Biaxially oriented polypropylene film, metallized film, and capacitor

Abstract

To provide a biaxially oriented polypropylene film that exhibits enhanced dielectric breakdown strength under both room-temperature and high-temperature conditions.SOLUTION: A biaxially oriented polypropylene film contains polypropylene and fullerenes.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a biaxially stretched polypropylene film, a metallized film, and a capacitor.

Background Art

[0002] Stretched films mainly composed of polypropylene have moisture-proof properties and further have rigidity, heat resistance, etc., and are therefore used in various industrial applications including packaging applications.

[0003] In particular, biaxially stretched polypropylene films are used for film capacitor applications (especially as dielectrics) due to their excellent electrical properties. In electronic devices, electrical devices, etc., for example, high-voltage capacitors; various switching power supplies; filter capacitors and smoothing capacitors such as converters and inverters, film capacitors made of biaxially stretched polypropylene films are used. Film capacitors are used in, for example, inverters and converters that control drive motors in electric vehicles, hybrid vehicles, etc., where the demand has been increasing in recent years.

[0004] Film capacitors, especially automotive film capacitors, are increasingly being used in high-temperature environments. For example, in devices (such as inverters and converters) that control the drive motors of automobiles, the use of high-temperature-resistant semiconductors (such as silicon carbide semiconductors) has been increasing in recent years. Along with this, capacitors used in these devices are also required to have excellent heat resistance at high temperatures of about 120°C (110°C to 130°C). Specifically, it is required that even when used for a long time at the above high temperatures, the decrease in the capacitance of the capacitor is suppressed (excellent life performance). Also, in order to obtain such performance, biaxially stretched polypropylene films are required to have excellent dielectric breakdown strength (dielectric breakdown strength) when a DC voltage is applied both at room temperature and at the above high temperatures, even when the film thickness is less than 20 μm. Hereinafter, in this specification, the temperature of about 120°C (110°C to 130°C) is referred to as "at high temperatures".

[0005] As prior art documents related to the present invention, Patent Documents 1 and 2 and Non-Patent Document 1 can be cited. Patent Document 1 discloses a biaxially stretched polypropylene film that satisfies the requirements of specific crystallite size and surface orientation coefficient. Patent Document 2 discloses a biaxially oriented film for electrical insulation obtained by adding fullerenes to a crystalline thermoplastic resin. Further, Non-Patent Document 1 discloses that when fullerene (C60) is added to a polypropylene compression film, the DC dielectric breakdown strength is improved as compared with the case where fullerene (C60) is not added.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, the biaxially stretched polypropylene film described in Patent Document 1 and the capacitor using the same do not satisfy the above excellent life performance as capacitor performance and the above excellent dielectric breakdown strength as film performance contributing to the manifestation of the life performance, and there is still room for improvement.

[0008] In addition, the biaxially oriented film for electrical insulation described in Patent Document 2 mentioned above includes an automotive capacitor film as one of its applications and is disclosed to contain fullerenes in the film. However, in order to ensure high breakdown voltage (dielectric strength), a polyester resin, particularly polyethylene terephthalate or polyethylene-2,6-naphthalenedicarboxylate (PEN), is described as being preferable. In particular, only the aspect of using the above PEN as a crystalline thermoplastic resin is disclosed in the examples. That is, the effects of using polypropylene as the resin constituting the film have not been confirmed.

[0009] Furthermore, the polypropylene compression film described in Non-Patent Document 1 mentioned above is described as containing fullerene (C60), but only the characteristics of the compression (press) film mainly applicable to HVDC (high-voltage direct current power transmission) applications are disclosed. That is, the effects in the state of a biaxially stretched film have not been confirmed. Also, a characteristic manufacturing method is disclosed in which after mixing xylene and fullerene (C60), polypropylene is further added, the mixture is heated to melt the polypropylene, and then a compression (press) film is formed by the method of a curing press.

[0010] The present invention has been made in view of the above-described problems, and an object thereof is to provide a biaxially stretched polypropylene film having improved dielectric strength both at room temperature and at high temperature.

[0011] Another object of the present invention is to provide a metallized film having the above biaxially stretched polypropylene film, a capacitor having the metallized film, and manufacturing methods thereof.

Means for Solving the Problems

[0012] The inventors of the present invention have conducted intensive studies on biaxially stretched polypropylene films. As a result, it has been found that a biaxially stretched polypropylene film containing polypropylene and fullerenes can achieve the above object, and in particular, a polypropylene film suitable for a film capacitor for high-frequency and high-current applications can be obtained among film capacitors. Further studies have been repeated to complete the present invention.

[0013] That is, the present invention relates to the following biaxially stretched polypropylene film, metallized film, and capacitor.

[0014] 1. A biaxially stretched polypropylene film containing polypropylene and fullerenes. 2. The biaxially stretched polypropylene film according to item 1 above, wherein the polypropylene contains linear polypropylene resin A. 3. The biaxially stretched polypropylene film according to item 2 above, wherein the polypropylene further contains branched-chain polypropylene resin B. 4. The biaxially stretched polypropylene film according to item 1 above, wherein the content of the fullerenes in the biaxially stretched polypropylene film is 0.0001% by mass or more and 0.05% by mass or less. 5. The biaxially stretched polypropylene film according to item 1 above, wherein the fullerenes contain 50% by mass or more of C60. 6. The biaxially stretched polypropylene film according to item 2 above, wherein the linear polypropylene resin A has a molecular weight distribution (Mw / Mn) of the weight average molecular weight Mw and the number average molecular weight Mn of 2.0 or more and 10.0 or less. 7. The biaxially stretched polypropylene film according to item 2 above, wherein the linear polypropylene resin A has a weight average molecular weight Mw of 200,000 or more and 400,000 or less. 8. The biaxially stretched polypropylene film according to item 2 above, wherein the linear polypropylene resin A has a number average molecular weight Mn of 30,000 or more and 54,000 or less. 9. The biaxially oriented polypropylene film according to item 2 above, wherein the linear polypropylene resin A has a melt flow rate at 230°C of 2.0 g / 10 min or more and 10.0 g / 10 min or less. 10. The biaxially oriented polypropylene film according to item 1 above, which is for a capacitor. 11. The biaxially oriented polypropylene film according to item 1 above, wherein the value of tanδ (%) measured in accordance with JIS C2151:2019 of the biaxially oriented polypropylene film is 0.01% or more and 0.1% or less, and it is for a film capacitor for high-frequency and high-current applications. 12. A metallized film having a metal layer on at least one side of the biaxially oriented polypropylene film according to item 1 above. 13. The metallized film according to item 12 above, having a thickness of 1.8 μm or more and 5.0 μm or less. 14. A capacitor including the metallized film according to item 12 or 13 above.

