Compositions for capacitors comprising polypropylene and cycloolefin polymers

Abstract

The present invention relates to a composition comprising 70 wt.-% to 95 wt.-% of a high isotactic propylene homopolymer, based on the total weight of the polypropylene composition, and 5 wt.-% to 30 wt.-% of a cycloolefin polymer composition, based on the total weight of the polypropylene composition, the cycloolefin polymer composition having a glass transition temperature in the range of 140 DEG C to 155 DEG C, where the content of the cycloolefin polymer composition is greater than or equal to 100%. The cycloolefin polymer composition includes at least a first cycloolefin polymer and a second cycloolefin polymer, the first cycloolefin polymer having a glass transition temperature of less than 140 DEG C. In addition, the present invention also relates to a cast film comprising the polypropylene composition and a biaxially oriented film comprising the polypropylene composition. In addition, a capacitor including the biaxially oriented film is also provided.

Description

TECHNICAL FIELD

[0001] The present invention relates to a composition comprising 70 wt.-% to 95 wt.-% of a high isotactic propylene homopolymer based on the total weight of the polypropylene composition, and 5 wt.-% to 30 wt.-% of a cyclic olefin polymer composition based on the total weight of the polypropylene composition, the cyclic olefin polymer composition having a glass transition temperature in the range of 140 °C to 155 °C, wherein the cyclic olefin polymer composition comprises at least a first cyclic olefin polymer and a second cyclic olefin polymer, and wherein the first cyclic olefin polymer has a glass transition temperature below 140 °C. Furthermore, the present invention relates to a cast film comprising the polypropylene composition and a biaxially oriented film comprising the polypropylene composition. In addition, a capacitor comprising the biaxially oriented film is provided. BACKGROUND

[0002] Polypropylene has been successfully used as dielectric in various capacitor film applications. However, a problem of known polypropylene capacitor grades is their limited maximum operating temperature. New generations of inverters, for example traction inverters for electric vehicles and inverters for renewable energy sources such as solar and wind energy, require capacitor films with higher temperature resistance.

[0003] So far, the demand for increasing the maximum operating temperature, or in other words, for higher durability at predetermined temperature levels, has been addressed in different ways.

[0004] CN 101724195 A proposes to compound polypropylene with a complex rubber powder in a weight ratio of 100:0.01 - 2.0. The complex rubber powder has a particle size of 20 nm to 100 nm and consists of a silicone rubber powder and a nucleating agent in a weight ratio of 99:1 to 90:10. It is prepared by mixing a radiation vulcanized silicone rubber latex with an aqueous solution or emulsion of a nucleating agent.

[0005] EP 2995641 A1 teaches that high heat resistance can be achieved by compounding a beta-nucleated high purity polypropylene with a long chain branched polypropylene. More specifically, it provides a polypropylene composition comprising 95.0 wt.-% to 99.9 wt.-% of a propylene homopolymer having a specific melt flow rate and a very low ash content, 0.1 wt.-% to 5.0 wt.-% of a long chain branched polypropylene, and up to 1000 ppm of a beta-nucleating agent.

[0006] WO 2018 / 210854 A1 and WO 2018 / 197034 A1 suggest that, in order to obtain a material suitable for high temperature capacitor applications, it can be preferable to blend polypropylene with an amount of a specific cyclic olefin copolymer. In this context, WO 2018 / 210854 A1 relies on ethylene norbornene copolymers having a specific norbornene content, while WO 2018 / 197034 A1 suggests the use of cyclic olefin polymers having a glass transition temperature in a broad range of 120 °C to 170 °C.

[0007] However, in the above-mentioned approaches, the improvement of the temperature resistance of the polypropylene composition for capacitors is achieved at the expense of other key properties. Mechanical properties such as low shrinkage and high stiffness as well as the ability to withstand high voltages are, for example, sacrificed to some extent.

[0008] Therefore, there is still a need to provide a polypropylene composition having an improved set of capacitor application properties, i.e. a polypropylene composition having high temperature resistance, high electric breakdown strength and suitable mechanical properties. It is therefore an object of the present application to overcome the disadvantages of known compositions and to provide a corresponding polypropylene composition. It is a further object of the present application to provide a polypropylene film having excellent properties in capacitor applications. Last but not least, the present application aims at providing a capacitor having an improved specification. Moreover, the capacitor film needs to have high electric breakdown strength and good mechanical properties such as low shrinkage and high stiffness. SUMMARY

[0009] The above-mentioned objects are solved by the polypropylene composition having the features of independent claim 1, the cast film having the features of claim 10, the biaxially oriented film having the features of claim 14 and the capacitor having the features of claim 15.

[0010] The polypropylene composition according to the present application comprises 70 wt.-% to 95 wt.-% of a high isotactic propylene homopolymer, based on the total weight of the polypropylene composition, and 5 wt.-% to 30 wt.-% of a cyclic olefin polymer composition, based on the total weight of the polypropylene composition, the cyclic olefin polymer composition having a glass transition temperature in the range of 140 °C to 155 °C, measured according to ISO 11357, wherein the cyclic olefin polymer composition comprises at least a first cyclic olefin polymer and a second cyclic olefin polymer, wherein the first cyclic olefin polymer has a glass transition temperature of less than 140 °C, measured according to ISO 11357. Preferably, the first cyclic olefin polymer has a glass transition temperature in the range of 60 °C to less than 140 °C, for example 100 °C to less than 140 °C, measured according to ISO 11357. Further, the second cyclic olefin polymer can have a glass transition temperature of at least 155 °C, measured according to ISO 11357. Preferably, the second cyclic olefin polymer has a glass transition temperature in the range of at least 155 °C to 190 °C, measured according to ISO 11357.

[0011] In embodiments, the polypropylene composition comprises 75 wt.-% to 90 wt.-%, preferably 79 wt.-% to 86 wt.-%, more preferably 81 wt.-% to 86 wt.-% of the high isotactic propylene homopolymer, and 10 wt.-% to 25 wt.-%, preferably 14 wt.-% to 21 wt.-%, more preferably 14 wt.-% to 19 wt.-% of the cyclic olefin polymer composition, based on the total weight of the polypropylene composition.

[0012] The cyclic olefin polymer composition having a glass transition temperature in the very specific and small range of 140 °C to 155 °C is selected and its compounding with the high isotactic propylene homopolymer is technically advantageous. On the one hand, the glass transition temperature range of the polypropylene composition is broadened compared to pure high isotactic propylene homopolymer, which leads to good mechanical properties and thus facilitates the processing of the composition into films for capacitor applications. On the other hand, the polypropylene composition according to the present application exhibits excellent structural and dielectric stability at high temperatures.

