Biaxially stretched polypropylene film

Abstract

Provided is a polypropylene film which can be stably formed over a longer period of time, and which has higher voltage resistance (in particular, voltage resistance at high temperatures) and higher yield. A biaxially stretched polypropylene film having a first surface and a second surface, in which the average maximum length of substantially elliptical spots on the first surface is 3.0 mm or less, and 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 [mu] m or less.

Description

TECHNICAL FIELD

[0001] The present application relates to a biaxially-stretched polypropylene film or the like. BACKGROUND

[0002] A polypropylene film has excellent electric properties such as high voltage resistance, low dielectric loss properties, and has high moisture resistance. Therefore, it is widely used for electronic devices, electric devices. Specifically, for example, it is used as a film for a high voltage capacitor; a capacitor for a filter of a power conversion circuit of a converter, an inverter, or the like, a smoothing capacitor, or the like.

[0003] In particular, in recent years, polypropylene films have begun to be widely used as capacitors for inverter power supply devices for controlling drive motors of electric cars, hybrid cars, or the like. The capacitors for inverter power supply devices used in cars or the like are required to be small, light, high capacity, and have high reliability over a long period of time.

[0004] Patent Document 1 discloses a capacitor-use biaxially-stretched polypropylene film in which the number of protrusions per 0.1 mm 2 and the ten-point average roughness satisfy a prescribed relationship. Patent Document 1 describes that, as an effect of the above-described capacitor-use biaxially-stretched polypropylene film, even if it is a film, the process adaptability is excellent, and high voltage resistance is exhibited even under a wide range of atmospheric temperature conditions from low temperature (-40°C) to high temperature (150°C).

[0005] PRIOR ART DOCUMENTS

[0006] PATENT DOCUMENTS

[0007] Patent Document 1: International Publication No. 2013 / 146367 SUMMARY

[0008] PROBLEMS TO BE SOLVED BY THE INVENTION

[0009] With respect to a polypropylene film used as a capacitor for an electric car, a hybrid car, or the like, with the miniaturization and high capacity of capacitors in recent years, it is required to thin the thickness of the film and increase the electrode area. In addition, in recent years, the desire for further cost reduction is strong, and as a production method of a film, a method of stretching a resin sheet at high speed has been studied, and it is required to be able to manufacture a capacitor element having voltage resistance (particularly, voltage resistance at high temperature) at a high yield.

[0010] Therefore, the present application aims to provide a polypropylene film that can be stably produced for a longer period of time and has higher voltage resistance (particularly, voltage resistance at high temperature) at a higher yield.

[0011] SOLUTION TO PROBLEM

[0012] The present inventors have found the following insights in the course of their research.

[0013] The polypropylene film for the above use is thin and soft in thickness, and therefore the cast sheet of the precursor thereof is also thin and soft. In the case of continuously conveying such a cast sheet, a bubble becomes likely to intrude into a state between the sheet and a conveying roll. In this case, the adhesion to the roll becomes uneven, and the non-adhered portion appears as an oval-shaped depression on the sheet surface.

[0014] In addition, the conveying roll is heated for the purpose of allowing β crystals to appear on the surface of the cast sheet and imparting plasticity to the sheet before stretching. In the case where the oval-shaped depression portion is dispersed on the surface of the sheet, the adhesion state of this portion to the conveying roll is different from that of the other portions, and therefore the generation of β crystals and the state of roughening at the time of stretching differ. In this case, oval-shaped spots are dispersed and appear on the surface of the film.

[0015] If the difference in surface roughness between the "spot" and the other portions is enlarged, the film has a tendency to be broken at the time of transverse stretching. It is considered that this is because, if portions shaped by different thermal histories exist in the sheet surface, the plasticity of the resin at the time of transverse stretching becomes locally uneven, and the stretching breakage of the film is induced.

[0016] In addition, the spots cause a decrease in the withstand voltage of the capacitor element in practice, and have a tendency to significantly decrease the yield. It is considered that if a difference in surface roughness locally exists in the sheet surface, a difference in the interlayer adhesion locally occurs in the inside of the metallized polypropylene film after winding, and the film is broken by the concentration of electric field.

[0017] The present inventors have further researched based on the above insights, and as a result, have found that the above problems can be solved if the biaxially stretched polypropylene film has a first surface and a second surface, wherein the average maximum length of the substantially oval-shaped spots of the first surface is 3.0 mm or less, and the difference between the average projected peak height Rpk outside the spots and the average projected peak height Rpk inside the spots is 0.040 μm or less. That is, the present application includes the following means.