Advantages of the Invention

[0015] The biaxially oriented polypropylene film of the present invention contains polypropylene and fullerenes, and thus has an improved dielectric breakdown strength both at room temperature and at high temperature compared to the case where no fullerenes are contained.

Modes for Carrying Out the Invention

[0016] Hereinafter, the biaxially oriented polypropylene film of the present invention, its manufacturing method, the metallized film, and the capacitor will be described in detail.

[0017] In this specification, "~" in a numerical range means "or more and or less". That is, the notation α~β means α or more and β or less, or β or more and α or less, and includes α and β as a range. Also, when a plurality of lower limit values and a plurality of upper limit values are separately described, any lower limit value and upper limit value can be selected and connected with "~".

[0018] In this specification, polypropylene may be abbreviated as PP, and polypropylene resin may be abbreviated as PP resin.

[0019] In this specification, the expressions "containing" and "comprising" include the concepts of "containing", "comprising", "substantially consisting of", and "consisting only of".

[0020] In this specification, the expression "capacitor" includes the concepts of "capacitor", "capacitor element", and "film capacitor".

[0021] In this specification, the directions of the polypropylene film are as follows. First, the machine direction of the film is the same as the Machine Direction (hereinafter referred to as the "MD direction"). The MD direction may be referred to as the length direction or the flow direction. Next, the transverse direction of the film is the same as the Transverse Direction (hereinafter referred to as the "TD direction"). The TD direction may be referred to as the width direction.

[0022] In this specification, the expression "at room temperature" means about 25°C (23°C to 27°C), and the expression "at high temperature" means about 120°C (110°C to 130°C).

[0023] 1. Biaxially oriented polypropylene film The biaxially stretched polypropylene film of the present invention is characterized by containing polypropylene and fullerenes. By containing fullerenes in addition to polypropylene, the biaxially stretched polypropylene film of the present invention has an improved dielectric breakdown strength both at room temperature and at high temperature compared to the case where no fullerenes are contained.

[0024] From the perspective of further improving the miniaturization and high capacitance of the capacitor when used in the capacitor, the upper limit of the thickness of the biaxially stretched polypropylene film of the present invention is preferably 6.0 μm or less, more preferably 5.5 μm or less, still more preferably 3.5 μm or less, particularly preferably 3.0 μm or less, and most preferably 2.8 μm or less. Also, from the perspective of manufacturing, the lower limit is preferably 0.8 μm or more, more preferably 1.0 μm or more, still more preferably 1.8 μm or more, and particularly preferably 2.0 μm or more. The method for measuring the thickness of the biaxially stretched polypropylene film in this specification is by the method described in the examples.

[0025] The density of the biaxially stretched polypropylene film is not particularly limited, but from the perspective of facilitating application to the capacitor, it is preferably 919 kg / m 3 or more and 930 kg / m 3 or less. The method for measuring the density of the polypropylene film in this specification is by the method described in the examples.

[0026] The polypropylene constituting the biaxially stretched polypropylene film of the present invention may be composed of one kind of polypropylene, or may be composed of a mixture of two or more kinds of polypropylenes. In a preferred embodiment, the polypropylene can be formed only from linear polypropylene A. Also, in another preferred embodiment, it can be formed from a mixture of two kinds of polypropylenes by mainly using linear polypropylene A and blending branched polypropylene B therewith. Here, when containing linear polypropylene resin A and branched polypropylene resin B, the linear polypropylene resin A with a higher content is referred to as the "main component polypropylene resin" or "base resin". Also, the relatively low-content branched polypropylene resin B is referred to as the "blend resin".

[0027] In the present invention, the content of the linear polypropylene resin A in the polypropylene is more than 50% by weight, preferably 55% by weight or more, more preferably 60% by weight or more, and still more preferably 65% by weight or more, based on 100% by weight of the polypropylene resin. Regarding the upper limit of the content of the linear polypropylene resin A, it may be 100% by weight (only linear polypropylene A) based on 100% by weight of the polypropylene resin. In the case of a blend of linear polypropylene A and branched polypropylene B, the content of linear polypropylene A is preferably 95% by weight or less, more preferably 90% by weight or less, still more preferably 80% by weight or less, and even more preferably 75% by weight or less.

[0028] (Linear polypropylene resin A) As the linear polypropylene resin A, for example, crystalline polypropylenes such as isotactic polypropylene and syndiotactic polypropylene can be used.

[0029] The weight average molecular weight Mw of the linear polypropylene resin A is preferably 200,000 or more and 400,000 or less, more preferably 250,000 or more and 350,000 or less, still more preferably 250,000 or more and 320,000 or less, and particularly preferably 250,000 or more and 300,000 or less. When the weight average molecular weight Mw of the linear polypropylene resin A is within the above range, in the manufacturing process of the biaxially stretched polypropylene film, it is easier to control the thickness of the cast sheet before biaxial stretching, and thickness unevenness is less likely to occur.

[0030] Note that the weight average molecular weight Mw of the linear polypropylene resin A is a characteristic of the linear polypropylene resin A as a raw material resin. Similarly, the number average molecular weight Mn, Z average molecular weight Mz, molecular weight distribution (Mw / Mn) of the weight average molecular weight Mw and the number average molecular weight Mn, melt flow rate (MFR) at 230°C, and melt tension at 230°C of the linear polypropylene resin A described below are also characteristics of the linear polypropylene resin A as a raw material resin.

[0031] The number average molecular weight Mn of the linear polypropylene resin A is preferably 30,000 or more and 54,000 or less, more preferably 33,000 or more and 52,000 or less, and even more preferably 33,000 or more and 47,000 or less. When the number average molecular weight Mn of the linear polypropylene resin A is within the above range, the change range of the capacitance of the manufactured capacitor at high temperature becomes smaller and the heat resistance is further improved.

[0032] The Z average molecular weight Mz of the linear polypropylene resin A is preferably 1,000,000 or more and 2,000,000 or less, and more preferably 1,200,000 or more and 1,800,000 or less. When the Z average molecular weight Mz of the linear polypropylene resin A is within the above range, the dielectric breakdown strength of the biaxially stretched polypropylene film at high temperature is further improved.

[0033] The molecular weight distribution (Mw / Mn) of the linear polypropylene resin A is preferably 2.0 or more and 10.0 or less. Among these, the lower limit value can preferably be set to 4.0 or more, more preferably 5.2 or more, even more preferably 5.3 or more, and most preferably 5.5 or more. Also, the upper limit value can preferably be set to 9.0 or less, more preferably 8.2 or less. When Mw / Mn is within the above range, the stretchability of the polypropylene film is further improved, and a thinner biaxially stretched polypropylene film can be manufactured.

