Biaxially oriented polypropylene film

Abstract

To provide a biaxially oriented polypropylene film having high processability and voltage resistance as well as low energy loss.SOLUTION: In a biaxially oriented polypropylene film, if tanδ(0) is a dielectric loss tangent before a voltage impression test and tanδ(174) is a dielectric loss tangent after the voltage impression test of applying electric voltage 174 V / μm, a rate of change ΔD(174) of the dielectric loss tangent before and after the voltage impression test expressed by equation (1) is 1.0 to 3.0: ΔD(174)=tanδ(174) / tanδ(0) ...(1).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a biaxially oriented polypropylene film that, when used as a dielectric in a capacitor, has high voltage resistance in a high-temperature, high-voltage environment. [Background technology]

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

[0003] In particular, in capacitor applications, it is particularly preferred as a capacitor dielectric due to its excellent high withstand voltage characteristics and low loss characteristics. Recently, various electrical equipment has been converted to inverters, which has led to an increased demand for smaller capacitors with larger capacities. Furthermore, particularly in automotive applications (including hybrid cars and electric vehicles), solar power generation, and wind power generation, the operating environment is becoming increasingly hot (meaning temperatures between 85°C and 125°C), and the demand for heat resistance in capacitors is also increasing. Therefore, dielectric films are required to be thinner, more heat-resistant, and have improved withstand voltage per thickness, as well as to have a reduced dielectric dissipation factor (tanδ) to minimize electrical energy loss.

[0004] As a technique for controlling the dielectric loss tangent tanδ of a film, a method of fragmenting a polymerization catalyst into nano-sized catalyst fragments and dispersing them in polypropylene (for example, Patent Document 1) and a method of depositing an inorganic dielectric on a substrate film to a thickness of 0.5% to 10% of the thickness of the substrate film (for example, Patent Document 2) have been proposed. Also, as a technique for controlling the dielectric loss tangent tanδ of a capacitor element, a technique of reducing the heat shrinkage rate of a biaxially oriented polypropylene film has been proposed to suppress metallikon peeling of the capacitor element (for example, Patent Documents 3 to 5). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Special Publication No. 2014-531480 [Patent Document 2] Japanese Patent Publication No. 2020-004743 [Patent Document 3] Japanese Patent Application Publication No. 10-156940 [Patent Document 4] Japanese Patent Publication No. 2020-124905 [Patent Document 5] Japanese Patent Application Publication No. 2020-124906 Summary of the Invention [Problem to be solved by the invention]

[0006] However, when the methods described in Patent Documents 1 and 2 are used to control the dielectric loss tangent tanδ of a film, the inclusion of catalyst residues and inorganic dielectrics tends to reduce the withstand voltage of the film. Furthermore, when the methods described in Patent Documents 3 to 5 are used to control the dielectric loss tangent tanδ of a capacitor element, it is not possible to suppress heat generation due to energy loss in the film, and the reduction in the dielectric loss tangent tanδ cannot be said to be sufficient in recent high-temperature, high-voltage environments. Therefore, an object of the present invention is to provide a biaxially oriented polypropylene film that has high processability, withstand voltage, and low energy loss. [Means for solving the problem]

[0007] The above-mentioned object can be achieved as follows: That is, the biaxially oriented polypropylene film of the present invention is a biaxially oriented polypropylene film characterized in that, when the dielectric loss tangent before a voltage application test is tanδ(0) and the dielectric loss tangent after a voltage application test in which a voltage of 174 V / μm is applied is tanδ(174), the rate of change in the dielectric loss tangent before and after a voltage application test, ΔD(174), as expressed by the following formula (1), is 1.0 or more and 3.0 or less. ΔD(174)=tanδ(174) / tanδ(0) ···(1) [Effects of the Invention]

[0008] The present invention provides a biaxially oriented polypropylene film that has a stable higher-order structure and can suppress heat generation even in high-temperature and high-voltage environments when used as a dielectric in a capacitor. Furthermore, the above-mentioned properties of the biaxially oriented polypropylene film can improve the life of the capacitor. DETAILED DESCRIPTION OF THE INVENTION

[0009] The biaxially oriented polypropylene film of the present invention will be described in more detail below. Note that, in the following numerical ranges expressed using "to", the upper and lower limits are included in the range, and the units of the upper and lower limits are the same.

[0010] The biaxially oriented polypropylene film of the present invention is a biaxially oriented polypropylene film obtained by stretching a cast sheet in two perpendicular directions, usually the longitudinal and width directions. In other words, biaxial orientation as used herein means stretching in two perpendicular directions, usually the longitudinal and width directions. The longitudinal direction refers to the direction in which the film runs during the manufacturing process (the winding direction in the case of a film roll), and the width direction refers to the direction in the plane of the film that is perpendicular to the longitudinal direction. Furthermore, polypropylene film refers to a sheet-like molded product whose main component is polypropylene resin, and polypropylene resin refers to a resin whose main structural unit is propylene units. The main structural unit refers to a structural unit that is present in an amount of more than 50 mol% but not more than 100 mol%, when all structural units constituting the resin are taken as 100 mol%. Hereinafter, the term "main structural unit" can be interpreted in the same way.

[0011] The polypropylene resin in the biaxially oriented polypropylene film of the present invention may include not only propylene homopolymers but also polypropylene copolymers and branched polypropylenes, which will be described later. In the present invention, the term "main component" refers to a component that accounts for more than 50% by mass and up to 100% by mass of all components of the film (100% by mass), more preferably 80% by mass to 100% by mass, even more preferably 90% by mass to 100% by mass, and particularly preferably 95% by mass to 100% by mass. Components other than the polypropylene resin in the film include the resins described below, as well as additives such as antioxidants and lubricants. When multiple polypropylene resins are contained in a film, if the combined content of all polypropylene resins exceeds 50% by mass, the film can be considered to be composed primarily of polypropylene resins.

[0012] As the polypropylene copolymer, a polypropylene copolymer copolymerized with other unsaturated hydrocarbons can be suitably used. Examples of copolymerization components of the polypropylene copolymer 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, 1-tetradecene, 1-hexadecene, 1-heptadecene, 1-octadecene, 1-eicosene, vinylcyclohexene, styrene, allylbenzene, cyclopentene, norbornene, and 5-methyl-2-norbornene.

