Biaxially oriented polypropylene film

Abstract

The object of the present invention is to provide a biaxially oriented polypropylene film that has excellent voltage resistance even at high temperatures and surface properties that enable uniform control of the amount of air and gap distance between film layers of a film capacitor, in order to obtain appropriate processability and safety mainly in large-capacity film capacitors. [Solution] A biaxially oriented polypropylene film characterized in that, on at least one side, the surface area ratio Sdr of the interface is 0.0004% or more and 0.0200% or less.

Description

Technical Field

[0001] The present invention relates to a biaxially oriented polypropylene film that has 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 therefore used in various applications such as packaging, tape, electrical applications including cable wrapping and capacitors.

[0003] Among these, in capacitor applications, due to its excellent high withstand voltage characteristics and low loss characteristics, it is particularly preferably used as a dielectric of a capacitor. Recently, various electrical equipment is being inverterized, and along with this, the requirements for miniaturization and large capacity of capacitors are becoming even stronger. Furthermore, especially in automotive applications (including hybrid cars and electric cars), solar power generation, and wind power generation applications, the use environment is becoming hotter (showing 85°C or higher and 125°C or lower), and the heat resistance requirements for capacitors are increasing.

[0004] Therefore, there is a demand for thinning, heat resistance improvement, and improvement of withstand voltage per thickness of the biaxially oriented polypropylene film as a dielectric, and there is also a demand for improvement of the safety of capacitors. Here, the safety of a capacitor is a property of maintaining insulation by scattering the deposited metal by the discharge energy during abnormal discharge in a metal vapor deposition capacitor having a metal vapor deposition film formed on a dielectric film as an electrode, and is an important property for preventing short circuits and breakdowns of capacitors. As a means of achieving both the withstand voltage per thickness and the safety of capacitors, it is considered effective to control the surface properties of the polypropylene film, and various studies have been conducted so far.

[0005] A method for controlling the surface properties of polypropylene films is known to utilize the crystal transition of polypropylene from the β-crystal to the α-crystal (hereinafter 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 biaxially oriented polypropylene films for capacitors (see, for example, Patent Documents 1 and 2).

[0006] As techniques that focus on the density of surface roughness and the uniformity of protrusions, methods such as adding branched polypropylene (see, for example, Patent Documents 3 and 4) and mixing polypropylenes with different molecular weights and molecular weight distributions (see, for example, Patent Document 5) have been proposed. With these methods, the spherulite size can be controlled to be small, so it is possible to form high-density protrusions with uniform height.

[0007] Furthermore, a technique focusing on the convex and concave parts of the surface roughness has been proposed, which involves high-temperature and high-pressure treatment of a longitudinally stretched sheet (see, for example, Patent Document 6). This method allows for uniform control of the height of the convex and concave parts. [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] WO2007 / 094072 publication [Patent Document 4] WO2012 / 121256 publication [Patent Document 5] Japanese Patent Publication No. 2014-231584 [Patent Document 6] Japanese Patent Publication No. 2019-172973 [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, crater-like protrusions and depressions are formed at a low density, making dielectric breakdown particularly likely in the depressions, and posing challenges to the voltage withstand characteristics under high-temperature conditions. Furthermore, when applying the methods described in Patent Documents 3, 4, and 5, which form protrusions of uniform height at high density, or the method described in Patent Document 6, which uniformly controls the height of the protrusions and depressions in the surface roughness, it is not possible to suppress localized coarse protrusions and depression shapes, and the control of the amount of air between film layers, which relates to processability during film capacitor manufacturing and voltage withstand and safety in recent high-temperature and high-voltage environments, cannot be said to be sufficient.

[0010] Therefore, the object of the present invention is to provide a biaxially oriented polypropylene film that has high processability and dielectric strength, and in order to obtain appropriate safety mainly in large-capacity capacitors, suppresses the formation of localized coarse protrusions and depressions on the film surface, and has a surface property that enables uniform control of the amount of air and gap distance between film layers of a film capacitor. [Means for solving the problem]

[0011] The above-mentioned problems can be solved as follows: The biaxially oriented polypropylene film of the present invention is characterized in that, on at least one side, the unfolded surface area ratio Sdr of the interface is 0.0004% or more and 0.0200% or less.

