to 30,000 meters cannot be produced.

[0007] According to WO 2018 / 197034 A1, foils with sufficient elongation can be produced, which can be used for capacitors at temperatures of up to 120 to 125° C. However, higher temperature resistance and smaller foil thicknesses are required for different applications, in particular for use in capacitors in automotive construction. In automotive construction, electronic circuits are often used in high-temperature environments, such as in the engine compartment or near the battery.

[0008] Purpose of the Invention

[0009] The object of the invention is to produce a foil having good electrical properties and having a small shrinkage at high temperatures. A high thermal stability is desirable. The foil should preferably be producible in conventional equipment for producing biaxially oriented foils. It is particularly important that it can be processed into long strips at high foil speeds. It is also advantageous that the foil can be produced with a high elongation. It is also possible to produce foils with a relatively low thickness. Finally, the invention is concerned with making it economically cheaper to produce foils, for example for capacitors. Summary of the invention

[0010] The subject of the present invention is a biaxially oriented foil comprising:

[0011] a) at least two cycloolefin polymers

[0012] as well as

[0013] b) at least one semicrystalline α-olefin polymer,

[0014] wherein the sum of the proportions of all cycloolefin polymers in the foil is in the range of 1 to 25 weight percent, and all cycloolefin polymers contained in the foil have a glass transition temperature in the range of 120 to 180° C., and the foil comprises at least a first cycloolefin polymer and a second cycloolefin polymer, wherein the second cycloolefin polymer has a higher glass transition temperature than the first cycloolefin polymer, and wherein the sum of the proportions of all semicrystalline α-olefin polymers in the foil is in the range of 75 to 99 weight percent, and all semicrystalline α-olefin polymers contained in the foil have a crystalline melting temperature in the range of 150 to 170° C., and wherein the weighted arithmetic mean of the glass transition temperatures of all cycloolefin polymers contained in the foil is less than or equal to the weighted arithmetic mean of the crystalline melting temperatures of all semicrystalline α-olefin polymers contained in the foil, and wherein all quantitative values ​​are relative to the total mass of the foil. If not otherwise stated, all proportions given in weight percent with respect to the present invention are respectively relative to the total mass of the foil.

[0015] The terms "orientation", "stretching", "pulling", "elongation" and "stretching" have the same meaning in this specification. This also applies to corresponding terms such as "stretching process", "stretching process", "stretching effect" and the like.

[0016] The foils according to the invention surprisingly offer a combination of good processability during production, high temperature resistance and good electrical properties, which foils from the prior art do not offer to such an extent. According to the prior art, the addition of cycloolefin polymers with a higher glass transition temperature reduces processability. However, as shown in the present invention, this surprisingly does not occur or not to the same extent when mixtures of polyolefin polymers are used, wherein the polyolefin polymers have different glass transition temperatures.

[0017] The good processability is shown in that the foil according to the invention allows the production with an area stretching ratio of about 50. The area stretching ratio is the product of the elongation of the product in the longitudinal direction and the elongation in the transverse direction. This higher area stretching ratio allows the production of thinner foils with a thickness of, for example, less than 5 μm. Smaller foil thicknesses are required in order to allow high capacitances at a small volume requirement (construction space) in capacitors. At the same time, such a foil allows the production at a foil speed of 280 m / min after stretching or when winding (a typical foil speed when producing foil is 250 m / min). The foil according to the invention can thus be produced in an economical manner.

[0018] At the same time, the foil of the present invention can also be used as an electrolyte in a capacitor at a temperature of up to 135°C and is therefore also superior to the foil from the prior art in terms of these properties. At this temperature, the foil of the present invention shows significantly better electrical properties than the foil from the prior art. Foil capacitors must meet increased requirements in terms of temperature and voltage, especially in the automotive field. The required operating temperature is 125°C and more particularly 135°C and higher. The present invention provides a biaxially stretched foil that meets these increased requirements. The present invention develops a foil comprising α-polyolefins and cycloolefin polymers, thereby opening up a new and broad range of applications. The foil also has a lower shrinkage value at high temperatures, especially in the transverse direction (TD). As a result, compared to the foil known from WO 2018 / 197034 A1, a thinner foil and possible applications at higher temperatures are produced in a low-cost preparation method. In addition, the foil of the present invention has good electrical properties and a favorable surface roughness.

[0019] Another effect of the invention is that, by combining two cycloolefin polymers, a smaller total amount of cycloolefin polymers can be used and thus improved processability on conveyor belts can be achieved with constant good temperature resistance. This has a positive influence on the achievable elongation, especially in the case of sequential stretching. In addition, costs are saved when using a smaller amount of cycloolefin polymer. There is no negative effect on the electrical properties.

[0020] The high proportion of polypropylene in the present foil allows the foil to be processed well during production and in particular to be stretched well. Surprisingly, the use of two polyolefin polymers with different glass transition temperatures provides advantageous high temperature properties without reducing processability.

[0021] The semicrystalline alpha-olefin polymer used in the present invention is preferably a polymer of a C3 to C8 polyolefin. The side chain can be linear or branched. It is generally polypropylene. Polypropylene gives good processability and good electrical properties to the foil of the present invention. It is usually especially a propylene homopolymer. However, polypropylene can contain up to 10 weight percent, and preferably up to 5 weight percent, of one or more C4 to C8 polyolefins as comonomers. The C4 to C8 polyolefin is preferably selected from the group consisting of 1-butene, 1-hexene and 1-octene.

[0022] Semi-crystalline polyolefins can be linear or branched. They can also be statistical copolymers or block copolymers. They can also be isotactic, syndiotactic or atactic polymers. Isotactic polypropylene is preferred. The crystallinity in polypropylene typically varies in the range of 35% to 80%, preferably in the range of 60 to 80%. The crystallinity is measured according to ISO 11357 by differential scanning calorimetry (DSC) at a heating rate of 10K / min. Particularly preferred is isotactic propylene with a crystallinity in the range of 60 to 80%. Usually, a propylene having a crystallinity of 0.895 g / cm 3 Up to 0.920g / cm 3 Polypropylene with a density within a range of 1000 % and 1000 % respectively. Density is measured according to ISO 1183.