[0013] In preferred embodiments, the polypropylene composition is obtained by melt blending the high isotactic propylene homopolymer and the cyclic olefin polymer composition. The equipment that can be used for performing the melt blending can be selected from the group consisting of a kneader, a mill and an extruder. Further, the cyclic olefin polymer composition itself can be obtained by melt blending at least two different cyclic olefin polymers.

[0014] Compared to dry blending processes, melt blending processes enable the production of polypropylene compositions with high homogeneity. By selecting the content of the cyclic olefin polymer composition to be in the range of 5 wt-% to 30 wt-%, the cyclic olefin polymer composition is finely and uniformly dispersed in the matrix of the highly isotactic propylene homopolymer. Generally, the state of dispersion of the cyclic olefin polymer composition is considered to have an influence on the mechanical and electrical properties of the polypropylene composition. The size and shape of the domains formed by the cyclic olefin polymer composition in the dispersion can be analyzed using atomic force microscopy (AFM) or other types of microscopy.

[0015] In view of the above, when analyzing a non-oriented cast film consisting of the polypropylene composition with a thickness of 250 µm using AFM, the domains of the cyclic olefin polymer, preferably more than 50% (d 50 , in particular more than 90% (d 90 , such as 95% (d 95 ) have a maximum dimension of less than 2.0 µm.

[0016] Cycloolefin polymer composition The cyclic olefin polymer composition is preferably a thermoplastic composition.

[0017] The cyclic olefin polymer composition can comprise or consist essentially of one type of cyclic olefin polymer or a blend of at least two different types of cyclic olefin polymers.

[0018] It is to be understood that the term "cyclic olefin polymer" refers to a polymer comprising cyclic olefin units. The cyclic olefin polymer can be a cyclic olefin homopolymer or a cyclic olefin copolymer.

[0019] In the present invention, the term "copolymer" refers to a polymer prepared from at least two different monomers. In other words, the term "copolymer" also encompasses terpolymers and copolymers having four or more different monomer units. Thus, the cyclic olefin polymer can be a copolymer comprising first monomer units, i.e. cyclic olefin units, and at least one second monomer unit different from the first monomer units, such as non-cyclic olefin units. A cyclic olefin copolymer comprising at least first cyclic olefin units and second cyclic olefin units different from the first cyclic olefin units is likewise encompassed by the term "cyclic olefin polymer".

[0020] The cyclic olefin polymer can also be obtained by ring-opening polymerization or ring-retaining polymerization of at least one cyclic olefin monomer. Among these alternatives, the cyclic olefin polymer obtained by ring-retaining polymerization is preferred.

[0021] In a further preferred embodiment, the at least one cyclic olefin monomer is selected from the group consisting of cyclopentadiene, tetracyclododecene, norbornene and derivatives thereof. Norbornene and derivatives thereof are particularly preferred examples of the at least one cyclic olefin monomer.

[0022] Norbornene and derivatives thereof are preferably compounds according to formula (I),

[0023] wherein n is 0 or 1, m is 0 or an integer, in particular 0 or 1, R 1 , R 2 , R 3 , R 4 , R 5 and R 6 are independently selected from the group consisting of hydrogen, halogen, alkyl, cycloalkyl, aryl and alkoxy, R 7 , R 8 , R 9 , R 10 , R 11 , R 12 , R 13 , R 14 , R 15 and R 16 are independently selected from the group consisting of hydrogen and alkyl, R 17 , R 18 , R 19 and R 20 are independently selected from the group consisting of hydrogen, halogen, alkyl, cycloalkyl and aryl, wherein R 17 and R 19 may alternatively form a cyclic ring or a cyclic system, which can be saturated or unsaturated.

[0024] A further and likewise preferred example of norbornene derivatives is a compound according to formula (II),

[0025] wherein R 21 and R 22 are independently selected from the group consisting of hydrogen, C5-C7 cycloalkyl, C5-C7 aryl and C1-C4 alkyl, and R 23 and R 24 are independently selected from the group consisting of hydrogen and C1-C4 alkyl.

[0026] The non-cyclic olefin monomer can be an a-olefin. Non-limiting examples of a-olefins are C2-C8 a-olefins such as a-olefins selected from the group consisting of ethylene, propylene, 1-butene, 2-methyl-1-propene (isobutylene), and 3-methyl-1-butene. Preferred a-olefins are linear C2-C8 a-olefins such as ethylene and propylene. Particularly preferred is ethylene as a-olefin.

[0027] In one embodiment, the cyclic olefin polymer composition comprises, consists of, or consists essentially of at least one cyclic olefin copolymer, preferably at least one cyclic olefin copolymer selected from the group consisting of norbornene-ethylene copolymers and tetracyclododecene-ethylene copolymers, or a blend of at least two of said cyclic olefin copolymers. In a particularly preferred embodiment, the cyclic olefin polymer composition comprises or consists essentially of one norbornene-ethylene copolymer or a blend of two norbornene-ethylene copolymers having different norbornene contents. Further, non-limiting examples of cyclic olefin polymers are polymers marketed by Mitsui Chemical under APEL® or by TOPAS Advanced Polymers under TOPAS®.

[0028] The glass transition temperature (hereinafter also referred to as “Tg”) of the cyclic olefin polymer composition can be controlled by the type of monomers used, for example by the type of at least one cyclic olefin monomer, and, optionally, if present, by the type of non-cyclic olefin monomer. If the cyclic olefin polymer composition is a blend of at least two cyclic olefin polymers, the weight ratio of the cyclic olefin polymers also influences the glass transition temperature of the composition. Further, in case the cyclic olefin polymer composition is a composition comprising at least one cyclic olefin copolymer, the amount of cyclic olefin monomers can be used to tailor the glass transition temperature of the overall composition. For norbornene-ethylene copolymers, for example, it is known that a higher glass transition temperature can be achieved by increasing the content of norbornene. The same applies to combinations of other cyclic olefin monomers with non-cyclic olefin monomers.

[0029] The cyclic olefin polymer composition can comprise cyclic olefin units (e.g. norbornene units) in an amount of 70 wt.-% to 90 wt.-%, preferably 75 wt.-% to 85 wt.-%, more preferably 75 wt.-% to 80 wt.-%, and non-cyclic olefin units (e.g. ethylene units) in an amount of 10 wt.-% to 30 wt.-%, preferably 15 wt.-% to 25 wt.-%, more preferably 20 wt.-% to 25 wt.-%, wherein each amount is based on the total weight of the cyclic olefin polymer composition.

