Biaxially oriented polypropylene film

Abstract

The object of this invention is to provide a biaxially oriented polypropylene film that can improve winding properties after aluminum deposition, processability when slit to form a wound body for film capacitors, and the element shape and performance of film capacitors. [Solution] A biaxially oriented polypropylene film characterized by comprising a single layer having at least one side having a surface aspect ratio (Str) of 0.15 or more and less than 0.30 (hereinafter referred to as side A), and a thickness (t) of 6.0 μm or more and less than 12.0 μm.

Description

Technical Field

[0001] The present invention relates to a biaxially oriented polypropylene film capable of improving the winding property after aluminum vapor deposition, the workability when slit into a wound body for a film capacitor, and the element shape and performance of the film capacitor, and a film capacitor using the same.

Background Art

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

[0003] Among these, 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 have been further strengthened. Furthermore, especially in applications such as electric railway applications such as the Shinkansen, automotive applications (including hybrid cars and electric vehicles), solar power generation, and wind power generation, the temperature of the use environment is increasing (showing 85°C or more and 125°C or less), and the requirement for heat resistance of film capacitors is also increasing.

[0004] Therefore, in addition to thinning, improving the heat resistance, and increasing the voltage withstand voltage per unit thickness of the diaxially oriented polypropylene film that serves as the dielectric, there is also a need to improve the safety of the film capacitor itself. Here, the safety of a film capacitor refers to the property of maintaining insulation in a metal vapor-deposited film capacitor, which uses a metal vapor-deposited film formed on a dielectric film as an electrode, by scattering the vapor-deposited metal with the discharge energy during abnormal discharge. This is an important property for preventing short circuits and failures of film capacitors. Controlling the surface properties of the diaxially oriented polypropylene film is considered an effective means of achieving both high voltage withstand voltage per unit thickness and safety of the film capacitor, and various studies have been conducted to date.

[0005] A method for controlling the surface properties of biaxially oriented polypropylene films is known, which utilizes the crystal transition of polypropylene from the β-crystal to the α-crystal (hereinafter sometimes referred to as the β-crystal method). This method utilizing the crystal transition does not require the addition of impurities such as additives that may worsen the dielectric strength, and is therefore preferred as a method for roughening the surface of biaxially oriented polypropylene films for film capacitors (see, for example, Patent Documents 1 and 2).

[0006] Other proposed techniques focusing on surface roughness density and protrusion uniformity include adding branched polypropylene (see, for example, Patent Documents 3 and 4), mixing polypropylenes with different molecular weights and molecular weight distributions (see, for example, Patent Document 5), and performing high-temperature heat treatment on the cast sheet before the longitudinal stretching process (see, for example, Patent Document 6). These methods allow for control of spherulite size, making it possible to form high-density protrusions with uniform height.

[0007] Furthermore, methods for controlling the shape pattern of the surface properties have been proposed, such as die-coating a resin solution onto a base film (see, for example, Patent Document 7). These methods allow for control of the coater gap and coating speed, making it possible to form a shape pattern of surface properties suitable for the processability of capacitors. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2008-133446 [Patent Document 2] Japanese Patent Publication No. 2014-077057 [Patent Document 3] International Publication No. 2007 / 094072 [Patent Document 4] International Publication No. 2012 / 121256 [Patent Document 5] Japanese Patent Publication No. 2014-231584 [Patent Document 6] Japanese Patent Publication No. 2020-132877 [Patent Document 7] Japanese Patent Publication No. 2023-141167 [Overview of the project] [Problems that the invention aims to solve]

[0009] However, when applying the β-crystallization method described in Patent Documents 1 and 2 to a film made of general linear polypropylene, steep convex and concave parts are formed in a crater-like shape at a low density, but the number of craters per unit area and the shape of the craters cannot be controlled. As a result, air is localized between the film layers, which presents challenges in terms of film winding properties and processability after aluminum deposition.

[0010] Regarding the methods described in Patent Documents 3 to 6 for forming high-density protrusions of uniform height, when using relatively thin and weak films with a thickness of 1 μm to 4 μm, high tension is applied during winding to prevent wrinkles, allowing for uniform control of the air volume by forming high-density protrusions. On the other hand, relatively thick films with a thickness of 7 μm or more are stiff and are wound at low tension, so the amount of air between film layers does not reach a sufficient level on surfaces where high-density protrusions of uniform height are formed. Furthermore, when winding such films at high tension, there are problems in that the weight per film increases, and blocking phenomena, where films stick together, frequently occur due to the increased contact area resulting from the formation of high-density protrusion surfaces.

[0011] Furthermore, when using the method described in Patent Document 7, which involves die-coating a substrate with a resin solution using a coater, although the amount of air can be optimized by controlling the surface properties, the production volume is limited by the coating, and the pattern formed during the solvent drying process after coating may be disrupted. In addition, voids containing air tend to form between the substrate and the coating layer, which can significantly reduce the dielectric strength of the resulting film. Moreover, the resins that can be dissolved in the solvent are limited, and these resins generally have inferior dielectric strength and low-loss characteristics compared to polypropylene, so they have not been sufficient to meet the requirements of recent years.

[0012] Therefore, the object of the present invention is to provide a biaxially oriented polypropylene film that can improve winding properties after aluminum deposition, processability when slit to form a wound body for film capacitors, and the element shape and performance of film capacitors. [Means for solving the problem]

[0013] The above-mentioned problems can be solved as follows. That is, the biaxially oriented polypropylene film of the present invention is a biaxially oriented polypropylene film characterized in that at least one side is a surface with an aspect ratio (Str) of surface properties of 0.15 or more and less than 0.30 (hereinafter referred to as side A), and consists of a single layer with a thickness (t) of 6.0 μm or more and less than 12.0 μm.

