JP7375599B2 - Biaxially oriented polypropylene film, metal film laminate film and film capacitor - Google Patents

Description

The present invention relates to a biaxially oriented polypropylene film, a metal film laminate film, and a film capacitor that have high voltage resistance under high temperature and high voltage environments when used as a dielectric of a capacitor.

Biaxially oriented polypropylene film has excellent transparency, mechanical properties, and electrical properties, so it is used in a variety of applications including packaging, tape, cable wrapping, and electrical applications such as capacitors.

Among these, it is particularly preferably used as a dielectric material for capacitors because of its excellent high withstand voltage characteristics and low loss characteristics. Recently, inverters are being used in various electrical equipment, and as a result, the demand for smaller capacitors and larger capacitors has become stronger. Furthermore, especially in automotive applications (including hybrid cars and electric vehicles), solar power generation, and wind power generation applications, the operating environment is becoming increasingly hot (85°C or higher and 125°C or lower), and there is an increasing demand for heat resistance for capacitors. There is. Therefore, the dielectric biaxially oriented polypropylene film is required to be thinner, more heat resistant, have higher withstand voltage per thickness, and improve the safety of capacitors. Here, the safety of capacitor elements refers to the function of maintaining insulation by scattering the deposited metal using discharge energy during abnormal discharge in a metal-deposited capacitor whose electrode is a metal-deposited film formed on a dielectric film. This is an important function in preventing short circuits and destruction of capacitors. In order to improve the safety of capacitors, it is known that it is important to control the amount of air and gap distance between the film layers that make up the capacitor, especially when the size is large such as a large capacity capacitor of 100 μF or more. The challenge is to uniformly control the amount of air and gap distance in the longitudinal and lateral directions of the capacitor.

In order to improve the withstand voltage per film thickness and the safety of capacitors, studies have been mainly conducted to control surface properties.

As a method for controlling the surface properties of a film, a method that utilizes crystal transition from β crystal to α crystal of polypropylene (hereinafter referred to as β crystal method) is known. This method that utilizes crystal transition is preferably used as a method for roughening biaxially oriented polypropylene films for capacitors because there is no need to mix in impurities such as additives that may cause deterioration of withstand voltage (e.g. , see Patent Documents 1 and 2).

Techniques that focus on the density of surface roughness and uniformity of protrusions include a method of adding branched polypropylene (for example, see Patent Documents 3 and 4) and a method of mixing polypropylene with different molecular weights and molecular weight distributions (for example, the patent (see document 5) has been proposed. Since these methods can control the spherulite size to a small size, it is possible to form convex portions with uniform height and high density.

As a technique for reducing the recesses formed by the β-crystal method, a method has been proposed in which the conditions for applying heat to the front and back surfaces of the film are controlled to optimize the amount of β-crystals formed (for example, see Patent Document 6). With this method, the withstand voltage of the film can be improved since the number of recesses can be reduced.

In addition, as a technology that focuses on the balance between the longitudinal and widthwise slipperiness of the film, a method has been proposed in which the in-plane slipperiness is isotropically controlled by adjusting the casting temperature and stretching temperature (for example, Patent Document 7). This method improves the roll transportability of the film, which improves device processability, and reduces unevenness in interlayer adhesion and residual stress when processed into devices.

Furthermore, in addition to controlling surface properties, a method of reducing heat shrinkage stress of a film has been proposed as a method of improving the heat resistance of a capacitor (see, for example, Patent Document 8). With this method, the film has high dimensional stability, so it can be heat treated at higher temperatures during capacitor element processing, and the longitudinal winding improves the uniformity of the interlayer gap of the film, improving the life of the capacitor. be able to.

Japanese Patent Application Publication No. 2008-133446 Japanese Patent Application Publication No. 2014-077057 WO2007-094072A1 WO2012-121256A1 Japanese Patent Application Publication No. 2014-231584 WO2017-077752A1 WO2016-158590A1 WO2014-148547A1

The method described in Patent Documents 1 and 2 mentioned above is because when the β-crystal method is applied using general linear polypropylene, steep crater-like convex portions and concave portions are formed at a low density. However, in recent years, it has not been possible to say that the amount of air between film layers has been sufficiently controlled to ensure voltage resistance and safety in high-temperature and high-voltage environments.

In addition, although the methods described in Patent Documents 3, 4, and 5 described above can form convex portions with uniform height at a high density, they also have concave portions at a high density and are prone to dielectric breakdown.・It could not be said that the amount of air between the film layers was sufficiently controlled for voltage resistance and safety in a high voltage environment.

Although the method described in Patent Document 6 can improve the withstand voltage of the film, it is necessary to form coarse protrusions in order to maintain workability, so when processed into a capacitor element, air between the film layers is removed. The amount tends to increase, and safety cannot necessarily be said to function properly in the high-temperature, high-voltage environments of recent years.

Although the method described in Patent Document 7 can reduce interlayer adhesion and uneven residual stress when processed into an element, it is not possible to reduce the unevenness of interlayer adhesion and residual stress when processed into an element. It is insufficient to make the amount of air and the gap distance between the film layers uniform when processed, and it cannot be said that the safety performance necessarily functions properly in the recent high temperature and high voltage environments.

Furthermore, although the method described in Patent Document 8 can improve the life of the capacitor, the film is relaxed at a high relaxation rate in the process of reducing heat shrinkage stress in the film manufacturing process, which weakens the orientation of the film. However, in recent years, the voltage resistance has not been sufficient in the high-temperature, high-voltage environment.

Therefore, an object of the present invention is to control the amount of air and the gap distance between the film layers of a capacitor in order to have high productivity, processability, and voltage resistance, and to obtain appropriate safety mainly in large-capacity capacitors. The object of the present invention is to provide a biaxially oriented polypropylene film having surface properties that are uniformly controlled in the longitudinal and lateral directions.

The above-mentioned problems can be achieved by the following.

A biaxially oriented polypropylene film containing a polypropylene resin as a main component, the biaxially oriented polypropylene having a surface having an aspect ratio (Str) of 0.55 to 0.95 on at least one side. film.

When the biaxially oriented polypropylene film of the present invention is used as a dielectric material of a capacitor, it has excellent productivity, processability, and voltage resistance. Since it can be controlled uniformly in the hand direction, it functions with high safety even in high temperature and high voltage environments, improving the life of the capacitor.

Hereinafter, the biaxially oriented polypropylene film, metal film laminate film, and film capacitor of the present invention will be explained in more detail.

The biaxially oriented polypropylene film of the present invention is a biaxially oriented polypropylene film obtained by stretching a cast sheet in two directions, the longitudinal direction and the width direction. In other words, the biaxial orientation here means stretching in the longitudinal direction and the width direction.