[0012] Furthermore, the biaxially oriented polypropylene film of the present invention can also have the following configuration in order to solve the above problems. (1) A biaxially oriented polypropylene film characterized in that, on at least one side, the surface area ratio Sdr of the interface is 0.0004% or more and 0.0200% or less. (2) The biaxially oriented polypropylene film according to (1), wherein the five-point valley region depth S5v is 70 nm or more and 1400 nm or less on at least one side. (3) A biaxially oriented polypropylene film according to (1) or (2), wherein the height S5p of the five-point peak region is 80 nm or more and 1000 nm or less on at least one side. (4) A biaxially oriented polypropylene film according to any of (1) to (3), wherein the film thickness (t) is 1.0 to 4.0 μm. [Effects of the Invention]

[0013] The present invention provides a biaxially oriented polypropylene film with high processability and voltage resistance. By using the biaxially oriented polypropylene film of the present invention as a dielectric for a capacitor, the amount of air between film layers and the interlayer distance can be uniformly controlled during capacitor processing. Therefore, when used as a capacitor, it exhibits high safety even in high-temperature and high-voltage environments, and its lifespan is also improved. [Modes for carrying out the invention]

[0014] The biaxially oriented polypropylene film, metal film laminated film, and film capacitor of the present invention will be described in more detail below. In the numerical ranges expressed using "~" below, the upper and lower limits are included within that range, and the units of the upper and lower limits are the same.

[0015] 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). 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.

[0016] Next, the polypropylene resin raw material 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), a high MFR polypropylene resin (B), a branched-chain polypropylene resin (H), and (I).

[0017] The linear polypropylene resin (A) means an isotactic polypropylene resin. This isotactic polypropylene resin is also known as a polypropylene resin commonly used in capacitor applications. The linear polypropylene resin (A) is a linear polypropylene resin, and it is preferable that the cold xylene soluble part (CXS) is 0.5% by mass or more and 4.0% by mass or less, the mesopentad fraction (mmmm) is 0.960 or more and 0.995 or less, and the melt flow index (MFR) is 0.5 g / 10 min or more and 5.0 g / 10 min or less. Specifically, examples of those that can be preferably used as the linear polypropylene resin (A) include "Borclean" (trademark) (such as HC300BF, HC318BF, etc.) manufactured by Borealis.

[0018] 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 particularly preferably 0.5% by mass or more and 2.0% by mass or less. CXS is the polypropylene component dissolved in xylene when the film 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 breakdown voltage resistance at high temperature of the film can be enhanced. Therefore, when used in a capacitor, relaxation under a high-temperature environment can be suppressed, the thermal dimensional stability can be improved, and the leakage current can be suppressed. Also, when the CXS of the linear polypropylene resin (A) is 0.5% by mass or more, deterioration of the drawability during film formation can be prevented.

[0019] 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. Let the mass of the polypropylene resin before dissolution in boiling xylene be X0 (g), and the mass of the polypropylene components dissolved in the filtrate be X (g). Then, CXS is determined from the following formula (1). Formula (1): CXS (mass%) = (X / X0) × 100.

[0020] 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 particularly 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. 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 high-temperature withstand voltage characteristics and dimensional stability. On the other hand, when the mesopentad fraction of the linear polypropylene resin (A) is 0.995 or less, the film-forming property is maintained, and it is easy to stably obtain a biaxially oriented polypropylene film. The mesopentad fraction can be measured by dissolving a polypropylene resin sample in a solvent and 13 measuring it using C-NMR. The detailed conditions and the like are shown in the examples.

[0021] The MFR of linear polypropylene resin (A), when measured under conditions of 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 particularly 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 makes it easier to obtain a stable biaxially oriented polypropylene film while maintaining film-forming properties. 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 dimensional stability and high-temperature withstand voltage characteristics.

[0022] Furthermore, the melt tension (MS) of the linear polypropylene resin (A) is preferably 1.5 cN or less, and 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 taken up at a speed of 6.5 m / min.

[0023] Next, we will explain high-MFR polypropylene resin (B). High-MFR polypropylene resin (B) is a polypropylene resin in which the MFR is 1800 g / 10 min or more and 2600 g / 10 min or less. As a specific example, "L-MODU" (registered trademark) S400 manufactured by Idemitsu Kosan Co., Ltd. is cited.

[0024] When the MFR of high-MFR polypropylene resin (B) is 1800 g / 10 min or higher, film-forming properties are maintained, and it is easy to obtain a stable biaxially oriented polypropylene film. When the MFR is 2600 g / 10 min or lower, dimensional stability and dielectric strength at high temperatures are easily maintained. In other words, by including such high-MFR polypropylene resin (B), it is possible to achieve both film-forming properties and dimensional stability and dielectric strength at high temperatures in the resulting biaxially oriented polypropylene film.

[0025] In the biaxially oriented polypropylene film of the present invention, the content of high-MFR polypropylene resin (B) is preferably 1.0% by mass or more and 10.0% by mass or less, more preferably 2.0% by mass or more and 8.0% by mass or less, even more preferably 3.0% by mass or more and 7.0% by mass or less, and particularly preferably 4.0% by mass or more and 6.0% by mass or less. When the content of high-MFR polypropylene resin (B) is 1.0% by mass or more, the stretchability during film formation is improved. On the other hand, when the content of high-MFR polypropylene resin (B) is 10.0% by mass or less, it is easier to maintain dielectric strength and dimensional stability at high temperatures. Furthermore, by keeping the content of high-MFR polypropylene resin (B) within the above range, the spherulite size of the polypropylene generated in the cooling process of the melt-extruded resin sheet can be controlled to be small, and the depth of the five-point valley region (S5v) on the film surface can be easily controlled to 70 nm or more and 1400 nm or less. Furthermore, if the film contains multiple components that fall under the category of high-MFR polypropylene resin (B), the content of these components shall be calculated by summing up all the relevant components, and this also applies to the branched-chain polypropylene resins (H) and (I) described later.

