Heat-treated biaxially oriented polypropylene film

Abstract

The present invention relates to a method for producing a heat-treated biaxially oriented polypropylene film and a heat-treated biaxially oriented polypropylene film obtained by said method.The present invention further relates to a capacitor comprising an insulating film comprising a layer of the heat-treated biaxially oriented polypropylene film of the present invention.The present invention further relates to the use of the heat-treated biaxially oriented polypropylene film of the present invention.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a heat-treated biaxially oriented polypropylene film and a heat-treated biaxially oriented polypropylene film obtained by this manufacturing method. The present invention further relates to a capacitor including an insulating film containing a layer of the heat-treated biaxially oriented polypropylene film of the present invention. The present invention further relates to the use of the heat-treated biaxially oriented polypropylene film of the present invention.

Background Art

[0002] Polypropylene is used in many applications. For example, since its chains do not have any kind of polar groups that orient under electric field stress, it is a material selected in the field of film capacitors. Inverters are mounted on various types of electrical devices, and with this trend, the demand for miniaturization and high capacitance of capacitors is increasing. Due to such market demand, biaxially oriented polypropylene films that are thinner and have improved both mechanical and electrical properties are preferably used in the field of capacitor applications.

[0003] Capacitor films need to withstand extreme conditions such as high temperatures and have a high dielectric breakdown field strength. In addition, capacitor films preferably also have good mechanical properties such as high rigidity and a high operating temperature. Many grades of capacitor films, i.e., power applications such as welding, electric vehicles, trains, ovens, wind turbines, solar panels, etc., use highly isotactic polypropylene resins. Apart from a balanced shrinkage rate and an optimized surface roughness, the main advantage associated with high isotacticity is the high heat resistance of the final film, which is related to high crystallinity, a high melting onset temperature, and a high melting peak temperature. There are also capacitor film grades based on highly isotactic polypropylene resins, in which case the lower layer polyolefin composition further contains long-chain branched polypropylene.

[0004] Dielectric breakdown occurs when the electric field applied to an insulator exceeds the dielectric breakdown field strength of the insulator, causing a current spark to penetrate the material and resulting in a short circuit. In general, the dielectric breakdown phenomenon as a physical process is not fully understood, and the correlation between the structure (morphology) of the insulator and its dielectric strength has not been established. The main reason is thought to be related to the fact that the dielectric breakdown phenomenon is the final result of multiple decomposition processes at the nanoscale and microscale, resulting in probabilistic behavior. Numerous factors, such as temperature, humidity, electrode metal, electrode geometry (shape, area), voltage rise rate, direct current (DC) or alternating current (AC) voltage, surface roughness, and sample thickness, affect the final dielectric breakdown field strength. In particular, high operating temperatures lead to dielectric breakdown and capacitor failure.

[0005] WO 2020 / 127862 A1 discloses a biaxially oriented polypropylene (BOPP) film having improved surface properties and a capacitor comprising an insulating film including a layer of the biaxially oriented polypropylene film. WO 2020 / 127862 A1 is silent about the heat treatment of the obtained BOPP film.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] Accordingly, an object of the present invention is to provide a biaxially oriented polypropylene film having improved permittivity and / or improved dielectric breakdown field strength, particularly for use as an insulating layer in a capacitor at a higher operating temperature.

[0008] A further object is to provide a method for obtaining such a biaxially oriented polypropylene film having improved dielectric constant and / or improved dielectric breakdown field strength, in particular a method for use as an insulating layer in a capacitor at a higher operating temperature.

Means for Solving the Problems

[0009] The present invention is based on the finding that the above problems can be solved by heat-treating the biaxially oriented polypropylene film according to the present invention.

Brief Description of the Drawings

[0010] [Figure 1] FIG. 1 shows BDS (broadband dielectric spectroscopy) measurements in Examples IE1 and IE2 and Comparative Example CE of the present invention. [Figure 2] FIG. 2 shows the results of SAXS measurements in IE1.

Modes for Carrying Out the Invention

[0011] Accordingly, the present invention is a method for producing a heat-treated biaxially oriented polypropylene film, the method comprising the following: (A) A step of providing a polypropylene composition, the polypropylene composition comprising (A1) a homopolymer of propylene having an isotactic pentad fraction content of 93 to 98% and a melt flow rate MFR2 of 0.4 to 10 g / 10 min, 90 to 99.99% by weight based on the total weight of the polypropylene composition, and (A2) a polymeric α-nucleating agent, 0.0000001 to 1% by weight based on the total weight of the polypropylene composition, and / or (A3) a β-nucleating agent, 0.0000001 to 0.0002% by weight based on the total weight of the polypropylene composition, comprising, preferably consisting of, the step, (B) A step of extruding the polypropylene composition into a film, (C) Simultaneously or sequentially orient the film to obtain a biaxially oriented polypropylene film (C1) Optionally metallize the biaxially oriented polypropylene film to obtain a metallized biaxially oriented polypropylene film (D) Heat-treat the metallized or non-metallized biaxially oriented polypropylene film as follows (D1) Heat the biaxially oriented polypropylene film to a first annealing temperature T1, where T1 is 80 - 150 °C (D2) Anneal the biaxially oriented polypropylene film at the first annealing temperature T1 for a first annealing time t1 to obtain a heat-treated biaxially oriented polypropylene film, where t1 is 10 - 120 minutes (D3) Cool the heat-treated biaxially oriented polypropylene film, preferably to room temperature (E) Recover the heat-treated biaxially oriented polypropylene film A method is provided that includes the above steps.

[0012] The present invention has many advantages. The heat treatment of the biaxially oriented polypropylene film leads to an improvement in the dielectric constant of the film, especially at operating temperatures above 100 °C. This is associated with permanent structural changes that occur during the treatment, as will be described later. The improvement in the dielectric constant in turn results in a higher capacitance for capacitors that include the heat-treated biaxially oriented polypropylene film as an insulating film.

[0013] A further advantage of the present invention is that, compared to prior art biaxially oriented polypropylene films having an operating temperature of about 115 °C, the structural changes in the heat-treated biaxially oriented polypropylene film according to the present invention occur only at high temperatures such as about 130 °C. Thus, the heat-treated biaxially oriented polypropylene film according to the present invention can advantageously be operated at high temperatures such as about 130 °C without causing dielectric breakdown or other capacitor failures.

[0014] Therefore, the heat-treated biaxially oriented polypropylene film according to the present invention can be used for improving the dielectric constant and / or the dielectric breakdown field strength of a capacitor, particularly an insulating film of a capacitor. The heat-treated biaxially oriented polypropylene film according to the present invention can be used particularly for a capacitor operating at a high temperature of over 100°C, preferably over 115°C.

[0015] The expression "homopolymer of propylene" as used herein relates to polypropylene consisting substantially, i.e., at least 99.5% by weight, more preferably at least 99.8% by weight, of propylene units. In a preferred embodiment, only propylene units are detectable in the homopolymer of propylene. The comonomer content can be measured using 13C NMR spectroscopy as described below. Further, it is understood that the homopolymer of propylene is linear polypropylene. 13 Furthermore, it is understood that the homopolymer of propylene is linear polypropylene.

[0016] In step (A) of the method according to the present invention, a polypropylene composition is provided.

[0017] The polypropylene composition preferably has a melt flow rate MFR2 of 0.5 to 10 g / 10 min, more preferably 1 to 7 g / 10 min.

[0018] The polypropylene composition contains a highly isotactic homopolymer (A1) of propylene, a polymeric α-nucleating agent (A2) and / or a β-nucleating agent (A3). Hereinafter, the homopolymer (A1) of propylene will also be referred to as the highly isotactic homopolymer (A1) of propylene.

[0019] The highly isotactic homopolymer (A1) of propylene is the main component of the polypropylene composition. The polypropylene composition contains 90 to 99.9% by weight, preferably 95 to 99.9% by weight, more preferably 98 to 99.9% by weight, particularly 99 to 99.9% by weight of the highly isotactic homopolymer (A1) of propylene.

[0020] The highly isotactic homopolymer of propylene (A1) has a melt flow rate MFR2 of 0.4 to 10 g / 10 min, preferably 1 to 7 g / 10 min, more preferably 2 to 6 g / 10 min. If the MFR is too low, the processability deteriorates. On the other hand, if the MFR is too high, sagging occurs at high temperatures used in the method for producing a biaxially oriented polypropylene film.

[0021] The highly isotactic homopolymer of propylene (A1) has a content of isotactic pentad fraction (mmmm-fraction) of 93 to 98%, preferably 94 to 98%, for example 95 to 98%. The content of the isotactic pentad fraction is calculated as the ratio of mmmm-pentads from all pentads. If the content of the isotactic pentad fraction is too low, the final crystallinity of the film becomes considerably low, resulting in a decrease in the tensile properties and modulus of elasticity of the film. On the other hand, if the isotactic pentad fraction is too high, the film may frequently break during film orientation in the machine direction and / or the transverse direction.

