Biaxially oriented polypropylene film, metal film laminated film, and film capacitor

Abstract

The object of this invention is to provide a biaxially oriented polypropylene film that can achieve both thinness and voltage resistance characteristics. [Solution] A biaxially oriented polypropylene film characterized by a melt flow rate of 6.0 to 8.0 g / 10 min measured at a temperature of 255°C and a load of 2.16 kg, and a melt flow rate of 12.0 to 16.0 g / 10 min measured at a temperature of 275°C and a load of 2.16 kg.

Description

Technical Field

[0001] The present invention relates to a biaxially oriented polypropylene film, a metal film laminated film, and a film capacitor using these, which have high withstand voltage properties in a high temperature and high voltage environment when used as a dielectric of a film capacitor.

Background Art

[0002] Biaxially oriented polypropylene films are excellent in transparency, mechanical properties, electrical properties, etc., and are used in various applications such as packaging applications, tape applications, and electrical applications including cable wrapping and film capacitors.

[0003] Among them, in the application of film capacitors, biaxially oriented polypropylene films are particularly preferably used as dielectrics because of their excellent high withstand voltage characteristics and low loss characteristics. Recently, various electrical equipment is being inverterized, and along with this, the requirements for miniaturization and large capacity of film capacitors are becoming even stronger. Furthermore, in applications such as automotive use (including hybrid cars and electric cars), solar power generation, and wind power generation, the use environment is getting hotter (showing 85°C or more and 125°C or less), and the requirement for heat resistance of film capacitors is increasing. The heat resistance of a film capacitor means an improvement in withstand voltage in a high temperature environment. To achieve both this and miniaturization simultaneously, it is necessary to achieve both thinning of the biaxially oriented polypropylene film used in the film capacitor and withstand voltage characteristics.

[0004] As a prior art, Patent Document 1 discloses a technique for obtaining an ultrathin biaxially oriented polypropylene film for capacitors with high dielectric strength at high temperatures by mixing at least two different isotactic polypropylene resins having different stereoregularities. Patent Documents 2 and 3 disclose a technique for obtaining a biaxially oriented polypropylene film for capacitors with excellent dielectric strength by using a polypropylene resin in which strain-curability parameters and gel fractions indicating the "degree of entanglement" of polypropylene molecular chains are defined. Furthermore, Patent Document 4 discloses a technique for obtaining a biaxially oriented polypropylene film for capacitors that exhibits high dielectric strength even at high temperatures by defining the melt tension and melt flow rate (MFR) of the polypropylene resin in addition to the strain-curability parameters. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2010-280795 [Patent Document 2] Japanese Patent Publication No. 2020-193342 [Patent Document 3] Japanese Patent Publication No. 2021-167419 [Patent Document 4] Japanese Patent Publication No. 2023-124523 [Overview of the project] [Problems that the invention aims to solve]

[0006] The biaxially oriented polypropylene films described in Patent Documents 1 to 4 can improve the voltage withstand characteristics in high-temperature environments to some extent by utilizing the properties of the polypropylene resin used as the raw material. However, the effect is insufficient, and it has been difficult to achieve this simultaneously with thin-film formation. Therefore, the present invention aims to provide a biaxially oriented polypropylene film for film capacitors that achieves both thin-film formation and high voltage withstand characteristics in high-temperature environments. [Means for solving the problem]

[0007] To solve the above problems, the biaxially oriented polypropylene film of the present invention has the following configuration. (1) A biaxially oriented polypropylene film characterized in that the melt flow rate measured at a temperature of 255°C and a load of 2.16 kg is 6.0 to 8.0 g / 10 min, and the melt flow rate measured at a temperature of 275°C and a load of 2.16 kg is 12.0 to 16.0 g / 10 min. (2) The biaxially oriented polypropylene film as described in (1), wherein the difference between the melting point (Tm) and the cooling crystallization temperature (Tmc) measured by differential scanning calorimeter (DSC) is 50°C or less. (3) A biaxially oriented polypropylene film according to (1) or (2), wherein the loss tangent (tanδ110) determined by dynamic viscoelasticity measurement at 110°C in the film width direction is 0.0960 or less. (4) A biaxially oriented polypropylene film according to any one of (1) to (3), comprising a linear polypropylene resin (A) with high stereoregularity as the main component, and further comprising a branched polypropylene resin (I) having a melt tension (MS) of 1.0 cN or more. (5) A metal film laminated film having a metal film on at least one side of a biaxially oriented polypropylene film as described in any of (1) to (4). (6) A film capacitor having a configuration in which the metal film laminated film described in (5) is laminated or wound. [Effects of the Invention]

[0008] The present invention provides a biaxially oriented polypropylene film with high processability and voltage resistance. Furthermore, by using the biaxially oriented polypropylene film of the present invention in a film capacitor, the film capacitor exhibits high safety even in high-temperature and high-voltage environments, and its lifespan is also improved. [Modes for carrying out the invention]

[0009] The biaxially oriented polypropylene film of the present invention is characterized by having a melt flow rate of 6.0 to 8.0 g / 10 min measured at a temperature of 255°C and a load of 2.16 kg, and a melt flow rate of 12.0 to 16.0 g / 10 min measured at a temperature of 275°C and a load of 2.16 kg. The biaxially oriented polypropylene film of the present invention will be described in detail below. Unless otherwise specified, in the present invention, when a numerical range is indicated with "~", the numerical range includes both the upper and lower limits, and if the unit is indicated only on the upper limit side, the unit on the lower limit side shall also be the same.

[0010] In this invention, a polypropylene film refers to a sheet-like molded article whose main component is polypropylene resin. The main component refers to a component that is present in an amount greater than 50% by mass and less than or equal to 100% by mass when the total constituent components of the object (in this case, the film) are set to 100% by mass, and the same interpretation can be applied to the main component hereafter. If multiple components that correspond to polypropylene resin are present, the product is considered to have polypropylene resin as its main component if their total amount is greater than 50% by mass.

[0011] Polypropylene resin refers to a resin whose primary constituent unit is the propylene unit. The primary constituent unit is defined as the constituent unit that accounts for more than 50 mol% but not exceeding 100 mol% when the total constituent units are set to 100 mol%. (This definition can be similarly interpreted for other olefin resins, except that the propylene unit is replaced by other olefin units.)

[0012] The biaxially oriented polypropylene film of the present invention is a biaxially oriented polypropylene film obtained by stretching a cast sheet in two orthogonal directions. In other words, biaxial orientation here means stretching in two orthogonal directions (mainly the longitudinal direction and the width direction). Whether or not a film has biaxial orientation can be determined, for example, by checking whether there is a difference in the plane orientation coefficients in the two orthogonal directions. The longitudinal direction refers to the direction in which the film travels during the film manufacturing process (the winding direction of the film in the case of a film roll), and the width direction refers to the direction parallel to the film surface and perpendicular to the longitudinal direction.