[0030] In a preferred embodiment, the glass transition temperature of the cyclic olefin polymer composition can be in the range of 140 °C to 150 °C, such as in the range of 140 °C to 144 °C, in particular in the range of 140 °C to 143 °C. In another preferred embodiment, the glass transition temperature of the cyclic olefin polymer composition can be in the range of 145 °C to 155 °C.

[0031] The first cyclic olefin polymer and the second cyclic olefin polymer can be formed into the cyclic olefin polymer composition by melt blending, preferably in a weight ratio of 1 :5 to 5:1, in particular in a weight ratio of 1 :3 to 3:1.

[0032] The use of two cyclic olefin polymers having different glass transition temperatures allows to widen the glass transition temperature range of the polypropylene composition and thus to extend the processing window of the polypropylene composition. Furthermore, the use of two cyclic olefin polymers having different glass transition temperatures allows to better control the high temperature properties of the material.

[0033] The MFR2 (260 °C, 2.16 kg) of the cyclic olefin polymer composition according to ISO 1133 is preferably in the range of 2 g / 10 min to 50 g / 10 min, more preferably in the range of 8 g / 10 min to 25 g / 10 min.

[0034] High isotactic propylene homopolymer The expression "homopolymer of propylene" as used in the present invention relates to a polypropylene consisting essentially of propylene units, the propylene unit content being at least 99.5 wt.-%, more preferably at least 99.8 wt.-%. Preferably, the homopolymer of propylene is a high isotactic homopolymer of propylene as described in the example section below. 13 The C NMR spectrum detects only propylene units in the homopolymer of propylene.

[0035] The high isotactic homopolymer of propylene is preferably a linear polypropylene, i.e. a polypropylene having no detectable amount of long chain or short chain branches.

[0036] In a preferred embodiment of the present invention, the high isotactic homopolymer of propylene is characterized by a rather high pentad isotacticity of at least 90 %, more preferably of at least 95 %. <mmmm>(through 13 More specifically, the high isotactic propylene homopolymer can have a fraction of isotactic pentad in the range of 93 % to 99.5 %, preferably 95 % to 99.0 %, more preferably 96 % to 98.5 %. In a particular embodiment, the pentad isotacticity of the high isotactic propylene homopolymer is in the range of 96.0 % to 98.5 %, still more preferably in the range of 97.0 % to 98.0 %.

[0037] The high isotacticity is preferably accompanied by a high crystallinity of the material. Furthermore, the high isotacticity is preferably accompanied by a low content of xylene cold soluble (XCS). Thus, in a further preferred embodiment of the present application, the high isotactic propylene homopolymer of the present application is characterized by a rather low XCS content, i.e. an XCS content equal to or below 2.0 wt.-%, more preferably equal to or below 1.8 wt.-%, still more preferably equal to or below 1.6 wt.-%. It is thus in particular understood that the XCS content of the high isotactic propylene homopolymer of the present application is in the range of 0.3 wt.-% to equal to or below 2.0 wt.-%, more preferably in the range of 0.3 wt.-% to equal to or below 1.8 wt.-%, still more preferably in the range of 0.4 wt.-% to equal to or below 1.6 wt.-%.

[0038] Preferably, the XCS content refers to the XCS content determined according to ISO 6427 at 23 °C. It is further understood that the XCS content indicates that the high isotactic propylene homopolymer is preferably free of any elastomeric polymer component, such as an ethylene-propylene rubber. In other words, the high isotactic propylene homopolymer does not correspond to a heterophasic polypropylene, i.e. a system consisting of a polypropylene matrix in which an elastomer phase is dispersed. Such systems are typically characterized by a rather high content of xylene cold soluble.

[0039] Alternatively or additionally, it is preferred that the high isotactic propylene homopolymer of the present application has a melt flow rate in a specific range. For example, the MFR2 (230 °C) of the high isotactic propylene homopolymer measured according to ISO 1133 can be in the range of 0.4 g / 10 min to 10 g / 10 min, preferably in the range of 0.5 g / 10 min to 5 g / 10 min. In a particularly preferred embodiment, the MFR2 (230 °C) of the high isotactic propylene homopolymer measured according to ISO 1133 is in the range of 2.0 g / 10 min to 4.5 g / 10 min. In this context, it is confirmed that MFR2 (230 °C) denotes the melt flow rate measured at a temperature of 230 °C and a load of 2.16 kg.

[0040] Furthermore, the high isotactic propylene homopolymer preferably has an ash content of 60 ppm or less, more preferably an ash content of 30 ppm or less, even more preferably an ash content of 20 ppm or less. Preferably, the ash content can be in the range of 5 ppm to 30 ppm, in particular in the range of 10 ppm to 20 ppm. A low ash content is crucial for dielectric properties like dielectric strength and dielectric loss. The ash content can be measured according to ISO 3451-1 (1997).

[0041] The melting temperature Tm of the high isotactic propylene homopolymer measured according to ISO 11357 can be in the range of 150 °C to 170 °C, preferably in the range of 160 °C to 170 °C. The crystallization temperature Tcof the high isotactic propylene homopolymer measured according to ISO 11357 can be in the range of 110 °C to 130 °C, preferably in the range of 110 °C to 120 °C.

[0042] Additive In one embodiment, the polypropylene composition comprises a high isotactic homopolymer, a cyclic olefin polymer composition, and an additive, the high isotactic homopolymer being present in an amount of 74.0 wt.-% to 89.0 wt.-%, preferably 78.5 wt.-% to 85.5 wt.-%, more preferably 80.5 wt.-% to 85.5 wt.-%, the cyclic olefin polymer composition being present in an amount of 10 wt.-% to 25 wt.-%, preferably 14 wt.-% to 21 wt.-%, more preferably 14 wt.-% to 19 wt.-%, and the additive being present in an amount of 0.0 wt.-% to 1.0 wt.-%, preferably 0.0 wt.-% to 0.5 wt.-%, wherein the amounts of the above-mentioned substances are again based on the total weight of the polypropylene composition. Preferably, the amounts of the high isotactic homopolymer, the cyclic olefin polymer composition, and the additive sum up to 100 wt.-%.

[0043] The additive can be a non-polymeric additive and / or a polymeric additive. For example, the additive can be selected from the group consisting of an antioxidant, a stabilizer, an acid scavenger, a colorant, a plasticizer, a slip agent, an antiscratch agent, a dispersant, a processing aid, a lubricant, a pigment, an antistatic agent, and the like. Preferably, the additive is selected from the group consisting of an antioxidant, a stabilizer, and an acid scavenger.

