JP7790219B2 - Biaxially oriented polypropylene film - Google Patents

Description

The present invention relates to a biaxially oriented polypropylene film that, when used as a dielectric in a capacitor, exhibits high voltage resistance in high-temperature, high-voltage environments.

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

In particular, its excellent high voltage resistance and low loss characteristics make it particularly suitable for use as a capacitor dielectric. Recently, various electrical equipment has been converted to inverters, which has led to an even stronger demand for smaller capacitors with higher capacitance. Furthermore, as temperatures (85°C to 125°C) continue to rise, particularly in automotive applications (including hybrid cars and electric vehicles), solar power generation, and wind power generation, capacitors are also required to be more heat-resistant. Therefore, dielectric films are being required to be thinner, more heat-resistant, and have improved voltage resistance per thickness, as well as to reduce the dielectric loss tangent tanδ in order to minimize electrical energy loss.

Technologies proposed for controlling the dielectric loss tangent tanδ of a film include fragmenting a polymerization catalyst into nano-sized catalyst fragments and dispersing them in polypropylene (e.g., Patent Document 1) and depositing an inorganic dielectric onto a substrate film at a thickness of 0.5% to 10% of the thickness of the substrate film (e.g., Patent Document 2). Furthermore, a technique for controlling the dielectric loss tangent tanδ of a capacitor element has been proposed, which involves reducing the thermal shrinkage rate of biaxially oriented polypropylene film to prevent metallikon peeling from the capacitor element (e.g., Patent Documents 3 to 5).

Special Publication No. 2014-531480 Japanese Patent Application Laid-Open No. 2020-004743 Japanese Patent Application Publication No. 10-156940 Japanese Patent Application Laid-Open No. 2020-124905 Japanese Patent Application Laid-Open No. 2020-124906

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 film's withstand voltage. 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 dielectric loss tangent tanδ cannot be said to be sufficient in today's high-temperature, high-voltage environments. Therefore, the object of the present invention is to provide a biaxially oriented polypropylene film that has high processability, withstand voltage properties, and low energy loss.

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) = tan δ (174)/tan δ (0) ... (1)

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

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 "~", the upper and lower limits are included in the range, and the units for the upper and lower limits are the same.

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, "biaxially oriented" here means stretched 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 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. "Main structural unit" refers to a structural unit that accounts for more than 50 mol% but not more than 100 mol%, when all structural units constituting the resin are taken as 100 mol%. Hereinafter, "main structural unit" can be interpreted in the same way.

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, as described below. In this specification, the term "major component" refers to a component that accounts for more than 50% by mass but not more than 100% by mass of the total components of the film (100% by mass), more preferably 80% by mass or more but not more than 100% by mass, even more preferably 90% by mass or more but not more than 100% by mass, and particularly preferably 95% by mass or more but not more than 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 types of 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.

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.

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 structural unit is a structural 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.

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 film is primarily composed of polypropylene resin. However, from the viewpoint of the voltage resistance 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 all structural units constituting the entire resin in the biaxially oriented polypropylene film are taken as 100 mol%. Whether the content satisfies the above requirement is determined by the number of structural units other than propylene units, regardless of the number of components of the polypropylene copolymer and other polymers, as long as the total amount of structural units other than propylene units is 20 mol% or less of all structural units constituting the entire resin in the biaxially oriented polypropylene film. From the above viewpoint, it is more preferable that the content of the polypropylene copolymer or other polymer be 1 mol% or less.

Furthermore, the biaxially oriented polypropylene film of the present invention preferably contains linear polypropylene resin A as the main component and branched polypropylene resin H. Linear polypropylene resin A and branched polypropylene resin H will be described later.

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, 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 the 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).

Here, "voltage application test applying a voltage of 174 V/μm" refers to 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 dielectric tangent before and after the voltage application test, ΔD(174), refers to the rate of change in 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 dielectric tangent before and after the voltage application test, ΔD(130)" can also be defined in the same way, except for the applied voltage.

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

Tan 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 tan δ, the greater the proportion of applied electrical energy that is lost as thermal energy. When a capacitor is used over an extended period of time, various factors can cause the tan δ of the dielectric to increase, generating a large amount of heat, which shortens the capacitor's lifespan.