[0026] Next, branched polypropylene resin (H) will be described. In this invention, branched polypropylene resin (H) is a branched polypropylene resin whose MFR (unit: g / 10 min) and MS (unit: cN) at 230°C satisfy the following formula (2). Formula (2): 0 <log(MS)≦-0.56×log(MFR)+0.74 。

[0027] While it is possible to determine whether a biaxially oriented polypropylene film contains branched-chain polypropylene resin (H) by measuring the polypropylene resin in the film, separation is not easy when the film contains multiple polypropylene resins. Therefore, it is also possible to determine this by measuring the MS and MFR of the polypropylene resin used as a raw material when manufacturing the biaxially oriented polypropylene film. The same method can be used to determine whether branched-chain polypropylene resin (I), as described later.

[0028] Furthermore, the MS of the branched polypropylene resin (H) is preferably 1.0 cN or more and 6.0 cN or less. By setting it within this range, excellent flow characteristics in the molten state are obtained, which can suppress uneven film thickness and film tearing. The MFR of the branched polypropylene resin (H) is preferably 4.0 g / 10 min or more and 10.0 g / 10 min or less. Setting the MFR of the branched polypropylene resin (H) to 4.0 g / 10 min or more makes it easier to obtain a stable biaxially oriented polypropylene film while maintaining film-forming properties. On the other hand, setting the MFR of the branched polypropylene resin (H) to 10.0 g / 10 min or less makes it easier to maintain dimensional stability and high-temperature withstand voltage characteristics.

[0029] The biaxially oriented polypropylene film of the present invention preferably contains 5.0% to 30.0% by mass of branched-chain polypropylene resin (H) in 100% by mass of all components constituting the film, more preferably 7.0% to 25.0% by mass, and even more preferably 10.0% to 22.0% by mass.

[0030] By setting the content of branched-chain polypropylene resin (H) to 5.0% by mass or more in 100% by mass of all components constituting the film, it becomes possible to form a film more stably when biaxially stretching a thin film. Furthermore, by setting the content of branched-chain polypropylene resin (H) to 30.0% by mass or less, the spherulite size does not become too small when the molten polymer is formed into a sheet, and the decrease in stereoregularity as a biaxially oriented polypropylene film is reduced, making it easier to maintain dielectric strength at high temperatures.

[0031] Next, branched polypropylene resin (I) will be described. In this invention, branched polypropylene resin (I) is a branched polypropylene resin whose MFR (unit: g / 10 min) and MS (unit: cN) at 230°C satisfy the following formula (3). Equation (3): log(MS)>-0.56×log(MFR)+0.74.

[0032] The MS of the branched-chain polypropylene resin (I) is preferably greater than 6.0 cN and 20 cN or less, more preferably greater than 6.0 cN and 15 cN or less, and even more preferably greater than 6.0 cN and 13 cN or less. By setting it within the above range, excellent flow characteristics in the molten state are obtained, which can suppress uneven film thickness and film tearing.

[0033] Furthermore, the MFR of the branched-chain polypropylene resin (I) is preferably 1.0 g / 10 min or more and less than 4.0 g / 10 min. Setting the MFR of the branched-chain polypropylene resin (I) to 1.0 g / 10 min or more makes it easier to maintain film-forming properties and obtain a stable biaxially oriented polypropylene film. On the other hand, setting the MFR of the linear-chain polypropylene resin (I) to 4.0 g / 10 min or less makes it easier to maintain dimensional stability and dielectric strength at high temperatures.

[0034] The biaxially oriented polypropylene film of the present invention preferably contains 1.0% to 10.0% by mass of branched-chain polypropylene resin (I) in 100% by mass of all components constituting the film, more preferably 2.0% to 8.0% by mass, and even more preferably 2.0% to 6.0% by mass.

[0035] By setting the content of branched-chain polypropylene resin (I) in the total mass of all components constituting the film to 1.0% or more by mass, it becomes possible to form a thin film more stably when biaxially stretching the thin film, and the spherulite size does not become too large when forming the molten polymer into a sheet, making it easier to control the surface shape to a suitable form. Furthermore, by setting the content of branched-chain polypropylene resin (I) to 10.0% or less by mass, the spherulite size does not become too small when forming the molten polymer into a sheet, and the decrease in stereoregularity as a biaxially oriented polypropylene film is reduced, making it easier to maintain dielectric strength at high temperatures.