[0013] The biaxially oriented polypropylene film of the present invention preferably has a thickness of 1.0 μm to 5.0 μm, more preferably 1.2 μm to 4.8 μm, and even more preferably 1.4 μm to 4.6 μm, from the viewpoint of film-forming properties, mechanical strength, high-temperature withstand voltage characteristics, and capacitance per unit volume when used as a dielectric in a film capacitor. By setting the thickness to 1.0 μm or more, the biaxially oriented polypropylene film can be made to have excellent mechanical strength and voltage withstand voltage characteristics in high-temperature environments, and its breakage during film formation and processing can be reduced. On the other hand, by setting the thickness to 5.0 μm or less, the capacitance per unit volume can be made larger when the biaxially oriented polypropylene film is used as a dielectric in a film capacitor, leading to miniaturization of the film capacitor. The thickness of the biaxially oriented polypropylene film can be measured by the micrometer method in accordance with JIS C 2330 (2014).

[0014] The thickness of the biaxially oriented polypropylene film can be adjusted, for example, by adjusting the slit width of the T-type slit die, the discharge rate from the T-type slit die, the rotation speed of the cast drum, and the product of the stretching ratio. More specifically, the thickness of the biaxially oriented polypropylene film can be reduced by decreasing the slit width of the T-type slit die, decreasing the discharge rate from the T-type slit die, increasing the rotation speed of the cast drum, and increasing the product of the stretching ratio. These methods may be used in appropriate combinations.

[0015] Next, the polypropylene resin raw materials used in the biaxially oriented polypropylene film of the present invention will be described. The biaxially oriented polypropylene film of the present invention preferably contains a linear polypropylene resin (A) and a branched polypropylene resin (I).

[0016] In the biaxially oriented polypropylene film of the present invention, the linear polypropylene resin (A) means an isotactic polypropylene resin. This isotactic polypropylene resin is also known as a polypropylene resin generally used for film capacitor applications. The linear polypropylene resin (A) preferably has a cold xylene soluble part (CXS) of 0.5% by mass or more and 4.0% by mass or less, a mesopentad fraction (mmmm) of 0.960 or more and 0.995 or less, and a melt flow rate (MFR) of 0.5 g / 10 min or more and 5.0 g / 10 min or less. Specifically, polypropylene resins that can be preferably used as the linear polypropylene resin (A) include "Borclean" (such as HC300BF and HC318BF) manufactured by Borealis.

[0017] The CXS of the linear polypropylene resin (A) is preferably 0.5% by mass or more and 4.0% by mass or less, more preferably 0.5% by mass or more and 3.0% by mass or less, and even more preferably 0.5% by mass or more and 2.0% by mass or less. CXS is the polypropylene component dissolved in xylene when the polypropylene resin is completely dissolved in xylene at 135°C and then precipitated at 20°C. That is, CXS is considered to correspond to a component that is difficult to crystallize due to reasons such as low stereoregularity and low molecular weight. When the CXS of the linear polypropylene resin (A) is 4.0% by mass or less, the heat resistance and withstand voltage at high temperature of the biaxially oriented polypropylene film can be enhanced. Therefore, when used in a film capacitor, relaxation under a high-temperature environment can be suppressed, thermal dimensional stability can be improved, and leakage current can be suppressed. Further, when the CXS of the linear polypropylene resin (A) is 0.5% by mass or more, deterioration of drawability during film formation can be prevented.

[0018] CXS can be quantified by the following procedure. First, 0.5 g of polypropylene resin is dissolved in 100 ml of boiling xylene at 135°C, allowed to cool, and then recrystallized in a constant temperature water bath at 20°C for 1 hour and filtered. Next, the polypropylene-based components dissolved in the filtrate are quantified by liquid chromatography, and CXS is determined from the following formula, where X0 (g) is the mass of the polypropylene resin before dissolution in boiling xylene and X (g) is the mass of the polypropylene component dissolved in the filtrate. CXS (mass%) = (X / X0) × 100.

[0019] The mesopentad fraction (mmmm) of the linear polypropylene resin (A) is preferably 0.960 or more and 0.995 or less, more preferably 0.965 or more and 0.995 or less, and still more preferably 0.970 or more and 0.995 or less. The mesopentad fraction (mmmm) is an index indicating the stereoregularity of the crystalline phase of polypropylene measured by nuclear magnetic resonance (NMR) method. Generally, the higher the value, the higher the crystallinity and melting point, and the better the withstand voltage characteristics at high temperatures. When the mesopentad fraction of the linear polypropylene resin (A) is 0.960 or more, it is easy to maintain the withstand voltage characteristics and dimensional stability in a high-temperature environment when made into a biaxially oriented polypropylene film. On the other hand, when the mesopentad fraction of the linear polypropylene resin (A) is 0.995 or less, the film-forming property is good, and it is easy to obtain a stable biaxially oriented polypropylene film. The mesopentad fraction can be measured by dissolving a polypropylene resin sample in a solvent and 13 measuring using C-NMR. The detailed measurement method, conditions, etc. will be described later.

[0020] The MFR of linear polypropylene resin (A), when measured at 230°C and 2.16 kg in accordance with JIS K 7210-1 (2014), is preferably 0.5 g / 10 min to 5.0 g / 10 min, more preferably 1.0 g / 10 min to 4.5 g / 10 min, and even more preferably 1.5 g / 10 min to 4.0 g / 10 min. Setting the MFR of linear polypropylene resin (A) to 0.5 g / 10 min or higher results in good film-forming properties and makes it easier to obtain stable biaxially oriented polypropylene films. On the other hand, setting the MFR of linear polypropylene resin (A) to 5.0 g / 10 min or lower makes it easier to maintain dielectric strength and dimensional stability in high-temperature environments when used as a biaxially oriented polypropylene film. The same measurement method and conditions for the MFR of branched polypropylene resin (I), described later, apply.

[0021] Furthermore, the melt tension (MS) of the linear polypropylene resin (A) is preferably 1.5 cN or less, more preferably 1.2 cN or less, and even more preferably 1.0 cN or less. When the MS of the linear polypropylene resin (A) is 1.5 cN or less, the flow characteristics are improved when the resin is melted, and the occurrence of uneven film thickness and film tearing can be suppressed. MS refers to the tension when the polypropylene resin is heated to 230°C to melt, the molten polypropylene is extruded as strands at an extrusion speed of 15 mm / min, and these strands are drawn back at a speed of 6.5 m / min. The measurement method and conditions for the melt tension are the same for the branched polypropylene resin (I) described later.