[0034] The melt flow rate (MFR) of the linear polypropylene resin A at 230°C is preferably 2.0 g / 10 min or more, more preferably 2.5 g / 10 min or more. Also, the melt flow rate (MFR) of the linear polypropylene resin A at 230°C is preferably 10.0 g / 10 min or less, more preferably 8.0 g / 10 min or less. Therefore, for example, it can be set to 2.0 g / 10 min or more and 10.0 g / 10 min or less. When the melt flow rate (MFR) of the linear polypropylene resin A at 230°C is within the above range, the volume resistivity of the biaxially stretched polypropylene film at high temperature is further improved. The method for measuring the melt flow rate (MFR) of the resin at 230°C in this specification is by the method described in the examples.

[0035] The melt tension of the linear polypropylene resin A at 230°C is preferably 1.0 g or less, and more preferably less than 1.0 g. When the melt tension of the linear polypropylene resin A at 230°C is within the above range, the flow characteristics in the molten state are excellent, so that unstable flow such as melt fracture hardly occurs. Therefore, since the film thickness uniformity is good, there is an advantage that it is difficult to form a thin portion where dielectric breakdown is likely to occur. The method for measuring the melt tension of the resin at 230°C in this specification is based on the method described in the examples.

[0036] (Branched-chain polypropylene resin B) The biaxially oriented polypropylene film of the present invention may contain a branched-chain polypropylene resin B in addition to the linear polypropylene resin A.

[0037] The branched-chain polypropylene resin B has the action of a crystallization nucleating agent. By containing the branched-chain polypropylene resin B, the "entanglement effect (pseudo-crosslinking effect)" and the "crystallization effect" of the molecular chains of polypropylene are promoted. Specifically, by containing the branched-chain polypropylene resin B, a large amount of β-crystals are formed in the cast sheet before biaxial stretching. Since the β-crystals are transferred to α-crystals by stretching the cast sheet containing the β-crystals, the biaxially oriented polypropylene film has high crystallinity and the crystal size of the α-crystals becomes large. Here, the β-crystal fraction means the ratio of β-crystals to the total of α-crystals and β-crystals.

[0038] In addition, a large amount of β-crystals are formed in the cast sheet before biaxial stretching. Since the β-crystals are transferred to α-crystals by stretching the cast sheet containing the β-crystals, (substantially) arc-shaped irregularities are formed on the polypropylene film obtained by stretching due to the difference in density between the β-crystals and the α-crystals, and the surface can be suitably roughened. This increases the surface roughness (Vmc) of the roughened surface and leads to an increase in the solid volume of the core part of the roughened surface. From this viewpoint as well, the biaxially oriented polypropylene film of the present invention is suitable as a material for a dielectric film for a capacitor.

[0039] As the branched-chain polypropylene resin B, a branched-chain polypropylene resin B obtained by polymerizing propylene using a metallocene catalyst is preferred.

[0040] When using, as the branched-chain polypropylene resin B, a branched-chain polypropylene resin B obtained by crosslinking modification with a peroxide instead of a branched-chain polypropylene resin B polymerized using a metallocene catalyst, due to the α-crystal nucleation effect of the branched-chain polypropylene resin B obtained by crosslinking modification with a peroxide, the formation of α-crystals is promoted and the formation of β-crystals is suppressed in the cast sheet before biaxial stretching. Even if the cast sheet containing α-crystals is stretched, the transfer of crystallites hardly occurs and unevenness is hardly formed. Therefore, from the viewpoint of roughening the biaxially stretched polypropylene film, a branched-chain polypropylene resin B polymerized using a metallocene catalyst can be preferably used.

[0041] In this regard, when using the branched-chain polypropylene resin B polymerized using a metallocene catalyst, the β-crystal fraction in the cast sheet before biaxial stretching is preferably 10% or more, more preferably 13% or more, still more preferably 15% or more, still more preferably 17% or more, and most preferably 19% or more.

[0042] Further, the metallocene catalyst is generally a metallocene compound that forms a polymerization catalyst for generating an olefin macromer. The branched-chain polypropylene resin B obtained by polymerizing propylene using a metallocene catalyst is preferred because the branched-chain length and branched-chain interval of the polypropylene become appropriate, the compatibility with linear polypropylene is further improved, and a more uniform composition and a more uniform surface shape can be easily obtained.

[0043] The melt tension of the above-mentioned branched-chain polypropylene resin B at 230 °C is preferably 3 g / cm 3 or more and 25 g / cm 3 or less, more preferably 5 g / cm 3 or more and 20 g / cm 3 or less, still more preferably 9 g / cm 3 or more and 20 g / cm 3The following is more preferable, 9 g / cm 3 or more and 17 g / cm 3 or less is particularly preferable. When the melt tension is within the above range, the volume resistivity of the biaxially stretched polypropylene film at high temperatures is further improved.

[0044] The melt flow rate (MFR) of the branched-chain polypropylene resin B at 230 °C is preferably 0.1 g / 10 min or more, more preferably 1.0 g / 10 min or more. Also, the melt flow rate (MFR) of the branched-chain polypropylene resin B at 230 °C is preferably 12.0 g / 10 min or less, more preferably 6.0 g / 10 min or less. When the melt flow rate at 230 °C is within the above range, the fluidity in the molten state is excellent, so unstable flow such as melt fracture is less likely to occur, and breakage during stretching is more suppressed. Therefore, since the film thickness uniformity is better, the formation of thin portions where dielectric breakdown is likely to occur is suppressed, and the volume resistivity of the biaxially stretched polypropylene film at high temperatures is further improved.

[0045] The weight average molecular weight Mw of the branched-chain polypropylene resin B is preferably 150,000 or more and 600,000 or less, more preferably 200,000 or more and 500,000 or less, still more preferably 250,000 or more and 500,000 or less, and particularly preferably 350,000 or more and 480,000 or less. When the weight average molecular weight Mw of the above-mentioned branched-chain polypropylene resin B is within the above range, the resin fluidity becomes more appropriate, the control of the thickness of the cast sheet is easier, and it becomes easier to produce a thin stretched film. Also, unevenness in the thickness of the cast sheet and the stretched film is less likely to occur, and more appropriate stretchability can be obtained.

[0046] The number average molecular weight Mn of the branched-chain polypropylene resin B is preferably 100,000 or more and 300,000 or less, more preferably 100,000 or more and 250,000 or less, still more preferably 100,000 or more and 200,000 or less. When the number average molecular weight Mn of the branched-chain polypropylene resin B is within the above range, the change range of the capacitance of the manufactured capacitor at high temperatures becomes smaller and the heat resistance is further improved.