[0018] Item 1. A biaxially stretched polypropylene film having a first surface and a second surface, wherein,

[0019] the average maximum length of the substantially oval-shaped spots of the first surface is 3.0 mm or less, and

[0020] the difference between the average projected peak height Rpk outside the spots and the average projected peak height Rpk inside the spots is 0.040 μm or less.

[0021] Item 2. The biaxially-stretched polypropylene film according to item 1, wherein the difference between the average protruding valley depth Rvk outside the aforementioned spot and the average protruding valley depth Rvk inside the aforementioned spot is 0.020 μm or less.

[0022] Item 3. The biaxially-stretched polypropylene film according to item 1, wherein the variation range of the slow axis angle is a range of 0.3° or more and 2.8° or less.

[0023] Item 4. The biaxially-stretched polypropylene film according to item 1, wherein the average number of approximately elliptical spots per 11.5 mm x 8.6 mm area of the aforementioned first surface is 0.6 or less.

[0024] Item 5. The biaxially-stretched polypropylene film according to item 1, wherein the polypropylene resin constituting the biaxially-stretched 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 under 230°C and a load of 2.16 kg of 7.0 g / 10 min or less, and a heptane-insoluble component of 96.0% or more and 99.5% or less.

[0025] Item 6. The biaxially-stretched polypropylene film according to item 1, which has a thickness of 1.7 μm or more and 6.5 μm or less.

[0026] Item 7. The biaxially-stretched polypropylene film according to item 1, which is a single-layer film.

[0027] Item 8. The biaxially-stretched polypropylene film according to any one of items 1 to 7, which is used for a capacitor.

[0028] Item 9. A metal layer-integrated polypropylene film, comprising: the biaxially-stretched polypropylene film according to any one of items 1 to 7; and, a metal layer disposed on one surface or both surfaces of the biaxially-stretched polypropylene film.

[0029] Item 10. A capacitor, comprising the metal layer-integrated polypropylene film according to item 9.

[0030] Item 11. A method of manufacturing the biaxially-stretched polypropylene film according to any one of items 1 to 7, comprising: obtaining a cast sheet using a casting drum having a micro-crack surface in which, at any position of the surface, an imaginary line of 0.1 mm in length is disposed in the width direction at any position, the imaginary line intersects with one or more grooves having a width of 1 μm or more and 10 μm or less; and, performing a biaxial stretching process on the cast sheet.

[0031] Effects of the Invention

[0032] According to the present application, it is possible to provide a polypropylene film with higher yield, which can be stably produced for a longer period of time and has higher voltage resistance, particularly higher voltage resistance at high temperatures. BRIEF DESCRIPTION OF DRAWINGS

[0033] A representative photograph showing roughly elliptical-shaped spots. Figure 1 A representative photograph showing roughly elliptical-shaped spots. A representative photograph showing roughly elliptical-shaped spots.

[0034] A representative photograph showing roughly elliptical-shaped spots. Figure 2 A representative photograph showing roughly elliptical-shaped spots. A representative photograph showing roughly elliptical-shaped spots.

[0035] A representative photograph showing roughly elliptical-shaped spots. Figure 3 A representative photograph showing roughly elliptical-shaped spots. DETAILED DESCRIPTION

[0036] In the present specification, the expressions "containing" and "including" include the concepts of "containing", "including", "consisting essentially of", and "consisting of".

[0037] In the present specification, ranges obtained by arbitrarily replacing upper limits and / or lower limits of ranges constituted by the upper limits and / or lower limits described for each parameter are also exemplified.

[0038] 1. Biaxially stretched polypropylene film

[0039] The present application relates, in one mode thereof, to a biaxially-stretched polypropylene film (in the present specification, sometimes also referred to as "the polypropylene film of the present application") having a first surface and a second surface, wherein the average maximum length of roughly elliptical-shaped spots of the aforementioned first surface is 3.0 mm or less, and the difference between the average protruding peak height Rpk outside the aforementioned spots and the average protruding peak height Rpk inside the aforementioned spots is 0.040 μm or less. Hereinafter, this will be described.

[0040] Of the two main surfaces of the polypropylene film of the present application, one surface is the first surface, and the other surface is the second surface. The first surface is the surface on which a metal layer is laminated when a capacitor is produced.

[0041] The average maximum length of roughly elliptical-shaped spots of the first surface of the polypropylene film of the present application is 3.0 mm or less (property 1).

[0042] The "spots" of property 1 refer to spots that can be visually recognized on the image of the measurement method described later, and are regions whose roughening degree and / or roughened surface shape are different from the surroundings. 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.

[0043] Further, in the preferred embodiment of the present application, the average number of the substantially elliptical spots per 11.5 mm x 8.6 mm area of the first surface is preferably 0.6 or less from the viewpoint of further improving the film production stability and / or the yield of the higher voltage resistance.