[0023] The crystalline melting temperature is preferably in the range of 160 to 165° C. It is known to those skilled in the art to change the crystalline melting temperature by utilizing copolymerization. The crystalline melting temperature is measured according to ISO 11357 according to differential scanning calorimetry (DSC) at a heating rate of 10 K / min. The semi-crystalline α-olefin polymer used preferably has a melt flow index (melt flow rate, Tg) in the range of 1-4 g / 10 min, the melt flow index being measured at a temperature of 230° C. under a load of 2.16 kg. If not otherwise stated, all melt flow indexes in this specification are measured according to ISO 1133.

[0024] In order to achieve the advantageous electrical properties of the foil, the foil is preferably particularly pure. For this purpose, high-purity qualities are preferably selected as α-olefin polymers. It is particularly preferred that the foil according to the invention has a low metal content, since even traces of metal in the dielectric of a capacitor can adversely affect the electrical properties of the capacitor. The total content of iron, cobalt, nickel, titanium, molybdenum, vanadium, chromium, copper and aluminum in the foil according to the invention is preferably less than 0.25 ppm.

[0025] Particularly suitable types are commercially available polymers, all from Borealis Corporation's " HC300BF"," HC312BF”, Polyolefins Company of Singapore (TPC) and polyolefins of Prime Polymer.

[0026] The cycloolefin polymer used in the present invention can be either a homopolymer or a copolymer. The cycloolefin polymer can be prepared by ring-opening polymerization or by ring-forming polymerization thereof. The cycloolefin polymer is preferably prepared by ring-forming copolymerization of a cyclic olefin (such as norbornene) and a non-cyclic olefin (such as α-olefin or ethylene), wherein ethylene is preferred. Information about cycloolefin polymers that can be used in the present invention can be found, for example, in WO 2018 / 197034 A1 and the documents cited therein. Cycloolefin polymers in which the double bonds remaining after polymerization have been hybridized can also be used.

[0027] Cyclic olefin polymers which can be prepared by ring-forming copolymerization of at least one olefin of the formula (I) with at least one α-olefin of the formula (II) are preferred.

[0028] Formula I:

[0029]

[0030] in

[0031] n is 0 or 1,

[0032] m is 0 or a positive integer, preferably 0 or 1,

[0033] R 1 , R 2 , R 3 , R 4 , R 5 and R 6 independently of one another, selected from the group consisting of hydrogen, halogen, alkyl, cycloalkyl, aryl and alkoxy,

[0034] R 17 , R 18 , R 19 and R 20 are independently selected from the group consisting of hydrogen and alkyl,

[0035] Where R 17 and R 19 Together they may form a ring or a ring system, wherein the ring may be saturated or unsaturated.

[0036] Formula (II):

[0037]

[0038] Where R 21 and R 22 are independently selected from the group consisting of hydrogen and alkyl.

[0039] Preferred are the following cycloolefin polymers, wherein n is 0, m is 0 or 1, R 21 is hydrogen and R 22 is selected from the group consisting of hydrogen and C1 to C8 alkyl. Among them, the following polymers are further preferred, wherein R 1 , R 2 , R 5 To R 8 and R 15 To R 20 Very particular preference is given to copolymers of norbornene and ethylene and of tetracyclododecene and ethylene.

[0040] Most preferred are foils of the invention wherein all cycloolefin polymers contained therein are copolymers comprising norbornene and ethylene as monomers. The polymerization is preferably carried out in the presence of a metallocene catalyst.

[0041] For the cycloolefin polymer of the foil of the invention, the monomers of formula (I) and (II) are preferably used in a molar ratio of 95:5 to 5:95. The monomers may contain up to 10 mol % of one or more comonomers selected from the group consisting of propylene, pentene, hexene, cyclohexene and styrene, relative to the total monomer amount.

[0042] Preferred cycloolefin polymers are known from DE 102 42 730 A1. Particularly preferred are the cycloolefin polymers from Topas Advanced Polymers GmbH, Frankfurt am Main, Germany. 6013, 6015, 6017 and 5013 type, Mitsui Chemicals' "APEL COC" polymer and JSR Corporation's "ARTON COC" polymer. Most preferred are 6013M07 / S04, 6015S04 and 6017S04. Also suitable are polymers which can be prepared by ring-opening polymerization of cyclopentadiene or norbornene.

[0043] The glass transition temperature of the cycloolefin polymer is preferably in the range of 120 to 170° C., particularly preferably in the range of 130 to 170° C. and most preferably in the range of 140 to 160° C. The glass transition temperature is measured in accordance with ISO 11357 by DSC at a heating rate of 10 K / min.

[0044] The cycloolefin polymer preferably has a melt flow index in the range of 0.3 and 4 g / 10 minutes, the melt flow index being measured at a temperature of 230° C. under a load of 2.16 kg.

[0045] In order to use the foil according to the invention in capacitors, a relatively low surface roughness is required. However, too low a surface roughness leads to poor processability of the foil on conveyor belts. In order to achieve a sufficient but not excessive surface roughness, it is advantageous to adapt the melt flow indexes of the polymers used to one another. The ratio of the melt flow indexes of the cycloolefin polymer to the semicrystalline α-olefin polymer is in the range of 1:2 to 2:1.

[0046] The second cycloolefin polymer differs from the first cycloolefin polymer at least in having a higher glass transition temperature. The glass transition temperature is preferably at least 3° C. higher, particularly preferably at least 5° C. higher. The glass transition temperature is particularly preferably at least 10° C. higher.

[0047] It is possible to use a plurality of semicrystalline α-olefin polymers. This can be used to optimize the properties of the foil. However, this is not necessarily necessary for achieving the effects of the invention. However, due to the use of a plurality of semicrystalline α-olefin polymers, the production process of the foil becomes somewhat more complicated. Therefore preferred is a foil according to the invention, wherein the foil contains only one semicrystalline α-olefin polymer and the weighted arithmetic mean of the glass transition temperatures of all cycloolefin polymers contained in the foil is less than or equal to the crystalline melting temperature of the semicrystalline α-olefin polymer. A foil according to the invention which contains only one semicrystalline α-olefin polymer which is polypropylene is particularly preferred.

