Biaxially oriented polypropylene film, metal film laminated film, and film capacitor

Abstract

The present invention aims to provide a biaxially oriented polypropylene film that can achieve high voltage resistance when used as a dielectric in film capacitors and has excellent productivity. [Solution] A biaxially oriented polypropylene film having a layer (layer A) containing both a polypropylene resin and a resin I having an alicyclic structure in its side chains, characterized in that the average aspect ratio of the domains in layer A is 2 or more and 10 or less in a cross-section in the direction of the main orientation axis and thickness.

Description

[Technical Field]

[0001] This invention relates to a biaxially oriented polypropylene film, a metal film laminate film, and a film capacitor using these, which offer excellent voltage resistance and productivity in high-temperature environments. [Background technology]

[0002] In recent years, the majority of electrical equipment has been converted to inverters, and consequently, the demand for smaller and larger capacity film capacitors has become even stronger. In response to this demand, particularly in fields such as automobiles (including electric vehicles and hybrid cars), electric aircraft, solar power generation, and wind power generation, there is a need for further thinning and improved heat resistance of the film, which is the dielectric of film capacitors, in addition to improving voltage resistance and productivity, and maintaining processability in the fabrication of film capacitor elements.

[0003] For film to be used as a dielectric in film capacitors in these fields, it is important that the film possesses excellent heat resistance (such as dimensional stability) at the ambient temperature and stable electrical performance (such as voltage resistance) in a temperature range 10°C to 20°C higher than the ambient temperature. Furthermore, considering future applications in power semiconductors using silicon carbide (SiC), it is said that the ambient temperature for film capacitors will be even higher, and it is estimated that the requirements for heat resistance will increase.

[0004] Currently, polypropylene film, which has relatively good heat resistance and voltage resistance among polyolefin films, is used as the dielectric for film capacitors. However, the upper limit of the operating temperature for polypropylene film is said to be about 110°C (Non-Patent Literature 1). However, due to the above circumstances, there is a need for further improvements in heat resistance and voltage resistance for film capacitors, and film capacitor films are required to have improved dielectric breakdown voltage in high-temperature environments exceeding 110°C. In other words, it has been extremely difficult for conventional polypropylene films to stably maintain voltage resistance in such temperature environments.

[0005] To miniaturize film capacitors and improve their heat resistance, various approaches have been considered, such as thinning the film, using films with high dielectric constants, and using films with glass transition temperatures exceeding the operating temperature range of film capacitors.

[0006] For example, a laminate has been proposed in which one layer is a cyclic olefin resin layer with a glass transition temperature exceeding 130°C, and the other layer is a polypropylene resin layer, with these layers stacked alternately (for example, Patent Document 1). Because such a laminate has a laminated structure in which two types of layers with different dielectric constants are stacked alternately, it can maintain a large capacitance while having excellent heat resistance and dielectric strength.

[0007] Furthermore, films with improved dielectric strength in high-temperature environments have been proposed by co-extruding cyclic olefin resin and polypropylene resin, followed by film formation and biaxial stretching (for example, Patent Documents 2 and 3). In addition, films with improved thermal dimensional stability in high-temperature environments have been proposed by using a polymer blend of cyclic olefin resin and linear polypropylene resin for film formation and biaxial stretching, or by using a compound resin raw material obtained by pre-kneading cyclic olefin resin and linear polypropylene resin for film formation and biaxial stretching (for example, Patent Documents 3 and 4). [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2015-012076 [Patent Document 2] Japanese Patent Publication No. 2018-034510 [Patent Document 3] Japanese Patent Publication No. 2020-521867 [Patent Document 4] Japanese Patent Publication No. 2024-047570 [Non-patent literature]

[0009] [Non-Patent Document 1] Motonobu Kawai, "The Leap Forward of Film Capacitors: From Automobiles to Energy," Nikkei Electronics, Nikkei BP, September 17, 2012 issue, pp. 57-62. [Overview of the Initiative] [Problems that the invention aims to solve]

[0010] However, the film described in Patent Document 1 is not a laminate formed by co-extrusion, but rather a laminate in which a cyclic olefin resin layer is formed on a polypropylene film by a coating method. As a result, the cyclic olefin resin layer is prone to peeling, and its processability in high-temperature environments, as well as its performance and reliability when used as a film capacitor, are not entirely satisfactory.

[0011] The films described in Patent Document 2 and some of the films described in Patent Document 3 have a laminated base layer made of a single cyclic olefin resin, making it difficult to increase the area stretching ratio. As a result, their voltage resistance in high-temperature environments is unsatisfactory, and their performance and reliability as film capacitors are also not entirely satisfactory. Furthermore, although Patent Document 2 discloses the inclusion of an elastomer to improve stretchability, even in this form, the voltage resistance in high-temperature environments is still unsatisfactory.

[0012] Some of the films described in Patent Document 3 are simply films made by blending cyclic olefin resin and polypropylene resin, making it difficult to increase the area stretching ratio during stretching. Furthermore, while it is possible to stretch at a high area stretching ratio by increasing the preheating temperature and stretching temperature during stretching in the width direction, films stretched at high temperatures experience a significant decrease in withstand voltage from room temperature to high temperatures, which poses a problem in that it shortens the lifespan when used for long periods as a film capacitor.

[0013] Although the film described in Patent Document 4 uses a compound resin raw material obtained by pre-mixing a cyclic olefin resin and a linear polypropylene resin, the dispersion uniformity of the cyclic olefin resin is insufficient. In particular, the dielectric strength in high-temperature environments is unsatisfactory in areas where the dispersion of the cyclic olefin resin is low, and the performance and reliability when used as a film capacitor are not sufficient.

[0014] Therefore, the objective of the present invention is to provide a biaxially oriented polypropylene film with excellent voltage resistance in high-temperature environments and productivity. [Means for solving the problem]

[0015] The above-mentioned problems can be solved as follows. That is, the biaxially oriented polypropylene film of the present invention is a biaxially oriented polypropylene film having a polypropylene resin and a layer (layer A) containing a resin I having an alicyclic structure in its side chains, wherein the average aspect ratio of the domains in the cross-section of layer A in the direction of the main orientation axis - thickness is 2 or more and 10 or less.

[0016] Furthermore, the biaxially oriented polypropylene film of the present invention can be in the following embodiments, or it can be a metal film laminated film or a film capacitor as described below. (1) A biaxially oriented polypropylene film having a layer (Layer A) containing both a polypropylene resin and a resin I having an alicyclic structure in its side chains, characterized in that the average aspect ratio of the domains in Layer A is 2 or more and 10 or less in a cross-section in the direction of the main orientation axis and thickness. (2) The biaxially oriented polypropylene film according to (1), wherein the glass transition temperature TgI (°C) of the resin I is 120°C or higher and 160°C or lower. (3) The biaxially oriented polypropylene film according to (1) or (2), wherein the content of resin I exceeds 2.0% by mass and is 10.0% by mass or less when all constituent components are considered to be 100% by mass. (4) The biaxially oriented polypropylene film according to any one of (1) to (3), having the A layer and a B layer mainly composed of a polypropylene resin, wherein at least one outermost layer is the B layer, and the B layer contains more polypropylene resin than the A layer. (5) The biaxially oriented polypropylene film according to any one of (1) to (4), having a thickness of 1.5 μm or more and 4.0 μm or less. (6) A metal film laminated film having a metal film on at least one side of the biaxially oriented polypropylene film according to any one of (1) to (5). (7) A film capacitor having a structure in which the metal film laminated film according to (6) is laminated or wound.

Advantages of the Invention

[0017] According to the present invention, it is possible to provide a biaxially oriented polypropylene film that achieves both high withstand voltage performance in a high-temperature environment and productivity. By using the biaxially oriented polypropylene film of the present invention as a dielectric of a film capacitor, a film capacitor that can be used for a long period of time even in a high-temperature and high-voltage environment can be obtained.

Embodiments for Carrying Out the Invention

[0018] The inventors of the present invention have intensively studied to solve the above-mentioned problems. Regarding the films described in Patent Documents 1 to 4 above, the reasons why they are likely to break during processing and the yield decreases, or the life of the film capacitor becomes short when used as a dielectric of the film capacitor for a long time are considered as follows.

[0019] In the film of Patent Document 1, in order to increase the withstand voltage at high temperatures, a cyclic olefin resin having a cyclic olefin in the main chain (hereinafter referred to as COP) is contained. Generally, COP has high brittleness and low impact resistance. Therefore, there is a concern that these films may break with the COP part as a starting point during processing, or dielectric breakdown may progress with the peeling part caused by impact as a starting point. Therefore, it is considered that the life of the film capacitor using such a film as a dielectric is short.

[0020] In the films described in Patent Document 2 and parts of the films described in Patent Document 3, the base layer is made of COP alone, and there is a difference in glass transition temperature between it and the surface polypropylene resin layer, making it difficult to increase the area stretching ratio. As a result, the molecular orientation of the surface polypropylene resin layer becomes insufficient, and it is thought that when such films are used as dielectrics for film capacitors, the leakage current increases and the lifespan of the film capacitors is shortened.

