Biaxially oriented polypropylene film

Abstract

To provide a polypropylene film which achieves all of the followings: stable film formation over a longer period of time; higher voltage resistance (especially, voltage resistance at high temperature); and a higher yield rate.SOLUTION: A biaxially oriented polypropylene film has a first surface and a second surface, where the average maximum length of substantially elliptical spots on the first surface is 3.0 mm or less, and where the difference between the average protruding peak height Rpk outside the spots and the average protruding peak height Rpk inside the spots is 0.040 μm or less.SELECTED DRAWING: None

Description

Technical Field

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

Background Art

[0002] Polypropylene films have excellent electrical properties such as high breakdown voltage and low dielectric loss characteristics, and also have high moisture resistance. Therefore, they are widely used in electronic and electrical equipment. Specifically, for example, they are used as films for high-voltage capacitors; capacitors for filters and smoothing capacitors in power conversion circuits such as converters and inverters.

[0003] Particularly in recent years, polypropylene films have begun to be widely used as capacitors for inverter power supply devices that control drive motors in electric vehicles, hybrid vehicles, etc. Capacitors for inverter power supply devices used in vehicles such as automobiles are required to be small, lightweight, have a high capacitance, and have high reliability over a long period of time.

[0004] Patent Document 1 discloses a biaxially stretched polypropylene film for a capacitor in which the number per 0.1 mm of protrusions and the 10-point average roughness satisfy a predetermined relationship. As an effect of the biaxially stretched polypropylene film for a capacitor having the above-described configuration, Patent Document 1 describes that even a thin film has excellent processability and exhibits high breakdown voltage under a wide range of ambient temperature conditions from low temperature (-40°C) to high temperature (150°C). 2

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] As a resin film for capacitors in electric vehicles, hybrid vehicles, etc., polypropylene films are required to have a thinner film thickness and a larger electrode area due to the recent miniaturization and higher capacitance of capacitors. In recent years, there has been a strong demand for further cost reduction. As a method for producing the film, a method of stretch-forming a resin sheet at high speed has been studied, and it is required to be able to manufacture capacitor elements having a high yield and withstand voltage characteristics (particularly, withstand voltage characteristics at high temperatures).

[0007] Therefore, an object of the present invention is to provide a polypropylene film that can be stably film-formed for a longer time and has a higher yield rate for a film having higher withstand voltage characteristics (particularly, withstand voltage characteristics at high temperatures).

Means for Solving the Problems

[0008] In the course of the research, the present inventor has found the following.

[0009] Since the polypropylene film for the above applications is thin and flexible, the cast sheet of the precursor before stretching is also thin and flexible. When such a cast sheet is continuously conveyed, air bubbles are likely to penetrate between the sheet and the conveying roll. In such a case, the adhesion to the roll becomes non-uniform, and the non-adhered portions appear as elliptical depressions on the sheet surface.

[0010] In addition, the conveying roll is heated for the purpose of expressing β-crystals on the surface of the cast sheet and imparting plasticity to the sheet before stretching. When the above-mentioned elliptical depression portions are scattered on the sheet surface, the adhesion state of these portions to the conveying roll is different from that of the other portions, so there is a difference in the generation of β-crystals and the roughening state during stretching. In such a case, elliptical spots appear scattered on the film surface.

[0011] ​When the difference in surface roughness between the "spots" and the other parts increases, the film tends to break during lateral stretching. This is thought to be because when there are parts formed with different heat histories within the sheet plane, the plasticity of the resin during lateral stretching becomes locally non-uniform, inducing stretching rupture of the film.

[0012] Also, the spots tend to cause a decrease in the withstand voltage of the capacitor element and significantly reduce the yield in practical use. When there is a local difference in surface roughness within the sheet plane, it is considered that a local difference in interlayer adhesion occurs inside the wound metallized polypropylene film, and the film is damaged due to electric field concentration.

[0013] As a result of further research based on the above findings, the inventor has found that a biaxially stretched polypropylene film having a first surface and a second surface, wherein the average maximum length of the substantially elliptical spots on the first surface is 3.0 mm or less, and the difference between the average peak height Rpk outside the spots and the average peak height Rpk inside the spots is 0.040 μm or less, can solve the above problems. That is, the present invention includes the following aspects.

[0014] Item 1. A biaxially stretched polypropylene film having a first surface and a second surface, wherein the average maximum length of the substantially elliptical spots on the first surface is 3.0 mm or less, and the difference between the average peak height Rpk outside the spots and the average peak height Rpk inside the spots is 0.040 μm or less, biaxially stretched polypropylene film.

[0015] Item 2. The biaxially stretched polypropylene film according to Item 1, wherein the difference between the average valley depth Rvk outside the spots and the average valley depth Rvk inside the spots is 0.020 μm or less.

[0016] Item 3. The biaxially stretched polypropylene film according to Item 1, wherein the range of the change width of the slow axis angle is 0.3° or more and 2.8° or less.

[0017] Item 4. The biaxially oriented polypropylene film according to Item 1, wherein the average number of substantially elliptical spots per 11.5 mm × 8.6 mm area of the first surface is 0.6 or less.

[0018] Item 5. The polypropylene resin constituting the biaxially oriented polypropylene film has a weight average molecular weight Mw of 250,000 or more and 450,000 or less, a ratio (Mw / Mn) of the weight average molecular weight Mw to the number average molecular weight Mn of 5.0 or more and 12.0 or less, a melt flow rate at 230 °C and a load of 2.16 kg of 7.0 g / 10 min or less, and a heptane insoluble content of 96.0% or more and 99.5% or less. The biaxially oriented polypropylene film according to Item 1.

[0019] Item 6. The biaxially oriented polypropylene film according to Item 1, having a thickness of 1.7 μm or more and 6.5 μm or less.

[0020] Item 7. The biaxially oriented polypropylene film according to Item 1, which is a single-layer film.

[0021] Item 8. The biaxially oriented polypropylene film according to any one of Items 1 to 7, which is for a capacitor.

[0022] Item 9. A metal layer integrated polypropylene film including the biaxially oriented polypropylene film according to any one of Items 1 to 7 and a metal layer disposed on one or both surfaces of the biaxially oriented polypropylene film.

[0023] Item 10. A capacitor including the metal layer integrated polypropylene film according to Item 9.

[0024] Item 11. A method for manufacturing the biaxially oriented polypropylene film according to any one of Items 1 to 7, including obtaining a cast sheet using a casting drum having a microcrack surface in which when a virtual line having a length of 0.1 mm in the widthwise direction is provided at an arbitrary position on the surface, the virtual line intersects one or more and 15 or less grooves and the groove width is 1 μm or more and 10 μm or less, and subjecting the cast sheet to a biaxial stretching treatment.

Effect of the Invention

[0025] According to the present invention, it is possible to provide a biaxially oriented polypropylene film that can be stably film-formed for a longer time and has a higher yield rate of a product having higher withstand voltage properties (particularly, withstand voltage properties at high temperatures).

Brief Description of the Drawings

[0026]

Figure 1

Figure 2

Figure 3

Mode for Carrying Out the Invention

[0027] In this specification, the expressions “containing” and “comprising” include the concepts of “containing”, “comprising”, “substantially consisting of”, and “consisting only of”.

[0028] In this specification, based on the ranges consisting of the upper limit and / or the lower limit described for each parameter, ranges in which the upper limit and / or the lower limit are arbitrarily interchanged between a plurality of ranges are also exemplified.

[0029] 1. Biaxially oriented polypropylene film In one aspect, the present invention relates to a biaxially oriented polypropylene film having a first surface and a second surface, wherein an average maximum length of substantially elliptical spots on the first surface is 3.0 mm or less, and a difference between an average protrusion peak height Rpk outside the spots and an average protrusion peak height Rpk inside the spots is 0.040 μm or less (in this specification, it may also be referred to as “the polypropylene film of the present invention”). This will be described below.

[0030] Of the two main surfaces of the polypropylene film of the present invention, one surface is the first surface and the other surface is the second surface. The first surface is the surface on which the metal layer is laminated during capacitor fabrication.

[0031] The average maximum length of the substantially elliptical spots on the first surface of the polypropylene film of the present invention is 3.0 mm or less (Characteristic 1).

[0032] The "spots" regarding Characteristic 1 are those that can be visually recognized on the image of the measurement method described later, and the periphery is a region where the roughening degree and / or the roughened surface shape are different. The maximum length (major axis) of the spots can be, for example, 10 mm or less, 8 mm or less, 7 mm or less, 6 mm or less, or 5 mm or less.

[0033] Furthermore, in a preferred embodiment of the present invention, from the viewpoint of further improving the production stability and / or the yield rate of those with higher withstand voltage, the average number of substantially elliptical spots per 11.5 mm × 8.6 mm region of the first surface is preferably 0.6 or less.

[0034] The measurement methods for the average number of spots and the average maximum length of the spots are as follows.

