Biaxially oriented polypropylene film

Abstract

To provide a biaxially oriented polypropylene film which exhibits superior processability and superior withstand voltage properties under high-temperature environments, and which can deliver proper safety when employed as a dielectric in film capacitors, particularly large-capacitance film capacitors.SOLUTION: A biaxially oriented polypropylene film comprises, on at least one side, a surface (termed X surface) in which the peak side surface area is 0.5% or more and 5.0% or less, with the peak height of 80 nm or more.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a biaxially oriented polypropylene film having high voltage resistance under high temperature and high voltage environments. [Background technology]

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

[0003] Among them, biaxially oriented polypropylene film is particularly preferred as a dielectric for film capacitor applications due to its excellent high voltage resistance and low loss characteristics. Recently, various electrical equipment is being converted to inverters, and as a result, there is an increasing demand for smaller film capacitors with larger capacities. Furthermore, particularly in automotive applications (including hybrid cars and electric cars), solar power generation, and wind power generation, the operating environment is becoming hotter (representing temperatures between 85°C and 125°C), and there is also an increasing demand for heat resistance for film capacitors.

[0004] Therefore, in addition to making the biaxially oriented polypropylene film, which is a dielectric, thinner, more heat-resistant, and improving the withstand voltage per thickness, there is also a demand for improving the safety of the film capacitor itself. Here, the safety of a film capacitor means the property of maintaining insulation by scattering the evaporated metal due to the discharge energy during abnormal discharge in a metal evaporated film capacitor, which has a metal evaporated film formed on a dielectric film as an electrode, and this property is important for preventing short circuits and destruction of the film capacitor. It is believed that controlling the surface properties of the biaxially oriented polypropylene film is an effective means of achieving both the withstand voltage per thickness of the film and the safety of the film capacitor, and various studies have been conducted so far.

[0005] A method that utilizes the crystal transition from β to α crystals of polypropylene (hereinafter, sometimes referred to as the β crystal method) is known as a method for controlling the surface properties of biaxially oriented polypropylene film. This method that utilizes the crystal transition does not require the incorporation of impurities such as additives that may deteriorate the withstand voltage, and is therefore preferably used 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 techniques that focus on the density of surface roughness and the uniformity of protrusions include a method of adding branched polypropylene (see, for example, Patent Documents 3 and 4) and a method of mixing polypropylenes with different molecular weights and molecular weight distributions (see, for example, Patent Document 5). These methods can control the spherulite size to be small, so that protrusions of uniform height can be formed at high density. Another technique that focuses on the height of protrusions and recesses is a method of subjecting a cast sheet to high-temperature pressure treatment (see, for example, Patent Document 6). This method can control the height of protrusions and recesses uniformly. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2008-133446 A [Patent Document 2] JP 2014-077057 A [Patent Document 3] International Publication No. 2007 / 094072 [Patent Document 4] International Publication No. 2012 / 121256 [Patent Document 5] JP 2014-231584 A [Patent Document 6] JP 2019-26211 A Summary of the Invention [Problem to be solved by the invention]

[0008] However, when the β-crystal method described in Patent Documents 1 and 2 is applied to a film made of a general linear polypropylene, steep convex and concave portions in a crater shape are formed at a low density. Therefore, when the biaxially oriented polypropylene film obtained by these methods is used as a derivative of a film capacitor, insulation breakdown is likely to occur, especially in the concave portions, and there is a problem with the voltage resistance characteristics under high temperature conditions. In addition, when the method described in Patent Documents 3 to 5, which forms convex portions of uniform height at a high density, is applied, or when the method described in Patent Document 6, which controls the height of the convex portions and concave portions uniformly, is applied, it is difficult to suppress the occurrence of localized coarse protrusions and concave shapes. Therefore, when the biaxially oriented polypropylene film obtained by these methods is used as a derivative of a film capacitor, the processability during the production of the film capacitor is insufficient, and the control of the amount of air between the film layers, which affects the voltage resistance and safety in recent high temperature and high voltage environments, was not sufficient.

[0009] Therefore, the object of the present invention is to provide a biaxially oriented polypropylene film that has excellent processability and voltage resistance characteristics in high-temperature environments, and can provide appropriate safety when used as a dielectric for film capacitors (mainly large-capacity film capacitors). [Means for solving the problem]

[0010] The above-mentioned objects can be achieved by the following: That is, the biaxially oriented polypropylene film of the present invention is a biaxially oriented polypropylene film characterized in that it has a surface (hereinafter referred to as X-surface) on at least one side in which the area of ​​peaks having a height of 80 nm or more is 0.5% to 5.0%.

[0011] In order to solve the above problems, the biaxially oriented polypropylene film of the present invention can be configured as follows, and can also be used to form a metal film laminated film and a film capacitor as described below. (1) A biaxially oriented polypropylene film having a surface (hereinafter referred to as X-face) on at least one side of which the area of ​​peaks having a height of 80 nm or more is 0.5% or more and 5.0% or less. (2) The biaxially oriented polypropylene film according to (1), wherein the variation in the area of ​​peaks having a height of 80 nm or more on the X-face is 2.0% or less in at least one of the longitudinal direction and the width direction. (3) The biaxially oriented polypropylene film according to (1) or (2), wherein the protruding valley depth (RvK) on the X-plane is 20 nm or more and 60 nm or less. (4) A biaxially oriented polypropylene film according to any one of (1) to (3), comprising a linear polypropylene resin (A), a low stereoregularity polypropylene resin (B), and two kinds of branched polypropylene resins (H) and (I), wherein the branched polypropylene resin (H) has a melt tension (MS) of 1.0 cN or more and 6.0 cN or less and a melt flow index (MFR) of 4.0 g / 10 min or more and 10.0 g / 10 min or less, and the branched polypropylene resin (I) has an MS of more than 6.0 cN and 20 cN or less and an MFR of 1.0 g / 10 min or more and less than 4.0 g / 10 min. (5) The biaxially oriented polypropylene film according to (4), comprising, based on 100% by mass of all components constituting the biaxially oriented polypropylene film, 70% by mass or more and less than 95% by mass of the linear polypropylene resin (A), more than 1.0% by mass and 10.0% by mass or less of the low stereoregularity polypropylene resin (B), 2.0% by mass or more and 10.0% by mass or less of the branched polypropylene resin (H), and 2.0% by mass or more and 10.0% by mass or less of the branched polypropylene resin (I). (6) A metal film laminated film having a metal film on at least one surface of the biaxially oriented polypropylene film according to any one of (1) to (5). (7) A film capacitor having a laminated or wound configuration of the metal film laminated film according to (6). Effect of the Invention

[0012] The present invention makes it possible to provide a biaxially oriented polypropylene film that has excellent processability and voltage resistance characteristics in high-temperature environments, and can provide appropriate safety when used as a dielectric for film capacitors (mainly large-capacity film capacitors). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] 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 following, in the ranges expressed using "~", the upper and lower limits are included in the range, and the units of the upper and lower limits are the same.

[0014] The biaxially oriented polypropylene film of the present invention is a biaxially oriented polypropylene film obtained by stretching a cast sheet in two perpendicular directions. In other words, biaxial orientation here means stretching in two perpendicular directions (mainly the longitudinal direction and the width direction). Whether a film has biaxial orientation can be determined, for example, by checking whether there is a difference in the plane orientation coefficient in the two perpendicular directions. In addition, the longitudinal direction refers to the direction in which the film runs in the film manufacturing process (the winding direction of the film in the state of a film roll), and the width direction refers to the direction parallel to the film surface and perpendicular to the longitudinal direction.

[0015] Next, the polypropylene resin 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 low stereoregularity polypropylene resin (B), and branched polypropylene resins (H) and (I) described below.

[0016] In addition, the biaxially oriented polypropylene film of the present invention preferably contains more than 70.0% by mass and less than 95.0% by mass of the linear polypropylene resin (A) in 100% by mass of all components constituting the biaxially oriented polypropylene film, more preferably 72.0% by mass or more and 93.0% by mass or less, even more preferably 74.0% by mass or more and 91.0% by mass or less, and particularly preferably 76.0% by mass or more and 90.0% by mass or less. By adopting such an embodiment, it becomes easy to achieve both heat resistance and voltage resistance at high temperatures of the biaxially oriented polypropylene film.

[0017] The linear polypropylene resin (A) means an isotactic polypropylene resin. This isotactic polypropylene resin is also known as a polypropylene resin generally used in film capacitor applications. The linear polypropylene resin (A) is preferably a linear polypropylene resin having a cold xylene soluble portion (CXS) of 0.5% by mass to 4.0% by mass, a mesopentad fraction (mmmm) of 0.960 to 0.995, and a melt flow index (MFR) of 0.5 g / 10 min to 5.0 g / 10 min. Specific examples of linear polypropylene resins that can be suitably used as the linear polypropylene resin (A) include "Borclean" (trademark) (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 a polypropylene component dissolved in xylene when the film is completely dissolved in xylene at 135°C and then precipitated at 20°C. In other words, CXS is considered to correspond to a component that is difficult to crystallize due to reasons such as low stereoregularity and molecular weight.