[0047] The molecular weight distribution (Mw / Mn) of the branched-chain polypropylene resin B is preferably 1.5 or more and 4.5 or less, more preferably 1.8 or more and 4.5 or less, and still more preferably 2.0 or more and 4.2 or less. When Mw / Mn is within the above range, the stretchability of the polypropylene film is further improved, and a thinner biaxially stretched polypropylene film can be produced.

[0048] The molecular weight, molecular weight distribution, etc. of the branched-chain polypropylene resin B can be controlled by adjusting the catalyst and polymerization conditions.

[0049] (Other resins) The biaxially stretched polypropylene film of the present invention may contain other resins (hereinafter also referred to as "other resins") other than the linear polypropylene resin A and the branched-chain polypropylene resin B. "Other resins" are resins other than the linear polypropylene resin A and the branched-chain polypropylene resin B, and are not particularly limited as long as the target biaxially stretched polypropylene film can be obtained. Examples of such other resins include other polyolefins other than polypropylene such as polyethylene, poly(1-butene), polyisobutene, poly(1-pentene), poly(1-methylpentene); copolymers of α-olefins such as ethylene-propylene copolymer, propylene-butene copolymer, ethylene-butene copolymer; random copolymers of vinyl monomers-diene monomers such as styrene-butadiene random copolymer; random copolymers of vinyl monomers-diene monomers-vinyl monomers such as styrene-butadiene-styrene block copolymer. When other resins are contained, the total amount of the linear polypropylene resin A and the branched-chain polypropylene resin B is preferably 95% by mass or more, and more preferably 98% by mass or more, based on 100% by mass of the total resin components. That is, the content of other resins is preferably 5% by mass or less, and more preferably 2% by mass or less, based on 100% by mass of the total resin components.

[0050] (Fullerenes) The biaxially stretched polypropylene film of the present invention contains fullerenes in addition to polypropylene. By containing fullerenes in addition to polypropylene, the biaxially stretched polypropylene film of the present invention has an improved dielectric breakdown strength both at room temperature and at high temperature as compared with the case where no fullerenes are contained. Examples of the fullerenes include fullerene, fullerene derivatives, and mixtures thereof.

[0051] Fullerene is a spherical or ellipsoidal carbon molecule, and examples thereof include C60, C70, C74, C76, C78, C80, C82, C84, C86, C88, C90, C92, C94, C96, C98, C100, and dimers and trimers of these compounds, etc., although it is not limited as long as the effects of the present invention can be obtained.

[0052] In the present invention, at least one selected from the group consisting of C60, C70, and dimers or trimers thereof is preferable among the above fullerenes. C60 and C70 are preferable because they are industrially easy to obtain and have excellent dispersibility in polypropylene. These fullerenes may be used alone or in combination of two or more, but it is preferable to contain 50% by mass or more of C60, preferably 70% by mass or more, more preferably 90% by mass or more, and may be 100% by mass. When a plurality of fullerenes are used in combination, it is preferable to contain 50% by mass or more of C60, and for example, it is preferable to use C60 and C70 in combination.

[0053] Examples of commercially available fullerenes include "nanom mix ST" (a mixture consisting of 50 to 60% by mass of C60, 15 to 20% of C70, and other higher fullerenes) manufactured by Frontier Carbon Corporation.

[0054] A fullerene derivative refers to a compound in which an atomic group composed of an atomic group forming a part of an organic compound or an atomic group composed of an inorganic element is bonded to at least one carbon constituting the fullerene. Such a fullerene derivative can be used in the same manner as fullerene as long as the film-forming property is not inhibited. The fullerene for obtaining the fullerene derivative is not limited as long as the effects of the present invention can be obtained, and any of the above-described fullerenes may be used. Examples of the fullerene derivative include hydrogenated fullerene, oxidized fullerene, hydroxylated fullerene, and halogenated (F, Cl, Br, I) fullerene. Further, the fullerene derivative may contain a substituent such as a carboxyl group, an alkyl group, or an amino group.

[0055] Fullerene can be obtained, for example, by extraction and separation from a fullerene-containing soot obtained by a resistance heating method, a laser heating method, an arc discharge method, a combustion method, or the like. Further, the fullerene derivative can be synthesized by using a conventionally known method with respect to fullerene. For example, a desired fullerene derivative can be obtained by utilizing a reaction with a nucleophile (nucleophilic addition reaction), a cycloaddition reaction, a photoaddition (cyclization) reaction, an oxidation reaction, or the like.

[0056] The content of fullerenes in the biaxially stretched polypropylene film of the present invention is not limited as long as the effects of the present invention can be obtained, but is preferably 0.001% by mass or more and 0.05% by mass or less. The lower limit is more preferably 0.003% by mass or more, and still more preferably 0.005% by mass or more. The upper limit is more preferably 0.03% by mass or less, and still more preferably 0.02% by mass or less. Note that the upper limit of the content of fullerenes can also be set to 0.01% by mass or less, or less than 0.01% by mass.

[0057] When the addition amount of fullerenes is less than 0.001% by mass, it may not be possible to sufficiently improve the dielectric breakdown strength at both room temperature and high temperature. On the other hand, when the addition amount of fullerenes exceeds 0.05% by mass, no further improvement in dielectric breakdown strength can be expected, and poor dispersion of fullerenes may occur during film formation, resulting in film breakage or a rough surface.

[0058] The fullerenes used in the present invention preferably have a dispersed particle diameter of 100 nm or less in the biaxially oriented film. When the dispersed particle diameter of the fullerenes exceeds 100 nm, the dispersibility of the fullerenes is not sufficient, and variations in the breakdown voltage characteristics may occur. Here, the dispersed particle diameter refers to the particle diameter of the aggregated secondary particles when the fullerenes are aggregated and dispersed. Such a dispersed particle diameter is obtained by observing a thickness cross-section parallel to the obtained MD direction (which may be referred to as the longitudinal direction or the film continuous film-forming direction) at 200 times magnification using an optical microscope (OPTPHOT-2 manufactured by Nikon Corporation), measuring the lengths in the MD direction of 100 fullerene dispersed phases, and calculating the value based on the number average length.

[0059] (Additive) The biaxially stretched polypropylene film of the present invention may further contain an additive. Examples of the additive include an antioxidant, a chlorine absorber, an ultraviolet absorber, a lubricant, a plasticizer, a flame retardant, an antistatic agent, a colorant, and the like.

[0060] (Characteristics of the biaxially stretched polypropylene film) By containing fullerenes in addition to polypropylene, the biaxially stretched polypropylene film of the present invention has an improved breakdown strength both at room temperature and at high temperature compared to the case where no fullerenes are contained.

