JP2024127627A - Biaxially oriented polypropylene film - Google Patents

Abstract

[Problem] To provide a biaxially stretched polypropylene film in which antioxidant bleed-out and blocking are suppressed and which has excellent long-term storage properties. [Solution] A biaxially stretched polypropylene film containing a secondary antioxidant A, characterized in that the secondary antioxidant A is a hindered phenol-based antioxidant having a carbonyl group, and the content of the secondary antioxidant A is 0.150 parts by mass or more and 0.300 parts by mass or less per 100 parts by mass of the resin component in the biaxially stretched polypropylene film. [Selected Figure] None

Description

The present invention relates to a biaxially oriented polypropylene film.

Biaxially oriented polypropylene film has excellent electrical properties, such as high voltage resistance and low dielectric loss, as well as high moisture resistance. Taking advantage of these properties, it is favorably used in electronic and electrical devices, for example as a dielectric film for capacitors such as high-voltage capacitors, various switching power supplies, filter capacitors for converters and inverters, and smoothing capacitors.

When biaxially oriented polypropylene film is used for applications such as capacitors, an antioxidant is added to improve long-term storage stability. For example, Patent Document 1 discloses that biaxially oriented polypropylene film for capacitors contains a specific antioxidant in an amount of 4000 ppm or more and 6000 ppm or less.

JP 2011-148896 A

However, biaxially oriented polypropylene film has the problem that it has poor long-term storage properties because blocking can occur due to the antioxidant bleeding out to the surface when stored for a long period of time.

In addition, if the amount of antioxidant that bleeds out onto the surface of the biaxially oriented polypropylene film is large, there is a problem that the voltage resistance of the biaxially oriented polypropylene film decreases.

Furthermore, when biaxially oriented polypropylene film is processed into a metallized film to manufacture a capacitor element, blocking between the metallized films is likely to occur, which can lead to problems such as reduced capacitor performance, including breakdown voltage and self-healing performance.

The present invention aims to provide a biaxially oriented polypropylene film that suppresses the bleeding out of antioxidants, suppresses blocking, and has excellent long-term storage properties.

As a result of extensive research aimed at solving the above problems, the inventors discovered that the above objective could be achieved by using a biaxially oriented polypropylene film containing a specific amount of a secondary antioxidant, and thus completed the present invention.

That is, the present invention relates to the following biaxially oriented polypropylene film.
1. A biaxially oriented polypropylene film containing a secondary antioxidant A,
The secondary antioxidant A is a hindered phenol-based antioxidant having a carbonyl group,
The content of the secondary antioxidant A is 0.15 parts by mass or more and 0.30 parts by mass or less based on 100 parts by mass of the resin component in the biaxially oriented polypropylene film.
A biaxially oriented polypropylene film.
2. The biaxially stretched polypropylene film according to item 1, wherein the secondary antioxidant A is pentaerythritol tetrakis[3-(3,5-di-tertiarybutyl-4-hydroxyphenyl)propionate].
3. The biaxially stretched polypropylene film according to item 1 or 2, wherein the biaxially stretched polypropylene film is a biaxially stretched polypropylene film derived from a polypropylene resin composition containing a polypropylene resin and the secondary antioxidant A, and the content of the secondary antioxidant A in the polypropylene resin composition is 0.20 parts by mass or more and 0.50 parts by mass or less with respect to 100 parts by mass of the polypropylene resin.
4. The biaxially stretched polypropylene film according to item 3, wherein the polypropylene resin composition further contains a primary antioxidant, and the content of the primary antioxidant in the polypropylene resin composition is 0.10 parts by mass or more and 0.20 parts by mass or less with respect to 100 parts by mass of the polypropylene resin.
5. The biaxially oriented polypropylene film according to item 4, wherein the primary antioxidant is 2,6-di-tert-butyl-p-cresol.
6. The biaxially stretched polypropylene film according to any one of items 1 to 5, wherein the value of the antioxidant peak intensity (m/z: 219) on the surface when subjected to vacuum heat aging treatment divided by the total ion intensity is 0.5 to 0.9.
7. The biaxially stretched polypropylene film according to any one of items 1 to 6, wherein the rate of change in dielectric breakdown strength (βVdc) after vacuum heat aging relative to the dielectric breakdown strength (αVdc) before vacuum heat aging, calculated by the following formula, is −10% to +10%, and the dielectric breakdown strength per thickness after vacuum heat aging is 540 Vdc or more.
[Rate of change (%)] = ((αVdc - βVdc) / αVdc) × 100
8. A metallized film for a capacitor, comprising the biaxially oriented polypropylene film according to any one of items 1 to 7 and a metal layer on one or both sides thereof.
9. A capacitor comprising the metallized film for a capacitor according to item 8.

