Biaxially oriented polypropylene film

Abstract

The present invention provides a biaxially oriented polypropylene film having high relative permittivity, suppressed breakage, and excellent dielectric breakdown strength.　The present invention provides a biaxially oriented polypropylene film having a polypropylene resin as the main component, wherein the biaxially oriented polypropylene film is characterized by containing from 0.5 mass% to less than 20 mass% of a metal oxide relative to the entire biaxially oriented polypropylene film and having a breaking strength of 45 MPa or higher.

Description

Technical Field

[0001] The present invention relates to a biaxially stretched polypropylene film.

Background Art

[0002] A biaxially stretched polypropylene film has excellent electrical properties such as high breakdown voltage and low dielectric loss characteristics, and high moisture resistance. Taking advantage of these characteristics, in electronic and electrical equipment, for example, it is preferably used as a dielectric film for capacitors such as filter capacitors and smoothing capacitors for high-voltage capacitors, various switching power supplies, converters, and inverters.

[0003] When the biaxially stretched polypropylene film is used for applications such as capacitors, inorganic particles are added to increase the relative permittivity for the purpose of obtaining a capacitor with a high capacitance. For example, Patent Document 1 discloses a high-dielectric constant composition obtained by mixing a high-dielectric constant substance composed of an organic substance characterized in that two or more kinds of organic molecules form hydrogen bonds with a polymer matrix, and it is disclosed that a polymer material obtained by mixing an inorganic high-dielectric constant substance such as titanium oxide or barium titanate in a polymer matrix exhibits a high dielectric constant.

[0004] Further, Patent Document 2 discloses that in a polypropylene film for capacitors, adding metal powder, inorganic particles, etc. reduces the breakdown voltage characteristics and processability of the obtained film.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, in these patent documents, not only is the relative permittivity increased by adding inorganic particles to the polypropylene film, but also means for obtaining a polypropylene film in which breakage is suppressed and which has excellent dielectric breakdown strength have not been studied. When metal powder or the like is added to the polypropylene film for the purpose of increasing the relative permittivity, the polypropylene film becomes liable to break, making biaxial stretching difficult, and there is also a problem that the breaking strength of the biaxially stretched polypropylene film obtained by biaxial stretching processing decreases. That is, the polypropylene film disclosed in the above patent documents has a problem that high relative permittivity, suppression of breakage, and dielectric breakdown strength cannot be achieved simultaneously.

[0007] An object of the present invention is to provide a biaxially stretched polypropylene film having a high relative permittivity, suppressed breakage, and excellent dielectric breakdown strength.

Means for Solving the Problems

[0008] As a result of intensive studies to solve the above problems, the present inventors have found that the above object can be achieved by a biaxially stretched polypropylene film containing a metal oxide in a specific range of content and having a breaking strength in a specific range, and have completed the present invention.

[0009] That is, the present invention relates to the following biaxially stretched polypropylene film. 1. A biaxially stretched polypropylene film mainly composed of a polypropylene resin, containing 0.5% by mass or more and less than 20% by mass of a metal oxide with respect to the entire biaxially stretched polypropylene film, having a breaking strength of 45 MPa or more, A biaxially stretched polypropylene film characterized by the above. 2. The biaxially stretched polypropylene film according to item 1, wherein the metal oxide is a metal oxide resulting from a hydration treatment. 3. The biaxially oriented polypropylene film according to claim 1 or 2, having a thickness of 1 to 20 μm. 4. The biaxially oriented polypropylene film according to any one of claims 1 to 3, having a relative permittivity of 2.50 or more. 5. The biaxially oriented polypropylene film according to any one of claims 1 to 4, having a protrusion valley depth on the surface of 0.1 to 1.5 μm. 6. A metallized film for a capacitor, having a metal film on one or both sides of the biaxially oriented polypropylene film according to any one of claims 1 to 5. 7. A capacitor comprising the metallized film for a capacitor according to claim 6.

Advantages of the Invention

[0010] The biaxially oriented polypropylene film of the present invention has a high relative permittivity. Therefore, when a capacitor element is manufactured using the biaxially oriented polypropylene film of the present invention, the capacitor element can exhibit a high capacitance. Further, when a capacitor element is manufactured using the biaxially oriented polypropylene film of the present invention, the number of turns of the element can be reduced, so that the capacitor element can be miniaturized. Furthermore, the biaxially oriented polypropylene film of the present invention is suppressed from breaking and exhibits excellent dielectric breakdown strength.

Embodiments for Carrying Out the Invention

[0011] In this specification, the expressions "containing" and "comprising" include the concepts of "containing", "comprising", "substantially consisting of", and "consisting only of".

[0012] In this specification, the expression "capacitor" includes the concepts of "capacitor", "capacitor element", and "film capacitor".

[0013] 1. Biaxially oriented polypropylene film The biaxially oriented polypropylene film of the present invention is a biaxially oriented polypropylene film mainly composed of a polypropylene resin, which contains a metal oxide in an amount of 0.5% by mass or more and less than 20% by mass based on the whole biaxially oriented polypropylene film, and has a breaking strength of 45 MPa or more.

[0014] Since the biaxially oriented polypropylene film of the present invention having the above characteristics contains a specific amount of a metal oxide, it has a high relative permittivity. Therefore, when a capacitor element is manufactured using the biaxially oriented polypropylene film of the present invention, the capacitor element can exhibit a high capacitance. Further, when a capacitor element is manufactured using the biaxially oriented polypropylene film of the present invention, the number of windings of the capacitor element can be reduced, so that the capacitor element can be miniaturized. Further, in the biaxially oriented polypropylene film of the present invention, since the content of the metal oxide is within a specific range, the breaking strength can be within a specific range despite the biaxially oriented polypropylene film containing the metal oxide, the breakage is suppressed, and the dielectric breakdown strength is excellent. That is, the biaxially oriented polypropylene film of the present invention combines the specific content of the metal oxide and the breaking strength within a specific range, has a high relative permittivity, suppresses breakage, and can exhibit excellent dielectric breakdown strength.

[0015] The layer structure of the biaxially oriented polypropylene film of the present invention is not particularly limited, and it may be formed of multiple layers or may be a single layer. From the viewpoint that the metal oxide is uniformly arranged in the whole biaxially oriented polypropylene film and has excellent dielectric properties, a single layer is preferable for the biaxially oriented polypropylene film of the present invention. Therefore, the single-layer biaxially oriented polypropylene film of the present invention can enjoy the effects of the present invention.

[0016] 1-1. Polypropylene resin The biaxially oriented polypropylene film of the present invention is mainly composed of a polypropylene resin. Preferably, the resin component constituting the biaxially oriented polypropylene film of the present invention is a polypropylene resin. The above-mentioned "main component" means that it is contained in the biaxially oriented polypropylene film in an amount of 50% by mass or more, preferably 70% by mass or more, more preferably 90% by mass or more, still more preferably 95% by mass or more, and particularly preferably 98% by mass or more in terms of solid content.

[0017] The polypropylene resin is not particularly limited, and those that can be used to form a biaxially oriented polypropylene film can be widely used. Examples of the polypropylene resin include propylene homopolymers such as isotactic polypropylene and syndiotactic polypropylene; long-chain branched polypropylene; ultra-high molecular weight polypropylene, etc. Preferably, a propylene homopolymer (propylene homopolymer) is mentioned. Among them, from the viewpoint of heat resistance, isotactic polypropylene is more preferably mentioned. Still more preferably, isotactic polypropylene obtained by homopolymerizing propylene in the presence of a catalyst for olefin polymerization is mentioned. The polypropylene resin may be a single type or a blend of two or more types.

[0018] The weight average molecular weight (Mw) of the polypropylene resin is preferably 200,000 or more and 450,000 or less. When such a polypropylene resin is used, appropriate resin fluidity can be obtained during biaxial orientation, and biaxial orientation becomes easy. For example, it becomes easier to obtain a thin-film biaxially oriented polypropylene film suitable for a small and high-capacity capacitor. In addition, it is preferable because unevenness in the thickness of the biaxially oriented polypropylene film is suppressed. The weight average molecular weight (Mw) of the polypropylene resin is more preferably 250,000 or more from the viewpoints of the thickness uniformity, mechanical properties, thermo-mechanical properties, etc. of the biaxially oriented polypropylene film. The weight average molecular weight (Mw) of the polypropylene resin is more preferably 400,000 or less from the viewpoints of the fluidity of the polypropylene resin and the stretchability when obtaining a thin-film biaxially oriented polypropylene film.