[0021] Furthermore, some of the films described in Patent Document 3 and Patent Document 4 improve heat resistance by incorporating COP into polypropylene resin and then stretching it. However, the dispersibility of COP in the polypropylene resin and the deformation-following ability of the COP domains during stretching may be insufficient. As a result, the stretchability of the unstretched film decreases, and when the temperature rises instantaneously, the molecular chains of the polypropylene resin relax. Therefore, when the resulting film is used as a dielectric in a film capacitor, the equivalent series resistance increases, and the lifespan of the film capacitor is shortened.

[0022] Based on the above considerations, the inventors conducted further studies and have invented a biaxially oriented polypropylene film that solves the above problems. The biaxially oriented polypropylene film of the present invention is a biaxially oriented polypropylene film having a layer (layer A) containing both a polypropylene resin and a resin I having an alicyclic structure in its side chains, and characterized in that the average aspect ratio of the domains in layer A is 2 or more and 10 or less in a cross-section in the direction of the main orientation axis - thickness direction.

[0023] The biaxially oriented polypropylene film of the present invention will be described in detail below. Hereinafter, the biaxially oriented polypropylene film may be simply referred to as polypropylene film or film. Furthermore, when upper and lower limits are specified separately for a preferred range, the combination thereof is arbitrary.

[0024] In this invention, biaxial orientation means that molecules are oriented in two orthogonal directions within the film plane, which can be achieved by stretching an unstretched film in two orthogonal directions (for example, the longitudinal direction and the width direction). The longitudinal direction refers to the direction in which the film travels during the manufacturing process (corresponding to the winding direction in the case of a film roll), and the width direction refers to the direction perpendicular to the longitudinal direction within the film plane. The biaxial orientation of a film can be confirmed by known indicators (for example, the stress of the maximum point strength in a tensile test).

[0025] In the present invention, a biaxially oriented polypropylene film refers to a sheet-like molded article that is biaxially oriented and mainly composed of polypropylene resin (preferably linear polypropylene resin). The main component refers to a component that is present in an amount greater than 50% by mass and less than or equal to 100% by mass when the total components constituting the film are considered to be 100% by mass (the same interpretation can be applied to the main component hereafter). If the film contains multiple components equivalent to polypropylene resin, even if each component is less than 50% by mass, if the sum of these components exceeds 50% by mass, the film shall be considered to be mainly composed of polypropylene resin.

[0026] Polypropylene resin refers to a resin that, when the total constituent units of the resin are set to 100 mol%, contains more than 50 mol% but no more than 100 mol% of propylene units, and does not fall under "Resin I, which has an alicyclic structure in its side chains," as described below. Hereafter, "Resin I, which has an alicyclic structure in its side chains" may simply be referred to as "Resin I" (details of Resin I will be described later).

[0027] The biaxially oriented polypropylene film of the present invention has a layer (layer A) containing both a polypropylene resin and a resin I having an alicyclic structure in its side chains. Generally, polypropylene resin has superior stretchability (productivity) compared to resin I, but inferior heat resistance. Therefore, by having a layer (layer A) containing both polypropylene resin and resin I, a biaxially oriented polypropylene film with excellent stretchability and voltage resistance in high-temperature environments can be obtained.

[0028] Here, "having layer A" in a biaxially oriented polypropylene film means that the biaxially oriented polypropylene film consists only of layer A or has a laminated structure including layer A. Furthermore, resin I having an alicyclic structure in its side chains refers to a resin that, when the total constituent units of the resin are set to 100 mol%, contains a total of 0.1 mol% to 100 mol% of constituent units 1 to 4 represented by the following chemical formulas. Note that any of the above constituent units 1 to 4 may be included in the molecular chain of resin I, and resin I contained in a biaxially oriented polypropylene film may be one type or more types. If there are multiple layers containing resin I, the layer with the highest content (mass%) shall be designated as layer A.

[0029] [ka]

[0030] Other constituent units of resin I are arbitrary, but examples include constituent units 5 and 6, which are represented by the following chemical formulas.

[0031] [ka]

[0032] A constituent unit refers to the smallest structural unit in the molecular chain of a resin that contains two or more carbon atoms. For example, in polyethylene, the constituent unit is constituent unit 6 where both R1 and R2 are H, and in polyethylene glycol, the constituent unit is (C2H4O). Furthermore, in cyclohexylethylene-ethylene-butylene-cyclohexylethylene copolymer, which is synthesized by hydrogenation of styrene-butylene-styrene copolymer, the constituent units are constituent unit 1 where R1 to R6 are H, constituent unit 6 where R1 to R2 are H, and constituent unit 6 where R1 and R2 are H and ethyl groups, respectively.

[0033] In constituent units 1 to 6, R1 to R8 represent H or any substituent, such as H, alkyl group, halogen group, nitro group, sulfone group, amide group, carbonyl group, carboxyl group, etc. Note that R1 to R8 may all be different, or two or more may be duplicates. Note that the wavy lines in constituent units 1 to 6 indicate that the chemical structure beyond that point is omitted.

[0034] However, from the viewpoint of increasing the degree of polymerization when manufacturing resin I, R1 is preferably H or a methyl group. Furthermore, from the viewpoint of improving dispersibility in polypropylene resin, R2 to R8 are preferably H, a linear alkyl group having 1 to 30 carbon atoms such as a methyl group or an ethyl group, an isopropyl group, an isobutyl group, a sec-butyl group, etc.

[0035] In the biaxially oriented polypropylene film of the present invention, when the total constituent units of resin I are set to 100 mol%, resin I is considered to be different if the difference is between 0.1 mol% and 100 mol%. Examples of embodiments in which resin I contains multiple types include, but is not limited to, embodiments that include a form containing 70 mol% of the above constituent unit 1 and a form containing 30 mol% of the above constituent unit 2, or embodiments that include a form containing 99 mol% of the above constituent unit 1 and a form containing 1 mol% of the above constituent unit 1.

[0036] In the biaxially oriented polypropylene film of the present invention, the glass transition temperature of resin I can be adjusted by adjusting the molecular structure of resin I. For example, the glass transition temperature of resin I can be increased by increasing the ratio of constituent units 1 to 4 in resin I or decreasing the ratio of constituent unit 6.

[0037] The dielectric strength (heat resistance) and film-forming properties of biaxially oriented polypropylene films in high-temperature environments are generally in a trade-off relationship, and the balance between the two can be adjusted by the content and type of resin I in layer A. For example, to improve the dielectric strength of biaxially oriented polypropylene films in high-temperature environments, it is effective to increase the content of resin I in layer A, increase the thickness ratio of layer A to the total thickness, and use resin I with a high proportion of constituent units 1-4 in layer A. Also, if layer A contains multiple resins I, it is effective to increase the mixing ratio of those with a high proportion of constituent units 1-4. On the other hand, to improve the film-forming properties of biaxially oriented polypropylene films, it is effective to decrease the content of resin I in layer A, decrease the thickness ratio of layer A to the total thickness, use resin I with a low proportion of constituent units 1-4 in layer A, and, if layer A contains multiple types of resin I, decrease the mixing ratio of those with a high proportion of constituent units 1-4.

[0038] The alicyclic structures of constituent units 1-4 in resin I not only increase the glass transition temperature and improve heat resistance, but also contribute to increased affinity with polypropylene resin. Furthermore, because resin I has such alicyclic structures in its side chains rather than its main chain, the decrease in molecular chain flexibility is suppressed, making it possible to achieve both stable stretchability during molding and a high glass transition temperature (heat resistance), leading to a balance between high voltage resistance and productivity in high-temperature environments.

[0039] The resin I used in the biaxially oriented polypropylene film of the present invention is preferably one containing constituent unit 1, as its synthesis method is industrially established and it can be synthesized by hydrogenating a styrene-based polymer for which high-quality raw materials can be procured. A styrene-based polymer refers to a polymer polymerized with styrene as one of its monomers, and examples include styrene-ethylene-butylene copolymer, styrene-ethylene copolymer, styrene-ethylene-propylene copolymer, styrene-butylene copolymer, styrene-ethylene-styrene copolymer, styrene-butylene-styrene copolymer, styrene-propylene-styrene copolymer, and polystyrene.

[0040] Resin I, which can be suitably used in the biaxially oriented polypropylene film of the present invention and is industrially available, can be more specifically described as polyvinylcyclohexane, polystyrene hydride, styrene-α-olefin copolymer hydride, etc. In particular, from the viewpoint of suppressing interfacial delamination in the polypropylene resin and improving dispersibility to increase the storage modulus in the direction of the main orientation axis, which will be described later, it is preferable to use a polymer synthesized by hydrogenating the unsaturated bonds derived from styrene and α-olefin in a styrene-α-olefin copolymer as resin I.

[0041] As for commercially available resins I that can be used industrially in the biaxially oriented polypropylene film of the present invention, examples include the "ViviOn" (registered trademark) series (0645, 1325, MDP-0011, etc.) from USI Corporation. The "ViviOn" (registered trademark) series is a copolymer containing a constituent unit 1 in which R1 to R6 are all H, and a constituent unit 6 in which R1 is H and R2 is an ethyl group.

[0042] In the biaxially oriented polypropylene film of the present invention, from the viewpoint of improving both the lifespan of the film capacitor and the productivity when used as a dielectric for a film capacitor, it is important that the average aspect ratio of domains in layer A in the main orientation axis direction-thickness direction cross section is 2 or more and 10 or less (hereinafter, "average aspect ratio of domains in layer A" may simply be referred to as "average aspect ratio of domains"). From the above viewpoint, the average aspect ratio of domains is preferably 3 or more and 9 or less, and more preferably 4 or more and 8 or less.