[0048] The foil according to the invention must contain at least two cycloolefin polymers. However, the foil may also contain more than two cycloolefin polymers, i.e., for example, 3 or 4. The cycloolefin polymers must have at least two glass transition temperatures that differ from one another. It may also contain, for example, more than two, i.e., for example, 3 or 4 cycloolefin polymers, which have glass transition temperatures that deviate from one another. However, the foil according to the invention generally contains only semicrystalline α-olefin polymers and no further cycloolefin polymers besides the first and second cycloolefin polymers.

[0049] Preferred is a foil according to the invention, wherein the foil comprises only one semicrystalline α-olefin polymer which is polypropylene, and the foil comprises no further cycloolefin polymers besides the first cycloolefin polymer and the second cycloolefin polymer, and the first cycloolefin polymer and the second cycloolefin polymer both comprise norbornene and ethylene as monomers. Such foil is particularly suitable for use in capacitors.

[0050] The biaxial orientation of the foil according to the invention is substantially influenced by both the proportion of the cycloolefin polymers and the glass transition temperature of the cycloolefin polymers. The foil according to the invention is particularly suitable for capacitors when the sum of the proportions of all cycloolefin polymers in the foil is in the range of 10 to 25 weight percent and the sum of the proportions of all semicrystalline α-olefin polymers in the foil is in the range of 75 to 90 weight percent. Very particularly preferred is a foil in which the sum of the proportions of all cycloolefin polymers in the foil is in the range of 10 to 20 weight percent and the sum of the proportions of all semicrystalline α-olefin polymers in the foil is in the range of 80 to 90 weight percent. Most preferred is a foil in which the sum of the proportions of all cycloolefin polymers in the foil is in the range of 15 to 20 weight percent and the sum of the proportions of all semicrystalline α-olefin polymers in the foil is in the range of 80 to 85 weight percent. It is also preferred that the first and second cycloolefin polymers are contained in an amount of at least 3 weight percent, and particularly preferably at least 5 weight percent, respectively.

[0051] In general, the foil according to the invention contains the first cycloolefin polymer and the second cycloolefin polymer in a ratio in the range of 3 to 22 weight percent, preferably in the range of 3 to 17 weight percent, particularly preferably in the range of 5 to 15 weight percent. If the cycloolefin polymers are used in these amounts, the effect produced by using more than one cycloolefin polymer is particularly prominent.

[0052] In addition to the above-mentioned polymer, the foil can also include conventional auxiliary agents and additives. Usually, auxiliary agents and additives are added in an amount of not more than 5 weight percent, preferably not more than 2 weight percent. Auxiliary agents and additives can be selected from the group consisting of fillers, pigments, dyes, oils, waxes, plasticizers, UV stabilizers, matte agents, preservatives, biocides, antioxidants, antistatic agents, fire retardants and reinforcing agents. However, in addition to the above-mentioned polymer, the foil preferably does not include other included substances. It is particularly preferred that the foil does not include solids (in addition to the above-mentioned polymer) and especially does not include anti-caking agents and fillers. Such solids may negatively affect electrical properties.

[0053] The foil according to the invention preferably has a thermal shrinkage of at most 5%, particularly preferably at most 3% and very particularly preferably 2% in the longitudinal direction. The foil according to the invention also preferably has a thermal shrinkage of at most 0.5%, particularly preferably at most 0.1% and very particularly preferably 0.05% in the transverse direction (TD). The thermal shrinkage in the transverse direction is decisive for the use in capacitors, since such thermal shrinkage leads to damage to the capacitors. The foil should in particular have these thermal shrinkage values ​​in both directions at the same time. Therefore, it is particularly preferred that the foil according to the invention has a thermal shrinkage of at most 5% in the longitudinal direction and a thermal shrinkage of at most 0.5% in the transverse direction. Most preferred is the foil according to the invention, which has a thermal shrinkage of at most 2% in the longitudinal direction and a thermal shrinkage of at most 0.05% in the transverse direction. The thermal shrinkage is measured at 130° C. and for a duration of 5 minutes in accordance with DIN ISO 11501. However, it is most preferred that the foil according to the invention has such thermal shrinkage values ​​in the transverse direction at a temperature of 140° C.

[0054] The roughness of the foil plays an important role for the electrical properties, and for this reason a lower roughness is preferred. However, since the foil according to the invention generally contains no anti-caking agents, at least a certain surface roughness must be present to ensure the processability of the foil. A foil that is too smooth slides on the conveyor rollers and can therefore be processed in industrial plants in practice. Preference is given to foils that have a surface roughness Ra in the range of 0.02 to 0.2 μm, preferably 0.04 to 0.12 μm. Ideally, the foil according to the invention has a surface roughness Ra in the range of 0.06 to 0.09. The surface roughness Ra is determined in accordance with DIN EN ISO 4287. Figure 1 or Figure 2As shown, the foil according to the invention has a uniform fibrillar surface structure on both sides. When measuring the surface roughness Ra, the measuring head is guided over a certain length of the surface in order to measure the unevenness. Since the elevations of the surface of the foil according to the invention are anisotropic, measurements in the longitudinal direction of the foil (conveying direction) and in a direction transverse to the longitudinal direction may result in different surface roughnesses. Preferably, two measured values ​​are present in each of the given areas.

[0055] As known to the person skilled in the art, the surface roughness of the foil can be set by appropriately selecting the temperature of the cooling rollers and the preheating temperature for the stretching. In this case, in the case of simultaneous stretching, the temperature of the cooling rollers and the preheating temperature of the furnace for simultaneous stretching (e.g., a LISIM system) are adapted to one another. In this case, in the case of sequential stretching, the temperature of the cooling rollers and the preheating temperature and the temperature of the stretching rollers in the longitudinal direction of stretching (MDO) are adapted to one another.

[0056] The foil according to the invention preferably has a thickness in the range of 2 μm to 10 μm. Particularly preferred are foils according to the invention having a thickness in the range of 2 to 5 μm. This relates to the foil thickness as determined in accordance with DIN 53370. A foil having such a thickness is optimally suitable for use as a dielectric.

[0057] The width of the foil according to the invention is typically in the range from 0.1 to 10 m and mostly in the range from 5 to 7 m.