[0019] The molecular weight distribution (Mw / Mn), calculated as the ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn) of the polypropylene resin, is preferably from 4 to 12. Further, the molecular weight distribution (Mw / Mn) is more preferably 5 or more, still more preferably 5.5 or more. Also, the molecular weight distribution (Mw / Mn) is more preferably 10 or less, still more preferably 9 or less, even more preferably 8 or less, and particularly preferably 7 or less. When such a polypropylene resin is used, appropriate resin fluidity can be obtained during biaxial stretching, and it becomes even easier to obtain a biaxially stretched propylene film that is thinned without thickness unevenness. Further, such a polypropylene resin is also preferable from the viewpoint of further improving the dielectric breakdown strength of the biaxially stretched polypropylene film.

[0020] The weight average molecular weight (Mw), number average molecular weight (Mn), and molecular weight distribution (Mw / Mn) of the polypropylene resin can be measured using a gel permeation chromatography (GPC) apparatus. More specifically, for example, it can be measured using the HLC-8121GPC-HT (trade name), a high-temperature GPC measuring machine with a built-in differential refractive index system (RI), manufactured by Tosoh Corporation. 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 flowed as the eluent at a flow rate of 1.0 ml / 10 minutes to obtain the measured values of Mw and Mn. A calibration curve regarding 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.

[0021] The melt flow rate (MFR) of the polypropylene resin at 230°C and a load of 2.16 kg is not particularly limited, but from the viewpoint of the stretchability of the resulting film, etc., it is preferably 7 g / 10 min or less, more preferably 5 g / 10 min or less, still more preferably 3 g / 10 min or less, and particularly preferably 1 g / 10 min or less. Further, from the viewpoint of improving the thickness accuracy of the biaxially stretched polypropylene film of the present invention, it is preferably 0.3 g / 10 min or more, and more preferably 0.5 g / 10 min or more. The MFR can be measured in accordance with JIS K 7210-1999.

[0022] 1-2. Metal oxide The biaxially stretched polypropylene film of the present invention contains a metal oxide. The metal oxide is not particularly limited, and examples include TiO 2 , SiO 2 , ZnO, MgO, Al 2 O 3 , TiBaO 3 , TiSrO 3 , etc. Regarding the metal oxide, it may contain one type of metal oxide, or may contain a mixture of two or more types of metal oxides combined. Among these, from the viewpoint of further improving the relative permittivity of the biaxially stretched polypropylene film, TiO 2 is more preferable.

[0023] The metal oxide is preferably a metal oxide resulting from a hydration treatment. In this specification, examples of the metal oxide resulting from a hydration treatment include: (1) a metal oxide prepared by impregnating polypropylene powder with a metal oxide precursor solution such as a metal alkoxide solution in the method for producing a biaxially stretched polypropylene film described below, removing the solvent, hydrating the metal oxide precursor by steam treatment, and further heating, so that the metal oxide precursor contained in the metal oxide precursor solution changes via a metal oxide precursor hydrate and a metal hydroxide; and (2) a metal oxide prepared by hydrating the metal oxide precursor by exposure treatment in a space containing moisture and further heating, so that the metal oxide precursor contained in the metal oxide precursor solution changes via a metal oxide precursor hydrate and a metal hydroxide. The biaxially stretched polypropylene film containing the metal oxide resulting from the hydration treatment makes it easier to further achieve both excellent breaking strength and a high relative permittivity. Specific treatments of the hydration treatment, namely steam treatment and exposure treatment, will be described in detail in the method for producing a biaxially stretched polypropylene film described below.

[0024] Examples of the metal oxide precursor include metal alkoxides. Examples of the metal alkoxide include metal methoxide, metal ethoxide, metal propoxide, metal butoxide, metal pentoxide, metal hexyloxide, metal heptyloxide, metal octyloxide, metal nonyloxide, metal decyloxide, metal undecyloxide, metal dodecyloxide, and the like. Among these, metal propoxide and / or metal ethoxide are preferable in terms of further excellent relative permittivity and the ability to reduce the average particle diameter of the metal oxide.

[0025] Examples of the metal contained in the metal oxide precursor include Ti, Si, Zn, Mg, Al, Sr, and the like. Among these, Ti and Al are preferable, and Ti is more preferable in terms of further excellent relative permittivity.

[0026] The metal oxide precursor is preferably titanium alkoxide or aluminum alkoxide. Examples of the titanium alkoxide include, for example, Ti(OEt) 4, Ti(OnPr) 4 , Ti(OiPr) 4 , Ti(OnBu) 4 , Ti(OiBu) 4 can be mentioned. Further, as the aluminum alkoxide, Al(OEt) 3 , Al(OnPr) 3 , Al(OiPr) 3 , Al(OnBu) 3 , Al(OiBu) 3 can be mentioned.

[0027] As the metal oxide contained in the biaxially stretched polypropylene film of the present invention prepared from the above metal oxide precursor, TiO 2 , Al 2 O 3 etc. can be mentioned. Among these, from the viewpoint of further improving the relative permittivity of the biaxially stretched polypropylene film, TiO 2 is preferable.

[0028] The metal oxide derived from the hydration treatment is preferably modified with a modifier. When the metal oxide is modified with a modifier, the relative permittivity of the biaxially stretched polypropylene film of the present invention is further improved. Examples of such a modifier include trimethoxysilane, trimethoxy(2-phenylethyl)silane, triethoxy(hexyl)silane, etc. Among these, trimethoxy(2-phenylethyl)silane is preferable in that it can further improve the relative permittivity of the biaxially stretched polypropylene film of the present invention.

[0029] In the biaxially oriented polypropylene film of the present invention, the average particle diameter of the metal oxide is preferably 5 nm or more, more preferably 15 nm or more, and even more preferably 25 nm or more. When the lower limit of the average particle diameter is within the above range, the relative permittivity of the biaxially oriented polypropylene film is further improved. Also, the average particle diameter of the metal oxide is preferably 100 nm or less, more preferably 80 nm or less, and even more preferably 60 nm or less. When the upper limit of the average particle diameter is within the above range, the biaxially oriented polypropylene film becomes even more difficult to break. The method for measuring the average particle diameter of the metal oxide in this specification is based on the method described in the examples.

[0030] The content of the metal oxide in the biaxially oriented polypropylene film is 0.5% by mass or more and less than 20% by mass based on the entire biaxially oriented polypropylene film. When the content of the metal oxide is less than 0.5% by mass, the relative permittivity of the biaxially oriented polypropylene film is inferior. When the content of the metal oxide is 20% by mass or more, the breakage of the biaxially oriented polypropylene film is not suppressed. The content of the metal oxide is preferably 1.5% by mass or more, more preferably 2% by mass or more, and even more preferably 3% by mass or more. Also, the content of the metal oxide is preferably 15% by mass or less, more preferably 10% by mass or less, and even more preferably 7% by mass or less. The method for measuring the content of the metal oxide in the biaxially oriented polypropylene film in this specification is based on the method described in the examples.

[0031] 1-3. Additive The biaxially oriented polypropylene film of the present invention may further contain an additive. The "additive" is generally an additive used in polypropylene resin and is not particularly limited as long as the biaxially oriented polypropylene film of the present invention can be obtained. Additives include, for example, antioxidants; catalysts for converting metal alkoxides into metal oxides, etc. The biaxially oriented polypropylene film of the present invention can contain such additives in an amount that does not have an adverse effect.

[0032] As the "antioxidant", there is no particular limitation as long as it is generally used for polypropylene and the biaxially stretched polypropylene film of the present invention can be obtained. Antioxidants are generally used for two purposes. One purpose is to suppress thermal degradation 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. The antioxidant that suppresses thermal degradation and oxidative degradation in the extruder is also referred to as the "primary agent", and the antioxidant that contributes to improving capacitor performance is also referred to as the "secondary agent". For these two purposes, two types of antioxidants may be used, or one type of antioxidant may be used for the two purposes.

[0033] Examples of the primary agent include 2,6-di-tert-butyl-p-cresol (common name: BHT), etc. The content of the primary agent can be included in an amount of about 1000 ppm to 4000 ppm. Most of the antioxidants for this purpose are consumed in the molding process in the extruder and hardly remain in the film after film forming (generally, the remaining amount is less than 100 ppm).

[0034] As the secondary agent, a hindered phenol-based antioxidant having a carbonyl group can be used.