[0043] Here, the principal orientation axis-thickness direction cross-section refers to the cross-section obtained when a biaxially oriented polypropylene film is cut with a plane parallel to both the principal orientation axis and the thickness direction. The principal orientation axis direction refers to the direction in which the resin molecules constituting the biaxially oriented polypropylene film are most strongly oriented, and can be defined as the direction in which the stress of the maximum point strength is greatest in a tensile test conducted at a tensile speed of 300 mm / min in an atmosphere of 23°C (details of the measurement method will be described later). In addition, the average aspect ratio of the domains can be measured by cutting the biaxially oriented polypropylene film using the microtome method to obtain an ultrathin section having a principal orientation axis-thickness direction cross-section, and observing and image analyzing the A layer portion of this section with a transmission electron microscope (TEM) (details of the measurement method will be described later).

[0044] If the average aspect ratio of the domains is greater than 10, insulation defects are more likely to occur, and when biaxially oriented polypropylene film is used as a dielectric in film capacitors, the lifespan of the film capacitors tends to decrease. On the other hand, if the average aspect ratio of the domains is less than 2, the film is more prone to rupture during the manufacturing process, reducing the productivity of biaxially oriented polypropylene film.

[0045] To achieve an average aspect ratio of 2 to 10 or within the preferred range described above, it is effective to use a biaxially oriented polypropylene film having layer A, control the dispersion state of resin I in layer A, and adjust the stretching conditions. The means for controlling the dispersion state of resin I in layer A are not particularly limited, but examples include adjusting the amount of resin I in the entire layer A to within the preferred range described later, using resin I with a glass transition temperature within the preferred range described later, and adding branched polypropylene resin in the compounding process of the raw materials for layer A. Furthermore, regarding the adjustment of stretching conditions, it is effective to adjust the heating time on the radiation heater during the longitudinal stretching process when forming the film to within the range described later. These methods may be used in combination as appropriate.

[0046] The thickness of the biaxially oriented polypropylene film of the present invention is preferably 1.0 μm to 5.0 μm, more preferably 1.5 μm to 4.5 μm, and even more preferably 1.5 μm to 4.0 μm, from the viewpoint of film-forming properties, mechanical strength, dielectric strength in high-temperature environments, and capacitance per unit volume when used as a dielectric in a film capacitor. By setting the thickness to 1.0 μm or more, the biaxially oriented polypropylene film can be made to have excellent mechanical strength and dielectric strength in high-temperature environments, and its breakage during film formation and processing can be reduced. On the other hand, by setting the thickness to 5.0 μm or less, the capacitance per unit volume can be increased when the biaxially oriented polypropylene film is used as a dielectric in a film capacitor, thereby improving the heat resistance and dielectric strength of the film capacitor. The thickness of the biaxially oriented polypropylene film can be measured by the micrometer method in accordance with JIS C 2330 (2014).

[0047] The thickness of the biaxially oriented polypropylene film can be adjusted, for example, by adjusting the slit width of the T-die, the discharge rate from the T-die, the rotation speed of the cast drum, and the product of the stretching ratio. More specifically, the thickness of the biaxially oriented polypropylene film can be reduced by decreasing the slit width of the T-die, decreasing the discharge rate from the T-die, increasing the rotation speed of the cast drum, and increasing the product of the stretching ratio. These methods may be used in appropriate combinations.

[0048] Next, we will describe the resin raw materials used in the A layer of the biaxially oriented polypropylene film of the present invention (sometimes simply referred to as the resin raw materials for the A layer). Note that the amounts of each component in the resin raw materials for the A layer are maintained before and after film formation, except for trace amounts of additives such as antioxidants; therefore, the same applies to the A layer.

[0049] The polypropylene resin that forms the main component of the resin raw material for layer A is not particularly limited, but it is preferable that the resin raw material for layer A is mainly composed of linear polypropylene resin. Here, "mainly composed of linear polypropylene resin" means that linear polypropylene resin is contained in more than 50% by mass but less than 93% by mass of the total resin components of the resin raw material for layer A, more preferably 70% by mass or more and 90% by mass or less, even more preferably 80.0% by mass or more and 90.0% by mass or less, particularly preferably 83.0% by mass or more and 89.0% by mass or less, and most preferably 83.0% by mass or more and 86.0% by mass or less. By adopting this configuration, it becomes easy to achieve both high voltage resistance in high-temperature environments and high voltage resistance in the resulting biaxially oriented polypropylene film.

[0050] Furthermore, "linear polypropylene resin" refers to a linear isotactic polypropylene resin, more preferably a linear polypropylene resin having a melt tension (MS) of 1.5 cN or less, preferably 1.1 cN or less, when measured at 230°C. Such isotactic polypropylene resins are known as polypropylene resins commonly used in film capacitor applications. MS refers to the tension when polypropylene resin is heated to 230°C to melt, the molten polypropylene is extruded as a strand at an extrusion speed of 15 mm / min, and this strand is drawn back at a speed of 6.5 m / min. MS can be measured with a known melt tension tester, and details of the measurement method will be described later.

[0051] From the viewpoint of improving the high-temperature dielectric strength of the resulting biaxially oriented polypropylene film, the linear polypropylene resin is preferably one in which the cold xylene soluble portion (CXS) is 0.5% to 4.0% by mass, the mesopentad fraction (mmmm) is 0.960 to 0.995, and the melt flow index (MFR) is 0.5 g / 10 min to 5.0 g / 10 min. Specific examples of linear polypropylene resins that can be suitably used as resin raw materials for layer A include Borclean polypropylene resins (HC300BF, HC318BF) manufactured by Borealis, Prime PolyPro® (registered trademark) polypropylene resins (F113G, F-300SP, F-704NP) manufactured by Prime Polymer Co., Ltd., and NOBLEN® (registered trademark) polypropylene resins FS2011DG3, WF836DG3, D101, W101, etc. manufactured by Sumitomo Chemical Co., Ltd.

[0052] From the above viewpoint, the CXS of the linear polypropylene resin is more preferably 0.5% by mass or more and 3.0% by mass or less, and even more preferably 0.5% by mass or more and 2.0% by mass or less. CXS is the polypropylene component dissolved in xylene when the polypropylene resin is completely dissolved in xylene at 135°C and then precipitated at 20°C (detailed measurement methods will be described later). In other words, CXS is considered to be a component that is difficult to crystallize due to reasons such as stereoregularity and low molecular weight. When the CXS of the linear polypropylene resin is 4.0% by mass or less, the dielectric strength of the resulting biaxially oriented polypropylene film in high-temperature environments can be improved. Therefore, when used in film capacitors, the relaxation of molecular chains in high-temperature environments is suppressed, improving thermal dimensional stability and reducing leakage current. In addition, when the CXS of the linear polypropylene resin is 0.5% by mass or more, deterioration of stretchability during film formation can be prevented.

[0053] The mesopentad fraction (mmmm) of the linear polypropylene resin is preferably 0.960 or more and 0.995 or less, more preferably 0.965 or more and 0.995 or less, and even more preferably 0.970 or more and 0.995 or less. The mesopentad fraction (mmmm) is an index indicating the stereoregularity of the crystalline phase of polypropylene, measured by nuclear magnetic resonance (NMR) spectroscopy. Higher values ​​indicate higher crystallinity and melting point, and superior dielectric strength under high-temperature environments. When the mesopentad fraction of the linear polypropylene resin is 0.960 or more, it is easier to maintain dielectric strength and dimensional stability under high-temperature environments when a biaxially oriented polypropylene film is formed. On the other hand, when the mesopentad fraction of the linear polypropylene resin is 0.995 or less, it is easier to maintain film-forming properties and stably obtain a biaxially oriented polypropylene film. The mesopentad fraction is determined by dissolving the polypropylene resin sample in a solvent. 13 The measurement can be performed using 1C-NMR, and the detailed conditions are shown in the examples.

[0054] The MFR of linear polypropylene resin, when measured at 230°C and 2.16 kg in accordance with JIS K 7210-1 (2014), is preferably 0.5 g / 10 min to 5.0 g / 10 min, more preferably 1.0 g / 10 min to 4.5 g / 10 min, and even more preferably 1.5 g / 10 min to 4.0 g / 10 min. Setting the MFR of linear polypropylene resin to 0.5 g / 10 min or higher allows for good film-forming properties and facilitates the stable acquisition of biaxially oriented polypropylene films. On the other hand, setting the MFR of linear polypropylene resin to 5.0 g / 10 min or lower makes it easier to maintain dimensional stability and dielectric strength in high-temperature environments when forming biaxially oriented polypropylene films. The measurement method and conditions for the MFR of branched polypropylene resin, as described later, are the same.

[0055] The resin raw material for layer A preferably contains branched polypropylene resin, from the viewpoint of achieving both film formation stability and dielectric strength under high-temperature environments. The branched polypropylene resin functions as a crystal nucleating agent and thus contributes to improving the above properties. From the viewpoint of flow characteristics in the molten state, the branched polypropylene resin used in the resin raw material for layer A preferably has a melt tension (MS) greater than 1.5 cN at 230°C.