[0035] From the central position in the width direction of each biaxially oriented polypropylene film to be measured, a 50 mm × 50 mm measurement sample is cut out at a total of 10 locations every 10 m in the longitudinal direction. Next, using a digital microscope (Digital Microscope VHX-2000 manufactured by Keyence Corporation), at a lens magnification of 30 times, a light quantity of 50% of the full range, a measurement method of reflection measurement, and a field of view range of 11.5 mm × 8.6 mm, observe an image of "halation removal + sharp mode (a process that makes the color vivid, emphasizes the contour, and removes glare)" on one side of each measurement sample (the side where the metal layer is laminated during capacitor production), and count the number of approximately elliptical "spots" observed within that field of view (a representative one is shown in Fig. 1). In addition, if a part of the spot does not fit within the field of view, that spot is not measured. Sum up the number of spots in the field of view of each measurement sample (a total of 10), and divide the obtained total value by 10 (= the number of measurement samples) to obtain the average number of elliptical spots per field of view (an area of 11.5 mm × 8.6 mm). Also, for each of the spots measured above, measure the diameter in the longitudinal direction and the diameter in the width direction in the biaxially oriented polypropylene film, and take the longer one (the major axis of the spot) as the maximum length of the spot (in the case of the representative example shown in Fig. 2, the diameter in the horizontal direction is the major axis). Sum up the maximum lengths of each spot, and divide the obtained total value by the number of spots to obtain the average maximum length of the spots.

[0036] From the viewpoints of film formation stability, yield rate, etc., the average number of spots is preferably 0 or more and 0.3 or less, more preferably 0 or more and 0.1 or less, and particularly preferably 0.

[0037] From the viewpoints of film formation stability, yield rate, etc., the average maximum length of the spots is preferably 0 mm or more and 1.0 mm or less, more preferably 0 mm or more and 0.5 mm or less, and particularly preferably 0 mm (= the average number of spots is 0).

[0038] The difference between the average peak height Rpk outside the spots and the average peak height Rpk inside the spots of the polypropylene film of the present invention is 0.040 μm or less (Characteristic 2).

[0039] The method for measuring the difference between the average peak height Rpk outside the spots and the average peak height Rpk inside the spots is as follows.

[0040] Using "VertScan2.0 (Model: R5500GML)" manufactured by Ryoka Systems, Ltd. as a laser interferometry non-contact surface shape measuring machine, measure the peak height Rpk of one surface of the biaxially stretched polypropylene film to be measured (the surface on which the metal layer is laminated during capacitor production). First, using the WAVE mode, apply a 530white filter and a 1×BODY lens barrel, and use a ×10 objective lens to perform measurements of 470.92 μm × 353.16 μm per field of view.

[0041] Regarding the inside of the spots, for each of all the spots measured by the above-described method for measuring the number of spots, measure at each of the 5 locations shown in Fig. 3 (the intersection of the major axis and the minor axis, the center of the line connecting one end of the major axis and the intersection, the center of the line connecting the other end of the major axis and the intersection, the center of the line connecting one end of the minor axis and the intersection, and the center of the line connecting the other end of the minor axis and the intersection).

[0042] Regarding the outside of the spots, perform measurements at the center of each of the measurement samples cut out by the above-described method for measuring the number of spots.

[0043] For the obtained data, perform noise removal processing using a median filter (3×3), and then perform Gaussian filter processing with a cut-off value of 30 μm to remove the waviness component. This makes it possible to appropriately measure the state of the measurement surface. Next, perform analysis using the analysis software "VS-Viewer" of "VertScan2.0", and calculate the average value of each value obtained at the above locations for the peak height Rpk, which is a lubricity evaluation parameter. Specifically, it is as follows.

[0044] Sum the measured values of the peak height Rpk inside the spots, and divide the obtained total value by X (= the number of measured spots × 5 (the number of measurement locations in one spot)) to obtain the average peak height Rpk inside the spots.

[0045] Sum the measured values of the peak height Rpk of the protruding mountain parts outside the spots of each sample for measurement, divide the obtained total value by 10 (= the number of samples for measurement), and obtain the average peak height Rpk of the protruding mountain parts outside the spots.

[0046] Subtract the average peak height Rpk inside the spots from the average peak height Rpk of the protruding mountain parts outside the spots to obtain the difference between the two.

[0047] The difference between the average peak height Rpk outside the spots and the average peak height Rpk inside the spots is preferably 0 μm or more and 0.030 μm or less, more preferably 0 μm or more and 0.010 μm or less, and particularly preferably 0 μm, from the viewpoints of film formation stability, yield rate, etc. In addition, when the average number of the spots is 0, the difference is 0 μm.

[0048] The average peak height Rpk outside the spots (= the average peak height Rpk of the film when the average number of the spots is 0) is preferably 0.010 μm or more and 0.095 μm or less, more preferably 0.015 μm or more and 0.090 μm or less, and still more preferably 0.020 μm or more and 0.085 μm or less, from the viewpoints of the high-temperature durability of the capacitor element, etc.

[0049] When the average number of the spots exceeds 0, the average peak height Rpk inside the spots is preferably 0.003 μm or more and 0.095 μm or less, more preferably 0.005 μm or more and 0.090 μm or less, and still more preferably 0.010 μm or more and 0.085 μm or less, from the viewpoints of film formation stability, yield rate, etc.

[0050] The combination of Property 1 and Property 2 described below can further improve the film-forming stability and the yield rate of products with higher breakdown voltage resistance. Although not wishing to be construed in a limiting sense, the reason is considered as follows. If Property 1 and Property 2 are satisfied, it is considered that the thermal history of the cast sheet, which is a stretching precursor, is appropriately homogenized in the film manufacturing process. As a result, the plasticity during transverse stretching is also homogenized, and film breakage is suppressed. Further, when a capacitor is manufactured using such a film, a capacitor having a breakdown voltage resistance with a high yield can be efficiently obtained. In a capacitor manufactured using such a film, it is considered that the voids between the film layers are appropriately homogenized inside the capacitor, and it becomes difficult for electric field concentration caused by local interlayer adhesion to occur. Thereby, it is considered that damage to the film due to local heat generation inside the capacitor is suppressed, and the above-described effects can be obtained.

[0051] From the viewpoint of further improving the film-forming stability and / or the yield rate of products with higher breakdown voltage resistance, the difference between the average protrusion valley depth Rvk outside the spots and the average protrusion valley depth Rvk inside the spots of the polypropylene film of the present invention is preferably 0.020 μm or less. This difference is more preferably 0 μm or more and 0.015 μm or less, still more preferably 0 μm or more and 0.010 μm or less, and particularly preferably 0 μm. When the average number of the spots is 0, this difference is 0 μm. Although not wishing to be construed in a limiting sense, by setting this difference within the above range, it becomes difficult for leakage current to occur from a locally thin portion, and the plasticity during transverse stretching is also homogenized. As a result, it is considered that the film-forming stability and / or the yield rate of products with higher breakdown voltage resistance can be further improved.

[0052] The method for measuring the difference between the average protrusion valley depth Rvk outside the spots and the average protrusion valley depth Rvk inside the spots conforms to the method for measuring the difference between the average protrusion peak height Rpk outside the spots and the average protrusion peak height Rpk inside the spots described above.

[0053] Sum the measured values of the protrusion valley depth Rvk within the spots, and divide the obtained total value by X (= the number of measured spots × 5 (the number of measurement points within one spot)) to obtain the average protrusion valley depth Rvk within the spots.

[0054] Sum the measured values of the protrusion valley depth Rvk outside the spots for each measurement sample, and divide the obtained total value by 10 (= the number of measurement samples) to obtain the average protrusion valley depth Rvk outside the spots.

[0055] Subtract the average protrusion valley depth Rvk within the spots from the average protrusion valley depth Rvk outside the spots to obtain the difference between the two.

[0056] The average protrusion valley depth Rvk outside the spots (= the average protrusion valley depth Rvk of the film when the average number of spots is 0) is preferably 0.005 μm or more and 0.060 μm or less, more preferably 0.010 μm or more and 0.050 μm or less, and still more preferably 0.015 μm or more and 0.040 μm or less from the viewpoints of the high-temperature durability of the capacitor element and the like.

[0057] The average protrusion valley depth Rvk within the spots when the average number of spots exceeds 0 is preferably 0.001 μm or more and 0.045 μm or less, more preferably 0.002 μm or more and 0.040 μm or less, and still more preferably 0.003 μm or more and 0.035 μm or less from the viewpoints of film formation stability and reduction of the number of insulation defects.

[0058] From the viewpoint of further improving the yield rate of those having film formation stability and / or higher withstand voltage, the polypropylene film of the present invention has a DC voltage of 600 V per 1 μm of thickness when a voltage is applied using an insulation defect inspection device having a unwind-windup mechanism. 2 The number of insulation defects per 10 m is preferably 1.0 or less. The number is more preferably 0 defects / 10 m 2 or more and 0.5 defects / 10 m 2 or less, more preferably 0 defects / 10 m 2 or more and 0.1 defects / 10 m 2 or less, particularly preferably 0 defects / 10 m 2Although not wishing to be limited thereto, a capacitor produced using such a film is less likely to suffer film breakage due to short-circuit breakdown even when used for a long time in a high-temperature environment, and high reliability can be obtained.