[0035] The "hindered phenol-based antioxidant having a carbonyl group" is usually a hindered phenol-based antioxidant having a carbonyl group, and there is no particular limitation as long as the biaxially stretched polypropylene film of the present invention can be obtained.

[0036] Examples of hindered phenolic antioxidants having a carbonyl group include triethylene glycol-bis[3-(3-tert-butyl-5-methyl-4-hydroxyphenyl)propionate] (trade name: Irganox 245), 1,6-hexanediol-bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (trade name: Irganox 259), pentaerythrityl tetrakis[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: Adeka Stab AO-50), and the like. As the hindered phenolic antioxidant having a carbonyl group, octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate and pentaerythrityl tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], which have a high molecular weight, are rich in compatibility with polypropylene, have low volatility, and are excellent in heat resistance, are most preferred.

[0037] For the purpose of suppressing deterioration that progresses with time during long-term use, the biaxially oriented polypropylene film of the present invention preferably contains one or more hindered phenolic antioxidants having a carbonyl group (secondary agents). Further, the content in the biaxially oriented polypropylene film is preferably 0.1 part by mass or more and 3 parts by mass or less, more preferably 0.5 part by mass or more and 1.5 parts by mass or less, based on 100 parts by mass of the polypropylene resin.

[0038] The "catalyst for converting metal alkoxide into metal oxide" is used in polypropylene, and as long as the polypropylene film of the present invention can be obtained, there are no particular limitations, and a base catalyst and / or an acid catalyst can be used. For example, as the base catalyst, bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate ("ADEKA STAB LA-77" manufactured by ADEKA Corporation) etc. can be exemplified.

[0039] The content of the base catalyst in the biaxially stretched polypropylene film is preferably 0.1 to 3% by mass based on 100% by mass of the biaxially stretched polypropylene film.

[0040] 1-4. Characteristics of biaxially oriented polypropylene film The biaxially stretched polypropylene film of the present invention has a breaking strength of 45 MPa or more. If the breaking strength is less than 45 MPa, the breaking strength of the biaxially stretched polypropylene film is insufficient. The breaking strength is preferably 47 MPa or more, more preferably 50 MPa or more. Also, the breaking strength is preferably 160 MPa or less, more preferably 120 MPa or less, even more preferably 105 MPa or less, further preferably 85 MPa or less, even further preferably 75 MPa or less, and particularly preferably 70 MPa or less. When the upper limit of the breaking strength is within the above range, the biaxially stretched polypropylene film can suitably contain the metal oxide within a specific range, and the relative permittivity is further improved. The method for measuring the breaking strength of the biaxially stretched polypropylene film in this specification is according to the method described in the examples.

[0041] The thickness of the biaxially stretched polypropylene film is preferably 1 to 20 μm, more preferably 2 to 19 μm. When the lower limit of the thickness is within the above range, the breakage of the biaxially stretched polypropylene film is further suppressed. When the upper limit of the thickness is within the above range, the relative permittivity of the biaxially stretched polypropylene film is further improved. From the viewpoint of moldability, the thickness of the biaxially stretched polypropylene film is more preferably 10 to 18 μm, particularly preferably 12 to 18 μm. From the viewpoint of the capacitance of the capacitor element, the thickness of the biaxially stretched polypropylene film is more preferably 2 to 4 μm, particularly preferably 2 to 3 μm. The method for measuring the thickness of the biaxially stretched polypropylene film in this specification is according to the method described in the examples.

[0042] The relative permittivity of the biaxially stretched polypropylene film is preferably 2.50 or more, more preferably 2.55 or more, further preferably 2.60 or more, and even more preferably 2.70 or more. Also, regarding the upper limit of the relative permittivity of the biaxially stretched polypropylene film, the higher the better, for example, it is 4.00 or less, etc. When the lower limit of the relative permittivity is within the above range, when a capacitor element is manufactured using the biaxially stretched polypropylene film, the capacitance is further improved. Also, when the lower limit of the relative permittivity is within the above range, when manufacturing a capacitor element, the number of turns of the element can be reduced, so that the capacitor element can be further miniaturized. The method for measuring the relative permittivity of the biaxially stretched polypropylene film in this specification is according to the method described in the examples.

[0043] The dielectric breakdown strength of the biaxially stretched polypropylene film is preferably 50 Vac / μm or more, more preferably 70 Vac / μm or more, and further preferably 85 Vac / μm or more. Also, regarding the upper limit value of the dielectric breakdown strength of the biaxially stretched polypropylene film, for example, it is 250 Vac / μm or less, 230 Vac / μm or less, 200 Vac / μm or less, 150 Vac / μm or less, 120 Vac / μm or less, etc. The method for measuring the dielectric breakdown strength of the biaxially stretched polypropylene film in this specification is according to the method described in the examples.

[0044] The protrusion valley depth (Rvk) on the surface of the biaxially oriented polypropylene film is preferably 0.1 to 1.5 μm, more preferably 0.1 to 1.1 μm. When the upper limit of the protrusion valley depth on the surface is within the above range, the dielectric breakdown strength of the biaxially oriented polypropylene film is further improved. The method for measuring the protrusion valley depth (Rvk) on the surface of the biaxially oriented polypropylene film in this specification is by the method described in the examples.

[0045] The biaxially oriented polypropylene film of the present invention described above has a high relative permittivity, suppressed breakage, and 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.

[0046] 1-5. Manufacturing method 1-5-1. Manufacturing method of biaxially oriented polypropylene film The biaxially oriented polypropylene film of the present invention is preferably produced by a production method including, but not limited to, the following steps 1 to 5. (Step 1) An impregnation step of impregnating polypropylene powder with a metal oxide precursor solution. (Step 2) A drying step of removing the solvent in the polypropylene powder-containing mixture obtained in Step 1. (Step 3) A polypropylene resin composition preparation step of heating and melting the polypropylene powder obtained through Step 2 to prepare a polypropylene resin composition. (Step 4) A film forming step of forming the polypropylene resin composition into a film to prepare a polypropylene film. (Step 5) A biaxial stretching step of biaxially stretching the polypropylene film.

[0047] In the above manufacturing method, in Step 1, the polypropylene powder is impregnated with a metal oxide precursor solution. In this specification, impregnation means that the polypropylene powder is immersed in the metal oxide precursor solution, and the metal oxide precursor solution penetrates into and exists inside the pores formed in the polypropylene powder. By using the polypropylene powder in which the pores are formed, the solvent of the metal oxide precursor solution impregnated into the pores in Step 1 is removed in Step 2 to become a metal oxide precursor, and the metal oxide precursor becomes a metal oxide by heat melting in Step 3. Through Steps 4 and 5, a biaxially stretched polypropylene film containing a metal oxide is manufactured. The biaxially stretched polypropylene film of the present invention manufactured through the above Steps 1 to 5 has a high relative permittivity, fracture is suppressed, and it can exhibit an excellent relative permittivity.

[0048] In the above manufacturing method, it is preferable to further have a hydration treatment step between Step 2 and Step 3. By having the hydration treatment step, the metal oxide precursor generated by removing the solvent contained in the polypropylene powder in Step 2 undergoes a hydration treatment and becomes a metal oxide precursor hydrate inside the pores of the polypropylene powder. By heat melting the polypropylene powder containing the metal oxide precursor hydrate inside the pores in Step 3, the metal oxide precursor hydrate becomes a metal oxide through a metal hydroxide, so the production efficiency of the metal oxide is further improved.

[0049] Hereinafter, a manufacturing method of a biaxially stretched polypropylene film having the above hydration treatment step will be exemplarily described.

[0050] (Step 1) Step 1 is an impregnation step of impregnating the polypropylene powder with a metal oxide precursor solution.

[0051] As a method for impregnating a polypropylene powder with a metal oxide precursor solution, for example, a method of mixing the polypropylene powder and the metal oxide precursor solution and stirring under heating to prepare a polypropylene powder-containing mixture can be mentioned. The impregnation is preferably carried out in a non-aqueous atmosphere. For example, the polypropylene powder and the metal oxide precursor solution may be mixed under a nitrogen atmosphere and stirred for 6 to 18 hours while heating at a temperature of 30 to 60°C to prepare a polypropylene powder-containing mixture.

[0052] Examples of the polypropylene powder used in Step 1 include the polypropylene powder described as the polypropylene resin used in the biaxially stretched polypropylene film of the present invention described above. The method for preparing the polypropylene powder used in Step 1 is not particularly limited, and it can be prepared by a conventionally known method. Examples of such methods include conventionally known methods such as the Ziegler-Natta method using a Ziegler-Natta catalyst and the Metallocene method using a Metallocene catalyst.