[0036] To obtain branched-chain polypropylene resins (H) and (I), preferred methods include using high-energy ionization radiation on the polypropylene resin (e.g., Japanese Patent Publication No. 62-121704), reacting the polypropylene resin with a specific organic peroxide (e.g., Japanese Patent Publication No. 2869606), reacting the polypropylene resin with a pyrolytic radical-forming agent and an ethylene-based polyfunctional unsaturated monomer (e.g., Japanese Patent Publication No. 10-330436), and using a specific catalyst during the polymerization of the polypropylene resin (e.g., Japanese Patent Publication No. 2009-057542).

[0037] More specifically, as the branched-chain polypropylene resin (H), products such as "WAYMAX" (registered trademark) (MFX3) manufactured by Nippon Polypropylene Co., Ltd. can be used. In addition, as the branched-chain polypropylene resin (I), products such as "Pro-fax" (registered trademark) (PF-814, etc.) manufactured by Lyondell Basell, "Daploy" (trademark) (WB130HMS, WB135HMS, etc.) manufactured by Borealis, and "WAYMAX" (registered trademark) (MFX6, MFX8, EX6000, EX8000, etc.) manufactured by Nippon Polypropylene Co., Ltd. can be used.

[0038] The branched polypropylene resins (H) and (I) contained in the biaxially oriented polypropylene film of the present invention preferably have a branched structure in their molecular chains. A polypropylene resin having a branched structure in its molecular chain is a polypropylene resin having five or fewer internal trisubstituted olefins per 10,000 carbon atoms, and the presence of these internal trisubstituted olefins is important. 1 This can be confirmed by the proton ratio of the H-NMR spectrum. The branched polypropylene resins (H) and (I) act as α-nucleating agents, and within a certain range of addition amounts, they also allow for the formation of rough surfaces due to crystalline morphology. In other words, by including the branched polypropylene resins (H) and (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.

[0039] In the biaxially oriented polypropylene film of the present invention, the difference obtained by subtracting the MS of branched-chain polypropylene resin (H) from the MS of branched-chain polypropylene resin (I) is preferably 1.0 or more and 15.0 or less, more preferably 3.0 or more and 12.0 or less, and even more preferably 4.0 or more and 10.0 or less. By keeping the difference obtained by subtracting the MS of branched-chain polypropylene resin (H) from the MS of branched-chain polypropylene resin (I) within the above range, it is easier to obtain a biaxially oriented polypropylene film with excellent film-forming properties and high-temperature voltage resistance characteristics.

[0040] Furthermore, in the biaxially oriented polypropylene film of the present invention, the difference obtained by subtracting the high-MFR polypropylene resin (B) from the sum of the mass percentages of the branched-chain polypropylene resin (I) and the branched-chain polypropylene resin (H) is preferably 5.0% by mass or more and 30.0% by mass or less, more preferably 10.0% by mass or more and 25.0% by mass or less, even more preferably 12.0% by mass or more and 22.0% by mass or less, and particularly preferably 12.0% by mass or more and 18.0% by mass or less. By setting the difference obtained by subtracting the high-MFR polypropylene resin (B) from the sum of the mass percentages of the branched-chain polypropylene resin (I) and the branched-chain polypropylene resin (H) to 5.0% by mass or more, it is easier to obtain a polypropylene film with excellent film-forming properties. By setting it to 30.0% by mass or less, it is easier to maintain high-temperature voltage resistance characteristics and dimensional stability. Furthermore, by setting the above range to the difference obtained by subtracting the high-MFR polypropylene resin (B) from the sum of the mass percentages of branched-chain polypropylene resin (I) and branched-chain polypropylene resin (H), the spherulite size and number do not become too large when forming the molten polymer into a sheet, making it easier to obtain a film surface shape in which the five-point peak region height (S5p) of the present invention is 80 nm to 1000 nm.

[0041] When biaxially oriented polypropylene film is used as a dielectric in capacitors, a metal vapor deposition is applied to its surface. However, biaxially oriented polypropylene film typically has 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 biaxially oriented polypropylene film after biaxial stretching. Specific surface treatment methods include, for example, corona discharge treatment, plasma treatment, glow treatment, and flame treatment.

[0042] From the viewpoint of achieving both processability and voltage resistance characteristics, it is important that the biaxially oriented polypropylene film of the present invention has an interface surface area ratio Sdr of 0.0004% or more and 0.0200% or less on at least one side. From the above viewpoint, it is more preferable that the interface surface area ratio Sdr of 0.0006% or more and 0.0100% or less on at least one side, even more preferable that it is 0.0008% or more and 0.0050% or less, and particularly preferable that it is 0.0010% or more and 0.0050% or less.

[0043] The interface unfolded surface area ratio Sdr is a composite parameter defined in ISO 25178-2 (2012) for evaluating three-dimensional surface properties (surface roughness), indicating how much the unfolded area (surface area) of the defined region has increased relative to the area of ​​the defined region. If there are no surface irregularities in the unfolded area of ​​the defined region, it will be the same as the area of ​​the defined region, and therefore Sdr = 0%. The measuring device for the interface unfolded surface area ratio Sdr is not particularly limited as long as it is capable of the above measurement, 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.