[0058] Preferred is a biaxially oriented foil comprising:

[0059] a) at least a first cycloolefin polymer and a second cycloolefin polymer, wherein both contain as monomers 5 to 95 weight percent norbornene and 5 to 95 weight percent ethylene, respectively, relative to all cycloolefin polymers,

[0060] and b) at least one semi-crystalline polypropylene,

[0061] wherein the sum of the proportions of all cycloolefin polymers in the foil is in the range of 10 to 25 weight percent, and all cycloolefin polymers contained in the foil have a glass transition temperature in the range of 120 to 180° C., wherein the second cycloolefin polymer has a glass transition temperature at least 3° C. higher than the first cycloolefin polymer, wherein the first cycloolefin polymer and the second cycloolefin polymer are contained in the foil in a proportion in the range of 5 to 15 weight percent, respectively, and wherein the sum of the proportions of all semicrystalline polypropylenes in the foil is in the range of 75 to 90 weight percent, and all semicrystalline α-olefin polymers contained in the foil have a crystalline melting temperature in the range of 150 to 170° C., and wherein the weighted arithmetic mean of the glass transition temperatures of all semicrystalline polypropylenes contained in the foil is less than or equal to the weighted arithmetic mean of the crystalline melting temperatures of all semicrystalline polypropylenes contained in the foil, and wherein the foil has a thickness in the range of 2 μm to 10 μm, and unless otherwise stated, all quantitative values ​​are relative to the total mass of the foil.

[0062] The foil according to the invention can have one or more layers and thus be a single-sublayer foil or a multi-sublayer foil. In a multi-sublayer foil, at least one layer has the composition of the foil according to the invention described above. A plurality of layers of such a multi-sublayer foil can also consist of a foil according to the invention.

[0063] Therefore, another embodiment of the present invention is a multilayer foil, which comprises at least one layer, which consists of the foil according to the invention described herein. In one embodiment, it is a multilayer foil, which has as a layer the foil according to the invention described above, which layer has an outer layer on only one side or on both sides, which outer layer contains one or more semi-crystalline α-olefin polymers, of which semi-crystalline polypropylene is particularly preferred, as already described above. The multilayer foil again preferably has the same properties as the single-layer foil, in particular with regard to shrinkage and surface roughness. A coextruded multilayer foil having at least one polypropylene outer layer and at least one layer having the composition of the above-described inventive foil has the following advantages: its surface properties can be set using known techniques common in polypropylene capacitor foils, for example by targeted control of crystallization and targeted formation of crystal spheres; and the mechanical and electrical properties of such a multilayer foil at higher temperatures can be improved by the presence of at least one layer of the above-described inventive foil.

[0064] Particularly preferred are multi-layer foils comprising an intermediate layer, which contains a semi-crystalline α-olefin polymer as described above, and which have two outer layers, each of which is composed of the foil according to the invention as described herein. The intermediate layer very particularly preferably contains only semi-crystalline polypropylene, as already described herein. This results in a foil which, while having very good dielectric properties, has improved temperature stability due to the outer layers, since the 100% polypropylene in the intermediate sublayer has a significant influence on this. Most preferred are foils which, apart from the intermediate layer and the outer layers, have no further layers. However, the foil can additionally have metallizations, as will be described in detail below.

[0065] Another embodiment of the foil according to the invention is a coated foil. This refers to a foil whose at least one outer layer has been coated with a material. In this sense, a coating is to be understood as a coating that has not yet been produced during the extrusion of the foil. The coating can be applied to the finished foil in a separate process after the production of the foil. A preferred embodiment is a foil according to the invention that is coated with a conductive layer at least on one side. Therefore, another embodiment of the invention is a foil according to the invention that is metallized on one or both sides.

[0066] For use in foil capacitors, metallization is necessary. Therefore, foils coated on one side with a metal layer are most preferred. For this purpose, the foil surface can be activated by means of a corona treatment after stretching and then the metal layer can be applied. Foils according to the invention that are metallized on both sides are also part of the invention.

[0067] Another subject of the invention is a capacitor, characterized in that it comprises a foil as described in the present description. The capacitor according to the invention preferably has a breakdown strength of greater than or equal to 500 V / μm at a foil thickness of 3.8 μm and a breakdown strength of greater than or equal to 400 V / μm at a foil thickness of 2.7 μm, the breakdown strength being measured at 23° C. under direct voltage according to DIN EN 60243-2. ​​In addition, the foil and capacitor according to the invention preferably have a breakdown strength of less than or equal to 1 to 2*10 -4 The dielectric loss factor is determined according to DIN EN 60674-2 (VDE). It is also preferred that the capacitor comprises a foil with a dielectric constant in the range of 2 to 2.5. The dielectric constant is likewise determined according to DIN EN 60674-2 (VDE).

[0068] Another subject of the present invention is a method for producing the foil according to the invention, comprising the following steps:

[0069] - producing a film, in particular by means of extrusion, comprising at least two cycloolefin polymers and at least one semicrystalline α-olefin polymer,

[0070] - stretching the film in the machine direction (MD) and in the transverse direction (TD) so as to obtain a biaxially stretched foil, and

[0071] - Winding up the biaxially stretched foil.

[0072] Features and advantages mentioned with regard to the foil also apply to the method and vice versa.

[0073] The area stretching ratio of the biaxially stretched foil, in particular the wound foil, is preferably at least 40.

[0074] The stretching in the longitudinal direction and in the transverse direction may be carried out simultaneously, or the stretching in the longitudinal direction may be carried out first and then the foil, which has been stretched in the longitudinal direction, may be stretched in the transverse direction.

[0075] To improve the properties of the foil, one or more of the following further steps may be performed after elongation or before winding:

[0076] - subjecting the biaxially stretched foil to a heat treatment,

[0077] - Relaxing the biaxially stretched foil, thereby reducing the area stretch ratio in a targeted manner.

[0078] For example, after relaxation, the area stretching ratio can also be 40.

[0079] The method is described in detail below:

[0080] (1) Formulation and feeding

[0081] As a possibility, the raw material for preparing the foil according to the invention can be provided as a compound comprising a semicrystalline alpha-olefin polymer and a cycloolefin polymer. It is also possible to use a batch feeding method, in which the semicrystalline alpha-olefin polymer and the cycloolefin polymer are mixed together according to the recipe directly before being added to the extruder. Plastic pellets or powders are used as raw materials.