[0056] The MFR of the branched polypropylene resin is preferably 5.0 g / 10 min or more and 10.0 g / 10 min or less. When the MFR of the branched polypropylene resin is 5.0 g / 10 min or more, film-forming properties are maintained and a stable biaxially oriented polypropylene film can be easily obtained. On the other hand, when the MFR of the branched polypropylene resin is 10.0 g / 10 min or less, the resulting biaxially oriented polypropylene film has excellent dimensional stability and dielectric strength in high-temperature environments.

[0057] The raw material for layer A preferably contains more than 5.0% by mass and 30% by mass or less of branched-chain polypropylene resin in 100% by mass of all resin components constituting the raw material for layer A, more preferably 5.0% by mass or more and 25% by mass or less, even more preferably 5.0% by mass or more and 20% by mass or less, and particularly preferably 5.0% by mass or more and 15% by mass or less.

[0058] To obtain branched-chain polypropylene resin, methods such as using high-energy ionization radiation on polypropylene resin (for example, the method described in Japanese Patent Publication No. 62-121704), reacting polypropylene resin with a specific organic peroxide (for example, the method described in Japanese Patent No. 2869606), reacting polypropylene resin with a pyrolytic radical-forming agent and an ethylene-based polyfunctional unsaturated monomer (for example, the method described in Japanese Patent Publication No. 10-330436), and using a specific catalyst during polymerization of polypropylene resin (for example, the method described in Japanese Patent Publication No. 2009-057542) are preferably used. Furthermore, as the branched-chain polypropylene resin, for example, "WAYMAX" (registered trademark) (EX4000, MFX3) manufactured by Nippon Polypropylene Co., Ltd. can be used.

[0059] The branched-chain polypropylene resin to be included in the resin raw material for layer A to obtain the biaxially oriented polypropylene film of the present invention is preferably a polypropylene resin having a branched structure in its molecular chain. A polypropylene resin having a branched structure in its molecular chain is a polypropylene resin having 5 or fewer internal trisubstituted olefins per 10,000 carbon atoms, and the presence of these internal trisubstituted olefins is important. 1 This can be confirmed by the proton ratio in the 1H-NMR spectrum.

[0060] Branched-chain polypropylene resin acts as an α-nucleating agent, while also allowing for the formation of a rough surface through crystalline morphology within a certain range of additive amounts. In other words, by including branched-chain polypropylene resin in the resin raw material for layer A, the size of the polypropylene spherulites generated during the cooling process of the melt-extruded resin sheet can be controlled to be small, resulting in a biaxially oriented polypropylene film with excellent voltage resistance in high-temperature environments.

[0061] In the biaxially oriented polypropylene film of the present invention, from the viewpoint of moldability, it is preferable that the glass transition temperature TgI of resin I is 120°C or higher and 160°C or lower. By adopting this configuration, the conformability of resin I to the polypropylene resin during molding can be improved. As a result, film capacitors using such biaxially oriented polypropylene films have improved reliability, and the decrease in leakage current when the temperature rises instantaneously is suppressed, thereby improving lifespan. Hereinafter, the glass transition temperature TgI of resin I may simply be referred to as TgI.

[0062] From the viewpoint of increasing the temperature at which the film capacitor can be used, it is more preferable that TgI be 125°C or higher, even more preferably 130°C or higher, particularly preferably 135°C or higher, and most preferably 138°C or higher.

[0063] Furthermore, generally speaking, when cyclic olefin resins have a glass transition temperature of 138°C or higher, their stretchability decreases significantly when dispersed in polypropylene resin, making it difficult to achieve both productivity and heat resistance sufficient for use as a dielectric in film capacitors under high-temperature conditions. The biaxially oriented polypropylene film of the present invention uses resin I as a resin with excellent heat resistance, allowing it to maintain stretchability even when its glass transition temperature is higher than 138°C, thus achieving both high voltage resistance and productivity sufficient for use as a dielectric in film capacitors under high-temperature conditions. On the other hand, from the viewpoint of moldability, a TgI of 150°C or lower is more preferable.

[0064] In this invention, the glass transition temperature of the resin can be measured using a differential scanning calorimeter in accordance with JIS K7121-1987 (details of the measurement method will be described later). The differential scanning calorimeter is not particularly limited and any known model can be used, for example, the EXSTAR DSC6220 manufactured by Seiko Instruments.

[0065] In the biaxially oriented polypropylene film of the present invention, from the viewpoint of achieving both film-forming properties and dielectric strength under high-temperature environments, it is preferable that the content of resin I is greater than 2.0% by mass and less than or equal to 10.0% by mass when all constituent components are considered as 100% by mass. Here, "all constituent components" refers to all components that constitute the biaxially oriented polypropylene film, and if multiple types of resin I are included, the content of resin I is calculated by summing all resin I.

[0066] In the biaxially oriented polypropylene film of the present invention, the resin I content is 10.0% by mass or less, which improves stability and stretchability during molding. From this viewpoint, the resin I content is more preferably 9.5% by mass or less, and even more preferably 7.5% by mass or less. On the other hand, the resin I content is greater than 2.0% by mass, which improves the voltage resistance in high-temperature environments when used as a film capacitor. From this viewpoint, the resin I content in the biaxially oriented polypropylene film is more preferably 3.0% by mass or more, and even more preferably 5.0% by mass or more.

[0067] In other words, by setting the resin I content in the biaxially oriented polypropylene film to more than 2.0% by mass and 10.0% by mass or within the above preferred range, the dielectric strength in high-temperature environments can be increased, making it easier to stably obtain a biaxially oriented polypropylene film that can be used as a dielectric for film capacitors with higher rated voltages. The resin I content in the biaxially oriented polypropylene film can be adjusted by adjusting the composition of the resin raw material for layer A, or by adjusting the thickness ratio of layer A to the biaxially oriented polypropylene film.

[0068] The biaxially oriented polypropylene film of the present invention may contain various additives, such as organic particles, inorganic particles, nucleating agents, antioxidants, heat stabilizers, chlorine scavengers, lubricants, antistatic agents, antiblocking agents, fillers, viscosity modifiers, and anticoloring agents, as long as they do not impair the objectives of the present invention. These additives may be used individually or in combination. Furthermore, if the biaxially oriented polypropylene film has a laminated structure, these additives may be included in the resin raw materials constituting any of the layers.

[0069] When antioxidants are included among these additives, their type and amount are preferably selected from the viewpoint of long-term heat resistance. For example, it is preferable to use sterically hindered phenolic antioxidants, and at least one of them is preferably a high molecular weight type with a molecular weight of 500 or more. Various specific examples can be given, but for example, it is preferable to use 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) in combination with 2,6-di-t-butyl-p-cresol (BHT: molecular weight 220.4).

[0070] The total content of high molecular weight antioxidants with a molecular weight of 500 or more is preferably in the range of 0.1 to 1.0% by mass relative to 100% by mass of all constituent components of the resin raw material for layer A. Too little antioxidant may result in poor long-term heat resistance, and too much antioxidant may adversely affect the film capacitor element due to blocking at high temperatures caused by the bleed-out of these antioxidants. From the above viewpoint, a more preferable content of the antioxidant is 0.2 to 0.7% by mass, and even more preferably 0.3 to 0.5% by mass, relative to 100% by mass of all constituent components. When the biaxially oriented polypropylene film has a laminated structure of two or more layers, it is preferable that each layer contains 0.3 to 0.5% by mass of high molecular weight antioxidants with a molecular weight of 500 or more, from the viewpoint of suppressing defects such as fisheyes and improving quality and dielectric strength.

[0071] The biaxially oriented polypropylene film of the present invention may contain linear polypropylene resins, branched polypropylene resins, and resins other than resin I, to the extent that the objectives of the present invention are not impaired. Specific resins that can be included in the biaxially oriented polypropylene film include, for example, vinyl polymer resins including various polyolefin resins, polyester resins, polyamide resins, polyphenylene sulfide resins, polyimide resins, and polycarbonate resins, with polymethylpentene and syndiotactic polystyrene being particularly preferred examples. In the case of a laminated structure of the biaxially oriented polypropylene film, these components may be added to layer A or to layers other than layer A.

[0072] The content of these resins is preferably 3% by mass or less, more preferably 2% by mass or less, and even more preferably 1% by mass or less, when the total resin components constituting the biaxially oriented polypropylene film are considered as 100% by mass. By keeping the content of these resins at 3% by mass or less, the influence of the domain interface can be suppressed, and the decrease in dielectric breakdown voltage under high-temperature environments can be reduced.

[0073] The biaxially oriented polypropylene film of the present invention may have either a single-layer or laminated structure. In the case of a single-layer structure, the biaxially oriented polypropylene film consists only of layer A. In the case of a laminated structure, the layer configuration of the biaxially oriented polypropylene film is not limited as long as it has layer A.

[0074] The biaxially oriented polypropylene film of the present invention has an A layer and a B layer mainly composed of polypropylene resin, wherein at least one of the outermost layers is the B layer, and it is preferable that the B layer contains more polypropylene resin than the A layer. If the biaxially oriented polypropylene film of the present invention has multiple layers containing polypropylene resin and resin I, the layer with the highest resin I content is designated as the A layer. This configuration enhances stability and stretchability during molding. From the above viewpoint, a two-layer configuration of A layer / B layer is preferred, and a three-layer configuration of B layer / A layer / B layer is more preferred.