[0053] The average particle size of the polypropylene powder is preferably 100 to 1000 μm, more preferably 200 to 800 μm. When the average particle size of the polypropylene powder is within the above range, the uniformity of the arrangement of the metal oxide described below in the biaxially stretched polypropylene film of the present invention is further improved. The method for measuring the average particle size of the polypropylene powder in this specification is based on the method described in the examples.

[0054] It is preferable that pores are formed in the polypropylene powder used in Step 1. When pores are formed in the polypropylene powder, the metal oxide precursor solution penetrates into the deep part of the powder, and the metal alkoxide solution is more easily impregnated.

[0055] As a method for producing the polypropylene powder having pores formed therein, for example, the above-mentioned Ziegler-Natta method or porous SiO 2Examples of methods include using a metallocene catalyst fixed to, etc. The polypropylene powder produced by this method has pores formed therein.

[0056] The average pore volume of the polypropylene powder is preferably 0.1 mL / g or more, more preferably 0.3 mL / g or more, and even more preferably 0.5 mL / g or more. Also, the average pore volume is preferably 3 mL / g or less, more preferably 1 mL / g or less, and even more preferably 0.7 mL / g or less. When the average pore volume of the polypropylene powder used in Step 1 is within the above range, the polypropylene powder can be appropriately impregnated with the metal alkoxide solution, and the relative dielectric constant of the produced biaxially stretched polypropylene film is further improved and breakage is further suppressed. The method for measuring the average pore volume of the polypropylene powder in this specification is according to the method described in the examples.

[0057] The average pore volume can be adjusted, for example, by changing the internal form of the solid catalyst component when producing the polypropylene powder by the above-mentioned Ziegler-Natta method. As a means for changing the internal form of the solid catalyst component, there is mentioned the conditions at the time of producing the particles containing magnesium ethoxide crystals which are the precursor of the above solid catalyst component, that is, the means for changing the type of alcohol reacted with metallic magnesium in the production of the above precursor.

[0058] The amount of the polypropylene powder in Step 1, with the total amount of the polypropylene powder, antioxidant, base catalyst, and metal oxide precursor being 100% by mass, is preferably 80 to 99.9% by mass, and more preferably 90 to 97% by mass.

[0059] The above metal oxide precursor solution is not particularly limited as long as the metal oxide precursor is dissolved in the solution. The metal oxide precursor solution contains a metal oxide precursor such as a metal alkoxide and a solvent.

[0060] As the metal oxide precursor, the metal oxide precursor described in the above biaxially stretched polypropylene film of the present invention can be used.

[0061] In Step 1, the amount of the metal oxide precursor is preferably 0.1 to 20% by mass, more preferably 1 to 10% by mass, in terms of metal oxide, with the total amount of the polypropylene powder, antioxidant, catalyst for converting the metal alkoxide into the metal oxide, and the amount of the metal oxide precursor being 100% by mass.

[0062] Examples of the solvent include heptane, methanol, ethanol, toluene, etc. Among these, heptane is preferable from the viewpoint of further improving the uniform dispersibility of the metal oxide in the polypropylene. The amount of the solvent in the metal oxide precursor solution is not particularly limited.

[0063] The metal oxide precursor solution preferably further contains a modifier. When the metal oxide precursor solution contains the modifier, the surface of the metal oxide contained in the biaxially stretched polypropylene film of the present invention is modified by the modifier, and the relative permittivity of the biaxially stretched polypropylene film of the present invention is further improved. As the modifier, the modifiers described in the above biaxially stretched polypropylene film can be used.

[0064] The content (molar ratio) of the modifier in the metal oxide precursor solution is preferably 0.001 to 1 mol, more preferably 0.01 to 0.1 mol, per 1 mol of the metal oxide precursor. When the lower limit of the content of the modifier is within the above range, the relative permittivity of the biaxially stretched polypropylene film of the present invention is further improved.

[0065] The metal oxide precursor solution may contain an antioxidant and a base catalyst. As the antioxidant and the base catalyst, those described in the above biaxially stretched polypropylene film of the present invention can be used.

[0066] The amounts of the antioxidant and the base catalyst in Step 1 are not particularly limited. With the total amount of the polypropylene powder, antioxidant, base catalyst, and the amount of the metal oxide precursor being 100% by mass, the antioxidant is preferably 0.001 to 2% by mass, and the base catalyst is preferably 0.001 to 2% by mass.

[0067] Through the above-described Step 1 (impregnation step), the polypropylene powder is impregnated with the metal oxide precursor solution. A polypropylene powder-containing mixture is prepared by Step 1 and is subjected to Step 2.

[0068] (Step 2) Step 2 is a drying step for removing the solvent in the polypropylene powder-containing mixture obtained in Step 1.

[0069] By Step 2, the solvent of the metal oxide precursor solution impregnated in the polypropylene powder is removed, and the metal oxide precursor is contained in the pores of the polypropylene powder.

[0070] Examples of the method for removing the solvent include known drying methods such as vacuum drying. The drying conditions are not particularly limited as long as the solvent can be removed. For example, drying may be performed under the conditions of a drying temperature of 0 to 40°C and a drying time of 3 to 10 hours.

[0071] By the above-described Step 2 (drying step), the solvent contained in the polypropylene powder is removed.

[0072] (Hydration treatment step) In the above manufacturing method, it is preferable to have a hydration treatment step for performing a hydration treatment on the metal oxide precursor contained in the polypropylene powder between Step 2 and Step 3 described below. By the hydration treatment step, the metal oxide precursor contained in the polypropylene powder is hydrated to become a metal oxide precursor hydrate.

[0073] When the metal oxide precursor is a metal alkoxide, the reaction in which the metal alkoxide contained in the polypropylene powder is hydrated to become a metal alkoxide hydrate in the hydration treatment step can be specifically represented by the following reaction formula. M(OR) n →M(OR) n Hydrate In the above reaction formula, M represents a metal atom, R represents an alkyl group, and n represents a number corresponding to the valence of metal M.

[0074] For example, when the metal alkoxide is titanium tetraisopropoxide, the above reaction formula is as follows. Ti[O(CH(CH 3 ) 2 )] 4 →Ti[O(CH(CH 3 ) 2 )] 4 Hydrate In addition, Ti[O(CH(CH 3 ) 2 )] 4 The hydrate may sometimes further proceed to Ti(OH) 4 .

[0075] Examples of the hydration treatment include steam treatment and exposure treatment. From the viewpoint of more sufficiently hydrating the metal oxide precursor, steam treatment is preferred.

[0076] Examples of the steam treatment method include a method of bringing the polypropylene powder from which the solvent has been removed in Step 2 into contact with steam in a chamber, and a method of exposing the polypropylene powder from which the solvent has been removed in Step 2 to humid air in a chamber.

[0077] As the conditions for the steam treatment, the humidity is preferably 40% RH or more, more preferably 50% RH or more, still more preferably 60% RH or more, and particularly preferably 70% RH or more. Also, the humidity may be 100% RH or less, and preferably 90% RH or less. Also, the temperature is preferably 30°C or more, more preferably 40°C or more, still more preferably 50°C or more. Also, the temperature is preferably 100°C or less, more preferably 80°C or less, and still more preferably 70°C or less. The time for the steam treatment is preferably 1 hour or more, more preferably 5 hours or more, still more preferably 10 hours or more, and particularly preferably 20 hours or more. Also, the time for the steam treatment is preferably 100 hours or less, and more preferably 50 hours or less.

[0078] As a method of exposure treatment, there is a method of exposing polypropylene powder from which a solvent has been removed in Step 2 in a space containing moisture. The space containing moisture may be an open space (outdoor) or a closed space (indoor). The humidity of the space containing moisture is preferably 40% RH or more, more preferably 50% RH or more, still more preferably 60% RH or more, and particularly preferably 70% RH or more. The humidity of the space containing moisture may be 100% RH or less, and preferably 90% RH or less. The exposure time in the space containing moisture is preferably 5 hours or more, more preferably 12 hours or more, and still more preferably 24 hours or more. The temperature of the space during the exposure is preferably -10 to 50°C, more preferably 0 to 40°C, and still more preferably 10 to 30°C.

[0079] By the hydration treatment step described above, the metal oxide precursor contained in the polypropylene powder is subjected to hydration treatment.