The biaxially oriented polypropylene film of the present invention has excellent long-term storage properties, as it suppresses the bleed-out of antioxidants and blocks. Therefore, when a capacitor element is produced using the biaxially oriented polypropylene film of the present invention, blocking between metallized films is suppressed, and the dielectric breakdown voltage and self-healing performance are improved, so that the capacitor element can exhibit excellent capacitor performance.

In this specification, the expressions "contain" and "include" include the concepts of "contain," "include," "consist essentially of," and "consist only of."

In this specification, the term "capacitor" includes the concepts of "capacitor," "capacitor element," and "film capacitor."

1. Biaxially oriented polypropylene film The biaxially oriented polypropylene film of the present invention is a biaxially oriented polypropylene film containing a secondary antioxidant A, wherein the secondary antioxidant A is a hindered phenol-based antioxidant having a carbonyl group, and the content of the secondary antioxidant A is 0.15 parts by mass or more and 0.30 parts by mass or less per 100 parts by mass of the resin component in the biaxially oriented polypropylene film.

The biaxially stretched polypropylene film of the present invention having the above characteristics contains a hindered phenol-based antioxidant having a carbonyl group as the secondary antioxidant A, so that bleed-out and blocking are suppressed, and the film has excellent long-term storage properties. In addition, the biaxially stretched polypropylene film of the present invention can exhibit excellent long-term storage properties because the lower limit of the content of the secondary antioxidant A is 0.15 parts by mass or more per 100 parts by mass of the resin component in the biaxially stretched polypropylene film. Furthermore, the biaxially stretched polypropylene film of the present invention can exhibit excellent long-term storage properties because the upper limit of the content of the secondary antioxidant A is 0.30 parts by mass or less per 100 parts by mass of the resin component in the biaxially stretched polypropylene film, so that bleed-out of the antioxidant is suppressed and blocking is suppressed. That is, the biaxially stretched polypropylene film of the present invention contains a specific secondary antioxidant A at a specific content, so that bleed-out of the antioxidant is suppressed and blocking is suppressed, and the film can exhibit excellent long-term storage properties.

When a capacitor element is produced using the biaxially oriented polypropylene film of the present invention exhibiting the above characteristics, blocking between metallized films is suppressed, and the breakdown voltage and self-healing performance are improved, so that the capacitor element can exhibit excellent capacitor performance. The self-healing performance is the performance that shows the insulation recovery (self-repairing ability) when insulation breakdown occurs inside the capacitor element. That is, when an insulation defect such as a conductive foreign body or a pinhole exists in a biaxially oriented polypropylene film, which is generally an insulator, applying a voltage to the capacitor element causes a current to concentrate at the location, and the vapor-deposited metal of the metallized film around the defect part is evaporated by the energy, instantly recovering the insulation and avoiding a short circuit. However, this requires a gap for the vapor-deposited metal to evaporate, and if blocking occurs between the metallized films and the films stick to each other, the evaporation of the vapor-deposited metal is hindered, making it difficult to electrically separate the insulation defect part (insulation recovery), and the self-healing performance is reduced (and in some cases, causing a short circuit).

Antioxidants are generally used for two purposes. One purpose is to suppress thermal and oxidative degradation in the extruder, and the other purpose is to contribute to suppressing degradation during long-term use as a capacitor film and improving capacitor performance. In this specification, antioxidants that suppress thermal and oxidative degradation in the extruder are referred to as "primary antioxidants," and antioxidants that contribute to improving capacitor performance are referred to as "secondary antioxidants."

The layer structure of the biaxially oriented polypropylene film of the present invention is not particularly limited, and may be formed of multiple layers or may be a single layer. The biaxially oriented polypropylene film of the present invention is preferably a single layer from the viewpoint that the metal oxide is uniformly distributed throughout the biaxially oriented polypropylene film and has excellent dielectric properties. Therefore, the biaxially oriented polypropylene film of the present invention consisting of a single layer can fully exert the effects of the present invention.

(Polypropylene resin)
The biaxially stretched polypropylene film of the present invention preferably contains a polypropylene resin as a main component. It is more preferable that the resin component constituting the biaxially stretched polypropylene film of the present invention is a polypropylene resin. The above-mentioned "main component" means that the biaxially stretched polypropylene film contains 50% by mass or more, preferably 70% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and particularly preferably 98% by mass or more, calculated as a solid content.