[0059] The method for measuring the number of insulation defects is as follows.

[0060] Using an insulation defect inspection apparatus having a pay-off and take-up mechanism, measure the number of insulation defects (number / 10 m 2 ) of the biaxially stretched polypropylene film to be measured. Pass the polypropylene film to be measured between a high-voltage electrode and a grounded metal roller, run the film, apply a DC voltage at that time, and measure the number of discharges occurring at the insulation defect portions with a counter attached to the apparatus. Divide the measured number by the measurement area to calculate the number of insulation defects (number / 10 m 2 ) of the film. The measurement conditions are as follows. · Distance between the high-voltage electrode and the grounded metal roller: 50 μm · Wrapping angle at which the film contacts the grounded metal roller: 120° · Shape of the high-voltage electrode: A metal plate with a thickness of 4 mm and the same width as the metal roller · Pay-off speed: 20 m / min · DC voltage: 600 V / μm · Measurement area: 472 m 2 · Test environment temperature: 20°C.

[0061] From the viewpoint of further improving the production stability and / or the yield rate of those having higher withstand voltage, the polypropylene film of the present invention preferably has a variation range of the slow axis angle in the range of 0.3° or more and 2.8° or less. The variation range is more preferably 0.3° or more and 2.0° or less, still more preferably 0.3° or more and 1.5° or less, and even more preferably 0.3° or more and 1.0° or less.

[0062] The slow axis angle of the biaxially stretched polypropylene film means the acute angle formed between the width direction of the biaxially stretched polypropylene film and the slow axis. The polypropylene film of the present invention is stretched biaxially in a first direction and a second direction orthogonal thereto. Since the polymer is oriented in the plane by the biaxial stretching, the biaxially stretched film has birefringence. In the plane of the film, the direction in which the refractive index is maximum is the direction in which the speed of light propagation is slow (the phase lags), and thus it is called the slow axis.

[0063] In the sequential biaxial stretching method, first, the cast base sheet is stretched in the flow direction (MD direction), and subsequently, the sheet is stretched in the transverse direction (TD direction). In this case, in the slow axis of the biaxially stretched polypropylene film, the refractive index in the transverse direction of the second direction tends to be larger than the refractive index in the flow direction of the first direction. Here, the transverse direction of the second direction becomes the slow axis.

[0064] In the stretching in the transverse direction (TD direction), when the stretching is completely performed in the transverse direction (when the stretching is completely performed in the direction orthogonal to the flow direction), the slow axis angle defined in this specification is 0°. However, in reality, shrinkage stress, mechanical external force, the thermoplasticity of the film, etc. act during stretching, and it is impossible to completely stretch in the transverse direction (TD direction), and the slow axis angle tends to be larger than 0°.

[0065] In a portion where the change width of the slow axis angle in the longitudinal direction is large, stretching is non-uniform, so the film is likely to generate distortion. The non-uniformity of the above stretching practically causes a decrease in the withstand voltage performance of the capacitor and tends to significantly reduce the yield. This is presumably because when the longitudinal stretching is non-uniform, the dimensional change inside the capacitor exposed to high temperature becomes non-uniform, electric field concentration occurs due to local interlayer adhesion, and the film is damaged. By controlling the change width of the slow axis angle in the longitudinal direction within the range of 0.3° or more and 2.8° or less, it is considered that the above troubles in the longitudinal direction are suppressed.

[0066] The method for measuring the change range of the slow axis angle and the average slow axis angle is as follows.

[0067] From the center of the biaxially oriented polypropylene film to be measured, measurement samples of 50 mm × 50 mm are cut out at 10 positions (positions at 0 m, 10 m, 20 m, 30 m, 40 m, 50 m, 60 m, 70 m, 80 m, and 90 m) every 10 m in the longitudinal direction. Next, with the width direction of the measurement sample being 0°, the acute angle formed by the width direction of the measurement sample and the slow axis is measured as the slow axis angle. Among the 10 measurement samples, the difference between the maximum value and the minimum value of the slow axis angle is taken as the "change range", and the average value is taken as the "average slow axis angle". The measuring device and measurement conditions are as follows.

[0068] Measuring device: Retardation measuring device RE-100 manufactured by Otsuka Electronics Co., Ltd. Light source: Laser-emitting diode (LED) Band-pass filter: 550 nm (measurement wavelength) Measurement interval: 0.1 sec Integration times: 10 times Number of measurement points: 15 points Gain: 10 dB Measurement environment: Temperature 23°C, humidity 60%.

[0069] The average slow axis angle is preferably 0° or more and 20° or less, more preferably 0° or more and 15° or less, and still more preferably 0° or more and 13° or less, from the viewpoint of the yield rate in film production and the like.

[0070] 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 polypropylene film of the present invention is preferably 6.5 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 1.7 μm or more, and particularly preferably 2.0 μm or more. Further, setting the thickness within the above range is also preferable from the perspective of film-forming stability and / or the yield rate of products with higher breakdown voltage resistance. The method for measuring the thickness of the biaxially stretched polypropylene film in this specification is based on the method described in the examples.

[0071] The layer structure of the polypropylene film of the present invention is not particularly limited. The polypropylene film of the present invention may be a single layer consisting of one layer, or may be a plurality of layers having the same or different compositions. The polypropylene film of the present invention is preferably a film composed of one or more film-shaped forming layers, and more preferably a single-layer film (a film composed of one film-shaped forming layer).

[0072] As long as the polypropylene film of the present invention contains polypropylene resin, its constituent materials are not particularly limited. The polypropylene resin is not particularly limited, and examples thereof include propylene homopolymers such as isotactic polypropylene, copolymers of propylene and ethylene, long-chain branched polypropylene, ultra-high molecular weight polypropylene, and the like. Among these, isotactic polypropylene is preferably mentioned from the perspective of heat resistance.

[0073] The content of the polypropylene resin is preferably 90% by mass or more, more preferably 95% by mass or more, based on the entire polypropylene film of the present invention (when the entire polypropylene film is 100% by mass). The upper limit of the content of the polypropylene resin is, for example, 100% by mass, 98% by mass, etc., based on the entire polypropylene film of the present invention.

[0074] The polypropylene resin may be a single type or a combination of two or more types.

[0075] Here, when there are two or more types of polypropylene resins contained in the polypropylene film of the present invention, the polypropylene resin with the higher content is referred to as the "main component polypropylene resin" in this specification. When there is one type of polypropylene resin contained in the polypropylene film of the present invention, the polypropylene resin is referred to as the "main component polypropylene resin" in this specification.

[0076] Hereinafter, in this specification, when the "polypropylene resin" is mentioned without specifically stating whether it is the main component or not, it means both the polypropylene resin as the main component and the polypropylene resin other than the main component, unless otherwise specified. For example, when it is described that "the weight average molecular weight Mw of the polypropylene resin is preferably 250,000 or more and 450,000 or less.", it means that the weight average molecular weight Mw of the polypropylene resin as the main component is preferably 250,000 or more and 450,000 or less, and the weight average molecular weight Mw of the polypropylene resin other than the main component is preferably 250,000 or more and 450,000 or less. When it is described that "the weight average molecular weight Mw of the polypropylene resin is preferably 250,000 or more and 450,000 or less.", it means both that the weight average molecular weight Mw of the polypropylene resin as the main component is preferably 250,000 or more and 450,000 or less, and that the weight average molecular weight Mw of the polypropylene resin other than the main component is preferably 250,000 or more and 450,000 or less.

[0077] The weight average molecular weight Mw of the polypropylene resin is preferably 250,000 or more and 450,000 or less, more preferably 250,000 or more and 420,000 or less, even more preferably 250,000 or more and 400,000 or less, and still even more preferably 260,000 or more and 390,000 or less, from the viewpoints of the thickness uniformity, mechanical properties, thermo-mechanical properties, etc. of the biaxially stretched polypropylene film. By using such a polypropylene resin, dielectric breakdown of the film is suppressed, and it becomes easy to obtain an extremely thin biaxially stretched polypropylene film suitable for a small and high-capacity capacitor. When two or more polypropylene resins are used, a polypropylene resin (preferably the main component polypropylene resin) having the above Mw of less than 250,000 and less than 330,000 (preferably 250,000 or more and 300,000 or less, more preferably 260,000 or more and 290,000 or less) and a polypropylene resin (preferably a polypropylene resin other than the main component) having the above Mw of 330,000 or more and 450,000 or less (preferably 350,000 or more and 420,000 or less, more preferably 370,000 or more and 400,000 or less, even more preferably 370,000 or more and 390,000 or less) are preferably used in combination.