[0022] In the biaxially oriented polypropylene film of the present invention, the branched-chain polypropylene resin (I) refers to a polypropylene resin having a branched structure in its molecular chain, and preferably has a melt tension (MS) of 1.0 cN or higher. Including such a polypropylene resin improves melting properties and enhances film moldability. The melt tension of the branched-chain polypropylene resin (I) refers to the value measured at 230°C (details of the measurement method will be described later), and although there is no particular upper limit, it is set at 40.0 cN from the viewpoint of feasibility. The MFR of the branched polypropylene resin (I) is preferably greater than 1.0 g / 10 min and less than or equal to 4.0 g / 10 min. If the MFR of the branched polypropylene resin (I) is greater than 1.0 g / 10 min, film-forming properties are maintained, and a stable biaxially oriented polypropylene film is easily obtained. On the other hand, if the MFR of the branched polypropylene resin (I) is 4.0 g / 10 min or less, the resulting biaxially oriented polypropylene film will have excellent dimensional stability and dielectric strength at high temperatures. In order to keep the MFR within the above range, methods such as controlling the weight-average molecular weight (Mw), number-average molecular weight (Mn), and molecular weight distribution (Mw / Mn) are preferred.

[0023] The biaxially oriented polypropylene film of the present invention preferably contains 1.0% to 25.0% by mass of branched-chain polypropylene resin (I) in 100% by mass of all components constituting the film, more preferably 3.0% to 20.0% by mass, even more preferably 5.0% to 20.0% by mass, particularly preferably 5.0% to 15.0% by mass, and particularly preferably 10% to 15% by mass. By setting the content of branched-chain polypropylene resin (I) in 100% by mass of all components constituting the film to 1.0% by mass or more, it becomes possible to form a film more stably when biaxially stretching a thin film, and when forming the molten resin composition into a sheet, the spherulite size does not become too large, making it easier to control to a suitable surface shape and resulting in good device processability. Furthermore, by limiting the branched-chain polypropylene resin (I) content to 25.0% by mass or less, the spherulite size does not become too small when the molten polymer is formed into a sheet, reducing the decrease in stereoregularity as a biaxially oriented polypropylene film, thus making it easier to maintain dielectric strength at high temperatures.

[0024] To obtain branched-chain polypropylene resin (I), methods such as using high-energy ionization radiation on polypropylene resin (for example, the method described in Japanese Patent Publication No. 62-121704), reacting polypropylene resin with a specific organic peroxide (for example, the method described in Japanese Patent No. 2869606), reacting polypropylene resin with a pyrolytic radical-forming agent and an ethylene-based polyfunctional unsaturated monomer (for example, the method described in Japanese Patent Publication No. 10-330436), and using a specific catalyst during polymerization of polypropylene resin (for example, the method described in Japanese Patent Publication No. 2009-057542) are preferably used. Suitable branched-chain polypropylene resins (I) include, for example, "Pro-fax" (registered trademark) (PF-814, etc.) from LyondellBasell, "Daploy" (trademark) (WB135HMS, WB140HMS, etc.) from Borealis, and "WAYMAX" (registered trademark) (MFX6, MFX8, EX6000, EX8000, etc.) from Nippon Polypropylene Co., Ltd.

[0025] The branched-chain polypropylene resin (I) contained in the biaxially oriented polypropylene film of the present invention has a branched structure in its molecular chain (linear polypropylene resin (A) does not have such a branched structure). A polypropylene resin having a branched structure in its molecular chain is a polypropylene resin having five or fewer internally trisubstituted olefins per 10,000 carbon atoms, and the presence of these internally trisubstituted olefins is significant. 1 This can be confirmed by the proton ratio of the H-NMR spectrum. The branched-chain polypropylene resin (I) acts as an α-crystal nucleating agent, and within a certain range of addition amounts, it is also possible to form a rough surface due to the crystalline form. In other words, by including the branched-chain polypropylene resin (I), the size of the polypropylene spherulites generated during the cooling process of the melt-extruded resin sheet can be controlled to be small, and a biaxially oriented polypropylene film with excellent voltage resistance characteristics at high temperatures can be obtained.

[0026] The biaxially oriented polypropylene film of the present invention may contain various additives, such as nucleating agents, antioxidants, heat stabilizers, antistatic agents, antiblocking agents, fillers, viscosity modifiers, and anticoloring agents, as long as they do not impair the objectives of the present invention. Furthermore, these components may be one type or multiple types, as long as they do not impair the effects of the present invention.

[0027] Among the additives mentioned above, the selection of the type of antioxidant and the adjustment of its content are important from the viewpoint of the long-term heat resistance of the biaxially oriented polypropylene film. Specifically, phenolic antioxidants with steric hindrance are preferred. Specifically, examples include 2,6-di-t-butyl-p-cresol (BHT: molecular weight 220.4), BASF Japan's "Irganox" (registered trademark) 1330 (molecular weight 775.2), and BASF Japan's "Irganox" (registered trademark) 1010 (molecular weight 1177.7). These antioxidants can be used alone or in combination as long as they do not impair the effects of the present invention. The total content of the above additives is preferably 0.01 parts by mass or more and 1.0 part by mass or less, more preferably 0.05 parts by mass or more and 0.9 parts by mass or less, and even more preferably 0.1 parts by mass or more and 0.8 parts by mass or less, when the resin component of the biaxially oriented polypropylene film is 100 parts by mass.

[0028] The biaxially oriented polypropylene film of the present invention is preferably obtained by molding a polypropylene resin composition mainly composed of the linear polypropylene resin (A) described above, and further containing a branched polypropylene resin (I), into a sheet and then biaxially stretching it. Any of the following biaxial stretching methods may be used: simultaneous inflation biaxial stretching, simultaneous tenter biaxial stretching, or sequential tenter biaxial stretching. However, from the viewpoint of film formation stability and thickness uniformity of the resulting biaxially oriented polypropylene film, sequential tenter biaxial stretching is preferred. In sequential tenter biaxial stretching, it is particularly preferable to stretch in the longitudinal direction first, followed by stretching in the width direction.

[0029] The biaxially oriented polypropylene film of the present invention is preferable for use as a dielectric for film capacitors due to its excellent dielectric strength in high-temperature environments. When the biaxially oriented polypropylene film of the present invention is used as a dielectric for film capacitors, a metal vapor deposition is applied to its surface. However, biaxially oriented polypropylene films typically have low surface energy, which can pose a challenge in terms of adhesion of the deposited metal. Therefore, it is preferable to apply a surface treatment to the biaxially oriented polypropylene film after biaxial stretching. Specific surface treatment methods include, for example, corona discharge treatment, plasma treatment, glow treatment, and flame treatment. These surface treatments may be used individually or in combination.