[0013] The biaxially oriented polypropylene film of the present invention may also contain a polymer (resin) other than polypropylene resin. The other polymer may be an unsaturated hydrocarbon other than propylene, such as a homopolymer or copolymer whose main constituent unit is a constituent unit derived from a monomer listed as a copolymerization component above. When the other polymer is a copolymer, propylene may also be used as a copolymerization component.

[0014] When the biaxially oriented polypropylene film of the present invention contains a polypropylene copolymer or other polymer, the content thereof is not particularly limited as long as the main component of the film is polypropylene resin. However, from the viewpoint of the withstand voltage characteristics and dimensional stability of the biaxially oriented polypropylene film, the content of these components is preferably an amount such that the total amount of structural units other than propylene units is 20 mol% or less, when the total amount of all structural units constituting the entire resin in the biaxially oriented polypropylene film is taken as 100 mol%. Whether the content satisfies the above requirement is determined by the fact that the total amount of structural units other than propylene units, based on the total amount of all structural units constituting the entire resin in the biaxially oriented polypropylene film, is 20 mol% or less, regardless of the number of components of the polypropylene copolymer and other polymers. From the above viewpoint, the content of the polypropylene copolymer or other polymer is more preferably an amount of 1 mol% or less.

[0015] Furthermore, the biaxially oriented polypropylene film of the present invention preferably contains linear polypropylene resin A as a main component and also contains branched polypropylene resin H. The linear polypropylene resin A and branched polypropylene resin H will be described later.

[0016] From the viewpoint of improving the life span of a capacitor when used in a biaxially oriented polypropylene film of the present invention, it is important that the rate of change ΔD(174) of the dielectric dissipation factor before and after a voltage application test, which is expressed by the following formula (1), is 1.0 or more and 3.0 or less, where tanδ(0) is the dielectric dissipation factor before a voltage application test and tanδ(174) is the dielectric dissipation factor after a voltage application test in which a voltage of 174 V / μm is applied. ΔD(174)=tanδ(174) / tanδ(0) ···(1).

[0017] Here, "voltage application test applying a voltage of 174 V / μm" means a test in which a DC voltage of 174 V / μm is applied to a film using a flat sample electrode for one hour, and the rate of change in the dielectric tangent before and after the voltage application test, ΔD(174), means the rate of change in the dielectric tangent due to the application of a voltage of 174 V / μm. Note that "voltage application test applying a voltage of 130 V / μm" and "rate of change in the dielectric tangent before and after the voltage application test, ΔD(130)" can also be defined in the same way, except for the applied voltage.

[0018] Hereinafter, a voltage application test in which a voltage of 174V / μm is applied may be simply referred to as the "174V / μm voltage application test," and the rate of change in dielectric tangent ΔD(174) before and after a voltage application test may be simply referred to as "ΔD(174)." Similarly, a voltage application test in which a voltage of 130V / μm is applied and the rate of change in dielectric tangent ΔD(130) before and after a voltage application test may be similarly referred to as the "130V / μm voltage application test" and "ΔD(130)." Details of the 174V / μm and 130V / μm voltage application tests will be provided later.

[0019] Tangent δ is an index that indicates the degree of dielectric loss, which refers to the loss of a portion of the electrical energy applied to a dielectric as thermal energy. Generally, the larger the value of tangent δ, the greater the proportion of the electrical energy lost as thermal energy. When a capacitor is used for an extended period of time, various factors can cause the tangent δ of the dielectric to increase, generating a large amount of heat, which shortens the capacitor's lifespan.

[0020] By setting ΔD(174) to 3.0 or less, heat generation during use can be suppressed when the capacitor is used, improving the capacitor's lifespan, particularly at high temperatures and high withstand voltages. Note that, since the dielectric loss tangent usually increases during a voltage application test of 174 V / μm, from the standpoint of feasibility, ΔD(174) is essentially 1.0 or more. From this perspective, ΔD(174) is preferably 1.0 or more and 2.7 or less, and more preferably 1.0 or more and 2.6 or less.

[0021] A suitable method for achieving ΔD(174) of 1.0 to 3.0 or within the preferred range is, for example, to incorporate branched polypropylene resin H into linear polypropylene resin A, form it into a cast sheet, and then stretch it longitudinally (stretch in the longitudinal direction) and quench it. When using this method, ΔD(174) is more likely to be reduced by appropriately controlling the amount of branched polypropylene resin H or by lowering the quenching temperature. More specifically, it is preferable to adjust the amount of branched polypropylene resin H to the preferred range described below. This embodiment promotes densification of polypropylene crystals and homogenization of the higher-order structure, stabilizing the higher-order structure of the film. As a result, it is easy to achieve ΔD(174) of 1.0 to 3.0 or within the preferred range. The quenching method is not particularly limited, but examples include a method in which the uniaxially oriented film is nipped with temperature-controlled nip rolls and passed through temperature-controlled transport rolls to perform a quenching treatment, with the transport roll temperature conditions being preferably 5°C to 15°C, more preferably 5°C to 13°C, and the nip roll temperature being preferably 5°C to 15°C, more preferably 7°C to 13°C.

[0022] In the biaxially oriented polypropylene film of the present invention, when the dielectric loss tangent after a voltage application test in which a voltage of 130 V / μm is applied is defined as tanδ(130), it is preferable that the rate of change in the dielectric loss tangent before and after the voltage application test, ΔD(130), as shown in the following formula (2), is 1.0 or more and 2.7 or less. ΔD(130)=tanδ(130) / tanδ(0) ···(2).

[0023] By setting ΔD(130) to 2.7 or less, heat generation during use can be suppressed when the capacitor is used, improving the capacitor's lifespan, particularly at high temperatures and high withstand voltages. Since the dielectric loss tangent usually increases during a voltage application test of 130 V / μm, ΔD(130) is essentially 1.0 or more from the standpoint of feasibility. From this perspective, ΔD(130) is preferably 1.0 or more and 2.6 or less, more preferably 1.0 or more and 2.5 or less, and even more preferably 1.0 or more and 2.4 or less.

