JP7743700B2 - Polypropylene film, metal film laminated film using same, and film capacitor - Google Patents

Description

The present invention relates to a polypropylene film that is particularly suitable for use in capacitors.

Polypropylene film has excellent transparency, mechanical properties, and electrical properties, making it suitable for a variety of applications, including packaging, tape, cable wrapping, and electrical applications such as capacitors.

Among these, polypropylene film is particularly preferred for use in capacitors, both for DC and AC high-voltage capacitors, due to its excellent voltage resistance and low loss characteristics.

Recently, various electrical devices have been converted to inverters, which has led to an even stronger demand for smaller capacitors with higher capacitance. Demand from these fields, particularly in automotive applications (including hybrid cars), solar power generation, and wind power generation, has made it essential for polypropylene film to be thinner, have improved breakdown voltage, and be highly reliable, able to maintain its properties even when used for long periods in high-temperature environments.

Among polyolefin-based films, polypropylene film is said to have high heat resistance and breakdown voltage. However, when applied to the aforementioned fields, it is important that it exhibits excellent dimensional stability at the ambient temperature of use and stable electrical performance, such as electrical resistance, even at temperatures 10 to 20°C higher than the ambient temperature. From the perspective of heat resistance, it is expected that the ambient temperature of use will become even higher in the future when considering power semiconductor applications using silicon carbide (SiC). Due to the demand for even higher heat resistance and high voltage resistance in capacitors, there is a need to improve the breakdown voltage of films in high-temperature environments exceeding 110°C. However, as described in Non-Patent Document 1, the upper limit of the operating temperature for polypropylene film is said to be approximately 110°C, and it has been extremely difficult to maintain a stable breakdown voltage in such a temperature environment.

In addition, during the vapor deposition process, the film's orientation can relax due to the thermal history caused by radiant heat, making it difficult for a film that is unstable to heat to fully utilize its inherent voltage resistance performance as a capacitor.

In order to obtain excellent performance in high-temperature environments when polypropylene film is used as a thin film capacitor, various methods have been proposed, such as improving heat resistance and rigidity by controlling the raw material properties, such as the mesopentad fraction, and the planar orientation coefficient of the polypropylene resin that makes up the film (see, for example, Patent Document 1). Another proposal has been made to improve rigidity by controlling the mesopentad fraction of the polypropylene resin, the orientation coefficient perpendicular to the film flow direction, and the planar orientation coefficient (see, for example, Patent Document 2). Furthermore, it has been proposed to improve voltage resistance in high-temperature environments by controlling the degree of crystal orientation and heat shrinkage rate (see, for example, Patent Document 3), and to improve heat resistance by increasing the raw material composition and the orientation of α-type crystals (see, for example, Patent Document 4).

Japanese Patent Application Laid-Open No. 2018-141122 Japanese Patent Application Publication No. 10-193451 International Publication No. 2016/182003 International Publication No. 2015/12324

Motonobu Kawai, "Film Capacitor Advances: From Automobiles to Energy," Nikkei Electronics, Nikkei BP, September 17, 2012, pp. 57-62

However, the polypropylene film described in Patent Document 1 does not have sufficient projection height, which can lead to wrinkles, particularly during the vapor deposition process, and can result in reduced reliability when made into a capacitor. Furthermore, the polypropylene film described in Patent Document 2 is not intended for use in high-temperature environments, and capacitor withstand voltage and reliability can be impaired at temperatures above 110°C. Furthermore, the polypropylene film described in Patent Document 3 has sufficient breakdown voltage in high-temperature environments of 110°C, but there is room for improvement in terms of breakdown voltage and rigidity in high-temperature environments above 110°C. The polypropylene film described in Patent Document 4 is not intended for use in high-temperature environments, and capacitor withstand voltage and reliability can be impaired at temperatures above 110°C.

The present invention aims to provide a polypropylene film that has excellent voltage resistance and reliability in high-temperature environments, has a structure with excellent heat stability, and is suitable for applications such as capacitors used at high temperatures and high voltages, and has suitable processability. It also aims to provide a metal film-laminated film and a film capacitor using the same.

The inventors conducted extensive research to solve the above problems and arrived at the following invention. The present invention is a polypropylene film in which, when measured by wide-angle X-ray diffraction after heating at 125°C for 60 minutes, the degree of crystal orientation of the α crystal (110) plane is 0.73 or more, and when the film is heated at 150°C for 10 minutes and then subjected to a tensile test at room temperature, the sum of the stress at an elongation of 5% in the longitudinal direction of the film (F5MD) and the stress at an elongation of 5% in the transverse direction of the film (F5TD) is 150 MPa or more when the film is heated at 150°C for 10 minutes and then subjected to a tensile test at room temperature.

The present invention provides a polypropylene film that has excellent voltage resistance and reliability in high-temperature environments, is suitable for applications such as capacitors used at high temperatures and high voltages, has excellent structural stability against heat, and is suitable for processability with minimal wrinkling during transportation processes, including vapor deposition. Also provided are a metal film-laminated film and a film capacitor using the film.

The inventors conducted extensive research to solve the above-mentioned problems, and came to the following conclusions about why the polypropylene films described in Patent Documents 1 to 4 above do not have sufficient breakdown voltage in high-temperature environments, and why, when used as capacitors, they do not have sufficient voltage resistance, reliability, or processability in high-temperature environments.

In other words, the polypropylene film described in Patent Document 1 has a smooth surface, which may not provide sufficient slipperiness. Furthermore, considering voltage resistance in high-temperature environments, the low mesopentad fraction of the raw materials used, inappropriate heat treatment during transverse stretching during film formation, and lack of slow cooling at the heat treatment temperature after transverse stretching may result in a high concentration of mobile amorphous components in the film, potentially resulting in low breakdown voltage at high temperatures. The polypropylene film described in Patent Document 2 was not designed for use in high-temperature environments, and the longitudinal and transverse stretching ratios during film formation were insufficient, potentially resulting in a reduction in voltage resistance due to the orientation relaxation of amorphous chains in high-temperature environments. The polypropylene film described in Patent Document 3 appears to have sufficient voltage resistance and reliability for use as a capacitor in a 110°C environment. However, considering voltage resistance in even higher-temperature environments, the stretching ratio and heat treatment during film formation, as well as the molecular chain orientation and structural fixation, are not necessarily sufficient, resulting in a reduction in the orientation relaxation of the amorphous chains in the film at higher temperatures, potentially resulting in a reduction in voltage resistance. Although Patent Document 4 enhances the orientation of molecular chains and improves heat resistance, the raw material composition does not take into consideration improved handleability and processability, and the stretching ratio and heat treatment during film production, as well as the orientation of molecular chains and fixation of the structure, are not necessarily sufficient, resulting in poor handleability and device processability. In addition, the rigidity in high-temperature environments is insufficient, resulting in a corresponding decrease in voltage resistance, which is thought to be a problem.

Based on the above considerations, the inventors conducted further research and found that the above problem can be solved by ensuring that, when a film is heated at 125°C for 60 minutes and then measured by wide-angle X-ray diffraction, the degree of crystal orientation of the α crystal (110) plane is 0.73 or more, and that when a film is heated at 150°C for 10 minutes and then subjected to a tensile test at room temperature, the sum of the stress (F5MD) (MPa) at an elongation of 5% in the longitudinal direction of the film and the stress (F5TD) (MPa) at an elongation of 5% in the transverse direction of the film when a tensile test is performed at room temperature after heating at 150°C for 10 minutes is 150 MPa or more.

In other words, the present invention is a polypropylene film in which, when measured by wide-angle X-ray diffraction after heating at 125°C for 60 minutes, the degree of crystal orientation of the α crystal (110) plane is 0.73 or more, and when the film is heated at 150°C for 10 minutes and then subjected to a tensile test at room temperature, the sum of the stress at an elongation of 5% in the longitudinal direction of the film (F5MD) and the stress at an elongation of 5% in the width direction of the film (F5TD) is 150 MPa or more when the film is heated at 150°C for 10 minutes and then subjected to a tensile test at room temperature.

The present invention will be explained in detail below. In this specification, "above" means the same as or greater than the indicated numerical value. Also, "below" means the same as or smaller than the indicated numerical value. Furthermore, "room temperature" means 23°C.

In this specification, polypropylene film may be referred to simply as film. The polypropylene film of the present invention is not a microporous film and therefore does not have numerous pores. In other words, the polypropylene film of the present invention refers to a polypropylene film other than a microporous film. Here, a microporous film is defined as a film having a pore structure that penetrates both surfaces of the film and has an air permeability of 5,000 seconds/100 ml or less, measured at 23°C and 65% relative humidity using a Type B Gurley tester according to JIS P 8117 (2009), when measuring the air permeation time of 100 ml of air.

The polypropylene film of the present invention has a crystal orientation degree of 0.73 or more in the α-crystal (110) plane when the film is heated at 125°C for 60 minutes and then measured by wide-angle X-ray diffraction. By achieving a crystal orientation degree of 0.73 or more, more preferably 0.75 or more, even more preferably 0.78 or more, and even more preferably 0.81 or more, the orientation order of the crystalline structure constituting the film is maintained at a high level, preventing the occurrence of areas prone to partial dielectric breakdown. When used as a capacitor, this prevents capacity loss and short-circuit breakdown even in high-temperature environments, prevents a decrease in voltage resistance, and ensures high reliability.

On the other hand, from the viewpoint of obtaining a stable film after biaxial stretching, the degree of crystal orientation of the α-crystal (110) plane when the polypropylene film is heated at 125°C for 60 minutes and then measured by wide-angle X-ray diffraction is preferably 0.95 or less. There is no particular lower limit for the degree of crystal orientation of the α-crystal (110) plane, but it is set to 0.60 from the viewpoint of reducing disorder in the orientation order of the crystalline structure that constitutes the film and preventing the occurrence of areas prone to electrical breakdown.