[0030] It is important that the biaxially oriented polypropylene film of the present invention has a melt flow rate (MFR) of 6.0 g / 10 min to 8.0 g / 10 min, measured at 255°C and 2.16 kg, and preferably 6.5 g / 10 min to 7.5 g / 10 min. Furthermore, it is important that the biaxially oriented polypropylene film of the present invention has a melt flow rate (MFR) of 12.0 g / 10 min to 16.0 g / 10 min, measured at 275°C and 2.16 kg, and preferably 12.5 g / 10 min to 15.0 g / 10 min, more preferably exceeding 13.5 g / 10 min and 15.0 g / 10 min or less. Considering the processability of the elements in film capacitor manufacturing, it is even more preferably exceeding 13.5 g / 10 min and 14.5 g / 10 min or less. 255°C and 275°C typically fall within the range of the melting point (Tm) + 100 ± 15°C of the biaxially oriented polypropylene film of the present invention. By specifying the MFR at each of the above temperatures within the aforementioned range, the molecular weight distribution during melting can be controlled, enabling a high level of both film-forming ability and dielectric strength under high-temperature conditions. If the aforementioned MFR is below the lower limit, film-forming ability deteriorates, making it impossible to stably obtain a biaxially oriented polypropylene film. If it exceeds the upper limit, the dielectric strength under high-temperature conditions decreases. The MFR shall be measured in accordance with JIS K 7210-1 (2014) under all temperature conditions.

[0031] The biaxially oriented polypropylene film of the present invention preferably has a difference of 50°C or less between its melting point (Tm) and cooling crystallization temperature (Tmc) as measured by differential scanning calorimeter (DSC), more preferably 49°C or less, and even more preferably 48°C or less (hereinafter, the melting point (Tm) and cooling crystallization temperature (Tmc) may be referred to as Tm and Tmc, respectively). When the difference between Tm and Tmc is 50°C or less, during the film manufacturing process, when the molten polypropylene resin (or polypropylene resin composition) is extruded from a T-type slit die and cooled in a cast drum to create a cast sheet, the solidification of the resin is not delayed, reducing the deterioration of thickness unevenness, and thus a cast sheet with a more uniform surface can be obtained. As a result, the processability of the element in the manufacturing of film capacitors is improved. Furthermore, although there is no particular lower limit to the difference between Tm and Tmc, it is preferably 35°C in order to reduce the deterioration of film manufacturing stability due to the progression of crystallization caused by the resin solidifying too quickly. Tm and Tmc can be measured using a differential scanning calorimeter (DSC), and details of the measurement method will be described later.

[0032] The biaxially oriented polypropylene film of the present invention preferably has a loss tangent (tanδ110) of 0.0960 or less, and more preferably 0.0955 or less, as determined by dynamic viscoelasticity measurement at 110°C in the film width direction. The loss tangent (tanδ) is the ratio of the storage modulus (E') to the loss modulus (E''), and is calculated from the formula "tanδ = E'' / E'". Hereinafter, the loss tangent (tanδ110) determined by dynamic viscoelasticity measurement at 110°C in the film width direction may simply be referred to as tanδ110.

[0033] A smaller loss tangent (tanδ) suggests that molecular motion is suppressed. In particular, for the biaxially oriented polypropylene film of the present invention, determining the loss tangent at Tm-60±10℃ serves as an indicator (boundary) for evaluating the dielectric strength under high-temperature conditions. Furthermore, 110℃ is usually included within the range of Tm-60±10℃. Therefore, by specifying tanδ110 to 0.0960 or less, the dielectric strength under high-temperature conditions can be improved. While there is no lower limit for tanδ110, setting it to 0.0100 is practical. Setting tanδ110 to 0.0100 or higher eliminates the need to excessively increase the stretching ratio during film formation, thus improving film formation stability. Note that tanδ110 can be measured from the viscoelastic-temperature curve obtained by the dynamic viscoelastic method, and details of the measurement method will be described later.

[0034] To achieve the MFR, the difference between Tm and Tmc, and tanδ110 of the biaxially oriented polyester film of the present invention within the above-mentioned range, it is preferable to include the aforementioned linear polypropylene resin (A) and branched polypropylene resin (I), with the amount of branched polypropylene resin (I) within the above-mentioned range, and then purge the extruder with an inert gas such as nitrogen to melt and knead the polypropylene resin. Normally, purging with an inert gas such as nitrogen suppresses molecular chain severance of the polypropylene resin, increasing the melt viscosity and raising the pressure, which can cause foreign matter and bubbles remaining in the system from the extruder to the T-type slit die to flow out, potentially worsening the film formation stability. However, this problem can be solved by controlling the temperature and residence time as melt extrusion conditions.

[0035] In the extrusion apparatus for manufacturing the biaxially oriented polypropylene film of the present invention, either a single-screw extruder or a twin-screw extruder (such as a vented extruder with a vacuum line) may be used. However, it is effective to use a so-called tandem extruder, in which the functions of melting the resin and maintaining the molten resin at a constant temperature are divided between the extruders. In particular, it is preferable to melt the resin at a high temperature inside the extruder and extrude it uniformly at a low temperature from the extruder outlet to the T-type slit die, and a tandem extruder is suitable for this purpose. The temperature and residence time depend on the specifications and arrangement from the extruder to the T-type slit die, but it is preferable to heat the temperature inside the extruder to the melting point of the main component, polypropylene resin, +90°C or higher (preferably the melting point of the polypropylene resin +95°C or higher), and from the extruder outlet to the T-type slit die to the melting point of the main component, polypropylene resin, +95°C or lower. Furthermore, from the viewpoint of adjusting the MFR of the polypropylene resin, it is preferable to adjust the residence time from the extruder outlet to the T-type slit die to 20.0 minutes or less, more preferably 18.0 minutes or less, and even more preferably 16.5 minutes or less. The residence time from the extruder outlet to the T-type slit die can be adjusted not only by the extrusion rate but also by the filter diameter, filter accuracy, number of filters used to filter the molten extruded polypropylene resin composition, and the amount of resin extruded. If the residence time to the T-type slit die becomes long, the temperature inside the extruder may be lowered to reduce resin degradation.

[0036] Next, the method for producing the biaxially oriented polypropylene film of the present invention will be described below, but is not necessarily limited thereto.

[0037] First, linear polypropylene resin (A) and branched polypropylene resin (I) are dry-blended in the above-mentioned mass ratio and supplied to a nitrogen-purged single-screw melt extruder, then supplied to a single-screw or twin-screw melt extruder, and melt extrusion is performed with the temperature inside the extruder at the melting point of the main component polypropylene resin + 90°C or higher (preferably at the melting point of the polypropylene resin + 95°C or higher), and the temperature from the extruder outlet to the T-type slit die at less than the melting point of the main component polypropylene resin + 95°C (an antioxidant may be mixed in at this time). Next, foreign matter and bubbles are removed using a filter installed in the middle of the polymer tube. Then, the molten polymer formed into a sheet is discharged from the T-type slit die onto a cast drum and cooled and solidified to form a cast sheet, which is then cooled with a cooling roll.