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. Furthermore, since the dielectric dissipation factor typically increases during a voltage application test of 174 V/μm, from the standpoint of feasibility, ΔD(174) should essentially be 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.

A suitable method for achieving ΔD(174) of 1.0 to 3.0 or within the preferred range described above 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) can be further 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 approach 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 described above. There are no particular restrictions on the quenching method, but one example is a method in which the uniaxially oriented film is nipped with temperature-controlled nip rolls and then passed through temperature-controlled transport rolls to perform the quenching treatment.The temperature conditions for this are preferably 5°C to 15°C, and more preferably 5°C to 13°C, for the transport rolls.The temperature conditions for the nip rolls are preferably 5°C to 15°C, and more preferably 7°C to 13°C.

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 represented by the following formula (2), is 1.0 or more and 2.7 or less.
ΔD(130)=tanδ(130)/tanδ(0) (2).

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 typically increases during a voltage application test of 130 V/μm, from the standpoint of feasibility, ΔD(130) should essentially be 1.0 or more. 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.

A suitable method for achieving ΔD(130) of 1.0 to 2.7 or within the preferred range 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) can be further 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 approach facilitates achieving ΔD(130) of 1.0 to 2.7 or within the preferred range, as described above. The quenching method and conditions described above for achieving ΔD(174) of 1.0 to 3.0 or within the preferred range can also be suitably employed.

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 preferable that the ratio of ΔD(174) to ΔD(130) (ΔD(174)/ΔD(130)) is 1.00 or more and 1.20 or less. From the above viewpoint, it is more preferable that the ratio of ΔD(174) to ΔD(130) (ΔD(174)/ΔD(130)) is 1.00 or more and 1.18 or less.

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, particularly at high temperatures and high withstand voltages. Furthermore, as the applied voltage in the voltage application test 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.

To keep ΔD(174)/ΔD(130) at 1.20 or less or within the preferred range described above, the same methods as those for adjusting ΔD(174) and ΔD(130) described above can be used.

From the viewpoint of achieving both mechanical strength and improved life when used in a capacitor, 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 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 capacitor dielectric, and as a result, it can be used favorably as a capacitor dielectric.

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 to improve adhesion to metals during metal vapor deposition. Polypropylene films typically have low surface energy, with a surface wetting 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 reduce the surface wetting 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.

Next, we will explain the polypropylene resin used in the biaxially oriented polypropylene film of the present invention. The biaxially oriented polypropylene film of the present invention preferably contains linear polypropylene resin A as the main component and branched polypropylene resin H.

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 (mmmm) of 96.0% to 99.5%, a melt flow index (MFR) of 0.5 g/10 min to 3.0 g/10 min, and a melt tension (MS) of 0.1 g to 1.2 g. Resins that can be suitably used as linear polypropylene resin A include, for example, "Borclean" (trademark) (e.g., HB300BF) manufactured by Borealis.

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

When measured in accordance with JIS K 7210-1 (2014), the melt flow rate (MFR) of linear polypropylene resin A is preferably 0.5 g/10 min or more and 3.0 g/10 min or less, more preferably 1.0 g/10 min or more and 2.9 g/10 min or less, and even more preferably 1.5 g/10 min or more and 2.8 g/10 min or less. 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, it is preferable to appropriately select the type and composition of the catalyst and the polymerization temperature, or to polymerize in the presence of hydrogen.

The melt tension (MS) of linear polypropylene resin A is preferably 1.0 g or less. When the MS of 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 achieve an MS of the polypropylene resin within the above range, it is preferable to appropriately select the type and composition of the catalyst and the polymerization temperature, or to polymerize in the presence of hydrogen.

In the biaxially oriented polypropylene film of the present invention, the content of linear polypropylene resin A, when the total amount of polypropylene resin constituting the film is taken as 100% by mass, is preferably more than 90% by mass but not more than 99% by mass, more preferably 92% by mass or more but not more than 98% by mass, and even more preferably 93% by mass or more but not more than 97% 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.