[0022] The highly isotactic homopolymer of propylene (A1) preferably has an ash content of 30 ppm or less, more preferably 20 ppm or less, particularly 15 ppm or less, for example 10 ppm or less. If the ash content is too high, especially when the ash content contains metal residues, it may have an adverse effect on the dielectric properties of the film. Such a film cannot be used for the production of capacitors. The ash content of the highly isotactic homopolymer of propylene (A1) is usually at least 1 ppm.

[0023] A particularly effective method for producing a propylene homopolymer suitable for the production of the film of the present invention, for example, the highly isotactic homopolymer of propylene (A1), is disclosed in EP-A-2543684, where a catalyst based on a solid component containing titanium trichloride is used in combination with an aluminum alkyl, an organic ether, and an alkyl methacrylate.

[0024] The polymerization is conveniently carried out in a slurry. In such a process, the catalyst, hydrogen and propylene monomer are contacted in a diluent essentially containing one or more alkanes having 4 to 15 carbon atoms, preferably 10 to 14 carbon atoms.

[0025] As used herein, "essentially containing" means that the diluent contains at least 90% by weight, preferably at least 95% by weight, more preferably at least 99% by weight of one or more of such alkanes.

[0026] The polymerization is typically carried out at a temperature of 50 to 100 °C, preferably 60 to 80 °C, and a pressure of 1 to 50 bar, preferably 3 to 15 bar.

[0027] Preferably, the process includes one or more washing steps. Washing is usually carried out by contacting the polymer slurry with a hydrocarbon diluent in one or more steps. After the contacting step, the excess diluent is typically removed, for example, by centrifugation. Preferably, the polymer slurry is contacted with the hydrocarbon diluent in at least two steps. When the washing includes a plurality of steps, in at least one step, it is preferred that an alcohol or an ether is present in addition to the hydrocarbon diluent. This facilitates the removal of the catalyst components from the polymer, thereby enabling a polymer having a very low ash content to be obtained.

[0028] The polypropylene composition provided in step (A) according to the present invention further contains a polymeric α-nucleating agent (A2) and / or a β-nucleating agent (A3).

[0029] The polypropylene composition contains, in one option, a polymeric α-nucleating agent (A2). The polymeric α-nucleating agent (A2) is preferably a polymer of a vinyl compound represented by the formula CH2=CH-CHR1R2, where R1 and R2 together form a 5- or 6-membered saturated, unsaturated or aromatic ring, or independently represent an alkyl group containing 1 to 4 carbon atoms. Preferably, the polymeric α-nucleating agent (A2) is a homopolymer of a vinyl compound represented by the formula CH2=CH-CHR1R2. The polypropylene composition contains 0.0000001 to 1% by weight (or 0.001 ppm to 10,000 ppm) of the polymeric α-nucleating agent, preferably 0.000001 to 0.01% by weight (or 0.01 ppm to 100 ppm), particularly preferably 0.000001 to 0.005% by weight (or 0.01 ppm to 50 ppm) of the polymeric α-nucleating agent (A2). Preferably, the polypropylene composition contains 0.000001 to 0.001% by weight (or 0.01 ppm to 10 ppm), more preferably 0.000001 to 0.0005% by weight (or 0.01 ppm to 5 ppm) of the polymeric α-nucleating agent (A2) based on the total weight of the polypropylene composition.

[0030] One method for incorporating the polymeric α-nucleating agent (A2) into the polypropylene composition involves prepolymerizing the polymerization catalyst by contacting it with a vinyl compound represented by the formula CH2=CH-CHR1R2, where R1 and R2 together form a 5- or 6-membered saturated, unsaturated, preferably non-aromatic unsaturated, or aromatic ring, or independently represent an alkyl group containing 1 to 4 carbon atoms. Propylene is then polymerized in the presence of such a prepolymerized catalyst.

[0031] In prepolymerization, the catalyst is prepolymerized to contain a prepolymer of up to 5 grams, preferably 0.1 to 4 grams of prepolymer per gram of the solid catalyst component. Next, the catalyst is contacted with a vinyl compound represented by the formula CH2=CH-CHR1R2 (wherein R1 and R2 are as defined above) under polymerization conditions.

[0032] Particularly preferably, both R1 and R2 are methyl groups, and the vinyl compound is thus 3-methyl-1-butene. Particularly preferably, R1 and R2 form a saturated 5- or 6-membered ring. Particularly preferably, the vinyl compound is vinylcyclohexane.

[0033] Preferably, the polymeric α-nucleating agent (A2) is selected from the group consisting of polyvinylcyclohexane, poly(3-methyl-1-butene), and mixtures thereof.

[0034] Particularly preferably, the catalyst contains 0.5 to 2 grams of the polymerized vinyl compound, such as poly(vinylcyclohexane), per gram of the solid catalyst component. Particularly preferably, the polymeric α-nucleating agent is selected from the group consisting of polyvinylcyclohexane, poly(3-methyl-1-butene), and mixtures thereof.

[0035] This approach enables the preparation of nucleated polypropylene as disclosed in EP-A-607703, EP-A-1028984, EP-A-1028985, and EP-A-1030878.

[0036] The polymeric α-nucleating agent according to the present invention is not a polymer of propylene.

[0037] Preferably, the prepolymerization is carried out in a slurry in an inert diluent at a temperature in the range of 20 to 80°C, preferably 35 to 65°C. The pressure is not critical and can be selected from atmospheric pressure to 50 bar. The reaction time is selected such that the amount of unreacted vinyl compound is less than a predetermined limit value of the reaction mixture, for example, less than 2000 ppm, or less than 1000 ppm.

[0038] As described above, by an approach in which a polymerization catalyst is prepolymerized and then propylene is polymerized in the presence of such a prepolymerized catalyst, nucleated polypropylene can be prepared.

[0039] Thus, according to one possible method, a polypropylene composition is produced by homopolymerizing propylene in the presence of such a prepolymerized catalyst. The propylene homopolymer is thereby nucleated by the polymeric α-nucleating agent (A2). In this case, the polypropylene composition preferably contains 0.1 to 200 ppm of the polymeric α-nucleating agent (A2), preferably poly(vinylcyclohexane), based on the total weight of the polypropylene composition. In that case, the polymerization method and the catalyst are preferably as described above. Thereby, a highly isotactic homopolymer (A1) of propylene is formed on a catalyst containing the polymeric α-nucleating agent (A2).

[0040] Alternatively, more preferably, the polypropylene composition is produced by homopolymerizing propylene in the presence of a vinyl compound as disclosed above and a polymerization catalyst that has not been prepolymerized, thereby producing a highly isotactic homopolymer (A1) of propylene. In such a case, the highly isotactic homopolymer (A1) of propylene is combined, before or during the extrusion step, with a further polymer, namely, a propylene homopolymer or copolymer (A3) produced by homopolymerizing propylene or copolymerizing propylene and a comonomer in the presence of a catalyst prepolymerized with a vinyl compound as described above.

[0041] The polypropylene composition, in one option, in addition to the homopolymer of propylene (A1), contains, based on the total weight of the polypropylene composition, a maximum of 0.0002% by weight of a β-nucleating agent (A3). The amount of the β-nucleating agent (A3) is usually at least 0.0000001% by weight based on the total weight of the polypropylene composition. Preferably, the amount of the β-nucleating agent (A3) is from 0.0000001% to 0.00015% by weight, more preferably from 0.0000001% to 0.000125% by weight, based on the total weight of the polypropylene composition.

[0042] The term "β-nucleating agent" refers to any nucleating agent suitable for inducing the crystallization of the hexagonal or pseudo-hexagonal modification of the propylene polymer. A mixture of such nucleating agents may be used.