The biaxially oriented polypropylene film of the present invention has a polypropylene resin as a main component. Note that the term "main component" means that the polypropylene resin is contained in 90% by mass or more and 100% by mass or less in 100% by mass of the film.

The biaxially oriented polypropylene film of the present invention is not particularly limited in the details of the polypropylene resin as long as it has a polypropylene resin as a main component, but the biaxially oriented polypropylene film contains a highly stereoregular polypropylene (A) and a high melt tension polypropylene (H). It is preferable. In the case where the biaxially oriented polypropylene film of the present invention contains highly stereoregular polypropylene (A) and high melt tension polypropylene (H), the highly stereoregular polypropylene (A) is contained in 100% by mass of the film. It is sufficient that the total content of high melt tension polypropylene (H) is 90% by mass or more and 100% by mass or less. The content of the polypropylene resin in the film is preferably 95% by mass or more and 100% by mass or less, more preferably 97% by mass or more and 100% by mass or less, particularly preferably 99% by mass or more and 100% by mass or less. The highly stereoregular polypropylene (A) and the high melt tension polypropylene (H) will be described later.

The biaxially oriented polypropylene film of the present invention may contain various additives, such as crystal nucleating agents, antioxidants, heat stabilizers, antistatic agents, antiblocking agents, fillers, viscosity It is preferable to contain a conditioning agent, a coloring inhibitor, etc.

Among the above-mentioned additives, selection of the type and content of the antioxidant is important from the viewpoint of long-term heat resistance. In other words, the antioxidant is preferably a phenolic one having steric hindrance, at least one of which is of a high molecular weight type having a molecular weight of 500 or more. Specifically, for example, 2,6-di-t-butyl-p-cresol (BHT: molecular weight 220.4), Irganox (registered trademark) 1330 (molecular weight 775.2) manufactured by BASF Japan, Ltd. It is preferable to use Irganox (registered trademark) 1010 (molecular weight 1177.7) alone or in combination. The total content of the above additives is preferably 0.01% to 1.0% by mass, more preferably 0.1% to 0.9% by mass, and more preferably 0.15% to 0.9% by mass based on 100% by mass of the film. It is more preferably 0.6% by mass, and particularly preferably 0.15 to 0.6% by mass.

The biaxially oriented polypropylene film of the present invention has a surface having an aspect ratio (Str) of 0.55 to 0.95 on at least one side. The aspect ratio (Str) of the surface texture is more preferably from 0.60 to 0.90, even more preferably from 0.65 to 0.85, and particularly preferably from 0.70 to 0.80.

The aspect ratio (Str) of the surface texture is one of the three-dimensional parameters of the surface texture defined by ISO25178-2 (2012), and is used as an index of the isotropy of the surface texture. The aspect ratio (Str) of the surface texture is defined as the horizontal distance (the minimum It is the value obtained by dividing the autocorrelation length (corresponding to Sal) by the horizontal distance in the direction of slowest decay to the value s. In other words, the aspect ratio (Str) of the surface texture has a value between 0 and 1, but when the aspect ratio (Str) of the surface texture is 0 or close to 0, the surface texture has a regular pattern with high anisotropy. On the other hand, when the aspect ratio (Str) of the surface texture is 1 or close to 1, it indicates that the surface texture has a highly isotropic random pattern. Further information regarding three-dimensional parameters including the aspect ratio (Str) of surface textures can be found, for example, in "New 3D Parameters and Filtration Techniques for Surface Metrology", Francois Blateyron, 2006.

By setting the aspect ratio (Str) of the surface texture to 0.55 or more, the surface texture becomes a moderately random pattern, which prevents unevenness in the amount of air and gap distance between film layers when laminating films in the capacitor element forming process. It is possible to prevent safety from becoming too effective or less effective when using a capacitor, prolonging the life of the capacitor, or making short-circuit damage less likely to occur. Furthermore, air trapped during film transport is easily released, suppressing meandering and wrinkles, and preventing deterioration of the winding appearance of the film roll and defects in the appearance and internal shape of the capacitor element. On the other hand, by setting the aspect ratio (Str) of the surface texture to 0.95 or less and leaving an appropriate amount of unevenness in the amount of air and gap distance between film layers when laminating films in the capacitor element formation process, it is possible to prevent the occurrence of simultaneous problems when using a capacitor. It is possible to prevent safety from being activated and extend the life of the capacitor.

The biaxially oriented polypropylene film of the present invention preferably has a surface texture with an aspect ratio (Str) of 0.55 to 0.95 and a minimum autocorrelation length (Sal) of 10 to 40 μm. The minimum autocorrelation length (Sal) is more preferably 15 to 35 μm, even more preferably 18 to 32 μm, and particularly preferably 20 to 30 μm.

The minimum autocorrelation length (Sal) is one of the three-dimensional parameters of the surface texture defined in ISO25178-2 (2012), and is a parameter that is an index of the period of the surface texture in the in-plane direction. The autocorrelation length (Sal) is the horizontal distance in the direction in which the autocorrelation function defined in ISO25178-2 (2012) attenuates fastest to a specific value s (s=0.2 in the present invention). A small value of the autocorrelation length (Sal) indicates that the surface texture is more finely uneven in the in-plane direction, while a larger value of the autocorrelation length (Sal) indicates that the surface texture is more fine in the in-plane direction. Indicates that the shape has a long period.

Setting the minimum autocorrelation length (Sal) to 10 μm or more prevents the density of unevenness on the film surface from becoming too high, prevents the amount of air between film layers from becoming too large when forming capacitor elements, and improves safety when using capacitors. Easy to prevent too much. On the other hand, setting the minimum autocorrelation length (Sal) to 40 μm or less prevents the density of unevenness on the film surface from becoming too low, prevents the amount of air between film layers from becoming too small when forming a capacitor element, and prevents short circuits when using a capacitor. Easy to prevent destruction.

The biaxially oriented polypropylene film of the present invention preferably has a surface texture with an aspect ratio (Str) of 0.55 to 0.95 and an arithmetic mean height (Sa) of 10 to 70 nm. The arithmetic mean height (Sa) is more preferably 15 to 65 nm, even more preferably 20 to 60 nm, and particularly preferably 25 to 55 nm. Setting the arithmetic mean height (Sa) to 10 nm or more maintains slipperiness, suppresses the occurrence of wrinkles during film transport during film formation and capacitor element processing, and prevents deterioration of the winding appearance of the film roll and the formation of capacitor elements. Defects in appearance and internal shape can be prevented. In addition, when the arithmetic mean height (Sa) is set to 70 nm or less, the film is prevented from slipping too much in the film transport process during film forming and processing, preventing the film from meandering, and preventing deterioration of the winding shape of the film roll and capacitors. Defects in the external appearance and internal shape of the element can be prevented.