[0044] By ensuring that the surface area ratio Sdr of the interface is 0.0004% or higher on at least one side, the slipperiness of the film is maintained. This prevents wrinkle formation and deterioration of the film roll's winding shape during the transport process when processing biaxially oriented polypropylene film, improving processability. It also prevents the interlayer gap of the film from narrowing during capacitor element formation, making short-circuit failure less likely when the capacitor is used. On the other hand, an Sdr of 0.0200% or lower at the interface means that there are no excessive irregularities on the surface, and this configuration moderately suppresses slipperiness. This reduces winding misalignment and meandering during the transport process when processing biaxially oriented polypropylene film. Furthermore, since the interlayer gap of the film does not widen excessively during capacitor element formation, safety is not excessive, reducing the decrease in voltage withstand characteristics caused by the irregularities, and consequently extending the lifespan of the capacitor. If the surface area ratio Sdr of the interface on at least one side is within the above range, appropriate irregularities can be maintained, improving processability and dielectric strength. However, if the surface area ratio Sdr of the interface on both sides is 0.0004% or more and 0.0200% or less, or satisfies the above preferred range, it results in even better processability and dielectric strength, which is more preferable.

[0045] To achieve an interface surface area ratio Sdr of 0.0004% to 0.0200% or within the preferred range described above, at least on one side, one method is to use the polypropylene resin described above and to set specific conditions for the preheating process during longitudinal stretching when forming the film, as will be described later. More specifically, it is effective to control the surface structure of a cast sheet made of the polypropylene resin described above by applying heat locally using a radiation heater or the like to instantaneously transfer the β crystals in the cast sheet to α crystals. In this case, the Sdr can be increased by increasing the processing time of the radiation heater.

[0046] The biaxially oriented polypropylene film of the present invention preferably has a five-point valley region depth S5v of 70 nm to 1400 nm on at least one side, more preferably 80 nm to 1200 nm, even more preferably 90 nm to 1000 nm, and particularly preferably 110 nm to 600 nm.

[0047] The five-point valley region depth S5v is a feature parameter for evaluating three-dimensional surface characteristics (surface roughness) as defined in ISO 25178-2 (2012), and represents the average of the valley region heights from the deepest valley point to the fifth deepest valley point within the defined region. The measuring device for the five-point valley region depth S5v is not particularly limited as long as it is capable of the above measurement, 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.

[0048] By setting the five-point valley region depth S5v to 70 nm or more on at least one side, blocking and transport wrinkles are suppressed during the film transport process when forming the film and processing the capacitor element, thereby reducing deterioration of the film roll's winding shape and defects in the appearance and internal shape of the capacitor element. Furthermore, it prevents the interlayer gap of the film from narrowing during capacitor element formation, thereby suppressing short-circuit failure when the capacitor is used. Setting the five-point valley region depth S5v to 1400 nm or less means that the depth of the recesses on the surface is shallow, and by adopting this configuration, the decrease in withstand voltage characteristics caused by these recesses is reduced, thus extending the lifespan of the capacitor. If the five-point valley region depth S5v is within the above range on at least one side, appropriate recesses can be maintained, resulting in excellent processability and withstand voltage performance. In particular, if the five-point valley region depth S5v is between 70 nm and 1400 nm on both sides or satisfies the above range, it results in even better processability and withstand voltage performance, which is more preferable.

[0049] To set the five-point valley region depth S5v to 70 nm to 1400 nm or within the preferred range described above, at least on one side, one method is to use the polypropylene resin described above and set specific conditions for the longitudinal stretching preheating process during film formation, as will be described later. More specifically, it is effective to control the surface structure by adjusting the content of the high-MFR polypropylene resin (B) described above. In this case, the higher the content of the high-MFR polypropylene resin (B), the smaller the spherulite size of the polypropylene generated in the cooling process of the melt-extruded resin sheet can be controlled, and thus S5v can be reduced.

[0050] The biaxially oriented polypropylene film of the present invention preferably has a five-point peak region height S5p of 80 nm to 1000 nm on at least one side, more preferably 90 nm to 700 nm, even more preferably 100 nm to 600 nm, and particularly preferably 100 nm to 550 nm.

[0051] The five-point peak region height S5p is a feature parameter for evaluating three-dimensional surface characteristics (surface roughness) as defined in ISO 25178-2 (2012), and represents the average of the five highest peak region heights in the defined region, starting from the highest peak. The measuring device for the five-point peak region height S5p is not particularly limited as long as it is capable of the above measurement, 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.