[0038] The temperature of the casting drum is preferably 80°C to 120°C, more preferably 85°C to 115°C, and even more preferably 85°C to 110°C, from the viewpoint of appropriately generating β-crystals and spherulites. Setting the casting drum temperature to 80°C or higher prevents the formation of too few β-crystals in the cast sheet, thus maintaining the slipperiness of the film obtained after biaxial stretching, and preventing wrinkles from forming and deterioration of the film roll's winding shape during the film transport process in film formation and processing. On the other hand, setting the casting drum temperature to 120°C or lower prevents the excessive formation of β-crystals in the cast sheet, reducing meandering during the film transport process in film formation and processing and minimizing deterioration of the film roll's winding shape.

[0039] The following section describes a method for more uniformly controlling the surface temperature of a cast drum in both the widthwise and circumferential directions, taking into account the mechanism by which the surface temperature of a cast drum becomes uneven. A cast drum typically has piping installed inside, either parallel to the widthwise or spirally (this type of cast drum is sometimes called a spiral type). The surface temperature of the cast drum can be controlled by introducing a temperature-controlled cooling medium from one end of this piping and releasing it from the other end. However, when using such a spiral-type cast drum, the heat transfer coefficient decreases in the piping sections parallel to the widthwise or spirally, and a temperature difference occurs between the introduction and release of the cooling medium due to the amount of heat generated during film cooling on the surface of the cast drum, resulting in an uneven surface temperature of the cast drum.

[0040] Furthermore, industrial water is commonly used as a cooling medium. This industrial water contains metal components such as iron and manganese, as well as foreign matter such as evaporation residue. Therefore, if industrial water is continuously flowed through the piping of a cooling drum for a long period of time, the aforementioned foreign matter may adhere to the inner sides of the piping and accumulate in certain areas, or the foreign matter may react with the metal on the sides of the piping to form aggregates. The formation and fall of these deposits and aggregates can cause localized temperature unevenness on the surface of the cooling drum. By regularly cleaning the piping, these deposits and aggregates can be removed early, allowing for more uniform temperature control on the surface of the cooling drum.

[0041] A more preferable method for controlling the surface temperature of a cast drum more uniformly is to use a cast drum having multiple jacket chambers within the thickness of the cast drum surface. In such a cast drum, a gas-liquid two-phase heat transfer medium is sealed in the jacket chambers. When the liquid phase heat transfer medium comes into contact with the cast drum surface, which has become locally hot due to heat conduction accompanying contact with the molten resin composition, the liquid phase heat transfer medium vaporizes and undergoes a phase change to the gas phase heat transfer medium. When this gas phase heat transfer medium comes into contact with the cast drum surface, which is still locally at a low temperature, it liquefies and undergoes a phase change again to the liquid phase heat transfer medium. In this way, the heat transfer medium vaporizes and absorbs latent heat when it comes into contact with the high-temperature cast drum surface, and then vaporizes and provides latent heat when it comes into contact with the low-temperature cast drum surface, thereby allowing for more uniform control of the cast drum surface temperature. Note that "thickness of the cast drum surface" refers to the cylindrical part that forms the side surface when the cast drum is considered as a cylinder with its rotation axis as the central axis.

[0042] Furthermore, after the molten sheet discharged from the T-type slit die lands on the cast drum, the time it remains in contact with the cast drum is preferably 0.5 seconds or more and 3.0 seconds or less, and more preferably 1.0 second or more and 3.0 seconds or less. If the contact time is 0.5 seconds or more, the molten sheet solidifies more easily, and breakage in the subsequent stretching process can be reduced. On the other hand, if the contact time is 3.0 seconds or less, the excessive formation of β crystals in the cast sheet can be prevented, and the occurrence of meandering in the film transport process during film formation and processing, as well as deterioration of the winding shape of the film roll, can be reduced.

[0043] Methods for adhering the molten sheet to the casting drum include electrostatic application, air knife method, nip roll method, and underwater casting method. However, the air knife method is preferred from the viewpoint of suppressing thickness unevenness, high-speed film formation, and controlling the surface properties of the film. The air temperature of the air knife is preferably between 60°C and 100°C. By setting the air temperature of the air knife to 60°C or higher, the amount of β crystals formed in the cast sheet is not reduced too much, and the slipperiness of the film obtained after biaxial stretching is maintained, thereby reducing the occurrence of wrinkles and deterioration of the winding shape of the film roll during the film transport process during film formation and processing. On the other hand, by setting the air temperature of the air knife to 100°C or lower, excessive β crystals are not formed in the cast sheet, thereby reducing the occurrence of meandering and deterioration of the winding shape of the film roll during the film transport process during film formation and processing.

[0044] The temperature difference between the cast drum temperature and the air knife temperature (the lower temperature minus the higher temperature) is preferably 30°C or less, more preferably 20°C or less, and even more preferably 15°C or less, from the viewpoint of maintaining high slipperiness when equivalent β-crystals are formed on both sides of the cast sheet to create a biaxially oriented polypropylene film. When this temperature difference is 30°C or less, the discrepancy in cooling conditions on both sides is suppressed, making it difficult for different irregularities to form on the front and back of the cast sheet, and making it easier for the slipperiness to be equivalent on both sides of the film. Therefore, when the resulting biaxially oriented polypropylene film is used in capacitor element processing, the amount of air drawn in during the winding process is stable, and the gaps between film layers and the amount of air are more likely to become uniform after the heat treatment process. As a result, high safety can be achieved when used as a film capacitor, and the reduction in the lifespan of the film capacitor is reduced.

[0045] On the other hand, there is no particular lower limit to this temperature difference, and theoretically it can be 0°C. However, considering the stability of film formation, it is preferable to make the cast drum temperature relatively higher than the air temperature of the air knife, and it is more preferable that the difference between the cast drum temperature and the air temperature of the air knife is 1°C or more, and even more preferable that it is 5°C or more. In the cooling process using a cast drum and an air knife, if the cast drum temperature is equal to the air temperature of the air knife, the cooling efficiency of the surface that is air-cooled by the air from the air knife is inferior to the surface that is cooled in close contact with the cast drum. Therefore, by making the cast drum temperature relatively higher than the air temperature of the air knife, it becomes easier to make the cooling efficiency of both sides roughly the same, and as a result of reducing the unevenness of the physical properties of the cast sheet due to differences in cooling conditions, the film formation becomes more stable.