[0061] The breakdown strength (V) of the biaxially stretched polypropylene film of the present invention at a direct current voltage at room temperature (for example, 23 °C) DC23℃) is, for example, when using a commercially available linear polypropylene as the polypropylene and a fullerene manufactured by Frontier Carbon Corporation, trade name "nanom mix ST" (a mixture consisting of 50 to 60% by mass of C60, 15 to 20% by mass of C70, and other higher fullerenes) as the fullerenes, preferably 620 V / μm or more, more preferably 630 V / μm or more. Here, it is preferable that the improvement rate of the dielectric breakdown strength based on the case without containing fullerenes (100%) is 10% or more. In addition, compared with an unstretched or uniaxially stretched polypropylene film, which is different from biaxial stretching, in the case of a biaxially stretched polypropylene film, when the content of fullerenes is the same, the improvement rate of the dielectric breakdown strength is high, which shows the superiority of the present invention that requires biaxial stretching (this is the same for the improvement rate of the dielectric breakdown strength at high temperatures).

[0062] The dielectric breakdown strength (V DC120℃ ) of the polypropylene film of the present invention at a direct current voltage at a high temperature (for example, 120 °C) is, for example, when using a commercially available linear polypropylene as the polypropylene and a fullerene manufactured by Frontier Carbon Corporation, trade name "nanom mix ST" (a mixture consisting of 50 to 60% by mass of C60, 15 to 20% by mass of C70, and other higher fullerenes) as the fullerenes, preferably 510 V / μm or more, more preferably 530 V / μm or more. The upper limit of the dielectric breakdown strength at a direct current voltage at a high temperature is preferably higher, for example, about 700 V / μm. Here, it is preferable that the improvement rate of the dielectric breakdown strength based on the case without containing fullerenes (100%) is 10% or more.

[0063] The value of tanδ (%) measured in accordance with JIS C2151:2019 for the biaxially stretched polypropylene film of the present invention is, for example, preferably 0.01% or more and 0.1% or less when using, as polypropylene, a commercially available linear polypropylene, and as fullerenes, fullerene manufactured by Frontier Carbon Corporation, trade name "nanom mix ST" (a mixture consisting of 50 to 60% by mass of C60, 15 to 20% by mass of C70, and other higher fullerenes). The method for measuring tanδ (%) of the biaxially stretched polypropylene film in accordance with JIS C2151:2019 in this specification is by the method described in the examples.

[0064] Here, tanδ (%) is a measure indicating power loss. When the value of tanδ is large, heat generation caused by current may cause a capacitor failure when the biaxially stretched polypropylene film is used for a capacitor. The biaxially stretched polypropylene film of the present invention uses polypropylene (excellent in high-frequency characteristics) as a resin and the value of tanδ is suppressed low. However, for example, when using polyethylene-2,6-naphthalenedicarboxylate (PEN; inferior in high-frequency characteristics compared to PP), which is a resin other than polypropylene, the value of tanδ becomes significantly larger and reaches about 0.50% when the content of fullerenes is the same. Therefore, the biaxially stretched polypropylene film of the present invention is useful for film capacitors for high-frequency and high-current applications from the characteristics of tanδ. Note that a film capacitor using PEN is for a surface-mount type medium-voltage film capacitor as common technical knowledge of those skilled in the art due to its high heat resistance characteristics, and is distinguished in application from a film capacitor for high-frequency and high-current applications using PP (generally a lead wire type).

[0065] 2. Method for producing biaxially oriented polypropylene film The method for manufacturing the biaxially stretched polypropylene film of the present invention is not particularly limited. For example, it can be specified as "a method for manufacturing a biaxially stretched polypropylene film having a step of biaxially stretching a cast sheet before biaxial stretching containing polypropylene and fullerenes." and can be specified as such.

[0066] Among them, from the perspective of the details of the manufacturing process, a polypropylene resin composition containing polypropylene and fullerenes is preferably melted at a temperature of 225°C or higher and 270°C or lower and a shear rate of 2000 s -1 or higher and 15000 s -1 or lower. It can be manufactured by a manufacturing method including a step of melting. Hereinafter, the above manufacturing method will be exemplarily described.

[0067] According to the above manufacturing method, a biaxially stretched polypropylene film showing a high volume resistivity even at high temperatures can be provided. The reason for being able to provide a biaxially stretched polypropylene film showing a high volume resistivity even at high temperatures is considered to be due to the sea-island phase separation structure (particularly, an appropriate island size) of the cast sheet by using two specific and different polypropylene resins.

[0068] The method of mixing the resins applied to the above manufacturing method is not particularly limited, but examples include a method of dry blending a base resin and a polymer powder or pellet of a blend resin using a mixer or the like, and a method of supplying a base resin and a polymer powder or pellet of a blend resin to a kneader and melt-kneading to obtain a kneaded product.

[0069] The above mixer and kneader are not particularly limited. The above kneader may be any of a single-screw type, a twin-screw type, and a multi-screw type with three or more axes. In the case of a twin-screw type or higher, it may be either co-rotating or counter-rotating.

[0070] In the case of kneading by melt-kneading, the kneading temperature is not particularly limited as long as a good kneaded product is obtained. Generally, it is in the range of 200°C or higher and 300°C or lower, and from the perspective of suppressing resin deterioration, 230°C or higher and 270°C or lower is preferable. Further, 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 generally be pelletized to an appropriate size using a known granulator. Thereby, a mixed polypropylene raw material resin pellet can be prepared.

[0071] The polypropylene resin composition may contain an additive. As the additive, the same additive as that described in the biaxially stretched polypropylene film of the present invention can be used. The above polypropylene resin composition may contain the above additive in a content that does not adversely affect the biaxially stretched polypropylene film.

[0072] In the method for producing the biaxially stretched polypropylene film, polypropylene resin pellets, dry-mixed polypropylene resin pellets, or mixed polypropylene resin pellets prepared by previously melt-kneading are supplied to an extruder together with fullerenes and heated and melted.

[0073] The above polypropylene resin composition is preferably melted at 170°C or higher and 320°C or lower. Specifically, the set temperature of the extruder during the heating and melting of the polypropylene resin composition is set to 225°C or higher and 270°C or lower. Thereby, a sea-island phase separation structure at the time of cast sheet forming described later is formed, and a biaxially stretched polypropylene film having a high volume resistivity even at a high temperature can be produced.