[0044] The antioxidant and the stabilizer can be selected from the group of hindered phenols, more preferably from the group of hindered phenols not containing phosphorous or sulfur.

[0045] The antioxidants and stabilizers are especially preferably one or more compounds selected from the group consisting of 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4- hydroxybenzyl)benzene (sold under the trade names Irganox 1330, Anox 330, Ethanox 330 and Kinox-30), tetra-(3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionic acid) pentaerythritol ester (sold under the trade names Irganox 1010, Anox 20, Ethanox 310TF and Kinox-10), octadecyl 3-(3',5'-di-tert-butyl-4-hydroxyphenyl)propionate (sold under the trade names Irganox 1076, Anox PP 18 and Kinox-16), butylated hydroxytoluene (sold under the trade names Ionol CP and Vulkanox BHT), 1,3,5-tris(3',5'-di-tert-butyl-4'-hydroxybenzyl) isocyanurate (sold under the trade names Irganox 3114, Anox IC-14, Ethanox 314 and Kinox-34), and 3,4-dihydro-2,5,7,8-tetramethyl-2-(4,8,12-trimethyltridecyl)-2H-1 -benzopyran-6-ol (sold under the trade names Irganox E 210 and alpha-tocopherol).

[0046] The total amount of antioxidants and stabilizers is preferably from 500 ppm to 8000 ppm, based on the total weight of the polypropylene composition. More preferably, the total amount of antioxidants and stabilizers is from 800 ppm to 7000 ppm, still more preferably from 1000 ppm to 6000 ppm, in particular from 1500 ppm to 6000 ppm, based on the total weight of the polypropylene composition. In view of the intended application of the composition in the field of capacitors, the antioxidants and stabilizers preferably do not comprise a phosphorous-containing auxiliary antioxidant such as tris(2,4-di-tert-butylphenyl) phosphite. While not wishing to be bound by theory, the latter class of compounds is believed to increase the dissipation of the final capacitor.

[0047] The acid scavengers can be salts of organic acids such as stearates. They are typically used to neutralize acids in the polymer. Examples of such compounds are calcium stearate, zinc stearate and zinc oxide. The acid scavengers are preferably used in an amount of from 50 ppm to 2000 ppm, more preferably from 50 ppm to 1000 ppm.

[0048] Film The present application also provides a film comprising the polypropylene composition as described above. The film can be a cast film or a biaxially oriented film (BOPP). Furthermore, the film preferably has a thickness of 1 pm to 1000 pm, more preferably of 1 pm to 300 pm, most preferably of 1 pm to 250 pm, in particular of 1 pm to 100 pm. This is especially advantageous because the current trend is to provide electrical systems with higher energy density and efficiency. Reducing the footprint of the film in the film capacitor helps to achieve this goal.

[0049] If a cast film having a thickness of 250 pm is provided, the film preferably has a dielectric breakdown voltage of 118 kV / mm or more, preferably of 125 kV / mm or more, measured according to IEC 60243-1 at 90 °C. For example, the dielectric breakdown voltage of a cast film having a thickness of 250 pm, measured according to IEC 60243-1 at 90 °C, can be in the range of 118 kV / mm to 250 kV / mm, preferably in the range of 125 kV / mm to 200 kV / mm.

[0050] It is further preferred that the tangent of the loss angle of the cast film is less than 0.001, preferably less than 0.0005, more preferably less than 0.0002, when the thickness of the cast film is 250 pm, wherein the tangent of the loss angle is measured at 20 Hz and 120 °C. For example, the tangent of the loss angle of the cast film can be in the range of 0.00001 to 0.001, preferably in the range of 0.00001 to 0.0005, more preferably in the range of 0.00005 to 0.0002, when measured at 20 Hz.

[0051] The electrical conductivity of the cast film according to the present application can be 2 fS / cm or less. For example, the electrical conductivity of the cast film can be in the range of 0.01 fS / cm to 2 fS / cm. The electrical conductivity can be measured with alternating current in the frequency range of 10 -2 Hz to 10 -1 Hz according to IEC 62631-3-2 or can be taken as the inverse of the direct current resistance measured according to ASTM D257, both measurements at a temperature of 90 °C and a film thickness of 250 pm.

[0052] The biaxially oriented polypropylene film according to the present application can be obtained in two steps. In a first step, a non-oriented film is produced by extruding the polypropylene composition through a flat die. In a second step, the non-oriented film is stretched in the machine direction (MD) and in the transverse direction (TD). The stretching or orientation of the non-oriented film can be performed simultaneously in the machine direction and in the transverse direction. The two steps of the process are preferably performed in a continuous process. Alternatively, the non-oriented film can be collected and cooled with rotating cooling rolls to solidify the film before stretching.

[0053] In embodiments, the biaxially oriented polypropylene film comprises at least 80 wt.-% of the polypropylene composition as defined by the present application, more preferably at least 90 wt.-% of the polypropylene composition, still more preferably consists of the polypropylene composition. Preferably, the biaxially oriented polypropylene film has a stretch ratio in the machine direction of at least 4, preferably at least 5; the biaxially oriented polypropylene film has a stretch ratio in the transverse direction of at least 4, preferably at least 5; more preferably, the biaxially oriented polypropylene film has a stretch ratio in the machine direction of at least 9 and a stretch ratio in the transverse direction of at least 5.

[0054] The biaxially oriented polypropylene film can be prepared by conventional drawing processes known in the art. Thus, the process for manufacturing the biaxially oriented polypropylene film can preferably employ the tenter method known in the art.

[0055] The tenter method is a method in which the polypropylene composition as defined herein is melt-extruded from a slot die, such as a T-die, and a non-oriented film is obtained by cooling on a cooling drum. The film is preheated, for example with heated metal rolls, and then stretched in the length direction between a plurality of rolls, a circumferential speed difference being established between the rolls, and then the two edges of the film are gripped with clamps and the transverse stretching is performed by means of a tenter oven, thereby obtaining a biaxially oriented film. The temperature of the film during the longitudinal drawing is preferably controlled in the temperature range of the melting point of the polypropylene composition.

[0056] Subsequently, the biaxially oriented film can be subjected to a corona discharge treatment in air, nitrogen, carbon dioxide gas or any mixture thereof. If the film is to be metallized for film capacitor applications, this can enhance the adhesion strength to the metal to be deposited.