[0044] The average number of the spots and the average maximum length of the spots are measured as follows.

[0045] From the center of each width direction of the biaxially-stretched polypropylene film as the measurement target, 50 mm x 50 mm measurement samples were cut out at a total of 10 points every 10 m in the length direction. Next, using a digital microscope (VHX-2000 manufactured by KEYENCE CORPORATION), under the lens magnification: 30 times, the light quantity: 50% of the full range, the measurement method: reflection measurement, the field of view range: 11.5 mm x 8.6 mm, for one surface (the surface on which the metal layer is laminated when a capacitor is produced) of each measurement sample, the image of "halo removal + clear mode (processing of color tone vividness, contour emphasis, and removal of glare)" was observed, and the number of substantially elliptical "spots" (a representative spot is shown in FIG. 2) observed in the field of view range was measured. Note that in the case where a part of the spot is not in the field of view, the spot was not measured. The number of spots in the field of view range of each measurement sample (a total of 10) was added up, and the total value obtained was divided by 10 (= the number of measurement samples) to obtain the average number of elliptical spots per field of view range (11.5 mm x 8.6 mm area). Figure 1 In addition, for each of the spots measured above, the diameter in the length direction and the diameter in the width direction of the biaxially-stretched 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 transverse direction is the major axis). The maximum length of each spot was added up, and the total value obtained was divided by the number of spots to obtain the average maximum length of the spots. Figure 2

[0046] From the viewpoint of the film production stability, the yield, and the like, the average number of the 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.

[0047] From the viewpoint of the film production stability, the yield, and the like, 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 the spots is 0).

[0048] The difference between the average protrusion peak part height Rpk outside the spot and the average protrusion peak part height Rpk inside the spot of the polypropylene film of the present application is 0.040 μm or less (property 2). ​

[0049] The method for measuring the difference between the average protrusion peak height Rpk outside the spot and the average protrusion peak height Rpk inside the spot is described below.

[0050] As the light interference type non-contact surface shape measuring machine, "VertScan 2.0 (Model: R5500GML)" manufactured by Ryoka Systems Inc. was used to measure the protrusion peak height Rpk of one face (the face on which the metal layer is laminated when the capacitor is manufactured) of the biaxially stretched polypropylene film as the measurement target. First, using the WAVE mode, a 530 white filter and a 1 x BODY lens barrel were applied, and a x 10 objective lens was used to perform measurement of 470.92 μm x 353.16 μm per field of view.

[0051] Regarding the inside of the spot, for all the spots measured by the above-described measurement method of the number of spots, the measurement was performed at the five positions shown in Figure 3 the figure (the intersection of the major axis and the minor axis, the center of the line connecting one end of the major axis and the aforementioned intersection, the center of the line connecting the other end of the major axis and the aforementioned intersection, the center of the line connecting one end of the minor axis and the aforementioned intersection, and the center of the line connecting the other end of the minor axis and the aforementioned intersection).

[0052] Regarding the outside of the spot, the measurement was performed at the center of each of the measurement samples cut by the above-described measurement method of the number of spots.

[0053] The obtained data was subjected to noise removal processing based on a median filter (3 x 3), and then, Gaussian filter processing based on a cutoff value of 30 μm was performed to remove the ripple component. Thereby, the state in which the measurement surface state could be appropriately measured was obtained. Next, analysis was performed using the analysis software "VS-Viewer" of "VertScan 2.0", and the average value of each value obtained at the above-described positions was calculated with respect to the protrusion peak height Rpk of the lubricity evaluation parameter. This is specifically described below.

[0054] The measured values of the protrusion peak height Rpk inside the spot were added, and the obtained total value was divided by X (= the number of measured spots x 5 (the measurement position inside 1 spot)) to obtain the average protrusion peak height Rpk inside the spot.

[0055] The measured values of the protrusion peak height Rpk outside the spot of each measurement sample were added, and the obtained total value was divided by 10 (= the number of measurement samples) to obtain the average protrusion peak height Rpk outside the spot.

[0056] The average protrusion peak height Rpk inside the spot was subtracted from the average protrusion peak height Rpk outside the spot to obtain the difference between the two.

[0057] From the viewpoints of film production stability, yield, and the like, the difference between the average protruding peak portion height Rpk outside the spot and the average protruding peak portion height Rpk inside the spot 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. Note that the difference is 0 μm in the case where the average number of spots is 0.

[0058] From the viewpoints of high-temperature durability of the capacitor element and the like, the average protruding peak portion height Rpk outside the spot (= average protruding peak portion height Rpk of the film in the case where the average number of 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 further preferably 0.020 μm or more and 0.085 μm or less.