[0043] Suitable types of β-nucleating agents are as follows: C5-C8-cycloalkyl monoamines or C6-C 12 - aromatic monoamines, and dicarboxylic acid derivative type diamide compounds from C5-C8-aliphatic dicarboxylic acids, C5-C8-alicyclic dicarboxylic acids or C6-C 12 - aromatic dicarboxylic acids, for example N,N'-di-C5-C8-cycloalkyl-2,6-naphthalenedicarboxamide compounds, for example N,N'-dicyclohexyl-2,6-naphthalenedicarboxamide and N,N'-dicyclooctyl-2,6-naphthalenedicarboxamide, etc., N,N'-di-C5-C8-cycloalkyl-4,4-biphenyldicarboxamide compounds, for example N,N'-dicyclohexyl-4,4-biphenyldicarboxamide, and N,N'-dicyclopentyl-4,4-biphenyldicarboxamide, etc., N,N'-di-C5-C8-cycloalkyl-terephthalamide compounds, for example N,N'-dicyclohexylterephthalamide, and N,N'-dicyclopentylterephthalamide, etc., N,N'-Di-C5-C8-cycloalkyl-1,4-cyclohexanedicarboxamide compounds, such as N,N'-dicyclohexyl-1,4-cyclohexanedicarboxamide, and N,N'-dicyclohexyl-1,4-cyclopentanedicarboxamide, etc., C5-C8-cycloalkylmonocarboxylic acid or C6-C 12 -aromatic monocarboxylic acid, and diamine derivative type diamide compounds from C5-C8-cycloaliphatic diamine or C6-C 12 -aromatic diamine, such as N,N'-C6-C 12 -arylene-bis-benzamide compounds, such as N,N'-p-phenylene-bis-benzamide and N,N'-1,5-naphthalene-bis-benzamide, etc., N,N'-C5-C8-cycloalkyl-bis-benzamide compounds, such as N,N'-1,4-cyclopentane-bis-benzamide and N,N'-1,4-cyclohexane-bis-benzamide, etc., N,N'-p-C6-C 12 -arylene-bis-C5-C8-cycloalkylcarboxamide compounds, such as N,N'-1,5-naphthalene-bis-cyclohexanecarboxamide and N,N'-1,4-phenylene-bis-cyclohexanecarboxamide, etc., and N,N'-C5-C8-cycloalkyl-bis-cyclohexanecarboxamide compounds, such as N,N'-1,4-cyclopentane-bis-cyclohexanecarboxamide and N,N'-1,4-cyclohexane-bis-cyclohexanecarboxamide, etc., C5-C8-alkylamino acid, C5-C8-cycloalkylamino acid or C6-C 12 -aryl amino acid, C5-C8-alkyl monocarboxylic acid chloride, C5-C8-cycloalkyl monocarboxylic acid chloride or C6-C 12- Aromatic monocarboxylic acid chloride and C5-C8-alkyl monoamine, C5-C8-cycloalkyl monoamine or C6-C 12 - Amino acid derivative type diamide compounds from the amidation reaction with aromatic monoamine, for example N-phenyl-5-(N-benzoylamino)pentanamide, and N-cyclohexyl-4-(N-cyclohexyl-carbonylamino)benzamide and the like.

[0044] Furthermore, suitable β-nucleating agents are quinacridone type compounds such as 5,12-dihydro-quino[2,3-b]acridine-7,14-dione (i.e., quinacridone), dimethylquinacridone and dimethoxyquinacridone, etc., quinacridone quinone type compounds such as quino[2,3-b]acridine-6,7,13,14(5H,12H)-tetrone (i.e., quinacridone quinone) and dimethoxyquinacridone quinone, etc., and dihydroquinacridone type compounds such as 5,6,12,13-tetrahydroquino[2,3-b]acridine-7,14-dione (i.e., dihydroquinacridone), dimethoxydihydroquinacridone and dibenzodihydroquinacridone.

[0045] Furthermore, suitable β-nucleating agents are dicarboxylates of metals from Group 11a of the periodic table, such as calcium pimelate and calcium suberate; and mixtures of dicarboxylic acids and salts of metals from Group 11a of the periodic table.

[0046] Furthermore, suitable β-nucleating agents are salts of metals from Group 11a of the periodic table and a compound of the formula

Chemical formula

[0047] Preferred β-nucleating agents are any one or a mixture of N,N'-dicyclohexyl-2,6-naphthalenedicarboxamide, the β-nucleating agent of EP 177961 and the β-nucleating agent of EP 682066.

[0048] Particularly preferred β-nucleating agents are 5,12-dihydro-quino[2,3-b]acridine-7,14-dione (CAS 1047-16-1) (i.e., quinacridone), quino[2,3-b]acridine-6,7,13,14(5H,12H)-tetrone (CAS 1503-48-6) (i.e., quinacridonequinone), 5,6,12,13-tetrahydroquino[2,3-b]acridine-7,14-dione (CAS 5862-38-4) (i.e., dihydroquinacridone), N,N-dicyclohexyl-2,6-naphthalenedicarboxamide (CAS 153250-52-3), and any one or a mixture of salts of dicarboxylic acids having at least 7 carbon atoms and metals of Group 11a of the periodic table, preferably calcium pimelate (CAS 19455-79-9).

[0049] Preferably, the β-nucleating agent is selected from the group consisting of 5,12-dihydro-quino[2,3-b]acridine-7,14-dione, quino[2,3-b]acridine-6,7,13,14(5H,12H)-tetrone, 5,6,12,13-tetrahydroquino[2,3-b]acridine-7,14-dione, and mixtures thereof.

[0050] Even more preferred β-nucleating agents are 5,12-dihydro-quino[2,3-b]acridine-7,14-dione (CAS 1047-16-1), quino[2,3-b]acridine-6,7,13,14(5H,12H)-tetrone (CAS 1503-48-6) and 5,6,12,13-tetrahydroquino[2,3-b]acridine-7,14-dione (CAS 5862-38-4), which are commercially available from BASF as Cinquasia Gold YT-923-D.

[0051] This β-nucleating agent is characterized by very high activity. Cinquasia Gold YT-923-D has very high activity and is very inexpensive, so it is preferred as a β-nucleating agent.

[0052] In step (B) according to the method of the present invention, the polypropylene composition provided in step (A) is extrusion molded into a film, preferably a flat film. In principle, any extruder suitable for extruding a film can be used in step (B). The die of the extruder is preferably a slot die or a flat die for extruding a film.

[0053] In step (C) according to the method of the present invention, the film extruded in step (B) is oriented simultaneously or sequentially, preferably in a continuous manner, to obtain a biaxially oriented polypropylene film. The orientation is carried out by stretching or drawing the film.

[0054] Preferably, in step (C), the film is oriented sequentially, and the sequential orientation is carried out first in the machine direction (MD) and then in the transverse direction (TD), or first in the transverse direction (TD) and then in the machine direction (MD), and more preferably, first in the machine direction (MD) and then in the transverse direction (TD).

[0055] Preferably, the draw ratio in the machine direction (MD) is at least 8.0 to 20.0, more preferably 9.0 to 15.0, and / or the draw ratio in the transverse direction (TD) is at least 8.0 to 20.0, more preferably 9.0 to 15.0.

[0056] Preferably, the film obtained in step (C) has a thickness of 0.5 to 10 μm, preferably 1 to 6 μm, and most preferably 2 to 5 μm.

[0057] Preferably, the orientation in step (C) is carried out in a continuous manner, preferably at a throughput of 20 kg / h to 900 kg / h, more preferably >25 kg / h to 500 kg / h. The continuous method may be any method that produces a product without interruption when properly executed and / or does not produce the product in batches.

[0058] Preferably, steps (B) and (C) are carried out as a tenter method.

[0059] An exemplary tenter method is as follows. The film is obtained by extruding a polypropylene composition through a flat die, the extrudate is recovered and cooled by a rotating cooling roll, and the film solidifies.

[0060] The cooling roll continuously conveys the non-oriented film to a tenter frame installed in an oven. The tenter frame is realized by two rails driven by a linear motor system, along which clips move in the machine direction. The two rails from the inlet to the outlet of the oven are arranged parallel to each other, diverging, and slightly converging to form a preheating, stretching, and relaxation zone. The biaxial stretching of the non-oriented cast film is achieved by feeding the non-oriented cast film into the preheating zone of the tenter, where at the inlet, the clamp grips both sides of the non-oriented cast film. The moving direction of the clamp is the extrusion direction, i.e., the machine direction (MD), and the distance between clips in the MD direction is constant in the preheating zone. When the film is biaxially drawn or stretched, the distance between the rails in the stretching zone increases compared to the preheating zone to achieve transverse direction (TD) stretching of the film, while the distance between clips in the MD direction increases for MD stretching of the film. The stretching can be either simultaneous or sequential as described above.

[0061] For example, the molten polypropylene composition is first extruded through a die onto the cooling roll. The surface temperature of the cooling roll is maintained at 10 to 100 °C, preferably 20 to 98 °C. The thickness of the film is 50 to 1000 μm, preferably 100 to 500 μm. Next, the film is sent to the tenter frame and stretched as described above. The temperature of the tenter oven is set at 160 to 175 °C. Before the biaxially oriented film is wound onto a mandrel, the biaxially oriented film is trimmed on both sides, and the non-oriented edges where the clips had fixed the film are removed.

[0062] Preferably, the biaxially oriented polypropylene film comprises at least one layer containing the polypropylene composition described in all of the above embodiments. Preferably, the layer containing the polypropylene composition contains 90 to 100% by weight of the polypropylene composition based on the total weight of the film layer. More preferably, the layer contains 95 to 100% by weight of the polypropylene composition, even more preferably 98 to 100% by weight, for example 99 to 100% by weight of the polypropylene composition. Particularly preferably, the layer consists of the polypropylene composition.

[0063] Preferably, the biaxially oriented polypropylene film may include additional layers such as polymer layers. The final additional polymer layer may be produced by any means known in the art. Thus, they may be coextruded with the film layer according to the invention, preferably coextruded. Alternatively, they may be laminated to form a film structure.

[0064] Optionally, the method according to the invention further comprises a step (C1) of metallizing the biaxially oriented polypropylene film to obtain a biaxially oriented polypropylene film further comprising a metal layer. In particular, when the biaxially oriented polypropylene film is used in the manufacture of capacitors, a metal layer is present.