[0057] Capacitor In another aspect, the present application relates to a capacitor comprising the biaxially oriented film as described above. The capacitor comprising the biaxially oriented film is expected to have a longer service life and a higher temperature resistance than conventional capacitors. Preferably, the biaxially oriented film as described above is metallized and the capacitor is a metallized film capacitor. The metallization of the biaxially oriented film can be carried out by any process known in the art, such as evaporation, electrodeposition, fusion, ion beam vacuum deposition, sputtering or ion plating. The thickness of the obtained metal layer can be from 100 A (0.01 pm) to 5000 A (0.5 pm). BRIEF DESCRIPTION OF DRAWINGS

[0058] Embodiments of the present application are illustrated by way of example and not limitation in the figures of the accompanying drawings in which like references indicate similar elements, and in which:

[0059] Figure 1 and Figure 2 Results of dielectric spectroscopy measurements using example materials are shown; Figure 3 The electrical breakdown behavior of several example materials can be compared; Figure 4 (a)- Figure 4 (d) Atomic force microscope images of several example materials are shown. DETAILED DESCRIPTION

[0060] Measurement method Quantification of the microstructure by NMR spectroscopy Quantitative nuclear magnetic resonance (NMR) spectroscopy was used to quantify the isotacticity of the polymers as well as the comonomer content.

[0061] Quantitative 1 H and 13 C NMR spectra were recorded in solution state using a Bruker Advance III 400 NMR spectrometer operating at 400.15 MHz and 100.62 MHz for 13 C{ 1 H} respectively. All spectra were recorded at 125 °C using a 13 C optimized 10 mm extended temperature probehead.

[0062] For polypropylene homopolymer, about 200 mg of material is dissolved in 1,2-tetrachloroethane-d2(TCE-d2). To ensure a homogeneous solution, after initial sample preparation in a heating zone, the NMR tube is further heated in a rotating furnace for at least 1 hour. Once the tube is inserted into the magnet, the tube is spun at a frequency of 10 Hz. This setting is chosen primarily for the high resolution required for tacticity distribution quantification (Busico, V., Cipullo, R., Prog. Polym. Sci. 26 (2001) 443; Busico, V.; Cipullo, R., Monaco, G., Vacatello, M., Segre, A.L., Macromoleucles 30 (1997) 6251). Standard single-pulse excitation is implemented with NOE and a two-level WALTZ16 decoupling scheme (Zhou, Z., Kuemmerle, R., Qiu, X., Redwine, D., Cong, R., Taha, A., Baugh, D. Winniford, B., J. Mag. Reson. 187 (2007) 225; Busico, V., Carbonniere, P., Cipullo, R., Pellecchia, R., Severn, J., Talarico, G., Macromol. Rapid Commun. 2007, 28, 11289). A total of 8192 (8k) transients are acquired for each spectrum.

[0063] The quantified 13 C{ 1 H} NMR spectra are processed, integrated and the relevant quantified properties are determined from the integrals.

[0064] For polypropylene homopolymer, all chemical shifts are inherently referenced to the methyl isotactic pentad at 21.85 ppm <mmmm>.

[0065] Characteristic signals corresponding to regiodefects (Resconi, L., Cavallo, L., Fait, A., Piemontesi, F., Chem. Rev. 2000, 100, 1253; Wang, W-J., Zhu, S., Macromolecules 33 (2000), 1157; Cheng, H.N., Macromolecules 17 (1984), 1950) or comonomer.

[0066] The tacticity distribution is quantified by integrating the methyl region between 23.6-19.7 ppm, correcting for any sites not related to the stereosequence of interest (Busico, V., Cipullo, R., Prog. Polym. Sci. 26 (2001) 443; Busico, V., Cipullo, R., Monaco, G., Vacatello, M., Segre, A.L., Macromolecules 30 (1997) 6251).

[0067] Specifically, the influence of regiodefects and comonomer on the quantification of tacticity distribution is corrected by subtracting representative regiodefects and comonomer integrals from the specific integral region of the stereosequence.

[0068] Isotacticity is determined at pentad level and reported as the percentage of isotactic pentad (mmmm) sequences relative to all pentad sequences: [mmmm] % = 100 * (mmmm / sum of all pentads) If needed, the method of Wang et al. (Wang, W-J., Zhu, S., Macromolecules 33 (2000), 1157) can be used to quantify the absence of ethylene in the high isotactic propylene homopolymer by integrating multiple signals of the entire spectral region of the NMR spectrum acquired under defined conditions. 13 C{ 1 H} spectrum acquired under defined conditions. This method is chosen for its accuracy, robustness and ability to account for the presence of regiodefects if needed. The integral region is slightly adjusted to increase applicability to a broader comonomer content range.

[0069] Ethylene incorporation (mole percent) is calculated as follows: E [mol%] = 100 * fE Ethylene incorporation (weight percent) is calculated as follows: E [wt%] = 100 * (fE * 28.05) / ( (fE * 28.05) + ((1-fE) * 42.08) ) Melt flow rate MFR 2 MFR2 is measured according to ISO 1133 (under a load of 2.16 kg) at a temperature of 230 °C (for high isotactic propylene homopolymers) or 260 °C (for cyclic olefin copolymer compositions).

[0070] Glass transition temperature Tg The glass transition temperature is measured using a differential scanning calorimeter and according to ISO 11357 with a scan rate of 10 K / min.

[0071] Melting temperature Tm, crystallization temperature Tc Differential scanning calorimetry (DSC) experiments are performed according to ISO 11357 / 1 on a TA Instruments Q2000 apparatus, calibrated with indium, zinc, tin. The measurements are done under a nitrogen atmosphere (50 mL / min) on 5 ± 0.5 mg samples in a heat / cool / heat cycle between -30 °C and 225 °C with a scan rate of 10 K / min according to ISO 11357 / 3. The peak values of the endothermic and exothermic peaks in the cooling and second heating cycle are taken as the melting temperature Tm and crystallization temperature Tc, respectively.

[0072] Ash content The ash content of the polymer is determined according to ISO 3451-1 (1997). The procedure is as follows: about 100 grams of polymer is weighed into a platinum crucible. The crucible is then heated on a Bunsen burner flame, causing the polymer to burn slowly. After the polymer has completely burned, the crucible is cooled, dried and weighed. The ash content is the weight of the residue divided by the weight of the polymer sample. At least two measurements are made, and if the difference between the two measurements is more than 7 ppm, a third measurement is made.