[0059] From the viewpoints of film production stability, yield, and the like, the average protruding peak portion height Rpk inside the spot in the case where the average number of spots is more than 0 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 further preferably 0.010 μm or more and 0.085 μm or less.

[0060] By combining Property 1 and Property 2 described later, the yield of the higher of film production stability and voltage resistance can be further improved. Although not intended to be a limiting explanation, the reason is considered to be as follows. It is considered that if Property 1 and Property 2 are satisfied, the thermal history of the cast sheet as a precursor before stretching is moderately homogenized in the film production process, and as a result, the plasticity at the time of transverse stretching is also homogenized, and film breakage is suppressed. In addition, in the case where a capacitor is produced using such a film, a capacitor having voltage resistance can be efficiently obtained with high yield. It is considered that a capacitor produced using such a film has pores between film layers inside the capacitor that are moderately homogenized, and becomes less likely to generate electric field concentration caused by local interlayer adhesion. It is thus considered that breakage of the film caused by local heat release inside the capacitor is suppressed, and the aforementioned effects can be obtained.

[0061] From the viewpoint of further improving the film production stability and / or the yield of a higher withstand voltage, the difference between the average projected valley depth Rvk outside the spot and the average projected valley depth Rvk inside the spot of the polypropylene film of the present application is preferably 0.020 μm or less. The difference is more preferably 0 μm or more and 0.015 μm or less, further preferably 0 μm or more and 0.010 μm or less, and particularly preferably 0 μm. Note that the difference is 0 μm in the case where the average number of spots is 0. Although not intended to be a limiting explanation, it is believed that by making the difference within the above range, leakage current is less likely to occur from a portion where the thickness is locally thin, and in addition, the plasticity at the time of transverse stretching is also homogenized, as a result, the film production stability and / or the yield of a higher withstand voltage can be further improved.

[0062] The method for measuring the difference between the average projected valley depth Rvk outside the spot and the average projected valley depth Rvk inside the spot is based on the method for measuring the difference between the average projected peak height Rpk outside the spot and the average projected peak height Rpk inside the spot.

[0063] The measured values of the projected valley depth Rvk inside the spot are added up, and the total value obtained is divided by X (= the number of measured spots x 5 (the number of measurement sites inside one spot)), to obtain the average projected valley depth Rvk inside the spot.

[0064] The measured values of the projected valley depth Rvk outside the spot of each measurement sample are added up, and the total value obtained is divided by 10 (= the number of measurement samples), to obtain the average projected valley depth Rvk outside the spot.

[0065] The average projected valley depth Rvk inside the spot is subtracted from the average projected valley depth Rvk outside the spot, to obtain the difference between the two.

[0066] From the viewpoint of the high-temperature durability of the capacitor element and the like, the average projected valley depth Rvk outside the spot (= the average projected valley depth Rvk of the film in the case where 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 further preferably 0.015 μm or more and 0.040 μm or less.

[0067] From the viewpoint of the film production stability and the reduction in the number of insulation defects and the like, the average projected valley depth Rvk inside the spot when the average number of spots is more than 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 further preferably 0.003 μm or more and 0.035 μm or less.

[0068] From the viewpoint of further improving the stability of film production and / or the yield of a higher voltage resistance, the polypropylene film of the present application preferably uses an insulation defect inspection device having an unwinding-winding mechanism, and the number of insulation defects per 10 m of the biaxially stretched polypropylene film as the measurement object is 1.0 or less when a direct current voltage of 600 V is applied per 1 μm thickness. The number is more preferably 0 / 10 m 2 or less, 0.5 / 10 m 2 or less, 0.1 / 10 m 2 or less, and 0 / 10 m 2 or less, 0.1 / 10 m 2 or less, and 0 / 10 m 2 or less. Although not intended to be a limiting explanation, a capacitor made using such a film is less likely to cause 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.

[0069] The measurement method of the number of insulation defects is described below.

[0070] The number of insulation defects (number / 10 m 2 ) of the biaxially stretched polypropylene film as the measurement object is measured using an insulation defect inspection device having an unwinding-winding mechanism. The polypropylene film as the measurement object is made to travel between a high voltage electrode and a metal roller grounded, and a direct current voltage is applied at this time. The number of discharges generated at the insulation defect portion is measured using a counter attached to the device. The number of insulation defects (number / 10 m 2 ) of the film is calculated by dividing the measured number by the measurement area. The measurement conditions are described below.