[0014] Furthermore, the biaxially oriented polypropylene film of the present invention can also be configured as follows to solve the above problems, and can also be used to make a metal film laminated film and a film capacitor as follows. (1) A biaxially oriented polypropylene film characterized in that at least one side is a surface with an aspect ratio (Str) of surface properties of 0.15 or more and less than 0.30 (hereinafter referred to as side A), and the film consists of a single layer with a thickness (t) of 6.0 μm or more and less than 12.0 μm. (2) The biaxially oriented polypropylene film according to (1), wherein the degree of sharpness kurtosis (Sku) on surface A is 5.0 or more and less than 10. (3) The biaxially oriented polypropylene film according to (1) or (2), wherein the average height (Spk) of the protruding peaks on surface A is 50 nm or more and less than 100 nm. (4) A biaxially oriented polypropylene film as described in (1) to (3), used as a dielectric for film capacitors. (5) A metal film laminated film having a metal film on at least one side of the biaxially oriented polypropylene film described in (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]

[0015] The present invention provides a biaxially oriented polypropylene film that can improve winding properties after interlayer aluminum deposition, processability when slit to form a wound body for film capacitors, and the element shape and performance of film capacitors. [Modes for carrying out the invention]

[0016] Hereinafter, the biaxially oriented polypropylene film, metal film laminated film, and film capacitor of the present invention will be described in more detail. In the numerical ranges expressed using "~" hereinafter, the upper limit value and the lower limit value are included in the range, and the units of the upper limit value and the lower limit value are the same.

[0017] The biaxially oriented polypropylene film of the present invention is a biaxially oriented film obtained by stretching a cast sheet in two directions perpendicular to each other. That is, the biaxial orientation here means stretching in two directions perpendicular to each other (mainly the longitudinal direction and the width direction). Whether the film has biaxial orientation can be specified, for example, by checking whether there is a difference in the surface orientation coefficient in two perpendicular directions. The longitudinal direction refers to the direction in which the film travels in the film manufacturing process (the winding direction of the film in the state of the film roll), and the width direction refers to the direction parallel to the film surface and perpendicular to the longitudinal direction.

[0018] The biaxially oriented polypropylene film of the present invention contains a polypropylene resin as the main component. The "main component" means a component contained in an amount of more than 50% by mass and 100% by mass or less in 100% by mass of all components constituting the film. The polypropylene resin refers to a resin containing more than 50 mol% and 100 mol% or less of propylene units when all the constituent units constituting the resin are 100 mol%.

[0019] The biaxially oriented polypropylene film of the present invention preferably contains a low-MFR polypropylene resin (A) as the main component and further contains a high-MFR polypropylene resin (B) having a higher melt flow index (MFR) at 230°C than the low-MFR polypropylene resin (A). Details of the low-MFR polypropylene resin (A) and the high-MFR polypropylene resin (B) will be described later. Here, MFR means the melt flow index, which can be measured under the conditions of 230°C and 2.16 kg in accordance with JIS K 7210-1 (2014) (unit: g / 10 min).

[0020] Here, "primarily composed of low-MFR polypropylene resin (A)" means that the biaxially oriented polypropylene film contains more than 75% by mass but less than 85% by mass of the low-MFR polypropylene resin (A) described later, more preferably 77% by mass or more but less than 83% by mass, and even more preferably 79% by mass or more but 81% by mass or less. By adopting this configuration, it becomes easier to improve the dielectric strength and control the surface properties of the film, and the resulting biaxially oriented polypropylene film has excellent dielectric strength and dimensional stability at high temperatures.

[0021] The biaxially oriented polypropylene film of the present invention may also contain various additives, such as nucleating agents, antioxidants, heat stabilizers, antistatic agents, antiblocking agents, fillers, viscosity modifiers, and anticoloring agents, as long as the objectives of the present invention are not impaired. These components may be used individually or in combination of multiple types.

[0022] Among the additives mentioned above, the selection of antioxidants and adjustment of their content are preferably carried out with consideration for the long-term heat resistance of the biaxially oriented polypropylene film. More specifically, it is preferable to use sterically hindered phenolic antioxidants alone or in combination of several types, with at least one of them being a high molecular weight type with a molecular weight of 500 or more. For example, it is preferable to use 2,6-di-t-butyl-p-cresol (BHT: molecular weight 220.4), BASF Japan's "Irganox" (registered trademark) 1330 (molecular weight 775.2), BASF Japan's "Irganox" (registered trademark) 1010 (molecular weight 1177.7), etc., alone or in combination. The total content of the above additives is preferably 0.01% to 1.0% by mass, more preferably 0.1% to 0.9% by mass, even more preferably 0.15% to 0.6% by mass, and particularly preferably 0.15% to 0.6% by mass, based on 100% by mass of all components of the biaxially oriented polypropylene film.

[0023] The following describes the polypropylene resin used in the biaxially oriented polypropylene film of the present invention. The low MFR polypropylene resin (A) is an isotactic polypropylene resin with an MFR of 3.0 g / 10 min or more and 5.0 g / 10 min or less. Such polypropylene resins are also known as polypropylene resins commonly used in film capacitor applications.

[0024] The low-MFR polypropylene resin (A) preferably has a cold xylene-soluble portion (CXS) of 0.5% to 4.0% by mass and a mesopentad fraction (mmmm) of 0.960 to 0.995. Specific examples of materials that can be suitably used as the low-MFR polypropylene resin (A) include Borclean™ (HC300BF, HC312BF, HC314BF, HC316BF, HC318BF, HC320BF, etc.) manufactured by Borealis.

[0025] The CXS of the low-MFR polypropylene resin (A) used in the biaxially oriented polypropylene film of the present invention 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 particularly preferably 0.5% by mass or more and 2.0% by mass or less. CXS refers to the polypropylene components dissolved in xylene when the resin is completely dissolved in xylene at 135°C and then precipitated at 20°C. In other words, CXS is considered to be a component that is difficult to crystallize due to reasons such as stereoregularity and low molecular weight.