[0065] Metallization, i.e., depositing a metal layer on the biaxially oriented polypropylene film, can be carried out by any method known in the art such as electrodeless vacuum evaporation, ion beam vacuum evaporation, sputtering or ion plating. The thickness of such a metal layer is typically from 100 Å (0.01 pm) to 5000 Å (0.5 pm).

[0066] Step (C1) is preferably carried out following step (C).

[0067] If any step (C1) is not carried out, the biaxially oriented polypropylene film obtained in step (C) is not metallized and can thus also be referred to as a non-metallized biaxially oriented polypropylene film.

[0068] The method according to the present invention further comprises a step (D) of heat-treating a biaxially oriented polypropylene film as follows: (D1) heating the biaxially oriented polypropylene film to an annealing temperature T ann where T ann is from 80°C to 150°C, (D2) annealing the biaxially oriented polypropylene film at the annealing temperature T ann for an annealing time t ann to obtain a heat-treated biaxially oriented polypropylene film, where t ann is from 10 to 140 minutes, (D3) cooling the heat-treated biaxially oriented polypropylene film, preferably to room temperature, Preferably, the annealing temperature T ann is from 90°C to 140°C, more preferably from 100°C to 120°C. Preferably, t ann is from 30 to 130 minutes, more preferably from 50 to 120 minutes.

[0069] Preferably, the heating to the annealing temperature T ann in step (D1) is carried out at a heating rate of 1°C / min to 10°C / min, more preferably 3°C / min to 7°C / min, and most preferably 5°C / min. Usually, the heating of the biaxially oriented polypropylene film in step (D1) is carried out from room temperature to the annealing temperature T ann .

[0070] Preferably, the cooling in step (D3) is carried out at a cooling rate of 1°C / min to 10°C / min, more preferably 3°C / min to 7°C / min, and most preferably 5°C / min.

[0071] Preferably, the heating step (D) is carried out in an inert atmosphere such as an N2 atmosphere.

[0072] The heating step (D) can be carried out in any suitable apparatus for heating, annealing, and cooling the biaxially oriented polypropylene film, preferably in an inert atmosphere as described herein, such as an oven.

[0073] In step (E) of the method according to the invention, a heat-treated biaxially oriented polypropylene film is recovered.

[0074] Preferably, the heat-treated biaxially oriented polypropylene film or the biaxially oriented polypropylene film further comprises a metal layer.

[0075] When the biaxially oriented polypropylene film further comprises a metal layer, the metal layer is preferably formed by any of the metallization steps (C1) described herein.

[0076] When the heat-treated biaxially oriented polypropylene film further comprises a metal layer, the metal layer is preferably formed by metallizing the heat-treated biaxially oriented polypropylene film. The metallization is preferably carried out as described in step (C1) herein.

[0077] Preferably, the polypropylene composition further comprises, based on the total weight of the polypropylene composition, up to 9.99% by weight of a propylene homopolymer or copolymer (A4) other than the homopolymer (A1) of propylene, i.e., other than the highly isotactic homopolymer (A1) of propylene.

[0078] The amount of the propylene homopolymer or copolymer (A4) is usually at least 0.01% by weight based on the total weight of the polypropylene composition.

[0079] The propylene homopolymer or copolymer (A4) typically serves as a carrier polymer for the polymeric α-nucleating agent (A2). The propylene homopolymer or copolymer (A4) preferably contains, based on the weight of the propylene homopolymer or copolymer (A4), 0.5 to 200 ppm, preferably 0.5 to 100 ppm, more preferably 1 to 200 ppm, for example 1 to 100 ppm of the polymeric α-nucleating agent (A2), preferably poly(vinylcyclohexane).

[0080] The amount of the homopolymer or copolymer (A4) of propylene is preferably 0.1 to 9.99% by weight, preferably 0.1 to 5% by weight, more preferably 0.2 to 4.99% by weight, even more preferably 0.2 to 1.99% by weight, particularly 0.2 to 0.99% by weight, based on the polypropylene composition.

[0081] The homopolymer or copolymer (A4) of propylene can be any homopolymer or copolymer of propylene. Preferably, the homopolymer or copolymer (A4) is relatively similar to the highly isotactic homopolymer (A1) of propylene. Therefore, the homopolymer or copolymer (A4) is preferably a homopolymer of propylene. Further, when the properties of the homopolymer or copolymer (A4) are substantially different from those of the highly isotactic homopolymer (A1) of propylene, the amount of the homopolymer or copolymer (A4) is preferably not more than 2% by weight based on the total weight of the polypropylene composition.

[0082] The propylene homopolymer or copolymer (A4) preferably has a branching index g of at least 0.9. Thereby, the propylene homopolymer or copolymer (A4) preferably substantially does not contain long-chain branches. In particular, the propylene homopolymer or copolymer (A4) does not contain a detectable amount of long-chain branches.

[0083] The propylene homopolymer or copolymer (A4) can be produced according to methods known in the art. As described above, according to one method, the propylene homopolymer or copolymer (A4) is produced by homopolymerizing propylene in the presence of a catalyst prepolymerized with a vinyl compound represented by the formula CH2=CH-CHR1R2 [wherein R1 and R2 are as defined above]. It is particularly preferred that the vinyl compound is vinylcyclohexane. Thereby, the propylene homopolymer or copolymer (A4) is nucleated by the polymeric α-nucleating agent (A2). The propylene homopolymer or copolymer (A4) preferably contains 0.1 to 200 ppm of the polymeric α-nucleating agent (A2), preferably poly(vinylcyclohexane), based on the total weight of the propylene homopolymer or copolymer (A4).

[0084] According to one suitable embodiment of the present invention, the polymerization method and catalyst for producing the propylene homopolymer or copolymer (A4) are the same as those described above for the highly isotactic homopolymer (A1) of propylene. Thereby, the propylene homopolymer or copolymer (A4) is formed on a catalyst containing the polymeric α-nucleating agent (A2).

[0085] According to a particularly preferred embodiment of the present invention, the propylene homopolymer or copolymer (A4) is produced by homopolymerizing propylene, or copolymerizing propylene with one or more comonomers selected from the group consisting of ethylene and α-olefins having 4 to 8 carbon atoms, in the presence of a Ziegler-Natta catalyst containing a solid component containing magnesium, titanium, and chlorine, and in the presence of an internal donor such as a phthalate, maleate, or citraconate, and in the presence of an aluminum alkyl such as triethylaluminum, and in the presence of an external donor such as a silicon ether, for example dicyclopentyldimethoxysilane, and this catalyst is prepolymerized with a small amount of a vinyl compound represented by the formula CH2=CH-CHR1R2 such as vinylcyclohexane.

[0086] Next, propylene is homopolymerized or copolymerized in one or more polymerization steps in the presence of a prepolymerization catalyst. The homopolymerization or copolymerization of propylene can be carried out by any suitable polymerization method known in the art, such as slurry polymerization, gas-phase polymerization, or a combination thereof.

[0087] Suitable methods for producing a propylene homopolymer or copolymer (A4) containing a polymeric α-nucleating agent (A2) are disclosed, inter alia, in WO-A-99 / 24479, WO-A-00 / 68315, EP-A-1801157, EP-A-1801155, and EP-A-1818365.

[0088] Typically, the propylene homopolymer or copolymer (A4) contains 0.1 to 200 ppm of the polymeric α-nucleating agent (A2), preferably 1 to 100 ppm, more preferably 5 to 50 ppm of the polymeric α-nucleating agent (A2). Also, in this embodiment, it is preferred that the propylene homopolymer or copolymer (A4) is a propylene homopolymer, and the propylene homopolymer preferably has a relatively high isotactic material content, as indicated by a high proportion of cold xylene-insoluble material, such as at least 96% by weight, or at least 97% by weight, or at least 98% by weight.

[0089] The polypropylene composition preferably further contains 0.1 to 9.99% by weight of a branched propylene polymer (A5) having a branching index g' of 0.9 or less, based on the total weight of the polypropylene composition. The branching index of the branched propylene polymer is usually at least 0.1.

[0090] The term "branched propylene polymer" used in the present invention refers to a branched polypropylene that is different from linear polypropylene in that the polypropylene main chain has side chains, while unbranched polypropylene, i.e., linear polypropylene, has no side chains. The side chains have a great influence on the rheology of polypropylene. Therefore, linear polypropylene and branched polypropylene can be clearly distinguished by their flow behavior under stress.

[0091] The branching index g’ defines the degree of branching and correlates with the amount of branching of the polymer. The branching index g’ is g’ = [IV]br / [IV]lin [where g’ is the branching index of the branched propylene polymer, [IV]br is the intrinsic viscosity of the branched propylene polymer, and [IV]lin is the intrinsic viscosity of a linear polypropylene having the same weight-average molecular weight (within the range of ±10%) as the branched propylene polymer]. Thus, a low g’ value is an indicator of a highly branched polymer. In other words, as the g’ value decreases, the branching of polypropylene increases. See B.H. Zimm and W.H. Stockmeyer, J. Chem. Phys. 17, 1301 (1949) in this regard. The intrinsic viscosity required to determine the branching index g’ is measured in accordance with DIN ISO 1628 / 1, October 1999 (at 135 °C in decalin).