[0071] • The interval between the high voltage electrode and the metal roller grounded: 50 μm

[0072] • The wrap angle at which the film contacts the metal roller grounded: 120°

[0073] • The shape of the high voltage electrode: a metal plate having a thickness of 4 mm and the same width as the metal roller

[0074] • The unwinding speed: 20 m / min

[0075] • The direct current voltage: 600 V / μm

[0076] • The measurement area: 472 m 2

[0077] • The test environment temperature: 20°C.

[0078] From the viewpoint of further improving the stability of film production and / or the yield of a higher voltage resistance, the variation range of the slow axis angle of the polypropylene film of the present application is preferably 0.3° or more and 2.8° or less. The variation range is more preferably 0.3° or more and 2.0° or less, further preferably 0.3° or more and 1.5° or less, and still further preferably 0.3° or more and 1.0° or less.

[0079] The slow axis angle of the biaxially stretched polypropylene film refers to the angle of the acute angle formed between the width direction of the biaxially stretched polypropylene film and the slow axis. The polypropylene film of the present application is stretched in two directions, a first direction and a second direction orthogonal thereto. By the aforementioned biaxial stretching, the polymer is oriented in the plane, and thus the biaxially stretched film becomes birefringent. In the plane of the film, the direction in which the refractive index is the largest becomes the direction in which the light advances slowly (phase delay), and thus is called the slow axis.

[0080] In the sequential biaxial stretching method, first, the cast base sheet is stretched in the flow direction (MD direction), and then the sheet is stretched in the transverse direction (TD direction). In this case, in the slow axis of the biaxially stretched polypropylene film, there is a tendency that the refractive index in the transverse direction of the second direction becomes 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.

[0081] In the stretching in the transverse direction (TD direction), in the case where the stretching is performed completely in the transverse direction (in the case where the stretching is performed completely in the direction orthogonal to the flow direction), the slow axis angle defined in the present specification becomes 0°. However, in reality, the shrinkage stress at the time of stretching, mechanical external force, the thermoplasticity of the film, and the like play a role, and the stretching cannot be performed completely in the transverse direction (TD direction), and there is a tendency that the slow axis angle becomes larger than 0°.

[0082] In the length direction, the stretching is not uniform at the site where the variation range of the slow axis angle is large, and thus strain is easily generated in the film. The aforementioned non-uniformity of the stretching leads to a decrease in the voltage resistance performance of the capacitor in practice, and there is a tendency that the yield is significantly decreased. It is considered that the reason is that, in the case where the stretching is not uniform in the length direction, the dimensional change becomes non-uniform in the inside of the capacitor exposed to high temperature, the electric field concentration is generated due to the local interlayer adhesion, and the film is broken. It is considered that, by controlling the variation range of the slow axis angle in the length direction to be in the range of 0.3° or more and 2.8° or less, the aforementioned failure in the length direction can be suppressed.

[0083] The measurement method of the variation range of the slow axis angle and the average slow axis angle is described below.

[0084] From the biaxially-stretched polypropylene film as the measurement target, 50 mm x 50 mm measurement samples were cut out every 10 m in the length direction from the center at a total of 10 points (0 m, 10 m, 20 m, 30 m, 40 m, 50 m, 60 m, 70 m, 80 m, and 90 m positions). Subsequently, the width direction of the aforementioned measurement sample was set to 0°, and the angle of the acute angle that the width direction of the aforementioned measurement sample made with the slow axis was measured as the slow axis angle. In 10 measurement samples, the difference between the maximum value and the minimum value of the slow axis angle was taken as the "variation amplitude", and the average value was taken as the "average slow axis angle". The measurement device and measurement conditions are described below.

[0085] Measurement device: Otsuka Electronics Co., Ltd. delay measurement device RE-100

[0086] Light source: laser light emitting diode (LED)

[0087] Bandpass filter: 550 nm (measurement wavelength)

[0088] Measurement interval: 0.1 seconds

[0089] Number of accumulations: 10 times

[0090] Number of measurement points: 15 points

[0091] Gain: 10 dB

[0092] Measurement environment: temperature 23°C, humidity 60%.

[0093] From the viewpoint of, for example, yield in film production, the average slow axis angle is preferably 0° or more and 20° or less, more preferably 0° or more and 15° or less, and further preferably 0° or more and 13° or less.

[0094] With respect to the thickness of the polypropylene film of the present application, from the viewpoint of further improving the miniaturization and high capacity of capacitors when used for capacitors, the upper limit is preferably 6.5 μm or less, more preferably 5.5 μm or less, further preferably 3.5 μm or less, particularly preferably 3.0 μm or less, and most preferably 2.8 μm or less. In addition, from the viewpoint of manufacturing, the lower limit is preferably 0.8 μm or more, more preferably 1.0 μm or more, further preferably 1.7 μm or more, and particularly preferably 2.0 μm or more. In addition, from the viewpoint of yield of those having higher film stability and / or voltage resistance, it is also preferable to set the thickness to the above range. The method for measuring the thickness of the biaxially-stretched polypropylene film in this specification is based on the method described in the examples.