[0073] Dielectric breakdown measurement Dielectric breakdown strength, equivalent to the breakdown field strength, usually expressed in kV / mm, is a property of gases, liquids and solids. When the electric field exceeds the breakdown field strength, a discharge channel is formed across the material connecting the electrodes, and a breakdown occurs. In solids, the discharge irreversibly destroys the material, while in gases and liquids the damage is temporary and reversible.

[0074] Generally, the occurrence of breakdown events is highly stochastic, i.e. when testing identical specimens to establish a breakdown distribution (see detailed description below), a high data scatter is observed and the data often do not follow a normal distribution (see Dissado L. A.; Fothergill J. C. "Electrical degradation and breakdown in polymers", IEEE Materials and Devices Series 9, Peter Peregrinus Ltd., 1992). The reason for the stochastic occurrence of breakdown events lies in the non-specific and localized breakdown initiation phenomena occurring at sites of electric field enhancement and low breakdown strength (see Dissado L. A.; Pothergill J. C. "Electrical degradation and breakdown in polymers", IEEE Materials and Devices Series 9, Peter Peregrinus Ltd., 1992). Generally, structural heterogeneities of the material, inclusions of contaminants or voids (see Chen G.; Davies A. E. The influence of defects on the short-term breakdown characteristics and long-term dc performance of LDPE insulation. IEEE Transactions on Electrical Insulation, 2000, 7, 401-407) and surface roughness (see Rytoluoto L; Gitsas A.; Pasanan S.; Lahti K. Effect of film structure and morphology on the dielectric breakdown characteristics of cast and biaxially oriented polypropylene films. European Polymer Journal, 2017, 95, 606-624) can all be potential breakdown sites, which are unavoidable in industrial production.

[0075] Breakdown test The breakdown strength of the solid is tested by a short-time voltage ramp test. Thin specimens of the cast film (250 pm thick) are clamped between electrodes and the voltage is increased (linear, exponential, stepwise, etc.) until breakdown occurs. The voltage at which breakdown occurs is the breakdown voltage (in kV) of the specimen (see IEC 60234-1 (2013) - Electric strength of insulating materials - Test methods - Part 1 : Tests at power frequencies).

[0076] Then, the thickness of the specimen at the breakdown point is measured to obtain the breakdown strength Eb (in kV / mm). This test is repeated on the same prepared material specimen to obtain the breakdown distribution of the material (see Rytoluoto T; Gitsas A.; Pasanan S.; Lahti K. Effect of film structure and morphology on the dielectric breakdown characteristics of cast and biaxially oriented polypropylene films. European Polymer Journal, 2017, 95, 606-624).

[0077] Statistical evaluation The breakdown distribution obtained from voltage ramp tests typically does not follow a normal distribution, but rather an extreme value distribution, where the Weibull distribution is commonly used (see Dissado L. A.; Fothergill J. C. "Electrical degradation and breakdown in polymers", IEEE Materials and Devices Series 9, Peter Peregrinus Ltd., 1992, and IEC 62539 (2007) - Guide for the statistical analysis of electrical insulation breakdown data). The Weibull distribution was originally proposed using three parameters, a scale parameter a, a shape parameter b and a location parameter d. The scale parameter a is the mean value of the Weibull distribution, used in analogy, but not equivalent to the mean of a normal distribution. The mean is the 50th percentile of a normal distribution, while the scale parameter a is the 63.2nd percentile of a Weibull distribution. When referring to the breakdown strength of a material, most authors essentially refer to the scale parameter a. The shape parameter b, as the name suggests, influences the shape of the Weibull distribution, which for low b values can look like an exponential distribution, or for high b values, the shape of the Weibull distribution looks almost bell-shaped like a normal distribution. Thus, a high b value implies low dispersion and bell shape. The location parameter d is a "shift constant" as it shifts the experimental distribution to the origin, i.e. when Eb - d = 0, the failure probability is zero. However, d is often not needed, assumed to be zero, and authors apply the Weibull distribution with two parameters, i.e. using only a and b.

[0078] Measurement details The measured breakdown strength and breakdown distribution are influenced by several experimental details such as the use of direct current or alternating current (Krentz T.; Khani M. M.; Bell M.; Benicewicz B. C.; Nelson J. K.; Zhao S.; Schadler L. S. Morphologically dependent alternating-current and direct-current breakdown strength in silica-polypropylene nanocomposites. Journal of Applied Polymer Science, 2017, 134), and the voltage ramp rate (Rytoluoto L; Ritamaki M.; Lahti K.; Karttunen M. Ramp rate effect on the breakdown response of SiCk-BOPP Nano composites, IEEE International conference on the properties of applications of dielectric materials, 2015, 496-499). Another important experimental detail is the measured volume. The sample thickness and (electrode) area influence the breakdown strength (see Lai honen S. J. et al. "Area dependence of breakdown strength of polymer films: automatic measurement method." IEEE Transactions on dielectrics and electrical insulation, 2007, 14, 263-274, and Rytoluoto L; Lahti K. Effect of film thickness and electrode area on the dielectric breakdown characteristics of metallized film capacitor films, 23rd Nordic Insulation Symposium (Nordis13), 2013, 33-38), and need to be reported together with the breakdown results.

[0079] Related to the volume effect of breakdown strength is the electrode geometry, such as shape and size. Typical electrode geometries include parallel plate-plate, cylinder-plate, and sphere-plate designs. In the latter, the effective electrode area is only the contact area between the sphere and the sample, so the effective area of the sample under stress is much smaller than the electrode diameter. In addition, breakdown can occur through the air gap between the electrode and the film outside the contact area, affecting the breakdown statistics.

[0080] In addition to electrode design, data collection techniques also play a role, such as how many samples are tested to build up the breakdown distribution. Generally, the more samples that are tested, the more extreme results are found, i.e., more very low Eb and very high Eb will be measured, which will affect the breakdown distribution. In the common voltage ramp test described above, samples are individually broken down one after the other. Each new sample starts at zero voltage and is subjected to a new voltage ramp, and breakdown will always occur at the weakest point of the sample, which stops the voltage ramp. Even though the sample has stronger points, they are not measured, i.e., the strongest parts of the sample (typically the material) are not tested. While the more samples that are tested, the higher the probability of high breakdown strength results in the distribution, this one sample at a time approach biases the breakdown distribution to lower breakdown strengths.