[0092] Branching can be achieved by using a specific catalyst, i.e., a specific single-site catalyst, or by chemical modification. For the production of branched propylene polymers obtained by using a specific catalyst, reference is made to EP 1 892 264. For branched propylene polymers obtained by chemical modification, EP 0 879 830 A1 is referred to. In such cases, the branched propylene polymer is also called high melt strength polypropylene. The branched propylene polymer according to the present invention is obtained by chemical modification as will be described in more detail below, and is thus high melt strength polypropylene (HMS-PP). Therefore, in the present invention, the terms "branched propylene polymer" and "high melt strength polypropylene (HMS-PP)" can be regarded as synonyms. The branched propylene polymer prepared in this way is also known as a long chain branched propylene polymer. Therefore, the branched propylene polymer of the present invention, i.e., high melt strength polypropylene (HMS-PP), has an F30 melt strength of more than 15.0 cN and a v30 melt extensibility of more than 200 mm / sec in order to impart a good strain hardening effect to the resulting polypropylene composition, preferably in the range of 15.0 to 50.0 cN, more preferably in the range of 20.0 to 45.0 cN, for example 25.0 to 40.0 cN, and a v30 melt extensibility in the range of 200 to 300 mm / sec, preferably 215 to 285 mm / sec, more preferably 235 to 275 mm / sec. The F30 melt strength and the v30 melt extensibility are measured in accordance with ISO 16790:2005.

[0093] The branched propylene polymer can be produced by any number of methods, for example, by treating an unmodified propylene polymer with a thermally decomposable radical former and / or by treatment with ionizing radiation, both of which treatments may optionally be accompanied by, or followed by, treatment with a bifunctional or polyfunctional ethylenically unsaturated monomer, such as butadiene, isoprene, dimethylbutadiene, divinylbenzene or trivinylbenzene.

[0094] As used herein, "bifunctional ethylenically unsaturated" means the presence of two non-aromatic double bonds, such as divinylbenzene or cyclopentadiene. Only such bifunctional ethylenically unsaturated compounds that can be polymerized with the aid of free radicals are used. The bifunctional unsaturated monomer is actually in a chemically bonded state rather than "unsaturated" because the two double bonds are each used for covalent bonding to the polymer chain of linear polypropylene.

[0095] Examples of branched propylene polymers are, in particular: Polypropylene modified by reaction with a bismaleimide compound in the molten state (EP-A-0 574 801 and EP-A-0 574 804), Polypropylene modified by treatment with ionizing radiation (EP 0 190 889 A2) Polypropylene modified by treatment with a peroxide in the solid phase (EP 0 384 431 A2) or polypropylene modified by treatment with a peroxide in the molten state (EP 0 142 724 A2), Polypropylene modified by treatment with a bifunctional ethylenically unsaturated monomer under the action of ionizing radiation (EP A 0 678 527), Polypropylene modified by treatment with a bifunctional ethylenically unsaturated monomer in the presence of a peroxide in the molten state (EP A 0 688 817 and EP A 0 450 342).

[0096] From the above list, branched propylene polymers obtained by treatment with a peroxide, particularly after treatment with a bifunctional ethylenically unsaturated monomer, are preferred.

[0097] Preferred branched propylene-based polymers are obtained by mixing linear polypropylene with 0.01 to 3% by weight of an organic peroxide thermally decomposable under the melting conditions of polypropylene and heating the mixture to melt.

[0098] Even more preferred branched propylene polymers are obtained by mixing linear polypropylene with 0.01 to 3% by weight of an organic peroxide thermally decomposable under the melting conditions of polypropylene and 0.2 to 3% by weight of a bifunctional ethylenically unsaturated monomer, and heating and melting the mixture.

[0099] The bifunctional ethylenically unsaturated monomer may be added at any timing before or during the heating and melting of the linear polypropylene / peroxide mixture. The bifunctional monomer may be added to the linear polypropylene before mixing with the peroxide.

[0100] According to a preferred embodiment, the bifunctional monomer is in a gaseous or liquid state absorbed by the polypropylene (which is still solid).

[0101] According to a preferred method, the branched propylene polymer is prepared by mixing particulate linear propylene polymer with 0.05 to 3% by weight of an organic peroxide (acyl peroxide, alkyl peroxide, perester and / or peroxicarbonate) thermally decomposable under the melting conditions of polypropylene based on the linear propylene polymer. The peroxide may optionally be solvated in an inert solvent. The mixing is carried out at a temperature of 30 to 100 °C, preferably 60 to 90 °C.

[0102] After mixing with the peroxide, the polypropylene / peroxide mixture is contacted with the bifunctional ethylenically unsaturated monomer. The bifunctional monomer may be in a gaseous or liquid state and may be applied in a pure or diluted state, for example, diluted with an inert gas or solvated in an organic solvent. The bifunctional monomer enables absorption by the particulate polypropylene at a temperature of 20 to 120 °C, preferably 60 to 100 °C. The actual sorption time is 10 to 1000 seconds, preferably 60 to 600 seconds. Thereby, typically, absorption amounts of 0.01 to 10% by weight and 0.05 to 2% by weight of the bifunctional monomer are obtained based on the linear propylene polymer, respectively.

[0103] Next, the polypropylene / peroxide / monomer mixture is heated from the adsorption temperature to 210 °C and melted in an atmosphere containing an inert gas, such as N2, and / or a bifunctional monomer. Thereby, the peroxide decomposes, free radicals are generated in the propylene polymer chain, and these react with the bifunctional monomer.

[0104] To remove unreacted monomers and decomposition products, this melt is heated to 280 °C and finally the melt is pelletized.

[0105] Linear polypropylene includes propylene homopolymers, copolymers of propylene with ethylene and / or α-olefins having 4 to 18 carbon atoms, and mixtures of such homopolymers and copolymers. According to a preferred embodiment of the present invention, the branched propylene polymer is prepared based on a propylene homopolymer having a low ash content, preferably an ash content of <60 ppm.

[0106] According to a more preferred embodiment of the present invention, the branched propylene polymer (A5) is prepared based on a highly isotactic homopolymer of propylene that is linear, such as the highly isotactic homopolymer of propylene (A1).

[0107] The particulate linear propylene polymer may have the shape of a powder, granule or grit.

[0108] The above method is preferably a continuous method carried out in a continuous reactor, mixer, kneader and extruder. However, batch production of the modified propylene polymer is equally possible.

[0109] Preferably, the bifunctional monomer is absorbed from the gas phase into the linear propylene polymer.

[0110] The difunctional ethylenically unsaturated monomer is preferably a C4-C10 diene and / or a C7-C10 divinyl compound. Particularly preferred are butadiene, isoprene, dimethylbutadiene or divinylbenzene.

[0111] The following peroxides are suitable for the above method: Acyl peroxides, such as dibenzoyl peroxide, benzoyl peroxide, 4-chlorobenzoyl peroxide, 3-methoxybenzoyl peroxide and methylbenzoyl peroxide, Alkyl peroxides, such as allyl tert-butyl peroxide, 2,2-di(tert-butylperoxy)butane, di-tert-amyl peroxide, 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, n-butyl-4,4-bis(tert-butylperoxy)valerate, diisopropylaminomethyl tert-amyl peroxide, dimethylaminomethyl tert-amyl peroxide, diethylaminomethyl tert-butyl peroxide, dimethylaminomethyl tert-butyl peroxide, 1,1-di(tert-amylperoxy)cyclohexane, tert-amyl peroxide, tert-butyl cumyl peroxide, methyl isobutyl ketone peroxide, 2,2-di(4,4-di(tert-butylperoxy)cyclohexyl)propane, tert-butyl peroxide and 1-hydroxybutyl n-butyl peroxide; Peresters and peroxyesters, for example, tert-butyl peroxyacetate, tert-butyl peroxy diethylacetate, cumyl peracetate, cumyl perpropionate, cyclohexyl peracetate, di-tert-butyl diperadipate, di-tert-butyl diperazelate, di-tert-butyl diperglutarate, di-tert-butyl diperphthalate, di-tert-butyl dipersebacate, 4-nitrocumyl perpropionate, 1-phenylethyl perbenzoate, phenylethyl nitrophthalate, bicyclo[2.2.1] tert-Butyl heptanecarboxylate, tert-butyl 4-carbomethoxybutyrate, tert-amyl peroxy-2-ethylhexyl carbonate, tert-amyl peroxy-2-ethylhexanoate, tert-butyl cyclobutanecarboxylate, 1,4-di(tert-butylperoxycarbonyl)cyclohexane, 1,1-di(tert-butylperoxy)-3,3,5-trimethyl-cyclohexane, 1,1-di(tert-butylperoxy)cyclohexane, tert-butyl cyclohexylperoxycarboxylate, tert-butyl cyclopentanecarboxylate, tert-butyl cyclopropanecarboxylate, tert-butyl peroxy-dimethylcinnamate, tert-butyl 2-(2,2-diphenylvinyl)perbenzoate, tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxy-3,5,5-trimethylhexanoate, tert-butyl 4-methoxyperbenzoate, tert-butyl peroxybenzoate, tert-butyl pernaphthoate, tert-butyl peroxyisopropyl carbonate, tert-butyl peroxy-2-ethylhexyl carbonate, tert-butyl peroxy-toluylate, tert-butyl 1-phenylcyclopropylpercarboxylate, tert-butyl 2-propylpent-2-enoate, tert-butyl 1-methylcyclopropylpercarboxylate, tert-butyl 4-nitrophenyl peracetate, tert-butyl nitrophenyl peroxycarbamate, tert-butyl A-succinimidopercarboxylate, tert-butyl percrotonate, tert-butyl permaleate, tert-butyl permethacrylate, tert-butyl peroctanoate, tert-butyl peroxyisobutyrate, tert-butyl peracrylate, tert-butyl perpropionate; and Mixtures of these peroxides.