[0052] By setting the five-point peak region height S5p to 80 nm or more on at least one side, blocking and transport wrinkles are suppressed during the film transport process when forming the film and processing capacitor elements, thereby reducing deterioration of the film roll's winding shape and defects in the appearance and internal shape of the capacitor elements. Furthermore, by setting the five-point peak region height S5p to 1000 nm or less, slipperiness is moderately suppressed, reducing winding misalignment and meandering during the transport process when processing biaxially oriented polypropylene film. In addition, since the interlayer gap of the film does not become excessively wide when forming capacitor elements, the safety is not excessive, and as a result, the lifespan of the capacitor can be extended. If the five-point peak region height S5p is within the above range on at least one side, appropriate protrusions can be maintained, resulting in excellent processability and withstand voltage performance. In particular, if the five-point peak region height S5p satisfies the above range on both sides, the processability and withstand voltage performance are even better, which is more preferable.

[0053] To ensure that the five-point peak region height S5p is between 80 nm and 1000 nm or within the preferred range described above, at least on one side, one method is to use the polypropylene resin described above and set specific conditions for the longitudinal stretching preheating process during film formation, as described later. More specifically, it is effective to control the surface structure by adjusting the difference obtained by subtracting the high-MFR polypropylene resin (B) from the sum of the mass percentages of the branched-chain polypropylene resin (I) and the branched-chain polypropylene resin (H) described above. In this case, the smaller the difference obtained by subtracting the high-MFR polypropylene resin (B) from the sum of the mass percentages of the branched-chain polypropylene resin (I) and the branched-chain polypropylene resin (H), the less the spherulite size and number will become excessive when the molten polymer is formed into a sheet, and the smaller S5p can be. In addition, the S5p value can also be increased by applying heat locally with a radiation heater or the like.

[0054] The biaxially oriented polypropylene film of the present invention is preferably obtained by molding a polypropylene resin composition consisting of the linear polypropylene resin (A), high MFR polypropylene resin (B), and branched polypropylene resins (H) and (I) into a sheet and then biaxially stretching it. The biaxial stretching can be performed by any of the following methods: simultaneous inflation biaxial stretching, simultaneous tenter biaxial stretching, or sequential tenter biaxial stretching. However, sequential tenter biaxial stretching is preferred from the viewpoint of film formation stability and thickness uniformity. In particular, it is preferable to stretch in the longitudinal direction first, and then in the width direction.

[0055] The biaxially oriented polypropylene film of the present invention preferably has a thickness of 1.0 μm to 4.0 μm, from the viewpoint of film-forming properties, mechanical strength, high-temperature withstand voltage characteristics, and capacitance per unit volume when used as a capacitor dielectric. From the above viewpoint, a thickness of 1.2 μm to 3.8 μm is more preferable, and a thickness of 1.4 μm to 3.0 μm is even more preferable. By setting the thickness to 1.0 μm or more, the biaxially oriented polypropylene film can be made to have excellent mechanical strength and high-temperature withstand voltage characteristics, and its breakage during film formation and processing can be reduced. On the other hand, by setting the thickness to 4.0 μm or less, the capacitance per unit volume can be made larger when used as a capacitor dielectric. The thickness shall be measured by the micrometer method in accordance with JIS C 2330 (2014).

[0056] The thickness of the biaxially oriented polypropylene film can be adjusted, for example, by adjusting the slit width of the T-die, the discharge rate from the T-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-die, decreasing the discharge rate from the T-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.

[0057] Next, the method for producing the biaxially oriented polypropylene film of the present invention will be described below, but is not necessarily limited thereto.

[0058] First, the linear polypropylene resin (A), high MFR polypropylene resin (B), and branched polypropylene resins (H) and (I) described above are dry-blended and supplied to a single-screw melt extruder, where melt extrusion is performed at 200-260°C. Next, foreign matter and modified polymers are removed using a filter installed in the middle of the polymer tube. Then, the molten polymer formed into a sheet is 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.

[0059] 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.

[0060] The molten sheet discharged from the T-die lands on the casting drum, and the time it remains in contact with the drum is preferably 0.8 seconds to 3.0 seconds, and more preferably 1.0 second to 3.0 seconds. A contact time of 0.8 seconds or more facilitates the solidification of the molten sheet and reduces breakage during the subsequent stretching process. On the other hand, a contact time of 3.0 seconds or less prevents the excessive formation of β-crystals in the cast sheet, reducing meandering during film transport processes and deterioration of the film roll's winding shape during film formation and processing.

[0061] 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. Setting the air knife temperature to 60°C or higher prevents excessive formation of β crystals in the cast sheet, maintaining the slipperiness of the film obtained after biaxial stretching. This reduces wrinkle formation and deterioration of the film roll's winding shape during film formation and processing. On the other hand, setting the air knife temperature to 100°C or lower prevents excessive formation of β crystals in the cast sheet, reducing meandering and deterioration of the film roll's winding shape during film formation and processing.

[0062] The temperature of the cooling roll used to further cool the cast sheet solidified by the casting drum is preferably between 10°C and 60°C. Setting the cooling roll temperature above 10°C 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 below 60°C reduces crystal formation in the cast sheet, making it easy to reduce the longitudinal variation of surface irregularities in the biaxially oriented polypropylene film obtained after biaxial stretching.