[0024] A suitable method for achieving ΔD(130) of 1.0 to 2.7 or the preferred range described above is, for example, to incorporate branched polypropylene resin H into high molecular weight polypropylene resin A, form it into a cast sheet, and then longitudinally stretch and quench it. When using this method, ΔD(130) is more likely to be reduced by appropriately controlling the amount of branched polypropylene resin H or by lowering the quenching temperature. More specifically, it is preferable to adjust the amount of branched polypropylene resin H to the preferred range described below. This embodiment facilitates achieving ΔD(130) of 1.0 to 2.7 or the preferred range described above, and the mechanism behind this is as described above. Regarding the quenching described here, the methods and conditions described for achieving ΔD(174) of 1.0 to 3.0 or the preferred range described above can also be suitably employed.

[0025] In order to improve the lifespan of a capacitor when used in a biaxially oriented polypropylene film of the present invention, the ratio of ΔD(174) to ΔD(130) (ΔD(174) / ΔD(130)) is preferably 1.00 or more and 1.20 or less. From the above viewpoint, the ratio of ΔD(174) to ΔD(130) (ΔD(174) / ΔD(130)) is more preferably 1.00 or more and 1.18 or less.

[0026] By keeping ΔD(174) / ΔD(130) at 1.20 or less, heat generation during use can be suppressed when the capacitor is used, improving the capacitor's lifespan, especially at high temperatures and high withstand voltages. Note that as the applied voltage in voltage tests increases, the rate of change of the dielectric tangent may increase but will not decrease, so ΔD(174) / ΔD(130) is effectively 1.00 or more.

[0027] To set ΔD(174) / ΔD(130) to 1.20 or less or within the above-mentioned preferred range, the same methods as those for adjusting ΔD(174) and ΔD(130) described above can be used.

[0028] The biaxially oriented polypropylene film of the present invention preferably has a thickness (t) of 1.0 μm or more and 3.0 μm or less from the viewpoint of achieving both mechanical strength and improved life when used in a capacitor. From the above viewpoints, the thickness (t) is more preferably 1.5 μm or more and 2.8 μm or less, and even more preferably 1.8 μm or more and 2.6 μm or less. By setting the thickness (t) to 1.0 μm or more, it is possible to appropriately control the mechanical strength and voltage resistance, and also to prevent film breakage during film formation and processing. On the other hand, by setting the thickness (t) to 3.0 μm or less, it is possible to increase the capacitance per volume when used as a dielectric in a capacitor, and as a result, it can be suitably used as a dielectric in a capacitor.

[0029] The biaxially oriented polypropylene film of the present invention is preferably subjected to a surface treatment such as corona discharge treatment, plasma treatment, glow discharge treatment, or flame treatment after biaxial stretching in order to improve adhesion to metals during metal vapor deposition. Polypropylene films typically have low surface energy, with a surface wet tension, an indicator of this, of approximately 30 mN / m, making adhesion to metals problematic. These surface treatments increase the surface energy of the polypropylene film, and by using these surface treatments to adjust the surface wet tension to 42 mN / m or more and 48 mN / m or less, adhesion to metals is improved during metal vapor deposition. These surface treatments may be used alone or in combination.

[0030] Next, the polypropylene resin used in the biaxially oriented polypropylene film of the present invention will be described. The biaxially oriented polypropylene film of the present invention preferably contains a linear polypropylene resin A as a main component and a branched polypropylene resin H.

[0031] The linear polypropylene resin A used in the biaxially oriented polypropylene film of the present invention refers to an isotactic polypropylene resin. This isotactic polypropylene resin is known as a polypropylene resin commonly used in polypropylene films for capacitor applications. The linear polypropylene A used in the present invention preferably has a mesopentad fraction (hereinafter, mmmm) of 96.0% to 99.5%, a melt flow index (hereinafter, MFR) of 0.5 g / 10 min to 3.0 g / 10 min, and a melt tension (hereinafter, MS) of 0.1 g to 1.2 g. Resins that can be suitably used as the linear polypropylene resin A include, for example, "Borclean" (trademark) (e.g., HB300BF) manufactured by Borealis.

[0032] The mesopentad fraction (mmmm) of the linear polypropylene resin A is preferably 96.0% or more, more preferably 97.0% or more, from the viewpoint of voltage resistance in a high-temperature environment. mmmm is an index of the stereoregularity of the crystalline phase of a polypropylene resin measured by nuclear magnetic resonance (NMR). The higher this value, the higher the crystallinity and melting point of the polypropylene resin, and when made into a film, the higher the voltage resistance, especially at high temperatures. To obtain such a polypropylene resin with high stereoregularity, methods such as washing the obtained resin powder with a solvent such as n-heptane, or appropriately selecting a catalyst and / or co-catalyst and selecting the composition are preferably employed.

[0033] The melt flow rate (MFR) of linear polypropylene resin A, as measured in accordance with JIS K 7210-1 (2014), is preferably 0.5 g / 10 min to 3.0 g / 10 min, more preferably 1.0 g / 10 min to 2.9 g / 10 min, and even more preferably 1.5 g / 10 min to 2.8 g / 10 min. When the MFR of linear polypropylene resin A is within the above preferred range, excellent film-forming properties are achieved, resulting in stable production of biaxially oriented polypropylene films, and the resulting biaxially oriented polypropylene films also exhibit excellent voltage resistance characteristics. To achieve an MFR of the polypropylene resin within the above range, a method of appropriately selecting the type and composition of the catalyst and the polymerization temperature, or a method of polymerization in the presence of hydrogen is preferably employed.

[0034] The melt tension (MS) of the linear polypropylene resin A is preferably 1.0 g or less. When the MS of the linear polypropylene resin A is 1.0 g or less, the flow characteristics are improved when the resin is melted, and the occurrence of uneven film thickness and film breakage can be suppressed. To set the MS of the polypropylene resin within the above range, a method of appropriately selecting the type and composition of the catalyst and the polymerization temperature, or a method of polymerization in the presence of hydrogen is preferably adopted.