The degree of crystal orientation of the α crystal (110) plane when the film after heating at 125°C for 60 minutes was measured by wide-angle X-ray diffraction as described above was, as will be described later, when, for example, polypropylene raw material A (sometimes referred to as polypropylene resin (A); the same applies to polypropylene raw material (B) and polypropylene raw material (C) described later) is used, which has a high mesopentad fraction and a cold xylene solubles (CXS) content of less than 3.0 mass%, is pre-stretched by 1.01 times or more and 1.10 times or less before longitudinal stretching, and the areal stretching ratio during biaxial stretching is 60. The stretch ratio in the width direction is set to 11.0 times or more, preferably 65 times or more, and the stretch ratio in the width direction is set to 11.0 times or more. In the heat setting and relaxation treatment steps after biaxial stretching, the film is first heat-treated at a temperature lower than the width direction stretching temperature (first stage) while undergoing relaxation treatment, and then heat-treated at a temperature lower than the first heat treatment temperature of 145°C or higher (second stage) while maintaining tension in the width direction, and then heat-treated at a temperature higher than the second heat treatment temperature of 80°C or higher (third stage) to obtain the desired film.

The polypropylene film of the present invention has a sum of the stress at 5% elongation in the longitudinal direction of the film (F5MD) when the film is heated at 150°C for 10 minutes and then subjected to a tensile test at room temperature, and the stress at 5% elongation in the transverse direction of the film (F5TD) when the film is heated under the same conditions and then subjected to a tensile test at room temperature, of 150 MPa or more. By making the sum of F5MD and F5TD 150 MPa or more, more preferably 170 MPa or more, even more preferably 180 MPa or more, and even more preferably 190 MPa or more, the film will have rigidity in high-temperature environments and will achieve high reliability when used as a capacitor.

On the other hand, the sum of F5MD and F5TD is preferably 300 MPa or less. By keeping it 300 MPa or less, it is possible to prevent the film from breaking during the film-forming process and reducing its formability.

The sum of F5MD and F5TD within the above range can be achieved, as described below, by using, for example, polypropylene raw material A having a high mesopentad fraction and less than 3.0% by mass of cold xylene solubles (CXS), pre-stretching the film to 1.01 to 1.10 times before longitudinal stretching, biaxially stretching the film to an areal stretch ratio of 65 times or more and a width stretch ratio of 11.0 times or more, and then, in the heat setting and relaxation treatment steps after biaxial stretching, first performing a heat treatment (first stage) at a temperature lower than the width direction stretching temperature while performing a relaxation treatment, followed by a heat treatment (second stage) at a temperature lower than the first heat treatment temperature of 145°C or higher while maintaining tension in the width direction, and then a further heat treatment (third stage) at 80°C or higher but lower than the second heat treatment temperature.

In the polypropylene film of the present invention, the "longitudinal direction" refers to the direction corresponding to the flow direction in the film manufacturing process (hereinafter sometimes referred to as "MD"), and the "width direction" refers to the direction perpendicular to the flow direction in the film manufacturing process (hereinafter sometimes referred to as "TD"). When a film sample is in the form of a reel, roll, or the like, the film winding direction can be considered the longitudinal direction. On the other hand, for films where it is unclear from the appearance which direction corresponds to the flow direction in the film manufacturing process, a slit-shaped film piece is sampled and its breaking strength is determined using a tensile tester. The direction giving the maximum breaking strength is considered to be the width direction and main orientation direction of the film, and the direction perpendicular to the width direction of the film is considered to be the longitudinal direction and the direction perpendicular to the main orientation direction. As will be described in detail later, if the sample width is less than 50 mm and breaking strength cannot be determined using a tensile tester, the crystal orientation of the α-crystal (110) plane is measured using wide-angle X-rays as follows, and the longitudinal and width directions of the film are determined based on the following criteria. That is, X-rays (CuKα rays) are incident perpendicular to the film surface, and the crystal peak at 2θ = approximately 14° (α crystal (110) plane) is scanned in the circumferential direction. The direction with the highest diffraction intensity in the obtained diffraction intensity distribution is defined as the film's width direction and main orientation direction, and the direction perpendicular to this is defined as the longitudinal direction and the direction perpendicular to the main orientation direction.

The polypropylene film of the present invention preferably contains a linear polypropylene resin as its main component. Here, "main component" refers to the component with the highest mass fraction (highest content) among the components constituting the polypropylene film.

Hereinafter, polypropylene film may be simply referred to as film.

Polypropylene resins are primarily composed of propylene homopolymers, but copolymerization components based on other unsaturated hydrocarbons may be used, or polymers other than propylene homopolymers may be blended, as long as the objectives of the present invention are not impaired. Examples of monomer components other than propylene that make up such copolymerization components or blends include ethylene, 1-butene, 1-pentene, 3-methylpentene-1, 3-methylbutene-1, 1-hexene, 4-methylpentene-1, 5-ethylhexene-1, 1-octene, 1-decene, 1-dodecene, vinylcyclohexene, styrene, allylbenzene, cyclopentene, norbornene, and 5-methyl-2-norbornene.

From the standpoint of dielectric breakdown voltage and heat resistance, the copolymerization amount or blend amount of components other than propylene is preferably 1 mol% or less, and the blend amount is preferably 1 mass% or less of the total resin constituting the film as the amount of components other than propylene.

The polypropylene film of the present invention preferably has a weight average molecular weight Mw and a z+1 average molecular weight Mz ₊₁ such that Mz ₊₁ /Mw is 3 or more and 10 or less. By _{setting Mz+1} /Mw to 10 or less, more preferably 7.9 or less, even more preferably 6.9 or less, particularly preferably 6.5 or less, and most preferably 6.1 or less, the molecular weight distribution is narrow and the film structure is uniform with little local unevenness, thereby suppressing thermal shrinkage and improving the breakdown voltage in a high-temperature environment.

A means for controlling M _z+1 /Mw within the above range can be achieved by using a raw material as polypropylene resin A having a mesopentad fraction of 0.97 or more or a chip melting point of 160°C or more, a cold xylene soluble fraction (CXS) of less than 3 mass%, a number average molecular weight (Mn) of 70,000 (7.0 × 10 ⁴ ) or less, and a Z+1 average molecular weight (M _z+1 ) of less than 3,000,000 (3.0 × 10 ⁶ ), and appropriately blending polypropylene resin B and/or polypropylene resin C within the preferred range of the present application.

The polypropylene film of the present invention may contain various additives, such as organic particles, inorganic particles, crystal nucleating agents, antioxidants, heat stabilizers, chlorine scavengers, slip agents, antistatic agents, antiblocking agents, fillers, viscosity modifiers, and color inhibitors, as long as the additives do not impair the objectives of the present invention.

When an antioxidant is added, the type and amount of antioxidant should be selected from the perspective of long-term heat resistance. Such antioxidants should be sterically hindered phenolic, with at least one of them preferably being a high-molecular-weight type with a molecular weight of 500 or more. Specific examples include various antioxidants, but it is preferable to use 2,6-di-t-butyl-p-cresol (BHT: molecular weight 220.4) in combination with 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene (e.g., BASF's "Irganox"® 1330: molecular weight 775.2) or tetrakis[methylene-3(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane (e.g., BASF's "Irganox"® 1010: molecular weight 1,177.7). The total content of these antioxidants is preferably 0.1% by mass or more and 1.0% by mass or less, based on the total amount of polypropylene. A content of 0.1% by mass or more, more preferably 0.2% by mass or more, provides excellent long-term heat resistance. A content of 1.0% by mass or less, more preferably 0.7% by mass or less, and even more preferably 0.4% by mass or less, can suppress blocking at high temperatures due to bleed-out of the antioxidants, preventing adverse effects on the capacitor element.

The polypropylene film of the present invention may contain resins other than polypropylene resins as long as the objectives of the present invention are not impaired. Examples of resins other than polypropylene resins include vinyl polymer resins, including various polyolefin resins, polyester resins, polyamide resins, polyphenylene sulfide resins, polyimide resins, and polycarbonate resins. Particularly preferred examples include polymethylpentene, cycloolefin copolymers, cycloolefin polymers, and syndiotactic polystyrene. The content of resins other than polypropylene resins is preferably less than 30% by mass, more preferably 19% by mass or less, even more preferably 15% by mass or less, and most preferably 9% by mass or less, based on 100% by mass of the total resin components constituting the polypropylene film. If the content of resins other than polypropylene resins is 30% by mass or more, the influence of the domain interface becomes significant, which may result in a decrease in the breakdown voltage in high-temperature environments.

The polypropylene film of the present invention preferably has a melting peak temperature (Tm) of 170°C or higher, obtained when the film is heated from 30°C to 260°C at a rate of 20°C/min using a differential scanning calorimeter (DSC). By setting the Tm to 170°C or higher, more preferably 171°C or higher, even more preferably 172°C or higher, even more preferably 173°C or higher, and even more preferably 174°C or higher, the breakdown voltage in high-temperature environments can be effectively improved. On the other hand, from the perspective of industrially producing polypropylene resin, Tm is preferably 200°C or lower.

A Tm in the above range can be achieved, for example, by using a polypropylene raw material A having a high mesopentad fraction and a cold xylene solubles (CXS) content of less than 3.0 mass%, as described below.

The polypropylene film of the present invention preferably has a difference (Tm - Tc) of 65°C or less between the peak melting temperature (Tm) of the film, obtained by heating the film from 30°C to 260°C at 20°C/min using a differential scanning calorimeter (DSC), and the peak crystallization temperature (Tc) obtained by cooling the film from 260°C to 30°C at 20°C/min. By keeping (Tm - Tc) at 65°C or less, more preferably 63°C or less, even more preferably 61°C or less, even more preferably 59°C or less, and even more preferably 57°C or less, the crystallization time during the resin cooling and solidification process can be shortened and the formation of coarse spherulites can be suppressed. Suppressing the formation of such coarse spherulites in cast sheets can reduce the generation of internal voids during the stretching process and the reduced reliability of capacitors due to the formation of coarse protrusions on the surface. Furthermore, it can also prevent the film's slipperiness from being reduced due to the increase in flat areas across the entire film surface, which can lead to poor processability.