The biaxially oriented polypropylene film of the present invention preferably has a thickness (t) of 1.0 to 3.0 μm. The thickness (t) is more preferably 1.2 to 2.8 μm, and even more preferably 1.5 to 2.5 μm. By setting the thickness (t) to 1.0 μm or more, the film can have excellent mechanical strength and high-temperature withstand voltage characteristics, and can also prevent film breakage during film formation and processing. On the other hand, by setting the thickness (t) to 3.0 μm or less, the capacitance per volume can be increased when used as a capacitor dielectric.

The biaxially oriented polypropylene film of the present invention preferably has surfaces on both sides having an aspect ratio (Str) of 0.55 to 0.95. When one side of the film is called the A side and the other side is called the B side, in a capacitor element, the A side and the B side of the film are usually in contact, and air exists in the gap between the layers. Therefore, if the aspect ratio (Str) of the surface texture is 0.55 to 0.95 on both the A side and B side of the film, unevenness in the interlayer air amount and gap distance during capacitor element formation can be more appropriately controlled. This is more preferable because it results in an improvement in the life of the capacitor, especially at high temperatures.

The biaxially oriented polypropylene film of the present invention preferably has surfaces on both sides having a minimum autocorrelation length (Sal) of 10 to 40 μm. When the minimum autocorrelation length (Sal) is 10 to 40 μm on both the A side and B side, the amount of interlayer air and gap distance during capacitor element formation can be controlled more appropriately, and the life of the capacitor can be improved, especially at high temperatures. This result is more preferable.

Next, the polypropylene raw material used in the biaxially oriented polypropylene film of the present invention will be explained. The biaxially oriented polypropylene film of the present invention preferably contains high melt tension polypropylene (H) in highly stereoregular polypropylene (A).

The highly stereoregular polypropylene (A) used as a raw material for the biaxially oriented polypropylene of the present invention means an isotactic polypropylene resin. This isotactic polypropylene resin is known as a polypropylene resin commonly used for capacitor applications. The highly stereoregular polypropylene (A) used in the present invention is a linear polypropylene resin, has a cold xylene soluble part (CXS) of 4% by mass or less, and a mesopendat fraction (mmmm) of 0.960 to 0. 995, preferably has a melt flow index (MFR) of 0.5 to 5.0 g/10 min.

The cold xylene soluble portion (CXS) of the highly stereoregular polypropylene (A) used in the biaxially oriented polypropylene film of the present invention is preferably 4% by mass or less, more preferably 3% by mass or less, It is particularly preferable that the content is 2% by mass or less. The cold xylene soluble portion (CXS) refers to 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, the cold xylene soluble portion (CXS) is considered to correspond to a component that is difficult to crystallize due to reasons such as stereoregularity and low molecular weight. When the cold xylene soluble portion (CXS) of the highly stereoregular polypropylene (A) is 4% by mass or less, the high temperature withstand voltage characteristics and dimensional stability of the biaxially oriented polypropylene film can be easily maintained.

The mesopentad fraction (mmmm) of the highly stereoregular polypropylene (A) used in the biaxially oriented polypropylene film of the present invention is preferably 0.960 to 0.995, and preferably 0.965 to 0.995. More preferably, it is from 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 method (NMR method). It is preferable because it has excellent withstand voltage characteristics. When the mesopentad fraction of the highly stereoregular polypropylene (A) is 0.960 or more, high-temperature withstand voltage characteristics and dimensional stability can be easily maintained. On the other hand, when the mesopentad fraction of the highly stereoregular polypropylene (A) is 0.995 or less, film formability is maintained and a biaxially oriented polypropylene film is easily obtained in a stable manner.

The melt flow index (MFR) of the highly stereoregular polypropylene (A) used in the biaxially oriented polypropylene film of the present invention is determined at 230°C and 2.16 kg according to JIS K 7210-1 (2014). When measured, it is preferably 0.5 to 5.0 g/10 minutes, more preferably 1.0 to 4.5 g/10 minutes, and 1.5 to 4.0 g/10 minutes. Particularly preferred. When the melt flow index (MFR) of the highly stereoregular polypropylene (A) is 0.5 g/10 minutes or more, a biaxially oriented polypropylene film can be easily obtained stably while maintaining film formability. On the other hand, when the MFR of the highly stereoregular polypropylene (A) is 5 g/10 minutes or less, it is easy to maintain dimensional stability and high-temperature withstand voltage characteristics.

In the biaxially oriented polypropylene film of the present invention, the content of the highly stereoregular polypropylene (A) is preferably 90 to 100% by mass. When the content of the highly stereoregular polypropylene (A) is 90% by mass or more, the heat resistance of the film is maintained and the withstand voltage at high temperatures is easily maintained.

The biaxially oriented polypropylene film of the present invention has a melt flow index (MFR, unit: g/10 min) and a melt tension (MS, unit: cN) at 230°C satisfying formula (1) in 100% by mass of the film. It is preferable to contain 5.0 to 15.0% by mass of high melt tension polypropylene (H).
log(MS)>-0.56×log(MFR)+0.74...Formula (1)
Here, high melt tension polypropylene (H) means a polypropylene resin that satisfies formula (1).

Note that it is possible to determine whether the polypropylene resin contained in the biaxially oriented polypropylene film corresponds to high melt tension polypropylene (H) by measuring the polypropylene resin in the film. If the film contains multiple polypropylene resins, it is not easy to separate them, so it is preferable to confirm by measuring the melt tension and melt flow index of the polypropylene resin that is the raw material for producing the film.

Furthermore, the melt tension (MS) of the high melt tension polypropylene (H) is preferably 5 to 18 cN.

The characteristics of high melt tension polypropylene (H) will be explained below.

The high melt tension polypropylene (H) contained in the biaxially oriented polypropylene film for capacitors of the present invention has a melt flow index (MFR) when measured at 230°C and a melt tension (MS) when measured at 230°C (unit: :cN) is a polypropylene resin satisfying the above formula (1). In order to obtain high melt tension polypropylene (H) that satisfies formula (1), a method using high-energy ionizing radiation on polypropylene resin (for example, JP-A-62-121704), or a method of reacting polypropylene resin with a specific organic peroxide. (e.g., Japanese Patent No. 2869606), a method of reacting a thermally decomposable radical forming agent with an ethylenically-based polyfunctional unsaturated monomer on polypropylene resin (e.g., Japanese Unexamined Patent Publication No. 10-330436), a method of reacting a specific catalyst during polymerization of polypropylene resin, The method used (for example, Japanese Patent Laid-Open No. 2009-057542) is preferably used. Specifically, "Profax (registered trademark) (PF-814, etc.)" manufactured by Lyondell Basell, "Daploy (trademark) (WB130HMS, WB135HMS, etc.)" manufactured by Borealis, "WAYMAX (trademark) (MFX3)" manufactured by Nippon Polypro Co., Ltd. , MFX6, MFX8, EX4000, EX6000, EX8000, etc.)" are exemplified.