[0078] The ratio (Mw / Mn) of the weight average molecular weight Mw to the number average molecular weight Mn of the polypropylene resin is preferably 5.0 or more and 12.0 or less, more preferably 5.0 or more and 10.0 or less, even more preferably 5.0 or more and 9.0 or less, from the viewpoint of obtaining appropriate resin fluidity during biaxial stretching and making it easy to obtain an extremely thin biaxially stretched propylene film without thickness unevenness. When two or more polypropylene resins are used, a polypropylene resin (preferably the main component polypropylene resin) having the above ratio of 5.0 or more and less than 7.0 (preferably 5.0 or more and 6.5 or less) and a polypropylene resin (preferably a polypropylene resin other than the main component) having the above ratio of 7.0 or more and 12.0 or less (preferably 7.5 or more and 10.0 or less, more preferably 7.5 or more and 9.0 or less) are preferably used in combination.

[0079] The measurement methods of the weight average molecular weight Mw and the number average molecular weight Mn of the polypropylene resin are in accordance with the methods described in the examples.

[0080] The melt flow rate (MFR) of the polypropylene resin at 230°C and a load of 2.16 kg is not particularly limited, but is preferably 7.0 g / 10 min or less from the viewpoint of stretchability and the like, and more preferably 0.5 g / 10 min or more and 6.0 g / 10 min or less from the viewpoint of improving the thickness accuracy of the polypropylene film of the present invention. When two or more polypropylene resins are used, the MFR is 4.0 g / 10 min or more and less than 7.0 g / 10 min (preferably 4.5 g / 10 min or more and 6.5 g / 10 min or less, more preferably 5.0 g / 10 min or more and 6.0 g / 10 min or less) of the polypropylene resin (preferably the main component polypropylene resin) and the MFR is 0.5 g / 10 min or more and less than 4.0 g / 10 min (preferably 1.0 g / 10 min or more and 3.5 g / 10 min or less, more preferably 1.5 g / 10 min or more and 3.0 g / 10 min or less) of the polypropylene resin (preferably the polypropylene resin other than the main component) are preferably used in combination. The method for measuring the melt flow rate of the polypropylene resin is according to the method described in the examples.

[0081] The heptane insoluble content (HI) of the polypropylene resin is preferably 96.0% or more and 99.5% or less, and more preferably 97.0% or more and 99.0% or less. Here, the higher the heptane insoluble content, the higher the stereoregularity of the resin. By using such a polypropylene resin, the crystallinity is moderately improved, and the initial withstand voltage and the long-term withstand voltage are improved. The method for measuring the heptane insoluble content (HI) is according to the method described in the examples.

[0082] The content of the main component polypropylene resin is preferably more than 50% by mass and 100% by mass or less, more preferably 55% by mass or more and 85% by mass or less, still more preferably 60% by mass or more and 75% by mass or less, and even more preferably 60% by mass or more and 70% by mass or less with respect to 100% by mass of the polypropylene resin.

[0083] The polypropylene resin can be produced using generally known polymerization methods. Examples of the polymerization method include gas phase polymerization method, bulk polymerization method, and slurry polymerization method.

[0084] The polymerization may be a single-stage polymerization using one polymerization reactor or a multi-stage polymerization using two or more polymerization reactors. Further, the polymerization may be carried out by adding hydrogen or a comonomer as a molecular weight regulator into the reactor.

[0085] As the catalyst for the polymerization, generally known Ziegler-Natta catalysts can be used, and there is no particular limitation as long as the polypropylene resin can be obtained. The catalyst may contain a cocatalyst component or a donor. By adjusting the catalyst and the polymerization conditions, the molecular weight, molecular weight distribution, stereoregularity, etc. can be controlled.

[0086] The molecular weight distribution, etc. of the polypropylene resin can be adjusted by resin blending (blending). For example, a method of blending two or more resins having different molecular weights and molecular weight distributions can be mentioned. Generally, a two-component polypropylene blend system in which a resin having a higher average molecular weight or a lower resin than the main resin is 55% by mass or more and 90% by mass or less based on the total resin is preferable because it is easy to adjust the amount of low molecular weight components.

[0087] In addition, when adopting the above mixing and adjusting method, the melt flow rate (MFR) may be used as a measure of the average molecular weight. In this case, from the viewpoint of convenience during adjustment, the difference in MFR between the main resin and the added resin is preferably about 1 to 30 g / 10 min.

[0088] The method of resin blending is not particularly limited, but examples include a method of dry blending the polymerization powder or pellets of the main resin and the added resin using a mixer or the like, and a method of supplying the polymerization powder or pellets of the main resin and the added resin to a kneader and obtaining a blended resin by melt kneading.

[0089] The mixer and the kneader are not particularly limited. The kneader may be of any of a single-screw type, a twin-screw type, or a multi-screw type with more than two screws. In the case of a twin-screw or more screw type, either a co-rotating or counter-rotating kneading type may be used.

[0090] In the case of a blend by melt kneading, if a good kneaded product is obtained, the kneading temperature is not particularly limited. Generally, it is in the range of 200°C to 300°C, and from the viewpoint of suppressing the deterioration of the resin, 230°C to 270°C is preferable. Further, in order to suppress the deterioration during the 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 pelletizer. Thereby, a mixed polypropylene raw material resin pellet can be obtained.

[0091] By using the above-mentioned polypropylene resin, it becomes easier to adjust to the above-mentioned film properties, and it is also preferable from the viewpoints of film-forming stability and / or the yield rate of products having higher withstand voltage properties.

[0092] The polypropylene film of the present invention may contain an additive. The "additive" is not particularly limited as long as it is an additive used for polypropylene resin in general terms.

[0093] Examples of the additive include an antioxidant, a light stabilizer, an ultraviolet absorber, a plasticizer, a lubricant, a crosslinking agent, a flame retardant, an antistatic agent, a heat resistance improver, an antiblocking agent, inorganic particles, resin particles, and the like. The polypropylene resin may contain the additive in an amount that does not adversely affect the polypropylene film of the present invention (for example, 10% by mass or less, 5% by mass or less, 1% by mass or less, or 0.1% by mass or less based on 100% by mass of the polypropylene film of the present invention).

[0094] 2. Method for producing biaxially oriented polypropylene film The biaxially stretched polypropylene film can be manufactured by obtaining a cast sheet which is a precursor before stretching from resin pellets, and then subjecting the cast sheet to a biaxial stretching process. As is clear from the results of the examples described later, by controlling the microcracks on the surface of the casting drum, the blowing air speed of the air knife, the distance between the blowing outlet of the air knife and the cast sheet, the stretching nip roll temperature, the hardness of the stretching nip roll, etc., specifically by manufacturing according to the following method, the biaxially stretched polypropylene film of the present invention can be obtained.

[0095] Among the above manufacturing conditions, the microcracks on the surface of the casting drum are important conditions. From this perspective, in one aspect of the present invention, when a virtual line with a length of 0.1 mm in the width direction is provided at an arbitrary position on the surface, the virtual line intersects with one or more and 15 or less grooves, and the groove width is 1 μm or more and 10 μm or less, the present invention relates to a method for manufacturing a biaxially stretched polypropylene film of the present invention, which includes obtaining a cast sheet using a casting drum having a microcrack surface and subjecting the cast sheet to a biaxial stretching process.

[0096] Hereinafter, the manufacturing method of the biaxially stretched polypropylene film of the present invention will be described in detail.

[0097] 2-1. Production of cast sheet The cast sheet can be formed using a known method. For example, polypropylene resin pellets, dry-mixed polypropylene resin pellets, or mixed polypropylene resin pellets prepared by previously melt-kneading are supplied to an extruder, heated and melted, foreign matters and modified polymers are removed through a filter, and then extruded in a sheet shape from a T-die, and cooled and solidified with at least one metal drum (casting drum) to form a cast sheet.

[0098] In an extruder, polypropylene resin is modified to some extent by thermal degradation and oxidative degradation. From the viewpoint of suppressing such polymer modification, the resin temperature during melt extrusion is 170°C or higher and 320°C or lower, preferably 200°C or higher and 300°C or lower. Further, it is possible to suppress degradation by nitrogen substitution in the extruder, screw shape, internal shape of the T-die during casting, addition amount of antioxidant, and the like.

[0099] The temperature of the casting drum is preferably 80°C or higher and 140°C or lower, and more preferably maintained at 90°C or higher and 105°C or lower. The β-crystal fraction of the cast sheet obtained within such a temperature range is about 5% or more and 20% or less by the X-ray method. In the range of the β-crystal fraction, the roughness of the film surface is moderately improved, and both the capacitor characteristics and the element winding processability can be satisfied.