[0035] In the biaxially oriented polypropylene film of the present invention, the content of linear polypropylene resin A is preferably more than 90% by mass and not more than 99% by mass, more preferably 92% by mass or more and 98% by mass or less, and even more preferably 93% by mass or more and 97% by mass or less, when the total polypropylene resin constituting the film is taken as 100% by mass. When the content of linear polypropylene resin A is within the above preferred range, the stereoregularity of the biaxially oriented polypropylene film is high and the voltage resistance is excellent.

[0036] Next, branched polypropylene resin H will be described. Branched polypropylene resin H used in the biaxially oriented polypropylene film of the present invention refers to a branched polypropylene resin having an MFR of 6.0 g / 10 min to 13.0 g / 10 min and an MS of 1.5 g to 8.0 g. To obtain such a branched polypropylene resin, methods such as using high-energy ionizing radiation on a polypropylene resin (e.g., JP 62-121704 A), reacting a polypropylene resin with a specific organic peroxide (e.g., JP 2869606 A), reacting a polypropylene resin with a thermally decomposable radical former and an ethylenically polyfunctional unsaturated monomer (e.g., JP 10-330436 A), or using a specific catalyst during polymerization of the polypropylene resin (e.g., JP 2009-057542 A) are preferably used. Resins that can be suitably used as the branched chain polypropylene resin H include, for example, "WAYMAX" (registered trademark) (EX4000, MFX3, etc.) manufactured by Japan Polypropylene Corporation.

[0037] The branched polypropylene resin H has a branched structure in the molecular chain. The branched polypropylene resin is a polypropylene resin having 5 or less internal tri-substituted olefins per 10,000 carbon atoms. The presence of the internal tri-substituted olefins is 1 This can be confirmed by the proton ratio in the H-NMR spectrum. Branched-chain polypropylene resin acts as an α crystal nucleating agent, and when added in a certain amount, it can also form a rough surface due to the crystalline morphology. In other words, the size of the polypropylene spherulites formed 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.

[0038] The MFR of branched polypropylene resin H is preferably 6.0 g / 10 min to 13.0 g / 10 min, more preferably 6.5 g / 10 min to 12.5 g / 10 min, and even more preferably 7.0 g / 10 min to 12.0 g / 10 min. When the MFR of branched polypropylene resin H is within the above range, polypropylene crystallization proceeds efficiently, stabilizing the higher-order structure of the film, making it easier to achieve the desired dielectric properties (ΔD(174), ΔD(130)). To achieve an MFR of the polypropylene resin within the above range, a method of appropriately selecting the type and composition of the catalyst and the polymerization temperature, or a method of polymerization in the presence of hydrogen are preferably used.

[0039] When the MFR of branched polypropylene resin H is MFR(H) and the MFR of linear polypropylene resin A is MFR(A), the difference in MFR between branched polypropylene resin H and linear polypropylene resin A (MFR(H) - MFR(A)) is preferably 5.0 g / 10 min or more and 10.0 g / 10 min or less, more preferably 5.5 g / 10 min or more and 9.5 g / 10 min or less, and even more preferably 6.0 g / 10 min or more and 9.5 g / 10 min or less. When MFR(H) - MFR(A) is within the above range, polypropylene crystallization proceeds efficiently, stabilizing the higher-order structure of the film, making it easier to achieve the desired dielectric properties (ΔD(174), ΔD(130)). To achieve MFR(H) - MFR(A) within the above range, the combination of linear polypropylene resin A and branched polypropylene resin H may be selected so that MFR(H) - MFR(A) falls within the above range.

[0040] The MS of branched-chain polypropylene resin H is preferably 1.5 g or more and 8.0 g or less, more preferably 2.0 g or more and 7.0 g or less, and even more preferably 3.0 g or more and 6.0 g or less. When the MS of branched-chain polypropylene resin H is within the above range, the polypropylene crystals become denser and the higher-order structure becomes more homogenous, stabilizing the higher-order structure of the film, making it easier to achieve the dielectric properties (ΔD(174), ΔD(130)) of the present application.

[0041] In the biaxially oriented polypropylene film of the present invention, the content of branched polypropylene resin H is preferably 1.0% by mass or more and 10% by mass or less, more preferably 2.0% by mass or more and 8.0% by mass or less, and even more preferably 3.0% by mass or more and 7.0% by mass or less, when the total polypropylene resin constituting the film is taken as 100% by mass. When the content of branched polypropylene resin H is within the above preferred range, the polypropylene crystals become denser and the higher-order structure becomes more homogenous, stabilizing the higher-order structure of the film and making it easier to achieve the dielectric properties (ΔD(174), ΔD(130)) of the present application.

[0042] The biaxially oriented polypropylene film of the present invention may preferably contain various additives, such as nucleating agents, antioxidants, heat stabilizers, lubricants, antistatic agents, antiblocking agents, fillers, viscosity modifiers, and color inhibitors, within the scope of the present invention.

[0043] Among the additives mentioned above, the type and amount of antioxidant selected is important from the viewpoint of long-term heat resistance. Specifically, antioxidants are preferably sterically hindered phenolic antioxidants, with at least one of them being a high-molecular-weight type with a molecular weight of 500 or more. Specifically, it is preferable to use, alone or in combination, 2,6-di-t-butyl-p-cresol (BHT: molecular weight 220.4), 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 1177.7). The total content of these antioxidants is preferably 0.03 to 1.0 part by mass, and more preferably 0.1 to 0.9 parts by mass, based on the total amount of polypropylene resin. When the content of the antioxidant in the polypropylene resin composition is 0.03 part by mass or more, the antioxidant effect is easily obtained and long-term heat resistance is easily maintained. On the other hand, when the content of the antioxidant in the polypropylene resin composition is 1.0 part by mass or less, high-temperature voltage resistance characteristics are easily maintained.