[0082] When the foil has multiple layers, each layer can be provided with its own raw material. Depending on the desired thickness of each layer, the raw material is delivered separately for each layer.

[0083] What has been said above for the foils of the invention applies to both the alpha-olefin polymers and the cycloolefin polymers used in the process.

[0084] (2) Extrusion

[0085] The raw materials are fed into the extruder. They are mixed and melted in the extruder. When the foil has only one layer, only one extruder is required.

[0086] When the foil has multiple layers, generally a separate extruder is used for each layer. However, when two or more layers have the same composition, multiple layers can be produced from the same plastic pellets. Then, the material of two or more of these layers can be extruded by the same extruder. Alternatively, when all layers have the same composition, all layers can be extruded by one extruder. Any arbitrary type of extruder can be used.

[0087] Preferred is an extruder selected from the group consisting of a single-screw extruder, a cascade extruder and a twin-screw extruder.

[0088] Other mixing and processing units can also be used, such as bus kneaders or planetary roller extruders. The extruder is preferably operated at a temperature in the range of 230 to 280° C., particularly preferably in the range of 240 to 270° C.

[0089] (3) Nozzle and cooling roller

[0090] The melt is ejected through a wide-slot nozzle and deposited on a cooling roll. When the foil has multiple layers, a multilayer nozzle is preferably used.

[0091] Preferably, a device for placing the film on the cooling drum (tie system) is used to improve the placement of the foil on the cooling roller. The preferred device is an air knife, ie a directed air stream that forces the film in the direction of the cooling roller.

[0092] The tie-down system produces a more uniform contact between the film and the cooling roll and thereby results in the formation of a smooth and flat cast foil.

[0093] The temperature of the cooling roller is preferably in the range of 80° C. to 100° C., preferably in the range of 85° C. to 95° C. and most preferably in the range of 90° C. to 95° C. To set the desired roughness, in the case of thinner foils, a higher temperature is selected within the specified temperature range. In the case of a foil thickness of 3.8 μm, as produced in the example, a temperature in the range of 90° C. to 95° C., in particular 93° C., is most suitable.

[0094] (4) Stretching

[0095] The cast foil thus produced is subsequently stretched in the longitudinal direction (machine direction; MD) and in the transverse direction (transverse direction; TD).

[0096] In the method of the present invention, the stretching ratio in the longitudinal direction is, for example, in the range of 3.0 to 7.0, preferably in the range of 5.0 to 7.0.

[0097] In the method of the present invention, the stretching ratio in the transverse direction is preferably in the range of 7.0 to 13.0, preferably in the range of 8.0 to 11.0.

[0098] The biaxial orientation or stretching in the longitudinal direction and in the transverse direction can be carried out successively to one another, in which case it is referred to as sequential stretching, or the foil can be stretched simultaneously in the longitudinal direction and in the transverse direction, in which case it is referred to as simultaneous stretching.

[0099] (4.1) Stretch sequentially

[0100] Sequential stretching is preferably performed as follows.

[0101] a) Stretch in the longitudinal direction

[0102] For example, the cast film produced as described above is first stretched in the longitudinal direction. For this purpose, the cast film is fed into a longitudinal stretching device, also called MDO (machine direction orienter), and stretched after sufficient heating in the machine direction. In the longitudinal stretching device, the foil is stretched, for example, between pairs of rollers with different rotation speeds.

[0103] Such a longitudinal stretching device is disclosed, for example, in DE 10 2011 109 385 A1.

[0104] b) Stretching in the transverse direction

[0105] The stretching in the transverse direction is preferably carried out temporally after the stretching in the longitudinal direction. During the stretching in the transverse direction, the foil stretched in the longitudinal direction is fed into a transverse elongation device 10, known as a TDO (Transverse Orienter).

[0106] As in Figure 1 As schematically shown in FIG. 1 , the transverse elongation device 10 generally has an oven 14 and two conveyor tracks 16 for the foil.

[0107] The conveyor track 16 is arranged in a mirror-symmetrical manner with respect to the symmetry axis S of the elongation device 10 and extends at least partially in the furnace 14. The conveyor track 16 extends outside the furnace 14 in an infeed zone 18 and an outfeed zone 20 for feeding the material strip 12 into and out of the elongation device 10.

[0108] In addition to the feed zone 18 and the discharge zone 20 , the transverse stretching device 10 has at least three further zones 22 , 24 , 26 .

[0109] The zones 22 , 24 , 26 are adjacent to one another so that, viewed along the longitudinal direction or usual withdrawal direction R of the transverse stretching device 10 , first the first zone 22 , then the second zone 24 , then the third zone 26 , and finally the delivery zone 20 are connected to the delivery zone 18 .

[0110] In a first zone 22 of the transverse stretching device 10 , which is adjacent to the feed zone 18 and is also referred to as a preheating zone, the conveying tracks 16 have a first distance relative to one another.

[0111] In a second zone 24 , also called elongation zone, the distance of the two conveyor tracks 16 relative to one another increases until finally reaching a second distance at the beginning of a third zone 26 , also called heat treatment zone.

[0112] At each of these transport tracks 16, a plurality of gripper units 28 are guided in a manner known per se. The gripper units 28 can be moved along the transport tracks 16 by suitable drives.

[0113] Such a transverse stretching device 10 is known, for example, from WO 2014094803 A1.

[0114] For transverse stretching, the foil that has been stretched in the longitudinal direction is fed into the transverse stretching device 10 in the feed zone 18 of the stretching device 10 in the withdrawal direction R. This is done, for example, in the following way: the foil is placed with its longitudinal edge in the withdrawal direction into the gripper unit 28 of the conveyor track 16 arranged at the corresponding side and the gripper unit 28 is closed. This process is also called "gripping". Thereby, the gripper unit 28 grips the foil and moves the foil into the furnace 14 of the transverse stretching device 10 or through the furnace.

[0115] In the infeed zone 18 , the foil 12 has a width E perpendicular to the withdrawal direction R which corresponds approximately to the first distance between the conveying tracks 16 .

[0116] The foil 12 is then guided through the first zone 22 and heated there.