[0100] The surface of the casting drum is not particularly limited, but a roll having a concavo-convex surface such as a sandblasted roll or a ceramic roll, or a micro-crack roll having a discharge path for entrained air is preferable in that the desired physical properties of the present invention can be easily obtained. The manufacturing methods of these rolls are known. For example, for a micro-crack roll, it can be manufactured according to or in accordance with the method described in, for example, Japanese Patent No. 6974939. Micro-cracks are caused by the stress of the metal plating layer, and the shape and number can be controlled by the thickness of the metal plating layer, plating conditions, heat treatment, chemical treatment, multi-layer formation, and the like. For example, in the case of chrome plating, the plating thickness is preferably 100 μm to 400 μm, more preferably 120 μm to 350 μm, and even more preferably 150 μm to 300 μm. By setting it within this range, the control of the shape and number of the grooves becomes easier, and when the roll is heated, cracking is less likely to occur.

[0101] Regarding the microcrack roll, when a virtual line with a length of 0.1 mm in the width direction is provided at any position on the roll surface, it is preferable that the virtual line intersects with one or more and 15 or fewer grooves, more preferably two or more and 10 or fewer grooves, and even more preferably three or more and 8 or fewer grooves. Also, the groove width is preferably 1 μm or more and 10 μm or less, more preferably 2 μm or more and 9 μm or less, and even more preferably 3 μm or more and 8 μm or less. By using those within the ranges of the number of grooves and the groove width, the air that enters between the casting drum and the cast sheet is appropriately discharged, and even for a thinned cast sheet, close conveyance becomes easy, and it becomes easy to obtain an extremely thin biaxially stretched polypropylene film. Also, even when the forming speed of the thinned cast sheet is increased, a phenomenon in which the cast sheet thickness undulates, so-called draw resonance (surge) phenomenon, is less likely to occur, and an extremely thin biaxially stretched polypropylene film having a uniform thickness in the longitudinal direction can be easily obtained.

[0102] As a method of adhering to the casting drum, any method such as an air knife method, a touch roll method, an electrostatic printing method, or a water-cooled casting method may be used, but the air knife method, which is easy to adjust regarding sheet adhesion and can be simply handled, is preferable.

[0103] When using an air knife, the wind speed of the blown air is preferably 70 m / second or more and 130 m / second or less, more preferably 80 m / second or more and 120 m / second or less, and even more preferably 90 m / second or more and 110 m / second or less. Also, the distance between the air outlet of the air knife and the cast sheet is preferably 2 mm or more and 5 mm or less, more preferably 2 mm or more and 4 mm or less. When using an air knife within such a range, the thinned resin extruded in a sheet shape from the T-die can be appropriately adhered to the casting drum, and the film vibration of the molten resin from the T-die outlet to the point of adhering to the casting drum can be suppressed. As a result, the above-described film characteristics can be easily obtained.

[0104] 2-2. Biaxial stretching treatment The biaxially stretched polypropylene film of the present invention is obtained by performing biaxial stretching in which the above-mentioned cast sheet is oriented biaxially in the longitudinal and transverse directions. Examples of the stretching method include simultaneous or sequential biaxial stretching methods. From the viewpoint of stably equalizing the thickness and increasing the mechanical strength of the film, the sequential biaxial stretching method is preferred.

[0105] As the sequential biaxial stretching method, first, the cast sheet is preheated through a conveying roll maintained at 70°C or higher and 135°C or lower, preferably 80°C or higher and 130°C or lower. Subsequently, it is preferably heated to 130°C or higher and 155°C or lower, preferably 140°C or higher and 150°C or lower, immediately before stretching in the longitudinal direction. By heating the cast sheet in this way, excessive thermal expansion of the cast sheet is suppressed, the planarity of the sheet is easily maintained before stretching in the longitudinal direction described later, and the sheet is easily adhered to the conveying roll uniformly.

[0106] From the viewpoint of easily obtaining the biaxially stretched polypropylene film of the present invention, immediately before stretching in the longitudinal direction preferably, a method of simultaneously heating both the front and back surfaces of the cast sheet is employed. The method is not particularly limited, but from the viewpoint of suppressing the difference in the thermal history within the plane of the cast sheet, a method of heating the stretching nip roll or a method of heating with electromagnetic radiation is preferred. From the viewpoint of being able to simultaneously fix the cast sheet immediately before stretching and adjust the temperature of the sheet, the method of heating the stretching nip roll is more preferred.

[0107] When heating the stretching nip roll, from the viewpoint of suppressing the difference in the thermal history between the front and back surfaces of the cast sheet, the temperature of the stretching nip roll is preferably 95°C or higher and 170°C or lower, more preferably 100°C or higher and 150°C or lower.

[0108] When heating the stretching nip roll, the rubber hardness of the roll is preferably 40° or higher and 80° or lower, more preferably 50° or higher and 70° or lower, so that the shape of the roll surface follows the shape of the surface of the cast sheet.

[0109] Thereafter, it is stretched 3 to 7 times, preferably 4 to 6 times in the longitudinal direction, and immediately cooled to room temperature.

[0110] After stretching in the longitudinal direction, the stretched film is led to a tenter, both ends are gripped with clips heated to 80°C or higher and 140°C or lower, and after preheating at a temperature of 140°C or higher and 185°C or lower, preferably 150°C or higher and 175°C or lower, it is stretched 6 to 12 times, preferably 8 to 11 times in the width direction at a temperature of 140°C or higher and 170°C or lower, preferably 150°C or higher and 160°C or lower.

[0111] Thereafter, relaxation and heat setting are performed and it is wound up. The wound film can be cut to a desired product width after being subjected to an aging treatment in an atmosphere of 20°C or higher and 45°C or lower.

[0112] 3. Polypropylene film integrated with metal layer In one aspect of the present invention, there is also provided a metal layer-integrated polypropylene film (which may also be referred to as "the metal layer-integrated polypropylene film of the present invention" in this specification) including the biaxially stretched polypropylene film of the present invention and a metal layer disposed on one or both sides of the biaxially stretched polypropylene film. Hereinafter, the metal layer-integrated polypropylene film of the present invention will be described in detail. A capacitor obtained by winding the metal layer-integrated polypropylene film of the present invention is excellent in initial withstand voltage property and long-term durability under high temperature and high voltage.

[0113] The 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 embodiment can be obtained, and electrodes usually used for manufacturing a capacitor can be used. Examples of the electrodes include metal foils, papers with at least one side metallized, and plastic films.

[0114] Since there is an increasing demand for smaller and lighter capacitors, it is preferable to form electrodes by directly metallizing one or both sides of the polypropylene film of the present invention. 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, etc., zinc and aluminum are preferable.

[0115] As methods for directly metallizing the surface of the polypropylene film, for example, a vacuum evaporation method and a sputtering method can be exemplified, and it is not particularly limited as long as the capacitor targeted by the present embodiment can be obtained. From the viewpoints of productivity and economy, etc., the vacuum evaporation method is preferable. As the vacuum evaporation method, generally, a crucible method, a wire method, etc. can be exemplified However, it is not particularly limited as long as the capacitor targeted by the present embodiment can be obtained, and an appropriate and optimal one can be selected as appropriate.

[0116] From the viewpoint of the electrical characteristics of the capacitor, the film resistance of the metal deposition film is preferably 1 Ω / sq or more and 100 Ω / sq or less. Even within this range, a higher value is desirable from the viewpoint of self-healing characteristics. Further, from the viewpoint of safety, the film resistance is more preferably 5 Ω / sq or more and 50 Ω / sq or less, and still more preferably 10 Ω / sq or more and 30 Ω / sq or less. The film resistance of the metal deposition film can be measured, for example, by the four-terminal method known to those skilled in the art during metal deposition. The film resistance of the metal deposition film can be adjusted, for example, by adjusting the output of the evaporation source to adjust the evaporation amount.

[0117] When forming a metal vapor deposition film on one side of a film, an insulating margin is formed by not vapor-depositing a certain width from one end of the film so that a capacitor is formed when the film is wound. Further, in order to firmly bond the metal layer integrated polypropylene film of the present invention 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 1 Ω / sq or more and 8 Ω / sq or less, and preferably 1 Ω / sq or more and 5 Ω / sq or less. The thickness of the metal film is not particularly limited, but is preferably 1 nm or more and 200 nm or less.

[0118] 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 so-called special margins such as a fishnet pattern and a T-margin pattern. When the metal vapor deposition film is formed on one side of the polypropylene film with a pattern including a special margin, the safety of the obtained capacitor is improved, which is effective also in terms of suppressing capacitor breakdown and short circuit, and is preferable.

[0119] 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.

[0120] The metal layer integrated polypropylene film of the present invention can be processed into the capacitor of the present invention described later through a winding process of winding along the long direction of the film. That is, two metal layer integrated polypropylene films of the present invention are used as a pair, and they are overlapped and wound so that the metal layer and the polypropylene film are alternately laminated. Then, a capacitor is obtained through a process of forming a pair of metallicon electrodes by metal spraying on both end faces to produce a film capacitor.

[0121] 4. Capacitor In one aspect, the present invention provides a capacitor including the metal layer integrated polypropylene film of the present invention (hereinafter, sometimes referred to as "the capacitor of the present invention"). Hereinafter, the capacitor of the present invention will be described in detail.