The high melt tension polypropylene (H) contained in the biaxially oriented polypropylene film of the present invention preferably has a branched structure in its molecular chain. Note that the polypropylene resin having a branched structure is a polypropylene resin having internal trisubstituted olefins at 5 or less positions per 10,000 carbon atoms, and the presence of this internal trisubstituted olefin indicates that protons in the 1H-NMR spectrum It can be confirmed by the ratio. Branched polypropylene (H) has an action as an α-crystal nucleating agent, and if the amount added is within a certain range, it is possible to form a rough surface due to the crystal morphology. That is, the spherulite size of the polypropylene produced in the cooling process of the melt-extruded resin sheet can be controlled to be small, and a biaxially oriented polypropylene film with excellent high-temperature withstand voltage characteristics can be obtained.

The biaxially oriented polypropylene film of the present invention preferably contains 5.0 to 15.0% by mass of high melt tension polypropylene (H) in 100% by mass of the film. The content of high melt tension polypropylene (H) is more preferably 5.0 to 13.0% by mass, even more preferably 5.0 to 10.0% by mass, and even more preferably 6.0 to 9.0% by mass. % is particularly preferable. By setting the content of high melt tension polypropylene (H) to 5.0% by mass or more, it is possible to prevent the spherulite size from becoming too large when forming the molten polymer into a sheet shape, and to improve the surface roughness suitable for the present invention. That is, it is easy to control the aspect ratio (Str), minimum autocorrelation length (Sal), and arithmetic mean height (Sa) of the surface properties to be suitable. In addition, by setting the content of high melt tension polypropylene (H) to 15.0% by mass or less, the spherulite size is prevented from becoming too small when forming the molten polymer into a sheet shape, and the surface unevenness of the present invention is prevented from becoming too small. is easily obtained, and the stereoregularity as a biaxially oriented polypropylene film is prevented from deteriorating, making it easier to maintain high-temperature withstand voltage.

The melt tension (MS) of the high melt tension polypropylene (H) contained in the biaxially oriented polypropylene film of the present invention is preferably 5 to 18 cN. The melt tension (MS) of the high melt tension polypropylene (H) is more preferably 5 to 15 cN, even more preferably 5 to 13 cN, and particularly preferably 6 to 10 cN. By setting the melt tension (MS) to 5 cN or more, the spherulite size is prevented from becoming too large when forming the molten polymer into a sheet shape, and the surface irregularities of the present invention can be easily obtained. In addition, by setting the melt tension (MS) to 18 cN or less, the spherulite size is prevented from becoming too small when forming the molten polymer into a sheet, and a suitable surface unevenness in the present invention, that is, a suitable surface quality is achieved. This makes it easier to control the aspect ratio (Str), minimum autocorrelation length (Sal), and arithmetic mean height (Sa), and also prevents the crystallinity of the biaxially oriented polypropylene film from becoming too high during vapor deposition processing. The film is less likely to break when processing capacitor elements.

The biaxially oriented polypropylene film of the present invention preferably has an ash content of 50 ppm or less (based on mass, the same applies hereinafter), more preferably 40 ppm or less, even more preferably 30 ppm or less, and particularly preferably 20 ppm or less. . When the ash content is 50 ppm or less, the high temperature withstand voltage characteristics of the biaxially oriented polypropylene film can be easily maintained. In order to keep the ash content within the above range, it is important to use raw materials with less catalyst residue. A method of thoroughly cleaning the path through which the polymer flows with polypropylene resin can be preferably employed.

The biaxially oriented polypropylene film of the present invention preferably has a surface wetting tension on at least one side of 38 to 52 mN/m, more preferably 40 to 50 mN/m, and even more preferably 42 to 48 mN/m. When the surface wetting tension is 38 mN/m or more, it is easy to make sufficient contact with metal during metal vapor deposition. On the other hand, when the surface wetting tension is 52 mN/m or less, high-temperature withstand voltage characteristics can be easily maintained. Note that a polypropylene film usually has a low surface energy and a surface wetting tension of about 30 mN/m. In order to keep the surface wetting tension within the above range, a method is preferably employed in which surface treatment is performed after biaxial stretching during film formation. Specifically, corona discharge treatment, plasma treatment, glow treatment, flame treatment, etc. can be employed.

The biaxially oriented polypropylene film of the present invention can be obtained by forming a polypropylene resin composition consisting of the above-described highly stereoregular polypropylene (A) and high melt tension polypropylene (H) into a sheet shape, and then biaxially stretching the polypropylene resin composition. preferable. Biaxial stretching can be achieved by simultaneous inflation biaxial stretching, simultaneous tenter biaxial stretching, or sequential tenter biaxial stretching. It is preferable to employ an axial stretching method. In particular, it is preferable to stretch in the width direction after stretching in the longitudinal direction.

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

First, the above-mentioned highly stereoregular polypropylene (A) and high melt tension polypropylene (H) are dry blended and supplied to a single-screw melt extruder, where melt extrusion is performed at 200 to 260°C. Next, a filter installed in the middle of the polymer tube removes foreign substances and modified polymers. Then, it is discharged from a T-die onto a cast drum to form a cast sheet, and is cooled by a cooling roll.

The temperature of the cast drum is preferably 60 to 100°C, more preferably 65 to 95°C, and even more preferably 70 to 90°C from the viewpoint of appropriately generating β crystals and spherulites. By setting the cast drum temperature to 60°C or higher, the amount of β-crystals formed in the cast sheet is prevented from decreasing too much, and in order to maintain the slipperiness of the film obtained after biaxial stretching, the film during film formation and processing is It is possible to prevent the occurrence of transport wrinkles in the transport process and deterioration of the winding appearance of the film roll. On the other hand, by setting the cast drum temperature to 100°C or less, it is possible to prevent excessive formation of β crystals in the cast sheet, and prevent the occurrence of meandering in the film conveyance process during film forming and processing. This makes it easier to prevent the appearance of the roll from deteriorating.