[0019] When the CXS of the linear polypropylene resin (A) is 4.0% by mass or less, the heat resistance and high-temperature voltage resistance of the biaxially oriented polypropylene film can be improved. Therefore, when the biaxially oriented polypropylene film of this embodiment is used in a film capacitor, relaxation in a high-temperature environment is suppressed, thermal dimensional stability is improved, and leakage current can be suppressed. In addition, when the CXS of the linear polypropylene resin (A) is 0.5% by mass or more, deterioration of stretchability during film formation can be reduced when producing a biaxially oriented polypropylene film.

[0020] 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 thermostatic water bath at 20°C for 1 hour and filtered. Next, the polypropylene component dissolved in the filtrate is quantified by liquid chromatography, and CXS is calculated from the following formula (1), where the mass of the polypropylene resin before dissolution in boiling xylene is X0 (g) and the mass of the polypropylene component dissolved in the filtrate is X (g). Formula (1): CXS(mass%)=(X / X0)×100.

[0021] The mesopentad fraction (mmmm) of the linear polypropylene resin (A) is preferably 0.960 to 0.995, more preferably 0.965 to 0.995, and even more preferably 0.970 to 0.995. The mesopentad fraction (mmmm) is an index showing the stereoregularity of the crystalline phase of polypropylene measured by nuclear magnetic resonance (NMR). Generally, the higher the value, the higher the crystallinity and melting point, and the more excellent the voltage resistance characteristics at high temperatures.

[0022] When the linear polypropylene resin (A) has a mesopentad fraction of 0.960 or more, the voltage resistance characteristics and dimensional stability are easily maintained in a high-temperature environment when the resin is made into a biaxially oriented polypropylene film. On the other hand, when the linear polypropylene resin (A) has a mesopentad fraction of 0.995 or less, the film formability is good, and it is easy to obtain a biaxially oriented polypropylene film stably. The mesopentad fraction can be determined by dissolving a polypropylene resin sample in a solvent and measuring the mesopentad fraction. 13 The measurement can be performed using C-NMR, and the detailed measurement method, conditions, etc. will be described later.

[0023] The MFR of the linear polypropylene resin (A), when measured in accordance with JIS K 7210-1 (2014) at 230°C and 2.16 kg, is preferably 0.5 g / 10 min or more and 5.0 g / 10 min or less, more preferably 1.0 g / 10 min or more and 4.5 g / 10 min or less, and even more preferably 1.5 g / 10 min or more and 4.0 g / 10 min or less.

[0024] When the MFR of the linear polypropylene resin (A) is 0.5 g / 10 min or more, the film formability is good and a biaxially oriented polypropylene film is easily obtained stably. On the other hand, when the MFR of the linear polypropylene resin (A) is 5.0 g / 10 min or less, the dimensional stability of the obtained biaxially oriented polypropylene film and the voltage resistance characteristics under high temperature environment are easily maintained.

[0025] The melt tension (MS) of the linear polypropylene resin (A) is preferably 1.5 cN or less, more preferably 1.2 cN or less, and even more preferably 1.0 cN or less. When the MS of the linear polypropylene resin (A) is 1.5 cN or less, the flow characteristics are improved when the polypropylene resin (A) is melted, and the thickness unevenness of the obtained biaxially oriented polypropylene film and the occurrence of film breakage during the production process can be reduced. Note that MS refers to the tension when the polypropylene resin is heated to 230°C to melt, the molten polypropylene is extruded into a strand at an extrusion speed of 15 mm / min, and the strand is taken up at a speed of 6.5 m / min, and the details of the measurement method will be described later.

[0026] Next, the low stereoregular polypropylene resin (B) will be described. In the present invention, the low stereoregular polypropylene resin (B) is a polypropylene resin having an MFR of 30 g / 10 min or more and 70 g / 10 min or less. Specific examples of polypropylene resins that can be suitably used as the low stereoregular polypropylene resin (B) in the present invention include "L-MODU" (registered trademark) S901 manufactured by Idemitsu Kosan Co., Ltd., and "Wintech" (registered trademark) WMG03 and WMG03UX manufactured by Japan Polypropylene Corporation.

[0027] When the MFR of the low stereoregular polypropylene resin (B) is 30 g / 10 min or more, good film-forming properties are maintained, and it is easy to obtain a biaxially oriented polypropylene film stably. On the other hand, when the MFR of the low stereoregular polypropylene resin (B) is 70 g / 10 min or less, the obtained biaxially oriented polypropylene film has excellent dimensional stability and voltage resistance properties at high temperatures. In other words, by including such a low stereoregular polypropylene resin (B), it is possible to achieve both dimensional stability and voltage resistance properties at high temperatures of the obtained biaxially oriented polypropylene film while maintaining film-forming properties.

[0028] In the biaxially oriented polypropylene film of the present invention, the content of the low stereoregular 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 9.0% by mass or less, even more preferably 3.0% by mass or more and 8.0% by mass or less, and particularly preferably more than 5.0% by mass and 8.0% by mass or less, when the total components constituting the film are taken as 100% by mass. When the content of the low stereoregular polypropylene resin (B) is 1.0% by mass or more, the stretchability during film formation is improved. As a result, the molecular orientation in each direction can be increased, and the voltage resistance characteristics at high temperatures are also improved. On the other hand, when the content of the low stereoregular polypropylene resin (B) is 10.0% by mass or less, the obtained biaxially oriented polypropylene film has excellent voltage resistance characteristics and dimensional stability at high temperatures. In addition, by setting the content of the low stereoregular polypropylene resin (B) within the above range, the size of the spherulites of the polypropylene resin generated in the cooling process of the melt-extruded resin sheet can be controlled to be small, and the area of ​​the peaks having a height of 80 nm or more can be easily controlled to 0.5 to 5.0%. When the film contains multiple components corresponding to the low stereoregular polypropylene resin (B), the content is determined by adding up all the corresponding components, and this also applies to the branched polypropylene resins (H) and (I) described later.

[0029] Next, the branched polypropylene resin (H) will be described. In the biaxially oriented polypropylene film of the present invention, the branched polypropylene resin refers to a polypropylene resin having a branched structure in the molecular chain. When the biaxially oriented polypropylene film of the present invention contains two types of branched polypropylene resins, the one with a relatively small MS value is defined as the branched polypropylene resin (H).

[0030] Whether or not a biaxially oriented polypropylene film contains a branched polypropylene resin (H) can be confirmed by measuring the composition of the polypropylene resin in the film, but since separation is not easy when the film contains multiple polypropylene resins, it may be confirmed by measuring the MS and MFR of the polypropylene resin used as the raw material when producing the biaxially oriented polypropylene film. This also applies to the branched polypropylene resin (I) described below.

[0031] Furthermore, the MS of the branched polypropylene resin (H) is preferably 1.0 cN or more and 6.0 cN or less. By setting the MS of the branched polypropylene resin (H) within the above range, the flow characteristics in the molten state are improved, and thickness unevenness and film breakage of the obtained biaxially oriented polypropylene film can be suppressed. In addition, 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. When the MFR of the branched polypropylene resin (H) is 4.0 g / 10 min or more, good film formability is maintained, so that a biaxially oriented polypropylene film can be stably obtained. On the other hand, when the MFR of the branched polypropylene resin (H) is 10.0 g / 10 min or less, the obtained biaxially oriented polypropylene film has excellent dimensional stability and high temperature voltage resistance characteristics.

[0032] The biaxially oriented polypropylene film of the present invention preferably contains 2.0% by mass or more and 10.0% by mass or less of branched polypropylene resin (H) based on 100% by mass of all components constituting the film, more preferably 3.0% by mass or more and 9.0% by mass or less, and even more preferably 4.0% by mass or more and 8.0% by mass or less.

[0033] By making the content of the branched polypropylene resin (H) 2.0% by mass or more out of 100% by mass of all components constituting the film, it becomes possible to more stably produce a thin film when biaxially stretching it, and it is easy to control the surface shape to a suitable shape without the spherulite size becoming too large when the molten polymer is formed into a sheet. In addition, by making the content of the branched polypropylene resin (H) 10.0% by mass or less, it is possible to prevent the spherulite size from becoming too small when the molten polymer is formed into a sheet, and the decrease in stereoregularity of the biaxially oriented polypropylene film is reduced. Therefore, the obtained biaxially oriented polypropylene film has excellent voltage resistance at high temperatures.

[0034] Next, the branched polypropylene resin (I) will be described.