On the other hand, the difference between Tm and Tc (Tm - Tc) is preferably 40°C or higher. A temperature of 40°C or higher results in excellent film formation stability.

Here, when the polypropylene film of the present invention is a film containing polypropylene and a thermoplastic resin that is immiscible with polypropylene, the melting peak temperature of the incompatible resin may be observed at a temperature different from the melting peak temperature of the polypropylene. However, in the present invention, the peak temperatures observed between 170°C and 200°C are defined as the melting peak temperature (Tm) and crystallization peak temperature (Tc) of the polypropylene film of the present invention. In this case, there may be cases where two or more peaks are observed within the above temperature range, or where the peak temperature is a peak observable on a multi-stage DSC chart known as a shoulder (observed when two or more peaks overlap), but in the present invention, the peak with the largest absolute value of the vertical axis heat flow (unit: mW) on the DSC chart is selected and defined as Tm and Tc, respectively.

The above-mentioned range of (Tm-Tc) can be achieved, for example, by using polypropylene raw material A and/or polypropylene raw material B having a high mesopentad fraction and a cold xylene solubles (CXS) content of less than 3.0 mass%, by including branched polypropylene raw material C, and by adjusting the ratio of these components.

The polypropylene film of the present invention exhibits a high breakdown voltage even in high-temperature environments. To ensure that a capacitor made from the film exhibits voltage resistance and reliability even in high-temperature environments, it is preferable that the skewness (Ssk) of at least one surface of the film, as defined by ISO 25178, is greater than -30 and less than 5. Here, Ssk is a parameter indicating the degree of unevenness of the surface. This Ssk represents the cube mean of Z(x, y) on a reference plane made dimensionless by the cube of the root-mean-square height Sq. It represents the degree of skewness and is a numerical value that indicates the symmetry of the peaks and valleys about the average plane. Therefore, when Ssk is less than 0, the film is skewed downward relative to the average line, meaning that there are more concave valleys than convex peaks. On the other hand, when Ssk is greater than 0, the film is skewed upward relative to the average line, meaning that there are more convex peaks than concave valleys. A skewness Ssk of 0 indicates symmetry (normal distribution) relative to the average line.

By setting the Ssk to a value greater than -30, more preferably at least -28, and even more preferably at least -26, it is possible to prevent the film surface from becoming too unevenly indented. This makes it less likely that voltage resistance will be impaired even in high-temperature environments, particularly in high-voltage capacitor applications, and also makes the film more easily slippery and easier to process.

On the other hand, by setting Ssk to less than 5, more preferably 4 or less, and even more preferably 3 or less, the presence of excessive convex shapes on the film surface is suppressed, and when made into a capacitor, gaps are suppressed between the film layers, preventing a decrease in capacitance in high-temperature environments and preventing the film's slipperiness from being impaired, resulting in a deterioration in voltage resistance and processability.

The Ssk of the polypropylene film of the present invention can be adjusted within the above range by, for example, using polypropylene raw material B having the preferred properties described below, setting the areal stretching ratio during biaxial stretching to 60 times or more, preferably 65 times or more, and the widthwise stretching ratio to 11.0 times or more, and controlling the casting drum temperature, the film's melting peak temperature (Tm), and the crystallization peak temperature (Tc) within preferred ranges.

In the polypropylene film of the present invention, it is preferable that the protruding peak heights SpkA and SpkB defined in ISO 25178 of the surface A on one side and the surface B on the other side satisfy the following relationship:
SpkA<SpkB
20 nm≦SpkA≦100 nm
80 nm≦SpkB≦150 nm
where:
SpkA: height of the protruding peak on surface A; SpkB: height of the protruding peak on surface B.

Spk is a functional parameter defined in ISO 25178, and indicates the average height of the portion (protruding peak) higher than the intersection of the equivalent line of the bearing curve of the height data and the line where the areal load ratio = 0%. Here, the bearing curve of the height data is calculated by accumulating the frequency at a certain height from the highest to the lowest, and expressing it as a percentage with the total number of all height data being 100%, and the areal load ratio at a certain height C is given by Smr(C). The equivalent line is the line with the smallest slope where the difference in areal load ratio (Smr) is 40%.

In the polypropylene film of the present invention, SpkA is more preferably 30 nm or more, and even more preferably 40 nm or more. Furthermore, SpkA is more preferably 90 nm or less, and even more preferably 80 nm or less.

In the polypropylene film of the present invention, SpkB is more preferably 90 nm or more, and even more preferably 100 nm or more. Furthermore, SpkB is more preferably 140 nm or less, and even more preferably 130 nm or less.

When SpkA and SpkB fall within the above ranges, the film's surface protrusion height is appropriately high, resulting in sufficient slipperiness, excellent processability when creating capacitor elements, suppressing excessively high protrusions on the film surface, and preventing a decrease in the capacitor's withstand voltage.

SpkA and SpkB can be controlled within the above ranges by, for example, using polypropylene raw material B with the preferred properties described below, setting the areal stretching ratio during biaxial stretching to 60 times or more, preferably 65 times or more, and the widthwise stretching ratio to 11 times or more, and controlling the casting drum temperature, film melting peak temperature (Tm), and crystallization peak temperature (Tc) within preferred ranges.

The polypropylene film of the present invention preferably has a total volume of 50 μm³ ^{or more} and 5,000 ^μm³ or less on at least one surface, calculated by adding up the volumes of valleys 20 nm or more deep in a 0.561 mm × 0.561 mm area, as measured by a scanning white light interference microscope. By making this total volume ⁵⁰ μm³ or more, more preferably ^{100 μm³} or more, and even more preferably 500 μm³ or more, the film has a moderate surface unevenness, which improves handling due to moderate film slippage and reduces wrinkle formation, improving device processability. Furthermore, even when used as a capacitor for a long period of time, the film suppresses significant capacitance change due to wrinkles, etc., and when the film is used as a laminated capacitor, the film has a moderate gap between the films, which allows for self-healing, improving the reliability of the capacitor. On the other hand, by setting the total volume of the valleys to 5,000 μm3 or ^less , preferably 4,000 ^μm3 or less, more preferably 3,500 μm3 ^{or less} , and particularly preferably 3,000 ^μm3 or less, it is possible to prevent localized thin portions from forming and causing dielectric breakdown. This improves the voltage resistance of the film, and can improve the voltage resistance and reliability in high-temperature environments even when used in high-voltage capacitor applications.

The total volume of valleys on the film surface can be adjusted to the preferred range described above, for example, by using polypropylene raw material B described below, setting the areal stretching ratio during biaxial stretching to 60 times or more, preferably 65 times or more, and the widthwise stretching ratio to 11.0 times or more, and controlling the casting drum temperature, the film's melting peak temperature (Tm), and the crystallization peak temperature (Tc) within the preferred ranges.

The polypropylene film of the present invention preferably has a widthwise heat shrinkage rate (HS125TD) of 1.0% or less after heat treatment at 125°C for 15 minutes. By keeping this heat shrinkage rate to 1.0% or less, more preferably 0.8% or less, even more preferably 0.6% or less, and even more preferably 0.4% or less, it is possible to suppress film shrinkage due to heat during the capacitor manufacturing process and use process, as well as a decrease in voltage resistance due to poor contact with the metallicon at the element end, and to prevent a decrease in capacity due to element tightening and a decrease in reliability due to short-circuit breakdown.

On the other hand, it is preferable that the thermal shrinkage rate in the width direction after heating at 125°C for 15 minutes be 0.02% or more. By keeping it at 0.02% or more, it is possible to prevent the winding state of the element from loosening due to heat during the capacitor manufacturing process or use process.

The heat shrinkage percentage in the width direction of the film after 15 minutes of heat treatment at 125°C can be achieved in the above range by, for example, using a polypropylene raw material A with a high mesopentad fraction and a cold xylene solubles (CXS) content of less than 3.0% by mass, pre-stretching the film to 1.01 to 1.10 times before longitudinal stretching, and biaxially stretching the film to an areal stretch ratio of 60 times or more, preferably 65 times or more, and a width direction stretch ratio of 11.0 times or more. In the heat setting and relaxation treatment steps after biaxial stretching, the film is first heat-treated at a temperature lower than the width direction stretching temperature (first stage) while undergoing relaxation treatment, followed by heat treatment at a temperature lower than the first heat treatment temperature of 135°C or higher (second stage) while maintaining tension in the width direction, and then heat treatment at a temperature higher than the first heat treatment temperature of 135°C or higher (third stage) at a temperature higher than the second heat treatment temperature.

In order to provide the polypropylene film of the present invention with a suitable degree of slipperiness to improve processability during the production of capacitor elements, it is preferable that the static friction coefficient (μ _s ) of at least one side of the film when the same sides are overlapped is 0.2 or more and less than 0.8. By making this static friction coefficient μ _s 0.2 or more, more preferably 0.4 or more, it is possible to prevent the film from slipping too much during film production, winding, or element processing. On the other hand, by making the static friction coefficient μ _s less than 0.8, more preferably 0.7 or less, and even more preferably 0.6 or less, it is possible to prevent an extreme decrease in the slipperiness of the film, thereby suppressing the occurrence of wrinkles and further improving handleability and element processability.

The polypropylene film of the present invention preferably has a breakdown voltage of 350 V/μm or more at 130°C. By setting this breakdown voltage to 350 V/μm or more, more preferably 375 V/μm or more, even more preferably 400 V/μm or more, and even more preferably 420 V/μm or more, when used as a capacitor, it is less likely to suffer short circuit breakdown, especially even when used for long periods of time in high-temperature environments, thereby maintaining voltage resistance and achieving high reliability. There is no particular upper limit to the breakdown voltage at 130°C, but it is generally around 800 V/μm.