[0112] Particularly preferred are dibenzoyl peroxide, tert-butyl peroxy-2-ethylhexanoate, tert-amyl peroxy-2-ethylhexanoate, tert-butyl peroxydiethylacetate, 1,4-di(tert-butylperoxycarbonyl)cyclohexane, tert-butyl peroxyisobutyrate, 1,1-di(tert-butylperoxy)-3,3,5-trimethyl-cyclohexane, methyl isobutyl ketone peroxide, 2,2-di(4,4-di(tert-butylperoxy)cyclohexyl)propane, 1,1-di(tert-butylperoxy)cyclohexane, tert-butyl peroxy-3,5,5-trimethylhexanoate, tert-amyl peroxy 2-ethylhexyl carbonate, 2,2-di(tert-butylperoxy)butane, tert-butyl peroxyisopropyl carbonate, tert-butyl peroxy 2-ethylhexyl carbonate, tert-butyl peracetate, tert-butyl peroxybenzoate, di-tert-amyl peroxide and mixtures thereof.

[0113] These peroxides are preferred because their mechanism of action has been found to be a compromise between the undesirable degradation of the polymer chain length and the desirable long-chain branching in this case.

[0114] Preferably, the polypropylene composition further comprises 0.01 to 1% by weight, preferably 0.05 to 0.8% by weight, of additive (A6) based on the total weight of the polypropylene composition.

[0115] The additive (A6) used according to the present invention is preferably selected from the group consisting of antioxidants, stabilizers, acid scavengers, and mixtures thereof. In this application, the nucleating agents disclosed herein are not considered additives.

[0116] The highly isotactic homopolymer (A1) of propylene according to the present invention has an isotacticity of 93 to 98%. In particular, when a homopolymer of propylene having an isotacticity of 95 to 98% is used in the production of a film, it has been found that the polymer is more easily decomposed than a conventional homopolymer of propylene having an isotacticity of 90 to 94%. Therefore, for the highly isotactic homopolymer (A1) of propylene, more effective stabilization is preferred.

[0117] The antioxidants and stabilizers used in the present invention are preferably selected from the group of hindered phenols, and more preferably selected from the group of hindered phenols that do not contain phosphorus or sulfur.

[0118] The antioxidants and stabilizers used in the present invention are particularly preferably 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), pentaerythrityl-tetrakis(3-(3<,>,5’-di-tert-butyl-4-hydroxyphenyl)-propionate (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), butylhydroxytoluene (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 2,5,7,8-tetramethyl-2(4<,>,8<,>,12’-trimethyltridecyl)chroman-6-ol (sold under the trade names Irganox E 210 and alpha-tocopherol), and are one or more compounds selected from the group consisting of these.

[0119] The antioxidants and stabilizers are preferably present in a total amount of 500 to 8000 ppm, based on the total weight of the polypropylene composition. More preferably, the antioxidants and stabilizers are present in a total amount of 800 to 7000 ppm, even more preferably 1000 to 6000 ppm, particularly 1500 to 6000 ppm, based on the total weight of the polypropylene composition. In particular, the antioxidants and stabilizers preferably do not contain a phosphorus-containing secondary antioxidant such as tris(2,4-di-tert-butylphenyl)phosphite, because such compounds increase the losses in the final capacitor.

[0120] The acid scavenger is usually a salt of an organic acid such as stearate. These have the function of neutralizing the acid in the polymer. Examples of such compounds are calcium stearate, zinc stearate and zinc oxide. The acid scavenger is typically used in an amount of 50 ppm to 2000 ppm, more preferably 50 ppm to 1000 ppm.

[0121] The present invention further provides a heat-treated biaxially oriented polypropylene film obtained by the method according to the present invention.

[0122] All preferred embodiments of the method according to the present invention are also, where applicable, preferred embodiments of the heat-treated biaxially oriented polypropylene film obtained by the method according to the present invention.

[0123] Preferably, the heat-treated biaxially oriented polypropylene film according to the present invention comprises at least one layer comprising a polypropylene composition as described in all of the above embodiments. Preferably, the layer comprising the polypropylene composition comprises 90 to 100% by weight of the polypropylene composition, based on the total weight of the film layer. More preferably, the layer comprises 95 to 100% by weight of the polypropylene composition, even more preferably 98 to 100% by weight, for example 99 to 100% by weight of the polypropylene composition. Particularly preferably, the layer consists of the polypropylene composition.

[0124] The present invention further provides a capacitor comprising an insulating film comprising a layer of the heat-treated biaxially oriented polypropylene film according to the present invention.

[0125] The present invention further provides the use of the heat-treated biaxially stretched film according to the present invention for increasing the dielectric constant of the insulating film of the capacitor and / or improving the dielectric breakdown field strength.

[0126] Finally, the present invention further provides the use of the heat-treated biaxially oriented polypropylene film according to the present invention as a layer of the insulating film of the capacitor.

[0127] All preferred embodiments of the method according to the present invention described above are also preferred embodiments of the use of the capacitor according to the present invention and the heat-treated biaxially stretched film according to the present invention.

[0128] Hereinafter, the present invention will be further described by way of examples.

[0129] <1. Definitions / Measurement Methods> The following definitions of terms and measurement methods apply to the above general description of the present invention and the following examples, unless otherwise defined.

[0130] [a) Melt Flow Rate] The melt flow rate MFR2 was measured according to ISO 1133 under a load of 230 °C and 2.16 kg.

[0131] [b) Quantification of Microstructure by NMR Spectroscopy] Quantitative nuclear magnetic resonance (NMR) spectroscopy was used to quantify the isotacticity and regio-regularity of propylene homopolymers.

[0132] Quantitative 13 C{ 1 H} NMR spectra were recorded in solution using a Bruker Advance III 400 NMR spectrometer operating at 400.15 and 100.62 MHz for 1 H and 13 C, respectively. All spectra were recorded using a 10 mm extended temperature probe head optimized for 13 C at 125 °C and nitrogen gas was used for all air pressures.

[0133] Approximately 200 mg of the material (propylene homopolymer) was dissolved in 1,2 - tetrachloroethane - d2 (TCE - d2). To ensure a homogeneous solution, after the first sample preparation in a heat block, the NMR tube was further heated in a rotary oven for at least 1 hour. After insertion into the magnet, the tube was rotated at 10 Hz. This setting was chosen mainly for the high resolution necessary for the quantification of the tacticity distribution (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). Standard single - pulse excitation was employed using the NOE and 2 - level WALTZ16 decoupling scheme (Zhou, Z., Kuemmerle, R., Qiu, X., Redwine, D., Cong, R., Taha, A., Baugh, D. Winniford, 15B., 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 were acquired per spectrum.

[0134] Quantitative 13 C{ 1 H} NMR spectra were processed, integrated, and the relevant quantitative characteristics were determined from the integral values using a proprietary computer program.

[0135] For the propylene homopolymer, all chemical shifts were referenced to the methyl isotactic pentad (mmmm) at 21.85 ppm as the internal standard.

[0136] Regio defects (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 characteristic signals corresponding to the comonomer were observed.

[0137] The methyl region between 23.6 - 19.7 ppm was integrated and any sites not related to the desired stereosequence were corrected to quantify the tacticity distribution (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).

[0138] Pentad isotacticity means the fraction of isotactic pentads (mmmm).

[0139] [c) Ash content] The ash content of the polymer was measured by burning the polymer in a weighed platinum crucible. Approximately 100 grams of the polymer are weighed into the crucible. The crucible is then heated in the flame of a Bunsen burner to slowly burn the polymer. 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 measured values exceeds 7 ppm, a third measurement is made.

[0140] [d) Melt strength and melt extensibility] The tests described herein comply with ISO 16790:2005. The strain hardening behavior is measured by the method described in the paper "Rheotens-Mastercurves and Drawability of Polymer Melts", M.H. Wagner, Polymer Engineering and Sience, Vol. 36, pp. 925-935. The strain hardening behavior of the polymer is analyzed by a Rheotens device (manufactured by Gottfert, Siemensstr. 2, 74711 Buchen, Germany), in which the molten strand is elongated by pulling it down with a defined acceleration.