[0026] When the CXS of the low-MFR polypropylene resin (A) is 4.0% by mass or less, the heat resistance and dielectric strength in high-temperature environments of the biaxially oriented polypropylene film can be improved. When such a biaxially oriented polypropylene film is used in a film capacitor, the relaxation of molecular chains in high-temperature environments is suppressed, improving thermal dimensional stability, and thus reducing leakage current associated with the shrinkage of the biaxially oriented polypropylene film. Furthermore, when the CXS of the low-MFR polypropylene resin (A) is 0.5% by mass or more, the deterioration of stretchability during film formation can be reduced when manufacturing the biaxially oriented polypropylene film.

[0027] 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, followed by filtration. Next, the polypropylene components dissolved in the filtrate are quantified by liquid chromatography, and the CXS is calculated from the following formula (1), with X0 (g) being the mass of polypropylene resin before dissolution in boiling xylene and X (g) being the mass of polypropylene components dissolved in the filtrate. Formula (1): CXS(mass%)=(X / X0)×100.

[0028] The mesopentad fraction (mmmm) of the low-MFR 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 even more preferably 0.970 or more and 0.995 or less. The mesopentad fraction (mmmm) is an index that indicates the stereoregularity of the crystalline phase of polypropylene, measured by nuclear magnetic resonance (NMR) spectroscopy. Generally, a higher value indicates higher crystallinity and melting point, and superior dielectric strength at high temperatures.

[0029] When the mesopentad fraction of low-MFR polypropylene resin (A) is 0.960 or higher, it is easier to maintain voltage resistance and dimensional stability in high-temperature environments when used as a biaxially oriented polypropylene film. On the other hand, when the mesopentad fraction of low-MFR polypropylene resin (A) is 0.995 or lower, film-forming properties are improved, and a more stable biaxially oriented polypropylene film can be obtained. The mesopentad fraction is determined by dissolving the polypropylene resin sample in a solvent. 13 The measurement can be performed using 1C-NMR, and the detailed measurement method and conditions will be described later.

[0030] The MFR of low MFR polypropylene resin (A), when measured in accordance with JIS K 7210-1 (2014) at 230°C and 2.16 kg, is preferably 3.0 g / 10 min to 5.0 g / 10 min, more preferably 3.3 g / 10 min to 4.7 g / 10 min, and even more preferably 3.6 g / 10 min to 4.4 g / 10 min.

[0031] By setting the MFR of the low-MFR polypropylene resin (A) to 3.0 g / 10 min or higher, film-forming properties are improved, and a more stable biaxially oriented polypropylene film can be obtained. On the other hand, by setting the MFR of the low-MFR polypropylene resin (A) to 5.0 g / 10 min or lower, the dimensional stability and dielectric strength characteristics under high-temperature environments of the resulting biaxially oriented polypropylene film can be easily maintained.

[0032] Next, the high-MFR polypropylene resin (B) will be described. In the present invention, the high-MFR polypropylene resin (B) is a polypropylene resin having an MFR of 5.5 g / 10 min or more and 7.5 g / 10 min or less. Specific examples of polypropylene resins that can be suitably used as the high-MFR polypropylene resin (B) in the present invention include "Wintec" (registered trademark) WFX4M, WXK1233, WFX4TA, WFW4M manufactured by Nippon Polypropylene Co., Ltd., and "Novatec" (registered trademark) FW4BAT, FB3B, etc. manufactured by Nippon Polypropylene Co., Ltd. The MFR of the high-MFR polypropylene resin (B), when measured under conditions of 230°C and 2.16 kg in accordance with JIS K 7210-1 (2014), is preferably 5.7 g / 10 min or more and 7.3 g / 10 min or less, and more preferably 5.9 g / 10 min or more and 7.1 g / 10 min or less.

[0033] When the MFR of high-MFR polypropylene resin (B) is 5.5 g / 10 min or higher, the fluidity in the molten state improves, making it easier to control the surface properties. In addition, it is possible to suppress the deterioration of film thickness unevenness and film tearing, thus enabling the acquisition of more stable biaxially oriented polypropylene films. On the other hand, when the MFR of high-MFR polypropylene resin (B) is 7.5 g / 10 min or lower, the fluidity does not become too high, which reduces unevenness at the sheet edges during sheet formation and improves film formation performance.

[0034] When a resin composition obtained by dry-blending a low-MFR polypropylene resin (A) and a high-MFR polypropylene resin (B) is melt-extruded using an extruder and supplied to a T-die, the molten resin composition expands in the width direction as the grooves of the T-die widen. At this time, stress is relieved as the molten resin composition expands in the width direction. However, as described above, when the molten resin composition contains polypropylene resins with different MFRs, the shear rate of the polypropylene resin with the relatively higher MFR is usually faster, so the polypropylene resin with the higher MFR expands more easily in the width direction compared to the one with the lower MFR. Therefore, the high-MFR polypropylene resin (B) is discharged from the die in a state that is more stretched in the width direction than the low-MFR polypropylene resin (A). In a molten resin composition discharged to the die in this state, the degree and size of spherulite formation during sheet molding and sheet stretching, which will be described later, changes, and the formation of crater-like protrusions is also affected.

[0035] Specifically, in an embodiment containing both low-MFR polypropylene resin (A) and high-MFR polypropylene resin (B), this mechanism forms crater-like protrusions on the surface of the biaxially oriented polypropylene film, with their long axes aligned in the width direction. As a result, a biaxially oriented polypropylene film having a surface (surface A) with an aspect ratio (Str) of 0.15 or more and less than 0.30 can be easily obtained.

[0036] From the viewpoint of controlling the aspect ratio (Str) of the surface properties of the resulting biaxially oriented polypropylene film to an appropriate range, the content of high MFR polypropylene resin (B) is preferably 15% by mass or more and less than 25% by mass, more preferably 17% by mass or more and 23% by mass or less, and even more preferably 19% by mass or more and 21% by mass or less, when the total components constituting the film are considered as 100% by mass.