To control the film's breakdown voltage at 130°C within the above range, for example, as described below, a polypropylene raw material A with a high mesopentad fraction and a cold xylene solubles (CXS) content of less than 3.0% by mass is used; the film is pre-stretched 1.01 to 1.10 times before longitudinal stretching; and during biaxial stretching, the areal stretch ratio is set to 65 times or more and the width direction stretch ratio is set to 11.0 times or more. In the heat setting and relaxation treatment steps after biaxial stretching, the film is first heat-treated (first stage) at a temperature lower than the width direction stretching temperature while undergoing relaxation treatment; then, while maintaining tension in the width direction, the film is heat-treated (second stage) at a temperature lower than the first heat treatment temperature and at 135°C or higher; and then, while maintaining tension in the width direction, a further heat treatment (third stage) at 80°C or higher but lower than the second heat treatment temperature. This can be achieved by appropriately subjecting the film to a multi-stage heat setting and relaxation treatment.

The polypropylene film of the present invention preferably has a mesopentad fraction of 0.970 or more as measured by nuclear magnetic resonance (NMR). The mesopentad fraction is an index that indicates the stereoregularity of the crystalline phase of polypropylene. By setting the mesopentad fraction to 0.970 or more, more preferably 0.975 or more, and even more preferably 0.981 or more, the degree of crystallinity and melting point are high, and the breakdown voltage in high-temperature environments can be improved. There is no particular upper limit for the mesopentad fraction.

In the present invention, polypropylene resins with a high mesopentad fraction are preferably those produced using a Ziegler-Natta catalyst, and a method of appropriately selecting the electron donor component in the catalyst is preferably employed. Polypropylene resins produced in this manner can have a molecular weight distribution (Mw/Mn) of 3.0 or more and a <2,1> erythro moiety deficiency of 0.1 mol% or less, and it is preferable to use such polypropylene resins.

The polypropylene film of the present invention preferably has a polypropylene component dissolved in xylene (CXS, also known as cold xylene solubles) of 3.0% by mass or less when the polypropylene film is completely dissolved in xylene and then precipitated at room temperature. Here, CXS is considered to be a component that is difficult to crystallize due to low stereoregularity, low molecular weight, or other reasons. By keeping the CXS content at 3.0% by mass or less, more preferably 1.5% by mass or less, even more preferably 1.3% by mass or less, particularly preferably 1.1% by mass or less, and most preferably 0.9% by mass or less, the film's heat resistance, high-temperature voltage resistance, and breakdown voltage can be improved. Therefore, when used in capacitors, relaxation in high-temperature environments is suppressed, improving thermal dimensional stability and suppressing leakage current. While there is no particular lower limit for CXS, a practical value is 0.1% by mass. Attempting to reduce the CXS content to less than 0.1% by mass can result in poor stretchability during film formation, potentially leading to tearing.

To achieve the CXS content within the above range, methods such as increasing catalytic activity when obtaining the polypropylene resin to be used, or washing the obtained polypropylene resin with a solvent or propylene monomer itself can be used.

The polypropylene film of the present invention is preferably surface-treated. Polypropylene film typically has low surface energy, making it difficult to stably apply metal vapor deposition to it. Therefore, in order to improve adhesion to the metal film, it is preferable to perform a surface treatment before vapor deposition. Specific examples of surface treatments include corona discharge treatment, plasma treatment, glow discharge treatment, and flame treatment.

Normally, the surface wetting tension of polypropylene film is around 30 mN/m, but for the polypropylene film of the present invention, these surface treatments are used to preferably increase the wetting tension to around 37 to 75 mN/m, more preferably 39 to 65 mN/m, and even more preferably 41 to 55 mN/m, as this provides excellent adhesion to the metal film and improves the safety of the capacitor.

The polypropylene film of the present invention preferably has a film thickness of 0.5 μm or more and less than 25 μm. By setting the film thickness to 0.5 μm or more and less than 25 μm, more preferably 0.6 μm or more and less than 6 μm, even more preferably 0.8 μm or more and less than 4 μm, and even more preferably 1 μm or more and less than 2.5 μm, it achieves an excellent balance between voltage resistance characteristics in high-temperature environments and the miniaturization of capacitor size due to thin film thickness, making it particularly suitable for thin-film heat-resistant film capacitors required for automotive applications (including hybrid car applications) used in high-temperature environments.

Film thickness can be adjusted, for example, by adjusting the extrusion discharge rate, the rotation speed of the casting drum, the lip gap of the die, and the stretch ratio. More specifically, film thickness can be reduced by reducing the extrusion discharge rate, increasing the rotation speed of the casting drum, narrowing the lip gap of the die, and increasing the stretch ratio. These methods can also be used in combination as appropriate.

The polypropylene film of the present invention is preferably in the form of a single-layer film, but may also be in the form of a laminate film.

The polypropylene film of the present invention is preferably used as a dielectric film for capacitors, but can be used in a variety of capacitor types. Specifically, from the perspective of electrode configuration, it may be used in either a laminated wound capacitor of metal foil and film or a metal-vapor-deposited film capacitor. It is also preferably used in oil-immersed capacitors impregnated with insulating oil or dry capacitors that do not use any insulating oil at all. From the perspective of making the most of the properties of the film of the present invention, it is particularly preferably used as a metal-vapor-deposited film capacitor. The film configuration in the capacitor may be wound or laminated.

The method for producing the polypropylene film of the present invention will now be described using an example.

First, we will explain the raw materials that are preferably used for the polypropylene film of the present invention. As mentioned above, in order to obtain the polypropylene film of the present invention, it is preferable to use multiple types of polypropylene raw materials, particularly those with different number average molecular weights.

Polypropylene raw material A refers to a linear polypropylene having a smaller number average molecular weight (Mn) than polypropylene raw material B described below. From the viewpoint of biaxially stretching the film, the Mn of polypropylene raw material A is preferably 30,000 (3.0 × 10 ⁴ ) or more, more preferably 40,000 (4.0 × 10 ⁴ ) or more, and even more preferably 50,000 (5.0 × 10 ⁴ ) or more. From the viewpoint of obtaining thermal stability in a high-temperature environment, the Mn of polypropylene raw material A is preferably 90,000 (9.0 × 10 ⁴ ) or less, more preferably 80,000 (8.0 × 10 ⁴ ) or less.

From the viewpoint of biaxially stretching the film, the Z+1 average molecular weight (Mz ₊₁ ) of the polypropylene raw material A is preferably 1,000,000 (1.0 × ¹⁰⁶ ) or more, and more preferably 1,500,000 (1.5 × ¹⁰⁶ ) or more. From the viewpoint of obtaining thermal stability in a high-temperature environment, the Mz ₊₁ of the polypropylene raw material A is preferably 2,500,000 (2.5 × ¹⁰⁶ ) or less, and more preferably 2,000,000 (2.0 × ¹⁰⁶ ) or less.

Polypropylene raw material A preferably has a cold xylene solubles (hereinafter referred to as CXS) content of 3% by mass or less. By keeping the CXS content at 3.0% by mass or less, more preferably 2.0% by mass or less, even more preferably 1.5% by mass or less, and even more preferably 1.0% by mass or less, excellent film formation stability is achieved, and the film strength, dimensional stability, and heat resistance are improved. The lower the CXS content, the better, but the lower limit is approximately 0.1% by mass.

To keep the CXS content within the above range, methods can be used to increase catalytic activity when obtaining the resin, or to wash the obtained resin with a solvent or the olefin monomer itself.

The mesopentad fraction of polypropylene raw material A is preferably 0.970 or greater. The mesopentad fraction is an index that indicates the stereoregularity of the crystalline phase of polypropylene as measured by nuclear magnetic resonance (NMR). By setting the mesopentad fraction to 0.970 or greater, more preferably 0.975 or greater, even more preferably 0.980 or greater, and even more preferably 0.983 or greater, the degree of crystallinity and melting point will be high, making the polypropylene suitable for use at high temperatures. There is no particular upper limit for the mesopentad fraction.

To obtain polypropylene resin with a high mesopentad fraction, methods such as washing the resulting resin powder with a solvent such as n-heptane, or appropriately selecting a catalyst and/or co-catalyst and composition are preferably used.

The melting point of the chips of polypropylene raw material A (meaning the melting point of the resin; the same applies hereinafter) is preferably 160°C or higher. By making the melting point of the chips 160°C or higher, more preferably 163°C or higher, and even more preferably 166°C or higher, it is possible to effectively obtain voltage resistance characteristics in high-temperature environments when made into a film.

The chip melting point is the melting peak temperature obtained when the chip is heated from 30°C to 260°C at 20°C/min using a differential scanning calorimeter (DSC). There may be cases where two or more melting peak temperatures are observed within the temperature range, or where the peak temperature is a shoulder that can be observed on a multi-stage DSC chart (observed when two or more peaks overlap). However, in the present invention, the chip melting point is defined as the temperature of the peak with the largest absolute value of the heat flow (unit: mW) on the vertical axis of the DSC chart.

The proportion of polypropylene raw material A in the raw materials for the polyolefin film of the present invention is preferably such that polypropylene raw material A is the main component, i.e., the largest component, relative to the polyolefin film.

In order to effectively obtain voltage resistance characteristics in high-temperature environments when formed into a film, it is also preferable that the raw materials for the polyolefin film of the present invention contain, in addition to polypropylene raw material A, polypropylene raw material B having a higher number average molecular weight (Mn) than polypropylene raw material A.

From the viewpoint of biaxially stretching the film, the number average molecular weight (Mn) of the polypropylene raw material B is preferably 50,000 (5.0 × 10 ⁴ ) or more, more preferably 60,000 (6.0 × 10 ⁴ ) or more, and even more preferably 70,000 (7.0 × 10 ⁴ ) or more. On the other hand, from the viewpoint of obtaining thermal stability of the film in a high-temperature environment, the Mn of the polypropylene raw material B is preferably 120,000 (12.0 × 10 ⁴ ) or less, more preferably 110,000 (11.0 × 10 ⁴ ) or less, and even more preferably 100,000 (10.0 × 10 ⁴ ) or less.

The Mn of polypropylene raw material B is larger than the Mn of polypropylene raw material A, preferably by 10,000 (1.0×10 ⁴ ) or more, and more preferably by 20,000 (2.0×10 ⁴ ) or more.