[0081] Advanced automated breakdown measurement methods exist (see Boggs S. A.; Ho J.; Jo T.R.; Overview of Laminar Dielectric Capacitors, Electrical Insulation Magazine, 2010, 26, 7-13, and Rytoluoto L; Lahti K. New approach to evaluate area-dependent breakdown characteristics of dielectric polymer films, Transactions on Dielectrics and Electrical Insulation, 2013, 20, 937-946, and Kerwien C.M.; Malandra D.L.; Broomall J.R., Large area DC dielectric breakdown voltage measurement of BOPP and PTFE thin films, IEEE Conference on Insulation and Dielectric Phenomena, 2016, 486-489, and Laihonen S. J. et al. “Area dependence of breakdown strength of polymer films: automatic measurement method.” IEEE Transactions on dielectrics and electrical insulation, 2007, 14, 263-274), one sub-type of which measures all individual breakdowns (multiple breakdowns) in one voltage ramp on the same sample (area), i.e. obtaining the complete breakdown distribution (see Boggs S. A.; Ho J.; Jo T.R.; Overview of Laminar Dielectric Capacitors, Electrical Insulation Magazine, 2010, 26, 7-13 and Rytoluoto L; Lahti K.New approach to evaluate area-dependent breakdown characteristics of dielectric polymer films, Transactions on Dielectrics and Electrical Insulation, 2013, 20, 937-946). With this method, after the weaker part of the sample is broken down, the stronger part of the sample is also broken down as the voltage continues to rise. Therefore, the breakdown distribution obtained by such a test usually includes very high breakdown strengths, thus this method makes the breakdown distribution biased towards higher breakdown strengths compared to the manual individual method.

[0082] In summary, when reporting breakdown data, it is possible to make direct comparisons between materials using the same test method, area, voltage ramp rate, and the same statistical evaluation, but not between different tests.

[0083] Electrical conductivity The conductivity is calculated as the inverse of the resistance determined according to ASTM D257.

[0084] Dielectric analysis (DEA) Dielectric analysis (DEA) was performed with a TA-Instruments model DEA 2070 instrument with a gold sputter device. The measurements were performed with a ramp of 3 °C / min up to 130 °C, a frequency of 50 Hz, and a constant spring force (F) of 100 N.

[0085] Dielectric analysis measures the capacitance and conductivity, which are two fundamental electrical properties of a material as a function of time, temperature, and frequency. The capacitance of a material refers to the ability to store charge, and the conductivity of a material refers to the ability to conduct charge. Capacitance and conductivity are important properties. Dissipation is independent of the thickness or orientation angle of the film.

[0086] The properties measured by dielectric analysis are as follows: e' = real part of the dielectric constant (relative permittivity) e" = imaginary part of the dielectric constant (loss factor) tan d = tangent of the loss angle (e" / e') s = ionic conductivity [1 / Ωcm].

[0087] Conductivity is directly proportional to the relative dielectric constant, while ionic conductivity is derived from the loss factor. Both the relative dielectric constant and the loss factor provide valuable information about the molecular motion. The relative dielectric constant determines the arrangement of dipoles, while the loss factor corresponds to the energy required to arrange dipoles and move ions. At low temperatures, the relative dielectric constant of a polymer is low because the dipoles are unable to move under the influence of an electric field to arrange themselves. The ionic conductivity is not significant until the polymer becomes a fluid, i.e. until it exceeds the glass transition temperature (Tg) and the melting temperature (Tm).

[0088] Above the glass transition temperature, the loss factor is used to calculate the bulk ionic conductivity, using the following equation:

[0089] σ = ionic conductivity ω = angular frequency (2πf) f = frequency (Hz) e0 = absolute permittivity of free space (8.85 x 10-12 F / m) Dielectric spectroscopy measurement Dielectric spectroscopy measurements were performed on the films using a Novocontrol Alpha frequency analyzer at 120 °C and atmospheric pressure. The frequency range of the dielectric spectroscopy measurements was 10 -2 Hz to 10 6 Hz. A suitably cut piece of film was placed between two identical electrodes with a diameter of 40 mm. The measurements were performed under a nitrogen atmosphere.

[0090] Atomic force microscopy image All specimens were taken directly from the film sample and cut using an ultra-low temperature microtome in the core of the film and in a direction perpendicular to the machine direction (MD). No surface treatment was performed. In addition to the "regular" scans, qualitative amplitude-frequency modulation (AM-FM) scans were performed, as this method allows for a better differentiation between the different phases. Comparisons were made based on one large area image of 20 x 20 μm2of the film.

[0091] Material The following table 1 lists the polymers that have been used to prepare the example materials.

[0092] Table 1: Polymers used to produce CE1, CE2 and IE1-IE3

[0093] The above polymers were mixed and melt blended in the amounts shown in Table 2 below to form example materials CE1, CE2 and IE1 -IE3. Example materials CE2 and IE1 -IE3 are polypropylene compositions comprising an amount of a cyclic olefin polymer. Example material CE1 is a pure high isotactic polypropylene homopolymer.

[0094] Table 2: Composition of sample materials CE1, CE2 and IE1 -IE3

[0095] Film samples were produced from the sample materials on a small laboratory blown film production line from COLLIN Lab & Pilot Solutions GmbH. The production line consists of an extruder with a 30 mm long screw (L / D ratio of 30). The extruder temperature was set to 235 °C, the melt temperature was 235 °C and recorded after 45 minutes of process stabilization. The extruder was followed by a die with a width of 300 mm. The die had a flexible die lip with a die gap of 0.5 mm - 1.5 mm. The production line was run at a constant throughput of 8 kg / h and a line speed of 10 m / min. The end width of the film was cut to 270 mm. The produced film thickness was 250 pm.

[0096] Subsequently, the electrical conductivity of the CE1, CE2 and IE1 -IE3 films was measured at different frequencies. The frequency dependence can be derived from Figure 1 It was found that the electrical conductivity of the propylene compositions comprising a cyclic olefin polymer (like CE2 and IE1 -IE3) is lower than the electrical conductivity of the pure high isotactic polypropylene homopolymer CE1 over the whole range of measured frequencies.

[0097] At the low frequency end of about 10 -2 to 10 -1 Hz, which is particularly important for PP capacitors, IE2 was the most effective.

[0098] The energy dissipation capability at different frequencies was evaluated based on the loss tangent tan d. Therefore, the imaginary part e' and the real part e" of the dielectric constant were measured and tan d was calculated as the ratio e' / e". The relationship of tan d with the frequency can be found in Figure 2 Generally, lower tan d values are expected to lead to better energy storage performance and a lower risk of damage due to local temperature increase. Therefore, the propylene composition IE2 scored the highest and has a particular advantage at the low frequency end compared to the other compositions and the pure high isotactic propylene polymer.