[0037] When the content of high-MFR polypropylene resin (B) is 15% by mass or more, the high-MFR polypropylene resin (B) is appropriately dispersed on the sheet surface when the molten polypropylene resin is formed into a sheet, making it easier to control the size and shape of the spherulites. As a result, it becomes easier to obtain a biaxially oriented polypropylene film with desirable surface properties, and the stretchability during film formation is also improved. On the other hand, when the content of high-MFR polypropylene resin (B) is 25% by mass or less, the high-MFR polypropylene resin (B) does not become excessive, making it easier to control the size and number of spherulites within an appropriate range. In addition, the fluidity of the molten resin composition becomes appropriate, and the stability during sheet formation is also improved. In other words, by keeping the content of high-MFR polypropylene resin (B) within the above range, the size and shape of the polypropylene resin spherulites generated in the cooling process of the melt-extruded resin sheet are optimized, making it easier to obtain a biaxially oriented polypropylene film having side A (details described later).

[0038] While it is possible to determine whether a biaxially oriented polypropylene film contains high MFR polypropylene resin (B) by measuring the polypropylene resin in the film, separation is not easy when the film contains multiple polypropylene resins. Therefore, it may also be possible to determine this by measuring the MFR of the polypropylene resin used as a raw material during the production of the biaxially oriented polypropylene film.

[0039] When the biaxially oriented polypropylene film of the present invention is used as a dielectric in a film capacitor, 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.

[0040] From the viewpoint of achieving both processability and dielectric strength, it is important that at least one side of the biaxially oriented polypropylene film of the present invention is a surface (hereinafter referred to as "Surface A") with an aspect ratio (Str) of 0.15 or more and less than 0.30. From the above viewpoint, it is more preferable that the aspect ratio of the surface is 0.16 or more and 0.28 or less, even more preferable that it is 0.17 or more and 0.26 or less, and particularly preferable that it is 0.17 or more and 0.23 or less. Here, "at least one side" means one side or both sides, and in the biaxially oriented polypropylene film of the present invention, as long as at least one side has Surface A, the other side is not particularly limited, but it is preferable that both sides be Surface A from the viewpoint that the side on which a metal film is formed by vapor deposition or the like can be freely selected. The same applies to the preferred range of the aspect ratio (Str) of the surface.

[0041] The surface texture aspect ratio (Str) is one of the three-dimensional parameters of surface texture as defined in ISO 25178-2 (2012), and is used as an indicator of the isotropy of surface texture. More specifically, the surface texture aspect ratio (Str) is the value obtained by dividing the horizontal distance in the direction in which the autocorrelation function specified in ISO 25178-2 (2012) decays fastest to a specific value (correlation value s: s=0.2 in this invention) by the horizontal distance in the direction in which it decays slowest to the correlation value s. That is, the surface texture aspect ratio (Str) is a value between 0 and 1. If the surface texture aspect ratio (Str) is 0 or close to 0, it indicates that the surface texture has a highly anisotropic, regular pattern, while if the surface texture aspect ratio (Str) is 1 or close to 1, it indicates that the surface texture has a highly isotropic, random pattern. Details regarding three-dimensional parameters, including the aspect ratio (Str) of surface texture, can be found, for example, on page 3 of "New 3D Parameters and Filtration Techniques for Surface Metrology" by Francois Blateyron, 2006.

[0042] The device used to measure the aspect ratio (Str) of the surface texture is not particularly limited, but for example, the non-contact surface and layer cross-sectional shape measurement system "VertScan" (registered trademark) 2.0 manufactured by Ryoka Systems Co., Ltd. can be used. Details of the method for measuring the aspect ratio (Str) of the surface texture using this device will be described later.

[0043] A surface aspect ratio (Str) of 0.15 or higher indicates that the surface texture of the biaxially oriented polypropylene film has a more uniform pattern, meaning that the major axes of the elliptical crater-like protrusions formed on the film surface are aligned in a specific direction (e.g., the width direction). This configuration results in a more uniform air content across the width direction when the biaxially oriented polypropylene film is made into a roll or reel. Therefore, wrinkles and winding misalignment are reduced during the conveying and winding processes when the biaxially oriented polypropylene film is manufactured, improving the quality of the film roll. Furthermore, when a biaxially oriented polypropylene film is made in this configuration, winding misalignment and winding crushing of metal film laminates with metals such as aluminum deposited on them can be suppressed during the winding and pressing processes in the formation of film capacitor elements, thereby improving processability.

[0044] On the other hand, if the aspect ratio (Str) of the surface texture is set to 0.30 or less, a moderate degree of unevenness remains in the directionality of the elliptical crater-like protrusions. Therefore, when a metal film laminated film, obtained by depositing metal onto the biaxially oriented polypropylene film of the present invention, is laminated or wound to form a film capacitor, the amount of air between the film layers does not become too large, and an excessive increase in the safety performance of the resulting film capacitor can be reduced. As a result, the film capacitor has a longer lifespan.

[0045] The method for forming a surface (hereinafter referred to as "surface A") having an aspect ratio (Str) of 0.15 or more and less than 0.30, and the method for achieving the above preferred range, are not particularly limited. For example, this could involve using the aforementioned low-MFR polypropylene resin (A) and high-MFR polypropylene resin (B), and setting specific conditions for the temperature of the casting drum in the film formation process, the temperature in the transverse stretching process, the stretching ratio, etc., as described later.

[0046] It is important that the biaxially oriented polypropylene film of the present invention consists of a single layer with a thickness (t) of 6.0 μm or more and less than 12.0 μm. Preferably, the thickness (t) of the biaxially oriented polypropylene film is 6.5 μm or more and 11.0 μm or less, and more preferably 7.0 μm or more and 10.0 μm or less. By setting the thickness (t) to 6.0 μm or more, the mechanical strength and stiffness of the biaxially oriented polypropylene film are improved, and wrinkles can be prevented without applying excessive transport tension during the transport and winding processes during film formation. Furthermore, by setting the thickness (t) to less than 12.0 μm, the capacitance per unit volume can be increased when the biaxially oriented polypropylene film is used as a dielectric in a film capacitor. In addition, by using a single layer of biaxially oriented polypropylene film, a decrease in withstand voltage performance due to voids that occur when air is trapped at the layer interface when multiple layers are used can be prevented. The thickness shall be measured by the micrometer method in accordance with JIS C 2330 (2014).