From the viewpoint of biaxially stretching the film, the Z+1 average molecular weight (Mz ₊₁ ) of the polypropylene raw material B is preferably 2.5 million (2.5 × ¹⁰ ) or more, more preferably 3.5 million (3.5 × ¹⁰ ) or more, even more preferably 4 million (4.0 × ¹⁰ ) or more, and still more preferably 4.5 million (4.5 × ¹⁰ ). On the other hand, from the viewpoint of obtaining thermal stability of the film in a high-temperature environment, the Mz ₊₁ of the polypropylene raw material B is preferably 8 million (8.0 × ¹⁰ ) or less, more preferably 7 million (7.0 × ¹⁰ ) or less.

From the viewpoint of obtaining thermal stability of the film in a high-temperature environment, Mz ₊₁ of polypropylene raw material B is preferably larger than the Mz ₊₁ average molecular weight of polypropylene raw material A, more preferably by 500,000 (0.5 × 10 ⁶ ) or more, even more preferably by 1,000,000 (1.0 × 10 ⁶ ) or more, and still more preferably by 1,500,000 (1.5 × 10 ⁶ ) or more.

Polypropylene raw material B preferably has a cold xylene solubles (CXS) content of 4.0% by mass or less. By keeping the CXS content in polypropylene raw material B to 4.0% by mass or less, and more preferably 3.0% by mass or less, excellent film formation stability is achieved, and the film strength, dimensional stability, and heat resistance are also improved. The lower the CXS content, the better, but the lower limit is approximately 0.1% by mass.

To keep the CXS content within the above range, methods can be used to increase catalytic activity when obtaining the resin, or to wash the obtained resin with a solvent or the olefin monomer itself.

The mesopentad fraction of polypropylene raw material B is preferably 0.940 or more, more preferably 0.95 or more, and even more preferably 0.960 or more.

The melting point of the chips of polypropylene raw material B is preferably 160°C or higher. By making the melting point of the chips 160°C or higher, more preferably 162°C or higher, and even more preferably 164°C or higher, the voltage resistance characteristics in high-temperature environments can be effectively improved when made into a film.

The content of polypropylene resin B in the raw materials for the polypropylene film of the present invention is preferably 1% by mass or more and 30% by mass or less, based on 100% by mass of the polypropylene film. The content of polypropylene raw material B is more preferably 2% by mass or more. Furthermore, the content of polypropylene raw material B is more preferably 25% by mass or less, and even more preferably 20% by mass or less.

The number average molecular weight (Mn), Z+1 average molecular weight (Mz ₊₁ ), cold xylene solubles (CXS), mesopentad fraction, chip melting point, and content of polypropylene raw material B are set within the above-mentioned preferred ranges, and further, prior to stretching, the film is pre-stretched at 1.01 to 1.10 times in the longitudinal direction, and during biaxial stretching, the areal stretching ratio is 65 times or more and the widthwise stretching ratio is 11.0 times or more. In the heat setting and relaxation treatment steps after biaxial stretching, first, relaxation treatment is performed while heat treatment (first stage) is performed at a temperature lower than the widthwise stretching temperature, and then the film is tensioned in the widthwise direction while the above-mentioned By appropriately subjecting the film to a multi-stage heat setting and relaxation treatment, in which a second heat treatment is performed at a temperature lower than the first heat treatment temperature of 135°C or higher, and a third heat treatment is performed at 80°C or higher but lower than the second heat treatment temperature (third heat treatment), appropriate protrusions are formed on the surface due to the difference in viscosity with polypropylene raw material A, and the number of tie molecules connecting crystals increases, which makes it easier to improve the orientation of molecular chains when the draw ratio is increased and makes it possible to increase the binding force of amorphous chains in a high-temperature environment.

The polyolefin film of the present invention may contain, in addition to polypropylene raw material A and polypropylene raw material B, a branched polypropylene raw material C.

There are several methods for producing polypropylene raw material C, including Ziegler-Natta catalyst systems and metallocene catalyst systems. However, from the perspective of using it in combination with polypropylene raw material A and polypropylene raw material B, metallocene catalyst systems are more preferred, as they have fewer low-molecular-weight and high-molecular-weight components and a narrow molecular weight distribution.

Commercially available polypropylene raw material C can be selected from, for example, Lyondell Basell's "Profax" (registered trademark) (PF-814, etc.), and metallocene catalyst systems can be selected from, for example, Borealis' "Daploy" (trademark) (WB130HMS, WB135HMS, WB140HMS, etc.) and Japan Polypropylene's "WAYMAX" (registered trademark) (MFX8, MFX6, MFX3, etc.).

The polypropylene raw material C preferably has a CXS of 5.0% by mass or less, more preferably 3.0% by mass or less. The lower the CXS, the better, but the lower limit is approximately 0.1% by mass. To control the CXS within this range, methods can be used that increase the catalytic activity when obtaining the resin, or that wash the obtained resin with a solvent or the olefin monomer itself.

From the viewpoint of stretching uniformity, it is preferable that polypropylene raw material C have a melt tension of 2 cN or more and 40 cN or less at 230°C. The melt tension is more preferably 3 cN or more, and even more preferably 5 cN or more. It is also more preferably 30 cN or less, and even more preferably 20 cN or less. In order to keep the melt tension of polypropylene raw material C within the above range, methods such as controlling the average molecular weight, molecular weight distribution, and degree of branching in the polypropylene raw material can be used.

The content of polypropylene resin C in the raw materials for the polypropylene film of the present invention is preferably 0.10% by mass or more, based on 100% by mass of the polypropylene film. The content of polypropylene raw material C is more preferably 0.15% by mass or more, even more preferably 0.20% by mass or more, and even more preferably 0.50% by mass or more. The content of polypropylene raw material C is preferably 10% by mass or less, more preferably 4.5% by mass or less, and even more preferably 3.0% by mass or less. When the content of polypropylene resin C is within the above range, it is possible to prevent spherulite size from becoming too large when the molten polymer is formed into a sheet, and to maintain high-temperature withstand voltage.

The method for producing the polypropylene film of the present invention using the raw materials described above will be explained in more detail below, but the present invention should not be construed as necessarily being limited thereto.

The polypropylene resin described above can be melt-extruded onto a support to form an unstretched polypropylene film.

Polypropylene raw material is melt-extruded from a single-screw extruder set at an extrusion temperature of preferably 220°C to 280°C, more preferably 230°C to 270°C, and then passed through a filtration filter. The extruded molten sheet is then extruded through a slit-shaped die at a temperature of preferably 200°C to 260°C, more preferably 210°C to 240°C. The molten sheet extruded from the slit-shaped die is solidified on a casting drum (cooling drum) controlled at a temperature of 30°C to 110°C, resulting in an unstretched polypropylene film. The molten sheet can be adhered to the casting drum using any of the following methods: electrostatic application, adhesion using the surface tension of water, air knife, press roll, underwater casting, and air chamber. However, the air knife method is preferred, as it provides good flatness and allows for control of surface roughness. It is also preferable to appropriately adjust the position of the air knife to allow air to flow downstream of the film production process to prevent vibration of the film. The temperature of the casting drum is more preferably 60°C or higher and 110°C or lower, and even more preferably 80°C or higher and 110°C or lower, from the viewpoint of improving device processability and voltage resistance by minimizing surface depressions and providing adequate slipperiness when formed into a film.

Biaxially oriented polypropylene film can be obtained by biaxially stretching unstretched polypropylene film, heat treating it, and relaxing it.

Biaxial stretching can be achieved by any of the following methods: inflation simultaneous biaxial stretching, tenter simultaneous biaxial stretching, and tenter sequential biaxial stretching. However, among these, sequential biaxial stretching using a tenter is preferred in terms of controlling the film's film formation stability, crystalline/amorphous structure, surface properties, and particularly the mechanical properties and thermal dimensional stability while increasing the stretch ratio in the width direction of the film. In sequential biaxial stretching, an unstretched polypropylene film is stretched in the longitudinal direction and then in the width direction.

In the longitudinal stretching process, it is preferable to perform multi-stage stretching, consisting of preliminary stretching and main stretching. The unstretched polypropylene film is preheated by passing it between rolls maintained at a temperature of preferably 70°C to 150°C, more preferably 80°C to 145°C, and then preliminary stretched in the longitudinal direction at a magnification of 1.01 to 1.10 times. This moderately promotes spherulite destruction in the unstretched sheet, pre-orienting the molecular chains, thereby further improving the crystallite size, crystalline orientation, and amorphous chain constraint of the stretched film obtained after the subsequent main stretching. This improves the film's withstand voltage and ensures a stable structure even in high-temperature environments.

The polypropylene film that has been pre-stretched in the longitudinal direction is then stretched between rolls maintained at a temperature of preferably 70°C to 150°C, more preferably 80°C to 145°C, preferably 2.0 to 15.0 times, more preferably 4.5 to 12.0 times, and even more preferably 5.5 to 10.0 times, in the longitudinal direction, and then cooled to room temperature.

Next, the film, uniaxially stretched in the longitudinal direction, is introduced into a tenter while both widthwise ends are held with clips. In the present invention, the temperature in the preheating step immediately prior to widthwise stretching is preferably the widthwise stretching temperature +5 to +15°C, more preferably +5 to +12°C, and even more preferably +5 to +10°C. This is because the fibril structure, which is highly oriented in the longitudinal direction by uniaxial stretching, can be further strengthened and the change in breakdown voltage before and after film heating can be suppressed. Furthermore, maintaining the temperature conditions as described above is also preferable from the perspective of improving thermal dimensional stability by stabilizing insufficiently oriented molecular chains after uniaxial stretching with high-temperature preheating. By setting the preheating temperature at least +5°C from the stretching temperature, the change in breakdown voltage before and after film heating can be suppressed, effectively improving thermal dimensional stability. On the other hand, by setting the preheating temperature at or below +15°C from the stretching temperature, film tearing during the stretching step can be suppressed.

Next, the film is stretched in the width direction while the edges are held with clips. The temperature during this process (width direction stretching temperature) is preferably 150°C or higher and 170°C or lower, more preferably 155°C or higher and 165°C or lower.