[0095] The layer constitution of the polypropylene film of the present application is not particularly limited. The polypropylene film of the present application can be a single layer formed of one layer, or can be a plurality of layers having the same or different compositions. The polypropylene film of the present application is preferably a film formed of one layer or a plurality of layers of a film-shaped molded layer, and more preferably a single layer film (a film formed of one layer of a film-shaped molded layer).

[0096] The polypropylene film of the present application is not particularly limited in the material thereof as long as it contains a polypropylene resin. As the polypropylene resin, there is no particular limitation, and examples thereof include isotactic polypropylene and the like, a homopolymer of propylene, a copolymer of propylene and ethylene, long-chain branched polypropylene, ultrahigh molecular weight polypropylene, and the like. Among these, from the viewpoint of heat resistance, isotactic polypropylene is preferable.

[0097] The content of the aforementioned polypropylene resin is preferably 90% by mass or more, and more preferably 95% by mass or more, with respect to the entire polypropylene film of the present application (when the entire polypropylene film is taken as 100% by mass). The upper limit of the content of the aforementioned polypropylene resin is, for example, 100% by mass, 98% by mass, or the like, with respect to the entire polypropylene film of the present application.

[0098] The aforementioned polypropylene resin can be a single kind, or can be a combination of two or more kinds.

[0099] Here, when the polypropylene resin contained in the polypropylene film of the present application is two or more kinds, the polypropylene resin having a larger content is referred to as "a polypropylene resin of a main component" in the present specification. In addition, when the polypropylene resin contained in the polypropylene film of the present application is one kind, the polypropylene resin is referred to as "a polypropylene resin of a main component" in the present specification.

[0100] Hereinafter, in the present specification, when referred to as "a polypropylene resin" without particularly specifying whether it is a polypropylene resin of a main component or a polypropylene resin other than a main component, it means both of them unless otherwise specified. For example, in the case where it is described that "the weight average molecular weight Mw of the aforementioned polypropylene resin is preferably 250,000 or more and 450,000 or less," it means both of the following: the weight average molecular weight Mw of the polypropylene resin of a 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 a main component is preferably 250,000 or more and 450,000 or less.

[0101] From the viewpoints of thickness uniformity, mechanical properties, thermal-mechanical properties, and the like of the biaxially-stretched polypropylene film, the weight average molecular weight Mw of the aforementioned polypropylene resin is preferably 250,000 or more and 450,000 or less, more preferably 250,000 or more and 420,000 or less, further preferably 250,000 or more and 400,000 or less, and still further preferably 260,000 or more and 390,000 or less. By using such a polypropylene resin, the dielectric breakdown of the film is suppressed, and a very thinned biaxially-stretched polypropylene film suitable for a small and high-capacity capacitor becomes easy to obtain. When two or more kinds of polypropylene resins are used, it is preferable to use in combination a polypropylene resin (preferably a polypropylene resin that is a main component) having an Mw of 250,000 or more and less than 330,000 (preferably 250,000 or more and 300,000 or less, and 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 an 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, and further preferably 370,000 or more and 390,000 or less).

[0102] From the viewpoint that a moderate resin flowability at the time of biaxial stretching is obtained, and a very thinned biaxially-stretched polypropylene film without thickness unevenness becomes easy to obtain, the ratio (Mw / Mn) of the weight average molecular weight Mw to the number average molecular weight Mn of the aforementioned polypropylene resin is preferably 5.0 or more and 12.0 or less, more preferably 5.0 or more and 10.0 or less, and further preferably 5.0 or more and 9.0 or less. When two or more kinds of polypropylene resins are used, it is preferable to use in combination a polypropylene resin (preferably a polypropylene resin that is a main component) having a 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 a ratio of 7.0 or more and 12.0 or less (preferably 7.5 or more and 10.0 or less, and more preferably 7.5 or more and 9.0 or less).

[0103] The method for measuring the weight average molecular weight Mw and the number average molecular weight Mn of the polypropylene resin is based on the method described in the Examples.