[0117] In the subsequent second zone 24, ie in the elongation zone, the foil 12 is stretched in the transverse direction, since the distance of the transport track 16 increases continuously. Thereby, the foil 12 becomes thinner and wider. At the end of the second zone 24, the foil 12 has a second width.

[0118] After the elongation has been completed, the foil 12 now passes through a third zone 26 , in which relaxation of the foil 12 can take place, before being unwound from the gripper unit 28 in the discharge zone 20 and leaving the transverse elongation device 10 at this width.

[0119] (4.2) Simultaneous stretching

[0120] The simultaneous stretching is carried out, for example, in the following manner, preferably by means of the pantograph method, the mandrel method, the LISIM method or the MESIM method. The foil speed is hereby at most 250 m / min.

[0121] The construction of the simultaneous stretching device corresponds to the construction described previously for the transverse stretching device in terms of the basic arrangement of the different zone-level tracks in the furnace. The difference between the transverse stretching device and the simultaneous stretching device lies in the length of the zones and / or the temperature in the zones. In addition, the conveying system differs in that the gripper units in the simultaneous stretching device can be accelerated individually on the tracks, for example by means of linear motors, in order to stretch the foil in the longitudinal direction. Such a simultaneous stretching device is described, for example, in WO2015007367A1, in particular in paragraphs

[0045] to

[0055] .

[0122] During simultaneous stretching, the unstretched cast film is moved into the feed zone of the simultaneous stretching device, where it is clamped into the clamp unit as described above.

[0123] The foil is then fed into a first zone and heated there.

[0124] The foil is subsequently fed into a second zone, the stretching zone, in order to be stretched there simultaneously in the transverse direction and in the longitudinal direction.

[0125] Analogously to what has been explained with regard to the transverse stretching device, the transverse stretching of the foil takes place as follows: In the stretching zone the distance of the transport tracks relative to one another is increased, thereby increasing the width of the foil.

[0126] In contrast to the transverse stretching device, in the stretching zone the gripper unit 28 is accelerated to a higher speed (also referred to as stretching speed) than the drive speed in the feed zone. The foil is thereby also stretched in the longitudinal direction.

[0127] In other words, the distance between the gripper units in the elongation zone increases not only in the longitudinal direction but also in the transverse direction, so that the foil is stretched in both the longitudinal and transverse directions. The distance between the gripper units increases here in relation to the desired elongation ratio in the longitudinal and transverse directions.

[0128] As described for the transverse stretching device, the biaxially stretched foil then passes through a third zone, in which a heat treatment or relaxation of the foil takes place. Here, the distance between the opposing gripper units in the transverse direction is reduced. The speed of the gripper units can also be reduced relative to the stretching zone.

[0129] In the discharge zone the foil is unwound by suitable means and then leaves the device while being stretched.

[0130] (5) Heat treatment

[0131] After stretching, the foil is preferably subjected to a heat treatment (also referred to as stabilization or annealing). For this purpose, the foil is kept at an elevated temperature for a certain period of time. This is preferably done directly after stretching in a third zone of a transverse stretching device or a simultaneous stretching device.

[0132] During the heat treatment, the stresses within the foil caused by the stretching are relieved, thereby making the properties uniform over the entire surface of the foil.

[0133] (6) Relaxation

[0134] Preferably, the foil is relaxed during the heat treatment, which means that the stress on the foil is reduced, thereby reducing the elongation ratio in a targeted manner.

[0135] During the subsequent stretching, the foil relaxes only in the transverse direction, for example by reducing the distance of the conveying tracks in the third zone.

[0136] During simultaneous stretching, relaxation is carried out in particular not only in the longitudinal direction but also in the transverse direction. For example, the distance between the gripper units relative to each other in the transverse direction is reduced and the speed of the gripper units is reduced relative to the speed in the elongation zone at the desired degree of relaxation.

[0137] It is also advantageous that the relaxation range in the method according to the invention is between 2% and 15%, preferably between 7 and 12%. In the case of simultaneous stretching, this preferably applies to both directions. By relaxing, the shrinkage of the foil is reduced.

[0138] (7) Rolling

[0139] After the foil has been biaxially stretched, the foil is coiled.

[0140] For this purpose, the edge of the foil gripped by the gripper unit is cut off in the longitudinal direction.

[0141] In some cases, it is also possible to treat the surface of the foil before final winding, if this is desired or necessary.

[0142] Finally the foil is wound up on a winder.

[0143] (8) Further processing

[0144] The biaxially oriented foil according to the invention produced in this way can be processed further by further work steps.

[0145] For example, the foils can be assembled into slit rolls before further processing.

[0146] Other possible processing steps are, in particular, metallization, lamination, coating, coating (barrier layer, protector), printing, surface treatment, laser processing, painting, etc. The foil is preferably metallized.

[0147] A further subject matter of the invention is the use of a foil, in particular a metallized foil, as described in the present description as a dielectric for producing a capacitor. BRIEF DESCRIPTION OF THE DRAWINGS

[0148] Figure 1 A transverse elongation device is schematically shown.

[0149] Figure 2 A micrograph of the surface of a foil according to the invention which was facing the cold roll during production is shown on the left, and the reverse side is shown on the right.

[0150] Figure 3 The maximum possible elongation in the longitudinal direction (MDx) of the foil, which contains only one cycloolefin polymer, is shown for sequential elongation and simultaneous elongation at a foil thickness of 3.8 μm.

[0151] List of abbreviations:

[0152] BO Biaxial Orientation

[0153] BOPP Biaxially Oriented Polypropylene

[0154] COC Cyclic olefin polymers, specifically cyclic olefin copolymers

[0155] MD Longitudinal direction, running direction, machine direction

[0156] PP Polypropylene

[0157] TD Transverse direction, transverse to the machine direction

[0158] Tg glass transition temperature DETAILED DESCRIPTION

[0159] Example:

[0160] Measurement method:

[0161] Foil thickness: DIN 53370; layer thickness: DIN EN ISO 3146; crystalline melting temperature, glass transition temperature and crystallinity: ISO 11357-1, -2, -3 (DSC, heating rate 10 K / min); melt flow index: ISO 1133; thermal shrinkage: DIN ISO 11501; surface roughness: DIN EN ISO 4287; electrical breakdown strength: DIN EN 60243-2; loss factor and dielectric constant: DIN EN 60674-2 (VDE).