[0044] The biaxially oriented polypropylene film of the present invention is preferably obtained by molding a polypropylene resin composition containing the above-mentioned linear polypropylene resin A as a main component and branched polypropylene resin H into a sheet and then biaxially stretching the sheet. As the biaxial stretching method, any of inflation simultaneous biaxial stretching, tenter simultaneous biaxial stretching, and tenter sequential biaxial stretching may be used, but tenter sequential biaxial stretching is preferred from the viewpoint of film formation stability and thickness uniformity. In particular, it is preferred to stretch the sheet in the longitudinal direction and then in the width direction.

[0045] Next, the method for producing the biaxially oriented polypropylene film of the present invention will be explained below, but the method is not necessarily limited thereto.

[0046] First, the linear polypropylene resin A and the branched polypropylene resin H are dry-blended in the preferred ratio described above and fed into a single-screw melt extruder, where they are melt-extruded at 200°C to 260°C. Next, foreign matter and modified polymers are removed using a filter installed midway through the polymer pipe. The molten sheet is then extruded from a T-die onto a casting drum, where it is cooled and solidified to form a cast sheet.

[0047] The temperature of the cast drum is preferably 70°C or higher and 110°C or lower, more preferably 75°C or higher and 105°C or lower, and even more preferably 80°C or higher and 100°C or lower, from the viewpoint of properly generating β crystals and spherulites. By setting the cast drum temperature to 70°C or higher, the formation of too few β crystals in the cast sheet is prevented, and the slipperiness of the film obtained after biaxial stretching is maintained, thereby preventing the occurrence of conveyance wrinkles during the film conveyance process during film production and processing and deterioration of the winding shape of the film roll. On the other hand, by setting the cast drum temperature to 110°C or lower, the formation of excessive β crystals in the cast sheet is prevented, and the occurrence of meandering during the film conveyance process during film production and processing and deterioration of the winding shape of the film roll are easily prevented.

[0048] The molten sheet discharged from the T-die lands on the casting drum and is preferably in close contact with the drum for 1.0 to 3.0 seconds. A close contact time of 1.0 second or more facilitates solidification of the molten sheet, making it easier to prevent breakage during the subsequent stretching process. On the other hand, a close contact time of 3.0 seconds or less can prevent excessive formation of β crystals in the cast sheet, making it easier to prevent meandering during the film transport process during film formation and processing, and deterioration of the film roll shape.

[0049] The molten sheet can be adhered to the casting drum using methods such as electrostatic application, air knife, nip roll, and underwater casting. However, the air knife method is preferred from the viewpoints of suppressing thickness unevenness, achieving high-speed film production, and controlling the surface properties of the film.

[0050] Next, in the longitudinal stretching step, the cast sheet is stretched in the machine direction (longitudinal stretching) to obtain a uniaxially oriented film. Specifically, the cast sheet is preferably passed through temperature-controlled longitudinal stretching rolls and stretched in the machine direction at a predetermined stretch ratio by controlling the peripheral speed difference between the rolls. The temperature of the longitudinal stretching rolls is preferably 135°C or higher and 155°C or lower. By setting the temperature of the longitudinal stretching rolls to 135°C or higher, insufficient heat during longitudinal stretching is prevented, and film breakage during the longitudinal stretching step is likely to be reduced. On the other hand, by setting the temperature of the longitudinal stretching rolls to 155°C or lower, the orientation of the uniaxially oriented film is enhanced, and the voltage resistance of the biaxially oriented polypropylene film obtained after biaxial stretching is likely to be improved.

[0051] The longitudinal stretching ratio is preferably 4.0 to 7.0 times, and more preferably 5.0 to 7.0 times. A longitudinal stretching ratio of 4.0 times or more tends to reduce the variation in the surface shape of the biaxially oriented polypropylene film obtained after biaxial stretching in the longitudinal and transverse directions, and the increased orientation tends to improve the voltage resistance of the biaxially oriented polypropylene film. On the other hand, a longitudinal stretching ratio of 7.0 times or less tends to reduce film breakage during the longitudinal stretching process and the subsequent transverse stretching process.

[0052] In order to obtain the biaxially oriented polypropylene film of the present invention, it is preferable to subject the uniaxially oriented film to a quenching treatment immediately after the longitudinal stretching step. Specifically, it is preferable to subject the uniaxially oriented film immediately after the longitudinal stretching step to a quenching treatment by passing it through temperature-controlled conveying rolls while nipping it with temperature-controlled nip rolls.

[0053] The temperature of the conveying roll during the cooling treatment is preferably 5°C or higher and 15°C or lower, and more preferably 5°C or higher and 13°C or lower. By setting the conveying roll temperature to 5°C or higher, it is possible to prevent the orientation of the uniaxially oriented film from becoming too high and suppress film tearing during transverse stretching, making it easier to maintain mass productivity. On the other hand, by setting the conveying roll temperature to 15°C or lower, it is possible to prevent the orientation of the film from becoming low, making it easier to form a stable higher-order structure that is less likely to change when a voltage is applied, and making it easier to achieve the dielectric properties (ΔD(174), ΔD(130)) of the biaxially oriented film of the present invention.

[0054] The nip roll temperature during the cooling treatment is preferably 5°C or higher and 15°C or lower, and more preferably 7°C or higher and 13°C or lower. By setting the nip roll temperature to 5°C or higher, curling of the cast sheet on the nip roll caused by the temperature difference between the front and back sides is reduced, thereby reducing tearing during the stretching process and ensuring mass productivity. On the other hand, by setting the nip roll temperature to 15°C or lower, the orientation of the film is prevented from becoming low, and a stable higher-order structure that is less likely to change when a voltage is applied is easily formed, making it easier to achieve the dielectric properties (ΔD(174), ΔD(130)) of the biaxially oriented film of the present invention.

[0055] The pressure of the nip rolls during the cooling treatment is preferably 0.30 MPa or more and 0.60 MPa or less, and more preferably 0.35 MPa or more and 0.55 MPa or less. By setting the nip roll pressure to 0.30 MPa or more, the occurrence of wrinkles and meandering of the film due to film shrinkage during the cooling treatment are suppressed, making it easier to reduce breakage in the longitudinal stretching step and ensure mass productivity. On the other hand, by setting the nip roll pressure to 0.60 MPa or less, film deformation due to pressure is suppressed, making it easier to reduce breakage in the stretching step and ensure mass productivity.