[0047] The method for forming a single layer having a thickness (t) of 6.0 μm or more and less than 12.0 μm or within the above preferred range is not particularly limited, but for example, it can be adjusted by using only one type of resin composition to constitute the film and adjusting the slit width of the T-die, the amount of material discharged from the T-die, the rotation speed of the cast drum, the product of the stretching ratio, etc. These methods may be used in appropriate combinations.

[0048] The biaxially oriented polypropylene film of the present invention preferably has a kurtosis (Sku) of 5.0 or more and less than 10 on surface A, more preferably 5.2 or more and 9.0 or less, and even more preferably 5.4 or more and 9.0 or less (hereinafter, kurtosis (Sku) may simply be referred to as Sku). Sku is one of the three-dimensional parameters of surface properties as defined in ISO 25178-2 (2012), and is calculated by normalizing the fourth-power mean of the height from the mean plane of the defined area. It represents kurtosis, which is a measure of surface sharpness, and expresses the sharpness of the height distribution. Generally, if Sku is greater than 3.0, it indicates that the height distribution is pointed, and that the protrusions on the film surface are sharp. Conversely, if Sku is less than 3.0, the height distribution of the surface irregularities is flattened, and it indicates that the protrusions on the film surface are blunt. Also, if Sku = 3.0, it indicates that the surface irregularities have an intermediate protrusion shape that is neither sharp nor blunt. If both sides are surface A, then if the Sku on one side is 5.0 or more and less than 10, it will be considered that "the Sku on surface A is 5.0 or more and less than 10," but it is preferable that this requirement is met on both sides (the same applies to the preferred range). The same treatment will be applied to other surface parameters related to surface A.

[0049] In the biaxially oriented polypropylene film of the present invention, setting the Sku of side A to 5.0 or higher results in sharper surface protrusions, which reduces the contact area between films and between the film and the roll, improving transportability and winding during film formation and processing into film capacitors. On the other hand, setting the Sku of side A to less than 10 prevents the surface protrusions from becoming too sharp, improving strength and preventing protrusion crushing.

[0050] The method for setting the Sku of side A to 5.0 or more and less than 10 or within the preferred range described above is not particularly limited, but examples include methods that set the temperature of the cast drum in the film manufacturing process, the preheating temperature of the longitudinal stretching process, the preheating temperature of the transverse stretching process, etc., to the conditions described later. These methods may be used in appropriate combinations.

[0051] The biaxially oriented polypropylene film of the present invention preferably has an average height (Spk) of the protruding peaks on surface A of 50 nm or more and less than 100 nm, more preferably 58 nm or more and 92 nm or less, and even more preferably 66 nm or more and 86 nm or less (hereinafter, the average height (Spk) of the protruding peaks may simply be referred to as Spk). By setting the Spk of surface A to 50 nm or more, the surface protrusions become of an appropriate height, and the amount of air is appropriately maintained when winding the biaxially oriented polypropylene film or a metal film laminated film formed thereon, thereby preventing adhesion between films and making it easier to prevent short-circuit failure when using film capacitors that use these as dielectrics. On the other hand, by setting the Spk of surface A to less than 100 nm, it is possible to prevent an excessive increase in air between film layers due to excessively high surface protrusion heights.

[0052] The method for setting the Spk of surface A to 50 nm or more and less than 100 nm or within the above preferred range is not particularly limited, but examples include setting specific conditions for the temperature of the cast drum in the film formation process described later, the preheating temperature in the longitudinal stretching process, the preheating temperature in the transverse stretching process, etc. These methods may be used in appropriate combinations.

[0053] The biaxially oriented polypropylene film of the present invention is preferably obtained by molding a polypropylene resin composition consisting of the low-MFR polypropylene resin (A) and the high-MFR polypropylene resin (B) described above 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.

[0054] Next, the method for producing the biaxially oriented polypropylene film of the present invention will be described below with specific examples, but the biaxially oriented polypropylene film of the present invention is not necessarily limited to those obtained by the following method.

[0055] First, the low-MFR polypropylene resin (A) and high-MFR polypropylene resin (B) described above are dry-blended to the aforementioned proportions and supplied to a single-screw melt extruder, where melt extrusion is performed at 220-260°C. Next, foreign matter and modified polymers are removed from the molten resin composition using a filter installed in the middle of the polymer tube. The molten resin composition, formed into a sheet, is then discharged from a T-die onto a cast drum and cooled and solidified to form a cast sheet, which is then cooled with a cooling roll.

[0056] The temperature of the casting drum is preferably 50°C to 70°C, more preferably 60°C to 69°C, and even more preferably 65°C to 68°C, from the viewpoint of appropriately generating β-crystals and spherulites. By setting the casting drum temperature to 50°C or higher, sufficient β-crystals can be formed in the cast sheet, thereby reducing the occurrence of meandering in the film transport process during film formation and processing, and the deterioration of the winding shape of the film roll. On the other hand, by setting the casting drum temperature to 70°C or lower, the excessive formation of β-crystals in the cast sheet is prevented, making it easier to control the surface properties and maintaining the slipperiness of the transported film and the resulting biaxially oriented polypropylene film. Therefore, the occurrence of wrinkles in the film transport process during film formation and processing, and the deterioration of the winding shape of the film roll can be prevented.

[0057] Furthermore, after the molten sheet discharged from the T-die lands on the cast drum, the time it remains in contact with the cast drum is preferably 3.0 seconds or more and 5.0 seconds or less, and more preferably 3.2 seconds or more and 4.0 seconds or less. If the contact time is 3.0 seconds or more, the molten sheet solidifies sufficiently, improving the sheet's formation and reducing breakage during the subsequent stretching process. On the other hand, if the contact time is 5.0 seconds or less, excessive contact between the cast sheet and the cast drum is prevented, improving the release properties of the cast sheet from the cast drum.

[0058] 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. Therefore, the occurrence of wrinkles and deterioration of the winding shape of the film roll are reduced in 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, and the occurrence of meandering and deterioration of the winding shape of the film roll are reduced in the film transport process during film formation and processing.