It is preferable that the molten sheet discharged from the T-die lands on the cast drum and stays in close contact with the drum for 1 to 3 seconds. When the time of close contact is 1 second or more, the molten sheet is easily solidified, and it is easy to prevent it from breaking in the subsequent stretching process. On the other hand, if the contact time is 3 seconds or less, excessive formation of β crystals in the cast sheet can be prevented, and the occurrence of meandering in the film conveyance process during film forming and processing, and the film roll. This makes it easier to prevent the appearance of the roll from deteriorating.

Methods such as electrostatic application, air knife method, nip roll method, and underwater casting method can be used to bring the molten sheet into close contact with the cast drum. The air knife method is preferred from the viewpoint of property control. The air temperature of the air knife is preferably 60 to 120°C. By setting the air knife temperature to 60°C or higher, it is possible to prevent the β crystals formed in the cast sheet from becoming too small, maintain the slipperiness of the film obtained after biaxial stretching, and transport the film during film formation and processing. This makes it easier to prevent the occurrence of wrinkles during transportation and deterioration of the winding appearance of the film roll during the process. On the other hand, by setting the air knife temperature to 120°C or lower, in order to prevent excessive formation of β-crystals in the cast sheet, it is possible to prevent meandering in the film conveyance process during film formation and processing, and to prevent winding of the film roll. It will be easier to prevent deterioration of your appearance.

The temperature difference between the cast drum temperature and the air knife temperature is preferably 20°C or less, more preferably 10°C or less, from the viewpoint of forming the same β crystals on both sides of the cast sheet. Setting the temperature difference between the cast drum temperature and the air knife temperature to 20°C or less prevents the formation of different unevenness on the front and back sides of the film, makes it easier for the front and back sides of the film to have the same slipperiness, and improves the winding process during capacitor element processing. The amount of trapped air is stabilized in the heat treatment process, and the gap between the film layers and the amount of air are more likely to be uniform after the heat treatment process. This prevents the safety from being too effective or becoming less effective when using the capacitor, which reduces the life of the capacitor. Hateful.

The cooling temperature of the cast sheet is preferably 10 to 50°C. When the cooling temperature is 10°C or more, it is easy to raise the film to a desired temperature in the subsequent longitudinal stretching process, and it is easy to prevent the film from breaking in the longitudinal stretching process. On the other hand, when the cooling temperature is set to 50° C. or less, it is easy to stop crystal formation in the cast sheet, and it is easy to prevent variations in the surface properties of the film obtained after the stretching process in the longitudinal direction.

In order to obtain the biaxially oriented polypropylene film of the present invention, it is preferable to subject the cast sheet to a high temperature heat treatment before the longitudinal stretching step. In general, the crater-like surface irregularities characteristic of biaxially oriented polypropylene films using the β-crystal method are caused by the formation of recesses due to the α-crystal transition of β-crystals (volume reduction due to the transition) and the recesses during the longitudinal stretching process. Mechanical deformation in the longitudinal stretching direction occurs at the same time, and wedge-shaped recesses are formed on the surface of the longitudinally stretched sheet. Next, in a lateral stretching process, the wedge-shaped recesses are stretched, and the edges of the recesses are deformed into protrusions, thereby forming crater-like surface irregularities.

In order to obtain the surface irregularities of the biaxially oriented polypropylene film of the present invention, it is preferable that the formation of recesses due to the α crystal transition of β crystals in the longitudinal stretching process and the mechanical deformation of the recesses in the longitudinal stretching direction occur in separate steps. . Furthermore, it is more preferable to apply an excessive amount of heat when transforming the β crystal to the α crystal to partially melt the concave portions caused by the α crystal transition.

Specifically, before the longitudinal stretching step, it is preferable that the cast sheet be heat-treated by passing it through a temperature-controlled transport roll while nipping it with temperature-controlled nip rolls. The diameter and number of conveyor rolls may be adjusted depending on the heat treatment time. Further, it is more preferable to cool the cast sheet after the heat treatment to stop partial melting.

The temperature of the conveyor roll during heat treatment is preferably 160 to 170°C, more preferably 160 to 165°C. If the conveyance roll temperature is 160°C or higher, not only the transition from β crystal to α crystal but also partial melting of the recesses formed by the α crystal transition is likely to occur, and the surface unevenness of the biaxially oriented polypropylene film of the present invention is likely to occur. Easy to obtain. On the other hand, when the conveyance roll temperature is 170° C. or lower, the entire cast sheet is prevented from melting and tearing, and mass productivity is easily maintained.

The temperature of the nip rolls is preferably 160 to 170°C, more preferably 160 to 165°C. When the conveyance roll temperature is 160° C. or higher, curling of the cast sheet caused by the temperature difference between the front and back sides is suppressed, tearing in the stretching process is prevented, and mass productivity is easily ensured. On the other hand, when the conveyance roll temperature is 170° C. or lower, the entire cast sheet is prevented from melting and tearing, and mass productivity is easily maintained.

The pressure of the nip rolls is preferably 0.30 to 0.60 MPa, more preferably 0.35 to 0.55 MPa. When the nip roll pressure is 0.30 MPa or more, the generation of wrinkles due to film expansion during heat treatment is suppressed, tearing during the longitudinal stretching process is prevented, and mass productivity is easily ensured. On the other hand, when the nip roll pressure is 0.60 MPa or less, deformation of the film due to pressure is prevented, tearing in the stretching process is suppressed, and mass productivity is easily ensured.

The heat treatment time for the cast sheet is preferably 1 to 10 seconds. By setting the heat treatment time to 1 second or more, not only the transition from β crystal to α crystal but also partial melting of the recesses due to volume reduction tends to occur, and the surface unevenness of the biaxially oriented polypropylene film of the present invention is easily obtained. On the other hand, by setting the heat treatment time to 10 seconds or less, the melting of the entire film and the occurrence of wrinkles due to film expansion can be prevented, and mass productivity can be easily ensured. Furthermore, it is easy to prevent excessive partial melting of the recessed portions and prevent excessive smoothing of the film after stretching.

The cooling temperature of the cast sheet after heat treatment is preferably 80 to 120°C. When the cooling temperature is 80° C. or higher, it is easy to raise the film to a desired temperature in the subsequent longitudinal stretching step, and it is easy to prevent breakage in the longitudinal stretching step. On the other hand, when the cooling temperature is 120° C. or less, partial melting of the surface of the cast sheet is easily stopped, and it is easy to prevent the surface properties of the film obtained after the stretching step from varying in the longitudinal direction.

Next, the cast sheet is stretched in the longitudinal direction in a longitudinal stretching step. The cast sheet is passed through rolls whose temperature is controlled at 125 to 145° C., and stretched in the longitudinal direction at a predetermined stretching speed and stretching ratio depending on the peripheral speed difference between the rolls.