[0063] Next, the obtained cast sheet is biaxially stretched. Specifically, the stretching conditions involve first controlling the temperature at which the cast sheet is stretched in the longitudinal direction. Methods for temperature control include using temperature-controlled rotary rolls or using a hot air oven.

[0064] In the longitudinal stretching preheating process, the cast sheet is preheated by passing it through rolls maintained at 100-125°C, preferably 100-120°C, and then through rolls maintained at 140-150°C. Furthermore, a radiation heater is installed on the rolls maintained at 135-150°C, preferably 140-150°C, to control the surface structure of the cast sheet by applying heat locally.

[0065] From the viewpoint of controlling the unfolded surface area ratio Sdr of the interface of the resulting biaxially oriented polypropylene film to an appropriate range, it is preferable to locally apply heat using a radiation heater. In this case, the processing time with the radiation heater is preferably 0.5 seconds or more and 2.0 seconds or less, more preferably 0.7 seconds or more and 1.8 seconds or less, and particularly preferably 0.8 seconds or more and 1.5 seconds or less. By setting the processing time with the radiation heater to 0.5 seconds or more, the amount of heat required for surface control can be reached, and the unfolded surface area ratio Sdr of the interface of the resulting biaxially oriented polypropylene film can be raised to an appropriate level. On the other hand, by setting the processing time with the radiation heater to 2.0 seconds or less, it is possible to prevent the induction of film tearing due to excessive heat and an excessive increase in the unfolded surface area ratio Sdr of the interface of the resulting biaxially oriented polypropylene film. The distance between the radiation heater and the cast sheet is preferably 1.0 to 10 mm, and the output of the radiation heater is preferably 1.0 to 10 kW.

[0066] Next, the cast sheet is stretched in the longitudinal direction in the longitudinal stretching process. The cast sheet, which has undergone the preheating process for longitudinal stretching, is passed through rolls controlled to a temperature of 120°C to 140°C, and stretched in the longitudinal direction (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 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.

[0067] Next, the uniaxially oriented film obtained by longitudinal stretching is held at both ends in the width direction with clips and stretched in the width direction at a stretching ratio of 5 to 15 times using a tenter-type stretcher controlled at a temperature of 140°C to 170°C. Furthermore, it is heat-set at a temperature of 150 to 170°C while being relaxed by 5 to 15% in the width direction.

[0068] Next, the biaxially oriented film is 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 onto a winding machine as a master roll. 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 the biaxially oriented polypropylene film of the present invention.

[0069] 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.

[0070] Next, a metal film laminate film using the biaxially oriented polypropylene film of the present invention will be described. 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 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.

[0071] Next, a film capacitor using the biaxially oriented polypropylene film of the present invention will be described. The film capacitor has a structure in which metal film laminates are stacked 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. 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 stacked and wound together so that the deposited portion extends beyond the margin portion in the width direction 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. 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]

[0072] The present invention will be described in detail below with reference to examples, but the present invention is not limited thereto. The properties were measured and evaluated by the following methods, and the following raw materials were used.

[0073] [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).

[0074] 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.

[0075] 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.

[0076] (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).

[0077] (3) 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).

[0078] (4) 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.

[0079] (5) Interfacial surface area ratio Sdr (unit: %), five-point valley region depth S5v (unit: nm), five-point peak region height S5p (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. Ten locations were randomly selected from the film roll corresponding to the center position in the width direction from the center position in the longitudinal direction. The unfolded surface area ratio (Sdr), five-point valley region depth (S5v), and five-point peak region height (S5p) of the interface at these ten locations were measured. The average of the obtained measurements for each parameter was calculated and used as the Sdr, S5v, and S5p of the film being measured. The detailed conditions for one measurement are as follows. Note that one field of view (field of view area: 939 μm vertical × 1,252 μm horizontal = 1,175,628 μm) was used for one measurement. 2 Measurements were taken of the following:

[0080] A. Measurement conditions 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.

[0081] B.Measurement method A dedicated sample holder was used to secure the film during measurement. The sample holder consists of two detachable metal plates with a circular hole in the center. The film was placed between these plates, ensuring it was wrinkle-free, and measurements were taken on the central circular portion of the film. 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.

[0082] 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 waviness components were removed using a Gaussian filter with a cutoff value of 250 μm. Next, the "ISOPara" function was used to calculate the unfolded surface area ratios Sdr, S5v, and S5p of the interface of the surface properties as defined in ISO25178-2 (2012). In the "ISOPara" function, the S-Filter was set to 6.0 μm.

[0083] (6) Thickness (unit: μm) The thickness was measured using the micrometer method in accordance with JIS C 2330 (2014).

[0084] (7) Evaluation of film-forming properties The film-forming properties of the film were evaluated according to the following criteria. The time between stopping and resuming film formation due to film tearing 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.