[0059] From the viewpoint of suitably controlling the surface of the resulting biaxially oriented polypropylene film, it is preferable to cool the cast sheet solidified by the cast drum with a cooling roll. From the above viewpoint, the temperature of the cooling roll is preferably 50°C to 100°C, more preferably 52°C to 85°C, and even more preferably 55°C to 70°C. By setting the temperature of the cooling roll to 50°C or higher, the size and number of spherulites formed in the sheet during the cooling process do not become too small or too few, and the parameters of the film surface can be controlled within an appropriate range while ensuring the stability of the film formation. Furthermore, by setting the temperature of the cooling roll to 100°C or lower, the size and number of spherulites formed in the sheet during the cooling process are suppressed, and the parameters of the film surface can be controlled within an appropriate range while ensuring the stability of the film formation.

[0060] Next, the cooled cast sheet is biaxially stretched. First, the cast sheet is preheated for longitudinal stretching. Methods of preheating include using a temperature-controlled rotary roll or using a hot air oven. For example, when using a temperature-controlled rotary roll, the cast sheet is preheated by passing it through a roll maintained at 125-155°C. After that, heat may be locally applied using a radiation heater to reduce film breakage during longitudinal stretching and control the surface structure of the cast sheet. The roll temperature in the preheating process for longitudinal stretching is preferably 125°C to 155°C, more preferably 127°C to 153°C, and even more preferably 129°C to 151°C. By setting the preheating temperature for longitudinal stretching to 125°C or higher, an appropriate amount of heat can be applied to the sheet, thereby suppressing film breakage during longitudinal stretching. Also, by setting it to 155°C or lower, it is possible to prevent the entire cast sheet from melting and tearing, making it easier to maintain mass production. Furthermore, it is also preferable to adjust the preheating temperature in multiple stages within the above range.

[0061] Next, the cast sheet is stretched in the longitudinal direction in the longitudinal stretching process. The cast sheet, which has passed through the preheating process, is passed through rolls controlled to a temperature of 150°C to 170°C, and stretched in the longitudinal direction at a predetermined stretching speed and stretching ratio due to the difference in peripheral speed between the rolls. The stretching ratio in the longitudinal direction 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 properties of the resulting biaxially oriented polypropylene film become more uniform, and the dielectric strength at high temperatures is 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.

[0062] Next, the widthwise ends of the uniaxially oriented film obtained by longitudinal stretching are gripped with clips, and the film is preheated in a tenter-type stretcher with the temperature controlled to 165°C or higher and less than 185°C, and then stretched in the widthwise direction with the temperature controlled to 150°C or higher and 170°C or lower. The temperature when preheating the uniaxially oriented film is preferably 165°C or higher and 185°C or lower, more preferably 167°C or higher and 183°C or lower, and even more preferably 169°C or higher and 181°C or lower. Setting the temperature to 165°C or higher ensures that the amount of heat supplied to the film is sufficient, preventing cracks on the film surface due to insufficient heat during stretching in the widthwise direction and maintaining appropriate surface properties. Furthermore, setting the temperature to 185°C or lower prevents changes in surface properties due to melting of the film surface that can occur when there is excessive heat, and prevents the aspect ratio (Str) of the surface properties from becoming excessively low. Furthermore, the temperature during stretching in the width direction is preferably 150°C to 170°C, more preferably 152°C to 168°C, and even more preferably 154°C to 166°C. Setting the stretching temperature in the width direction to 150°C or higher ensures an appropriate film temperature during stretching, reducing film breakage. Additionally, setting the temperature to 170°C or lower ensures appropriate residual stress in the film, resulting in a dimensional change rate suitable for processing into film capacitors.

[0063] The stretching ratio in the width direction (lateral stretching ratio) is preferably 5.0 times or more and 15 times or less, more preferably 7.0 times or more and 13 times or less, and even more preferably 9.0 times or more and 11 times or less. By setting the lateral stretching ratio to 5.0 times or more, the protrusions formed on the film surface can be stretched further in the width direction, making it easier to control the aspect ratio (Str) of the surface properties within an appropriate range. On the other hand, by setting the lateral stretching ratio to 15.0 times or less, the protrusions are not stretched excessively, so they can be controlled to an appropriate shape. In addition, film breakage due to excessive stretching is also greatly reduced.

[0064] Subsequently, the biaxially stretched film is heat-set at a temperature of 120-160°C while being relaxed in the width direction by a rate of 7.0-12.0%. The relaxation rate in the width direction is more preferably 8.0-11.0%, and even more preferably 9.0-10.0%. By setting the relaxation rate in the width direction to 7.0% or more, residual stress within the film can be reduced, and dimensional changes of the film over time can be suppressed. Furthermore, by setting the relaxation rate to 12.0% or less, tears and scratches caused by the film relaxing at the exit of the tenter-type stretcher, and wrinkles during film transport can be suppressed. In addition, by setting the relaxation rate within the above range, the film does not shrink too much, making it easier to control the aspect ratio (Str) of the surface properties within an appropriate range.

[0065] Next, the biaxially oriented film is slowly cooled to room temperature as needed, subjected to corona discharge treatment in air, nitrogen, carbon dioxide, or a mixture thereof, the edges of the film held with clips are cut and removed, and the film with the edges removed is wound 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, wound onto a core as a film roll, and a roll of the biaxially oriented polypropylene film of the present invention is obtained. These treatments may be performed on both sides or on one side. The winding length and core diameter can be appropriately adjusted according to the thickness and application of the biaxially oriented polypropylene film, and are not particularly limited, but are preferably, for example, 2,000 m to 100,000 m and 2 inches to 16 inches, respectively.

[0066] 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.

[0067] Next, a metal film laminated film using the biaxially oriented polypropylene 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. The metal film may be formed on any side of the biaxially oriented polypropylene film, but if only one side is side A, it is preferable to form it on side A.