The stretching ratio in the longitudinal direction is preferably 4.0 to 7.0 times, more preferably 5.0 to 7.0 times. The higher the stretching ratio, the more uniform the surface properties of the film and the better its high-temperature withstand voltage characteristics. When the longitudinal stretching ratio is 7.0 times or less, it is easy to prevent film breakage in the longitudinal stretching process and in the next transverse stretching process.

Next, the ends of the film of the longitudinally stretched sheet are held with clips, and stretched in the width direction at a stretching ratio of 8 to 15 times using a tenter type stretching machine whose temperature is controlled at 140 to 165°C.

Next, the biaxially stretched polypropylene film is subjected to corona discharge treatment in air, nitrogen, carbon dioxide, or a mixture of these gases, and the edges of the film held with clips are cut and removed. The film from which the portion has been removed is wound up as a master roll using a winding machine. Finally, the film unwound from the master roll is slit to a specific width using a slitter, and wound around a core as a film roll to obtain the biaxially oriented polypropylene film of the present invention.
The biaxially oriented polypropylene film of the present invention is preferably used as a dielectric for capacitors, but is not limited to the type of capacitor. Specifically, from the viewpoint of electrode configuration, either a foil-wound capacitor or a metal vapor-deposited film capacitor may be used, and it is also preferable to use an oil-immersed type capacitor containing insulating oil or a dry type capacitor that does not use any insulating oil. used. Moreover, from the viewpoint of shape, it does not matter if it is a winding 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 vapor deposited film capacitor.

In the present invention, it is preferable to provide a metal film on at least one side of the biaxially oriented polypropylene film to obtain a metal film laminate film. The method is not particularly limited, but preferably used is, for example, a method of depositing aluminum on at least one side of the film to provide a metal film such as an aluminum deposited film that becomes an internal electrode of the film capacitor. At this time, other metal components such as nickel, copper, gold, silver, chromium, and zinc can also be deposited simultaneously or sequentially with the aluminum. Furthermore, a protective layer may be provided on the deposited film using oil or the like.

The thickness of the metal film of the metal film laminate film is preferably 20 to 100 nm from the viewpoint of electrical properties and safety of the capacitor. Further, for the same reason, it is preferable that the surface resistance value of the metal film is 1 to 20 Ω/□. The surface resistance value can be controlled by the metal type and film thickness used.

In the present invention, after forming the metal film, the metal film laminated film may be subjected to aging treatment at a specific temperature or heat treatment, if necessary. Furthermore, for insulation or other purposes, at least one side of the metal film laminate film may be coated with polyphenylene oxide or the like.

The metal film laminated film thus obtained can be laminated or wound by various methods to obtain a film capacitor. In other words, the film capacitor of the present invention is composed of the metal film laminated film of the present invention, and the film capacitor of the present invention includes a laminated type obtained by laminating metal film laminated films, and a laminated type obtained by laminating metal film laminated films. This includes both wound-type film capacitors obtained by winding film. A preferred method of manufacturing a wound capacitor will be described below, but the method is not necessarily limited thereto.

First, aluminum is vacuum deposited on one side of the biaxially oriented polypropylene film of the present invention. At this time, aluminum is vapor-deposited in the form of stripes with margins running in the longitudinal direction of the film. Next, a blade is inserted into the center of each vapor-deposited part and the center of each margin part on the surface to make a slit, thereby producing a tape-shaped take-up reel whose surface has a margin on one side. Two tape-shaped take-up reels with margins on the left or right are wound, one on the left margin and one on the right margin, overlapping each other so that the vapor-deposited part protrudes from the margin in the width direction to create a wound body. get. After the wound body is heat-treated, metallicon is thermally sprayed on both end faces 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 uses, including vehicles, home appliances (TVs, refrigerators, etc.), general miscellaneous protection, automobiles (hybrid cars, power windows, wipers, etc.), and power supplies. Capacitors can also be suitably used for these purposes.

Hereinafter, the present invention will be explained in detail with reference to Examples. In addition, the characteristics were measured and evaluated by the following method.

(1) Mesopentad fraction (mmmm)
A polypropylene resin sample was dissolved in a solvent, and the mesopentad fraction (mmmm) was determined using 13C-NMR under the following conditions (Reference: New Edition Polymer Analysis Handbook, Japan Society for Analytical Chemistry, Polymer Analysis Research Edited by the Council, 1995, pp. 609-611).

A. Measurement conditions device: Bruker DRX-500
Measurement nucleus: 13C nucleus (resonance frequency: 125.8MHz)
Measured concentration: 10wt%
Solvent: benzene/heavy orthodichlorobenzene = mass ratio 1:3 mixed solution Measurement temperature: 130°C
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 times Measurement mode: complete decoupling
B. Analysis Conditions Fourier transform was performed with LB (line broadening factor) set to 1.0, and the mmmm peak was set at 21.86 ppm. Peak division is performed using WINFIT software (manufactured by Bruker). At that time, we performed peak division as shown below starting from the peak on the high magnetic field side, and then performed automatic fitting using the attached software. After optimizing the peak division, the sum of mmmm peak fractions was determined. The above measurement was performed five times, and the average value was taken as the mesopentad fraction (mmmm) of this sample.
Peak (a) mrrm
(b) (c) rrrm (split as two peaks)
(d)rrrr
(e) mrmr
(f) mrmm+rmrr
(g)mmrr
(h)rmmr
(i)mmmr
(j)mmmm
(2) Melt flow index (MFR) (unit: g/10min)
Measured at 230°C and 2.16 kg in accordance with JIS K 7210-1 (2014).

(3) Melt tension (MS) (unit: cN)
Using a Toyo Seiki Seisakusho Co., Ltd. melt tension tester (capillary diameter 2.1 mm, cylinder diameter 9.55 mm), polypropylene resin was heated to 230°C, the molten polypropylene was discharged into a strand at an extrusion speed of 15 mm/min, and this strand was The tension when taken off at a speed of 6.5 m/min was measured and defined as melt tension (MS).