[0074] The polypropylene resin composition is in a state where the temperature is 225°C or higher and 270°C or lower, and the shear rate is 2000 s -1 or more and 15000 s -1 or less. Thereby, a sea-island phase separation structure at the time of cast sheet forming described later is formed, and a polypropylene film having a high volume resistivity even at a high temperature can be produced. When the shear rate is less than 2000 s -1 , the extrusion amount is not constant, and the shape and dimensions of the raw sheet may become irregular or fluctuate regularly, and there is a risk of breakage during the conveyance of the raw sheet or breakage during stretching. Also, when the shear rate is 15000 s -1If it exceeds this value, unmelted materials may be extruded due to a phenomenon called breakup inside the extruder, making it difficult to obtain a uniform raw sheet and increasing the likelihood of breakage during stretching. Alternatively, excessive heat generation may occur when passing through the chip clearance, significantly deteriorating the polypropylene resin composition. Even if a uniform raw sheet is obtained, the volume resistivity of the film obtained by stretching may decrease. The shear rate can be adjusted by the cylinder diameter of the extruder, the screw rotation speed, and the groove depth of the screw.

[0075] The above shear rate is 2000 s -1 or more and 10000 s -1 or less is preferable, and 2000 s -1 or more and 2300 s -1 or less is more preferable. By being within the range of the shear rate, it becomes easier to obtain a biaxially stretched polypropylene film with a weight fraction w of 2.6% or more and 4.0% or less, and the heat resistance of a film capacitor using the biaxially stretched polypropylene film as a capacitor dielectric is further improved.

[0076] Next, the molten polypropylene resin composition is extruded into a sheet using a T-die and cooled and solidified with at least one metal drum to form an unstretched cast sheet. The surface temperature of the above metal drum (the temperature of the metal drum that first contacts after extrusion) is preferably 10°C or more and 105°C or less, and more preferably 15°C or more and 100°C or less. The surface temperature of the above metal drum can be determined according to the physical properties of the polypropylene resin used, etc. If the surface temperature of the metal drum is less than 10°C, it is difficult to obtain good sheet formability of the raw sheet, so there is a risk of uneven stretching and breakage during film stretching.

[0077] The thickness of the above cast sheet is not particularly limited, preferably 0.05 mm or more and 2 mm or less, and more preferably 0.1 mm or more and 1 mm or less.

[0078] The biaxially oriented polypropylene film can be manufactured by subjecting the above-mentioned cast sheet to a biaxial stretching treatment. The stretching is biaxial stretching for biaxially orienting in the longitudinal and transverse directions, and as the stretching method, a sequential biaxial stretching method is preferable. As the above-mentioned sequential biaxial stretching method, for example, first, the cast sheet is maintained at a temperature of 100°C or higher and 180°C or lower (preferably 120°C or higher and 170°C or lower), and passed between rolls with a speed difference to be stretched in the flow direction. The stretching ratio in the flow direction is preferably 3.0 times or more and 5.0 times or less, more preferably 4.0 times or more and 4.9 times or less, and still more preferably 4.6 times or more and 4.8 times or less. By setting the stretching ratio in the flow direction to 5.0 times or less, even when containing the branched-chain polypropylene resin B, when performing the stretching treatment in the width direction following the stretching treatment in the flow direction, the occurrence of stretching unevenness can be suppressed, and film breakage is easily suppressed. Next, the sheet is guided to a tenter and stretched in the transverse direction. The temperature during the transverse stretching is preferably 160°C or higher and 180°C or lower, and the stretching ratio in the transverse direction is preferably 3.0 times or more and 11.0 times or less. Next, after the transverse stretching, relaxation and heat setting are performed, and then it is wound up.

[0079] By the manufacturing method described above, a biaxially oriented polypropylene film can be manufactured.

[0080] In the above-mentioned biaxially oriented polypropylene film, from the viewpoint of further improving the adhesion characteristics in post-processes such as a metal vapor deposition process, corona discharge treatment may be performed online or offline after the stretching and heat setting processes. The corona discharge treatment can be performed using a known method. It is preferable to perform it using air, carbon dioxide gas, nitrogen gas, or a mixed gas thereof as the atmospheric gas.

[0081] The biaxially oriented polypropylene film of the present invention produced by the above manufacturing method contains fullerenes in addition to polypropylene, and has an improved dielectric breakdown strength both at room temperature and at high temperature as compared with the case where fullerenes are not contained. Thereby, it is possible to provide a metallized film using the biaxially oriented polypropylene film and a high-life capacitor having excellent heat resistance even at high temperatures. Therefore, the biaxially oriented polypropylene film of the present invention is suitable for capacitor applications, and can be particularly used as a dielectric of a capacitor constituting an inverter in a hybrid vehicle or an electric vehicle.

[0082] 3. Metallized film, capacitor and method for producing them The metallized film of the present invention is a metallized film having a metal layer on at least one side of the above biaxially oriented polypropylene film.

[0083] The metal layer functions as an electrode. As the metal used for the metal layer, for example, a single metal such as zinc, lead, silver, chromium, aluminum, copper, nickel, a mixture of plural kinds thereof, an alloy thereof, etc. can be used. Among these, zinc and aluminum are preferable in terms of excellent environmental load, economy, and capacitor performance.

[0084] The method for laminating the 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 a vacuum evaporation method and a sputtering method. From the viewpoint of excellent productivity and economy, the vacuum evaporation method is preferable. Examples of the vacuum evaporation method include a crucible method and a wire method, and an appropriate optimum one can be selected.

[0085] The margin pattern when laminating the metal layer by vapor deposition is not particularly limited, and from the viewpoint of further improving the safety of the capacitor and further suppressing the breakdown and short circuit of the capacitor, a pattern including a so-called special margin such as a fishnet pattern and a T-margin pattern is preferably applied on one surface of the biaxially oriented polypropylene film.

[0086] The method for forming the margin is not particularly limited, and it may be formed by a known method such as a tape method or an oil method.

[0087] The thickness of the metallized film of the present invention is not particularly limited, preferably 1.8 μm or more and 5.0 μm or less, and more preferably 2.0 μm or more and 3.0 μm or less.

[0088] The capacitor of the present invention is a capacitor including the above metallized film. The metallized film of the present disclosure can be laminated by a conventionally known method or wound to form a film capacitor.

[0089] The above film capacitor may have a configuration in which a plurality of metallized films are laminated, or may have a wound metallized film. Such a film capacitor can be suitably used for a capacitor for an inverter power supply device that controls a drive motor of an electric vehicle, a hybrid vehicle, etc. It can also be suitably used in applications such as for railway vehicles, wind power generation, solar power generation, and general household appliances.

Examples

[0090] Examples and comparative examples are shown below to specifically explain the present invention. However, the present invention is not limited to the examples.