[0080] (Step 3) Step 3 is a polypropylene resin composition preparation step of heating and melting the polypropylene powder obtained through the hydration treatment step to prepare a polypropylene resin composition.

[0081] By heating and melting, the metal oxide precursor hydrate contained in the polypropylene powder becomes a metal oxide through a metal hydroxide.

[0082] When the metal oxide precursor is a metal alkoxide, the reaction in Step 3 in which the metal alkoxide hydrate contained in the polypropylene powder becomes a metal oxide through a metal hydroxide can be specifically represented by the following reaction formula. M(OR) n Hydrate → M(OH) n → MO m In the above reaction formula, M is a metal atom, R is an alkyl group, n is a number corresponding to the valence of metal M, and m represents a number that is 1 / 2 of n.

[0083] For example, when the metal alkoxide is titanium tetraisopropoxide, the above reaction formula is as follows. Ti[O(CH(CH 3 ) 2 )] 4 Hydrate → Ti(OH) 4 → TiO 2

[0084] The heating temperature during the heat melting is not particularly limited as long as it is equal to or higher than the melting temperature of polypropylene. For example, the above heating temperature is preferably 160 °C or higher, more preferably 170 °C or higher. Also, the above heating temperature is preferably 300 °C or lower, more preferably 280 °C or lower, and even more preferably 250 °C or lower. The heating time is not particularly limited as long as the metal oxide precursor hydrate can sufficiently change to the metal oxide via the metal hydroxide. For example, the above heating time is preferably 1 minute or longer, more preferably 5 minutes or longer, and even more preferably 10 minutes or longer. Also, the above heating time is preferably 120 minutes or shorter, more preferably 60 minutes or shorter, and even more preferably 30 minutes or shorter.

[0085] The method of heat melting is not particularly limited, and it may be heat melted and kneaded by a conventionally known method using a twin-screw kneader, a melt extruder, or the like.

[0086] In Step 3, an antioxidant may be added to the polypropylene resin composition. Examples of the antioxidant used in Step 3 include the antioxidants described as the antioxidants used in the biaxially stretched polypropylene film of the present invention described above. The addition amount of the antioxidant is preferably 0.01 part by mass or more, more preferably 0.1 part by mass or more, based on 100 parts by mass of the polypropylene resin composition. Also, the addition amount of the antioxidant is preferably 5 parts by mass or less, more preferably 3 parts by mass or less, and even more preferably 2 parts by mass or less, based on 100 parts by mass of the polypropylene resin composition.

[0087] The polypropylene resin composition may be subjected to Step 4 described below in a heated and melted state and formed into a film. Alternatively, it may be extruded into a thin shape such as a string, used as a strand, and then subjected to Step 4. The strand may be cut and formed into pellets before being subjected to Step 4. Further, it may be once formed into pellets using a granulator and then subjected to Step 4. When the polypropylene resin composition is formed into a strand or pellets and then subjected to Step 4, it may be reheated and melted before Step 4.

[0088] Through Step 3 (polypropylene resin composition preparation step) described above, a polypropylene resin composition is prepared.

[0089] (Step 4) Step 4 is a film forming step of forming the polypropylene resin composition into a film to prepare a polypropylene film.

[0090] The method of forming the polypropylene resin composition into a film is not particularly limited, and it can be formed by a conventionally known film forming method. Examples of such film forming methods include hot press forming in which the polypropylene resin composition is formed into a film using a hot press machine, cast forming in which molten resin is extruded into a sheet using a single-layer or multi-layer T-die or I-die connected to a melt extruder, calendar forming, rolling forming, inflation forming, and the like. In addition, from the viewpoint that the metal oxide is uniformly arranged throughout the biaxially stretched polypropylene film and has excellent dielectric properties by forming a single-layer biaxially stretched polypropylene film using a single-layer T-die or I-die, it is preferable to use a single-layer T-die or I-die.

[0091] The temperature of the polypropylene resin composition during film forming is preferably 180°C or higher, more preferably 200°C or higher. Also, the heating temperature is preferably 300°C or lower, more preferably 280°C or lower, and even more preferably 250°C or lower. When the temperature of the polypropylene resin composition is within the above range, film forming is easy and thermal degradation of the polypropylene resin can be suppressed.

[0092] The thickness of the polypropylene film prepared by Step 4 is not limited. The thickness of the polypropylene film prepared by Step 4 is preferably 1 mm or less, more preferably 500 μm or less. Also, the thickness of the polypropylene film prepared by Step 4 is preferably 100 μm or more, more preferably 200 μm or more. When the thickness of the polypropylene film is within the above range, breakage is further suppressed during biaxial stretching in Step 5 described below.

[0093] Through Step 4 (film forming step) described above, the polypropylene resin composition is formed into a film, and a polypropylene film can be prepared.

[0094] (Step 5) Step 5 is a biaxial stretching step of performing biaxial stretching on the polypropylene film. The biaxial stretching method is not particularly limited, and biaxial stretching can be performed by a conventionally known biaxial stretching method using a biaxial stretching machine such as a batch type biaxial stretching machine or a continuous type biaxial stretching machine. Also, the biaxial stretching may be either simultaneous biaxial stretching in which stretching is performed simultaneously in the longitudinal and transverse directions or sequential biaxial stretching in which stretching is performed sequentially in the longitudinal and transverse directions.

[0095] In Step 5, the stretching ratio in the longitudinal direction is preferably 2 to 7 times, more preferably 3 to 5 times. Also, the stretching ratio in the transverse direction is preferably 3 to 11 times, more preferably 5 to 9 times. Also, the stretching temperature is preferably 150°C or higher, more preferably 160°C or higher. Also, the stretching temperature is preferably 180°C or lower, more preferably 170°C or lower.

[0096] Through Step 5 (biaxial stretching step) described above, biaxial stretching is performed on the polypropylene film, and a biaxially stretched polypropylene film is manufactured.

[0097] In the biaxially oriented polypropylene film of the present invention, in order to enhance the adhesion characteristics in post-processes such as a metal vapor deposition process, corona discharge treatment can be performed online or offline after the stretching and heat setting processes are completed. The corona discharge treatment can be performed using a known method. As the atmospheric gas, it is preferable to use air, carbon dioxide gas, nitrogen gas, and a mixed gas thereof.

[0098] 2. Metallized film for capacitor In one aspect of the present invention, it relates to a metallized film for a capacitor (which may also be referred to as "the metallized film of the present invention" in this specification) having a metal film on one or both sides of the biaxially oriented polypropylene film of the present invention. Hereinafter, this will be described in detail.

[0099] The biaxially oriented polypropylene film of the present invention can be provided with electrodes on one or both sides for processing into a capacitor. Such electrodes are not particularly limited as long as the capacitor targeted by the present invention can be obtained, and usually, electrodes used for manufacturing a capacitor can be used. Examples of the electrodes include metal foils, papers metallized on at least one side, and plastic films.

[0100] Since capacitors are increasingly required to be smaller and lighter, it is preferable to directly metallize one or both sides of the biaxially oriented polypropylene film of the present invention to form electrodes. As the metal to be used, for example, simple metals such as zinc, lead, silver, chromium, aluminum, copper, and nickel, mixtures of plural kinds thereof, and alloys thereof can be used. However, considering the environment, economy, and capacitor performance, zinc and aluminum are preferable.

[0101] As a method of directly metallizing the surface of the biaxially oriented polypropylene film of the present invention, for example, a vacuum evaporation method and a sputtering method can be exemplified, and it is not particularly limited as long as the capacitor aimed at by the present invention can be obtained. From the viewpoints of productivity and economy, the vacuum evaporation method is preferable. As the vacuum evaporation method, generally, a crucible method, a wire method, etc. can be exemplified, but it is not particularly limited as long as the capacitor aimed at by the present invention can be obtained, and an appropriate optimum one can be selected as appropriate.

[0102] From the viewpoint of the electrical characteristics of the capacitor, the film resistance of the metal vapor deposition film is preferably about 1 to 100 Ω / sq. Even within this range, it is desirable to be on the higher side from the viewpoint of self-healing characteristics, and the film resistance is more preferably 5 Ω / sq. or more, and even more preferably 10 Ω / sq. or more. Also, from the viewpoint of the safety of the capacitor, the film resistance is more preferably 50 Ω / sq. or less, and even more preferably 30 Ω / sq. or less. The film resistance of the metal vapor deposition film can be measured during metal vapor deposition by, for example, the four-terminal method known to those skilled in the art. The film resistance of the metal vapor deposition film can be adjusted, for example, by adjusting the output of the evaporation source to adjust the evaporation amount.