[0085] (8) 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 more wound materials were produced in the same manner, 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 △: Two or more defective items but less than three. ×: 4 or more defective items.

[0086] (9) Life evaluation of film capacitors A winding with a capacitance of 120 μF was obtained by the method described in (8). 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) before and after voltage application was calculated from the following formula (4). The capacitance was measured using an LCR high-tester 3522-50 manufactured by HIOKI E.E. CORPORATION. Equation (4): 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.

[0087] [Raw materials] (1) Resin Linear polypropylene resin (A): Borclean (trademark) 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, and an MS of 1.0 cN. High MFR polypropylene resin (B): Idemitsu Kosan Co., Ltd.'s "L-MODU" (registered trademark) S400 is a high-MFR polypropylene resin with an MFR of 2000g / 10min. Branched polypropylene resin (H): "WAYMAX" (registered trademark) (MFX3), manufactured by Nippon Polypropylene Co., Ltd., is a branched-chain polypropylene resin with an MFR of 9.0 g / 10 min and an MS of 5.0 cN. Branched polypropylene resin (I): "WAYMAX" (registered trademark) (EX6000), manufactured by Nippon Polypropylene Co., Ltd., is a branched-chain polypropylene resin with an MFR of 2.9 g / 10 min and an MS of 9.0 cN.

[0088] (2) Antioxidants Antioxidant 1: BASF Japan's "Irganox" (registered trademark) 1010 Antioxidant 2: 2,6-di-t-butyl-p-cresol (BHT).

[0089] (Example 1) A polypropylene resin mixture was prepared by mixing linear polypropylene resin (A), high MFR polypropylene resin (B), branched polypropylene resin (H), and branched polypropylene resin (I) in a mass ratio of 75.0:5.0:15.0:5.0. 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 it was melt-extruded at 250°C. Afterward, foreign matter was removed from the extruded molten polypropylene resin composition using a 25 μm cut sintered filter, and the mixture 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 92°C using an air knife with an air temperature of 80°C, and then cooled on a cooling roll maintained at 30°C to obtain a cast sheet. At this time, the time the sheet-like molten polypropylene resin composition was in close contact with the cast drum and the cooling roll was 1.0 second each. (Hereafter, the side that was in contact with the cast drum will be referred to as the drum surface (D surface), and the side that was not in contact with the drum will be referred to as the non-drum surface (non-D surface)).

[0090] Next, in the preheating process before longitudinal stretching, the cast film was preheated by passing it through a roll maintained at 120°C, and then through another roll maintained at 145°C. Furthermore, the cast sheet was heated for 1.0 second on the roll maintained at 145°C using a radiation heater. At this time, the distance between the cast sheet and the radiation heater was 5.0 mm, and the output of the radiation heater was 5.0 kW. Afterward, the cast sheet that had undergone the preheating process before longitudinal stretching was stretched longitudinally at a stretching ratio of 5.6 times using a longitudinal stretching roll at a temperature of 140°C to obtain a uniaxially oriented film.

[0091] Furthermore, the uniaxially oriented film was guided to a tenter by gripping the widthwise end with clips and stretched in the widthwise direction at a temperature of 159°C and a stretching ratio of 11 times. Next, a 12% relaxation treatment was performed in the widthwise direction at a temperature of 158°C, and after slow cooling to room temperature, the D-side was stretched at 25 W·min / m 2 Corona discharge treatment was performed at the specified treatment intensity. The widthwise end of the obtained biaxially oriented polypropylene film, held with clips, was cut off and wound up on a winding machine. Next, it was slit using a slitter to a film width of 0.82 m, 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 2.0 μm. The physical properties of the obtained biaxially oriented polypropylene film and the results of each evaluation are shown in Table 1.

[0092] (Examples 2-7, Comparative Examples 1-4) A biaxially oriented polypropylene film was obtained in the same manner as in Example 1, except that the composition of the polypropylene resin and the preheating process before longitudinal stretching 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.

[0093] [Table 1] [Industrial applicability]

[0094] The present invention provides a biaxially oriented polypropylene film with high processability and voltage resistance. By using the biaxially oriented polypropylene film of the present invention as a dielectric for a capacitor, the amount of air between film layers and the interlayer distance can be uniformly controlled during capacitor processing. Therefore, when used as a capacitor, it 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 in that, on at least one side, the unfolded surface area ratio Sdr of the interface is 0.0004% or more and 0.0200% or less.

2. The biaxially oriented polypropylene film according to claim 1, wherein the five-point valley region depth S5v is 70 nm or more and 1400 nm or less on at least one side.

3. The biaxially oriented polypropylene film according to claim 1 or 2, wherein the height S5p of the five-point peak region is 80 nm or more and 1000 nm or less on at least one side.

4. A biaxially oriented polypropylene film according to claim 1 or 2, wherein the film thickness (t) is 1.0 to 4.0 μm.

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