[0046] The cast sheet solidified by the casting drum is preferably further cooled with a cooling roll, and the temperature of the cooling roll is preferably between 10°C and 60°C. Setting the cooling roll temperature to 10°C or higher makes it easier to raise the film to the desired temperature in the subsequent high-temperature heat treatment process. On the other hand, setting the cooling temperature to 60°C or lower reduces crystal formation in the cast sheet, making it easy to reduce longitudinal variations in the surface shape of the biaxially oriented polypropylene film obtained after biaxial stretching.

[0047] Next, the cast sheet is stretched in the longitudinal direction in the longitudinal stretching process. Generally, the characteristic crater-like surface shape of biaxially oriented polypropylene films using the β-crystal method is created by the formation of depressions (volume reduction due to the transition) caused by the transition from β-crystals to α-crystals generated through the cast drum during the longitudinal stretching process. Methods for controlling the temperature and temperature unevenness of the longitudinal stretching roll include using a longitudinal stretching roll with multiple jacket chambers within the surface thickness, similar to the cast drum described above.

[0048] In the longitudinal stretching process, the cast sheet is passed through preheated rolls controlled to a temperature of 100°C to 150°C, preferably 110°C to 150°C, and stretched longitudinally (longitudinal stretching) at a predetermined stretching speed and stretching ratio due to the difference in peripheral speed between the rolls. The longitudinal stretching ratio is preferably 4.0 to 7.0 times, and more preferably 5.0 to 7.0 times. By setting the stretching ratio to 4.0 times or higher, the surface shape of the film becomes uniform and the high-temperature withstand voltage characteristics are also improved. If the longitudinal stretching ratio is 7.0 times or lower, film breakage in the longitudinal stretching process and the subsequent transverse stretching process is reduced. Preheating may be performed using only rolls with a constant temperature, or it may be performed using multiple rolls with different temperatures.

[0049] Next, it is preferable to grasp both ends in the width direction of the uniaxially oriented film obtained by longitudinal stretching with clips and stretch it in the width direction at a stretching ratio of 5.0 to 15.0 times using a tenter-type stretcher controlled at a temperature of 140°C to 170°C. Furthermore, it is preferable to heat-set the film at a temperature of 150 to 170°C while allowing it to relax by 5 to 15% in the width direction. By performing stretching and heat-setting under these conditions, it is possible to improve thinness and thermal dimensional stability.

[0050] Next, the biaxially oriented film is slowly cooled to room temperature as needed, and then subjected to corona discharge treatment in air, nitrogen, carbon dioxide, or a mixture thereof. This treatment may be performed on both sides or on one side. After that, using a known slitter or the like, the film held by the clip is cut parallel to the longitudinal direction, and both ends in the width direction are removed, and it is wound up as a master roll on a winding machine. Finally, the film unwound from the master roll is slit to a specific width using a slitter, and wound onto a core as a film roll to obtain a roll of the biaxially oriented polypropylene film of the present invention.

[0051] The biaxially oriented polypropylene film of the present invention is preferably used as a dielectric for film capacitors, but is not limited to the type of film capacitor. Specifically, in terms of electrode configuration, it may be either a foil-wound film capacitor or a metal-deposited film capacitor, and it is also preferably used in oil-immersion type film capacitors containing insulating oil or dry-type film capacitors that do not use insulating oil at all. Furthermore, in terms of shape, it may be either a wound type or a laminated type. Due to the properties of the biaxially oriented polypropylene film of the present invention, it is particularly preferably used as a metal-deposited film capacitor.

[0052] Next, the metal film laminated film of the present invention will be described. The metal film laminated film of the present invention has a metal film on at least one side of the biaxially oriented polypropylene film of the present invention. As a method for forming the metal film, a method of depositing a metal such as aluminum onto at least one side of the biaxially oriented polypropylene film to provide a metal film that will serve as the internal electrode of the film capacitor is preferably used. At this time, other metal components such as nickel, copper, gold, silver, chromium, and zinc can be deposited simultaneously or sequentially with the aluminum. In addition, a protective layer such as oil can be provided on the metal film. The thickness of the metal film is preferably 20 nm to 100 nm from the viewpoint of the electrical characteristics and safety of the film capacitor. Also, for the same reason, the surface resistance of the metal film is preferably 1 Ω / sq to 20 Ω / sq. The surface resistance can be controlled by the type of metal used and the film thickness.

[0053] In this invention, if necessary, after forming a metal film, the metal film laminated film can be subjected to aging treatment at a specific temperature or heat treatment. Furthermore, a coating such as polyphenylene oxide can be applied to at least one side of the metal film laminated film for insulation or other purposes.

[0054] The film capacitor of the present invention has a structure in which the metal film laminate of the present invention is laminated or wound. That is, the film capacitor of the present invention includes both a laminated film capacitor obtained by laminating the metal film laminate of the present invention and a wound film capacitor obtained by winding the metal film laminate. A preferred manufacturing method for the wound film capacitor will be described below, but the film capacitor of the present invention is not necessarily limited thereto.

[0055] First, aluminum is vacuum-deposited onto one side of a biaxially oriented polypropylene film. The aluminum is deposited in stripes with margins running along the longitudinal direction of the film. Next, slits are made by inserting a blade into the center of each deposited area and the center of each margin on the surface, creating a tape-shaped reel with a margin on one side. Two of these tape-shaped reels, one with a left margin and one with a right margin, are stacked and wound together so that the deposited portion extends beyond the margin in the width direction, thereby obtaining a wound-type film capacitor. After heat treatment of the wound-type body, metallicon is sprayed onto both ends in the width direction to form external electrodes, and lead wires are welded to the metallicon to obtain a wound-type film capacitor.

[0056] Film capacitors have a wide range of applications, including vehicles, home appliances (such as televisions and refrigerators), general dust protection, automobiles (hybrid cars, power windows, wipers, etc.), and power supplies. Film capacitors using the biaxially oriented polypropylene film of the present invention can also be suitably used in these applications. [Examples]

[0057] The present invention will now be described in more detail with reference to examples, but the present invention is not limited to these examples. Prior to describing each example, the methods for evaluating each physical property will be described.

[0058] [Method for evaluating characteristics] (1) Thickness (unit: μm) The thickness of the biaxially oriented polypropylene film was measured using the micrometer method in accordance with JIS C 2330 (2014).