[0056] The cooling time is preferably 0.1 to 1.0 seconds, more preferably 0.2 to 0.9 seconds, and even more preferably 0.3 to 0.8 seconds. A cooling time of 0.1 seconds or longer prevents the film from becoming less oriented, and facilitates the formation of a stable higher-order structure that is less susceptible to change when a voltage is applied, making it easier to achieve the dielectric properties (ΔD(174), ΔD(130)) of the biaxially oriented film of the present invention. On the other hand, a cooling time of 1.0 seconds or shorter suppresses the occurrence of wrinkles and film meandering due to film shrinkage during the cooling process, thereby reducing breakage during the longitudinal stretching process and ensuring mass productivity.

[0057] Next, the uniaxially oriented film is gripped at both widthwise ends with clips and stretched in the widthwise direction (transverse stretching) at a stretching ratio of 8.0 to 15 times using a tenter-type stretching machine controlled at a temperature of 155°C to 165°C, and then heat-set at a temperature of 150°C to 170°C and a relaxation rate of 5% to 15%, to obtain a biaxially oriented polypropylene film.

[0058] Next, the biaxially oriented polypropylene film is subjected to a corona discharge treatment in air, nitrogen, carbon dioxide gas, or a mixture thereof, and then both widthwise ends held by clips are cut and removed, and the film is wound up as an intermediate product on a winder. Finally, the biaxially oriented polypropylene film unwound from the intermediate product is slit to a specific width on a slitter and wound around a core as a film roll to obtain the biaxially oriented polypropylene film roll of the present invention.

[0059] The biaxially oriented polypropylene film of the present invention is preferably used as a dielectric for a capacitor, but is not limited to the type of capacitor. Specifically, from the viewpoint of electrode configuration, it may be either a foil-wound capacitor or a metal-vapor-deposited film capacitor, and is also preferably used in an oil-immersion type capacitor containing insulating oil or a dry capacitor that does not use insulating oil at all. In addition, from the viewpoint of shape, it may be a wound type or a laminated type. Due to the properties of the biaxially oriented polypropylene film of the present invention, it is particularly preferably used as a metal-vapor-deposited film capacitor.

[0060] Next, we will explain the metal film laminated film using the biaxially oriented polypropylene film of the present invention. A preferred method for forming the metal film is to vapor-deposit a metal such as aluminum onto at least one side of the biaxially oriented polypropylene film to form a metal film that will serve as an internal electrode for the film capacitor. Other metal components, such as nickel, copper, gold, silver, chromium, and zinc, can also be vapor-deposited simultaneously with or sequentially with the aluminum. A protective layer, such as oil, can also be formed on the metal film. From the perspective of the electrical characteristics and safety of the capacitor, the thickness of the metal film is preferably 20 nm to 100 nm. For the same reason, the surface resistivity of the metal film is preferably 1 Ω / sq to 20 Ω / sq. The surface resistivity can be controlled by the type of metal used and the film thickness.

[0061] Next, we will explain a film capacitor using the biaxially oriented polypropylene film of the present invention. The film capacitor has a laminated or wound configuration with a metal film-laminated film. An example of a manufacturing method for a wound film capacitor is described below. First, aluminum is vacuum-deposited on one side of a biaxially oriented polypropylene film. The aluminum is deposited in stripes with margins running longitudinally along the film. Next, a blade is used to slit the surface at the center of each deposited area and the center of each margin, creating a tape-like take-up reel with a margin on one side. Two tape-like take-up reels with left and right margins are stacked and wound together so that the deposited area extends beyond the margin in the width direction to obtain a wound body. After heat-treating the wound body, metallicon is sprayed on both widthwise end faces to form external electrodes, and lead wires are welded to the metallicon to obtain a wound film capacitor. Film capacitors have a wide range of applications, including vehicles, home appliances (such as televisions and refrigerators), general noise prevention, automobiles (such as hybrid cars, power windows, and wipers), and power supplies, and film capacitors using the biaxially oriented polypropylene film of the present invention can also be suitably used for these applications. [Example]

[0062] The present invention will be described in detail below with reference to examples. The properties were measured and evaluated by the following methods.

[0063] [Evaluation method for each characteristic] (1) Mesopentad fraction (mmmm) A polypropylene resin sample is dissolved in a solvent. 13 The mesopentad fraction (mmmm) was determined using C-NMR under the following conditions (Reference: New Edition Polymer Analysis Handbook, edited by the Japan Society for Analytical Chemistry and the Polymer Analysis Research Forum, 1995, pp. 609-611). A. Measurement conditions Equipment: Bruker DRX-500 Measurement nuclei: 13 C nucleus (resonance frequency: 125.8MHz) Measured concentration: 10wt% Solvent: Benzene / d-o-dichlorobenzene = 1:3 mass ratio mixed solution Measurement temperature: 130℃ Spin speed: 12Hz NMR sample tube: 5mm tube Pulse width: 45° (4.5 μs) Pulse repetition time: 10 seconds Data points: 64K Conversion count: 10,000 times Measurement mode: complete decoupling B.Analysis conditions Fourier transformation was performed with a line broadening factor (LB) of 1.0, and the mmmm peak was determined to be 21.86 ppm. Peak splitting was performed using WINFIT software (Bruker). Peak splitting was performed as follows, starting from the peak on the high magnetic 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. The above measurement was performed five times, and the average value was used as the mesopentad fraction (mmmm) of this sample. peak (a)mrrm (b)(c) rrrm (split into two peaks) (d)rrrr (e)mrmr (f)mrmm+rmrr (g)mmrr (h)rmmr (i) mmmr (j)mmmm.

[0064] (2) Melt flow rate (MFR) (unit: g / 10 min) Measurements were performed in accordance with JIS K 7210-1 (2014) at a temperature of 230°C and a load of 2.16 kg.