[0103] When forming a metal vapor deposition film on one side of the biaxially oriented polypropylene film, an insulating margin is formed without vapor deposition over a certain width from one end of the film so that a capacitor is formed when the film is wound. Further, in order to strengthen the bonding between the metallized polypropylene film and the metallicon electrode, it is preferable to form a heavy edge structure at the end opposite to the insulating margin. The film resistance of the heavy edge is usually about 1 to 8 Ω / sq., and preferably about 1 to 5 Ω / sq. The thickness of the metal vapor deposition film is not particularly limited, but is preferably 1 to 200 nm.

[0104] There is no particular limitation on the margin pattern of the metal vapor deposition film to be formed. However, from the viewpoint of improving characteristics such as the safety of the capacitor, it is preferable to use a pattern including a so-called special margin such as a fishnet pattern or a T margin pattern. When the metal vapor deposition film is formed on one side of the polypropylene film of the present invention using a pattern including a special margin, the safety of the resulting capacitor is improved, which is effective and preferable in terms of suppressing capacitor breakdown and short circuits.

[0105] As a method for forming the margin, known methods such as a tape method in which masking is performed with a tape during vapor deposition and an oil method in which masking is performed by applying oil can be used without any limitation.

[0106] The metallized film of the present invention can be processed into the capacitor of the present invention described below through a winding process in which it is wound along the longitudinal direction of the metallized film. That is, two metallized films of the present invention are used as a pair, and they are overlapped and wound so that the metal vapor deposition film and the biaxially stretched polypropylene film are alternately laminated. Thereafter, a pair of metallicon electrodes are formed on both end faces by metal spraying to obtain a capacitor through a process of manufacturing a film capacitor.

[0107] 3. Capacitor In one aspect, the present invention relates to a capacitor (which may also be referred to as "the capacitor of the present invention" in this specification) including the metallized film of the present invention. This will be described in detail below.

[0108] In the process of manufacturing the capacitor, a winding process of the metallized film for the capacitor is performed. For example, two metallized films of the present invention are overlapped and wound so that the metal film in the metallized film of the present invention and the biaxially stretched polypropylene film of the present invention are alternately laminated, and further, so that the insulating margin portion is on the reverse side. At this time, it is preferable to laminate the two metallized films of the present invention as a pair with a shift of 1 to 2 mm. The winding machine to be used is not particularly limited, and for example, an automatic winding machine 3KAW-N2 type manufactured by Minato Seisakusho Co., Ltd. can be used.

[0109] When manufacturing a flat capacitor, after winding, usually, pressing is applied to the obtained wound product. Pressing promotes winding tightening and element shaping of the capacitor. From the viewpoint of controlling and stabilizing the interlayer gap, the pressure to be applied varies depending on the thickness of the biaxially stretched polypropylene film of the present invention, etc., but is 2 to 20 kg / cm 2 is.

[0110] Subsequently, a capacitor is manufactured by spraying metal on both end faces of the wound product to provide metallized electrodes.

[0111] The capacitor may be further subjected to a predetermined heat treatment. For example, in the process of manufacturing a capacitor, a step of subjecting the capacitor to heat treatment under vacuum at a temperature of 80 to 125°C for 1 hour or more (hereinafter, sometimes referred to as "thermal aging") may be included.

[0112] In the above step of subjecting the capacitor to heat treatment, the temperature of the heat treatment is usually 80°C or higher, and preferably 90°C or higher. On the other hand, the temperature of the heat treatment is usually 130°C or lower, and preferably 125°C or lower. By performing heat treatment at the above temperature, the effect of thermal aging can be obtained. Specifically, the voids between the films constituting the capacitor based on the metallized film of the present invention are reduced, corona discharge is suppressed, and moreover, the internal structure of the metallized film of the present invention changes and crystallization progresses. As a result, it is considered that the dielectric breakdown strength is improved. When the temperature of the heat treatment is lower than the predetermined temperature, the above effects of thermal aging cannot be sufficiently obtained. On the other hand, when the temperature of the heat treatment is higher than the predetermined temperature, thermal decomposition, oxidative degradation, etc. may occur in the biaxially stretched polypropylene film.

[0113] As a method of subjecting the capacitor to heat treatment, for example, a known method including a method using a constant temperature bath or a method using high-frequency induction heating in a vacuum atmosphere may be appropriately selected. Specifically, it is preferable to adopt a method using a constant temperature bath.

[0114] The heat treatment time is preferably 1 hour or more, more preferably 10 hours or more, from the viewpoint of obtaining mechanical and thermal stability, but more preferably 20 hours or less from the viewpoint of further preventing molding defects such as heat wrinkles and molding.

[0115] Leads are usually welded to the metallized electrodes of the capacitors subjected to heat aging. Further, in order to impart weather resistance and particularly to prevent humidity deterioration, it is preferable to enclose the capacitor in a case and pot it with an epoxy resin.

[0116] The capacitor of the present invention is a small and large-capacity capacitor based on the metallized film of the present invention and has a high dielectric breakdown strength.

Example

[0117] The present invention will be described more specifically by way of examples, but these examples are for explaining the present invention and do not limit the present invention in any way.

[0118] (1) Manufacture of biaxially oriented polypropylene film <Example 1> (Preparation of powder of propylene homopolymer before impregnation) Propylene was put into a polymerization reactor and polymerized using a Ziegler-Natta catalyst (TiCl 4 / MgCl 2 / dibutyl phthalate type) to prepare a powder of propylene homopolymer before impregnation. The density of the prepared propylene homopolymer before impregnation was 0.9 g / cm 3 , and the melt flow rate (MFR) was 0.5 g / 10 min (JIS K 7210, 1999). Further, the prepared propylene homopolymer before impregnation had Mw = 2.6×10 5 , Mw / Mn = 5.69, and mmmm = 98 mol%. Further, the average particle diameter of the prepared powder of propylene homopolymer before impregnation was 480 μm, and the average pore volume was 0.56 mL / g.

[0119] The average particle diameter and average pore volume of the propylene homopolymer powder before impregnation were measured by the following method.

[0120] (Average particle diameter) The average particle diameter of the propylene homopolymer powder before impregnation was measured using a particle size distribution measuring device (Partica LA-950V2 manufactured by HORIBA). Ethanol was used as the solvent. The average particle diameter is represented as D 50 and D 10 , D 50 , D 90 correspond to the particle diameters at 10%, 50%, and 90% in the cumulative distribution, respectively. The particle size distribution (RSF) is defined as (D 90 - D 10 ) / D 50 .

[0121] (Average pore volume) The average pore volume of the propylene homopolymer powder before impregnation was measured by the mercury intrusion method using mercury porosimetry (Shimadzu, Autopore IV 9505). Specifically, it is calculated as [the amount of mercury injected into the pores of the PP powder filled in the sample cell] ÷ [the weight of the PP powder filled in the sample cell].

[0122] (Process 1) Powders of propylene homopolymer before impregnation, an antioxidant, octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate (ADEKA's "ADEKA STAB AO-50"), a base catalyst, bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate (ADEKA's "ADEKA STAB LA-77"), titanium tetraisopropoxide as a metal oxide precursor, trimethoxysilane as a modifier, and heptane as a solvent were added into a reaction vessel, stirred at 50 °C for 12 hours under a nitrogen atmosphere to prepare a mixture containing polypropylene powder. The addition amount of each reagent was based on the total blending amount of the powder of propylene homopolymer, antioxidant, base catalyst, and metal oxide precursor being 100% by mass. The powder of propylene homopolymer was 96.5% by mass, the antioxidant (AO-50) was 1.0% by mass, the base catalyst (LA-77) was 0.5% by mass, and the metal oxide precursor (titanium tetraisopropoxide) was 2.0% by mass in terms of titanium oxide conversion (converted as if it were converted to 100% titanium oxide). Also, trimethoxysilane was added in an amount of 0.06 mol (0.00008 mol) per 1 mol of titanium tetrapropoxide, and 5 ml of heptane was added.

[0123] (Process 2) The mixture containing polypropylene powder obtained in Process 1 was dried under vacuum at 23 °C for 6 hours to remove the solvent (heptane) in the mixture containing polypropylene powder, and a powder of propylene homopolymer was obtained.