[0035] The MS of the branched polypropylene resin (I) is preferably more than 6.0 cN and not more than 20 cN, more preferably more than 6.0 cN and not more than 15 cN, and even more preferably more than 6.0 cN and not more than 13 cN. By setting the MS of the branched polypropylene resin (I) within the above range, the flow characteristics in the molten state become good, so that the thickness unevenness of the obtained biaxially oriented polypropylene film can be reduced and film breakage during the production process can be suppressed.

[0036] The MFR of the branched polypropylene resin (I) is preferably 1.0 g / 10 min or more and less than 4.0 g / 10 min. When the MFR of the branched polypropylene resin (I) is 1.0 g / 10 min or more, good film formability is maintained and a biaxially oriented polypropylene film can be stably obtained. On the other hand, when the MFR of the branched polypropylene resin (I) is less than 4.0 g / 10 min, the dimensional stability of the biaxially oriented polypropylene film and the voltage resistance characteristics at high temperatures are improved.

[0037] The biaxially oriented polypropylene film of the present invention preferably contains 2.0% by mass or more and 10.0% by mass or less of branched polypropylene resin (I) based on 100% by mass of all components constituting the film, more preferably 3.0% by mass or more and 9.0% by mass or less, even more preferably 4.0% by mass or more and 8.0% by mass or less, and particularly preferably more than 4.0% by mass but less than 6.0% by mass.

[0038] By making the content of the branched polypropylene resin (I) 2.0% by mass or more out of 100% by mass of all components constituting the film, it becomes possible to more stably produce a thin film when biaxially stretching it, and the spherulite size does not become too large when the molten polymer is formed into a sheet, making it easy to control the surface shape to a suitable shape. Also, by making the content of the branched polypropylene resin (I) 10.0% by mass or less, the spherulite size does not become too small when the molten polymer is formed into a sheet, reducing the decrease in stereoregularity as a biaxially oriented polypropylene film, making it easy to maintain the withstand voltage at high temperatures.

[0039] In order to obtain the branched polypropylene resins (H) and (I), a method of using high-energy ionizing radiation on a polypropylene resin (e.g., JP 62-121704 A), a method of reacting a specific organic peroxide with a polypropylene resin (e.g., JP 2869606 A), a method of reacting a thermally decomposable radical former and an ethylenically polyfunctional unsaturated monomer with a polypropylene resin (e.g., JP 10-330436 A), a method of using a specific catalyst during polymerization of a polypropylene resin (e.g., JP 2009-057542 A), and the like are preferably used.

[0040] More specifically, as the branched polypropylene resin (H), "WAYMAX" (registered trademark) (MFX3) manufactured by Japan Polypropylene Corporation can be suitably used. In addition, as the branched polypropylene resin (I), "Pro-fax" (registered trademark) (PF-814, etc.) manufactured by Lyondell Basell, "Daploy" (trademark) (WB130HMS, WB135HMS, etc.) manufactured by Borealis, "WAYMAX" (registered trademark) (MFX6, MFX8, EX6000, EX8000, etc.) manufactured by Japan Polypropylene Corporation can be suitably used.

[0041] The branched polypropylene resins (H) and (I) contained in the biaxially oriented polypropylene film of the present invention have a branched structure in the molecular chain. The polypropylene resin having a branched structure in the molecular chain is a polypropylene resin having 5 or less internal tri-substituted olefins per 10,000 carbon atoms. The presence of the internal tri-substituted olefins is 1 This can be confirmed by the proton ratio in the H-NMR spectrum. The branched polypropylene resins (H) and (I) act as α crystal nucleating agents, and also enable the formation of rough surfaces through crystal morphology if added in a certain range of amounts. In other words, by including the branched polypropylene resins (H) and (I), the size of the polypropylene spherulites formed 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 properties at high temperatures can be obtained.

[0042] In the branched polypropylene resin (H) and the branched polypropylene resin (I) constituting the biaxially oriented polypropylene film of the present invention, the difference obtained by subtracting the MS of the branched polypropylene resin (H) from the MS of the branched 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 setting the difference obtained by subtracting the MS of the branched polypropylene resin (H) from the MS of the branched polypropylene resin (I) within the above range, film formability is improved, and a biaxially oriented polypropylene film having excellent high-temperature voltage resistance characteristics is easily obtained.

[0043] In addition, in the low stereoregular polypropylene resin (B), branched polypropylene resin (H), and branched polypropylene resin (I) that constitute the biaxially oriented polypropylene film of the present invention, the value obtained by subtracting the content (mass%) of the low stereoregular polypropylene resin (B) from the sum of the contents (mass%) of the branched polypropylene resin (I) and the branched polypropylene resin (H) is preferably 0.0 mass% or more and 5.0 mass% or less, more preferably 1.0 mass% or more and 4.0 mass% or less, and even more preferably 2.0 mass% or more and 4.0 mass% or less.

[0044] By setting the difference obtained by subtracting the content (mass%) of the low stereoregular polypropylene resin (B) from the sum of the contents (mass%) of the branched polypropylene resin (I) and the branched polypropylene resin (H) to 0.0% by mass or more, a biaxially oriented polypropylene film with excellent film formability is easily obtained. On the other hand, by setting the value to 5.0% by mass or less, the high-temperature withstand voltage characteristics and dimensional stability of the obtained biaxially oriented polypropylene film can be improved. In addition, by setting the difference obtained by subtracting the content (mass%) of the low stereoregular polypropylene resin (B) from the sum of the contents (mass%) of the branched polypropylene resin (I) and the branched polypropylene resin (H) to the above range, the size and number of spherulites do not become too large when the molten resin composition is formed into a sheet, and it is easy to form a surface (X-plane) with a peak side area of ​​0.5% to 5.0% when the molten resin composition is formed into a sheet, and further it is easy to set the variation in the peak side area of ​​80 nm or more on the X-plane to 2.0% or less in at least one of the longitudinal direction and the width direction.

[0045] In addition, when the biaxially oriented polypropylene film of the present invention is used as a dielectric for a film capacitor, a metal is vapor-deposited on the surface. However, the biaxially oriented polypropylene film usually has a low surface energy, and the adhesion of the vapor-deposited metal may become an issue. Therefore, it is preferable to subject the biaxially oriented polypropylene film to a surface treatment after biaxial stretching. Specific surface treatment methods that can be used include, for example, corona discharge treatment, plasma treatment, glow discharge treatment, and flame treatment.

[0046] From the viewpoint of achieving both processability and voltage resistance, it is important that the biaxially oriented polypropylene film of the present invention has an X-face on at least one side, in which the area of ​​the peaks having a height of 80 nm or more is 0.5% to 5.0%. From the above viewpoint, the area of ​​the peaks having a height of 80 nm or more on the X-face is more preferably 0.7% to 3.0%, and even more preferably 0.9% to 2.0%. Here, "at least one side" means one side or both sides. In addition, as long as there is an X-face on at least one side, there is no particular restriction on the other side, but from the above viewpoint and the viewpoint of being able to freely select the side on which the metal film is formed by deposition or the like, it is preferable that both sides are X-faces. The same applies to the preferred range of the area of ​​the peaks having a height of 80 nm or more.

[0047] The device for measuring the area of ​​peaks having a height of 80 nm or more is not particularly limited, but for example, a non-contact surface / layer cross-sectional shape measuring system "VertScan" (registered trademark) 2.0 manufactured by Ryoka Systems Co., Ltd. can be used. The method for measuring the area of ​​peaks using this device will be described in detail later.

[0048] By making the area of ​​the peaks of 80 nm or more 0.5% or more, the slipperiness of the film is maintained. Therefore, during the conveying process when processing the biaxially oriented polypropylene film, the occurrence of wrinkles and deterioration of the winding shape of the film roll can be reduced, improving the processability. In addition, since the surface of the peaks of 80 nm or more with an area of ​​0.5% or more has moderate unevenness, when a metal film laminated film having a metal film formed on the surface is used for a film capacitor element, it is possible to prevent the gaps of the laminated or wound metal film laminated film from becoming excessively narrow. Therefore, the obtained film capacitor element is less likely to be broken by short circuit during use.

[0049] On the other hand, the area of ​​the peaks with a height of 80 nm or more being 5.0% or less means that there are no excessive peaks on the surface, and this embodiment appropriately suppresses the slipperiness. Therefore, the occurrence of winding deviation and meandering can be reduced during the conveying process when processing the biaxially oriented polypropylene film. Furthermore, when a metal film laminated film having a metal film formed on the surface is used in a film capacitor element, the gaps between the laminated or wound metal film laminated film do not become excessively wide. Therefore, the safety of the film capacitor element is not excessive, and the decrease in the withstand voltage characteristics caused by the peaks is reduced. That is, as a result, the life of the film capacitor element is improved.

[0050] There are no particular limitations on the method for forming an X-face on at least one side in which the area of ​​peaks having a height of 80 nm or more is 0.5% to 5.0%, and on the method for setting the area of ​​peaks having a height of 80 nm or more on the X-face within the above-mentioned preferred range. For example, there may be mentioned a method in which the above-mentioned polypropylene resin is used and the cooling step with a cooling roll downstream of the cast drum in the film production is performed under specific conditions as described later.