[0065] (3) Melt tension (MS) (unit: g) Using a Capillograph 1B (capillary diameter 2.0 mm, capillary length 40 mm, cylinder diameter 9.55 mm) manufactured by Toyo Seiki Seisaku-sho, polypropylene resin was heated to 230°C, and the molten polypropylene was extruded at an extrusion rate of 20 mm / min to form an extruded strand. The tension was then measured when this strand was taken up at a rate of 4.0 m / min.

[0066] (4) Change in dielectric tangent (ΔD) before and after voltage application test On a film roll corresponding to the center position in the width direction of the master roll, measurements A to F described below were carried out in order at 10 points randomly taken from the center position of the film roll in the longitudinal direction. ΔD was calculated for each measurement point, and the average value of the 10 points was taken as ΔD for that sample.

[0067] A. Measurement of dielectric tangent (tanδ(0)) before voltage application test Dielectric relaxation measurements were performed on samples taken from the film roll using an Agilent Technologies Precision LCR Meter E4980A at a temperature of 25°C under the following conditions. The dielectric loss tangent (tanδ = imaginary part ε" / real part ε') at 2000 Hz was calculated from the real part (resistance) ε' and imaginary part (reactance) ε" of the obtained complex impedance, and this was used as the dielectric loss tangent (tanδ(0)) before the voltage application test. Applied voltage: 2V Frequency: 20Hz~2MHz Measurement mode: Cp-D Time: Long Average: 16.

[0068] B. Voltage application test at 130V / μm The sample whose dielectric loss tangent (tanδ(0)) was measured before the voltage application test was set in a HIOKI SEM-8310 plate sample electrode connected to an Element Ltd. DC high-voltage power supply ELS19-10K05B1, and a voltage was applied at a temperature of 25°C and a DC voltage of 130 V / μm for 1 hour. When the thickness of the sample to be measured was 2.3 μm, the power supply voltage was set to 300 V.

[0069] C. Measurement of dielectric loss tangent (tanδ(130)) after voltage application test at 130V / μm For the sample after the voltage application test of 130 V / μm, dielectric relaxation measurement was performed at a temperature of 25°C under the conditions described above using a Precision LCR meter E4980A manufactured by Agilent Technologies, Inc., and the dielectric loss tangent (tanδ = imaginary part ε" / real part ε') at 2000 Hz was calculated from the real part (resistance) ε' and imaginary part (reactance) ε" of the obtained complex impedance, and this was used as the dielectric loss tangent (tanδ(130)) after the voltage application test of 130 V / μm.

[0070] D. Voltage application test at 174V / μm Voltage 1 30 Dielectric tangent (tanδ(1 30 The sample used to measure ) was set in a HIOKI SEM-8310 plate sample electrode connected to an Element Ltd. DC high-voltage power supply ELS19-10K05B1, and a voltage of 174 V / μm was applied at 25°C for 1 hour. When the sample thickness was 2.3 μm, the power supply voltage was set to 400 V.

[0071] E. Measurement of dielectric tangent (tanδ(174)) after voltage application test at 174V / μm For the sample after the voltage application test of 174 V / μm, dielectric relaxation measurement was performed at a temperature of 25°C under the conditions described above using a Precision LCR meter E4980A manufactured by Agilent Technologies, Inc., and the dielectric loss tangent (tanδ = imaginary part ε" / real part ε') at 2000 Hz was calculated from the real part (resistance) ε' and imaginary part (reactance) ε" of the obtained complex impedance, and this was used as the dielectric loss tangent (tanδ(174)) after the voltage application test of 174 V / μm.

[0072] F. Calculation of the rate of change in dielectric tangent (ΔD) before and after the voltage application test Using the tan δ(0), tan δ(130), and tan δ(174) obtained in A, C, and E above, ΔD was calculated according to the following formulas (1) and (2). ΔD(174)=tanδ(174) / tanδ(0) ···(1) ΔD(130)=tanδ(130) / tanδ(0) ···(2).

[0073] (5) Thickness (t) (unit: μm) The thickness was measured by the micrometer method in accordance with JIS C 2330 (2014).

[0074] (6) Wetting tension (unit: mN / m) According to JIS K 6768 (1999), the wetting tension of the corona-treated surface of biaxially oriented polypropylene film was measured using a test mixture of ethylene glycol monoethyl ether, formamide, and methanol.

[0075] (7) Element processability in capacitor manufacturing Aluminum was vacuum-deposited on the corona-treated side of the biaxially oriented polypropylene film to a resistivity of 15 Ω / sq using a vacuum deposition machine manufactured by ULVAC, Inc. Aluminum was deposited in stripes with longitudinal margins (79.0 mm wide deposited strips, 1.0 mm wide margins, repeated). The film was then slit with a blade at the center of each deposited strip and the center of each margin to produce tape-like take-up reels with a total width of 40 mm and 0.5 mm margins on either the left or right edge. Two of the resulting 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, resulting in a wound assembly with a capacitance of 120 μF. The elements were wound using a KAW-4NHB coil manufactured by Kaito Seisakusho, Inc. Finally, the film was heat-treated at 130°C in a reduced-pressure atmosphere for 10 hours to obtain capacitor elements.

[0076] (8) Capacitor life test Aluminum was vacuum-deposited on the corona-treated side of biaxially oriented polypropylene film to a resistance of 15 Ω / sq using a vacuum deposition machine manufactured by ULVAC, Inc. Aluminum was deposited in stripes with longitudinal margins (79.0 mm wide strips, repeated 1.0 mm wide strips). The film was then slit with a blade at the center of each deposited strip and at the center of each margin to produce tape-like take-up reels with a total width of 40 mm and 0.5 mm margins on either the left or right edge. Two reels from the left and right margins of the resulting reels were overlapped and wound together so that the deposited portion extended 0.5 mm beyond the margins in the width direction, resulting in a wound capacitor with a capacitance of 120 μF. The elements were wound using a KAW-4NHB manufactured by Kaito Seisakusho, Inc. Finally, the film was heat-treated for 10 hours in a reduced-pressure atmosphere at 130°C. Metallicon was spray-coated on both widthwise end faces to form external electrodes, and lead wires were welded to the metallicon to obtain capacitor elements. Next, the capacitor characteristics of 10 capacitor elements were evaluated. First, the capacitance (C0) was measured at room temperature. Next, a voltage of 200 VDC / μm (460 V when the thickness (t) was 2.3 μm) was applied to the capacitor elements at a high temperature of 125°C for 800 hours. After that, the capacitance (C) was measured at room temperature, and the rate of change in capacitance (ΔC) before and after voltage application was calculated using the following formula. ΔC=((C0-C) / C0)×100 The average value of the rate of change (ΔC) in capacitance before and after voltage application for 10 capacitor elements was taken as the rate of change for that sample, and evaluation was performed according to the following criteria. The evaluation was conducted in the order of ◎, ○, △, and ×. ◎: ΔC is less than 2% ○: ΔC is 2% or more and less than 3% △: ΔC is 3% or more and less than 5% ×: ΔC is 5% or more.