[0141] The Rheotens experiment simulates industrial spinning and extrusion processes. In principle, the melt is pressed or extruded through a round die and the resulting strand is drawn off. The stress applied to the extrudate is recorded as a function of the melt properties and the measurement parameters (in particular, the ratio of the output and the draw-off speed, which is actually a measure of the elongation rate). The material was extruded using an experimental extruder FLAAKE Polylab system and a gear pump with a cylindrical die (L / D = 6.0 / 2.0 mm). To measure the F30 melt strength and the v30 melt extensibility, the pressure at the extruder outlet (= gear pump inlet) was set to 30 bar by bypassing a part of the extruded polymer. To measure the F200 melt strength and the v200 melt extensibility, the pressure at the extruder outlet (= gear pump inlet) was set to 200 bar by bypassing a part of the extruded polymer. The gear pump was pre-adjusted to a strand extrusion speed of 5 mm / s and the melt temperature was set to 200 °C. The spinline length between the die and the Rheotens wheel was 80 mm. At the start of the experiment, the winding speed of the Rheotens wheel was adjusted to the speed of the extruded polymer strand (zero tension): then the experiment was started by slowly increasing the winding speed of the Rheotens wheel until the polymer filament broke. Since the acceleration of the wheel was sufficiently small, the tensile force was measured in a quasi-steady state. The acceleration of the pulled-down molten strand (2) was 120 mm / s 2It is. Rheotens was operated in combination with the PC program EXTENS. This is a real-time data acquisition program that displays and stores measurement data of tensile force and draw-down speed. The end points of the Rheotens curve (force vs. pulley rotation speed) at which the polymer strand breaks are obtained as the values of F30 melt strength and v30 melt extensibility, or the values of F200 melt strength and v200 melt extensibility, respectively.

[0142] [e) Intrinsic viscosity] The intrinsic viscosity required to determine the branching index g’ is measured according to DIN ISO 1628 / 1, October 1999 (in decalin, 135 °C).

[0143] [f) Broadband dielectric spectroscopy (BDS)] BDS measurements were performed using a Novocontrol Alpha frequency analyzer at different temperatures in the range of 20 - 130 °C within an error of ±0.1 °C in the frequency range of 10 -1 ~10 6 Hz under atmospheric pressure and nitrogen atmosphere. The film samples were coated with a metal layer (50 nm Au, pressure 2×10 -2 mbar, using magnetron sputtering), and measured using a pin as the upper electrode to obtain better contact between the film surface and the electrode. On the other hand, a circular electrode with a diameter of 4 mm was used on the bottom surface. The pin electrode has a rounded tip characterized by a curvature κ = 0.002 μm -1 (radius of curvature R κ = 500 μm). This reduces the contact area between the Au layer and the pin electrode with a diameter d ~ 50 μm. Also, the rounded tip shows a very small roughness (<1 μm) and is equipped with a controllable pressure, ensuring that its use does not cause any damage to the Au coating during BDS measurements. Each measurement of the permittivity ε’ and loss ε’’ was performed 6 times, and the average values were recorded. The loss tangent was calculated as tanδ = ε’’ / ε’.

[0144] The influence of heat treatment on the dielectric properties of BOPP films was investigated using the following heat protocol: The films were heated from room temperature to 110 °C at a rate of 5 °C / min. After heating, the samples were held at 110 °C for an annealing time t ann = 120 minutes in a BDS cell. After annealing, the samples were cooled to room temperature at 5 °C / min. The dielectric properties were measured at 100 °C at the start of the cycle, 110 °C during heating, and at room temperature after completion of the cycle.

[0145] [e) Small-angle X-ray scattering (SAXS)] SAXS measurements were carried out using CuK α radiation (Rigaku MicroMax 007 X-ray generator; Osmic Confocal Max-Flux curved multilayer optics). 2D diffraction patterns were recorded on a Mar345 image plate detector at a sample-detector distance of 2060 mm. The intensity distribution as a function of the coefficient of the total scattering vector, q = (4π / λ)sin(2θ / 2), [where 2θ is the scattering angle] was obtained by radial averaging of the 2D data set. Several stacks of BOPP films with a thickness of about 0.3 mm were used in the X-ray beam. To investigate the effect of annealing, SAXS measurements during heating and cooling were employed on BOPP films. The samples were heated from 30 °C to 150 °C and then cooled to 30 °C. The cavity size in the BOPP films was extracted using Lorentz-corrected curves.

Examples

[0146] <2. Examples> [a) Materials] The following materials and compounds were used in the examples. iHPP: A highly isotactic homopolymer of propylene, manufactured according to Reference Example 1 of WO 2017 / 064224, nPP: A nucleated propylene homopolymer, manufactured according to Reference Example 3 of WO 2017 / 064224, HMS: A high melting strength polypropylene, a branched propylene polymer, manufactured according to Reference Example 4 of WO2017 / 064224, The β-nucleating agent used in the examples is the β-nucleating agent quinacridone quinone CGNA-7588 commercially available from BASF, Pentaerythrityl-tetrakis(3-(3’,5’-di-tert.butyl-4-hydroxyphenyl)-propionate, commercially available from BASF as Irganox 1010.

[0147] [b) Polypropylene composition] Example IE1 of the present invention is a polypropylene composition which is a melt blend of 99.5 wt% iHPP, 0.5 wt% nPP, 1000 ppm butylhydroxytoluene, 75 ppm calcium stearate and 4500 ppm Irganox 1010. IE1 has an MFR2 of 3.3 g / 10 min, an isotactic pentad fraction of 97.1%, and an ash content of 17%.

[0148] Example IE2 of the present invention is a polypropylene composition which is a melt blend of 98.9 wt% iHPP, 1.0 wt% HMS, and 0.0001 wt% (1 ppm) β-nucleating agent, 1000 ppm butylhydroxytoluene, 75 ppm calcium stearate and 4500 ppm Irganox 1010. IE2 has an MFR2 of 3.3 g / 10 min, an isotactic pentad fraction of 97.4%, and an ash content of 13%.

[0149] Comparative Example CE is a non-nucleated polypropylene composition which is a melt blend of 99.5 wt% iHPP, 1000 ppm butylhydroxytoluene, 75 ppm calcium stearate and 4500 ppm Irganox 1010. CE has an MFR2 of 3.3 g / 10 min, an isotactic pentad fraction of 97.1%, and an ash content of 17%.

[0150] The above polypropylene composition was melt-blended at a melting temperature of 235°C using a twin-screw extruder.

[0151] It was processed using a pilot-scale biaxially oriented line including an MDO (Machine Direction Orientation) unit installed between the cast film extrudate and the tenter frame, owned and operated by Bruckner Maschinenbau GmbH. The film was extruded onto a cooling roll maintained at 90°C at a rate of 35 kg / h into a sheet with a thickness of 240 μm at a cooling roll / film speed of 10 m / min. This cast film was continuously fed into an MDO unit consisting of 12 rolls, of which the first 6 were heated to 95 - 130°C to preheat the film, the subsequent 2 were maintained at 140°C for stretching, and the last 4 were maintained at 110 - 124°C for annealing. The MD stretching process was carried out between the 8th and 9th rolls, and the 9th to 12th rolls were operated at 50 m / min, thereby producing an MDO or MD-stretched film. The MDO film was continuously fed into the tenter frame using 180 - 175°C for preheating, 175 - 165°C for stretching, and 165 - 170°C for relaxation. In the tenter operation, the MD clip distance was kept constant, and in the diverging stretching zone of the tenter, the MDO film was stretched only in the TD direction. The engineering draw ratios in MD and TD were 5.0×9.0.

[0152] The obtained BOPP films are hereinafter referred to as "SEQ" representing sequential orientation. For example, when the draw ratio in the machine direction is 6.5 and the draw ratio in the transverse direction is 9.0, it is referred to as "SEQ6.5×9.0" (see Table 1 below).

[0153] All the BOPP films tested had a thickness of about 4.8 μm. This thickness is obtained by providing flat films of different thicknesses and applying different draw ratios by changing the machine direction (MD) and the transverse direction (TD) accordingly to obtain a BOPP film with a desired thickness of about 4.8 μm.

[0154]

Table 1

[0155] Subsequently, the BOPP films of Example IE1 and IE2 of the present invention and Comparative Example CE were heat-treated as follows. The BOPP film was introduced into the BDS apparatus as described above. The film was heated from room temperature to an annealing temperature T of 110° C. at a rate of 5° C. / min ann to. After heating, the sample was held in the BDS cell at 110° C. for an annealing time, t ann = 120 minutes. After annealing, the sample was cooled to room temperature at 5° C. / min. The dielectric properties were measured at the start of the cycle, during heating at 100° C. and 110° C., and at room temperature after the end of the cycle (see FIG. 1 below). The results of the heat treatment are shown in Table 2 below.