Next, we will explain branched polypropylene resin H. 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 or more and 13.0 g/10 min or less and an MS of 1.5 g or more and 8.0 g or less. To obtain such branched polypropylene resin, preferred methods include using high-energy ionizing radiation on polypropylene resin (e.g., JP 62-121704 A), reacting polypropylene resin with specific organic peroxides (e.g., JP 2869606 A), reacting polypropylene resin with a thermally decomposable radical former and an ethylenically polyfunctional unsaturated monomer (e.g., JP 10-330436 A), and using a specific catalyst during polymerization of polypropylene resin (e.g., JP 2009-057542 A). Examples of resins that can be suitably used as branched polypropylene resin H include "WAYMAX" (registered trademark) (EX4000, MFX3, etc.) manufactured by Japan Polypropylene Corporation.

Branched polypropylene resin H has a branched structure in the molecular chain. Branched polypropylene resin is a polypropylene resin having five or fewer internal tri-substituted olefins per 10,000 carbon atoms, and the presence of this internal tri-substituted olefin can be confirmed by the proton ratio in the ¹ H-NMR spectrum. Branched polypropylene resin acts as an α-crystal nucleating agent, and can also form a rough surface due to the crystalline form if added in a certain amount. In other words, the size of the polypropylene spherulites generated during the cooling process of the melt-extruded resin sheet can be controlled to be small, and a biaxially oriented polypropylene film with excellent voltage resistance can be obtained.

The MFR of branched polypropylene resin H is preferably 6.0 g/10 min or more and 13.0 g/10 min or less, more preferably 6.5 g/10 min or more and 12.5 g/10 min or less, and even more preferably 7.0 g/10 min or more and 12.0 g/10 min or less. 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 and 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, it is preferable to appropriately select the type and composition of the catalyst and the polymerization temperature, or to polymerize in the presence of hydrogen.

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 and making it easier to achieve the desired dielectric properties (ΔD(174), ΔD(130)). To achieve MFR(H)-MFR(A) within the above range, simply select a combination of linear polypropylene resin A and branched polypropylene resin H so that MFR(H)-MFR(A) falls within the above range.

The MS of branched 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 polypropylene resin H is within the above range, the polypropylene crystals become more dense and the higher-order structure becomes more homogenous, stabilizing the higher-order structure of the film and making it easier to achieve the dielectric characteristics (ΔD(174), ΔD(130)) of the present application.

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 more compact and the higher-order structure becomes more homogenous, stabilizing the higher-order structure of the film and making it easier to achieve the dielectric characteristics (ΔD(174), ΔD(130)) of the present application.

The biaxially oriented polypropylene film of the present invention may also preferably contain various additives, such as nucleating agents, antioxidants, heat stabilizers, lubricants, antistatic agents, antiblocking agents, fillers, viscosity modifiers, and color inhibitors, as long as the additives do not impair the objectives of the present invention.

Among the additives listed above, the type and amount of antioxidant selected is important from the perspective of long-term heat resistance. Specifically, antioxidants should be sterically hindered phenolic compounds, with at least one of these being a high molecular weight compound with a molecular weight of 500 or greater. Specifically, 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), and tetrakis[methylene-3(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane (e.g., BASF's "Irganox"® 1010: molecular weight 1177.7), used alone or in combination, are preferred. 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, relative to the total amount of polypropylene resin. When the antioxidant content in the polypropylene resin composition is 0.03 parts by mass or more, it is easy to obtain the antioxidant effect and maintain long-term heat resistance. On the other hand, when the antioxidant content in the polypropylene resin composition is 1.0 part by mass or less, it is easy to maintain high-temperature voltage resistance characteristics.

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 it. 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 viewpoints of film formation stability and thickness uniformity. It is particularly preferred to stretch the film in the longitudinal direction and then in the width direction.

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

First, the linear polypropylene resin A and branched polypropylene resin H described above 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 or higher and 260°C or lower. 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.

From the perspective of properly generating β crystals and spherulites, 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. Setting the cast drum temperature to 70°C or higher prevents excessive formation of β crystals in the cast sheet and maintains the slipperiness of the film obtained after biaxial stretching, thereby preventing the occurrence of conveyance wrinkles during the film conveyance process during film production and processing and deterioration of the wound shape of the film roll. On the other hand, setting the cast drum temperature to 110°C or lower prevents excessive formation of β crystals in the cast sheet and makes it easier to prevent the occurrence of meandering during the film conveyance process during film production and processing and deterioration of the wound shape of the film roll.