(4) Cold xylene soluble part (CXS)
0.5 g of polypropylene resin is dissolved in 100 ml of boiling xylene at 135°C, left to cool, and then recrystallized in a constant temperature water bath at 20°C for 1 hour. Polypropylene components dissolved in the filtrate were quantified by liquid chromatography. When the mass of the polypropylene resin before dissolving in boiling xylene is X0 (g) and the mass of the polypropylene component dissolved in the filtrate is X (g), the cold xylene soluble part (CXS) is calculated from the following formula (2). I asked for it.
CXS (mass%) = (X/X0) x 100...Formula (2)
(5) Surface texture aspect ratio (Str), autocorrelation length (Sal), arithmetic mean height (Sa),
The measurement was performed using a non-contact surface/layer cross-sectional shape measurement system VertScan 2.0 (model: R3300GL-Lite-AC) manufactured by Ryoka System Co., Ltd. On the film roll corresponding to the center position in the width direction of the master roll, 10 points randomly sampled in the longitudinal direction from the center position of the film roll are taken as measurement points, and the average of the measured values at those 10 points is the surface texture of the sample. The aspect ratio (Str), the autocorrelation length (Sal), and the arithmetic mean height (Sa) were taken as. The detailed conditions for one measurement were as follows. Note that one field of view (field area: 939 μm vertically×1,252 μm horizontally = 1,175,628 μm ² ) was measured for each measurement.

A. Measurement conditions CCD camera: SONY HR-57 1/2”
Objective lens: 10X
Lens barrel: 0.5X BODY
Wavelength filter: 530 white
Measurement mode: Wave
Field of view size: 640 x 480
Scan range: (start) 5μm, (stop) -5μm
B. Measurement method: A special sample holder is used for film measurement. The sample holder was two removable metal plates with a circular hole in the center, and the film was sandwiched and fixed between them without wrinkles, and the film in the central circular part was measured. Note that the film and sample holder were installed so that the longitudinal direction of the film roll coincided with the longitudinal direction of the measurement field of view.

C. Analysis method The data obtained from the above measurements were analyzed using VertScan 2.0 image analysis software VS-Viewer. First, noise was removed using a median filter (5×5), and undulation components were removed using a Gaussian filter with a cutoff value of 250 μm. Next, the aspect ratio (Str), autocorrelation length (Sal), and arithmetic mean height (Sa) of the surface texture defined in ISO25178-2 (2012) were calculated using the "ISOPara" function. Note that in the "ISOPara" function, the S-Filter was set to 6.0 μm.

(6) Thickness (t)
The thickness was measured by a micrometer method according to JIS C 2330 (2014).

(7) Ash content A biaxially oriented polypropylene film with an initial mass of W0 is placed in a platinum crucible, first sufficiently burned in a gas burner, and then treated in an electric furnace at 750 to 800°C for 1 hour to completely incinerate. The mass W1 of the ash was measured and calculated from the following formula (3).
Ash content = (W1/W0) x 1,000,000 (ppm) ... Formula (3)
(8) Element processability in capacitor manufacturing Aluminum was vacuum deposited on the corona-treated side of the biaxially oriented polypropylene film using a vacuum deposition machine manufactured by ULVAC Co., Ltd. to a thickness of 15 Ω/□. At that time, aluminum was vapor-deposited in a stripe shape having a margin part running in the longitudinal direction (width of the vapor-deposited part was 79.0 mm, and width of the margin part was 1.0 mm repeatedly). Then, a blade was inserted into the center of each vapor deposition part and the center of each margin part to make a slit, thereby producing a tape-shaped take-up reel having a total width of 40 mm and having a margin part of 0.5 mm at either the left or right end. One reel from the left margin and one from the right margin of the obtained reel were wound by overlapping each other so that the vapor-deposited part protruded 0.5 mm from the margin part in the width direction to form a wound body with a capacitance of 120 μF. I got it. Note that KAW-4NHB manufactured by Kaito Seisakusho Co., Ltd. was used for winding the element. Finally, heat treatment was performed in a reduced pressure atmosphere at 140° C. for 10 hours to obtain a capacitor element. Capacitor elements with wrinkles or distortion in shape on the outside or inside were rejected, and element workability was evaluated based on the number of rejected capacitor elements. In addition, the ratio to the total number of manufactured items was expressed as a percentage and was used as an index of processability. Note that 50 capacitor elements were produced and evaluated according to the following criteria.
◎: No defective products 〇: 1 defective product △: 2 to 3 defective products ×: 4 or more defective products.

(9) Evaluation of Capacitor Characteristics Aluminum was vacuum deposited on the corona-treated side of the biaxially oriented polypropylene film using a vacuum deposition machine manufactured by ULVAC Co., Ltd. to give a resistance of 15 Ω/□. At that time, aluminum was vapor-deposited in a stripe shape having a margin part running in the longitudinal direction (width of the vapor-deposited part was 79.0 mm, and width of the margin part was 1.0 mm repeatedly). Then, a blade was inserted into the center of each vapor deposition part and the center of each margin part to make a slit, thereby producing a tape-shaped take-up reel having a total width of 40 mm and having a margin part of 0.5 mm at either the left or right end. One reel from the left margin and one from the right margin of the obtained reel were wound by overlapping each other so that the vapor-deposited part protruded 0.5 mm from the margin part in the width direction to form a wound body with a capacitance of 120 μF. I got it. Note that KAW-4NHB manufactured by Kaito Seisakusho Co., Ltd. was used for winding the element. Finally, it was heat treated in a reduced pressure atmosphere at 130° C. for 10 hours, metallicon was thermally sprayed on both end faces in the width direction to form external electrodes, and lead wires were welded to the metallicon to obtain a capacitor element. Next, the capacitor characteristics of 10 capacitor elements were evaluated. First, capacitance (C0) was measured at room temperature. Next, a voltage of 200 VDC/μm (when the thickness (t) was 2.0 μm, the applied voltage was 400 V) was applied to the capacitor element for 400 hours at a high temperature of 125° C. Thereafter, the capacitance (C) was measured at room temperature, and the rate of change in capacitance (ΔC) before and after voltage application was calculated from the following formula (4).
ΔC=((C0-C)/C0)×100...Formula (4)
The average value of the capacitance change rate (ΔC) before and after voltage application of 10 capacitor elements was taken as the change rate of the sample, and evaluated according to the following criteria.
◎: 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.

(Example 1)
The mesopentad fraction is 0.980, the melt flow index (MFR) is 2.6 g/10 min, the cold xylene soluble part (CXS) is 1.5 wt%, the weight average molecular weight is 400,000, and the number average molecular weight is 6.5. 91.5% by mass of polypropylene resin manufactured by Prime Polymer Co., Ltd. (hereinafter referred to as highly stereoregular polypropylene (A1)), which has a melt flow index (MFR) of 2.9g/10 min, and a melt tension ( EX6000 (hereinafter referred to as high melt tension polypropylene (H1)) manufactured by Nippon Polypropylene Co., Ltd. having MS) of 9 cN was dry blended in an amount of 8% by mass and supplied to a single-screw melt extruder.

Note that the high melt tension polypropylene (H1) satisfies the above-mentioned formula (1).