[0075] The biaxially oriented polypropylene film of the present invention can be preferably used as a dielectric in film capacitors. The type of film capacitor is not limited; specifically, from the viewpoint of electrode configuration, it may be a wound film capacitor with metal foil and film, or a metal-deposited film capacitor. It can also be preferably used in oil-immersion type film capacitors impregnated with insulating oil, or in dry-type capacitors that do not use insulating oil at all. Among these, due to the properties of the biaxially oriented polypropylene film of the present invention, it is particularly preferably used as a metal-deposited film capacitor. From the viewpoint of shape, it may be a wound type or a laminated type (the film capacitor of the present invention will be described later).

[0076] Biaxially oriented polypropylene films typically have low surface energy, making it difficult to stably apply metal vapor deposition. Therefore, it is preferable to perform surface treatment before vapor deposition to improve adhesion with the metal film. Examples of surface treatments include corona discharge treatment, plasma treatment, glow treatment, and flame treatment.

[0077] The biaxially oriented polypropylene film of the present invention can be obtained, for example, by acquiring a polypropylene resin sheet using a resin composition mainly composed of linear polypropylene resin, branched polypropylene resin, and resin I, and then performing biaxial stretching, heat treatment, and relaxation treatment. As for the biaxial stretching method, any of the following methods may be used: simultaneous inflation biaxial stretching, simultaneous tenter biaxial stretching, or sequential tenter biaxial stretching. Among these, sequential tenter biaxial stretching and simultaneous tenter biaxial stretching are preferred in terms of controlling the film formation stability, crystalline / amorphous structure, surface properties, and especially the mechanical properties and thermal dimensional stability while increasing the stretching ratio of the present invention.

[0078] The method for producing the biaxially oriented polypropylene film of the present invention will be described in more detail below. However, the biaxially oriented polypropylene film of the present invention is not limited to those obtained by the following method.

[0079] First, in manufacturing the biaxially oriented polypropylene film of the present invention, it is preferable to improve the dispersion state of resin I and polypropylene resin in the resin raw material for layer A, from the viewpoint of increasing the dielectric breakdown voltage of the resulting biaxially oriented polypropylene film at high temperatures. For this reason, it is preferable to obtain the resin raw material for layer A by compounding linear polypropylene resin, branched polypropylene resin, resin I and an antioxidant in advance.

[0080] While single-screw extruders and twin-screw extruders can be used for compounding, it is particularly preferable to use a twin-screw extruder from the viewpoint of achieving a good dispersion state. The resin temperature during compounding is preferably within the following temperature range from the viewpoint of improving the dispersion state of resin I and further increasing the dielectric breakdown voltage of the resulting biaxially oriented polypropylene film at high temperatures. First, it is preferably 300°C or lower, and more preferably 280°C or lower. On the other hand, it is preferably 200°C or higher, and more preferably 230°C or higher.

[0081] From the viewpoint of the dielectric strength of the resulting biaxially oriented polypropylene film in a high-temperature environment, the content of resin I in the resin composition obtained by compounding is preferably more than 2.0% by mass, more preferably 3.0% by mass or more, and even more preferably 5.0% by mass or more, when the total components to be compounded are considered as 100% by mass. On the other hand, from the viewpoint of improving the dispersibility of resin I and improving stretchability, the content of resin I in the resin raw material for layer A obtained by compounding is preferably 10.0% by mass or less, more preferably 9.5% by mass or less, and even more preferably less than 7.5% by mass.

[0082] Antioxidants may be added to the resin composition obtained by compounding. The amount of antioxidant is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, and even more preferably 0.4% by mass or more, based on 100% by mass of all components obtained by compounding. The upper limit is preferably 1.0% by mass. Furthermore, setting the mesopentade fraction of the linear polypropylene resin to 0.960 or more is preferable because the resulting biaxially oriented polypropylene film has a high melting point and is suitable for use at high temperatures.

[0083] Next, linear polypropylene resin or branched polypropylene resin is added to the resin composition obtained by compounding to adjust the amount of resin I to a desired level and obtain a resin raw material for layer A, which is then supplied to a single-screw extruder. In this case, the melting time is preferably 10 minutes or more and 60 minutes or less. A melting time of 10 minutes or more allows resin I to diffuse sufficiently. On the other hand, a melting time of 60 minutes or less suppresses the deterioration of the polypropylene resin due to the heat generated during kneading. Therefore, by setting the melting time within the above range, the resulting biaxially oriented polypropylene film is likely to produce a high-performance film capacitor when used as a dielectric for a film capacitor.

[0084] In this manner, the resin raw material for layer A is supplied to a single-screw extruder, passed through a filtration filter, and then melt-extruded into a sheet from a slit-shaped die at a temperature between 200°C and 270°C. At this time, the shear rate of the molten sheet material extruded from the slit-shaped die is 500 s. -1 More than 1000s -1 Preferably, the following conditions apply: Shear rate of 500 s -1 By doing so, resin I will diffuse sufficiently. On the other hand, the shear rate will be 1000 s. -1 The following measures can suppress the degradation of polypropylene resin due to shear heating.

[0085] When the biaxially oriented polypropylene film of the present invention has a laminated structure, a two-layer structure of two types, such as layer A / layer B, or a three-layer structure of three types, such as layer B / layer A / layer B, is preferred from the viewpoint of suppressing film breakage when the area stretching ratio is increased. In the above configuration, at least layer A contains both polypropylene resin and resin I, and preferably layer A contains linear polypropylene resin, branched polypropylene resin, and resin I. In the above configuration, the resin I content in layer B is less than that of layer A, preferably 3% by mass or less, more preferably 1% by mass or less, and it is most preferable that layer B does not contain resin I, when the total mass of layer B is taken as 100% by mass. When layer B is laminated on both sides, the compositions of the layers B on both sides may be the same or different from each other. Furthermore, when the biaxially oriented polypropylene film of the present invention is manufactured in a laminated structure, resin I may be contained in only one of the multiple layers, or in two or more layers (if there are multiple layers containing polypropylene resin and resin I, the layer with the highest resin I content shall be designated as resin layer A).

[0086] The method for forming a laminated structure of the biaxially oriented polypropylene film of the present invention is not particularly limited, but for example, the following method can be employed. A resin composition obtained by compounding linear polypropylene resin, branched polypropylene resin, and resin I (if necessary, the mass ratio of resin I may be further adjusted with linear polypropylene resin and branched polypropylene resin) is supplied to a single-screw extruder as the resin raw material for layer A, and linear polypropylene resin and branched polypropylene resin are supplied to another single-screw extruder as the raw material for layer B. Subsequently, a method can be used in which the molten resin is laminated in a two-layer structure of layer A / layer B, or a three-layer structure of layer B / layer A / layer B, using a feed block method by melt co-extrusion. In this case, from the viewpoint of film-forming properties and dielectric strength at high temperatures, the thickness of layer A relative to the total thickness of the biaxially oriented polypropylene film is preferably 80% to 94%. The molten laminate thus obtained is extruded in a sheet shape from a slit-shaped die and solidified on a temperature-controlled casting drum to obtain an unstretched polypropylene film (cast sheet). Furthermore, when using a biaxially oriented polypropylene film as a single layer, the resin raw material for layer A described above can be solidified on the casting drum in the same manner.

[0087] Regardless of whether the film has a single-layer or laminated structure, from the viewpoint of appropriately controlling crystal growth while cooling and solidifying the molten resin composition, the temperature of the casting drum is preferably 30°C to 120°C, and more preferably 60°C to 95°C.

[0088] Any of the following methods can be used to adhere the molten sheet to the casting drum: electrostatic application, adhesion using the surface tension of water, air knife method, press roll method, underwater casting method, or air chamber method. However, the air knife method is preferred because it provides good flatness and allows for control of surface roughness. It is also preferable to appropriately adjust the position of the air knife so that air flows downstream of the film formation to prevent vibration of the film. The air temperature of the air knife is preferably between 30°C and 120°C.

[0089] The cast sheet solidified by the casting drum is preferably further cooled with a cooling roll, and the temperature of the cooling roll is preferably between 10°C and 60°C. Setting the cooling roll temperature to 10°C or higher makes it easier to raise the film to the desired temperature in the subsequent high-temperature heat treatment process. On the other hand, setting the cooling temperature to 60°C or lower reduces crystal formation in the cast sheet, making it easy to reduce longitudinal variations in the surface shape of the biaxially oriented polypropylene film obtained after biaxial stretching.

[0090] Next, the unstretched polypropylene film is biaxially stretched to achieve biaxial orientation. For biaxial orientation, either sequential biaxial stretching, where the film is stretched sequentially in the longitudinal and width directions, or simultaneous biaxial stretching, where the film is stretched simultaneously, may be used. The following describes the case of sequential biaxial stretching.

[0091] First, during stretching, the unstretched polypropylene film is brought into contact with a roll set to a predetermined longitudinal stretching temperature and stretched longitudinally at a predetermined magnification. The longitudinal stretching temperature is preferably 120°C or higher, and more preferably 140°C or higher from the viewpoint of suppressing film breakage. In particular, if the glass transition temperature of resin I is 140°C or higher, it is preferable to set the temperature to be above the glass transition temperature of resin I from the viewpoint of improving the conformability of resin I to the polypropylene resin and thereby increasing the high-temperature dielectric strength of the resulting biaxially oriented polypropylene film. On the other hand, from the viewpoint of preventing melting of the polypropylene resin, the longitudinal stretching temperature is preferably 160°C or lower, and more preferably 155°C or lower.