From the viewpoint of suppressing the change in breakdown voltage before and after heating the film, the widthwise stretching ratio is preferably 11.0 times or more and 20.0 times or less. By setting the widthwise stretching ratio to 11.0 times or more, more preferably 11.5 times or more, and even more preferably 12.0 times or more, the orientation contribution of the fibril structure highly oriented in the longitudinal direction by uniaxial stretching is reduced, thereby effectively suppressing the change in breakdown voltage before and after heating the film. By increasing the widthwise stretching ratio, orientation is imparted in the widthwise direction while maintaining high longitudinal orientation, thereby increasing in-plane molecular chain tension and further improving structural stability against heat. Therefore, setting the widthwise stretching ratio as described above is preferable from the viewpoint of obtaining the effect of improving the thermal shrinkage characteristics, which are a trade-off. On the other hand, setting the widthwise stretching ratio to 20.0 times or less, more preferably 19.0 times or less, and even more preferably 18.0 times or less, can reduce film breakage during film formation.

In the production of the polypropylene film of the present invention, the areal stretch ratio is preferably 65 times or more. The areal stretch ratio is the longitudinal stretch ratio multiplied by the widthwise stretch ratio. By setting the areal stretch ratio to 65 times or more, more preferably 66 times or more, even more preferably 68 times or more, and even more preferably 72 times or more, the change in breakdown voltage before and after heating the film can be suppressed, and when made into a capacitor, it can have excellent reliability for long-term use in high-temperature environments.

In the production of polypropylene film of the present invention, the subsequent heat treatment and relaxation treatment steps involve heat setting (first-stage heat treatment) at a temperature of 145°C to 165°C, but below the width-direction stretching temperature, while applying a relaxation of 2% to 20% in the width direction while the film is held taut in the width direction with clips; then, while again holding the film taut in the width direction with clips, heat treatment (second-stage heat treatment) is performed at 135°C or higher, but below the heat setting temperature (first-stage heat treatment temperature); and, while still holding the film taut, heat setting (third-stage heat treatment) is performed at 80°C or higher, but below the heat setting temperature (second-stage heat treatment temperature). This multi-stage heat treatment is preferred from the perspectives of suppressing changes in breakdown voltage before and after heating the film, improving structural stability against heat, and ensuring voltage resistance and reliability when made into a capacitor.

In the relaxation treatment, a relaxation rate of 2% or more and 20% or less is preferable from the perspective of increasing structural stability against heat. By setting the relaxation rate to 20% or less, more preferably 18% or less, and even more preferably 15% or less, it is possible to prevent the film from relaxing too much inside the tenter, causing wrinkles in the product and causing unevenness during vapor deposition. It also prevents a decrease in mechanical properties. On the other hand, by setting the relaxation rate to 2% or more, more preferably 5% or more, and even more preferably 8% or more, structural stability against heat is achieved, and when made into a capacitor, it is possible to prevent a decrease in capacity and short-circuit damage in high-temperature environments.

After undergoing a multi-stage heat treatment that lowers the temperature, the film is led to the outside of the tenter, where the clips on both ends of the film width are released in a room temperature atmosphere. The film edges are slit in the winding process, and a film product roll with a film thickness of 0.5 μm or more and less than 10 μm is wound up. Before winding the film, it is preferable to perform a corona discharge treatment in air, nitrogen, carbon dioxide gas, or a mixture of these gases to improve the adhesion of the vapor-deposited metal to the surface to be vapor-deposited.

Specific examples of preferred production conditions to be focused on in order to obtain the polypropylene film of the present invention are as follows.
- The melt extrusion temperature should be lowered in multiple stages: before the filter, after the filter, and at the die.
- The mesopentad fraction of polypropylene resin A is 0.970 or more.
- The CXS of polypropylene resin A is less than 3.0 mass%.
- Preliminary stretching of 1.01 times or more and 1.10 times or less is carried out before longitudinal stretching.
The area stretching ratio is 65 times or more.
The stretching ratio in the width direction is 11.0 times or more.
The preheating temperature before stretching in the width direction is the width direction stretching temperature +5 to +15°C.
The first stage heat treatment temperature is 145°C or higher and 165°C or lower, and is lower than the width direction stretching temperature.
The second stage heat treatment temperature is 135°C or higher and lower than the first stage heat treatment temperature.
The third stage heat treatment temperature is 80°C or higher and lower than the second stage heat treatment temperature.
- In the first heat treatment step, a relaxation treatment of 2% to 20% in the width direction is performed.

Next, we will explain the metal film laminated film made using the polypropylene film of the present invention, the film capacitor made using the same, and the manufacturing methods for the same.

The metal film-laminated film of the present invention has a metal film on at least one side of the polypropylene film of the present invention. This metal film-laminated film can be obtained by providing a metal film on at least one side of the polypropylene film of the present invention described above.

In the present invention, the method for applying a metal film is not particularly limited. However, a preferred method is to deposit a metal film, such as a vapor-deposited film of aluminum or an aluminum-zinc alloy, on at least one side of the polypropylene film to form an internal electrode of the film capacitor. In this case, other metal components, such as nickel, copper, gold, silver, or chromium, can also be deposited simultaneously with or sequentially to the aluminum. A protective layer, such as oil, can also be deposited on the vapor-deposited film. If the surface roughness of the polypropylene film differs between the front and back, it is preferable to deposit a metal film on the smoother surface to form a metal film laminated film, from the perspective of improving voltage resistance.

In the present invention, if necessary, after forming the metal film, the metal film laminated film can be annealed or heat treated at a specific temperature. Furthermore, for insulation or other purposes, a resin coating such as polyphenylene oxide can be applied to at least one side of the metal film laminated film.

The film capacitor of the present invention uses the metal film laminated film of the present invention. In other words, the film capacitor of the present invention has the metal film laminated film of the present invention.

For example, the film capacitor of the present invention can be obtained by laminating or winding the metal film laminated film of the present invention described above using various methods. Examples of preferred methods for manufacturing wound film capacitors are as follows:

Aluminum is vapor-deposited under reduced pressure on one side of a polypropylene film. This is done in stripes with margins running longitudinally. Next, a blade is used to slit the center of each vapor-deposited area on the surface and the center of each margin, creating a tape-like take-up reel with a margin on one side of the surface. Two tape-like take-up reels with a left or right margin are stacked on top of each other and wound together so that the vapor-deposited area extends beyond the margin in the width direction, producing a wound body.

When vapor deposition is performed on both sides, one side is vapor deposited in stripes with a margin running lengthwise, and the other side is vapor deposited in stripes so that the longitudinal margin is positioned in the center of the vapor deposition area on the back side. Next, a blade is cut into the center of the margins on both sides to create a tape-like take-up reel with a margin on one side on each side (for example, if there is a margin on the right side of the front side, there will be a margin on the left side on the back side). The resulting reel and one unvapor-deposited laminated film are then overlapped and wound together so that the metallized film extends beyond the laminated film in the width direction to obtain a wound body.

The core material is removed from the wound body produced as described above and pressed, and metallicon is sprayed onto both end surfaces to form external electrodes. Lead wires are welded to the metallicon to obtain a wound film capacitor. Film capacitors have a wide range of uses, including railway vehicles, automobiles (hybrid cars, electric cars), solar power generation, wind power generation, and general home appliances, and the film capacitor of the present invention can be suitably used for these applications. The polypropylene film of the present invention can also be used in a variety of applications, such as packaging film, release film, processing film, sanitary products, agricultural products, construction products, and medical products, and is particularly suitable for applications that involve a heating process in film processing.

The present invention will be explained in more detail below with reference to examples.

[Measurement and evaluation method]
The methods for measuring the characteristic values and evaluating the effects in the present invention are as follows.

(1) Film Thickness The thickness of ten randomly selected points on the polypropylene film was measured in an atmosphere of 23°C and 65% RH using a contact-type electronic micrometer (K-312A model) manufactured by Anritsu Corporation. The arithmetic mean value of the thicknesses at the ten points was taken as the film thickness of the polypropylene film.

(2) Crystal Orientation of the α-Crystal (110) Plane After Heating The film was heat-treated at 125°C for 60 minutes using a 20mm wide square metal frame with a thickness of 2mm, outer dimensions of 300mm x 300mm, and inner dimensions of 280mm x 280mm, cut out from the center. Double-sided tape (Nichiban "Nice Tack" (registered trademark) NW-H15, adhesive strength 02) was attached to the four sides of the frame, and the film was attached so that it covered the entire surface of the metal frame. The film was then sandwiched between metal frames of the same dimensions. The film was attached carefully to prevent wrinkles. Next, a sample was created by clamping and fixing the four sides of the frame in the metal frame/double-sided tape/film/metal frame configuration with clips, and left in an oven heated to 125°C for 60 minutes. The sample was then removed and left at room temperature for 5 minutes, after which the film was cut out along the inner edge of the metal frame to obtain the film after heat treatment at 125°C. The film after heat treatment at 125°C was cut into strips 4 cm long and 1 mm wide, and the strips were stacked to a thickness of 1 mm to prepare a sample. The film sample was placed between the X-ray source and detector so that the X-rays would penetrate the polypropylene film, and X-rays were incident perpendicular to the film surface. The crystal peak at 2θ = approximately 14° (α crystal (110) plane) was scanned in the circumferential direction to obtain the half-width H (°) of the orientation peak. The orientation peak was calculated using the following formula.
Crystal orientation degree = (180°-H)/180°
The measurement equipment and conditions are shown below.

(Measuring device)
・X-ray diffractometer: Rigaku Corporation, Model 4036A2 X-ray source: CuKα ray (using Ni filter)
Output: 40kV-30mA
・Goniometer: Rigaku Corporation, Model 2155D, Slit: 2mmφ-1°-1°
Detector: Scintillation counter/counting recorder RAD-C type manufactured by Rigaku Denki Co., Ltd. (measurement conditions)
Circumferential scan (2θ = approximately 14°)
Scanning method: Step scan Measurement range: 0 to 360°
Step: 0.5°
Accumulation time: 2 seconds.