[0122] In the process of manufacturing a capacitor, winding of the film is performed. For example, two pairs of the metal layer integrated polypropylene films of the present invention are overlapped and wound so that the metal layer and the polypropylene film in the metal layer integrated polypropylene film of the present invention are alternately laminated, and further, the insulating margin portion is on the reverse side. At this time, it is preferable to laminate the two pairs of the metal layer integrated polypropylene films of the present invention with a shift of 1 to 2 mm. The winding machine to be used is not particularly limited. For example, an automatic winding machine 3KAW-N2 type manufactured by Minato Seisakusho Co., Ltd. can be used.

[0123] When manufacturing a flat capacitor, after winding, usually, pressing is performed on the obtained wound product. Pressing promotes winding and element forming of the capacitor. From the viewpoint of controlling and stabilizing the interlayer gap, the applied pressure varies depending on the thickness of the polypropylene film and the like, but is, for example, 2 to 20 kg / cm2.

[0124] Subsequently, a capacitor is manufactured by spraying metal on both end faces of the wound product to provide a metallicon electrode. The capacitor is further subjected to a predetermined heat treatment. That is, in the present embodiment, it includes a step of performing heat treatment on the capacitor at a temperature of 80 to 125 ° C for 1 hour or more under vacuum (hereinafter, sometimes referred to as "thermal aging").

[0125] In the step of performing heat treatment on the capacitor, the temperature of the heat treatment is 80°C or higher and 130°C or lower, preferably 90°C or higher and 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 metal layer integrated polypropylene film of the present invention are reduced, corona discharge is suppressed, and the internal structure of the metal layer integrated polypropylene film of the present invention changes and crystallization proceeds. As a result, it is considered that the withstand voltage property is improved. When the temperature of the heat treatment is lower than the predetermined temperature, the above effects due to 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 polypropylene film.

[0126] As a method of performing heat treatment on the capacitor, 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.

[0127] The time for performing the heat treatment is preferably 1 hour or longer, more preferably 10 hours or longer, from the viewpoint of obtaining mechanical and thermal stability, but more preferably 20 hours or shorter from the viewpoint of preventing molding defects such as heat wrinkles and molding. Usually, a lead wire is welded to the metallicon electrode of the capacitor subjected to thermal aging. Further, in order to impart weather resistance and particularly prevent humidity degradation, it is preferable to enclose the capacitor in a case and pot it with an epoxy resin. The capacitor of the present invention is a small and large-capacity capacitor based on the metal layer integrated polypropylene film of the present invention, and has an initial withstand voltage property and long-term durability under high temperature and high voltage.

[0128] The capacitor of the present invention using the polypropylene film of the present invention is preferably used in a high-temperature environment, and can be made into a small-sized and high-capacity capacitor (for example, the capacitance is 5 μF or more, preferably 10 μF or more, more preferably 20 μF or more, even more preferably 30 μF or more, and particularly preferably 40 μF or more. The upper limit of the capacitance is not particularly limited, and is, for example, 100 μF, 80 μF, 70 μF, or 60 μF). Therefore, the capacitor of the present invention can be used as a high-voltage capacitor, a filter capacitor and a smoothing capacitor for various switching power supplies, converters and inverters, etc., which are used in electronic devices, electrical devices, etc. Further, the capacitor of the present invention can also be preferably used as a capacitor for an inverter, a capacitor for a converter, etc., which control a drive motor of an electric vehicle, a hybrid vehicle, etc., for which the demand has been increasing in recent years.

Examples

[0129] Hereinafter, the present invention will be described in detail based on examples, but the present invention is not limited by these examples.

[0130] (1) Measurement of resin properties (1-1) Measurement of weight average molecular weight (Mw), number average molecular weight (Mn), and molecular weight distribution (Mw / Mn) of polypropylene resin Using GPC (gel permeation chromatography), the weight average molecular weight (Mw), number average molecular weight (Mn), and molecular weight distribution (Mw / Mn) of the polypropylene resin used as a raw material in the examples and comparative examples were measured under the following conditions.

[0131] Specifically, an HLC-8121GPC-HT type, which is a high-temperature GPC device equipped with a differential refractometer (RI) manufactured by Tosoh Corporation, was used. As the column, TSKgel manufactured by Tosoh Corporation Three GMHHR-H(20)HTs were connected and used. Measurement was carried out by flowing trichlorobenzene as an eluent at a flow rate of 1.0 ml / min at a column temperature of 140°C. A calibration curve was prepared using standard polystyrene manufactured by Tosoh Corporation, and the measured molecular weight values were converted to polystyrene values to obtain the weight average molecular weight (Mw) and the number average molecular weight (Mn). The molecular weight distribution (Mw / Mn) was obtained using these values of Mw and Mn.

[0132] (1-2) Measurement of heptane insoluble content (HI) For the polypropylene resins used as raw materials in the examples and comparative examples, a measurement sample of about 3 g was prepared by press molding into a size of 10 mm × 35 mm × 0.3 mm. Next, about 150 mL of heptane was added, and Soxhlet extraction was performed for 8 hours. The heptane-insoluble content was calculated from the sample masses before and after extraction.

[0133] (1-3) Measurement of melt flow rate (MFR) For the polypropylene resins used as raw materials in the examples and comparative examples, the melt flow rate (MFR) in the form of raw material resin pellets was measured in accordance with Condition M of JIS K 7210 using a melt indexer manufactured by Toyo Seiki Co., Ltd. Specifically, first, a sample weighed to 4 g was inserted into a cylinder with a test temperature of 230°C and preheated under a load of 2.16 kg for 3.5 minutes. Then, the weight of the sample extruded from the bottom hole in 30 seconds was measured to obtain the MFR (g / 10 min). The above measurement was repeated 3 times, and the average value was taken as the measured value of MFR.

[0134] (2) Preparation of casting drum As the metal drum (casting drum) used for manufacturing the sheet (cast sheet) to be subjected to the biaxial stretching treatment in the manufacturing process of the biaxially stretched polypropylene film, casting drums A to E were prepared.

[0135] At any position on the surface of the casting drum, when a virtual line with a length of 0.1 mm in the width direction was provided at an arbitrary position, the number of grooves intersecting the virtual line and the average value of the groove widths were measured. Specifically, the measurement was carried out as follows. Twelve positions were observed in total, including four positions in the circumferential direction of the casting drum (0 degrees, 90 degrees, 180 degrees, 270 degrees on the clock) and three positions in the width direction (10 (one end), 50 (center), 90 (one end) with respect to a surface length of 100). A virtual line with a width of 0.1 mm was drawn at the center of the screen, and the number of intersecting grooves and the groove widths were measured, and the average values were calculated.

[0136] The number of grooves and the groove widths are shown in Table 1.

[0137]

Table 1

[0138] (3) Production of biaxially oriented polypropylene film A biaxially oriented polypropylene film was produced according to the manufacturing conditions in Table 2. The thickness of the biaxially oriented polypropylene film was measured in accordance with JIS-C2330 using a micrometer (JIS-B7502).

[0139] <Example 1> Polypropylene resin A (Mw = 270,000, Mw / Mn = 5.7, heptane-insoluble content = 97.8%, MFR = 5.6 g / 10 min, manufactured by Prime Polymer) and polypropylene resin B (Mw = 380,000, Mw / Mn = 8.3, heptane-insoluble content = 98.8%, MFR = 2.3 g / 10 min, manufactured by Korea Petrochemical) were supplied to an extruder at a mass ratio of A:B = 65:35 and melted at a resin temperature of 230°C. Then, after removing foreign substances and modified polymers with a filter installed in the middle of the polymer tube, it was extruded using a T-die and wound around a casting drum with a surface temperature maintained at 92°C to be solidified, and a cast sheet with a thickness of 0.1 mm was produced at a speed of 60 m / min. Incidentally, the casting drum used was A in Table 1.

[0140] Also, as a method of adhering to the casting drum, an air knife was used, the wind speed of the blown air was 110 m / s, and the distance between the outlet of the air knife and the cast sheet was 3 mm.

[0141] The obtained cast sheet was preheated at a temperature of 130 °C, sandwiched between a conveying roll heated to 145 °C and a stretching nip roll with a rubber hardness of 70° heated to 110 °C, stretched 5 times in the longitudinal direction, and immediately returned to room temperature.

[0142] Thereafter, the stretched film was guided to a tenter, both ends were gripped with clips at 110 °C and preheated at 170 °C, stretched 10 times in the width direction at a temperature of 155 °C, and then relaxed and heat-fixed to wind up a biaxially stretched polypropylene film with a thickness of 2.3 μm in a roll shape.

[0143] <Example 2> In the production of the cast sheet, a biaxially stretched polypropylene film was obtained in the same manner as in Example 1 except that the distance between the outlet of the air knife and the sheet was set to 5 mm.

[0144] <Example 3> In the production of the cast sheet, a biaxially stretched polypropylene film was obtained in the same manner as in Example 2 except that the wind speed of the blown air of the air knife was set to 70 m / s.