Further, as antioxidants, 0.4% by mass of Irganox (registered trademark) 1010 manufactured by BASF Japan, and 0.1% by mass of 2,6-di-t-butyl-p-cresol (BHT) were contained. Polypropylene resin was melted at a temperature of 250° C., and foreign matter was removed using a sintered filter with a cut of 25 μm. Next, the molten resin was melt-extruded into a sheet through a T-shaped slit die, solidified by adhesion with an air knife at an air temperature of 80°C on a cast drum maintained at a temperature of 80°C, and then passed through a cooling roll maintained at a temperature of 30°C. Cooled on top. The time period during which the cast drum and the molten sheet were in close contact with each other was 1.5 seconds. Here, the surface on the side that touches the cast drum was defined as the drum surface (D surface), and the surface on the side that does not touch the ground is defined as the non-drum surface (non-D surface).

The obtained cast sheet was heat-treated on a conveyance roll at a temperature of 165°C for 5 seconds while being nipped at a pressure of 0.45 MPa with a nip roll at a temperature of 165°C, and then cooled on a cooling roll at a temperature of 100°C.
Next, the sheet was stretched 5.5 times in the longitudinal direction using longitudinal stretching rolls at a temperature of 145° C., and the ends of the longitudinally stretched sheet were held with clips and stretched 11 times in the width direction at 160° C. Further, 12% relaxation was performed in the width direction at 158°C. Thereafter, the drum surface (D surface) side of the film was slowly cooled to room temperature, and corona discharge treatment was applied to the drum surface (D surface) side at a treatment intensity of 25 W min/ ^m2 , and the edges of the film held with clips were cut and removed. The film from which the portion was removed was wound up using a winding machine. Next, the film was slit to a film width of 0.82 m using a slitter, and wound around a core by 30,000 m in the longitudinal direction as a film roll to obtain a biaxially oriented polypropylene film with a thickness of 2.0 μm. Table 1 shows the physical properties of the obtained film.

(Example 2)
A biaxially oriented polypropylene film with a thickness of 2.0 μm was obtained in the same manner as in Example 1 except that the heat treatment time of the cast sheet was 10 seconds. Table 1 shows the physical properties of the obtained film.

(Example 3)
A biaxially oriented polypropylene film having a thickness of 2.0 μm was obtained in the same manner as in Example 1 except that the heat treatment time of the cast sheet was 1 second. Table 1 shows the physical properties of the obtained film.

(Example 4)
Instead of the high melt tension polypropylene (H1) in Example 1, WB135HMS manufactured by Borealis (hereinafter referred to as high melt tension polypropylene (hereinafter referred to as high melt tension polypropylene A biaxially oriented polypropylene film having a thickness of 2.0 μm was obtained in the same manner as in Example 1 except that H2) was used. Table 1 shows the physical properties of the obtained film.

Note that the high melt tension polypropylene (H2) satisfies the above-mentioned formula (1).

(Example 5)
A biaxially oriented polypropylene film having a thickness of 2.0 μm was obtained in the same manner as in Example 4 except that the heat treatment time of the cast sheet was 10 seconds. Table 1 shows the physical properties of the obtained film.

(Example 6)
In place of the high melt tension polypropylene (H1) in Example 2, EX4000 manufactured by Nippon Polypropylene Co., Ltd. (hereinafter referred to as high melt tension polypropylene) having a melt induction index (MFR) of 6.2 g/10 min and a melt tension (MS) of 4 cN was used. A biaxially oriented polypropylene film with a thickness of 2.0 μm was obtained in the same manner as in Example 2 except that (denoted as (H3)) was used. Table 1 shows the physical properties of the obtained film.

Note that the high melt tension polypropylene (H3) satisfies the above-mentioned formula (1).

(Comparative example 1)
A biaxially oriented polypropylene film with a thickness of 2.0 μm was obtained in the same manner as in Example 1 except that it did not contain high melt tension polypropylene (H1) and the cast sheet was not heat-treated before longitudinal stretching. Table 1 shows the physical properties of the obtained film.

(Comparative example 2)
A biaxially oriented polypropylene film with a thickness of 2.0 μm was obtained in the same manner as in Example 1 except that high melt tension polypropylene (H1) was not contained. Table 1 shows the physical properties of the obtained film.

(Comparative example 3)
A biaxially oriented polypropylene film having a thickness of 2.0 μm was obtained in the same manner as in Example 1 except that the cast sheet was not heat-treated before longitudinal stretching. Table 1 shows the physical properties of the obtained film.

(Comparative example 4)
A film was produced in the same manner as in Example 1 except that the cast sheet was not nipped with nip rolls during heat treatment before longitudinal stretching, but the film broke during the longitudinal stretching process and a biaxially oriented polypropylene film could not be obtained.

Since the biaxially oriented polypropylene film of the present invention has high productivity, processability, and voltage resistance, it can be suitably used as a dielectric material of a film capacitor.

Claims (10)

A biaxially oriented polypropylene film containing a polypropylene resin as a main component, the biaxially oriented polypropylene having a surface having an aspect ratio (Str) of 0.55 to 0.95 on at least one side. film. The biaxially oriented polypropylene film according to claim 1, wherein the aspect ratio (Str) of the surface texture is 0.55 to 0.95 and the minimum autocorrelation length (Sal) of the surface is 10 to 40 μm. The biaxially oriented polypropylene film according to claim 1 or 2, wherein the aspect ratio (Str) of the surface texture is 0.55 to 0.95 and the arithmetic mean height (Sa) of the surface is 10 to 70 nm. The biaxially oriented polypropylene film according to any one of claims 1 to 3, having a thickness (t) of 1.0 to 3.0 μm. The biaxially oriented polypropylene film according to any one of claims 1 to 4, having surfaces on both sides having a surface texture aspect ratio (Str) of 0.55 to 0.95. Biaxially oriented polypropylene film according to any one of claims 1 to 5, characterized in that it has surfaces on both sides with a minimum autocorrelation length (Sal) of 10 to 40 μm. In 100% by mass of the film, 5% of high melt tension polypropylene (H) whose melt flow index (MFR, unit: g/10min) and melt tension (MS, unit: cN) at 230°C satisfy formula (1) was added. The biaxially oriented polypropylene film according to any one of claims 1 to 6, characterized in that it contains .0 to 15.0% by mass.
log(MS)>-0.56×log(MFR)+0.74...Formula (1) The biaxially oriented polypropylene film according to claim 7, wherein the high melt tension polypropylene (H) has a melt tension (MS) of 5 to 18 cN. A metal film laminate film obtained by forming a metal film on at least one side of the biaxially oriented polypropylene film according to any one of claims 1 to 8. A film capacitor comprising the metal film laminate film according to claim 9.