[0059] (2) Mesopentad fraction (mmmm) Dissolve the polypropylene resin sample in a solvent. 13 The mesopentade fraction (mmmm) was determined using 1C-NMR under the following conditions (Reference: New Edition Polymer Analysis Handbook, edited by The Japan Society for Analytical Chemistry and the Polymer Analysis Research Group, 1995, pp. 609-611). A. Measurement conditions • Equipment: Bruker DRX-500 • Measurement nucleus: 13 C nucleus (resonance frequency: 125.8MHz) ·Measurement concentration: 10% by mass • Solvent: Benzene / deuthorthodichlorobenzene = mass ratio 1:3 mixed solution ·Measurement temperature: 130℃ • Spin speed: 12Hz • NMR sample tube: 5mm tube • Pulse width: 45° (4.5μs) • Pulse repetition time: 10 seconds • Data points: 64K • Number of conversions: 10,000 • Measurement mode: complete decoupling B.Analysis conditions A Fourier transform was performed with a line broadening factor (LB) of 1.0, resulting in a mmmm peak of 21.86 ppm. Peak splitting was then performed using WINFIT software (Bruker). The peak splitting was performed starting from the high-field side, and further automatic fitting was performed using the accompanying software. After optimizing the peak splitting, the sum of the mmmm peak fractions was calculated. This measurement was performed five times, and the average value was taken as the mesopentad fraction (mmmm) of this sample. (Peak splitting) (a)mrrm (b)(c)rrrm (split into two peaks) (d)rrrr (e)mrmr (f)mrmm+rmrr (g)mmrr (h)rmmr (i)mmmr (j)mmmm.

[0060] (3) Melt Flow Rate (MFR) (Unit: g / 10 min) Measurements were taken in accordance with JIS K 7210-1 (2014) under conditions of 230°C, 255°C, 275°C, and a load of 2.16 kg (MFR of resin was measured at 230°C, and MFR of film was measured at 255°C and 275°C).

[0061] (4) Melt tension (MS) (unit: cN) The following procedure was used to measure the tension using a melt tension tester (capillary diameter 2.1 mm, cylinder diameter 9.55 mm) manufactured by Toyo Seiki Seisakusho Co., Ltd. First, polypropylene resin was heated to 230°C and melted. Next, the molten polypropylene resin was extruded into strands at an extrusion speed of 15 mm / min, and the tension was measured when these strands were pulled back at a speed of 6.5 m / min. The obtained value was defined as the MS (Magnetic Stress Test).

[0062] (5) Cold xylene soluble portion (CXS) (Unit: mass%) 0.5 g of polypropylene resin was dissolved in 100 ml of boiling xylene at 135°C, allowed to cool, and then recrystallized in a constant temperature water bath at 20°C for 1 hour. Afterward, solid matter such as crystals was removed by filtration, and the polypropylene components dissolved in the filtrate were quantified by liquid chromatography. Let X0 (g) be the mass of the polypropylene resin before dissolution in boiling xylene, and X (g) be the mass of the polypropylene components dissolved in the filtrate. CXS was calculated using the following formula. CXS(mass%)=(X / X0)×100.

[0063] (6) Melting point: Tm (unit: °C) A 5 mg film sample was weighed and measured using a differential scanning calorimeter (DSC6220) manufactured by Hitachi High-Tech Science Corporation. The temperature was increased at a rate of 20°C / min under a nitrogen atmosphere within the range of 25 to 250°C. The temperature of the endothermic peak associated with crystal melting was read and defined as the melting point (Tm). The melting point of the resin was measured using the same method.

[0064] (7) Cooling crystallization temperature: Tmc (unit: °C) A 5 mg film sample was weighed and melted at 250°C for 5 minutes under a nitrogen atmosphere using a differential scanning calorimeter DSC6220 manufactured by Hitachi High-Tech Science Corporation. The sample was then cooled at a cooling rate of 20°C / min, and the temperature of the exothermic peak associated with crystallization was read and defined as the cooling crystallization temperature (Tmc).

[0065] (8) Loss tangent (tanδ) A test specimen (width (short side) 5 mm × length (long side) 20 mm), cut from a biaxially oriented polypropylene film with the film width direction as the long side, was mounted in the apparatus chuck under a 23°C atmosphere. It was then cooled to -60°C, and measurements were started when the temperature reached -50°C after the start of heating. A viscoelastic-temperature curve was drawn using the dynamic viscoelastic method, and the storage modulus (E'110) and loss modulus (E''110) were read. The loss tangent at 110°C (tanδ110) was calculated using the following formula. The measurement apparatus and conditions were as follows. (tanδ110)=(E”110) / (E'110) <Measurement device, conditions> • Device: Rheogel-E4000 (manufactured by UBM) • Geometry: Tension • Chuck spacing: 10mm • Frequency: 10Hz Distortion: 0.1-0.2% Temperature range: -50 to 150°C • Heating rate: 3°C / min • Measurement environment: Under nitrogen.

[0066] (9) Evaluation of film-forming properties The film-forming properties of biaxially oriented polypropylene films were evaluated according to the following criteria. The time between stopping and resuming film formation due to film tearing during the process was excluded from the observation time. ◎: No film tears occurred for more than 48 hours. ○: Film tears occurred 1 to 2 times in 48 hours. △: Film tears occurred 3 to 5 times in 48 hours. ×: More than 6 film tears occurred within 48 hours.

[0067] (10) Evaluation of processability of components in film capacitor manufacturing Aluminum was vacuum-deposited onto the corona-treated side of a biaxially oriented polypropylene film using a vacuum deposition machine manufactured by ULVAC, Inc., to achieve a surface resistance of 15 Ω / sq. The aluminum was deposited in a stripe pattern with a margin running along the longitudinal direction (a repeating pattern of a deposited section width of 79.0 mm and a margin section width of 1.0 mm). Next, slits were made by inserting a blade into the center of each deposited section and each margin section to create a tape-like winding reel with a total width of 40 mm and a 0.5 mm margin at either the left or right end. Two of these reels, one with the left margin and one with the right margin, were overlapped and wound together so that the deposited portion extended 0.5 mm beyond the margin in the width direction, resulting in a winding body with a capacitance of 120 μF. KAW-4NHB manufactured by Kaito Seisakusho Co., Ltd. was used for winding. Finally, the winding body was heat-treated for 10 hours in a reduced-pressure atmosphere at 140°C. The winding body was visually inspected, and any with wrinkles or distortions in appearance or shape were considered defective. 200 wound bodies were produced using the same method, and the same evaluation was repeated. The processability of the wound bodies was then evaluated according to the following criteria. ◎: No defective products ○: Less than 1 defective item △: Two or more defective items but less than three. ×: 4 or more defective items.