[0162] Material:

[0163] Polypropylene: HC300BF (crystal melting temperature 164°C; melt flow index (230°C / 2.16kg): 3.3g / 10min; metallizable PP for dielectric films of capacitors; extremely high purity, ash content <20ppm, metal content: titanium <3ppm, aluminum <3ppm, chlorine <3ppm;

[0164] Cyclic polyolefin polymer 1 (COC1): Topas COC 6013 M07

[0165] Copolymer of norbornene and ethylene

[0166] Glass transition temperature 142°C,

[0167] Melt flow index (260°C, 2.16kg): 13.26g / 10min;

[0168] Cyclic polyolefin polymer 2 (COC2): Topas COC 6015 S04

[0169] Copolymer of norbornene and ethylene

[0170] Glass transition temperature 158℃

[0171] Melt flow index (260°C, 2.16kg): 4.08g / 10min;

[0172] Preparation of the foil of the present invention:

[0173] The polypropylene, cycloolefin polymer 1 and cycloolefin polymer 2 in the form of pellets were melted (dosed in batches) at 250 to 260° C. by means of a twin-screw extruder and applied to a cold roll by means of a wide-slot nozzle. To assist in extruding the foil onto the cold roll, a conventional high-pressure air knife was used. The temperature of the cold roll surface was maintained at a temperature in the range of 90 to 95° C. No cooling bath was used.

[0174] The foil is fed into a simultaneous elongation device (Examples 1, 3, 4 and 5 only). The simultaneous elongation device is divided into heating zones, in which the foil is heated to the temperatures shown in Table 1. The foil is heated uniformly from top to bottom. The previously described first zone 22, second zone 24 and third zone 26 are divided into different heating zones in the following.

[0175] Table 1: Thermal characteristic curves during elongation (simultaneous; LISIM method)

[0176] Heating zone Temperature [℃] Processing steps 1-2 170 Preheat 3 171-172 Preheat 4 172-173 Elongated Area 5 174 Elongated Area 6 168 Annealing / Relaxation 7 170-172 Annealing / Relaxation 8 172 Annealing / Relaxation 9 166-164 Annealing / Relaxation

[0177] In Examples 1, 4 and 5, the foil was stretched by means of simultaneous stretching in the longitudinal direction by a factor of 6.1 and in the transverse direction by a factor of 9.9. Relaxation (stress removal) was carried out in the longitudinal and transverse directions at a value of 10%. Thus, the effective stretching ratios in the longitudinal direction were 5.5 and in the transverse direction were 9, and the area stretching ratio was 50. In Example 3, a stretching ratio of 5 in the longitudinal direction was used.

[0178] In Example 2, sequential stretching was used. For this purpose, the foil was first fed into a longitudinal stretching device (MDO). In the longitudinal stretching device, the foil was guided over rollers which were heated and had different speeds and thus stretched the foil in the longitudinal direction. The rollers had the temperatures shown in Table 2.

[0179] Table 2(MDO):

[0180]

[0181]

[0182] The foil, which has been stretched in the longitudinal direction, is then stretched in the transverse direction. For this purpose, it is fed into a transverse stretching device (TDO). Here, the foil is preheated again in a preheating zone (first zone 22), stretched in a stretching zone (second zone 24) and heat treated (so-called annealing) in a third zone 26 and relaxed by 10% in the transverse direction. In the various zones, the foil is heated to the temperatures shown in Table 3. The foil is heated uniformly from top to bottom.

[0183] Table 3 (TDO):

[0184] district Temperature [℃] Processing steps 1 182 Preheat 2 180 Preheat 3 178-176 Preheat 4 174-173 Elongated Area 5 172-171 Elongated Area 6 170 annealing 7 172 annealing 8 172 annealing 9 166-162 annealing

[0185] In order to ensure stable foil production, cracks must be avoided. For this reason, the area elongation ratio needs to be reduced by reducing the elongation ratio in the longitudinal direction in Examples 2 and 3. In Example 2, an elongation ratio of 3.7 in the longitudinal direction is used.

[0186] Two foils according to the invention were produced and three comparative experiments were carried out. The data for the foils can be gathered from Table 4 below.

[0187] Table 4: Composition and properties

[0188]

[0189]

[0190] *Comparative experiment

[0191] **Weighted arithmetic mean of Tg of COC1 and COC2

[0192] ***sim=simultaneously;seq=sequentially

[0193] In Example 1 and Example 3, only one cycloolefin polymer is used according to the prior art (WO 2018 / 197034 A1) and the amount of the polymer in the polymer blend is increased to improve temperature resistance. It is shown that when the cycloolefin polymer ratio is increased by only 5 weight percent, the area elongation ratio needs to be reduced during simultaneous stretching in order to ensure stable production (sufficient operational stability). Only in this way can a longer foil strip be produced without foil cracks. In particular, the elongation in the machine direction must be reduced. As can be seen from WO2018 / 197034A1, instead of reducing the elongation, the foil thickness can also be increased to prevent the foil from tearing during production. However, a larger foil thickness is disadvantageous in the case of a foil for a capacitor because the capacitor has a larger volume when the foil is thicker at the same capacitance. This is disadvantageous because the construction space is strictly limited in many applications, including in automobile manufacturing. In Examples 4 and 5 according to the present invention, it is not necessary to reduce the area elongation ratio at the same foil thickness. In addition, Example 5 provides a breakdown voltage that is increased by about 10%.

[0194] As shown in Table 4, the shrinkage values ​​at 140° C. in the transverse direction are also slightly improved in the examples according to the invention relative to the non-inventive examples.

[0195] In order to study the influence of the invention on the electrical properties of the foil, the obtained foil was subjected to a typical metallization process for capacitor foils (profile metallization for DC link applications), and capacitors were prepared from each metallized foil. The capacitors were exposed to different test voltages at 125° C. and 135° C. in a hot box for 3000 hours. The different field strengths given in the table were thus generated. The field strength is the quotient of the voltage in [V] and the foil thickness [μm] (here 3.8 μm). If a short circuit is measured, the capacitor is counted as failed. At defined time intervals (1000 hours / 2000 hours / 3000 hours), the capacitance drift (dC / C) and the change in the loss factor of the capacitor were studied. In the case of large deviations of these data from the initial values, the capacitor was also counted as failed. Table 5 shows the experimental results. Examples 1 to 3 are comparative experiments.