It is preferable that the time that the molten sheet discharged from the T-die lands on the casting drum and remains in close contact with the drum is 1.0 second or more and 3.0 seconds or less. A close contact time of 1.0 second or more makes it easier to solidify 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 makes it possible to prevent excessive formation of β crystals in the cast sheet, making it easier to prevent meandering during the film transport process during film production and processing, and deterioration of the winding shape of the film roll.

Methods that can be used to adhere the molten sheet to the casting drum include electrostatic application, air knife, nip roll, and underwater casting, but the air knife method is preferred from the standpoints of suppressing thickness unevenness, enabling high-speed film production, and controlling the surface properties of the film.

Next, in the longitudinal stretching process, 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 using the difference in peripheral speed between the rolls. The temperature of the longitudinal stretching rolls is preferably 135°C or higher and 155°C or lower. Setting the temperature of the longitudinal stretching rolls to 135°C or higher prevents insufficient heat during longitudinal stretching and makes it easier to reduce film breakage during the longitudinal stretching process. On the other hand, setting the temperature of the longitudinal stretching rolls to 155°C or lower enhances the orientation of the uniaxially oriented film, making it easier to improve the voltage resistance of the biaxially oriented polypropylene film obtained after biaxial stretching.

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

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 process. Specifically, it is preferable to subject the uniaxially oriented film immediately after the longitudinal stretching process to a quenching treatment by passing it through temperature-controlled conveying rolls while nipping it with temperature-controlled nip rolls.

The temperature of the transport rolls during the cooling process is preferably 5°C or higher and 15°C or lower, and more preferably 5°C or higher and 13°C or lower. By maintaining a transport roll temperature of 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 maintaining a transport roll temperature of 15°C or lower, it is possible to prevent the orientation of the film from becoming too low and to easily form 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.

The nip roll temperature during the cooling process 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, setting the nip roll temperature to 15°C or lower prevents the film from becoming less oriented, making it easier to form a stable higher-order structure that is less susceptible 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.

The nip roll pressure during the cooling process 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 process are suppressed, making it easier to reduce breakage during the longitudinal stretching process 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 during the stretching process and ensure mass productivity.

The cooling time is preferably 0.1 seconds or more and 1.0 seconds or less, more preferably 0.2 seconds or more and 0.9 seconds or less, and even more preferably 0.3 seconds or more and 0.8 seconds or less. Setting the cooling time to 0.1 seconds or more prevents the film from becoming less oriented, and makes it easier to form 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, setting the cooling time to 1.0 second or less suppresses the occurrence of wrinkles and film meandering due to film shrinkage during the cooling process, thereby reducing tearing during the longitudinal stretching process and making it easier to ensure mass productivity.

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

Next, the biaxially oriented polypropylene film is subjected to a corona discharge treatment in air, nitrogen, carbon dioxide, or a mixture of these gases, after which both widthwise ends held by the clips are cut and removed, and the film is then 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, yielding a roll of biaxially oriented polypropylene film of the present invention.

The biaxially oriented polypropylene film of the present invention is preferably used as a dielectric for capacitors, but is not limited to the type of capacitor. Specifically, from the perspective of electrode configuration, it may be either a foil-wound capacitor or a metal-vapor-deposited film capacitor, and is also preferably used in oil-immersed capacitors that contain insulating oil, or dry capacitors that do not use insulating oil at all. Furthermore, from the perspective of shape, it may be either 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.

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 the internal electrode of 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.

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 protection, automobiles (hybrid cars, power windows, wipers, etc.), and power supplies, and film capacitors using the biaxially oriented polypropylene film of the present invention can also be suitably used in these applications.

The present invention will be explained in detail below using examples. The properties were measured and evaluated using the following methods.