The polypropylene resin is not particularly limited, and any polypropylene resin that can be used to form a biaxially oriented polypropylene film can be widely used. Examples of polypropylene resins include propylene homopolymers such as isotactic polypropylene and syndiotactic polypropylene; long-chain branched polypropylene; and ultra-high molecular weight polypropylene. Of these, propylene homopolymers are preferred, and from the viewpoint of heat resistance, isotactic polypropylene is more preferred, and isotactic polypropylene obtained by homopolymerizing propylene in the presence of an olefin polymerization catalyst is even more preferred. The polypropylene resin may be one type alone or a blend of two or more types.

The biaxially stretched polypropylene film of the present invention can be formed from a polypropylene resin composition containing a polypropylene resin and a secondary antioxidant A (hereinafter, in this specification, also simply referred to as a "polypropylene resin composition") used in the manufacturing method described below. In other words, the biaxially stretched polypropylene film of the present invention can be a biaxially stretched polypropylene film derived from a polypropylene resin composition containing a polypropylene resin and a secondary antioxidant A.

The weight average molecular weight (Mw) of the polypropylene resin in the polypropylene resin composition is preferably 200,000 to 550,000, more preferably 200,000 to 500,000, even more preferably 200,000 to 450,000, and particularly preferably 250,000 to 400,000. When such a polypropylene resin is used, appropriate resin fluidity is obtained during biaxial stretching, making biaxial stretching easier. For example, it becomes easier to obtain a thin biaxially stretched polypropylene film suitable for a small and high-capacity capacitor. In addition, it is preferable because unevenness in the thickness of the biaxially stretched polypropylene film is suppressed. In addition, the weight average molecular weight (Mw) of the polypropylene resin in the polypropylene resin composition is more preferably 250,000 or more from the viewpoint of the uniformity of the thickness of the biaxially stretched polypropylene film, mechanical properties, thermo-mechanical properties, etc. The weight average molecular weight (Mw) of the polypropylene resin in the polypropylene resin composition is more preferably 400,000 or less from the viewpoint of the fluidity of the polypropylene resin and the stretchability when obtaining a thin biaxially stretched polypropylene film.

The number average molecular weight (Mn) of the polypropylene resin in the polypropylene resin composition is preferably 30,000 to 60,000, more preferably 40,000 to 60,000, and even more preferably 40,000 to 50,000. When such a polypropylene resin is used, appropriate resin fluidity is obtained during biaxial stretching, making biaxial stretching easier. For example, it becomes easier to obtain a thin biaxially stretched polypropylene film suitable for a small, high-capacity capacitor. In addition, this is preferable because unevenness in the thickness of the biaxially stretched polypropylene film is suppressed. In addition, the number average molecular weight (Mn) of the polypropylene resin in the polypropylene resin composition is more preferably 50,000 or less from the viewpoint of the fluidity of the polypropylene resin and the stretchability when obtaining a thin biaxially stretched polypropylene film.

The average molecular weight distribution (Mw/Mn) of the polypropylene resin in the polypropylene resin composition, calculated as the ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn), is preferably 4 to 12, more preferably 5.5 to 10, and even more preferably 6 to 8. When such a polypropylene resin is used, appropriate resin fluidity is obtained during biaxial stretching, making it easier to obtain a thin biaxially stretched polypropylene film with uniform thickness. In addition, such a polypropylene resin is also preferable from the viewpoint of further improving the dielectric breakdown strength of the biaxially stretched polypropylene film.

The weight average molecular weight (Mw), number average molecular weight (Mn), and molecular weight distribution (Mw/Mn) of the polypropylene resin in the polypropylene resin composition can be measured using a gel permeation chromatograph (GPC) device. More specifically, for example, a high-temperature GPC measuring device with built-in differential refractive index (RI) manufactured by Tosoh Corporation, HLC-8121GPC-HT (product name), can be used for measurement. As the GPC column, three TSKgel GMHHR-H (20) HT manufactured by Tosoh Corporation are connected and used. The column temperature is set to 140°C, and trichlorobenzene is passed as the eluent at a flow rate of 1.0 ml/10 min to obtain the measured values of Mw and Mn. A calibration curve for the molecular weight M is created using standard polystyrene manufactured by Tosoh Corporation, and the measured values are converted to polystyrene values to obtain Mw and Mn. Mw/Mn can be calculated from Mw and Mn.

(Secondary Antioxidant A)
The biaxially oriented polypropylene film of the present invention contains a secondary antioxidant A. In the present invention, the secondary antioxidant A is a hindered phenol-based antioxidant having a carbonyl group.