(3) Stress of the heated film when the elongation in the longitudinal direction of the film is 5% (F5MD) and stress of the heated film when the elongation in the width direction of the film is 5% (F5TD)
The method for heat-treating the film at 150°C for 10 minutes involves using a 20mm wide square metal frame with a thickness of 2mm, outer dimensions of 300mm x 300mm, and inner dimensions of 280mm x 280mm, with a cutout. Double-sided tape (Nichiban "Nice Tack" (registered trademark) NW-H15 adhesive strength 02) is applied to the four sides of the frame, and the film is attached so that it covers the entire surface of the metal frame. The film is then sandwiched between metal frames of the same dimensions. The film is attached so as not to wrinkle. Next, a sample is created by clamping and fixing the four sides of the frame in a metal frame/double-sided tape/film/metal frame configuration with clips, and this is left in an oven heated to 150°C for 10 minutes. The sample is then removed and left at room temperature for 5 minutes. The film is then cut along the inner edge of the metal frame, and used as the film after heat treatment at 150°C. The film after heat treatment at 150°C is cut into a rectangle measuring 50mm long x 10mm wide in the test direction to prepare a sample. Next, the film was subjected to a tensile test in a tensile tester for rectangular samples (Tensilon UCT-100 manufactured by Orientec) with an initial chuck distance of 20 mm, at a tension rate of 300 mm/min in an atmosphere of 23°C. The load applied to the film when the sample was elongated by 5% was read and divided by the cross-sectional area of the sample before the test (film thickness x width (10 mm)) to calculate the stress at 5% elongation (F5 value, unit: MPa). Measurements were performed five times for each sample in the longitudinal direction and width direction, and the average values were calculated to calculate F5MD and F5TD, respectively. The film thickness used to calculate the F5 value was the value measured in (1) above.

(4) Melting peak temperature (Tm) and crystallization peak temperature (Tc) of polypropylene resin and film
Using a differential scanning calorimeter (Seiko Instruments, EXSTAR DSC6220), 3 mg of polypropylene film was heated in a nitrogen atmosphere from 30°C to 260°C at a rate of 20°C/min, then held at 260°C for 5 minutes, and then cooled to 30°C at a rate of 20°C/min. The endothermic peak temperature obtained during the heating process was defined as the melting peak temperature of the polypropylene film, and the exothermic peak temperature obtained during the cooling process was defined as the crystallization peak temperature of the polypropylene film. In this specification, Tm and Tc were calculated from the average values obtained from n=3 measurements. In some cases, two or more peak temperatures were observed within the temperature range, or a peak temperature observable on a multi-stage DSC chart called a shoulder (observed in a chart where two or more peaks overlap) was observed. However, in the present invention, the temperature of the peak with the largest absolute value of the vertical axis heat flow (unit: mW) on the DSC chart was defined as Tm and Tc, respectively. The (Tm) and (Tc) of polypropylene resins were also measured in the same manner.

(5) Skewness (Ssk) of the surface protrusion shape
Skewness (Ssk) is defined in accordance with ISO 25178. Measurements were performed using a scanning white light interference microscope VS1540 manufactured by Hitachi High-Tech Science Corporation. The attached analysis software was used to remove waviness components from the captured image using polynomial fourth-order approximation surface correction, followed by processing with a median (3 x 3) filter. Then, interpolation processing was performed, i.e., pixels for which height data could not be obtained were supplemented with height data calculated from surrounding pixels. Measurements were taken at five randomly selected locations on one surface, and the average value was calculated. Measurements were performed on both surfaces of the film.
The measurement conditions are as follows:
Manufacturer: Ryoka Systems Co., Ltd. Distributor: Hitachi High-Tech Science Corporation Device name: Scanning white light interference microscope VS1540
Measurement conditions: Objective lens 10x
1x telescope tube
Zoom lens 1x
Wavelength filter 530nm white
Measurement mode: Wave
Measurement software: VS-Measure Version 10.0.4.0
Analysis software: VS-Viewer Version 10.0.3.0
Measurement area: 0.561 mm x 0.561 mm.

(6) Cold xylene soluble portion of film (CXS)
0.5 g of the polypropylene resin in the case of the raw material, or the film sample in the case of the film, was dissolved in 100 ml of xylene at 135°C, allowed to cool, and then allowed to stand in a thermostatic water bath at 20°C for 1 hour to recrystallize, after which the polypropylene component dissolved in the filtrate was quantified by liquid chromatography. The amount of the polypropylene component dissolved in the filtrate was determined by the following formula: CXS (mass%) = (X/ _X0 ) × 100, where X (g) is the amount of the polypropylene component dissolved in the filtrate and _X0 (g) is the precise weight of 0.5 g of the sample.
was calculated from

(7) Height of protruding peaks on the film surface (SpkA), (SpkB)
The protruding peak height (Spk) is defined in ISO 25178. Measurements were performed using a VS1540 scanning white light interference microscope manufactured by Hitachi High-Tech Science Corporation. The attached analysis software was used to remove waviness components from the captured image using polynomial fourth-order approximation surface correction, followed by processing with a median (3 x 3) filter and then interpolation (a process in which height data for pixels for which no height data could be obtained is supplemented with height data calculated from surrounding pixels). Measurements were taken at five arbitrary locations on one surface, and the average value was calculated. Both surfaces of the film were measured, and the value of the surface with the lower value was designated SpkA, and the value of the opposite surface was designated SpkB. The measurement conditions were the same as those for (5) Degree of bias of surface protrusion shape.

(8) Total volume of valleys 20 nm or deeper on the film surface Measurements were performed using a VS1540 scanning white light interference microscope manufactured by Hitachi High-Tech Science Corporation. The attached analysis software removed waviness components from the captured image using a polynomial fourth-order approximation surface correction, then processed with a median (3 x 3) filter. Then, interpolation processing was performed, i.e., pixels for which height data could not be obtained were supplemented with height data calculated from surrounding pixels. Analysis was then performed using the bearing function, an analysis tool in the analysis software. To specify valley-side voids 20 nm or deeper, the valley-side height threshold was set to -20 nm in the height region specification. The volume value of the analyzed valleys that met this threshold was then read. Measurements were performed in five randomly selected 0.561 x 0.561 ^mm2 regions on one surface, and the average of the total volume within the region was used as the total volume of valleys 20 nm or deeper on the film surface.

Both sides of the film were measured, and if the total valley volume on one side was within the range of 50 to 5,000 ^μm³ , the value for the side that was within the range was recorded; if both sides were within the range, the value for the side with the smaller value was recorded; and if neither side was within the range, the value for the side with the total valley volume closest to the range of 50 to 5,000 ^μm³ was recorded. The measurement conditions were the same as those for (5) Degree of bias of surface protrusion shape above.

(9) Heat shrinkage in the width direction after heat treatment at 125°C for 15 minutes HS125TD
Five samples, each 200 mm long and 10 mm wide, were cut out so that the width direction of the film was the long side, and marked with lines 25 mm from both ends. The distance between the marked lines was measured with a universal projector to determine the test length (L1). Next, a load of 3 g was applied to one end (the lower end) of the test piece in the longitudinal direction, and the test piece was heated in a suspended state for 15 minutes in an oven maintained at 125 ° C. The test piece was removed and cooled at room temperature. The dimension (L2) between the marked lines was measured with a universal projector, and the heat shrinkage rate HS125TD of each sample was calculated using the following formula. The arithmetic average value of the five samples was calculated as the heat shrinkage rate HS125TD in the measurement direction.
HS125TD={(L1-L2)/L1}×100
Here,
HS125TD: Heat shrinkage rate (%) in the width direction after heat treatment at 125°C for 15 minutes
L1: Distance between the gauge lines before heat treatment (150 mm)
L2: The distance between the gauge lines after heat treatment.

(10) Static friction coefficient ( _μs )
Measurements were made at 25°C and 65% RH using a slip tester manufactured by Toyo Seiki Co., Ltd. in accordance with JIS K 7125 (1999). The measurements were made in the longitudinal direction of the films, with different sides overlapping each other. The same measurement was made five times for each sample, and the average value was calculated to be the static friction coefficient ( _μs ) of the sample.

(11) Mesopentad Fraction A polypropylene resin in the case of a raw material, or a film sample in the case of a film, was freeze-pulverized into powder, extracted with n-heptane at 60°C for 2 hours to remove impurities and additives in the polypropylene, and then dried under reduced pressure at 130°C for 2 hours or more to obtain a sample. The sample was dissolved in a solvent, and the mesopentad fraction (mmmm) was determined using ^13C -NMR under the following conditions.

Measurement conditions and equipment: Bruker DRX-500
・Measurement nucleus: ¹³ C nucleus (resonance frequency: 125.8MHz)
・Measurement concentration: 10% by mass
Solvent: Benzene: deuterated orthodichlorobenzene = 1:3 mixed solution (volume ratio)
・Measurement temperature: 130℃
Spin speed: 12Hz
NMR sample tube: 5 mm tube Pulse width: 45° (4.5 μs)
Pulse repetition time: 10 seconds Data points: 64K
Accumulation count: 10,000 times Measurement mode: complete decoupling.

The analysis was performed as follows. Fourier transformation was performed with LB (line broadening factor) set to 1, and the mmmm peak was determined to be 21.86 ppm. Peak splitting was performed using WINFIT software (manufactured by Bruker). Peak splitting was performed as follows, starting from the peak on the high magnetic field side. Further, automatic fitting of the software was performed to optimize the peak splitting, and the sum of the mmmm peak fractions was determined to be the mesopentad fraction (mmmm).
(1) mrrm
(2), (3) rrrm (split into two peaks)
(4) rrrr
(5) mrmr
(6) mrmm+rmrr
(7) mmrr
(8) rmmr
(9) mmmr
(10) mmmm
The same measurement was carried out five times for the same sample, and the average of the obtained mesopentad fractions was taken as the mesopentad fraction of the sample.