[0091] The raw materials used in the examples and comparative examples are as follows. · Polypropylene (linear polypropylene of commercial product) · Fullerenes (as fullerenes, manufactured by Frontier Carbon Corporation, trade name "nanom mix ST" (a mixture consisting of 50 to 60% by mass of C60, 15 to 20% by mass of C70, and other higher fullerenes))

[0092] [Examples 1 to 5 and Comparative Example 1] <Manufacture of biaxially stretched polypropylene film> The above raw materials were dry-blended at the mixing ratios (mass ratios) shown in Table 1. Subsequently, they were fed into an extruder, melted at a resin temperature of 230°C, then extruded using a T-die, and wound around a metal drum maintained at a surface temperature of 23°C for solidification to produce an unstretched cast sheet with a thickness of approximately 300 μm. Next, the unstretched cast sheet was stretched 4.6 times in the flow direction and then 9.4 times in the transverse direction at a temperature of 165°C using a batch-type biaxial stretcher KARO IV manufactured by Brueckner to produce a biaxially stretched polypropylene film with a thickness of 7.0 μm.

[0093] [Comparative Examples 2 and 3] <Manufacture of Unstretched Polypropylene Film> The above raw materials were dry-blended at the mixing ratios (mass ratios) shown in Table 1. Subsequently, they were fed into an extruder, melted at a resin temperature of 230°C, then extruded using a T-die, and wound around a metal drum maintained at a surface temperature of 23°C for solidification to produce an unstretched cast sheet with a thickness of approximately 10 μm. This was used as an unstretched polypropylene film (comparative product).

[0094]

Table 1

[0095] <Measurement Method> For each example and comparative example, various properties were evaluated under the following measurement conditions.

[0096] (1) Characteristic evaluation of resin (Measurement of the number-average molecular weight (Mn), weight-average molecular weight (Mw), z-average molecular weight (Mz), molecular weight distribution (Mw / Mn), and differential distribution value of polypropylene) Using GPC (gel permeation chromatography), the number-average molecular weight (Mn), weight-average molecular weight (Mw), z-average molecular weight (Mz), molecular weight distribution (Mw / Mn), and differential distribution value of the distribution curve of polypropylene were measured under the following conditions.

[0097] The HLC-8121GPC-HT type, a differential refractometer (RI)-equipped high-temperature GPC device manufactured by Tosoh Corporation, was used. As columns, three TSKgel GMHHR-H(20)HT manufactured by Tosoh Corporation were connected in series, and one TSKgel guardcolumn HHR(30) was also used. At a column temperature of 140 °C, using 1,2,4-trichlorobenzene containing 0.05 wt% of 2,6-di-tert-butyl-p-cresol (common name: BHT) as the eluent and flowing it at a flow rate of 1.0 ml / min for measurement, the number average molecular weight (Mn), weight average molecular weight (Mw), and z average molecular weight (Mz) were obtained. Using the values of Mw and Mn, the molecular weight distribution (Mw / Mn) was obtained.

[0098] The measurement conditions are as follows. GPC device: HLC-8121GPC / HT (manufactured by Tosoh) Light scattering detector: DAWN EOS (manufactured by Wyatt Technology) Column: 1 TSKgel guardcolumn HHR(30) (7.8 mm ID × 7.5 cm) + 3 TSKgel GMHHR-H(20)HT (7.8 mm ID × 30 cm) (manufactured by Tosoh) Eluent: 1,2,4-trichlorobenzene containing 0.05 wt% of BHT Flow rate: 1.0 mL / min Sample concentration: 2 mg / mL Injection volume: 300 μL Column temperature: 140 °C System temperature: 40 °C Pretreatment: The sample was precisely weighed, the eluent was added, and it was shaken and dissolved at 140 °C for 1 hour, followed by heat filtration using a 0.5-μm sintered metal filter. Calibration curve: A calibration curve of a fifth-order approximation curve was created using standard polystyrene manufactured by Tosoh Corporation. However, the molecular weight was converted to the molecular weight of polypropylene using the Q-factor.

[0099] From the obtained calibration curve and SEC chromatogram, using the analysis software for the measuring device, the integral value of the concentration fraction was plotted on the vertical axis and the molecular weight (logarithmic value) was plotted on the horizontal axis to obtain an integral molecular weight distribution curve. Also, the differential value (the slope of the integral molecular weight distribution curve) of the integral molecular weight distribution curve at each molecular weight was determined, the differential value was plotted on the vertical axis and the molecular weight (logarithmic value) was plotted on the horizontal axis to obtain a differential molecular weight distribution curve.

[0100] From these curves, the number average molecular weight Mn, the weight average molecular weight Mw, and the Z average molecular weight Mz were obtained. Using the values of Mw and Mn, the molecular weight distribution (Mw / Mn) was obtained. Also, using the values of Mz and Mn, the molecular weight distribution (Mz / Mn) was obtained.

[0101] (Melt Tension) Using a Capirograph 1B manufactured by Toyo Seiki Co., Ltd., under the following conditions, the tension detected by the pulley when the resin of the raw materials used in the examples and comparative examples was extruded into a string and wound around a roller was defined as the melt tension. Capillary: diameter 2.0 mm, length 40 mm Cylinder diameter: 9.55 mm Cylinder extrusion speed: 20 mm / min Winding speed: 4.0 m / min Temperature: 230 °C.

[0102] In addition, when the melt tension is extremely high, at a take-up speed of 4.0 m / min, the resin may break. In such a case, the take-up speed was decreased, and the tension at the highest speed at which it could be taken up was defined as the melt tension.

[0103] (Melt Flow Rate (MFR)) The melt flow rate (MFR) of the resin pellets of the raw materials used in the examples and comparative examples was measured in accordance with Condition M of JIS K 7210 using a melt index manufactured by Toyo Seiki Co., Ltd. Specifically, first, a 4 g sample was inserted into a cylinder set at a test temperature of 230°C and preheated for 3.5 minutes under a load of 2.16 kg. Then, the weight of the sample extruded from the bottom hole in 30 seconds was measured, and the MFR (unit: g / 10 min or g / 10 min) was determined. The above measurement was repeated 3 times, and the average value was taken as the measured value of MFR.

[0104] (2) Characteristic evaluation of biaxially oriented polypropylene film (Measurement of Dielectric Breakdown Strength: DC, at Room Temperature) The measurement conditions and procedures were as follows.