The secondary antioxidant A is not particularly limited as long as it is a hindered phenol-based antioxidant having a carbonyl group, and examples thereof include triethylene glycol-bis[3-(3-tertiary-butyl-5-methyl-4-hydroxyphenyl)propionate] (trade name: Irganox 245), 1,6-hexanediol-bis[3-(3,5-di-tertiary-butyl-4-hydroxyphenyl)propionate] (trade name: Irganox 259), pentaerythritol tetrakis[3-(3,5-di-tertiary-butyl 2,2-thio-diethylenebis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (trade name: Irganox 1010), 2,2-thio-diethylenebis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (trade name: Irganox 1035), N,N'-hexamethylenebis(3,5-di-tert-butyl-4-hydroxy-hydrocinnamamide) (trade name: Irganox 1098), octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate (trade name: Adekastab AO-50), and the like. Among these, from the viewpoints of high molecular weight, excellent compatibility with polypropylene, low volatility and excellent heat resistance, octadecyl-3-(3,5-di-tertiary-butyl-4-hydroxyphenyl)propionate and pentaerythritol tetrakis[3-(3,5-di-tertiary-butyl-4-hydroxyphenyl)propionate] are preferred, with pentaerythritol tetrakis[3-(3,5-di-tertiary-butyl-4-hydroxyphenyl)propionate] being more preferred.

The content of the secondary antioxidant A in the biaxially stretched polypropylene film is 0.150 parts by mass or more and 0.300 parts by mass or less per 100 parts by mass of the resin component in the biaxially stretched polypropylene film. If the content is less than 0.150 parts by mass, the long-term storage stability of the biaxially stretched polypropylene film is reduced. If the content is more than 0.300 parts by mass, the bleed-out of the antioxidant cannot be sufficiently suppressed. The content of the secondary antioxidant A is preferably 0.160 parts by mass or more and 0.290 parts by mass or less, more preferably 0.180 parts by mass or more and 0.270 parts by mass or less, even more preferably 0.200 parts by mass or more and 0.250 parts by mass or less, and particularly preferably 0.200 parts by mass or more and 0.240 parts by mass or less. The method for measuring the content of the secondary antioxidant A in the biaxially stretched polypropylene film is the method described in the Examples.

The content of secondary antioxidant A in the polypropylene resin composition before curing is preferably 0.200 parts by mass or more and 0.500 parts by mass or less, and more preferably 0.250 parts by mass or more and 4000 parts by mass or less, per 100 parts by mass of polypropylene resin. By setting the content of secondary antioxidant A in the polypropylene resin composition within the above range, it becomes easier to adjust the content of secondary antioxidant A in the biaxially oriented polypropylene film formed by curing the polypropylene resin composition to 0.150 parts by mass or more and 0.300 parts by mass or less.

(Primary Antioxidant)
The biaxially oriented polypropylene film of the present invention may contain a primary antioxidant, which further suppresses thermal and oxidative degradation of the polypropylene resin composition in an extruder for producing the biaxially oriented polypropylene film.

As a primary antioxidant, it is preferable to use, for example, 2,6-di-tert-butyl-p-cresol (BHT) to suppress deterioration of the polypropylene resin product in the extruder.

The content of the primary antioxidant in the polypropylene resin composition is not particularly limited. The content of the primary antioxidant in the polypropylene resin composition before curing is preferably about 0.10 parts by mass or more and 0.20 parts by mass or less per 100 parts by mass of polypropylene resin. Most of the primary antioxidant is consumed in the molding process in the extruder, and almost none remains in the film after film formation, so the content (residual amount) of the primary antioxidant in the biaxially oriented polypropylene film of the present invention is about 100 ppm or less.

(Additives)
The biaxially oriented polypropylene film of the present invention may further contain additives, such as chlorine absorbers, ultraviolet absorbers, lubricants, plasticizers, flame retardants, antistatic agents, and colorants.

(Characteristics of biaxially oriented polypropylene film)
In the biaxially stretched polypropylene film of the present invention, the value obtained by dividing the antioxidant peak intensity (m/z: 219) on the surface by the total ion intensity when subjected to vacuum heat aging is preferably 0.5 to 1.0, more preferably 0.5 to 0.9, and even more preferably 0.5 to 0.8. When the lower limit of the above value is within the above range, the long-term storage stability of the biaxially stretched polypropylene film is further improved. When the upper limit of the above value is within the above range, the bleed-out of the antioxidant in the biaxially stretched polypropylene film is further suppressed, and blocking is further suppressed. The method for measuring the above value in this specification is the method described in the Examples.