[0051] More specifically, the cooling on the cooling roll downstream of the casting drum is effective in rapidly cooling the cast sheet solidified by the casting drum so as to reduce the surface temperature unevenness of the cooling roll that further cools the cast sheet solidified by the casting drum. At this time, the peak area of ​​the biaxially oriented polypropylene film having a height of 80 nm or more can be reduced by lowering the cooling temperature and reducing the temperature unevenness of the film leaving the cooling roll. From the viewpoint of suitably adjusting the peak area of ​​the biaxially oriented polypropylene film having a height of 80 nm or more, the preferred temperature of the cooling roll is 15°C or more and 40°C or less, more preferably 20°C or more and 35°C or less, and even more preferably 23°C or more and 30°C or less. In addition, a heat pipe type cooling roll (details will be described later) can be used as a means for reducing the temperature unevenness of the film leaving the cooling roll. These methods can be used in combination as appropriate.

[0052] In the biaxially oriented polypropylene film of the present invention, the variation in the peak area of ​​80 nm or more in the X-plane is preferably 2.0% or less, more preferably 1.5% or less, and even more preferably 1.0% or less in at least one of the longitudinal and transverse directions. Hereinafter, the "peak area variation of 80 nm or more in the longitudinal direction" may be referred to as "R-MD" and the "peak area variation of 80 nm or more in the transverse direction" may be referred to as "R-TD". The lower limits of R-MD and R-TD are theoretically 0%, but considering compatibility with the peak area of ​​80 nm or more being 0.5% to 5.0%, 0.3% is preferable.

[0053] The variation in the peak area of ​​80 nm or more in each direction can be measured by the following method. First, a biaxially oriented polypropylene film sample is taken from a film roll in a rectangular shape of 100 mm (longitudinal direction) × width length (width direction), and the sample is divided into 30 equal parts parallel to the longitudinal direction to obtain 30 test pieces of 100 mm × 1 / 30 mm of the width length. Next, the peak area of ​​the center of the obtained test piece is measured 10 times, and the average value is taken as the peak area of ​​the corresponding test piece. After that, the same measurement is performed on all test pieces, and the maximum and minimum values ​​of the obtained peak area are taken as the "maximum peak area in the width direction" and the "minimum peak area in the width direction", respectively. Furthermore, the longitudinal direction and the width direction are swapped, and 30 test pieces are similarly obtained from the center of the width direction and similar measurements are performed to obtain the "maximum peak area in the longitudinal direction" and the "minimum peak area in the longitudinal direction". After the measurement is completed, R-MD and R-TD are calculated from the following formulas (4) and (5), respectively. The above-mentioned device can be used to measure the hillside area, and the details of the measurement method using this device will be described later. Formula (4): R-MD = Maximum area of ​​the mountain side in the longitudinal direction - Minimum area of ​​the mountain side in the longitudinal direction Equation (5): R-TD = maximum value of the area of ​​the peak side in the width direction - minimum value of the area of ​​the peak side in the width direction.

[0054] By making at least one of R-MD and R-TD on the X-plane 2.0% or less or the above-mentioned preferred range, when the metal film laminated film having the metal film formed on the X-plane is used in a film capacitor element, the amount of air and the gap distance between the laminated or wound metal film laminated film can be suitably suppressed. Therefore, by using the biaxially oriented polypropylene film of such an embodiment in a film capacitor, the safety of the film capacitor can be prevented from being too effective or not being effective enough when the film capacitor is used, the life of the film capacitor can be extended, or short circuit destruction can be prevented from occurring. In addition, by making it such an embodiment, the occurrence of winding deviation and meandering can be reduced in the conveying process during processing of the biaxially oriented polypropylene film. That is, when at least one of R-MD and R-TD is 2.0% or less or the above-mentioned preferred range, the biaxially oriented polypropylene film has excellent processability and voltage resistance.

[0055] The above-mentioned effects can be obtained by making at least one of R-MD and R-TD on the X-plane 2.0% or less or in the above-mentioned preferred range, but from the viewpoint of further enhancing the above-mentioned effects, it is more preferable that both R-MD and R-TD are 2.0% or less or in the above-mentioned preferred range. From the same viewpoint and from the viewpoint of increasing the degree of freedom in selecting the surface on which the metal film is formed, when both sides are X-planes, it is preferable that at least one of R-MD and R-TD is 2.0% or less or in the above-mentioned preferred range on both sides, and it is more preferable that both R-MD and R-TD are 2.0% or less or in the above-mentioned preferred range on both sides.

[0056] In order to make at least one of R-MD and R-TD 2.0% or less or within the above-mentioned preferred range, the above-mentioned polypropylene resin is used, and the cooling process with the cooling roll downstream of the casting drum during film production is set under specific conditions as described below. More specifically, it is effective to cool the cast sheet solidified by the casting drum by minimizing the surface temperature unevenness of the cooling roll. In other words, by minimizing the temperature unevenness of the film leaving the cooling roll, the R-MD and R-TD of the biaxially oriented polypropylene film can be reduced.

[0057] From the viewpoint of suitably adjusting the peak side area of ​​the biaxially oriented polypropylene film having a height of 80 nm or more and further suitably adjusting the R-MD and R-TD, the preferred temperature of the cooling roll is 15°C to 40°C, more preferably 20°C to 35°C, and even more preferably 23°C to 30°C. A heat pipe type cooling roll can be used as a means for reducing the temperature unevenness of the film leaving the cooling roll. As another method, it is also effective to control the surface structure by adjusting the difference obtained by subtracting the content (mass%) of the low stereoregular polypropylene resin (B) from the sum of the contents (mass%) of the above-mentioned branched chain polypropylene resin (I) and branched chain polypropylene resin (H) within the above-mentioned range. In this case, the smaller the value of the difference, the smaller the size and number of spherulites formed when the molten polymer is formed into a sheet, and therefore the smaller the R-MD and R-TD can be. These methods can be used in combination as appropriate.

[0058] In the biaxially oriented polypropylene film of the present invention, the protruding valley depth (RvK) in the X-plane is preferably 20 nm or more and 60 nm or less, more preferably 25 nm or more and 54 nm or less, and even more preferably 30 nm or more and 50 nm or less. Hereinafter, the protruding valley depth (RvK) may be simply referred to as RvK.

[0059] The protruding valley depth RvK is a feature parameter for evaluating a roughness curve defined in JIS B0671-2 (2002), and is an index showing how deep the part is in the defined area compared to the average unevenness of the surface. The measuring device for the protruding valley depth RvK is not particularly limited as long as it is capable of the above measurement, but for example, the non-contact surface / layer cross-sectional shape measuring system "VertScan" (registered trademark) 2.0 manufactured by Ryoka Systems Co., Ltd. can be used. The details of the measurement method when using this device will be described later.

[0060] By making the protruding valley depth RvK 20 nm or more on the X-plane, blocking and wrinkles caused by transport can be suppressed during the film transport process in film formation and film capacitor element processing. As a result, deterioration of the winding shape of the film roll and defects in the external and internal shapes of the film capacitor element can be reduced.

[0061] On the other hand, setting the protruding valley depth RvK on the X-face to 60 nm or less means that the recesses present on the X-face are shallow, and this embodiment reduces the deterioration of the withstand voltage characteristics caused by the recesses. Therefore, by using a biaxially oriented polypropylene film of this embodiment in a film capacitor, the life of the film capacitor can be extended. If RvK is set within the above range on at least one X-face, the surface can maintain an appropriate recess, improving processability and withstand voltage performance. In particular, when both sides are X-faces, if RvK is set to 20 nm or more and 60 nm or less or satisfies the above preferred range on both sides, processability and withstand voltage performance are further improved.

[0062] In order to make the RvK of the X-side 20 nm or more and 60 nm or less or the above-mentioned preferred range, the above-mentioned polypropylene resin is used, and the cooling process with the cooling roll downstream of the casting drum during film production is set under specific conditions as described below. More specifically, it is effective to rapidly cool the cast sheet solidified by the casting drum so as to reduce the surface temperature unevenness of the cooling roll that further cools the cast sheet. At this time, the RvK of the X-side of the biaxially oriented polypropylene film can be reduced by lowering the cooling temperature and reducing the temperature unevenness of the film leaving the cooling roll.