[0099] Figure 3 The bar graph, from which conclusions can be drawn regarding the breakdown durability of the films made from CE1, CE2 and IE1, shows that the breakdown voltage of IE1 is increased by about 10% over that of CE1 and CE2.

[0100] The morphology of the films made from CE2 and IE1 -IE3 was examined using atomic force microscopy (AFM). From the AFM images in Figure 4 (a)- Figure 4 the AFM images in (d) the following conclusions can be drawn: Figure 4 (b) - the dispersion quality of the compositions corresponding to IE1 -IE3 in Fig. (d) is far superior to that of the composition corresponding to CE2 in (a). Without wishing to be bound by theory, it is believed that this indicates that IE1 -3 have a higher homogeneity and improved overall macroscopic dielectric properties. Figure 4 (b) - the dispersion quality of the compositions corresponding to IE1 -IE3 in Fig. (d) is far superior to that of the composition corresponding to CE2 in (a). Without wishing to be bound by theory, it is believed that this indicates that IE1 -3 have a higher homogeneity and improved overall macroscopic dielectric properties.< / mmmm> < / mmmm>

Claims

1. A polypropylene composition comprising: 70 wt.-% to 95 wt.-% of a high isotactic propylene homopolymer (A), based on the total weight of the polypropylene composition, and 5 wt.-% to 30 wt.-% of a cyclic olefin polymer composition (B), based on the total weight of the polypropylene composition, the cyclic olefin polymer composition (B) having a glass transition temperature in the range of 140 °C to 155 °C, measured according to ISO 11357, wherein the cyclic olefin polymer composition (B) comprises at least a first cyclic olefin polymer (bl) and a second cyclic olefin polymer (b2), wherein the first cyclic olefin polymer (bl) has a glass transition temperature below 140 °C, measured according to ISO 11357.

2. The polypropylene composition according to claim 1, characterized in that The cyclic olefin polymer composition (B) has a glass transition temperature in the range of 140 °C to 150 °C, preferably 140 °C to 144 °C, more preferably 140 °C to 143 °C, or in the range of 145 °C to 155 °C.

3. The polypropylene composition according to one of the preceding claims, characterized in that The cyclic olefin polymer composition (B) has a MFR2(260 °C) in the range of 2 g / 10 min to 50 g / 10 min, preferably 8 g / 10 min to 25 g / 10 min, measured according to ISO 1133.

4. The polypropylene composition according to one of the preceding claims, characterized in that The cyclic olefin polymer composition (B) comprises: cyclic olefin units, such as norbornene units, in an amount of 70 wt.-% to 90 wt.-%, preferably 75 wt.-% to 85 wt.-%, more preferably 75 wt.-% to 80 wt.-%, and non-cyclic olefin units, such as ethylene units, in an amount of 10 wt.-% to 30 wt.-%, preferably 15 wt.-% to 25 wt.-%, more preferably 20 wt.-% to 25 wt.-%, wherein the amounts are each based on the total weight of the cyclic olefin polymer composition (B).

5. The polypropylene composition according to one of the preceding claims, characterized in that The polypropylene composition comprises: the high isotactic propylene homopolymer (A) in an amount of 75 wt.-% to 90 wt.-%, preferably 79 wt.-% to 86 wt.-%, more preferably 81 wt.-% to 86 wt.-%, and the cyclic olefin polymer composition (B) in an amount of 10 wt.-% to 25 wt.-%, preferably 14 wt.-% to 21 wt.-%, more preferably 14 wt.-% to 19 wt.-%, wherein the amounts are each based on the total weight of the polypropylene composition.

6. The polypropylene composition according to one of the preceding claims, characterized in that The cyclic olefin polymer composition (B) comprises or consists of at least one cyclic olefin copolymer, preferably at least one cyclic olefin copolymer is selected from the group consisting of norbornene-ethylene copolymers and tetracyclododecene-ethylene copolymers, or the cyclic olefin polymer composition (B) comprises or consists of a blend of at least two cyclic olefin copolymers.

7. The polypropylene composition according to one of the preceding claims, characterized in that The first cyclic olefin polymer (b1 ) has a glass transition temperature measured according to ISO 11357 of 60 °C to less than 140 °C and / or the second cyclic olefin polymer (b2) has a glass transition temperature measured according to ISO 11357 of at least 155 °C, in particular at least 155 °C to 190 °C.

8. The polypropylene composition according to one of the preceding claims, characterized in that The high isotactic propylene homopolymer (A) has at least one of the following: use 13 The content of the isotactic five-unit fraction measured by C NMR is 93% to 99.5%, preferably 95% to 99.0%, and more preferably 96% to 98.5%. an MFR2 (230 °C) measured according to ISO 1133 of 0.4 g / 10 min to 10 g / 10 min, preferably of 0.5 g / 10 min to 5 g / 10 min, an ash content measured according to ISO 3451-1 (1997) of 60 ppm or less, preferably of 30 ppm or less, a crystallization temperature (Tc) measured according to ISO 11357 of 110 °C to 130 °C, preferably of 110 °C to 120 °C.

9. The polypropylene composition according to one of the preceding claims, characterized in that The polypropylene composition is obtained by melt blending the high isotactic propylene homopolymer (A) with the cyclic olefin polymer composition (B), wherein, optionally, the cyclic olefin polymer composition (B) itself is obtained by melt blending at least two different cyclic olefin polymers.

10. A cast film comprising the polypropylene composition according to one of the preceding claims.

11. The cast film according to claim 10, characterized in that The cast film has a dielectric breakdown voltage of 118 kV / mm or more, preferably of 125 kV / mm or more, wherein the dielectric breakdown voltage is measured according to IEC 60243-1 at 90 °C on the cast film having a thickness of 250 pm.

12. The cast film according to one of claims 10 and 11, characterized in that The cast film has a loss tangent of less than 0.001, preferably of less than 0.0005, more preferably of less than 0.0002, wherein the loss tangent is measured according to the method described in the description at 20 Hz and 120 °C.

13. The cast film according to one of claims 10 to 12, characterized in that The cast film has an electrical conductivity of 2 fS / cm or less, when the electrical conductivity is measured with alternating current in a frequency range of 10 -2 Hz to 10 -1 Hz according to IEC 62631-3-2, or when the electrical conductivity is taken as the inverse of the direct current resistance measured according to ASTM D257, both measurements being carried out at a temperature of 90 °C and on the cast film having a thickness of 250 pm.

14. A biaxially oriented film comprising the polypropylene composition according to one of claims 1 to 9.

15. A capacitor comprising the biaxially oriented film according to claim 14.

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