In the biaxially stretched polypropylene film of the present invention, the rate of change of the dielectric breakdown strength (βVdc) after the vacuum heat aging treatment relative to the dielectric breakdown strength (αVdc) before the vacuum heat aging treatment calculated by the following formula is preferably -10% to +10%, more preferably -6% to +10%. By having the rate of change in the above range, the decrease in dielectric breakdown strength due to the heat load during the processing (thermal aging) of the biaxially stretched polypropylene film into a capacitor element can be more suppressed, so that even after the processing of the capacitor element, the dielectric breakdown withstand voltage of the biaxially stretched polypropylene film is more easily guaranteed, and the dielectric breakdown withstand voltage performance is more easily maintained. Note that the dielectric breakdown strength in this specification is determined by the method described in the Examples.
[Rate of change (%)] = ((αVdc - βVdc) / αVdc) × 100

In the biaxially oriented polypropylene film of the present invention, the dielectric breakdown strength per thickness after vacuum heat aging is preferably 540 Vdc or more, more preferably 560 Vdc or more. When the lower limit of the dielectric breakdown strength is in the above range, the dielectric breakdown voltage of the capacitor manufactured using the biaxially oriented polypropylene film is further improved. In addition, the upper limit of the dielectric breakdown strength is not particularly limited, and is about 580 Vdc. In this specification, the dielectric breakdown strength per thickness after vacuum heat aging is measured by the method described in the Examples.

The thickness of the biaxially oriented polypropylene film is preferably 1 to 20 μm, more preferably 2 to 10 μm, and even more preferably 3 to 8 μm. When the thickness is within the above range, breakage of the biaxially oriented polypropylene film is further suppressed, and the capacitance of the capacitor element is further improved. Note that the method for measuring the thickness of the biaxially oriented polypropylene film in this specification is the method described in the Examples.

The biaxially oriented polypropylene film of the present invention described above has a high dielectric constant, is less likely to break, and has excellent dielectric breakdown strength. Therefore, the biaxially oriented polypropylene film of the present invention can be suitably used as a biaxially oriented polypropylene film for capacitors.

2. Manufacturing Method of Biaxially Stretched Polypropylene Film The manufacturing method of the biaxially oriented polypropylene film of the present invention is not particularly limited, and for example, the biaxially oriented polypropylene film of the present invention can be manufactured by a manufacturing method including a step of melting a polypropylene resin composition containing at least a polypropylene resin and a secondary antioxidant A at a temperature of preferably 225° C. or more and 270° C. or less.

There are no particular limitations on the method for preparing the polypropylene resin composition, but examples include a method in which polymer powder or pellets of polypropylene resin are mixed with a secondary antioxidant and other additives added as necessary, and then dry-blended using a mixer or the like, or a method in which the mixture is fed into a kneader and melt-kneaded.

The above mixer and kneader are not particularly limited. The above kneader may be a single screw type, a twin screw type, or a multi-screw type having three or more screws. In the case of a twin or more screw type, the screws may rotate in the same direction or in opposite directions.

In the case of melt kneading, the kneading temperature is not particularly limited as long as a good kneaded product is obtained. In general, the temperature is in the range of 200°C to 300°C, and from the viewpoint of suppressing deterioration of the resin, 230°C to 270°C is preferable. In addition, in order to suppress deterioration during kneading and mixing of the resin, an inert gas such as nitrogen may be purged into the kneader. The melt kneaded resin may be pelletized to an appropriate size using a commonly known granulator. This allows the preparation of polypropylene resin pellets.

In the above-mentioned method for producing biaxially oriented polypropylene film, polypropylene resin pellets, dry-mixed polypropylene resin pellets, or mixed polypropylene resin pellets prepared in advance by melt kneading are first fed into an extruder and heated to melt.

The polypropylene resin composition is preferably melted at 170°C or higher and 320°C or lower. Specifically, the extruder temperature setting during heating and melting the polypropylene resin composition is preferably 225°C or higher and 270°C or lower. This further improves the dielectric breakdown strength of the biaxially oriented polypropylene film produced.

Next, the molten polypropylene resin composition is extruded into a sheet using a T-die, and cooled and solidified on at least one metal drum to form an unstretched cast sheet. The surface temperature of the metal drum (the temperature of the metal drum that first comes into contact after extrusion) is preferably 10°C or higher and 105°C or lower, and more preferably 15°C or higher and 100°C or lower. The surface temperature of the metal drum can be determined depending on the physical properties of the polypropylene resin used, etc.

The thickness of the cast sheet is not particularly limited, but is preferably 50 μm or more and 2000 μm or less, and more preferably 100 μm or more and 1000 μm or less.

The biaxially stretched polypropylene film can be produced by subjecting the cast sheet to a biaxial stretching process. The stretching is preferably biaxial stretching in which the sheet is oriented biaxially in the longitudinal and transverse directions, and the stretching method is preferably a sequential biaxial stretching method. In the sequential biaxial stretching method, for example, the cast sheet is first kept at a temperature of 100°C to 180°C (preferably 120°C to 170°C) and stretched in the flow direction by passing it between rolls with a speed difference. The stretching ratio in the flow direction is preferably 3.0 times to 7.0 times, more preferably 4.0 times to 6.0 times. The sheet is then guided to a tenter and stretched in the transverse direction. The temperature during transverse stretching is preferably 160°C to 180°C, and the stretching ratio in the transverse direction is preferably 3 times to 11 times. Next, after the transverse stretching, the sheet is relaxed, heat-set, and wound up.