[0104] The melt flow rate (MFR) of the aforementioned polypropylene resin at 230°C under a load of 2.16 kg is not particularly limited, and is preferably 7.0 g / 10 minutes or less from the viewpoint of stretchability and the like, and more preferably 0.5 g / 10 minutes or more and 6.0 g / 10 minutes or less from the viewpoint of improving the thickness accuracy of the polypropylene film of the present application. When two or more kinds of polypropylene resins are used, it is preferable to use in combination a polypropylene resin having an MFR of 4.0 g / 10 minutes or more and less than 7.0 g / 10 minutes (preferably 4.5 g / 10 minutes or more and 6.5 g / 10 minutes or less, more preferably 5.0 g / 10 minutes or more and 6.0 g / 10 minutes or less) (preferably a polypropylene resin that is the main component) and a polypropylene resin having an MFR of 0.5 g / 10 minutes or more and less than 4.0 g / 10 minutes (preferably 1.0 g / 10 minutes or more and 3.5 g / 10 minutes or less, more preferably 1.5 g / 10 minutes or more and 3.0 g / 10 minutes or less) (preferably a polypropylene resin other than the main component). The method for measuring the melt flow rate of the aforementioned polypropylene resin is based on the method described in the examples.

[0105] The heptane-insoluble component (HI) of the aforementioned polypropylene resin is preferably 96.0% or more and 99.5% or less, more preferably 97.0% or more and 99.0% or less. Here, the more the heptane-insoluble component, the higher the stereoregularity of the resin. By using such a polypropylene resin, the crystallinity is moderately increased, and the initial voltage resistance and the voltage resistance over a long period of time are improved. The method for measuring the heptane-insoluble component (HI) is based on the method described in the examples.

[0106] The content of the polypropylene resin that is the main component 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, further preferably 60% by mass or more and 75% by mass or less, and more further preferably 60% by mass or more and 70% by mass or less, with respect to 100% by mass of the polypropylene resin.

[0107] The aforementioned polypropylene resin can be generally produced using a known polymerization method. As the aforementioned polymerization method, for example, a gas phase polymerization method, a bulk polymerization method, and a slurry polymerization method can be exemplified.

[0108] The polymerization can be single-stage (one-stage) polymerization using one polymerization reactor, or can be multistage polymerization using two or more polymerization reactors. In addition, the polymerization can be performed with the addition of hydrogen or a comonomer as a molecular weight modifier in the reactor.

[0109] As the catalyst at the time of polymerization, a publicly known Ziegler-Natta catalyst can be used without particular limitation as long as the aforementioned polypropylene resin can be obtained. The aforementioned catalyst can also include a cocatalyst component, a donor. By adjusting the catalyst, the polymerization conditions, it is possible to control the molecular weight, the molecular weight distribution, the stereoregularity, and the like.

[0110] The molecular weight distribution and the like of the aforementioned polypropylene resin can be adjusted by resin mixing (blending). For example, a method in which two or more resins that differ from each other in terms of the molecular weight, the molecular weight distribution, and the like can be cited. Generally, in a case where a master resin in which a resin having a higher average molecular weight or a resin having a lower average molecular weight than the master resin is mixed at 55% by mass or more and 90% by mass or less of the entire resin is taken as 100% by mass, a two-polypropylene mixed system is easy to adjust the amount of the low-molecular-weight component, and thus is preferred.

[0111] Note that, in a case where the aforementioned mixing adjustment method is employed, a melt flow rate (MFR) can also be used as a criterion for the average molecular weight. In this case, from the viewpoint of convenience at the time of adjustment, it is preferred that the difference between the MFRs of the master resin and the added resin be approximately 1 to 30 g / 10 minutes in advance.

[0112] There is no particular limitation on the method of performing resin mixing, and a method in which the polymerized powder or pellets of the master resin and the added resin are dry mixed using a mixer or the like can be cited. A method in which the polymerized powder or pellets of the master resin and the added resin are supplied to a kneader and subjected to melt kneading to obtain a blended resin can also be cited.

[0113] The aforementioned mixer and the aforementioned kneader are not particularly limited. The aforementioned kneader can be any of a single-screw type, a twin-screw type, and a multi-screw type of three or more. In a case of a screw type of two or more screws, the kneader can be of any of a co-rotating type and a counter-rotating type.

[0114] In a case where blending is performed using melt kneading, the kneading temperature is not particularly limited as long as a good kneaded product can be obtained. Generally, the range of 200°C to 300°C is preferred from the viewpoint of suppressing the degradation of the resin, and 230°C to 270°C is more preferred. In addition, in order to suppress the degradation of the resin at the time of kneading and mixing, a non-active gas such as nitrogen can also be purged into the kneader. The resin after melt kneading can generally be pelletized into an appropriate size using a publicly known pelletizer. Thereby, a mixed polypropylene raw resin pellet can be obtained.