[0068] (11) Life evaluation of film capacitors A winding with a capacitance of 120 μF was obtained by the method described in (10). The winding was then heat-treated for 10 hours in a reduced-pressure atmosphere at 140°C, and metallicon was sprayed onto both ends in the width direction to form external electrodes. Lead wires were then welded to the metallicon to obtain a film capacitor. Next, the life evaluation of 15 film capacitors was performed using the following procedure. First, the capacitance (C0) was measured at room temperature. Then, a voltage of 325 VDC / μm (650 V when the thickness is 2.0 μm) was applied to the film capacitor at a high temperature of 120°C for 1000 hours. After that, the capacitance (C) was measured at room temperature, and the rate of change of capacitance (ΔC in %) before and after voltage application was calculated from the following formula (6). The capacitance was measured using an LCR high-tester 3522-50 manufactured by HIOKI E.E. CORPORATION. ΔC = ((C0 - C) / C0) × 100 The average of the rate of change (ΔC) of capacitance before and after voltage application for 15 film capacitors was used as the rate of change of capacitance before and after voltage application for that sample, and it was evaluated according to the following criteria. A smaller rate of change (ΔC) of capacitance before and after voltage application indicates that the decrease in capacitance at high temperatures is suppressed, and the lifespan evaluation of the film capacitor is considered good. ◎◎: ΔC is less than 1.5% ◎: ΔC is between 1.5% and less than 2% ○: ΔC is 2% or more but less than 3% △: ΔC is between 3% and less than 5% ×: ΔC is 5% or more.

[0069] [Raw materials] (1) Resin • Linear polypropylene resin (A) Borclean HC300BF, manufactured by Borealis, is a linear polypropylene resin with a mesopentad fraction of 0.980, a CXS of 1.2% by mass, an MFR of 3.3 g / 10 min, an MS of 1.0 cN, and a melting point of 165.1 °C. • Branched polypropylene resin (I) Borealis' "Daploy" WB140HMS: branched-chain polypropylene resin with an MFR of 2.1 g / 10 min, an MS of 36.0 cN, and a melting point of 166.3 °C. (2) Antioxidants • Antioxidant 1: BASF Japan's “Irganox” (registered trademark) 1010 • Antioxidant 2: 2,6-di-t-butyl-p-cresol (BHT) (Example 1) A polypropylene resin mixture, obtained by mixing linear polypropylene resin (A) and branched polypropylene resin (I) in a mass ratio of 90.0:10.0, antioxidant 1, and antioxidant 2 were dry-blended in a mass ratio of 99.5:0.4:0.1 and supplied to a nitrogen-purged single-screw melt extruder. The extruder temperature was set to 265°C, and the temperature from the extruder outlet to the T-type slit die was set to 250°C for melt extrusion. Subsequently, foreign matter was removed from the extruded molten polypropylene resin composition using a 25 μm cut sintered filter, and the composition was further discharged in a sheet form from the T-type slit die. Furthermore, the sheet-like molten polypropylene resin composition was solidified by adhering it to a cast drum maintained at a surface temperature of 90°C using an air knife with an air temperature of 80°C, and then cooled on a cooling roll maintained at a temperature of 45°C to obtain a cast sheet. At this time, the sheet-like molten polypropylene resin composition was in contact with the cast drum and the cooling roll for 1.5 seconds each. The cast drum used had multiple jacket chambers within its thickness and contained a gas-liquid two-phase heat transfer medium. Its surface temperature was controlled by controlling the temperature of the cooling water circulating inside the cast, and temperature unevenness was controlled by controlling the amount of cooling water circulating inside the cast. Next, the obtained cast sheet was preheated by passing it through a roll maintained at a surface temperature of 120°C, and then again by passing it through a roll maintained at a surface temperature of 145°C. After that, it was stretched in the longitudinal direction at a stretching ratio of 5.5 times on a stretching roll maintained at a surface temperature of 140°C to produce a uniaxially oriented film. In the longitudinal stretching process, similar to the cast drum described above, a longitudinal stretching roll with multiple jacket chambers within its thickness and containing a gas-liquid two-phase heat transfer medium was used. Its surface temperature was controlled by controlling the temperature of the steam circulating inside the longitudinal stretching roll, and temperature unevenness was controlled by controlling the amount of steam circulating inside the longitudinal stretching roll. Furthermore, the uniaxially oriented film was guided to a tenter by gripping its widthwise end with clips, and stretched in the widthwise direction at a temperature of 160°C and a stretching ratio of 11.0 times. Next, a 12% relaxation treatment was performed in the widthwise direction at a temperature of 158°C, and after simmering to room temperature, the film was stretched at 25 W·min / m on the drum surface (the side that was in contact with the cast drum). 2Corona discharge treatment was performed at the specified treatment intensity. The widthwise end of the obtained biaxially oriented polypropylene film was cut off using clips and wound on a winding machine to obtain a master roll with a widthwise length of 6,500 mm. Next, it was slit using a slitter to a film width of 820 mm, and 30,000 m was wound onto a core in the longitudinal direction to obtain a biaxially oriented polypropylene film roll with a thickness of 2.3 μm. The physical properties of the obtained biaxially oriented polypropylene film and the results of each evaluation are shown in Table 1.

[0070] (Examples 2-10, Comparative Examples 1-6) A biaxially oriented polypropylene film roll with a thickness of 2.3 μm was obtained using the same manufacturing method as in Example 1, except that the raw materials and manufacturing conditions were as shown in Table 1. The physical properties of the obtained biaxially oriented polypropylene film and the results of each evaluation are shown in Table 1. The residence time up to the T-type slit die was adjusted by the resin extrusion rate (the residence time decreases as the resin extrusion rate increases).

[0071] [Table 1]

[0072] In the production of the biaxially oriented polypropylene films in each example and comparative example, antioxidants are included, but only in trace amounts, and some are lost during the manufacturing process. Therefore, the composition of the polypropylene resin mixture is shown (this is substantially equivalent to the composition of the obtained biaxially oriented polypropylene film). [Industrial applicability]

[0073] The present invention provides a biaxially oriented polypropylene film with high processability and voltage resistance. Furthermore, by using the biaxially oriented polypropylene film of the present invention in a film capacitor, the film capacitor exhibits high safety even in high-temperature and high-voltage environments, and its lifespan is also improved.

Claims

1. A biaxially oriented polypropylene film characterized by a melt flow rate of 6.0 to 8.0 g / 10 min measured at a temperature of 255°C and a load of 2.16 kg, and a melt flow rate of 12.0 to 16.0 g / 10 min measured at a temperature of 275°C and a load of 2.16 kg.

2. The biaxially oriented polypropylene film according to claim 1, wherein the difference between the melting point (Tm) and the cooling crystallization temperature (Tmc), as measured by differential scanning calorimetry (DSC), is 50°C or less.

3. The biaxially oriented polypropylene film according to claim 1 or 2, wherein the loss tangent (tanδ110) determined by dynamic viscoelasticity measurement at 110°C in the film width direction is 0.0960 or less.

4. A biaxially oriented polypropylene film according to claim 1 or 2, comprising a linear polypropylene resin (A) with high stereoregularity as the main component, and further comprising a branched polypropylene resin (I) having a melt tension (MS) of 1.0 cN or more.

5. A metal film laminated film having a metal film on at least one side of a biaxially oriented polypropylene film according to claim 1 or 2.

6. A film capacitor having a configuration in which the metal film laminated film described in claim 5 is laminated or wound.

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