[0063] From the viewpoint of suitably adjusting the peak side area of ​​the biaxially oriented polypropylene film having a height of 80 nm or more and further suitably adjusting the RvK of the X-plane, the preferred temperature of the cooling roll is 15°C to 40°C, more preferably 20°C to 35°C, and even more preferably 23°C to 30°C. A heat pipe type cooling roll can be used as a means for reducing the temperature unevenness of the film leaving the cooling roll. In addition, as another method, it is also effective to control the surface structure by adjusting the difference obtained by subtracting the content (mass%) of the low stereoregular polypropylene resin (B) from the sum of the contents (mass%) of the above-mentioned branched chain polypropylene resin (I) and branched chain polypropylene resin (H) within the above-mentioned range. In this case, the smaller the value of the difference, the smaller the size and number of spherulites formed when the molten polymer is formed into a sheet, and therefore the smaller the RvK can be. These methods can be used in combination as appropriate.

[0064] The biaxially oriented polypropylene film of the present invention is preferably obtained by forming the polypropylene resin composition consisting of the linear polypropylene resin (A), the low stereoregular polypropylene resin (B), and the branched polypropylene resins (H) and (I) into a sheet and biaxially stretching the sheet. The biaxial stretching method may be any of the inflation simultaneous biaxial stretching method, the tenter simultaneous biaxial stretching method, and the tenter sequential biaxial stretching method, but it is preferable to adopt the tenter sequential biaxial stretching method from the viewpoint of film formation stability and thickness uniformity of the obtained biaxially oriented polypropylene film. In the tenter sequential biaxial stretching method, it is particularly preferable to stretch the film in the longitudinal direction and then in the transverse direction.

[0065] The biaxially oriented polypropylene film of the present invention preferably has a thickness of 1.0 μm or more and 4.0 μm or less from the viewpoints of film formability, mechanical strength, high-temperature voltage resistance, and capacitance per volume when used as a dielectric of a film capacitor. From the above viewpoints, the thickness is more preferably 1.2 μm or more and 3.8 μm or less, and even more preferably 1.4 μm or more and 3.0 μm or less. By making the thickness 1.0 μm or more, the biaxially oriented polypropylene film can be made excellent in mechanical strength and high-temperature voltage resistance, and breakage during film formation and processing can be reduced. On the other hand, by making the thickness 4.0 μm or less, when the biaxially oriented polypropylene film is used as a dielectric of a film capacitor, the capacitance per volume can be increased, and the film capacitor can be easily miniaturized. The thickness of the biaxially oriented polypropylene film can be measured by the micrometer method in accordance with JIS C 2330 (2014).

[0066] The thickness of the biaxially oriented polypropylene film can be adjusted, for example, by adjusting the slit width of the T-die, the discharge amount from the T-die, the rotation speed of the cast drum, the product of the stretching ratio, etc. More specifically, the thickness of the biaxially oriented polypropylene film can be reduced by narrowing the slit width of the T-die, reducing the discharge amount 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 combination.

[0067] Next, a method for producing the biaxially oriented polypropylene film of the present invention will be described below, but the biaxially oriented polypropylene film of the present invention is not necessarily limited to the one obtained by the method described below.

[0068] First, the linear polypropylene resin (A), the low stereoregular polypropylene resin (B), the branched polypropylene resin (H), and (I) are dry-blended so that the low stereoregular polypropylene resin (B), the branched polypropylene resin (H), and (I) are in the amounts described above, and then fed to a single-screw melt extruder, and melt extruded at 200 to 260 ° C. (The amount of the linear polypropylene resin (A) can be appropriately adjusted so that the amounts of the low stereoregular polypropylene resin (B), the branched polypropylene resin (H), and (I) are in the ranges described above.) Next, foreign matter and modified polymers are removed from the molten polymer composition by a filter installed in the middle of the polymer tube. Then, the molten polymer composition molded into a sheet shape is discharged from a T-die onto a casting drum and cooled and solidified to form a cast sheet, which is then cooled by a cooling roll.

[0069] The temperature of the cast drum is preferably 80°C or more and 120°C or less from the viewpoint of appropriately generating β crystals and spherulites, more preferably 85°C or more and 115°C or less, and even more preferably 85°C or more and 110°C or less. By setting the cast drum temperature at 80°C or more, the amount of β crystals formed in the cast sheet is not too small, and the slipperiness of the transport film and the obtained biaxially oriented polypropylene film is maintained, so that the occurrence of wrinkles in the film transport process during film production and processing and the deterioration of the winding shape of the film roll can be prevented. On the other hand, by setting the cast drum temperature at 120°C or less, the excessive formation of β crystals in the cast sheet can be prevented, and the occurrence of meandering in the film transport process during film production and processing and the deterioration of the winding shape of the film roll can be reduced.

[0070] A specific example of a method for controlling the surface temperature of the casting drum in the width direction and circumferential direction more uniformly will be described below, taking into consideration the mechanism by which the surface temperature of the casting drum becomes non-uniform. A casting drum usually has piping installed inside it parallel to the width direction or in a spiral shape (such a casting drum system is sometimes called a spiral type). The surface temperature of the casting drum can be controlled by introducing a temperature-controlled cooling medium into one end of the piping and letting it flow out from the other end. However, the heat transfer coefficient is reduced in the piping part parallel to the width direction or in a spiral shape, and a temperature difference occurs between the inflow and outflow of the cooling medium due to the amount of heat generated during film cooling on the casting drum surface, resulting in non-uniform surface temperature of the casting drum.

[0071] In addition, industrial water is generally used as the cooling medium. This industrial water contains foreign matter such as evaporation residues as well as metal components such as iron and manganese. Therefore, if industrial water is continuously flowed through the piping of the cooling drum for a long period of time, the above-mentioned foreign matter may adhere to and partially accumulate on the inner side surface of the piping of the cooling drum, or the foreign matter may react with the metal on the side surface of the piping to form agglomerates. The occurrence and fall of these deposits and agglomerates causes localized temperature unevenness on the surface of the cooling drum, so that the temperature of the surface of the cooling drum can be more uniformly controlled by removing these deposits and agglomerates early by periodically cleaning the piping, etc.

[0072] A more preferred method for controlling the surface temperature of the casting drum more uniformly is to use a casting drum having a plurality of jacket chambers within the thickness of the casting drum surface. In such a casting drum, a gas-liquid two-phase heat medium is sealed in the jacket chamber, and when the liquid-phase heat medium comes into contact with the casting drum surface that has become locally hot due to heat conduction accompanying contact with the molten polymer composition, the liquid-phase heat medium is vaporized and phase-changed to a gas-phase heat medium. When this gas-phase heat medium comes into contact with the casting drum surface that is still locally in a low temperature state, it is liquefied and phase-changed back to a liquid-phase heat medium. In this way, the heat medium vaporizes when it comes into contact with the high-temperature casting drum surface, taking away latent heat, and then vaporizes when it comes into contact with the low-temperature casting drum surface, providing latent heat, thereby making it possible to control the surface temperature of the casting drum more uniformly. The "thickness of the casting drum surface" refers to the cylindrical portion that forms the side when the casting drum is likened to a cylinder with a rotation axis as the central axis.

[0073] In addition, after the molten sheet discharged from the T-die lands on the casting drum, the time of adhesion to the casting drum is preferably 0.8 seconds or more and 3.0 seconds or less, and more preferably 1.0 seconds or more and 3.0 seconds or less. When the adhesion time is 0.8 seconds or more, the molten sheet is easily solidified, and breakage in the subsequent stretching process can be reduced. On the other hand, when the adhesion time is 3.0 seconds or less, excessive formation of β crystals in the cast sheet can be prevented, and the occurrence of meandering during the film transport process during film production and processing and the deterioration of the winding shape of the film roll can be reduced.

[0074] As a method for adhering the molten sheet to the cast drum, methods such as electrostatic application, air knife method, nip roll method, and underwater casting method can be adopted, but the air knife method is preferred from the viewpoints of suppressing thickness unevenness, high-speed film production, and controlling the surface properties of the film. The air temperature of the air knife is preferably 60°C or more and 100°C or less. By setting the air temperature of the air knife to 60°C or more, the amount of β crystals formed in the cast sheet is not too small, and the slipperiness of the film obtained after biaxial stretching is maintained, so that the occurrence of wrinkles and the deterioration of the winding shape of the film roll are reduced in the film conveying process during film production and processing. On the other hand, by setting the air temperature of the air knife to 100°C or less, excessive β crystals are not formed in the cast sheet, and the occurrence of meandering and the deterioration of the winding shape of the film roll are reduced in the film conveying process during film production and processing.

[0075] From the viewpoint of controlling at least one side of the obtained biaxially oriented polypropylene film as the X-face and the peak side area having a height of 80 nm or more, and the R-MD and R-TD in an appropriate range, it is preferable to rapidly cool the cast sheet solidified by the cast drum. From the above viewpoint, the temperature of the cooling roll is preferably 15°C or more and 40°C or less, more preferably 20°C or more and 35°C or less, and even more preferably 23°C or more and 30°C or less. By setting the temperature of the cooling roll to 15°C or more, the size and number of spherulites formed in the sheet during the cooling process do not become too small, and the peak side area having a height of 80 nm or more, the R-MD and R-TD can be controlled in an appropriate range while ensuring the film formation stability of the film. In addition, by setting the temperature of the cooling roll to 40°C or less, the size and number of spherulites formed in the sheet during the cooling process are suppressed, and the peak side area having a height of 80 nm or more, the R-MD and R-TD can be controlled in an appropriate range while ensuring the film formation stability.