[Evaluation method for each characteristic]
(1) Mesopentad fraction (mmmm)
A polypropylene resin sample was dissolved in a solvent, and the mesopentad fraction (mmmm) was determined using ¹³ C-NMR under the following conditions (reference: New Edition Polymer Analysis Handbook, edited by the Japan Society for Analytical Chemistry and Polymer Analysis Research Forum, 1995, pp. 609-611).
A. Measurement conditions Apparatus: Bruker DRX-500
Measurement nucleus: ^13C nucleus (resonance frequency: 125.8MHz)
Measured concentration: 10wt%
Solvent: benzene/d-o-dichlorobenzene = mass ratio 1:3 mixed solution Measurement temperature: 130°C
Spin speed: 12Hz
NMR sample tube: 5 mm 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 LB (line broadening factor) set to 1.0, and the mmmm peak was determined to be 21.86 ppm. Peak division was performed using WINFIT software (manufactured by Bruker). Peak division 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 division, 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) mmr
(j) mmmm.

(2) Melt flow rate (MFR) (unit: g/10 min)
Measurement was carried out in accordance with JIS K 7210-1 (2014) at a temperature of 230°C and a load of 2.16 kg.

(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, Ltd., 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.

(4) Rate of change in dielectric loss 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.

A. Measurement of dielectric loss tangent (tan δ(0)) before voltage application test Dielectric relaxation measurement was performed on a sample taken from the film roll at a temperature of 25°C under the following conditions using a Precision LCR meter E4980A manufactured by Agilent Technologies, Inc., and the dielectric loss tangent (tan δ = 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 (tan δ(0)) before the voltage application test.
Applied voltage: 2V
Frequency: 20Hz to 2MHz
Measurement mode: Cp-D
Time: Long
Average: 16.

B. Voltage Application Test at 130 V/μm The sample whose dielectric loss tangent (tan δ(0)) had been measured before the voltage application test was set in a HIOKI Corporation plate sample electrode SEM-8310 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.

C. Measurement of dielectric loss tangent (tan δ(130)) after voltage application test at 130 V/μm Dielectric relaxation measurement was performed on the sample after the voltage application test at 130 V/μm using a Precision LCR meter E4980A manufactured by Agilent Technologies Inc. at a temperature of 25° C. under the conditions described above, and the dielectric loss tangent (tan δ=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 (tan δ(130)) after the voltage application test at 130 V/μm.

D. Voltage Application Test at 174 V/μm The sample for which the dielectric loss tangent (tan δ(1 30 )) was measured after a voltage application test at 1 30 V/μm was set in a HIOKI SEM-8310 plate sample electrode connected to an Element LLC DC high-voltage power supply ELS19-10K05B1, and a voltage was applied at 25°C and a DC voltage of 174 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 400 V.

E. Measurement of Dielectric Loss Tangent (tan δ(174)) After Voltage Application Test at 174 V/μm For the sample after the voltage application test at 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 (tan δ=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 (tan δ(174)) after the voltage application test at 174 V/μm.

F. Calculation of the rate of change in dielectric loss tangent (ΔD) before and after voltage application test ΔD was calculated by the following formulas (1) and (2) using tan δ (0), tan δ (130), and tan δ (174) obtained in A, C, and E above.
ΔD (174) = tan δ (174)/tan δ (0) ... (1)
ΔD(130)=tanδ(130)/tanδ(0) (2).

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

(6) Wet tension (unit: mN/m)
In accordance with JIS K 6768 (1999), the wetting tension of the corona-treated surface of a biaxially oriented polypropylene film was measured using a test mixture of ethylene glycol monoethyl ether, formamide, and methanol.

(7) Element Processability in Capacitor Manufacturing Aluminum was vacuum-deposited onto the corona-treated side of the biaxially oriented polypropylene film using a vacuum deposition machine manufactured by ULVAC Corporation to a density of 15 Ω/sq. Aluminum was deposited in stripes with longitudinal margins (79.0 mm wide deposited strips, 1.0 mm wide margins, repeated). Next, a blade was inserted into the center of each deposited strip and the center of each margin to create a tape-like take-up reel with a total width of 40 mm and a 0.5 mm margin on either the left or right end. 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 manufactured by Kaito Seisakusho Co., Ltd. Finally, the reel was heat-treated for 10 hours in a reduced-pressure atmosphere at 130°C to obtain a capacitor element.