[0077] [Resins used in the production of biaxially oriented polypropylene film] The resins and the like used in the production of the biaxially oriented polypropylene films of each Example and Comparative Example are as follows. Linear polypropylene resin A1: Polypropylene resin with mmmm of 98%, MFR of 2.5 g / 10 min, and MS of 1.0 g (Borealis "Borclean" (trademark) HB300BF) Branched Polypropylene Resin H1: Polypropylene resin with an MFR of 9.0 g / 10 min and an MS of 5.0 g ("WAYMAX" (registered trademark) MFX3 manufactured by Japan Polypropylene Corporation) Branched Polypropylene Resin H2: Polypropylene resin with an MFR of 6.2 g / 10 min and a melt tension MS of 4.0 g ("WAYMAX" (registered trademark) EX4000, manufactured by Japan Polypropylene Corporation) Branched Polypropylene Resin H3: Polypropylene resin with an MFR of 2.5 g / 10 min and a melt tension MS of 32 g ("Daploy" (trademark) WB135HMS manufactured by Borealis).

[0078] Example 1 Linear polypropylene resin A1 and branched polypropylene resin H1 were dry-blended at a mass ratio of 95:5 and fed to a single-screw melt extruder. The antioxidants included 0.4 parts by mass of BASF Japan's "Irganox" (registered trademark) 1010, based on 100 parts by mass of the total polypropylene resin, and 0.1 parts by mass of 2,6-di-t-butyl-p-cresol (BHT). The polypropylene resin mixture was melted at 250°C in the single-screw melt extruder, and foreign matter was removed using a sintered filter with a 25 μm cutoff. The molten polypropylene resin mixture was then extruded into a sheet through a T-slit die and solidified by contacting it with a casting drum maintained at 90°C using an air knife. The time the molten sheet remained in contact with the casting drum was 1.5 seconds. The surface that contacted the casting drum was designated the drum surface (D surface), and the surface that did not contact the casting drum was designated the non-drum surface (non-D surface). The cast sheet was then stretched in the machine direction at a stretch ratio of 6.0 using longitudinal stretching rolls at a temperature of 145°C. The resulting uniaxially oriented film was placed on a conveyor roll at a temperature of 10°C and quenched for 0.5 seconds while being nipped with nip rolls at a temperature of 10°C at a pressure of 0.45 MPa. The resulting machine-stretched sheet was then clamped at both widthwise ends with clips and introduced into a tenter, where it was stretched in the machine direction at a stretch ratio of 11 at 160°C and further relaxed by 12% in the machine direction at a temperature of 158°C. It was then gradually cooled to room temperature, and the drum surface (D-side) of the film was stretched at 25 W·min / m 2 The film was subjected to a corona discharge treatment at a treatment strength of 1000 kJ / min, and the edge portions of the film held by clips were cut and removed. The film with the removed edges was then wound up on a winder. The film was then slit to a width of 0.82 m using a slitter, and 30,000 m was wound longitudinally around a core to form a film roll, yielding a biaxially oriented polypropylene film with a thickness of 2.3 μm. The evaluation results are shown in Table 1.

[0079] (Examples 2 to 4, Comparative Examples 1 to 4) A biaxially oriented polypropylene film having a thickness of 2.3 μm was obtained in the same manner as in Example 1, except that the polypropylene resin composition and the cooling temperature of the uniaxially oriented film were as shown in Table 1. The evaluation results are shown in Table 1.

[0080] [Table 1]

[0081] The polypropylene resin composition is shown as a value assuming the entire polypropylene resin as 100% by mass. [Industrial Applicability]

[0082] The biaxially oriented polypropylene film of the present invention has high voltage resistance and can therefore be suitably used as a dielectric for film capacitors.

Claims

1. A biaxially oriented polypropylene film characterized in that, when the dielectric loss tangent before a voltage application test is tanδ(0) and the dielectric loss tangent after a voltage application test in which a voltage of 174 V / μm is applied is tanδ(174), the rate of change in the dielectric loss tangent before and after a voltage application test, ΔD(174), as expressed by the following formula (1), is 1.0 or more and 3.0 or less. However, for tanδ(174), the biaxially oriented polypropylene film sample is set on a flat sample electrode connected to a DC high-voltage power supply, and a voltage of 130 V / μm is applied at 25°C for 1 hour. The biaxially oriented polypropylene film sample is then set on a flat sample electrode connected to a DC high-voltage power supply, and a voltage of 174 V / μm is applied at 25°C for 1 hour. The tanδ(174) of the biaxially oriented polypropylene film sample is used. ΔD (174) = tan δ (174) / tan δ (0) ... (1)

2. 2. The biaxially oriented polypropylene film according to claim 1, characterized in that, when the dielectric loss tangent after a voltage application test in which a voltage of 130 V / μm is applied is tanδ(130), the rate of change ΔD(130) of the dielectric loss tangent before and after the voltage application test, which is represented by the following formula (2), is 1.0 or more and 2.7 or less. ΔD(130)=tanδ(130) / tanδ(0)...(2)

3. The biaxially oriented polypropylene film according to claim 1 or 2, characterized in that the ratio of the ΔD(174) to the ΔD(130) (ΔD(174) / ΔD(130)) is 1.00 or more and 1.20 or less.

Images