The manufacturing method described above allows the production of biaxially oriented polypropylene film.

The biaxially stretched polypropylene film may be subjected to corona discharge treatment online or offline after the stretching and heat setting processes are completed, in order to further improve the adhesive properties in subsequent processes such as the metal deposition processing process. The corona discharge treatment can be performed using a known method. It is preferable to use air, carbon dioxide gas, nitrogen gas, or a mixture of these gases as the atmospheric gas.

The biaxially oriented polypropylene film of the present invention can be produced by the manufacturing method described above.

3. Metallized film, capacitor, and manufacturing method thereof The metallized film of the present invention is a metallized film having a metal layer on one or both sides of the biaxially oriented polypropylene film described above. The metallized film having such a configuration can be usefully used as a metallized film for a capacitor.

The metal layer functions as an electrode. Examples of metals that can be used for the metal layer include simple metals such as zinc, lead, silver, chromium, aluminum, copper, and nickel, mixtures of multiple types of these, and alloys of these. Among these, zinc and aluminum are preferred in terms of their environmental impact, economic efficiency, and excellent capacitor performance.

The method for laminating a metal layer on at least one side (one side or both sides) of the biaxially oriented polypropylene film is not particularly limited, and examples thereof include vacuum deposition and sputtering. From the viewpoint of excellent productivity and economic efficiency, vacuum deposition is preferred. Examples of vacuum deposition methods include the crucible method and the wire method, and the most suitable method can be selected as appropriate.

The margin pattern used when laminating the metal layer by vapor deposition is not particularly limited, but from the viewpoint of further improving the safety of the capacitor and further suppressing damage and short circuits of the capacitor, it is preferable to apply a pattern including a so-called special margin, such as a fishnet pattern or a T-margin pattern, to one side of the biaxially oriented polypropylene film.

There are no particular limitations on the method for forming the margin, and it may be formed by any known method such as the tape method or oil method.

The thickness of the metallized film of the present invention is not particularly limited, but is preferably 1.8 μm or more and 6.0 μm or less, more preferably 2.0 μm or more and 5.0 μm or less, and even more preferably 2.3 μm or more and 2.8 μm or less.

The capacitor of the present invention is a capacitor that includes the metallized film for capacitors. The metallized film of the present invention can be laminated or rolled by a conventionally known method to form a film capacitor.

The film capacitor may have a structure in which multiple metallized films are laminated, or may have a wound metallized film. Such a film capacitor can be suitably used as a capacitor for inverter power devices that control the drive motors of electric vehicles, hybrid vehicles, etc. It can also be suitably used in applications such as railway vehicles, wind power generation, solar power generation, and general home appliances.

The present invention will be explained in more detail with reference to examples. However, these examples are for the purpose of explaining the present invention and are not intended to limit the present invention in any way.

The raw materials used in the examples and comparative examples are as follows.
(Polypropylene resin)
Polypropylene resin A: Prime Polymer Co., Ltd. MFR 5g/10min
Polypropylene resin B: Prime Polymer Co., Ltd. MFR 4g/10min
(Primary Antioxidant)
BHT: 2,6-di-tertiary-butyl-para-cresol (secondary antioxidant)
Secondary antioxidant A: BASF Japan Ltd., product name Irganox 1010, a hindered phenol-based antioxidant having a carbonyl group, pentaerythritol tetrakis[3-(3,5-di-tertiarybutyl-4-hydroxyphenyl)propionate]
Secondary antioxidant B: BASF Japan Co., Ltd., product name Irganox 1330, a hindered phenol-based antioxidant not having a carbonyl group, 1,3,5-trimethyl-2,4,6-tris(3,5-di-tertiarybutyl-4-hydroxybenzyl)benzene

Production of biaxially oriented polypropylene film <Examples and Comparative Examples>
The above-mentioned raw materials were mixed in the amounts (parts by mass) shown in Table 1. Then, the mixture was fed to an extruder, melt-kneaded at a temperature of 250°C, extruded using a T-die, and wound around a cast roll whose surface was kept at 90°C at a take-up speed of 1.0 m/min to produce a cast raw sheet having a thickness of 300 μm. The cast raw sheet was then stretched 5 times in the machine direction at a temperature of 140°C, and stretched 10 times in the transverse direction at a temperature of 165°C to produce a biaxially stretched polypropylene film having a thickness of 6 μm.