[0156]

Table 2

[0157] In the case of dipoles with weak interactions, it is expected that ε' will decrease when heated according to the following relationship:

Equation

[0158] The product Δ(T·ε') is defined as a comparison parameter. Theoretically, this parameter T·ε', that is, the product of temperature and permittivity, should be constant. However, as shown in Table 2, in IE1 and IE2, this parameter increases significantly compared to CE. This is because in the frequency range 10 -1 ~10 6Applicable to Hz; 10 3 Examples at Hz are shown in the table. This change in dielectric constant is due to the internal stress of the film generated at high temperature (T > 110°C) and the irreversible increase in the cavity size described later.

[0159] The results of SAXS measurements at IE1 are shown in Figure 2. As can be seen, the initial cavity thickness at room temperature is about 47 nm. When the temperature is raised to 150°C, the cavity thickness increases to about 54 nm, and this increase occurs at about 130°C. Even when cooled to room temperature, the cavity thickness remains at a higher level, thus proving that an irreversible structural change has occurred. Preferred embodiments of the present specification include at least the following: [1] A method for manufacturing a heat-treated biaxially oriented polypropylene film, the method comprising the following steps: (A) A step of providing a polypropylene composition, the polypropylene composition comprising (A1) a propylene homopolymer having an isotactic pentad fraction content of 93 to 98% measured by NMR spectroscopy and a melt flow rate MFR2 of 0.4 to 10 g / 10 min measured in accordance with ISO 1133, 90 to 99.99% by weight based on the total weight of the polypropylene composition, and (A2) a polymeric α-nucleating agent, 0.0000001 to 1% by weight based on the total weight of the polypropylene composition, and / or (A3) a β-nucleating agent, 0.0000001 to 0.0002% by weight based on the total weight of the polypropylene composition, comprising the step (B) Extruding the polypropylene composition into a film, (C) Simultaneously or sequentially orienting the film to obtain a biaxially oriented polypropylene film, (D) Heat-treating the biaxially oriented polypropylene film as follows (D1) Heating the biaxially oriented polypropylene film to an annealing temperature T ann where T ann is 80 to 150 °C, (D2) Annealing the biaxially oriented polypropylene film at the annealing temperature T ann for an annealing time t ann to obtain a heat-treated biaxially oriented polypropylene film, where t ann is 10 to 140 minutes, (D3) Cooling the heat-treated biaxially oriented polypropylene film, (E) Recovering the heat-treated biaxially oriented polypropylene film comprising the method. [2] The polypropylene composition (A4) A propylene homopolymer or copolymer other than the propylene homopolymer (A1), up to 9.99% by weight based on the total weight of the polypropylene composition, further comprising the method according to [1].[[]END]] [3] The polypropylene composition (A5) A branched propylene polymer having a branching index g' of 0.9 or less as measured as described herein, 0.1 to 9.99% by weight based on the total weight of the polypropylene composition comprising the method according to [1] or [2].[[]END]] [4] The polypropylene composition (A6) An additive, 0.01 to 1% by weight based on the total weight of the polypropylene composition further comprising the method according to any one of [1] to [3].[[]END]] [5]In step (C), the film is sequentially oriented, and the sequential orientation is carried out either first in the machine direction (MD) and then in the transverse direction (TD), or first in the transverse direction (TD) and then in the machine direction (MD), by the method according to any one of [1] to [4]. [6]The draw ratio in the machine direction (MD) is at least 8.0 to 20.0, and / or the draw ratio in the transverse direction (TD) is at least 8.0 to 20.0, by the method according to any one of [1] to [5]. [7]The polymeric α-nucleating agent (A2) is a polymer of a vinyl compound represented by the formula CH 2 =CH-CHR 1 R 2 wherein R 1 and R 2 together form a 5- or 6-membered saturated, unsaturated or aromatic ring, or independently represent an alkyl group containing 1 to 4 carbon atoms, by the method according to any one of [1] to [6]. [8]The polymeric α-nucleating agent (A2) is selected from the group consisting of polyvinylcyclohexane, poly(3-methyl-1-butene) and mixtures thereof, by the method according to [7]. [9]The β-nucleating agent is selected from the group consisting of 5,12-dihydro-quino[2,3-b]acridine-7,14-dione, quino[2,3-b]acridine-6,7,13,14(5H,12H)-tetrone, 5,6,12,13-tetrahydro-quino[2,3-b]acridine-7,14-dione, and mixtures thereof, by the method according to any one of [1] to [8].

[10] The film has a thickness of 0.5 to 10 μm, by the method according to any one of [1] to [9].

[11] The heat-treated biaxially oriented polypropylene film or biaxially oriented polypropylene film further comprises a metal layer, by the method according to any one of [1] to [9].

[12] A heat-treated biaxially oriented polypropylene film obtained by the method according to any one of [1] to

[11] .

[13] A capacitor comprising an insulating film, wherein the insulating film comprises a layer of the heat-treated biaxially oriented polypropylene film according to

[12] .

[14] Use of the heat-treated biaxially stretched film according to

[12] for increasing the dielectric constant of the insulating film of the capacitor and / or improving the dielectric breakdown field strength.

[15] Use of the heat-treated biaxially oriented polypropylene film according to

[12] as a layer of the insulating film of the capacitor.

Claims

1. A method for manufacturing a heat-treated biaxially oriented polypropylene film, the method comprising the following steps: (A) Providing a polypropylene composition, the polypropylene composition comprising (A1) a propylene homopolymer having an isotactic pentad fraction content of 93 to 98% as measured by NMR spectroscopy and a melt flow rate MFR2 of 0.4 to 10 g / 10 min as measured in accordance with ISO 1133, 90 to 99.99% by weight based on the total weight of the polypropylene composition, and (A2) a polymeric α-nucleating agent, 0.0000001 to 1% by weight based on the total weight of the polypropylene composition, and / or (A3) a β-nucleating agent, 0.0000001 to 0.0002% by weight based on the total weight of the polypropylene composition, containing, (B) Extruding the polypropylene composition into a film, (C) Simultaneously or sequentially orienting the film to obtain a biaxially oriented polypropylene film, (D) Heat-treating the biaxially oriented polypropylene film as follows (D1) Heating the biaxially oriented polypropylene film to an annealing temperature T ann where T ann is 80 to 150 °C, (D2) Annealing the biaxially oriented polypropylene film at an annealing temperature T ann for an annealing time t ann to obtain a heat-treated biaxially oriented polypropylene film, where t ann is 10 to 140 minutes, (D3) Cooling the heat-treated biaxially oriented polypropylene film, (E) Recovering the heat-treated biaxially oriented polypropylene film including, a method.

2. The polypropylene composition is (A4) A homopolymer or copolymer of propylene other than the homopolymer of propylene (A1), up to 9.99% by weight based on the total weight of the polypropylene composition, The method according to claim 1, further comprising.

3. The polypropylene composition is (A5) A branched propylene polymer having a branching index g' of 0.9 or less as measured as described herein, 0.1 to 9.99% by weight based on the total weight of the polypropylene composition The method according to claim 1, comprising.

4. The polypropylene composition is (A6) An additive, 0.01 to 1% by weight based on the total weight of the polypropylene composition The method according to claim 1, further comprising.

5. In step (C), the film is sequentially oriented, and the sequential orientation is performed either first in the machine direction (MD) and then in the transverse direction (TD), or first in the transverse direction (TD) and then in the machine direction (MD). The method according to claim 1.

6. The draw ratio in the machine direction (MD) is at least 8.0 to 20.0, and / or the draw ratio in the transverse direction (TD) is at least 8.0 to 20.

0. The method according to claim 1.

7. The polymeric α-nucleating agent (A2) is a polymer of a vinyl compound represented by the formula CH ２ =CH-CHR １ R ２ wherein R １ and R ２ together form a 5- or 6-membered saturated, unsaturated or aromatic ring, or independently represent an alkyl group containing 1 to 4 carbon atoms. The method according to claim 1.

8. The polymeric α-nucleating agent (A2) is selected from the group consisting of polyvinylcyclohexane, poly(3-methyl-1-butene) and mixtures thereof. The method according to claim 7.

9. The method according to claim 1, wherein the β-nucleating agent is selected from the group consisting of 5,12-dihydro-quino[2,3-b]acridine-7,14-dione, quino[2,3-b]acridine-6,7,13,14(5H,12H)-tetrone, 5,6,12,13-tetrahydro-quino[2,3-b]acridine-7,14-dione, and mixtures thereof.

10. The method according to claim 1, wherein the film has a thickness of 0.5 to 10 μm.

11. The method according to claim 1, wherein the heat-treated biaxially oriented polypropylene film or biaxially oriented polypropylene film further comprises a metal layer.

12. A heat-treated biaxially oriented polypropylene film obtained by the method according to claim 1.

13. A capacitor comprising an insulating film, wherein the insulating film comprises a layer of the heat-treated biaxially oriented polypropylene film according to claim 12.

14. Use of the heat-treated biaxially stretched film according to claim 12 for increasing the dielectric constant of the insulating film of the capacitor and / or improving the dielectric breakdown field strength.

15. Use of the heat-treated biaxially oriented polypropylene film according to claim 12 as a layer of the insulating film of the capacitor.

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