[0092] From the viewpoint of controlling the average aspect ratio of domains in the main orientation axis-thickness direction cross-section of the resulting biaxially oriented polypropylene film, it is preferable to install a radiation heater on the longitudinal stretching roll and apply heat locally. The processing time with the radiation heater is preferably 0.2 seconds to 1.0 seconds, more preferably 0.3 seconds to 0.8 seconds, and even more preferably 0.4 seconds to 0.7 seconds, when the distance between the unstretched polypropylene film and the radiation heater is 5.0 mm and the output of the radiation heater is 5.0 kW. By setting the processing time with the radiation heater to 0.2 seconds or more, it is possible to prevent the induction of film tearing due to insufficient heat. On the other hand, by setting the processing time with the radiation heater to 1.0 second or less, it is possible to prevent excessive changes in the average aspect ratio of domains in the main orientation axis-thickness direction cross-section of the resulting biaxially oriented polypropylene film.

[0093] The distance between the radiation heater and the cast sheet is preferably 1.0 to 10 mm, and the output of the radiation heater is preferably 1.0 to 10 kW. When the distance between the radiation heater and the cast sheet and the output of the radiation heater are appropriately adjusted within this range, the processing time can be adjusted to provide the same amount of heat as when these are set to 5.0 mm and 5.0 kW, respectively, and the processing time is between 0.2 seconds and 1.0 second.

[0094] Furthermore, the longitudinal stretching ratio is preferably 4.0 times or more, more preferably 4.3 times or more, and even more preferably 4.5 times or more, from the viewpoint of increasing the area stretching ratio and thus increasing the dielectric breakdown voltage at high temperatures. On the other hand, from the viewpoint of suppressing film breakage, the longitudinal stretching ratio is preferably 7.0 times or less, and more preferably 6.5 times or less. After stretching the unstretched polypropylene film in the longitudinal direction in this manner, it is cooled to room temperature on a cooling roll to obtain a uniaxially oriented polypropylene film.

[0095] The longitudinal stretching speed is preferably 10,000% / s to 35,000% / s, and more preferably 15,000% / s to 35,000% / s, from the viewpoint of uniform stretching and stable film formation. Setting the longitudinal stretching speed to 10,000% / s or more makes it easier to obtain a uniform film surface shape and maintain dielectric strength in high-temperature environments. On the other hand, setting the longitudinal stretching speed to 35,000% / s or less prevents film breakage during film formation. The longitudinal stretching speed can be calculated from the following formula (3). Note that when stretching using a rotary roll system, the stretching section is defined as the tangential distance between rolls with different peripheral speeds, and it is assumed that the stretching speed is uniform within the stretching section. Equation (3): Longitudinal stretching rate (% / min) = (MDX-1) × 100 / (L / V) MDX: Longitudinal stretching ratio (times) L: Extension section distance (m) V: Film formation rate after stretching (m / min).

[0096] Next, the obtained uniaxially oriented polypropylene film is guided into the tenter's preheating chamber while its widthwise ends are held by multiple clips and preheated. Then, while still being held by the clips at both widthwise ends, the uniaxially oriented polypropylene film is introduced into the stretching chamber and stretched in the widthwise direction by widening the spacing between opposing clips. At this time, the ambient temperature of the stretching chamber (stretching temperature in the widthwise direction) is preferably 140°C or higher, more preferably 150°C or higher, more preferably 155°C or higher, and even more preferably 160°C or higher, from the viewpoint of uniformly stretching resin I, which has a high glass transition temperature, and improving the constant voltage properties of the biaxially oriented polypropylene film in a high-temperature environment. On the other hand, from the viewpoint of preventing melting of the polypropylene resin, the stretching temperature in the widthwise direction is preferably 175°C or lower, more preferably 172°C or lower. The preheating temperature can also be adjusted to a similar extent.

[0097] From the viewpoint of increasing the dielectric breakdown voltage of the resulting biaxially oriented polypropylene film, the stretching ratio in the width direction is preferably 5.0 times or more, more preferably 6.5 times or more, and even more preferably 8.0 times or more. On the other hand, from the viewpoint of stable film formation, the stretching ratio in the width direction is preferably 15.0 times or less, more preferably 13.5 times or less, and even more preferably 12.0 times or less.

[0098] The area stretching ratio is preferably 30.0 times or more, from the viewpoint of thinning the resulting biaxially oriented polypropylene film and suppressing the relaxation of the polypropylene resin orientation when the temperature rises instantaneously. In this invention, the area stretching ratio is the longitudinal stretching ratio multiplied by the widthwise stretching ratio. In this invention, the widthwise stretching ratio refers to the stretching ratio after stretching in the widthwise direction and before the relaxation treatment. From the viewpoint of increasing the usable temperature, the area stretching ratio is more preferably 35 times or more, and even more preferably 40 times or more. There is no particular upper limit to the area stretching ratio, but from the viewpoint of feasibility, it is 80 times for sequential biaxial stretching and 150 times for simultaneous biaxial stretching, and both are preferably 60 times.

[0099] In the manufacturing of the biaxially oriented polypropylene film of the present invention, it is preferable to include a heat treatment and a relaxation treatment step after biaxial stretching. In this step, it is preferable to perform a heat treatment at a temperature of 120°C to 175°C in a tenter atmosphere while tension-holding both ends in the width direction with clips and applying a relaxation of 5 to 20% in the width direction, from the viewpoint of improving the reliability of the film capacitor when the biaxially oriented polypropylene film is used as a dielectric for a film capacitor. From the above viewpoint, the heat treatment temperature is preferably 140°C to 165°C. Furthermore, from the viewpoint of improving the reliability and lifespan when the biaxially oriented polypropylene film is used as a dielectric for a film capacitor, the relaxation treatment rate is preferably 7% or more, more preferably 8% or more, and even more preferably 9% or more. On the other hand, from the viewpoint of increasing the dielectric breakdown voltage of the biaxially oriented polypropylene film, the relaxation treatment rate is more preferably 18% or less, and even more preferably 16% or less.

[0100] After heat treatment and relaxation treatment, the biaxially oriented polypropylene film is guided to the outside of the tenter, and the clips at both ends in the width direction are released in a room temperature atmosphere. Then, the film edge is slit in the winding process and the biaxially oriented polypropylene film is wound into a roll. Before slitting the film edge or before winding the biaxially oriented polypropylene film, it is preferable to perform a surface treatment such as corona discharge treatment on at least one side in air, nitrogen, carbon dioxide, or a mixture thereof in order to improve the adhesion of the vapor-deposited metal.

[0101] Furthermore, a biaxially oriented polypropylene film roll of a desired width can be obtained by unwinding the film from the resulting biaxially oriented polypropylene film roll, slitting it parallel to the longitudinal direction, and winding it again. The number of biaxially oriented polypropylene film rolls obtained at this time can be one or multiple, and the number can be adjusted as appropriate by adjusting the width of the slits.

[0102] Next, a metal film laminated film using the biaxially oriented polypropylene film of the present invention, a film capacitor using the same, and a method for manufacturing them will be described.

[0103] The metal film laminate of the present invention has a metal film on at least one side of the biaxially oriented polypropylene film of the present invention. This metal film laminate can be obtained by providing a metal film on at least one side of the biaxially oriented polypropylene film of the present invention described above.

[0104] In the present invention, the method for forming the metal film is not particularly limited, but a preferred method is to deposit aluminum or an alloy of aluminum and zinc onto at least one side of a biaxially oriented polypropylene film to form a metal film such as a deposited film that will serve as the internal electrode of a film capacitor. At this time, other metal components such as nickel, copper, gold, silver, and chromium can be deposited simultaneously or sequentially with the aluminum. A protective layer such as oil can also be provided on the deposited film. If the surface roughness of the biaxially oriented polypropylene film differs between the front and back sides, it is preferable to provide the metal film on the smoother surface side to form a metal film laminated film, from the viewpoint of improving dielectric strength.

[0105] The metal film laminate of the present invention can be annealed or heat-treated at a specific temperature as needed after the metal film is formed. The annealing temperature is preferably in the range of T°C to (T+50)°C, where T[°C] is the temperature at which the film is intended to be used. More preferably, the annealing temperature is in the range of (TgA-50)°C to (TgA+20)°C, where TgA[°C] is the glass transition temperature of the resin I used in the metal film laminate of the present invention. Furthermore, a resin coating, such as polyphenylene oxide, can be applied to at least one side of the metal film laminate for insulation or other purposes.

[0106] The film capacitor of the present invention has a structure in which the metal film laminate of the present invention is laminated or wound. For example, the film capacitor of the present invention can be obtained by laminating or winding the metal film laminate of the present invention described above in various ways. A preferred manufacturing method for a wound film capacitor is as follows.

[0107] Aluminum is deposited onto one side of a biaxially oriented polypropylene film under reduced pressure. The aluminum is deposited in a stripe pattern with a margin running along the longitudinal direction. Next, a blade is inserted into the center of each deposited area and the center of each margin to create a tape-shaped reel with a margin on one side of the surface. Two of these tape-shaped reels, one with a left margin and one with a right margin, are stacked and wound together so that the deposited portion extends beyond the margin in the width direction, thereby obtaining a wound body.