[0076] As a method for rapidly cooling a cast sheet, it is effective to use a heat pipe type cooling roll with high cooling performance. The inside of the heat pipe type cooling roll is vacuum and the working fluid is sealed, and the sealed working fluid sticks to the inner wall of the outer cylinder by centrifugal force due to the rotation of the roll. When a high-temperature cast sheet comes into contact with this heat pipe type cooling roll, heat is transferred from the cast sheet and the working fluid evaporates, and the evaporated working fluid comes into contact with a cooling water pipe provided further inside the outer cylinder and is condensed, and the condensed working fluid is blown outward (toward the outer cylinder) by centrifugal force and sticks to the inner wall again. Therefore, the heat pipe type cooling roll is less likely to cause temperature unevenness that occurs with the spiral type cooling roll, and it is relatively easy to keep the surface temperature constant.

[0077] By cooling the cast sheet using such a heat pipe type cooling roll, the size and number of spherulites can be controlled within a suitable range, so that the obtained biaxially oriented polypropylene film has an X-face on at least one side, and further, the area of ​​the peaks on the X-face with a height of 80 nm or more, and the R-MD and R-TD can be adjusted within a suitable range. As a result, the obtained biaxially oriented polypropylene film can reduce the occurrence of winding slippage and meandering during the transport process during processing.

[0078] In addition, by performing such cooling, when the metal film laminated film obtained by forming a metal film on the X-face of the biaxially oriented polypropylene film is used in a film capacitor element, it is possible to suitably suppress the unevenness of the amount of air and the gap distance between the laminated or wound metal film laminated film. Therefore, the safety of the film capacitor is not excessive, and the deterioration of the withstand voltage characteristics caused by the peaks is reduced, and as a result, the life of the film capacitor can be extended.

[0079] A typical cooling roll is a spiral type cooling roll in which piping is provided in parallel to the width direction or in a spiral shape. In such a cooling roll, the surface temperature of the cooling roll can be controlled by inflowing a temperature-controlled cooling medium from one end of the piping and letting it out from the other end. However, in a spiral type cooling roll, the heat transfer coefficient is reduced in the spiral piping portion, and a temperature difference occurs between the inflow and outflow of the cooling medium due to the amount of heat generated during film cooling on the surface, resulting in an uneven surface temperature of the cooling roll. From the viewpoint of improving the withstand voltage characteristics of the obtained biaxially oriented polypropylene film in a high-temperature environment, it is desirable that the surface temperature of the cooling roll is low, but in a conventional cooling roll, unevenness occurs in the surface temperature, and when the cooling temperature is lowered, the temperature unevenness generated at the inflow and outflow portions becomes large, and breakage occurs frequently in the transverse stretching process due to uneven stretching. Therefore, by using a heat pipe type cooling roll, the surface temperatures of the inflow and outflow portions become more uniform, making it possible to lower the cooling temperature.

[0080] After the cast sheet contacts the cooling roll, the time for which it is in close contact with the cooling roll is preferably 1.0 seconds or more and 5.0 seconds or less, and more preferably 1.2 seconds or more and 4.8 seconds or less. When the time for close contact is 1.0 seconds or more, the size of the spherulites formed in the cooling sheet is not too small or the number is not too small, and the peak area with a height of 80 nm or more, R-MD and R-TD can be controlled within an appropriate range while ensuring film formation stability. On the other hand, when the time for close contact is 5.0 seconds or less, the size of the spherulites formed in the cooling sheet is not too large or the number is not too large, and the peak area with a height of 80 nm or more, R-MD and R-TD can be controlled within an appropriate range while ensuring film formation stability.

[0081] In addition, immediately after the exit of the cooling roll, the temperature unevenness of the cast sheet in the width direction is preferably 3.0°C or less, more preferably 2.0°C or less, and even more preferably 1.0°C or less. By adopting such an embodiment, the size and number of β crystals and spherulites formed in the cast sheet are uniform in the width direction, and it is easy to prevent the surface shape of the film from varying in the longitudinal and width directions. The smaller the temperature unevenness, the more preferable it is, and the lower limit is 0°C. The temperature unevenness can be obtained from the cast sheet temperature in the width direction measured using a known visual radiation thermometer, and the measurement and calculation methods will be described later. The temperature unevenness can be adjusted, for example, by using a heat pipe type cooling roll and setting the temperature of the cooling roll to 15°C or more and 40°C or more.

[0082] Next, the cast sheet after cooling is biaxially stretched. As specific stretching conditions, first, the temperature at which the cast sheet is stretched in the longitudinal direction (longitudinal stretching) is controlled. The temperature control method includes a method using a temperature-controlled rotating roll, a method using a hot air oven, and the like.

[0083] For example, when a temperature-controlled rotating roll is used in the preheating step of the longitudinal stretching, the cast sheet is preheated by passing it through a roll maintained at 100 to 125° C., preferably 100 to 120° C., and then preheated by passing it through a roll maintained at 140 to 150° C. Thereafter, the surface structure of the cast sheet may be controlled by locally applying heat by a radiation heater on the roll maintained at 135 to 150° C., preferably 140 to 150° C.

[0084] Next, the cast sheet is stretched in the longitudinal direction in the longitudinal stretching process. The cast sheet that has been through the preheating process for longitudinal stretching is passed through rolls whose temperature is controlled to 120°C or more and 150°C or less, and is stretched in the longitudinal direction at a predetermined stretching speed and stretch ratio depending on the peripheral speed difference between the rolls. The stretch ratio in the longitudinal direction is preferably 4.0 times or more and 7.0 times or less, and more preferably 5.0 times or more and 7.0 times or less. By setting the stretch ratio at 4.0 times or more, the surface properties of the obtained biaxially oriented polypropylene film become more uniform, and the voltage resistance characteristics at high temperatures are also improved. By setting the longitudinal stretch ratio at 7.0 times or less, the breakage of the film in the longitudinal stretching process and the subsequent transverse stretching process is reduced.

[0085] Next, both widthwise ends of the uniaxially oriented film obtained by the longitudinal stretching are held with clips, and the film is stretched in the widthwise direction at a stretch ratio of 5.0 to 15 times using a tenter-type stretching machine controlled at a temperature of 140 to 170° C. Furthermore, the film is heat-set at a temperature of 150 to 170° C. while being relaxed by 5 to 15% in the widthwise direction.

[0086] Next, the biaxially stretched film is slowly cooled to room temperature as necessary, and is subjected to a corona discharge treatment in air, nitrogen, carbon dioxide gas, or a mixture of these gases, the edge portions of the film held by the clips are cut and removed, and the film with the ends removed is wound up as a master roll by a winder. Finally, the film unwound from the master roll is slit to a specific width by a slitter, and wound around a core as a film roll to obtain a roll of the biaxially oriented polypropylene film of the present invention. Note that these treatments may be applied to both sides or one side.

[0087] The biaxially oriented polypropylene film of the present invention is preferably used as a dielectric for a film capacitor, but is not limited to the type of film capacitor. Specifically, from the viewpoint of electrode configuration, it may be either a foil-wrapped film capacitor or a metal-deposited film capacitor, and it is also preferably used for an oil-immersed type film capacitor containing insulating oil or a dry type film capacitor that does not use insulating oil at all. In addition, from the viewpoint of shape, it may be a wound type or a laminated type. Due to the characteristics of the biaxially oriented polypropylene film of the present invention, it is particularly preferably used as a metal-deposited film capacitor.

[0088] Next, the 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 on at least one side of the biaxially oriented polypropylene film to provide a metal film that will become an 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 also be deposited simultaneously with or successively to the aluminum. Also, a protective layer can be provided on the metal film using oil or the like. The thickness of the metal film is preferably 20 nm or more and 100 nm or less from the viewpoint of the electrical characteristics and safety of the film capacitor. Also, for the same reason, the surface resistance value of the metal film is preferably 1 Ω / sq or more and 20 Ω / sq or less. The surface resistance value can be controlled by the type of metal used and the film thickness. The metal film may be formed on either side of the biaxially oriented polypropylene film, but if only one side is an X-face, it is preferable to form it on the X-face.