[0105] The dielectric breakdown voltage (BDV) of the polypropylene films according to the examples and comparative examples was measured 16 times under the following test conditions with the electrode configuration described in JIS C2151 (2006) 17.2.2 (flat electrode method). The applied voltage at the time when the leakage current of the following upper limit reference value was detected during the voltage increase was defined 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 among the 16 measurement results was taken as the strength of dielectric breakdown ES (VDC / μm). Test piece: Approximately 150 mm × 150 mm Conditioning of test piece: 30 minutes under atmospheric conditions Power supply: DC Atmosphere: In air, at room temperature (23°C) Testing machine: DC withstand voltage / insulation resistance tester TOS9213AS manufactured by Kikusui Electronics Industry Co., Ltd. Voltage increase rate: 100 V / s Current detection response speed: MID Upper limit reference value: 5 mA

[0106] (Measurement of Dielectric Breakdown Strength: DC, at High Temperature) The measurement conditions and procedures were as follows. With the electrode configuration described in JIS C2151(2006) 17.2.2 (Flat Electrode Method), the breakdown voltage (BDV) of the polypropylene films according to the examples and comparative examples was measured 16 times under the following test conditions. The applied voltage at the time when the leakage current of the following upper limit reference value was detected during voltage increase was defined 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 among the 16 measurement results was defined as the breakdown strength ES (VDC / μm). Test piece: Approximately 150 mm × 150 mm Conditioning of test piece: 30 minutes under atmospheric conditions Power supply: DC Atmosphere: In air, at high temperature (120 °C) Testing machine: DC withstand voltage / insulation resistance tester TOS9213AS manufactured by Kikusui Electronics Industry Co., Ltd. Voltage increase rate: 100 V / s Current detection response rate: MID Upper limit reference value: 5 mA

[0107] (Film thickness) Under the environment of temperature 23 ± 2 °C and humidity 50 ± 5% RH, the thickness of the biaxially oriented polypropylene film was measured using a paper thickness gauge MEI-11 (measurement pressure 100 kPa, lowering speed 3 mm / second, measurement terminal φ = 16 mm, measuring force 20.1 N) manufactured by Citizen Seimitsu Co., Ltd. The samples were cut out from the roll with 10 or more sheets stacked, and were handled so that no wrinkles or air entered the film during cutting. For the 10-sheet stacked samples, 5 measurements were performed, and the average value of the 5 measurements was divided by 10 to calculate the thickness. Atmosphere: In air, 120 °C Testing machine: DC withstand voltage / insulation resistance tester TOS9213AS manufactured by Kikusui Electronics Industry Co., Ltd. Voltage increase rate: 100 V / s Current detection response rate: MID Upper limit reference value: 5 mA

[0108] (Film density) The density of the film was measured by the D method of JIS K7112(1999). Measuring device: Density gradient tube type specific gravity measuring device A type manufactured by Shibayama Scientific Instruments Co., Ltd. Gradient liquid: Ethanol aqueous solution Measurement temperature: 23 ± 0.5 °C Number of measurements: n = 3

[0109] (Measurement of tanδ) The measurement conditions and procedures were as follows. Conditions not otherwise specified were measured in accordance with "18 Dielectric Dissipation Factor and Relative Permittivity" of JIS C2151:2019.

[0110] First, a measurement jig (hereinafter also simply referred to as a jig) was placed in a thermostatic bath at 23 °C. The configuration of the jig is as follows. Also, an impedance analyzer, model number "E4990A" manufactured by Keysight Technologies, was connected to the jig using a coaxial cable.

[0111] <Measurement jig> Main electrode (φ3 mm) Counter electrode (φ3 mm) Each electrode is made of copper plated with gold, and the main electrode and the counter electrode face each other. The measurement jig (electrode) was connected to the impedance analyzer via a coaxial connector.

[0112] Next, the polypropylene films according to the examples and comparative examples (hereinafter also referred to as samples) were placed in an environment of 23 °C and 50% RH for 24 hours. Then, the sample was sandwiched between the jigs in the thermostatic bath. Specifically, the main electrode was brought into close contact with one surface of the sample, the counter electrode was brought into close contact with the other surface, and the sample and each electrode were brought into close contact with a load of 2 kgf. Then, it was left standing for 30 minutes. Next, an AC voltage of 0.5 V and 1 kHz was applied from the built-in power supply of the impedance analyzer E4990A, and the value of tanδ was read.

[0113] (Discussion) As is clear from the results in Table 1 above, the biaxially stretched polypropylene film of the present invention contains fullerenes in addition to polypropylene, and thus has an improved dielectric breakdown strength both at room temperature and at high temperature as compared with Comparative Example 1 that does not contain fullerenes. Further, in comparison with Comparative Examples 2 and 3 which are unstretched films, Examples 1 to 5 which require biaxial stretching are excellent in dielectric breakdown strength both at room temperature and at high temperature, regardless of whether they contain fullerenes (Comparative Example 2) or do not contain fullerenes (Comparative Example 3).

Claims

1. A biaxially oriented polypropylene film containing polypropylene and fullerenes.

2. The biaxially oriented polypropylene film according to Claim 1, wherein the polypropylene contains a linear polypropylene resin A.

3. The biaxially oriented polypropylene film according to Claim 2, wherein the polypropylene further contains a branched-chain polypropylene resin B.

4. The biaxially oriented polypropylene film according to Claim 1, wherein the content of the fullerenes in the biaxially oriented polypropylene film is 0.0001% by mass or more and 0.05% by mass or less.

5. The biaxially oriented polypropylene film according to Claim 1, wherein the fullerenes contain 50% by mass or more of C60.

6. The biaxially oriented polypropylene film according to Claim 2, wherein the linear polypropylene resin A has a molecular weight distribution (Mw / Mn) of the weight average molecular weight Mw and the number average molecular weight Mn of 2.0 or more and 10.0 or less.

7. The biaxially oriented polypropylene film according to Claim 2, wherein the linear polypropylene resin A has a weight average molecular weight Mw of 200,000 or more and 400,000 or less.

8. The biaxially oriented polypropylene film according to Claim 2, wherein the linear polypropylene resin A has a number average molecular weight Mn of 30,000 or more and 54,000 or less.

9. The biaxially oriented polypropylene film according to Claim 2, wherein the linear polypropylene resin A has a melt flow rate at 230 °C of 2.0 g / 10 min or more and 10.0 g / 10 min or less.

10. The biaxially oriented polypropylene film according to Claim 1, which is for a capacitor.

11. The biaxially oriented polypropylene film according to Claim 1, wherein the value of tanδ (%) measured in accordance with JIS C2151:2019 of the biaxially oriented polypropylene film is 0.01% or more and 0.1% or less, and it is for a film capacitor for high-frequency and high-current applications.

12. A metallized film having a metal layer on at least one side of the biaxially oriented polypropylene film according to Claim 1.

13. The metallized film according to Claim 12, having a thickness of 1.8 μm or more and 5.0 μm or less.

14. A capacitor including the metallized film according to Claim 12 or 13.

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