[0068] Next, the film capacitor of the present invention will be described. The film capacitor of the present invention has a structure in which the metal film laminate of the present invention is laminated or wound. Below, an example of a method for manufacturing a wound film capacitor will be described. First, aluminum is vacuum deposited on one side of the biaxially oriented polypropylene film of the present invention. At that time, the aluminum is deposited in a stripe shape having a margin portion running in the longitudinal direction of the film. Next, a blade is made to slit the center of each deposited portion and the center of each margin portion on the surface, and a tape-shaped winding reel with a margin on one side of the surface is produced. Two tape-shaped winding reels, one with a left margin and one with a right margin, are overlapped so that the deposited portion extends beyond the margin portion in the width direction and wound together to obtain a wound body. After heat treatment of the wound 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 film capacitor.

[0069] 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]

[0070] The present invention will be described in detail below with reference to examples, but the present invention is not limited to the embodiments described below. The properties were measured and evaluated by the following methods, and the following raw materials were used.

[0071] [Measurement and evaluation methods] (1) 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).

[0072] A. Measurement conditions Equipment: Bruker DRX-500 Nucleus for measurement: 13 C nucleus (resonance frequency: 125.8MHz) Measured concentration: 10% by mass Solvent: Benzene / deuterated orthodichlorobenzene = 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.

[0073] 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 automatic fitting was then 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.

[0074] (2) Melt Flow Index (MFR) (Unit: g / 10min) Measurements were taken at 230°C and with a weight of 2.16 kg, in accordance with JIS K 7210-1 (2014).

[0075] (3) 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. After that, 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 polypropylene resin before dissolution in boiling xylene, and X (g) be the mass of polypropylene components dissolved in the filtrate. CXS was calculated from the following formula (1). Formula (1): CXS(mass%)=(X / X0)×100.

[0076] (4) Surface texture aspect ratio (Str, unit: -), sharpness kurtosis (Sku, unit: -), average height of protruding peaks (Spk, unit: nm) Measurements were performed using the non-contact surface and layer cross-sectional shape measurement system "VertScan" (registered trademark) 2.0 (model: R3300GL-Lite-AC) manufactured by Ryoka Systems Co., Ltd. The specific measurement procedure is as follows. First, a biaxially oriented polypropylene film sample was taken from a film roll with a width of 0.82 m in a rectangular shape of 100 mm (longitudinal direction) x 0.82 m (width direction). This sample was divided into 30 equal parts parallel to the longitudinal direction to obtain 30 test pieces of 100 mm x 27.3 mm. Next, the aspect ratio (Str), kurtosis (Sku), and average height of protruding peaks (Spk) of the surface properties of each obtained test piece were measured 10 times at arbitrarily changed positions under the following conditions, and the average values ​​were calculated. Subsequently, the same measurements and calculation of average values ​​were performed for all 30 divided test pieces. The average values ​​of each test specimen were further averaged to determine the "aspect ratio of surface properties (Str)," "curtosis (Sku)," and "average height of protruding peaks (Spk)" of the biaxially oriented polypropylene film.

[0077] A. Measurement conditions The detailed conditions for each measurement were as follows: CCD camera: SONY HR-57 1 / 2 Objective lens: 10X Lens tube: 0.5X BODY Wavelength filter: 530 white Measurement mode: Wave Field of view size: 640 x 480 Scan range: (Start) 5μm, (Stop) -5μm Correlation value s: 0.2 Measurement field of view (field of view area): 0.939 mm (vertical) x 1.252 mm (horizontal) = 1.176 μm 2 .

[0078] B. Fixing the test specimen A dedicated sample holder was used to secure the test specimen during measurement. The sample holder consisted of two detachable metal plates with a circular hole in the center. The test specimen was sandwiched between these plates, ensuring there were no wrinkles, and measurements were taken at the central circular portion of the specimen. The film and sample holder were positioned so that the longitudinal direction of the film roll aligned with the vertical direction of the measurement field of view.

[0079] C. Analysis method The data obtained from the above measurements was analyzed using the image analysis software VS-Viewer of "VertScan" (registered trademark) 2.0. First, noise was removed using a median filter (5x5), and then undulation components were removed using a Gaussian filter with a cutoff value of 250 μm. Next, the "Aspect Ratio (Str)," "Curtosis (Sku)," and "Average Height of Protruding Peaks (Spk)" as defined in ISO25178-2 (2012) were calculated using the "ISOPara" function. In the "ISOPara" function, the S-Filter was set to 6.0 μm.

[0080] (5) Thickness (unit: μm) The thickness of the biaxially oriented polypropylene film was measured using the micrometer method in accordance with JIS C 2330 (2014).

[0081] (6) 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. 〇: Film tears occurred 1 to 3 times in 48 hours. △: Film tears occurred 4 to 6 times in 48 hours. ×: More than 7 film tears occurred within 48 hours.

[0082] (7) 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 79.0 mm width in the deposited area and 1.0 mm width in the margin). Next, slits were made by cutting into the center of each deposited area and each margin, creating 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. These wound materials were visually inspected, and any with wrinkles or distortions in appearance or shape were deemed defective. 200 wound materials were produced using the same method, and the same evaluation was repeated. The processability of the wound materials was then evaluated according to the following criteria. ◎: No defective products 〇: Less than 1 defective product △: 2 or more defective items and 3 or less defective items ×: 4 or more defective items.

[0083] (8) Characterization of film capacitors A winding with a capacitance of 120 μF was obtained by the method described in (7). 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 characteristics of 15 film capacitors were evaluated using the following procedure. First, the capacitance (C0) was measured at room temperature. Then, a voltage of 2100 VDC was applied to the film capacitors at a temperature of 85°C for 2000 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 (2). The number of capacitors with dielectric breakdown was also checked. The capacitance was measured using an LCR high-tester 3522-50 manufactured by HIOKI E.E. CORPORATION. Equation (2): 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 2% ○: ΔC is between 2% and less than 3% △: ΔC is between 3% and less than 5% ×: ΔC is 5% or more.