[0124] (Hydration treatment process) The powder of propylene homopolymer obtained through Processes 1 and 2 was subjected to steam treatment. Specifically, the powder of propylene homopolymer obtained through Processes 1 and 2 was exposed in a chamber at a humidity of 100% and 50 °C for 24 hours, thereby subjecting the titanium tetraisopropoxide impregnated in the propylene powder to steam treatment. By this treatment, the titanium tetraisopropoxide in the powder of propylene homopolymer was hydrated.

[0125] (Step 3) To 100 parts by mass of the powder of the propylene homopolymer obtained through the water treatment step, 1.0 part by mass of pentaerythrityl tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (manufactured by BASF Japan Ltd., "Irganox 1010") was further added as an antioxidant, and melt kneaded at 180 °C and 200 rpm for 15 minutes using a twin-screw kneader (Micro Compounder IM5, manufactured by Xplore Instruments), and then extruded into a string-like shape to prepare a polypropylene resin composition (a strand with a diameter of about 5 mm). The titanium tetraisopropoxide hydrate contained in the powder of the propylene homopolymer was converted to titanium oxide by melt kneading in the polypropylene resin composition.

[0126] (Step 4) The polypropylene resin composition (strand) was cut to a length of about 1 to 2 cm, and after the cut polypropylene resin composition (strand) was supplied to a hot press machine, it was melted at 230 °C for 5 minutes. Next, while maintaining the temperature at 230 °C, the gas in the strand was removed, and then melt pressing was performed at 20 MPa for 5 minutes. Then, the melt-pressed film-like polypropylene resin composition was annealed (slowly cooled) at 100 °C for 5 minutes, and further allowed to stand at 0 °C for 3 minutes to obtain a single-layer non-stretched polypropylene film with a thickness of 300 μm.

[0127] (Step 5) The polypropylene film obtained in Step 4 was preheated at 165 °C using a biaxial stretching machine (batch type stretching machine "KARO", manufactured by Bruckner Maschinenbau GmbH & Co. KG). It was biaxially stretched under the conditions of stretching 4 times in the MD direction (longitudinal axis direction) at 165 °C and then stretching 7 times in the TD direction (transverse axis direction) at 165 °C to produce a single-layer biaxially stretched polypropylene film. The thickness of the biaxially stretched polypropylene film was 12 μm.

[0128] <Example 2> A biaxially oriented polypropylene film was produced in the same manner as in Example 1, except that trimethoxy(2-phenylethyl)silane was used as the modifier and the thickness of the biaxially oriented polypropylene film was changed to 13 μm.

[0129] <Example 3> A biaxially oriented polypropylene film was produced in the same manner as in Example 1, except that triethoxy(hexyl)silane was used as the modifier and the thickness of the biaxially oriented polypropylene film was changed to 14 μm.

[0130] <Example 4> A biaxially oriented polypropylene film was produced in the same manner as in Example 1, except that trimethoxy(2-phenylethyl)silane was used as the modifier, the amount of the modifier was 0.00012 mol, the addition amount of titanium tetraisopropoxide was 3.0% by mass in terms of titanium oxide, and accordingly the content of the powder of the propylene homopolymer was adjusted, and the thickness of the biaxially oriented polypropylene film was changed to 15 μm.

[0131] <Example 5> A biaxially oriented polypropylene film was produced in the same manner as in Example 1, except that trimethoxy(2-phenylethyl)silane was used as the modifier, the amount of the modifier was 0.00020 mol, the addition amount of titanium tetraisopropoxide was 5.0% by mass in terms of titanium oxide, and accordingly the content of the powder of the propylene homopolymer was adjusted, and the thickness of the biaxially oriented polypropylene film was changed to 18 μm.

[0132] <Example 6> A biaxially oriented polypropylene film was produced in the same manner as in Example 1, except that no modifier was used, the addition amount of titanium tetraisopropoxide was 3.0% by mass in terms of titanium oxide, and accordingly the content of the powder of the propylene homopolymer was adjusted, and the thickness of the biaxially oriented polypropylene film was changed to 13 μm.

[0133] <Example 7> Without using a modifier, the addition amount of titanium tetraisopropoxide was 5.0% by mass in terms of titanium oxide, and accordingly, the content of the propylene homopolymer powder was adjusted. A biaxially stretched polypropylene film was produced in the same manner as in Example 1, except that the thickness of the biaxially stretched polypropylene film was changed to 14 μm.

[0134] <Example 8> A biaxially stretched polypropylene film was produced in the same manner as in Example 1, except that the thickness of the biaxially stretched polypropylene film was changed to 14 μm, and instead of the steam treatment, the exposure treatment was carried out by exposing it to air (50% RH) in an open space for 24 hours.

[0135] <Example 9> As the modifier, trimethoxy(2-phenylethyl)silane was used, the amount of the modifier was 0.00004 mol, the addition amount of titanium tetraisopropoxide was 1.0% by mass in terms of titanium oxide, and accordingly, the amount of the propylene homopolymer powder was adjusted. A biaxially stretched polypropylene film was produced in the same manner as in Example 1, except that the thickness of the biaxially stretched polypropylene film was changed to 11 μm.

[0136] <Example 10> Instead of Step 1 of Example 1, the following Step 1 was carried out. (Project 1) The powder of the propylene homopolymer before impregnation, and as antioxidants, octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate (manufactured by Adeka Corporation, "Adekastab AO-50"), as a base catalyst, bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate (manufactured by Adeka Corporation, "Adekastab LA-77"), aluminum triisopropoxide as a metal oxide precursor, trimethoxy(2-phenylethyl)silane as a modifier, and heptane as a solvent were added into a reaction vessel, and stirred at 50 °C for 12 hours under a nitrogen atmosphere to prepare a polypropylene powder-containing mixture. The addition amount of each reagent was based on the total blending amount of the powder of the propylene homopolymer, antioxidant, base catalyst, and metal oxide precursor being 100% by mass. The powder of the propylene homopolymer was 97.5% by mass, the antioxidant (AO-50) was 1.0% by mass, the base catalyst (LA-77) was 0.5% by mass, and the metal oxide precursor (aluminum triisopropoxide) was 1.0% by mass in terms of aluminum oxide conversion (converted as if it were converted to 100% aluminum oxide). Also, trimethoxy(2-phenylethyl)silane was added in an amount of 0.06 mol per 1 mol of aluminum triisopropoxide (0.00003 mol), and 5 ml of heptane was added.

[0137] Also, the thickness of the biaxially stretched polypropylene film was changed to 10 μm. Otherwise, in the same manner as in Example 1, a biaxially stretched polypropylene film was produced.

[0138] <Example 11> The amount of the modifier was 0.00006 mol, the addition amount of aluminum triisopropoxide was 2.0% by mass in terms of aluminum oxide conversion, and accordingly, the amount of the powder of the propylene homopolymer was adjusted. A biaxially stretched polypropylene film was produced in the same manner as in Example 10 except that the thickness of the biaxially stretched polypropylene film was changed to 13 μm.

[0139] <Comparative Example 1> Without using a modifier and without adding titanium tetraisopropoxide (0% by mass in terms of titanium oxide), and adjusting the amount of the powder of the propylene homopolymer accordingly, a biaxially stretched polypropylene film was produced in the same manner as in Example 1 except that steam treatment was not performed.

[0140] <Comparative Example 2> In the same manner as in Example 1, a powder of the propylene homopolymer before impregnation was prepared. Next, a commercially available titanium oxide powder (AEROXIDE TiO 2 p25 with an average particle diameter of 21 nm) was added, and melt kneading was performed at 180 °C and 200 rpm for 15 minutes using a twin-screw kneader (Micro Compounder IM5, manufactured by Xplore Instruments) to prepare a polypropylene resin composition. Next, the polypropylene resin composition was extruded into a string shape to obtain a strand. The addition amount of the titanium oxide powder was 2.0% by mass based on 100% by mass of the polypropylene resin composition.

[0141] Using the above polypropylene resin composition (strand), the same process as in Step 4 of Example 1 was performed to obtain a single-layer non-stretched polypropylene film with a thickness of 301 μm.

[0142] Using the polypropylene film obtained in Step 4, an attempt was made to perform biaxial stretching by the same process as in Step 5 of Example 1, but breakage occurred and a homogeneous biaxially stretched polypropylene film could not be produced. The thickness of the broken edge during biaxial stretching was 20 μm. In the following measurement method, for Comparative Example 2, the broken edge was used.

[0143] (2) Measurement method Regarding the obtained biaxially stretched polypropylene film, the properties were evaluated under the following measurement conditions.