[0089] Next, a film capacitor using the metal film laminated film of the present invention will be described. The film capacitor of the present invention has a configuration in which the metal film laminated film of the present invention is laminated or wound. An example of a manufacturing method of a wound film capacitor will be described below. First, aluminum is vacuum-deposited on one side of the biaxially oriented polypropylene film of the present invention. At that time, aluminum is deposited in stripes having margins running in the longitudinal direction of the film. Next, a blade is inserted into the center of each deposition part on the surface and the center of each margin part to slit the surface, and a tape-shaped winding reel having a margin on one side is produced. Two tape-shaped winding reels having a left margin and a right margin are overlapped and wound in the width direction so that the deposition part protrudes from the margin part, and a wound body is obtained. After heat treatment of the wound body, metallicon is sprayed on both end faces in the width direction to form external electrodes, and a lead wire is welded to the metallicon to obtain a wound film capacitor.

[0090] The applications of film capacitors are diverse, including vehicles, home appliances (such as televisions and refrigerators), general noise control, automobiles (such as hybrid cars, power windows, and wipers), and power sources, and film capacitors using the biaxially oriented polypropylene film of the present invention can also be suitably used for these applications. EXAMPLES

[0091] 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.

[0092] [Measurement and evaluation method] (1) Mesopentad fraction (mmmm) A polypropylene resin sample is dissolved in a solvent. 13 The mesopentad fraction (mmmm) was determined using C-NMR under the following conditions (Reference: New Edition Polymer Analysis Handbook, edited by the Japan Society for Analytical Chemistry and the Polymer Analysis Research Forum, 1995, pp. 609-611).

[0093] A. Measurement conditions Equipment: Bruker DRX-500 Measurement nuclei: 13 C nucleus (resonance frequency: 125.8MHz) Measured concentration: 10% by mass Solvent: Benzene / d-o-dichlorobenzene = 1:3 mass ratio 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 Conversion count: 10,000 times Measurement mode: complete decoupling.

[0094] B.Analysis conditions Fourier transformation was performed with LB (line broadening factor) set to 1.0, the mmmm peak was set to 21.86 ppm, and peak division was performed using WINFIT software (Bruker). At that time, peak division was performed as follows from the peak on the high magnetic field side, and automatic fitting was further performed using the attached software. After optimizing the peak division, the total mmmm peak fraction was calculated. The above measurement was performed five times, and the average value was taken as the mesopentad fraction (mmmm) of this sample. (peak division) (a)mrrm (b)(c) rrrm (split into two peaks) (d)rrrr (e)mrmr (f)mrmm+rmrr (g)mmrr (h)rmmr (i) mmmr (j)mmmm.

[0095] (2) Melt flow rate (MFR) (unit: g / 10 min) Measurements were performed at 230°C and 2.16 kg in accordance with JIS K 7210-1 (2014).

[0096] (3) Melt tension (MS) (unit: cN) Measurements were performed using a melt tension tester (capillary diameter 2.1 mm, cylinder diameter 9.55 mm) manufactured by Toyo Seiki Seisakusho Co., Ltd., in the following manner. First, polypropylene resin was heated to 230°C to melt it. Next, the molten polypropylene resin was extruded into a strand at an extrusion speed of 15 mm / min, and the tension was measured when this strand was taken up at a speed of 6.5 m / min, and the obtained value was taken as MS.

[0097] (4) Cold xylene solubles (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 thermostatic water bath at 20°C for 1 hour. Solids such as crystals were then removed by filtration, and the polypropylene components dissolved in the filtrate were quantified by liquid chromatography. The mass of the polypropylene resin before dissolution in boiling xylene was X0 (g), and the mass of the polypropylene components dissolved in the filtrate was X (g), and CXS was calculated from the following formula (1). Formula (1): CXS(mass%)=(X / X0)×100.

[0098] (5) Area of ​​peaks with heights of 80 nm or more (unit: %), protruding valley depth RvK (unit: nm) The measurement was performed using a non-contact surface / layer cross-sectional shape measuring 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) × 0.82 m (width direction), and the sample was divided into 30 equal parts parallel to the longitudinal direction to obtain 30 test pieces of 100 mm × 27.3 mm. Next, the peak side area with a height of 80 nm or more and the protruding valley depth RvK of the obtained test piece were measured 10 times at arbitrary positions under the following conditions, and the average values ​​were taken as the "peak side area with a height of 80 nm or more" and the "protruding valley depth RvK" of the test piece, respectively.

[0099] After that, the same measurements were performed on all the test pieces, and the maximum and minimum values ​​of the peak side area with a height of 80 nm or more were respectively defined as the "maximum peak side area in the width direction" and the "minimum peak side area in the width direction". Furthermore, the longitudinal and width directions were swapped to take a biaxially oriented polypropylene film sample of the same size from the center of the width direction, and the same measurements were performed on the test pieces obtained by dividing this into 30 equal parts parallel to the width direction, and the "maximum peak side area in the longitudinal direction" and "minimum peak side area in the longitudinal direction" were similarly obtained. Next, the average values ​​were calculated from the peak side area with a height of 80 nm or more and the protruding valley depth RvK of all the test pieces divided into 30 equal parts parallel to the longitudinal direction and all the test pieces divided into 30 equal parts parallel to the width direction, and the obtained values ​​were respectively defined as the "peak side area with a height of 80 nm or more" and the "protruding valley depth (RvK)" of the biaxially oriented polypropylene film.

[0100] After the measurement, the longitudinal area variation of peaks with a height of 80 nm or more (R-MD) and the widthwise area variation of peaks with a height of 80 nm or more (R-TD) were calculated using the following formulas (4) and (5). Note that the measurement was performed on both sides. Formula (4): R-MD = Maximum area of ​​the mountain side in the longitudinal direction - Minimum area of ​​the mountain side in the longitudinal direction Equation (5): R-TD = maximum value of the area of ​​the peak side in the width direction - minimum value of the area of ​​the peak side in the width direction.

[0101] A. Measurement conditions The detailed conditions for one 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 Measurement field of view (field of view area): 0.939 mm (vertical) x 1.252 mm (horizontal) = 1.176 μm 2 .

[0102] B. Fixation of the test specimen A dedicated sample holder was used to fix the test piece during measurement. The sample holder is two detachable metal plates with a circular hole in the center, and the film is sandwiched between them without any wrinkles and fixed in place, and the film in the central circular part was measured. The film and sample holder were placed so that the longitudinal direction of the film roll and the vertical direction of the measurement field of view were aligned.

[0103] C. Analysis method The data obtained by the above measurement was analyzed with the image analysis software VS-Viewer of "VertScan" (registered trademark) 2.0. First, noise was removed using a median filter (5 x 5), and waviness components were removed using a Gaussian filter with a cutoff value of 250 μm. Next, analysis was performed using the bearing function, which is an analysis tool of the attached analysis software. In order to specify a mountain side region with a height of 80 nm or more, the mountain side height threshold was set to +80 nm in the height region specification. Next, the analyzed mountain side area value was read and rounded to two significant digits. Next, RvK was calculated using the bearing function, which is an analysis tool of the attached analysis software.

[0104] (6) Thickness (unit: μm) The thickness of the biaxially oriented polypropylene film was measured by the micrometer method according to JIS C 2330 (2014).

[0105] (7) Film-forming evaluation The film formability of the biaxially oriented polypropylene film was evaluated according to the following criteria. Note that the time from when film formation was stopped due to the occurrence of film breakage during the process until film formation was restarted was excluded from the observation time. ⊚: No film tear occurred after 48 hours. ○: Film breakage occurred 1 to 3 times within 48 hours. △: The film broke 4 to 6 times within 48 hours. ×: The film broke 7 or more times within 48 hours.

[0106] (8) Evaluation of element processability in film capacitor manufacturing On the corona-treated side of the biaxially oriented polypropylene film, aluminum was vacuum-deposited using a vacuum deposition machine manufactured by ULVAC Co., Ltd. so that the surface resistance was 15Ω / sq. At that time, aluminum was deposited in stripes with margins running in the longitudinal direction (repeated with a width of 79.0 mm for the deposited part and a width of 1.0 mm for the margin part). Next, a blade was inserted into the center of each deposited part and the center of each margin part to slit the film, and a tape-like take-up reel with a total width of 40 mm and a margin part of 0.5 mm on either the left or right end was produced. Two pieces of the left margin and one piece of the right margin of the obtained reel were overlapped and wound so that the deposited part protruded 0.5 mm beyond the margin part in the width direction, and a wound body with a capacitance of 120μF was obtained. Note that a KAW-4NHB manufactured by Kaito Seisakusho Co., Ltd. was used for winding. Finally, the wound body was heat-treated for 10 hours in a reduced pressure atmosphere at 140°C. The wound bodies were visually observed, and those with wrinkles or distortions in appearance or inside were determined to be defective. 200 wound bodies were produced in the same manner and the same evaluation was repeated, and the workability of the wound bodies was evaluated according to the following criteria. ◎: No defective products 〇: Less than 1 defective product △: 2 to less than 3 defective items ×: 4 or more defective items.