[0108] When vapor deposition is performed on both sides, one side is vapor-deposited in a stripe pattern with a margin running along the longitudinal direction, and the other side is vapor-deposited in a stripe pattern so that the longitudinal margin is located in the center of the vapor-deposited area on the back side. Next, a blade is inserted into the center of the margin on both the front and back sides to make a slit, and a tape-shaped winding reel is created on both sides, each with a margin on one 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 of the back side). The obtained reels and one unvapor-deposited laminated film are stacked on top of each other so that the metallized film extends beyond the laminated film in the width direction, and wound onto a core material to obtain a wound body.

[0109] One method for obtaining a film capacitor of the present invention using the metal layer laminated film of the present invention is to remove the core material from the wound body prepared as described above, press it, spray Metallicon onto both end faces to form external electrodes, and weld lead wires to the Metallicon to form a wound film capacitor. Film capacitors have a wide range of applications, including power control units for electric vehicles such as electric cars, hybrid cars, and fuel cell vehicles, electric aircraft such as drones, railway vehicles, solar and wind power generation, and general home appliances, and the film capacitor of the present invention can be suitably used in these applications. In addition, the biaxially oriented polypropylene film of the present invention can be used in various applications such as packaging films, release films, process films, sanitary products, agricultural products, building materials, and medical products, and can be particularly preferably used in applications that include a heating process in film processing. [Examples]

[0110] The present invention will be described in more detail below using examples, but the present invention is not limited to the embodiments described below.

[0111] [Measurement and evaluation methods] (1) The principal orientation axis direction and the direction perpendicular to the principal orientation axis of the biaxially oriented polypropylene film The principal orientation axis of the biaxially oriented polypropylene film was determined as follows, according to the method described below (tensile test). The direction perpendicular to the principal orientation axis of the biaxially oriented polypropylene film was defined as the direction perpendicular to the principal orientation axis within the film plane.

[0112] <Tensile Test> First, cut a biaxially oriented polypropylene film into a rectangle measuring 50mm in length and 10mm in width to create a sample. <1> Sample <1> The direction of the longer side was defined as 0°. Next, a sample rectangle of the same size was created such that the direction of the longer side was rotated 15° to the right from the 0° direction. <2> Collect a sample, and then rotate the rectangular sample by 15° in the direction of the longer side in the same manner as below. <3> ~ <12> Samples were taken. Next, each rectangular sample was placed in a tensile testing machine with an initial chuck distance of 20 mm so that the longer side was the tensile direction (measurement direction), and a tensile test was performed at a tensile speed of 300 mm / min in an atmosphere of 23°C. At this time, the maximum load until the rectangular sample broke was read, and this value was divided by the cross-sectional area of ​​the sample before the test (film thickness × width) to calculate the stress at the point of maximum strength. The longer side direction of the sample with the highest value was defined as the principal orientation axis of the biaxially oriented polypropylene film.

[0113] (2) Average aspect ratio of the domain in the cross-section in the principal orientation axis direction and thickness direction Using a microtome, ultrathin sections with a cross-section in the principal orientation axis direction and thickness direction were collected from a biaxially oriented polypropylene film (the principal orientation axis direction and thickness direction refers to a direction parallel to the principal orientation axis and perpendicular to the film surface). The collected sections were stained with RuO4, and the cross-section of layer A was observed using a transmission electron microscope (TEM) under the following observation conditions, and images were acquired. At this time, resin I stained blacker than the polypropylene resin, so the black-stained areas were used as domains to identify layer A and perform the following measurements. <Observation conditions> • Equipment: Hitachi, Ltd. Transmission Electron Microscope (TEM) HT7700 • Acceleration voltage: 100kV • Magnification: 20,000x To measure the average aspect ratio of domains in the principal orientation axis direction and thickness direction, five domains were selected upwards from the center of the acquired image, in order of proximity to the center of the field of view, without moving the field of view from the center of the layer containing both polypropylene resin and resin I, and then five domains were selected downwards. The length in the principal orientation axis direction and thickness direction was measured for the selected domains, the aspect ratio was calculated, and the average value was taken as the average aspect ratio of the resin I domains. If a selected domain had an end outside the field of view, the field of view was moved from one end to the other to acquire multiple images, and the length in the principal orientation axis direction and thickness direction of the domain was determined from the stitched-together image. If 10 domains could not be selected in one field of view, the field of view was moved to another, and observation continued until the measurement of 10 domains was completed.

[0114] (3) Thickness of biaxially oriented polypropylene film The thickness of the biaxially oriented polypropylene film was measured using the micrometer method in accordance with JIS C 2330 (2014).

[0115] (4) Mesopentadione fraction (mmmm) Dissolve the polypropylene resin sample in a solvent. 13The mesopentade fraction (mmmm) was measured and calculated using 1C-NMR (Reference: New Edition Polymer Analysis Handbook, edited by the Japan Society for Analytical Chemistry and the Polymer Analysis Research Group, 1995, pp. 609-611). The measurement equipment and conditions, as well as the analysis conditions, are as follows.

[0116] A. Measuring device and conditions Equipment: Bruker DRX-500 Nucleus for measurement: 13 C nucleus (resonance frequency: 125.8MHz) Measured concentration: 10% by mass Solvent: Benzene / deuterated orthodichlorobenzene = mass ratio 1:3 mixed solution Measurement temperature: 130℃ Spin speed: 12Hz NMR sample tube: 5mm tube Pulse width: 45° (4.5μs) Pulse repetition time: 10 seconds Data points: 64K Number of conversions: 10,000 Measurement mode: complete decoupling.

[0117] B.Analysis conditions A Fourier transform was performed with a line broadening factor (LB) of 1.0, resulting in a mmmm peak of 21.86 ppm. Peak splitting was then performed using WINFIT software (Bruker). The peak splitting was performed starting from the high-field side, and automatic fitting was then performed using the accompanying software. After optimizing the peak splitting, the sum of the mmmm peak fractions was calculated. This measurement was performed five times, and the average value was taken as the mesopentad fraction (mmmm) of the sample. (Peak splitting) (a)mrrm (b)(c)rrrm (split into two peaks) (d)rrrr (e)mrmr (f)mrmm+rmrr (g)mmrr (h)rmmr (i)mmmr (j)mmmm.

[0118] (5) Melt Flow Index (MFR) (Unit: g / 10min) Measurements were taken at 230°C and with a weight of 2.16 kg, in accordance with JIS K 7210-1 (2014).

[0119] (6) Melt tension (MS) (unit: cN) The following procedure was used to measure the tension using a melt tension tester (capillary diameter 2.1 mm, cylinder diameter 9.55 mm) manufactured by Toyo Seiki Seisakusho Co., Ltd. First, polypropylene resin was heated to 230°C and melted. Next, the molten polypropylene resin was extruded at an extrusion speed of 15 mm / min to form strands, and the tension obtained when these strands were pulled back at a speed of 6.5 m / min was measured and defined as the MS (Magnetic Stress Metric).

[0120] (7) Cold xylene soluble portion (CXS Unit: mass%) 0.5 g of polypropylene resin was dissolved in 100 ml of boiling xylene at 135°C, allowed to cool, and then recrystallized in a constant temperature water bath at 20°C for 1 hour. After that, solid matter such as crystals was removed by filtration, and the polypropylene components dissolved in the filtrate were quantified by liquid chromatography. Let X0 (g) be the mass of polypropylene resin before dissolution in boiling xylene, and X (g) be the mass of polypropylene components dissolved in the filtrate. CXS was calculated from the following formula (1). Formula (1): CXS(mass%)=(X / X0)×100.

[0121] (8) Melting point and glass transition temperature (Tg) of resin The melting point and glass transition temperature (Tg) of the resin were measured in accordance with JIS K7121-1987. Using a differential scanning calorimeter (Seiko Instruments EXSTAR DSC6220), 3 mg of film or polymer was heated from 30°C to 260°C at a rate of 20°C / min in a nitrogen atmosphere, then held at 260°C for 5 minutes, and then cooled to 30°C at a rate of 20°C / min. After holding at 20°C for another 5 minutes, the temperature was raised again from 30°C to 260°C at a rate of 20°C / min. In the DSC curve obtained during the reheating process, the peak temperature of the endothermic peak was taken as the melting point of the resin, and the glass transition temperature (Tg) was calculated using the following formula (2). Equation (2): Glass transition temperature = (Extracorporeal glass transition start temperature + Extracorporeal glass transition end temperature) / 2. If multiple endothermic peaks are observed in a single measurement, the peak temperature of the endothermic peak with the highest peak temperature was used as the peak temperature for that measurement.