For each example and comparative example, various characteristics were evaluated under the following measurement conditions.

Characterization of biaxially oriented polypropylene film (thickness)
Measurements were made using an outside micrometer (high-precision digital micrometer MDH-25MB manufactured by Mitutoyo Corporation) in accordance with JIS K 7130:199 Method A.

(Heat aging treatment)
One biaxially oriented polypropylene film produced in the Examples and Comparative Examples was sandwiched between aluminum foil and left in a vacuum thermostatic chamber for a certain period of time under the following thermal aging conditions, assuming device processing. Then, it was opened to the atmosphere and cooled for 1 hour to prepare a sample after thermal aging treatment.
[Heat aging conditions]
・Equipment: Espec vacuum thermostatic chamber (VAC-201)
-Heat aging temperature: 120℃
Vacuum conditions: Continuous evacuation from inside the chamber Aging time: 15 hours

(Antioxidant content (residual amount) of biaxially oriented polypropylene film)
The biaxially oriented polypropylene film was cut into pieces to prepare samples for extraction, which were then subjected to Soxhlet extraction with dichloromethane. The resulting extract was dissolved in methanol, and the content of secondary antioxidant A was quantitatively analyzed by high performance liquid chromatography (HPLC) under the conditions shown below.
[HPLC measurement conditions]
Measurement equipment: Waters Acquity UPLC-SQDetector2
Detector: Photodiode array detector Acquity PDA
Column: ACQUITY BEH Phenyl (length 100 mm, inner diameter 2.1 mm, particle size 1.7 μm)
Mobile phase: 100mmol/L ammonium bicarbonate (containing 2.5% ammonia)/acetonitrile/water = 3/45/52 (v/v/v) → 3/94/3 (v/v/v)
・Flow rate: 0.6 mL/min, injection volume: 2 μL, quantitative wavelength: 220 nm

(Amount of antioxidant bleed-out on the surface of biaxially oriented polypropylene film before and after heat aging treatment)
The surfaces of the produced biaxially oriented polypropylene films and the biaxially oriented polypropylene films that had been subjected to a heat aging treatment were subjected to time-of-flight secondary ion mass spectrometry (TOF-SIMS) under the following measurement conditions, and the amount of bleed-out of the antioxidant was measured by the following method.
(1) Characteristic peaks ( ²¹⁹ C ₁₅ H ₂₃ O ⁺ , ²³³ C ₁₆ H ₂₅ O ⁺ ) reflecting the functional groups of the antioxidant compound are detected, and the peak intensity on the surface of the biaxially oriented polypropylene film is confirmed.
(2) The positive characteristic peak m/z: 219 was used as an indicator of antioxidants, and the ratio of this peak to the total ion intensity on the surface of a biaxially oriented polypropylene film was calculated and compared before and after thermal aging treatment.
[TOF-SIMS measurement conditions]
Measurement depth: 1 to 2 nm
・Measurement area: 100μmφ square ・Primary ion source: Au+

(Dielectric breakdown strength of biaxially oriented polypropylene film before and after heat aging treatment)
The dielectric breakdown strength of the produced biaxially oriented polypropylene film and the biaxially oriented polypropylene film that had been subjected to a heat aging treatment was measured by the following measurement method.
[Method for measuring the dielectric breakdown strength (direct current (DC)) of biaxially oriented polypropylene film (DC voltage resistance evaluation)]
The dielectric breakdown voltage (BDV) of biaxially oriented polypropylene film was measured under the following test conditions using an electrode configuration conforming to JIS C2151 (2006) 17.2.2 (flat electrode method). The measurement was performed 16 times. The applied voltage at the point when the leakage current of the upper limit standard value below was detected during voltage increase was taken as the BDV. The BDV was divided by the film thickness (μm), and the average value of 12 points excluding the top 2 points and the bottom 2 points out of the 16 measurement results was taken as the dielectric breakdown strength ES (VDC/μm).
[Measurement conditions]
・Test piece: approx. 150mm x 150mm
Conditioning of test specimen: 30 minutes under atmospheric conditions Power supply: DC Atmosphere: Air, 25°C
・Testing machine: Kikusui Electronics Co., Ltd. DC withstand voltage/insulation resistance testing machine TOS9213AS
・Voltage rise speed: 100V/s
Current detection response speed: MID
Upper limit: 5mA

In addition, the rate of change in the dielectric breakdown strength (βVdc) after the vacuum heat aging treatment relative to the dielectric breakdown strength (αVdc) before the vacuum heat aging treatment measured by the above method was calculated by the following formula.
[Rate of change (%)] = ((αVdc - βVdc) / αVdc) x 100