(8) Capacitor Life Test: Aluminum was vacuum-deposited onto the corona-treated side of biaxially oriented polypropylene film using a vacuum deposition machine manufactured by ULVAC Co., Ltd. to a capacitance of 15 Ω/sq. Aluminum was deposited in stripes with longitudinal margins (79.0 mm wide deposited strips, 1.0 mm wide margins, repeated). Next, a blade was used to slit the center of each deposited strip and the center of each margin, producing tape-like take-up reels with a total width of 40 mm and 0.5 mm margins on either the left or right end. Two of the resulting reels, one from each of the left and right margins, were overlapped and wound together so that the deposited portion extended 0.5 mm beyond the margin in the width direction, yielding a wound assembly with a capacitance of 120 μF. A KAW-4NHB manufactured by Kaito Seisakusho Co., Ltd., was used for winding the elements. Finally, the capacitor was heat-treated for 10 hours in a reduced-pressure atmosphere at 130°C. Metallicon was sprayed onto both end faces in the width direction to form external electrodes, and lead wires were welded to the metallicon to obtain capacitor elements. The capacitor characteristics of 10 capacitor elements were then 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. Thereafter, 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 of 10 capacitor elements before and after the application of voltage was taken as the rate of change for that sample, and the sample was evaluated according to the following criteria. The evaluation was conducted in the order of excellent, good, fair, △, and poor.
◎: ΔC is less than 2%. ○: ΔC is 2% or more but less than 3%. △: ΔC is 3% or more but less than 5%. ×: ΔC is 5% or more.

[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 a mmmm content of 98%, an MFR of 2.5 g/10 min, and an 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 having an MFR of 2.5 g/10 min and a melt tension MS of 32 g ("Daploy" (trademark) WB135HMS manufactured by Borealis).

Example 1
Linear polypropylene resin A1 and branched polypropylene resin H1 were dry blended at a ratio of 95:5 (by mass) and fed to a single-screw melt extruder. Furthermore, as antioxidants, 0.4 parts by mass of "Irganox" (registered trademark) 1010 manufactured by BASF Japan Ltd., and 0.1 parts by mass of 2,6-di-t-butyl-p-cresol (BHT) were added, based on 100 parts by mass of the total polypropylene resin. In the single-screw melt extruder, the polypropylene resin mixture was melted at a temperature of 250 ° C., and foreign matter was removed using a sintered filter with a 25 μm cutoff. Next, the molten polypropylene resin mixture was extruded into a sheet shape through a T-slit die and solidified by adhering it to a cast drum maintained at a temperature of 90 ° C. using an air knife. The time during which the cast drum and the molten sheet were in close contact with each other was 1.5 seconds. Here, the surface that contacts the cast drum was designated the drum surface (D surface), and the surface that does not contact the cast drum was designated the non-drum surface (non-D surface). The cast sheet was then stretched in the longitudinal direction at a stretch ratio of 6.0 times using a longitudinal stretching roll at a temperature of 145 ° C. The uniaxially oriented film thus obtained was placed on a conveying roll at a temperature of 10 ° C. and quenched for 0.5 seconds while being nipped with a nip roll at a temperature of 10 ° C. at a pressure of 0.45 MPa. The obtained longitudinally stretched sheet was then gripped at both widthwise ends with clips and introduced into a tenter, stretched in the widthwise direction at a stretch ratio of 11 times at 160 ° C., and further relaxed in the widthwise direction at a temperature of 158 ° C. by 12%. The film was then gradually cooled to room temperature, and the drum surface (D surface) of the film was subjected to a corona discharge treatment at a treatment intensity of 25 W min / m ^2. The edge portions of the film gripped with the clips were cut and removed, and the film with the removed edges was wound up on a winder. The film was then slit using a slitter to a width of 0.82 m, and 30,000 m of the film was wound around a core in the longitudinal direction as a film roll to obtain a biaxially oriented polypropylene film with a thickness of 2.3 μm. The evaluation results are shown in Table 1.

(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.

For the polypropylene resin composition, values are listed assuming the total polypropylene resin as 100% by mass.

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 (3)

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. 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) = tan δ (130)/tan δ (0) ... (2) 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.