[0196] Table 5: Electrical characteristics

[0197]

[0198] * Field Strength

[0199] Normal = After 3000 hours the capacitor is still within all limits

[0200] Failure = Failure due to short circuit or due to high capacitance drift or excessive loss factor

[0201] -Undetermined

[0202] As can be seen from Table 5, the foil according to the present invention can have significantly better electrical properties at high temperatures. The foils according to the present invention of Examples 4 and 5 fully withstood the tests at a temperature of 125° C. and a field strength of 200 V / μm and 250 V / μm without failure. The foils of Comparative Experiments 1 to 3 all failed at 250 V / μm and only the foil of Comparative Experiment 1 withstood the test at 200 V / μm. The foil of Comparative Experiment 3 was not measured at 200 V / μm, but could not be used at 125° C. and 250 V / μm due to a short circuit.

[0203] At 135° C., none of the comparative foils withstood the tests carried out. The foils of Example 5 withstood the tests at all field strengths, and the foils of Example 4 still withstood the tests at a field strength of 140 V / μm. They failed only at 200 V / μm due to excessively high capacitance drift. The foils according to the invention therefore have significantly better electrical properties at high temperatures. Thus, unlike capacitors according to the prior art, the foils according to the invention can be used in the automotive industry for high-temperature applications.

Claims

1. Biaxially oriented foil, comprising: a) at least two cycloolefin polymers, all of which are copolymers comprising norbornene and ethylene as monomers, as well as b) a semicrystalline alpha-olefin polymer which is polypropylene, wherein the sum of the proportions of all cycloolefin polymers in the foil is in the range of 10 to 25 weight percent, and all cycloolefin polymers contained in the foil have a glass transition temperature in the range of 120 to 180° C., and the foil comprises at least a first cycloolefin polymer and a second cycloolefin polymer, wherein the second cycloolefin polymer has a higher glass transition temperature than the first cycloolefin polymer, and wherein the sum of the proportions of all semi-crystalline α-olefin polymers in the foil is in the range of 75 to 90 weight percent, and all semi-crystalline α-olefin polymers contained in the foil have a crystalline melting temperature in the range of 150 to 170° C., and wherein the weighted arithmetic mean of the glass transition temperatures of all cyclic olefin polymers contained in the foil is less than or equal to the weighted arithmetic mean of the crystalline melting temperatures of all semi-crystalline alpha-olefin polymers contained in the foil, and All quantities therein are relative to the total mass of the foil.

2. The foil according to claim 1, wherein the foil comprises only one semicrystalline α-olefin polymer and the weighted arithmetic mean of the glass transition temperatures of all cyclic olefin polymers contained in the foil is less than or equal to the weighted arithmetic mean of the crystalline melting temperatures of the semicrystalline α-olefin polymers.

3. The foil according to claim 1 or 2, wherein the second cycloolefin polymer has a glass transition temperature at least 3°C ​​higher than the first cycloolefin polymer.

4. The foil according to claim 1 or 2, wherein the foil comprises only one semi-crystalline α-olefin polymer which is polypropylene and the foil comprises no further cycloolefin polymers besides the first cycloolefin polymer and the second cycloolefin polymer, and the first cycloolefin polymer and the second cycloolefin polymer both comprise norbornene and ethylene as monomers. 5 . The foil according to claim 1 , wherein the first cycloolefin polymer and the second cycloolefin polymer are contained in the foil in a ratio ranging from 5 to 15 weight percent, respectively.

6. Multilayer foil comprising at least one layer consisting of a foil according to any one of claims 1 to 5.

7. The foil according to claim 1 or 6, wherein the foil has a thermal shrinkage in the longitudinal direction of at most 5% and in the transverse direction of at most 0.5% after measurement at 130°C for 5 minutes according to ISO 11501.

8. The foil according to claim 1 or 6, wherein the foil has a thermal shrinkage of at most 2% in the longitudinal direction and at most 0.05% in the transverse direction after measurement at 130°C for 5 minutes according to ISO 11501.

9. The foil according to claim 1 or 6, wherein the foil has a surface roughness R in the range of 0.02 to 0.2 μm in the longitudinal direction and in the transverse direction, measured in accordance with DIN EN ISO 4287. a .

10. The foil according to claim 1 or 6, wherein the foil has a surface roughness R in the range of 0.06 μm to 0.09 μm in the longitudinal direction and in the transverse direction, measured in accordance with DIN EN ISO 4287. a .

11. The foil according to claim 1 or 6, wherein the foil has a thickness in the range of 2 μm to 10 μm.

12. The foil according to claim 1 or 6, wherein the foil has a thickness in the range of 2 to 5 μm.

13. The foil of claim 1 or 6, wherein the foil is metallized on one or both sides.

14. Use of the foil according to any one of 1 to 13 as a dielectric for producing a capacitor.

15. A capacitor, characterized in that The capacitor comprises a foil according to one of claims 1 to 13 .

16. Method for producing a foil according to any one of claims 1 to 13, characterized in that The foil is prepared according to a method comprising the following steps: - producing a film, in particular by extrusion, comprising at least two cycloolefin polymers and at least one semicrystalline α-olefin polymer, - stretching the film in the longitudinal direction (MD) and in the transverse direction (TD) so as to obtain a biaxially stretched foil, and - Winding up the biaxially stretched foil.

17. The method according to claim 16, characterized in that The biaxially stretched foil has an area stretch ratio of at least 40.

18. The method according to claim 16 or 17, characterized in that Stretching in the longitudinal direction and in the transverse direction is carried out simultaneously, or stretching in the longitudinal direction is first carried out and then the foil stretched in the longitudinal direction is stretched in the transverse direction.

19. The method according to claim 17, characterized in that One or more of the following additional steps: - subjecting the biaxially stretched foil to a heat treatment, - Relaxing the biaxially stretched foil, thereby reducing the area stretch ratio in a targeted manner.

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