[0084] [Raw materials] (1) Resin Low MFR polypropylene resin (A): Borclean (trademark) HC320BF, manufactured by Borealis, is a linear polypropylene resin with a mesopentad fraction of 0.980, a CXS of 1.2% by mass, and an MFR of 3.7 g / 10 min. High MFR polypropylene resin (B), etc.: (Used in Examples 1-7, Comparative Examples 1, 4-6) "Wintech" (registered trademark) WFX4M, manufactured by Nippon Polypropylene Co., Ltd., is a high-MFR polypropylene resin with an MFR of 7.0g / 10min. (Used in Comparative Example 2) "Wintech" (registered trademark) WFX6 polypropylene resin manufactured by Nippon Polypropylene Co., Ltd. has an MFR of 2.0g / 10min. (Used in Comparative Example 3) "Wintech" (registered trademark) WMX03, manufactured by Nippon Polypropylene Co., Ltd., is a polypropylene resin with an MFR of 25.0 g / 10 min. (Used in Examples 8-10) Novatec (registered trademark) FB3B is a high-MFR polypropylene resin manufactured by Nippon Polypropylene Co., Ltd., with an MFR of 7.5g / 10min.

[0085] (2) Antioxidants Antioxidant 1: BASF Japan's "Irganox" (registered trademark) 1010 Antioxidant 2: 2,6-di-t-butyl-p-cresol (BHT).

[0086] (Example 1) A polypropylene resin mixture, obtained by mixing low-MFR polypropylene resin (A) and high-MFR polypropylene resin (B) in a mass ratio of 80:20, antioxidant 1, and antioxidant 2 were dry-blended in a mass ratio of 99.5:0.4:0.1 and supplied to a single-screw melt extruder, where melt extrusion was performed at 250°C. Afterward, foreign matter was removed from the extruded molten polypropylene resin composition using a 25 μm cut sintered filter, and the composition was further extruded in a sheet form through a T-type slit die. The sheet-like molten polypropylene resin composition was then solidified by adhering it to a cast drum maintained at 66°C using an air knife with an air temperature of 80°C, thereby obtaining a cast sheet. Next, the cast sheet was cooled on a cooling roll maintained at 55°C to obtain a cooled sheet. The time the sheet-like molten polypropylene resin composition was in contact with the cast drum and the cooling roll was 3.5 seconds and 2.0 seconds, respectively (hereinafter, the side in contact with the cast drum is referred to as the drum surface (D surface), and the side not in contact is referred to as the non-drum surface (ND surface)).

[0087] Next, in the preheating process before longitudinal stretching, the cast film was preheated by passing it through rolls maintained at 130°C and then at 147°C. Furthermore, the cast sheet was heated for 0.4 seconds on the roll maintained at 147°C using a radiation heater. After that, the cast sheet that had passed through the preheating process before longitudinal stretching was stretched in the longitudinal direction at a stretching ratio of 4.8 times on a longitudinal stretching roll at a temperature of 150°C to obtain a uniaxially oriented film. Furthermore, the uniaxially oriented film was guided to a tenter by gripping both ends in the width direction with clips, and stretched in the width direction under the conditions of a preheating temperature of 172°C, a stretching temperature of 162°C, and a stretching ratio of 9.5 times. Subsequently, a 9.8% relaxation treatment was performed in the width direction at a temperature of 150°C, and it was slowly cooled to room temperature, with a load of 25 W·min / m on the D-side. 2 Corona 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 the film was wound up using a winding machine to obtain an intermediate product with a width of 6.0 m. Next, the film was unwound from the intermediate product, and the unwound film was slit parallel to the longitudinal direction using a slitter so that the width (length in the widthwise direction) was 0.82 m. The biaxially oriented polypropylene film was wound onto a core to obtain a biaxially oriented polypropylene film roll with a thickness of 7.8 μm. The physical properties of the obtained biaxially oriented polypropylene film and the results of each evaluation are shown in Table 1.

[0088] (Examples 2-10, Comparative Examples 1-6) A biaxially oriented polypropylene film was obtained in the same manner as in Example 1, except that the composition of the polypropylene resin mixture and the conditions for the casting, longitudinal stretching, and transverse stretching processes 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.

[0089] [Table 1]

[0090] However, regarding high-MFR polypropylene resin (B), the resins used in Comparative Examples 2 and 3 do not qualify as high-MFR polypropylene resin (B) in terms of MFR values. Furthermore, while antioxidants are included in the production of the biaxially oriented polypropylene films in each example and comparative example, the amount is trace and partially disappears during the manufacturing process, thus indicating the composition of the polypropylene resin mixture (which is substantially equivalent to the composition of the resulting biaxially oriented polypropylene film). [Industrial applicability]

[0091] The present invention provides a biaxially oriented polypropylene film that improves windability after aluminum deposition, processability when slit to form a wound body for film capacitors, and the element shape and performance of film capacitors. Therefore, by using the biaxially oriented polypropylene film of the present invention as a dielectric for film capacitors, the film capacitors exhibit high safety even in high-temperature and high-voltage environments, and their lifespan is also improved. For this reason, the biaxially oriented polypropylene film of the present invention can be suitably used in applications where the operating environment tends to be high-temperature, such as electric railway applications including Shinkansen bullet trains, automotive applications (including hybrid cars and electric vehicles), solar power generation, wind power generation, etc.

Claims

1. A biaxially oriented polypropylene film characterized by comprising a single layer having at least one side that is a surface with an aspect ratio (Str) of 0.15 or more and less than 0.30 (hereinafter referred to as "Side A"), and having a thickness (t) of 6.0 μm or more and less than 12.0 μm.

2. The biaxially oriented polypropylene film according to claim 1, wherein the degree of sharpness (Sku) on surface A is 5.0 or more and less than 10.

3. The biaxially oriented polypropylene film according to claim 1 or 2, wherein the average height (Spk) of the protruding peaks on surface A is 50 nm or more and less than 100 nm.

4. A biaxially oriented polypropylene film according to claim 1 or 2, used as a dielectric for film capacitors.

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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