[0145] <Example 4> In the production of the cast sheet, a biaxially stretched polypropylene film was obtained in the same manner as in Example 1 except that the casting drum was set to B in Table 1.

[0146] <Example 5> In the production of the cast sheet, a biaxially stretched polypropylene film was obtained in the same manner as in Example 1 except that the casting drum was set to C in Table 1.

[0147] <Example 6> In the production of the cast sheet, except that the casting drum was E in Table 1, the wind speed of the blowing air of the air knife was 130 m / s, the distance between the blowing outlet of the air knife and the sheet was 2 mm, and in the longitudinal stretching process, the temperature of the stretching nip roll was 130°C, a biaxially stretched polypropylene film was obtained in the same manner as in Example 1.

[0148] <Example 7> A biaxially stretched polypropylene film was obtained in the same manner as in Example 6, except that in the longitudinal stretching process, the rubber hardness of the stretching nip roll was 50°.

[0149] <Example 8> In the production of the cast sheet, except that the casting drum was D in Table 1, the blowing wind speed of the air knife was 70 m / s, and the distance between the blowing outlet of the air knife and the sheet was 5 mm, a biaxially stretched polypropylene film was obtained in the same manner as in Example 1.

[0150] <Example 9> A biaxially stretched polypropylene film was obtained in the same manner as in Example 1, except that the thickness of the finally wound biaxially stretched polypropylene film was 1.8 μm.

[0151] <Example 10> A biaxially stretched polypropylene film was obtained in the same manner as in Example 1, except that the thickness of the finally wound biaxially stretched polypropylene film was 6 μm.

[0152] <Example 11> In the production of the cast sheet, a biaxially stretched polypropylene film was obtained in the same manner as in Example 1, except that the resin supplied to the extruder was A.

[0153] <Comparative Example 1> In the production of the cast sheet, the procedure was the same as in Example 1, except that the wind speed of the blowing air of the air knife was set to 140 m / s. In this case, since the air of the air knife entered between the resin extruded in a sheet form from the T-die and the casting drum, and the film vibration of the molten resin became remarkable until it adhered to the casting drum from the T-die outlet, it was not possible to produce a biaxially stretched film.

[0154] <Comparative Example 2> In the production of the cast sheet, a biaxially stretched polypropylene film was obtained in the same manner as in Example 4, except that the wind speed of the blowing air of the air knife was 70 m / s and the distance between the blowing outlet of the air knife and the sheet was 5 mm.

[0155] <Comparative Example 3> In the production of the cast sheet, a biaxially stretched polypropylene film was obtained in the same manner as in Example 5, except that the wind speed of the blowing air of the air knife was 70 m / s and the distance between the blowing outlet of the air knife and the sheet was 5 mm.

[0156] <Comparative Example 4> In the longitudinal stretching process, a biaxially stretched polypropylene film was obtained in the same manner as in Example 6, except that the temperature of the stretching nip roll was set to 90°C.

[0157] <Comparative Example 5> A biaxially stretched polypropylene film was obtained in the same manner as in Example 1, except that the thickness of the finally wound biaxially stretched polypropylene film was set to 1.6 μm.

[0158] <Comparative Example 6> In the longitudinal stretching process, a biaxially stretched polypropylene film was obtained in the same manner as in Example 12, except that the temperature of the stretching nip roll was 90°C and the hardness of the stretching nip roll was 50°.

[0159]

Table 2

[0160] (4) Measurement of properties of biaxially oriented polypropylene film (4-1) Measurement of spots From the central position in the width direction of each of the biaxially oriented polypropylene films of the examples and comparative examples, measurement samples of 50 mm × 50 mm were cut out at a total of 10 locations every 10 m in the longitudinal direction. Next, using a digital microscope (Digital Microscope VHX-2000 manufactured by Keyence Corporation), at a lens magnification of 30 times, a light quantity of 50% of the full range, a measurement method of reflection measurement, and a field of view range of 11.5 mm × 8.6 mm, an image of "halation removal + sharp mode (a process that makes the color vivid, emphasizes the contour, and removes the glare)" was observed for one surface (the surface on which the metal layer is laminated during capacitor production) of each measurement sample, and the number of substantially elliptical "spots" (a representative one is shown in Fig. 1) observed within the field of view was counted. In addition, when a part of the spot did not fit within the field of view, that spot was not counted. The total number of spots in the field of view of each measurement sample (a total of 10) was summed up, and the obtained total value was divided by 10 (= the number of measurement samples) to obtain the average number of elliptical spots per unit field of view (an area of 11.5 mm × 8.6 mm).

[0161] Also, for each of the spots measured above, the diameter in the longitudinal direction and the diameter in the width direction in the biaxially oriented polypropylene film were measured, and the longer one (the major axis of the spot) was taken as the maximum length of the spot (in the case of the representative example shown in Fig. 2, the diameter in the horizontal direction is the major axis). The maximum lengths of all the spots were summed up, and the obtained total value was divided by the number of spots to obtain the average maximum length of the spots.

[0162] (4-2) Measurement of peak height Rpk of protrusion and valley depth Rvk of depression As a laser interference non-contact surface shape measuring machine, "VertScan2.0 (Model: R5500GML)" manufactured by Hishikawa System Co., Ltd. was used to measure the peak height Rpk and valley depth Rvk of the protruding ridges on one side of the biaxially stretched polypropylene film (the side where the metal layer is laminated during capacitor production) in the examples and comparative examples. First, using the WAVE mode, a 530white filter and a 1×BODY lens barrel were applied, and with a ×10 objective lens, measurements were taken at 470.92μm × 353.16μm per field of view.

[0163] Regarding within the spots, for each of all the spots measured in the above (4-1), measurements were taken at each of the 5 locations shown in Figure 3 (the intersection of the major axis and minor axis, the center of the line connecting one end of the major axis and the intersection, the center of the line connecting the other end of the major axis and the intersection, the center of the line connecting one end of the minor axis and the intersection, and the center of the line connecting the other end of the minor axis and the intersection).

[0164] Regarding outside the spots, measurements were taken at the center of each measurement sample.

[0165] For the obtained data, noise removal processing was performed using a median filter (3×3), and then Gaussian filter processing was performed with a cut-off value of 30μm to remove the waviness component. As a result, the state of the measurement surface was made suitable for measurement. Next, analysis was performed using the analysis software "VS-Viewer" of "VertScan2.0", and for the peak height Rpk and valley depth Rvk of the lubricity evaluation parameters, the average value of each value obtained at the above locations was calculated. Specifically, it is as follows.

[0166] The measured values of the peak height Rpk within the spots were summed, and the obtained total value was divided by X (= the number of measured spots × 5 (the number of measurement locations within one spot)) to obtain the average peak height Rpk within the spots.

[0167] The measured values of the peak height Rpk outside the spots of each measurement sample were summed, and the obtained total value was divided by 10 (= the number of measurement samples) to obtain the average peak height Rpk outside the spots.

[0168] The average peak height Rpk inside the spots was subtracted from the average peak height Rpk outside the spots to obtain the difference between the two.

[0169] Also, the average peak height Rpk outside the spots and the average peak height Rpk inside the spots were added together, and the resulting value was divided by 2 to obtain the average peak height Rpk of the film. When the average number of spots was 0, the average measured value at the center of each measurement sample was taken as the average peak height Rpk of the film.

[0170] The measured values of the valley depths Rvk inside the spots were summed up, and the resulting total value was divided by X (= the number of spots measured × 5 (the number of measurement points inside one spot)) to obtain the average valley depth Rvk inside the spots.

[0171] The measured values of the valley depths Rvk outside the spots of each measurement sample were summed up, and the resulting total value was divided by 10 (= the number of measurement samples) to obtain the average valley depth Rvk outside the spots.

[0172] The average valley depth Rvk inside the spots was subtracted from the average valley depth Rvk outside the spots to obtain the difference between the two.

[0173] Also, the average valley depth Rvk outside the spots and the average valley depth Rvk inside the spots were added together, and the resulting value was divided by 2 to obtain the average valley depth Rvk of the film. When the average number of spots was 0, the average measured value at the center of each measurement sample was taken as the average valley depth Rvk of the film.

[0174] (4-3) Measurement of number of insulation defects Using an insulation defect inspection device having a pay-off - take-up mechanism, the number of insulation defects (number / 10 m) of the biaxially stretched polypropylene films of the examples and comparative examples 2) was measured. The polypropylene film to be tested was run between a high-voltage electrode and a grounded metal roller through the film, and a DC voltage was applied at that time. The number of discharges occurring at the insulation defect sites was measured with a counter attached to the device. The measured number was divided by the measurement area to calculate the number of insulation defects of the film (number / 10 m 2 ) was calculated. The measurement conditions were as follows. · Distance between the high-voltage electrode and the grounded metal roller: 50 μm · Holding angle at which the film contacts the grounded metal roller: 120° · Shape of the high-voltage electrode: A metal plate with a thickness of 4 mm and the same width as the metal roller · Unwinding speed: 20 m / min · DC voltage: 600 V / μm · Measurement area: 472 m 2 · Test environment temperature: 20°C.