[0122] (9) Life evaluation of film capacitors Aluminum was vacuum-deposited onto the corona-treated side of a biaxially oriented polypropylene film using a vacuum deposition machine manufactured by ULVAC, Inc., to achieve a surface resistance of 15 Ω / sq. The aluminum was deposited in a stripe pattern with a margin running along the longitudinal direction (a repeating pattern of 79.0 mm width in the deposited area and 1.0 mm width in the margin). Next, slits were made by cutting into the center of each deposited area and each margin, creating a tape-like winding reel with a total width of 40 mm and a 0.5 mm margin at either the left or right end. Two of these reels, one from the left margin and one from the right margin, were overlapped and wound together so that the deposited portion extended 0.5 mm beyond the margin in the width direction, obtaining a winding body with a capacitance of 120 μF. The winding body was then heat-treated for 10 hours in a reduced-pressure atmosphere at 140°C. Metallicon was sprayed onto both ends in the width direction to form external electrodes, and lead wires were welded to the Metallicon to obtain a film capacitor. Next, the lifespan of 15 film capacitors was evaluated using the following procedure. First, the capacitance (C0) was measured at room temperature. Next, a voltage of 200VDC / μm (for example, 400V when the thickness is 2.0μm) was applied to the film capacitor at a high temperature of 130°C for 1000 hours. After that, the capacitance (C) was measured at room temperature, and the rate of change of capacitance (ΔC) before and after voltage application was calculated from the following formula (4). The capacitance was measured using a Hioki E.E. Corporation LCR high tester 3522-50. Equation (4): ΔC=((C0-C) / C0)×100 The average of the rate of change (ΔC) of capacitance before and after voltage application for 15 film capacitors was used as the rate of change of capacitance before and after voltage application for that sample, and it was evaluated according to the following criteria. A smaller rate of change (ΔC) of capacitance before and after voltage application indicates that the decrease in capacitance at high temperatures is suppressed, and the lifespan evaluation of the film capacitor is considered good. ◎: ΔC is less than 2% ○: ΔC is between 2% and less than 3% △: ΔC is between 3% and less than 5% ×: ΔC is 5% or more.

[0123] (10) Film formation stability of biaxially oriented polypropylene film The film formation stability of biaxially oriented polypropylene films was evaluated according to the following criteria. The time from when film formation was stopped due to film tearing until it was resumed was excluded from the observation time. ◎: No film tears occurred for more than 48 hours from the start of film formation. ○: One film tear occurred 48 hours after the start of film formation. △: Two to three film tears occurred within 48 hours of the start of film formation. ×: Film tearing occurred four or more times within 48 hours of the start of film formation, or film formation itself was impossible.

[0124] [Resin etc.] The following resins and other materials were used to produce the biaxially oriented polypropylene films in each example and comparative example.

[0125] <Linear polypropylene resin> Borclean HC300BF, manufactured by Borealis, is a highly stereoregular polypropylene resin with a mesopentad fraction of 0.980, a CXS of 1.2% by mass, an MFR of 3.3 g / 10 min, and an MS of 1.0 cN.

[0126] <Branched chain polypropylene resin> "WAYMAX" (registered trademark) (MFX3), manufactured by Nippon Polypropylene Co., Ltd., is a branched-chain polypropylene resin with an MFR of 9.0 g / 10 min and an MS of 5.0 cN.

[0127] <Resin I> Resin I1 USI's "ViviOn" (registered trademark) (0645) is a resin with a glass transition temperature of 148°C and an MFR of 6.0 g / 10 min, and has an alicyclic structure in its side chains. Resin I2 USI's "ViviOn" (registered trademark) (1325) is a resin with a glass transition temperature of 128°C and an MFR of 13 g / 10 min, and has an alicyclic structure in its side chains.

[0128] <Cyclic Olefin Resin> Cyclic olefin resin 1 Polyplastic "TOPAS" (registered trademark) (6017S-04) is a copolymer resin of ethylene and norbornene with a glass transition temperature of 178°C and an MFR of 1.5 g / 10 min. Cyclic olefin resin 2 Polyplastic "TOPAS" (registered trademark) (8007S-04) is a copolymer resin of ethylene and norbornene with a glass transition temperature of 78°C and an MFR of 32 g / 10 min.

[0129] <Ingredients other than resin components> Antioxidant 1: BASF Japan's "Irganox" (registered trademark) 1010 Antioxidant 2: 2,6-di-t-butyl-p-cresol (BHT).

[0130] <Pre-mixing raw materials> Linear polypropylene resin, branched polypropylene resin, resin I (or cyclic olefin resin), antioxidant 1, and antioxidant 2 were mixed in a mass ratio of 50.0:19.7:30.0:0.15:0.15. The mixture was kneaded and extruded in a twin-screw extruder set to 260°C. The molten resin composition was then discharged from the die in strand form, cooled and solidified in a 25°C water bath, and cut into chips. Here, resin I refers to either resin I1 or resin I2, and cyclic olefin resin refers to either cyclic olefin resin 1 or cyclic olefin resin 2. All resins were kneaded under the same conditions.

[0131] (Example 1) A linear polypropylene resin, a branched polypropylene resin, and a preliminary kneaded raw material in which Resin I1 has a mass ratio of 84.0:10.0:6.0 were supplied to a single-screw melt extruder for the A layer after adjusting the concentration with the linear polypropylene resin and the branched polypropylene resin. A polypropylene resin mixture obtained by mixing a linear polypropylene resin and a branched polypropylene resin at a mass ratio of 90.0:10.0, an antioxidant 1, and an antioxidant 2 were dry-blended at a mass ratio of 99.7:0.15:0.15 and supplied to a single-screw melt extruder for the B layer. Then, each resin mixture was melt-extruded at an extrusion temperature of 255°C for 20 minutes. Thereafter, foreign substances were removed from the extruded molten polypropylene resin composition with a 25-μm cut sintered filter, and the B layer / A layer / B layer was laminated in a thickness ratio of 1 / 12 / 1 using a feed block type B / A / B composite T-die, and discharged in a sheet form at a shear rate of 800 s -1 Then, the molten sheet was adhered to a casting drum whose surface temperature was maintained at 90°C by an air knife (air temperature: 80°C) and solidified, and then cooled on a cooling roll maintained at a temperature of 50°C to obtain an unstretched polypropylene film. The unstretched polypropylene film was heated to a temperature of 150°C with a plurality of roll groups, and the unstretched polypropylene film was heated by a radiation heater on the roll for 0.5 seconds. At this time, the distance between the cast sheet and the radiation heater was 5.0 mm, and the output of the radiation heater was 5.0 kW. Thereafter, it was stretched in the longitudinal direction at a magnification of 4.5 times and a stretching speed of 15000% / s between rolls with a peripheral speed difference to obtain a uniaxially oriented polypropylene film. After the uniaxially oriented polypropylene film was cooled to room temperature on a cooling roll at 25°C, both end portions in the width direction were gripped with a plurality of clips, and the uniaxially oriented polypropylene film was guided to a tenter, preheated at 170°C, and then stretched 9.0 times in the width direction at the same temperature. Further, heat treatment was performed at 165°C while giving a 12% relaxation in the width direction, and it was guided outside the tenter and the clips were released. After the polypropylene film stretched in the width direction was cooled to room temperature, 25 W·min / m was applied to the drum surface (the surface in contact with the casting drum) side 2Corona discharge treatment was performed at the specified treatment intensity. The obtained biaxially oriented polypropylene film was clipped and both ends in the width direction were cut off, and the film was wound up to obtain a master roll with a width of 5,000 mm. Next, the film was unwound from the master roll, and the film was slit parallel to the longitudinal direction using a slitter to a width of 0.62 m, and wound onto a core with a longitudinal length of 30,000 m to obtain a biaxially oriented polypropylene film roll with a thickness of 3.5 μm. The physical properties of the biaxially oriented polypropylene film constituting the obtained biaxially oriented polypropylene film roll and the results of each evaluation are shown in Table 1.

[0132] (Examples 2-11, Comparative Examples 1-5) A biaxially oriented polypropylene film was obtained in the same manner as in Example 1, except that the raw material formulation (corresponding to the film composition) and manufacturing conditions were as shown in Table 1. The evaluation results of the obtained biaxially oriented polypropylene films are shown in Table 1. In Comparative Examples 1 and 4, biaxially oriented polypropylene films could not be obtained because the film broke during stretching, and therefore, the measurements and evaluations for the biaxially oriented polypropylene films were not performed.

[0133] [Table 1]

[0134] Since some of the antioxidant is lost during the manufacturing process of biaxially oriented polypropylene film, and its amount is negligible compared to the total resin content, the composition of each layer is almost identical to that of the resin. Therefore, the amount of antioxidant is not indicated when describing the composition in the table. [Industrial applicability]

[0135] The present invention provides a biaxially oriented polypropylene film that achieves both high voltage resistance and productivity. By using the biaxially oriented polypropylene film of the present invention as a dielectric in a film capacitor, a film capacitor that can be used for a long period of time even in high-temperature and high-voltage environments can be obtained.

Claims

1. A biaxially oriented polypropylene film having a layer (Layer A) containing both a polypropylene resin and a resin I having an alicyclic structure in its side chains, characterized in that the average aspect ratio of the domains in Layer A is 2 or more and 10 or less in a cross-section in the direction of the main orientation axis and thickness.

2. The biaxially oriented polypropylene film according to claim 1, wherein the glass transition temperature TgI (°C) of the resin I is 120°C or higher and 160°C or lower.

3. The biaxially oriented polypropylene film according to claim 1 or 2, wherein, when all constituent components are considered as 100% by mass, the content of resin I is greater than 2.0% by mass and less than or equal to 10.0% by mass.

4. The biaxially oriented polypropylene film according to claim 1 or 2, having the aforementioned A layer and a B layer mainly composed of polypropylene resin, wherein at least one of the outermost layers is the B layer, and the B layer contains more polypropylene resin than the A layer.

5. A biaxially oriented polypropylene film according to claim 1 or 2, having a thickness of 1.5 μm or more and 4.0 μm or less.

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

7. A film capacitor having a configuration in which the metal film laminated film described in claim 5 is laminated or wound.

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