[0144] (Film thickness) Measurement was carried out in accordance with JIS K 7130:199 A method using an external micrometer (Mitutoyo Corporation, high-precision digital micrometer MDH-25MB).

[0145] (Metal oxide content) Using a differential scanning thermogravimetric analyzer (manufactured by Seiko Instruments Inc., TG / DTA-6200), the content of metal oxides contained in the biaxially stretched polypropylene films of the examples and comparative examples was measured. Specifically, using the differential scanning thermogravimetric analyzer, the biaxially stretched polypropylene films of each example and comparative example were heated in dry air at 10 °C / min under the condition of a measurement temperature of 20 to 600 °C. The weight of the ash after heating was considered as the weight of the metal oxide, and the mass percentage of the ash after heating with respect to 100% by mass of the biaxially stretched polypropylene film before heating was calculated and taken as the content of the metal oxide.

[0146] (Tensile strength) The tensile strength was measured in accordance with JIS K 7127:1999. Specifically, a rectangular sample with a length of 150 mm and a width of 10 mm was cut out from the biaxially stretched polypropylene films of the examples and comparative examples. At this time, the sample was cut out so that the MD direction became the length direction. After setting the sample in the chuck of a tensile testing machine (Tensilon universal testing machine RTG-1210 manufactured by A&D Company, Limited) with a chuck interval of 50 mm, a tensile test was performed at a test speed of 300 mm / min. The temperature during the tensile test was 23 °C. The breaking strength (MPa) was calculated by dividing the load value at the time of sample fracture in the tensile test by the cross-sectional area of the sample before the tensile test (the thickness of the sample before the tensile test × width 10 mm). The tensile test was performed 5 times, and the average value was taken as the breaking strength (MPa).

[0147] (Relative permittivity εr) On both sides of the biaxially stretched polypropylene films (hereinafter also referred to as "samples" or "films") of the examples and comparative examples, a mold with an opening of φ30 mm was applied respectively, and the two molds were fixed so that the openings of the two molds overlapped with the sample sandwiched therebetween. Next, argon gas was introduced into a sputtering apparatus (manufactured by Vacuum Devices Co., Ltd., model: MSP-20-TK) equipped with a gold target, and a gold layer was formed on each side of the sample so that the resistance value became 2 Ω / □. As a result, a capacitor having a gold electrode with a diameter of 30 mm on each side of the sample was prepared.

[0148] To measure the capacitance of the prepared capacitor, a measurement circuit was configured as follows. That is, a brass table was placed on a workbench covered with insulating rubber (product number: YS-231-23-21 manufactured by Yotsugi Co., Ltd.), and a conductive rubber of φ40 mm was placed on the brass table. Next, the capacitor and a brass cylinder (φ27 mm, mass 250 g) were placed on the conductive rubber in this order so that their centers coincided. At this time, the flat part of the brass cylinder was made to contact the gold electrode of the capacitor. Next, each lead wire of a capacitance measuring device (manufactured by Hioki Electric Co., Ltd., LCR Hi Tester 3522-50) was connected to the brass table and the brass cylinder respectively, and the capacitance C of the capacitor was measured. The measurement was performed under the measurement conditions of an applied voltage of 1 V and a measurement frequency of 1 kHz after leaving it to stand in a thermostat set at 23 °C for 30 minutes. Next, the relative permittivity εr of the sample was calculated by the following formula. The permittivity of vacuum ε0 was a value of 8.85×10 -12 F / m was used. (εr)=[(capacitance C)×(film thickness)] / [(permittivity of vacuum ε0)×(electrode area)]

[0149] (Dielectric breakdown strength (AC withstand voltage evaluation)) The breakdown voltage (BDV) of biaxially oriented polypropylene films in Examples and Comparative Examples was measured 12 times under the following test conditions according to the electrode configuration described in JIS S C2152 (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 voltage increase was defined as the BDV. The BDV was divided by the thickness (μm) of the biaxially oriented polypropylene film, and the average value of 8 numerical values excluding the top 2 points and the bottom 2 points among the 12 measurement results was defined as the breakdown strength ES (V AC / μm). Test piece: Approximately 150 mm × 150 mm Conditioning of test piece: 60 minutes under atmospheric conditions Power supply: Alternating current (frequency 60 Hz) Atmosphere: 100 °C Testing machine: Dielectric withstand voltage tester TOS5051A manufactured by Kikusui Electronics Industry Co., Ltd. Voltage increase rate: 100 V / second Upper limit reference value: 5 mA

[0150] (Depth of valley of surface protrusion [Rvk]) The depth of valley of surface protrusion (Rvk) of the biaxially oriented polypropylene film was measured by the following measurement method. That is, “VertScan2.0 (model: R5500GML)” manufactured by Ryoka Systems Co., Ltd. was used as an optical interference type non-contact surface shape measuring machine. As a measurement sample, the biaxially oriented polypropylene film was cut out to an arbitrary size of about 20 cm square, and in a state where wrinkles were fully stretched, it was set on the measurement stage using an electrostatic adhesion plate or the like.

[0151] The WAVE mode was used for measurement, a 530white filter and a 1×BODY lens barrel were applied, and measurement was performed for each field of view (353.16 μm × 470.92 μm) using a 10× objective lens. This operation was performed at 10 locations at 1 cm intervals in the flow direction from the center of the surface of the measurement sample in both the flow direction and the width direction.

[0152] Next, noise removal processing was performed on the obtained data using a median filter (3×3), and then Gaussian filter processing was performed with a cut-off value of 30 μm to remove the undulation component. As a result, the surface state was made suitable for measurement.

[0153] Next, analysis was performed using the "ISO parameter" in the "Bearing" plug-in function of the analysis software "VS-Viewer" of "VertScan2.0" to obtain Rvk, and the average value of the values obtained at the above 10 locations was calculated and used as the protrusion valley depth (Rvk) of the surface of the biaxially stretched polypropylene film.

[0154] (Average particle diameter of metal oxide (TiO 2 , Al 2 O 3 ) The average particle diameter of the metal oxide (TiO 2 , Al 2 O 3 ) in the biaxially stretched polypropylene film was measured by the following measurement method. That is, using the biaxially stretched polypropylene film, an ultra-thin section with a thickness of 100 nm for observation was prepared using an ultramicrotome (ULTRACUTS FCS manufactured by Leica). The ultra-thin section was observed using a transmission electron microscope (TEM) (H-7100 manufactured by Hitachi), the particle diameters of 100 particles of the metal oxide were counted, and the average value was taken as the average particle diameter of the metal oxide.

[0155] (3) Measurement results The results of the above characteristic evaluations of the examples and comparative examples are shown in Table 1.

[0156]

Table 1

[0157] In Comparative Example 2, biaxial stretching was attempted, but the polypropylene film broke. Therefore, each characteristic in Comparative Example 2 in Table 1 was measured using the cut end of the film that broke during stretching.

[0158] In Comparative Example 2, titanium oxide powder with an average particle diameter of 21 nm was used. However, since the titanium oxide powder aggregated during melt-kneading, the measured value of the average particle diameter of the titanium oxide became 40.5 nm.

Claims

1. A biaxially oriented polypropylene film containing a polypropylene resin as a main component, containing 0.5% by mass or more and 10% by mass or less of a metal oxide with respect to the entire biaxially oriented polypropylene film, The metal oxide is TiO ２ , ZnO, MgO, Al ２ O ３ , TiBaO ３ , and TiSrO ３ and is at least one selected from the group consisting of wherein the metal oxide is a hydrated metal oxide, the average particle diameter of the metal oxide is 5 nm or more and 100 nm or less, the breaking strength is 45 MPa or more, and the thickness is 1 to 20 μm, characterized in that it is a biaxially oriented polypropylene film.

2. The biaxially oriented polypropylene film according to Claim 1, wherein the average particle diameter of the metal oxide is 15 nm or more and 80 nm or less.

3. The biaxially oriented polypropylene film according to Claim 1 or 2, containing 7% by mass or less of the metal oxide with respect to the entire biaxially oriented polypropylene film.

4. The biaxially oriented polypropylene film according to any one of Claims 1 to 3, having a relative permittivity of 2.50 or more.

5. The biaxially oriented polypropylene film according to any one of Claims 1 to 4, wherein the depth of the protrusions and valleys on the surface is 0.1 to 1.5 μm.

6. A metallized film for a capacitor, having a metal film on one or both surfaces of the biaxially oriented polypropylene film according to any one of Claims 1 to 5.

7. A capacitor, comprising the metallized film for a capacitor according to Claim 6.

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