[0175] (4-4) Measurement of slow axis angle and change width of slow axis angle From the center of each of the biaxially oriented polypropylene films of the examples and comparative examples, measurement samples of 50 mm × 50 mm were cut out at 10 locations (at positions of 0 m, 10 m, 20 m, 30 m, 40 m, 50 m, 60 m, 70 m, 80 m, and 90 m) every 10 m in the longitudinal direction. Next, the widthwise direction of the measurement sample was set to 0°, and the acute angle formed by the widthwise direction of the measurement sample and the slow axis was measured as the slow axis angle. Among the 10 measurement samples, the difference between the maximum value and the minimum value of the slow axis angle was obtained as the "change width". The measuring device and measurement conditions are as follows.

[0176] Measuring device: Retardation measuring device RE-100 manufactured by Otsuka Electronics Co., Ltd. Light source: Laser-emitting diode (LED) Band-pass filter: 550 nm (measurement wavelength) Measurement interval: 0.1 sec Integration times: 10 times Number of measurement points: 15 points Gain: 10 dB Measurement environment: Temperature 23°C, humidity 60%.

[0177] (4-5) Measurement results Table 3 shows the measurement results of the properties of the biaxially oriented polypropylene film.

[0178]

Table 3

[0179] (5) Evaluation of process passability The production of the biaxially oriented films of the examples and comparative examples was started. From the time when the obtained film thickness reached the target thickness (Table 2) ±2%, the time until the film broke during stretching (continuous film-forming time) was measured. The time when the thickness reached the target thickness ±2% was measured and confirmed according to the thickness measurement method in (3) above. Based on the obtained time, the process passability was evaluated according to the following evaluation criteria. The results are shown in Table 4 described later. A++: The film could be formed without breaking during stretching even after more than 48 hours. A+: The film could be formed without breaking during stretching after more than 32 hours and less than 48 hours. A: The film could be formed without breaking during stretching after more than 24 hours and less than 32 hours. B: The film could be formed without breaking during stretching after more than 16 hours and less than 24 hours. C: Stretching breakage occurred after more than 8 hours and less than 16 hours. D: Stretching breakage occurred within less than 8 hours. E: The film could not be formed.

[0180] (6) Evaluation of withstand voltage property Capacitors were fabricated using the biaxially oriented films of the examples and comparative examples, and the breakdown voltage resistance of the capacitors was evaluated.

[0181] (6-1) Fabrication of capacitor Using a vacuum evaporation machine manufactured by ULVAC, Inc., an aluminum film with a T-margin evaporation pattern was deposited on the measurement sample obtained in (4-1) above at an evaporation resistance of 20 Ω / □ to obtain a metallized film containing a metal film on one side of the biaxially oriented polypropylene film.

[0182] After slitting to a width of 50 mm, two metallized films were aligned, and using an automatic winder model 3KAW-N2 manufactured by Minato Seisakusho Co., Ltd., winding was performed at a winding tension of 210 g for 840 turns. The wound element was heat-treated at 120 °C for 15 hours while being pressed, and then zinc metal was sprayed on the end face of the element to obtain a flat capacitor. Lead wires were soldered to the end faces of the flat capacitor, and then it was sealed with epoxy resin. Incidentally, the capacitance of the obtained capacitor was 50 μF.

[0183] (6-2) Evaluation of initial withstand voltage property The initial capacitance (C0) of the capacitor before the test was measured using an LCR high tester 3522-50 manufactured by Hioki Electric Co., Ltd. Next, a DC voltage of 450 V / μm was applied to the capacitor for 10 seconds. The capacitance (C1) of the capacitor after the voltage application was measured in the same manner, and the capacitance change rate before and after the voltage application was calculated by the following formula.

[0184]

Equation

[0185] The above-mentioned capacitance change rate ΔC was measured for 100 elements and evaluated according to the following criteria. The number of capacitors for each of A+ to C was calculated, and the ratio (initial breakdown voltage yield) of A+ and A capacitors was determined. An initial breakdown voltage yield of 95% or more was considered qualified. A+: ΔC is less than -0.2%. A: ΔC is -0.5% or less and less than -1%. B: ΔC is -1% or less and less than -2%. C: ΔC is -2% or less.

[0186] (6-3) Evaluation of long-term withstand voltage property The initial capacitance (C0) of the capacitor before the test was measured using an LCR high tester 3522-50 manufactured by Hioki Electric Co., Ltd. Next, in a high-temperature bath at 115 °C, a DC voltage of 320 V / μm was continuously applied to the capacitor for 1000 hours. The capacitance of the capacitor after 1000 hours The capacitance (C 1000 ) was similarly measured, and the capacitance change rate (ΔC 1000 ) before and after the voltage load was calculated by the following formula.

[0187] [Number]

[0188] The above capacitance change rate ΔC 1000 was measured for 100 elements and evaluated according to the following criteria. The number of capacitors in each of A+ to C was calculated, and the ratio (long-term withstand voltage yield) of capacitors of A+ and A was obtained. A long-term withstand voltage yield of 90% or more was considered qualified. A+: ΔC 1000 is less than 0.5%. A: ΔC 1000 is -0.5% or less and less than -5%. B: ΔC 1000 is -5% or less and less than -10%. C: ΔC 1000 is -10% or less.

[0189] (7) Evaluation results The evaluation results of process passability and withstand voltage are shown in Table 4.

[0190] [Table 4]

Claims

1. A method for manufacturing a biaxially oriented polypropylene film having a first surface and a second surface, comprising obtaining a cast sheet using a casting drum having a microcrack surface in which, when a virtual line having a length of 0.1 mm in the widthwise direction is provided at an arbitrary position on the surface, the virtual line intersects one or more and 15 or fewer grooves, and the groove width is 1 μm or more and 10 μm or less, and subjecting the cast sheet to a biaxial stretching treatment.

2. The biaxially oriented polypropylene film, wherein the average maximum length of the substantially elliptical spots on the first surface is 3.0 mm or less, and / or the difference between the average peak height Rpk outside the spots and the average peak height Rpk inside the spots is 0.040 μm or less, is a biaxially oriented polypropylene film, according to the method of Claim 1.

3. The biaxially oriented polypropylene film, wherein the average maximum length of the substantially elliptical spots on the first surface is 3.0 mm or less, and the difference between the average peak height Rpk outside the spots and the average peak height Rpk inside the spots is 0.040 μm or less, is a biaxially oriented polypropylene film, according to the method of Claim 1.

4. The method according to Claim 2, wherein the difference between the average valley depth Rvk outside the spots and the average valley depth Rvk inside the spots is 0.020 μm or less.

5. The method according to Claim 1, wherein the change range of the slow axis angle of the biaxially oriented polypropylene film is in the range of 0.3° or more and 2.8° or less.

6. The method according to Claim 1, wherein the average number of substantially elliptical spots per 11.5 mm × 8.6 mm area of the first surface is 0.6 or less.

7. The polypropylene resin constituting the biaxially oriented polypropylene film has a weight average molecular weight Mw of 250,000 or more and 450,000 or less, a ratio (Mw / Mn) of the weight average molecular weight Mw to the number average molecular weight Mn of 5.0 or more and 12.0 or less, a melt flow rate at 230 °C and a load of 2.16 kg of 7.0 g / 10 min or less, and a heptane insoluble content of 96.0% or more and 99.5% or less, according to the method of Claim 1.

8. The method according to Claim 1, wherein the thickness of the biaxially oriented polypropylene film is 1.7 μm or more and 6.5 μm or less.

9. The method according to Claim 1, wherein the biaxially oriented polypropylene film is a single-layer film.

10. At any position on the surface, when a virtual line with a length of 0.1 mm in the width direction is provided at an arbitrary position, obtaining a cast sheet using a casting drum having a microcrack surface in which the virtual line intersects one or more and 15 or fewer grooves and the groove width is 1 μm or more and 10 μm or less, and obtaining a biaxially stretched polypropylene film by a method including biaxially stretching the cast sheet, and placing a metal layer on one or both surfaces of the biaxially stretched polypropylene film, A method for manufacturing a metal layer-integrated polypropylene film.

11. At any position on the surface, when a virtual line with a length of 0.1 mm in the width direction is provided at an arbitrary position, obtaining a cast sheet using a casting drum having a microcrack surface in which the virtual line intersects one or more and 15 or fewer grooves and the groove width is 1 μm or more and 10 μm or less, and obtaining a biaxially stretched polypropylene film by a method including biaxially stretching the cast sheet, obtaining a metal layer-integrated polypropylene film by a method including disposing a metal layer on one or both surfaces of the biaxially stretched polypropylene film, and winding the metal layer-integrated polypropylene film, A method for manufacturing a capacitor.

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