[0107] (9) Life evaluation of film capacitors A wound body with a capacitance of 120 μF was obtained by the method described in (8). The wound body was then heat-treated for 10 hours in a reduced pressure atmosphere at 140° C., and metallicon was sprayed on both end faces in the width direction to form external electrodes, and a lead wire was welded to the metallicon to obtain a film capacitor. Next, a life evaluation was performed on 15 film capacitors in the following procedure. First, the capacitance (C0) was measured at room temperature. Next, a voltage of 325 VDC / μm (when the thickness was 2.0 μm, the applied voltage was 650 V) was applied to the film capacitor at a high temperature of 120° C. for 1000 hours. Thereafter, the capacitance (C) was measured at room temperature, and the change rate of the capacitance before and after the voltage application (ΔC unit: %) was calculated from the following formula (6). The capacitance was measured using an LCR high tester 3522-50 manufactured by Hioki E.E. Corporation. Equation (6): ΔC=((C0-C) / C0)×100 The average rate of change in capacitance (ΔC) before and after voltage application for 15 film capacitors was taken as the rate of change in capacitance for that sample before and after voltage application, and was evaluated according to the following criteria. The smaller the rate of change in capacitance (ΔC) before and after voltage application, the more suppressed the decrease in capacitance at high temperatures is, and the better the life evaluation of the film capacitor can be said to be. ◎: ΔC is less than 2% ○: ΔC is 2% or more and less than 3% △: ΔC is 3% or more and less than 5% ×: ΔC is 5% or more.

[0108] (10) Temperature unevenness of the cast sheet leaving the cooling roll The temperature of the cast sheet leaving the cooling roll was measured in the width direction parallel to the sheet using a Fluke visual radiation thermometer VT06. The maximum and minimum values ​​were identified from the obtained thermographic image, and the difference between them was taken as the temperature unevenness of the cast sheet leaving the cooling roll.

[0109] [Raw materials] (1) Resin Linear polypropylene resin (A): "Borclean" (trademark) HC300BF manufactured by Borealis: A linear polypropylene resin having a mesopentad fraction of 0.980, a CXS of 1.2 mass%, an MFR of 3.3 g / 10 min, and an MS of 1.0 cN. Low stereoregularity polypropylene resin (B): "L-MODU" (registered trademark) S901 manufactured by Idemitsu Kosan Co., Ltd. A low stereoregular polypropylene resin having an MFR of 50 g / 10 min. Branched polypropylene resin (H): "WAYMAX" (registered trademark) (MFX3) manufactured by Japan Polypropylene Corporation: A branched polypropylene resin having 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 Japan Polypropylene Corporation: A branched polypropylene resin having an MFR of 2.9 g / 10 min and an MS of 9.0 cN.

[0110] (2) Antioxidants Antioxidant 1: "Irganox" (registered trademark) 1010 manufactured by BASF Japan Ltd. Antioxidant 2: 2,6-di-t-butyl-p-cresol (BHT).

[0111] Example 1 A polypropylene resin mixture obtained by mixing linear polypropylene resin (A), low stereoregular polypropylene resin (B), branched polypropylene resin (H), and branched polypropylene resin (I) at 83.0:7.0:5.0:5.0 (mass ratio), antioxidant 1, and antioxidant 2 were dry-blended at 99.5:0.4:0.1 (mass ratio) and fed to a single-screw melt extruder, and melt extrusion was performed at 250 ° C. Thereafter, foreign matter was removed from the extruded molten polypropylene resin composition with a sintered filter with a cut of 25 μm, and the composition was further discharged in a sheet form from a T-shaped slit die. Furthermore, the sheet-shaped molten polypropylene resin composition was brought into close contact with a cast drum maintained at a temperature of 90 ° C. by an air knife with an air temperature of 80 ° C., and solidified to obtain a cast sheet. The cast drum used had a plurality of jacket chambers in the wall thickness and contained a gas-liquid two-phase heat transfer medium, and the surface temperature was controlled by controlling the temperature of the cooling water passed inside the cast drum, and the temperature unevenness was controlled by controlling the amount of cooling water passed inside the cast drum. The cast sheet was then cooled on a cooling roll maintained at a temperature of 25°C to obtain a cooling sheet. At this time, the time that the sheet-shaped molten polypropylene resin composition was in close contact with the cast drum and the cooling roll was 1.5 seconds, respectively (hereinafter, the surface that was in contact with the cast drum is referred to as the drum surface (D surface), and the surface that was not in contact with the cast drum is referred to as the non-drum surface (non-D surface)). In addition, a heat pipe type cooling roll maintained at a temperature of 25°C was used as the cooling roll.

[0112] Next, in the preheating process before longitudinal stretching, the cast film was preheated by passing it through rolls maintained at 120°C and then through rolls maintained at 145°C. After that, the cast sheet that had passed through the preheating process before longitudinal stretching was stretched in the longitudinal direction at a stretch ratio of 5.6 times using longitudinal stretching rolls at a temperature of 145°C to produce a uniaxially oriented film. Furthermore, with both ends in the width direction held by clips, the uniaxially oriented film was introduced into a tenter and stretched in the width direction at a temperature of 159°C and a stretch ratio of 11 times. Next, a 12% relaxation treatment was performed in the width direction at a temperature of 158°C, and the film was gradually cooled to room temperature, and the D-side side was stretched at 25 W·min / m 2The biaxially oriented polypropylene film was subjected to a corona discharge treatment at a treatment strength of 1000 μm. The widthwise end of the obtained biaxially oriented polypropylene film, which was held by the clip, was cut off and wound up by a winder to obtain an intermediate product having a width of 6.0 m. Next, the film was unwound from the intermediate product, and the unwound film was slit in parallel to the longitudinal direction by a slitter so that the width (length in the width direction) was 0.82 m, and 30,000 m of biaxially oriented polypropylene film was wound around a core in the longitudinal direction to obtain a biaxially oriented polypropylene film roll having a thickness of 2.1 μm. The physical properties and evaluation results of the obtained biaxially oriented polypropylene film are shown in Table 1.

[0113] (Examples 2 to 9, Comparative Examples 1 to 5) 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 conditions for the cooling step of the cast sheet were as shown in Table 1. The physical properties and evaluation results of the obtained biaxially oriented polypropylene film are shown in Table 1.

[0114] [Table 1]

[0115] In producing the biaxially oriented polypropylene films of each Example and Comparative Example, an antioxidant was added, but since this was a small amount and some of it was lost during the production process, the composition of the polypropylene resin mixture is shown (this is substantially equivalent to the composition of the obtained biaxially oriented polypropylene film). [Industrial Applicability]

[0116] The present invention provides a biaxially oriented polypropylene film that is excellent in processability and voltage resistance characteristics under high-temperature environments, and can provide appropriate safety when used as a dielectric for a film capacitor (mainly a large-capacity film capacitor). Therefore, when the biaxially oriented polypropylene film of the present invention is used as a film capacitor, it exhibits high safety even under high-temperature and high-voltage environments, and also improves its lifespan. Therefore, the biaxially oriented polypropylene film of the present invention can be suitably used for film capacitors in applications where the environment is prone to high temperatures, such as automobile applications (including hybrid cars and electric cars), solar power generation, and wind power generation.

Claims

1. A biaxially oriented polypropylene film, characterized in that it has a surface (hereinafter referred to as X-surface) on at least one side, in which the area of ​​peaks having a height of 80 nm or more is 0.5% or more and 5.0% or less.

2. 2. The biaxially oriented polypropylene film according to claim 1, wherein the variation in the area of ​​peaks having a height of 80 nm or more on the X-plane is 2.0% or less in at least one of the longitudinal direction and the width direction.

3. The biaxially oriented polypropylene film according to claim 1 or 2, wherein the protruding valley depth (RvK) is 20 nm or more and 60 nm or less in the X-plane.

4. The present invention comprises a linear polypropylene resin (A), a low stereoregular polypropylene resin (B), and two branched polypropylene resins (H) and (I), The branched polypropylene resin (H) has a melt tension (MS) of 1.0 cN or more and 6.0 cN or less and a melt flow rate (MFR) of 4.0 g / 10 min or more and 10.0 g / 10 min or less, The biaxially oriented polypropylene film according to claim 1 or 2, wherein the branched polypropylene resin (I) has an MS of more than 6.0 cN and not more than 20 cN and an MFR of 1.0 g / 10 min or more and less than 4.0 g / 10 min.

5. The biaxially oriented polypropylene film according to claim 4, comprising, based on 100% by mass of all components constituting the biaxially oriented polypropylene film, 70% by mass or more and less than 95% by mass of the linear polypropylene resin (A), more than 1.0% by mass and 10.0% by mass or less of the low stereoregularity polypropylene resin (B), 2.0% by mass or more and 10.0% by mass or less of the branched polypropylene resin (H), and 2.0% by mass or more and 10.0% by mass or less of the branched polypropylene resin (I).

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

7. A film capacitor having a laminated or wound configuration of the metal film laminated film according to claim 6.

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