(12) Breakdown voltage at 130°C (V/μm)
The film was heated for 1 minute in an oven maintained at 130°C, and then measured in that atmosphere in accordance with JIS C2330 (2014) 6.2 and JIS C2151 (2019) 17.2 B (plate electrode method) cited therein. However, for the lower electrode, a conductive rubber (E-100<65> manufactured by Togawa Rubber Co., Ltd.) of the same dimensions was placed on the aluminum foil described in JIS C2151 (2019) 17.2.2 Figure 3-C was used as the electrode. The breakdown voltage test was performed 30 times, and the obtained value was divided by the film thickness (measured in (1) above) and converted to units of V/μm. Of the 30 measured values (calculated values), the five largest and five smallest were excluded, and the average of the 20 points was used as the film breakdown voltage at 130°C.

(13) Molecular weight and molecular weight distribution of polypropylene resin and film: These were evaluated and calculated using GPC (gel permeation chromatography) under the following measurement conditions. As sample pretreatment under the following measurement conditions, the sample was weighed, a solvent (1,2,4-TCB with 0.1% BHT added) was added, and the sample was dissolved by shaking at 140°C for 1 hour. Next, the sample was heated and filtered using a sintered filter with a pore size of 0.5 μm, and fractionated by molecular size.

<Apparatus and measurement conditions>
・Apparatus: HLC-8321GPC/HT (detector: RT)
・Column:
TSKgel guard column _{H HR} (30) HT (7.5 mm ID x 7.5 mm) x 1 tube + TSKgel GMH _HR -H (20) HT (7.8 mm ID x 30 cm) (Tosoh Corporation) x 3 tubes; Eluent: 1,2,4-trichlorobenzene (Fujifilm Wako Pure Chemical Industries, Ltd., for GPC) + BHT (0.05%)
・Flow rate: 1.0mL/min.
Detection condition: polarity = (-)
・Injection volume: 0.3mL
Column temperature: 140°C
System temperature: 40°C
Sample concentration: 1 mg/mL
A calibration curve was prepared using standard polystyrene (manufactured by Tosoh Corporation), and the measured molecular weight values were converted into polystyrene values to obtain the Z+1 average molecular weight (Mz ₊₁ ), weight average molecular weight (Mw), and number average molecular weight (Mn). The molecular weight distribution (Mz ₊₁ /Mw) was then calculated using the Mz ₊₁ and Mw values.

(14) Evaluation of film capacitor characteristics (voltage resistance at 120°C, reliability, processability)
Aluminum was vapor-deposited on one side of the film using a vacuum vapor deposition machine (manufactured by ULVAC, Inc.), and a vapor deposition pattern with a film resistance of 10 Ω/sq and a margin section in the direction perpendicular to the longitudinal direction, namely a T-shaped margin (longitudinal pitch (period) of 17 mm, fuse width of 0.5 mm), was formed using masking oil. When the wetting tension differed between the front and back surfaces, vapor deposition was performed on the side with the higher wetting tension.

The film with the vapor deposition pattern formed was slit to obtain a vapor deposition reel with a film width of 50 mm and an edge margin width of 2 mm.

The capacitor element was then wound up on this reel using an element winding machine (KAW-4NHB, manufactured by Kaito Seisakusho Co., Ltd.), metallicon was applied, and the element was then heat-treated under reduced pressure at 128°C for 12 hours, after which lead wires were attached to complete the capacitor element.

A so-called step-up test was conducted using 10 capacitor elements thus obtained. A voltage of 250 VDC was applied to the capacitor elements at a high temperature of 120°C, and after 10 minutes at that voltage, the applied voltage was gradually increased in steps of 50 VDC per minute.

<Voltage resistance evaluation>
In the step-up test, the change in capacitance at this time was measured and plotted on a graph. The voltage at which the capacitance reached 80% of the initial value was divided by the film thickness ((1) above) to determine the withstand voltage, which was evaluated as follows:
S: The withstand voltage is 400 V/μm or more.
A: The withstand voltage is 390 V/μm or more and less than 400 V/μm.
B: The withstand voltage is 380 V/μm or more and less than 390 V/μm.
C: The withstand voltage is less than 380 V/μm.
S, A, and B are usable. C is inferior in practical performance.

<Reliability evaluation>
After increasing the voltage until the capacitance was reduced to 15% or less of the initial value, the capacitor element was disassembled to check the state of breakdown, and the reliability was evaluated as follows.
S: The element shape remains unchanged, and no penetration damage is observed.
A: There is no change in the shape of the element, and breakage of the film in a penetrating manner is observed in 1 to 5 layers.
B: There is no change in the shape of the element, and penetration damage is observed through 6 to 10 film layers.
C: Changes in element shape are observed, or breakage that penetrates more than 10 layers is observed, or the element shape changes significantly and breaks. S can be used without problems, and A and B can be used depending on the conditions. C has poor practical performance.

<Processability evaluation>
The evaluation was based on the following criteria: A capacitor element was prepared in the same manner as above, and the shape of the element was visually confirmed.
S: No misalignment, wrinkles or deformation of the end surface film of the capacitor element, causing no problems in subsequent processes. Level A: No deformation of the capacitor element, slight wrinkles but usable without problems. Level B: Slight deformation and wrinkles of the capacitor element but usable. Level C: Severe deformation and wrinkles of the capacitor element causing problems in subsequent processes. Levels S and A mean that the capacitor can be used without problems, B means that it can be used depending on the conditions, and C means that it is difficult to put into practical use.

[Polypropylene raw material]
The raw materials shown in Table 1 below were used to produce the polypropylene films of the Examples and Comparative Examples. Four types of raw polypropylene materials A (A1, A2, A3, A4), three types of raw polypropylene materials B (B1, B2, B3), and two types of raw polypropylene materials C (C1: Ziegler-Natta catalyst system, C2: metallocene catalyst system) were used.

[Example 1]
The raw materials and film-forming conditions used in this example are as shown in Tables 1 and 2. First, 93 parts by mass of polypropylene raw material A1, 4 parts by mass of polypropylene raw material B1, and 3 parts by mass of polypropylene raw material C1 were dry-blended. The blended raw materials were fed into a single-screw extruder at 260°C, melted, passed through a filtration filter, passed through a pipe set at a temperature of 255°C, and melt-extruded into a sheet from a T-slit die set at 250°C. This sheet-like material was placed on a casting drum maintained at 99°C, adhered with an air knife, and cooled and solidified to obtain an unstretched polypropylene film. The unstretched polypropylene film was preheated stepwise to 143°C using a group of rolls, passed between rolls with different peripheral speeds, pre-stretched 1.08 times at 130°C, and then stretched 6.1 times in the longitudinal direction at 143°C. The film was then introduced into a tenter, preheated at 169°C (TD stretching temperature + 7°C) while both ends of the film were held with clips, and then stretched 12.3 times in the width direction at 163°C. The first heat treatment was performed at 159°C while providing 15% relaxation in the width direction, and the second heat treatment was performed at 150°C while both ends of the film were held with clips in the width direction. The third heat treatment was performed at 111°C, after which the film was introduced to the outside of the tenter, the clips on the film ends were released, and the film surface (the side contacting the casting drum) was subjected to a corona discharge treatment in the atmosphere at a treatment intensity of 25 W min/ ^m² , and the resulting polypropylene film with a thickness of 2.3 µm was wound into a film roll. The evaluation results for each item are shown in Table 4.

[Examples 2 to 9, Comparative Examples 1 to 7]
Polypropylene films having the thicknesses shown in Tables 2 and 3 were obtained in the same manner as in Example 1, except that the raw material compositions and film-forming conditions were as shown in Tables 2 and 3. The thickness was adjusted by increasing or decreasing the rotation speed of the single-screw extruder. The same applies to the other examples and comparative examples below. The evaluation results for each item are shown in Tables 4 to 6.

Claims (10)

A polypropylene film in which, when measured by wide-angle X-ray diffraction after heating at 125°C for 60 minutes, the degree of crystal orientation of the α crystal (110) plane is 0.73 or more, and the sum of the stress at 5% elongation in the longitudinal direction of the film (F5MD) when the film is heated for 10 minutes at 150°C and then subjected to a tensile test at room temperature, and the stress at 5% elongation in the transverse direction of the film (F5TD) when the film is heated for 10 minutes at 150°C and then subjected to a tensile test at room temperature, is 150 MPa or more. 2. The polypropylene film according to claim 1, wherein the melting peak temperature (Tm) of the film obtained by heating the film from 30°C to 260°C at 20°C/min using a differential scanning calorimeter (DSC) and the crystallization peak temperature (Tc) obtained by cooling the film from 260°C to 30°C at 20°C/min satisfy the following relationship:
Tm-Tc≦65(℃) The polypropylene film according to claim 1 or 2, wherein the skewness (Ssk) as defined by ISO 25178 on at least one surface of the film is greater than -30 and less than 5. The polypropylene film according to any one of claims 1 to 3, wherein when the polypropylene film is completely dissolved in xylene and then precipitated at room temperature, the polypropylene component (CXS) dissolved in xylene is 3.0 mass% or less. The polypropylene film according to any one of claims 1 to 4, wherein the protruding peak heights SpkA and SpkB defined in ISO 25178 of the surface A on one side and the surface B on the other side satisfy the following relationship.
SpkA<SpkB
20 nm≦SpkA≦100 nm
80 nm≦SpkB≦150 nm
where:
SpkA: height of protruding peak on surface A SpkB: height of protruding peak on surface B 6. The polypropylene film according to any ^one of claims 1 to ⁵ , wherein the total volume of valleys having a depth of 20 nm or more in an area of 0.561 mm x 0.561 mm measured by a scanning white light interference microscope on at least one surface is 50 μm or more and 5,000 μm or less. The polypropylene film according to any one of claims 1 to 6, having a heat shrinkage rate in the width direction (HS125TD) after heating at 125°C for 15 minutes of 1.0% or less. The polypropylene film according to any one of claims 1 to 7, wherein the static friction coefficient (μs) when the films are stacked is 0.2 or more and less than 0.8. A metal film laminated film having a metal film on at least one side of the polypropylene film described in any one of claims 1 to 8. A film capacitor made using the metal film laminated film described in claim 9.