(Evaluation of blocking of biaxially oriented polypropylene film (long-term storage stability of rolled film))
The rolled biaxially oriented polypropylene film was left in the same environment for six months, and the state of sticking (blocking) of the biaxially oriented polypropylene film when unrolled was evaluated. Specifically, the peel load was measured with a tension gauge, and the film was evaluated according to the following evaluation criteria. Note that a rating of A to C indicates that there is no problem in practical use, while a rating of D or E indicates that the film is poorly suited for processing in practical use.
A: No sticking
B: There is some sticking, but the peel load is 10g or less.
C: Peel load is more than 10g and 100g or less
D: Peel load is more than 100g
E: Breaking occurs

Capacitor element characteristic evaluation (element blocking evaluation)
A 620 mm wide biaxially oriented polypropylene film was wound and metal was evaporated using a winding vacuum evaporation machine (model number: EWE-060) manufactured by ULVAC, Inc. Al metal was evaporated to 20 Ω/□ on the active part and Zn metal was evaporated to 5 Ω/□ on the heavy edge part to produce a metallized film with an insulating margin and a divided electrode. The metallized film was then slit into a 30 mm wide strip using a Nuintek slitter (model number: NT-750) to produce a small roll of metallized film. Next, two 30 mm wide metallized films were stacked together using an automatic winding machine (model number: 3KAW-N2) manufactured by Kaito Seisakusho, and the number of turns was set so that the element capacitance was 50 μF under the conditions of a winding speed of 4 m/sec, winding tension of 180 g, and contact roller contact pressure of 260 g. The metallized film was then wound. The wound element was flattened by pressing, and then, with a press load still applied, zinc metal was sprayed onto the element end faces to form electrode lead-outs, and heat-treated at 120°C for 15 hours to harden the element. After heat-hardening, the element was unwound, and the state of sticking (blocking) between the films due to unwinding was checked, and the peel load was measured with a tension gauge. Evaluation was performed according to the following evaluation criteria. Note that a rating of A to C indicates that there is no problem in practical use, while a rating of D or E indicates that the processing suitability is poor in practical use.
A: No sticking
B: There is some sticking, but the peel load is 50g or less.
C: Peel load is more than 50g and 100g or less
D: Peel load is more than 100g and 300g or less
E: Peel load exceeds 300g or tear occurs due to adhesion

Measurement Results The results of the above characteristic evaluations of the Examples and Comparative Examples are shown in Tables 1 and 2.

Claims (9)

A biaxially oriented polypropylene film containing a secondary antioxidant A,
The secondary antioxidant A is a hindered phenol-based antioxidant having a carbonyl group,
The content of the secondary antioxidant A is 0.150 parts by mass or more and 0.300 parts by mass or less based on 100 parts by mass of the resin component in the biaxially oriented polypropylene film.
A biaxially oriented polypropylene film. The biaxially oriented polypropylene film according to claim 1, wherein the secondary antioxidant A is pentaerythritol tetrakis[3-(3,5-di-tertiarybutyl-4-hydroxyphenyl)propionate]. The biaxially stretched polypropylene film according to claim 1, which is a biaxially stretched polypropylene film derived from a polypropylene resin composition containing a polypropylene resin and the secondary antioxidant A, and the content of the secondary antioxidant A in the polypropylene resin composition is 0.200 parts by mass or more and 0.500 parts by mass or less per 100 parts by mass of the polypropylene resin. The biaxially stretched polypropylene film according to claim 3, wherein the polypropylene resin composition further contains a primary antioxidant, and the content of the primary antioxidant in the polypropylene resin composition is 0.10 parts by mass or more and 0.20 parts by mass or less per 100 parts by mass of the polypropylene resin. The biaxially oriented polypropylene film according to claim 4, wherein the primary antioxidant is 2,6-di-tert-butyl-p-cresol. The biaxially oriented polypropylene film according to claim 1, in which the value obtained by dividing the antioxidant peak intensity (m/z: 219) on the surface by the total ion intensity after vacuum thermal aging treatment is 0.5 or more and 0.9 or less. The biaxially oriented polypropylene film according to claim 1, wherein the rate of change in dielectric breakdown strength (βVdc) after vacuum heat aging treatment relative to the dielectric breakdown strength (αVdc) before vacuum heat aging treatment calculated by the following formula is −10% to +10%, and the dielectric breakdown strength per thickness after vacuum heat aging treatment is 540 Vdc or more.
[Rate of change (%)] = ((αVdc - βVdc) / αVdc) × 100 A metallized film for capacitors having a metal layer on one or both sides of the biaxially oriented polypropylene film according to claim 1. A capacitor comprising the metallized film for capacitors according to claim 8.