[0115] By using the aforementioned polypropylene resin, it becomes easy to adjust the aforementioned film properties, and thus is also preferred from the viewpoint of the stability of film production and / or the yield of a higher voltage resistance.

[0116] The polypropylene film of the present application can also contain an additive. The "additive" is not particularly limited as long as it is an additive generally used for polypropylene resins.

[0117] As the aforementioned additive, for example, 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, an inorganic particle, a resin particle, and the like can be given. The aforementioned polypropylene resin can contain the aforementioned additive in an amount that does not adversely affect the polypropylene film of the present application (for example, 10% by mass or less, 5% by mass or less, 1% by mass or less, or 0.1% by mass or less, relative to 100% by mass of the polypropylene film of the present application).

[0118] 2. Method for producing biaxially stretched polypropylene film

[0119] The biaxially stretched polypropylene film can be produced by obtaining a cast sheet corresponding to a stretching precursor from a resin pellet, and then performing a biaxial stretching process on the cast sheet. As a result of the examples described later, it was found that by controlling the microcracks on the surface of the casting drum, the air blowing speed of the air knife, the distance between the air outlet of the air knife and the cast sheet, the temperature of the stretching nip roll, the hardness of the stretching nip roll, and the like, and specifically, by producing according to the following method, the biaxially stretched polypropylene film of the present application can be obtained.

[0120] Among the aforementioned production conditions, the microcracks on the surface of the casting drum are an important condition. From this viewpoint, the present application relates, in one mode thereof, to a method for producing the biaxially stretched polypropylene film of the present application, which includes: obtaining a cast sheet using a casting drum having a microcracked surface, the casting drum having microcracks on the surface, the microcracks intersecting with one or more grooves having a width of 1 μm or more and 10 μm or less; and performing a biaxial stretching process on the aforementioned cast sheet.

[0121] Hereinafter, the method for producing the biaxially stretched polypropylene film of the present application will be described in detail.

[0122] 2-1. Production of cast sheet

[0123] The cast sheet can be molded by using a publicly known method. For example, a polypropylene resin pellet, a polypropylene resin pellet after dry mixing, or a mixed polypropylene resin pellet prepared by preliminary melt kneading can be supplied to an extruder, heated and melted, and then extruded from a T-die into a sheet shape after removing foreign matter and modified polymers through a filter, and cooled and solidified on at least one metal drum (casting drum) to be molded into a cast sheet.

[0124] The polypropylene resin is modified to some extent by thermal degradation, oxidative degradation in the extruder. From the viewpoint of inhibiting such polymer modification, the resin temperature at the time of melt extrusion is 170°C or higher and 320°C or lower, and is preferably 200°C or higher and 300°C or lower. In addition, the degradation can be inhibited by nitrogen replacement in the extruder, screw shape, internal shape of the T-die at the time of casting, antioxidant addition amount, and the like.

[0125] The temperature of the casting drum is preferably 80°C or higher and 140°C or lower, and is more preferably maintained at 90°C or higher and 105°C or lower. With respect to the β crystal fraction of the cast sheet obtained in such a temperature range, in the range where the aforementioned β crystal fraction becomes 5% or higher and 20% or lower by the X-ray method, the roughness of the film surface is moderately improved, and both the capacitor characteristics and the element winding processability can be satisfied.

[0126] The surface of the casting drum is not particularly limited, and in terms of being able to easily obtain the properties desired in the present application, a roll having a concave-convex surface such as a sandblasted roll or a ceramic roll, or a microcrack roll having an air-entraining discharge path is preferred. The manufacturing method of these rolls is known, and for example, for a microcrack roll, it can be manufactured, for example, in accordance with or based on the method described in Japanese Patent 6974939. The microcracks are generated due to the stress of the metal plating layer, and by the thickness of the metal plating layer, plating conditions, heat treatment, chemical treatment, multilayering, and the like, the shape and number can be controlled. For example, in the case of chromium plating, the plating thickness is preferably set to 100 μm to 400 μm, more preferably 120 μm to 350 μm, and further preferably 150 μm to 300 μm. By setting to this range, the control of the shape and number of the grooves becomes easier, and when the roll is heated, cracks are less likely to occur.

[0127] As for the micro crack roll, when an imaginary line of 0.1 mm in length is provided at any position on the roll surface in the width direction, it is preferable that the imaginary line intersects with 1 or more and 15 or less grooves, more preferable that the imaginary line intersects with 2 or more and 10 or less grooves, and further preferable that the imaginary line intersects with 3 or more and 8 or less grooves. In addition, 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 further preferably 3 μm or more and 8 μm or less. By using the micro crack roll of the above-mentioned groove number and groove width range, the air intruding between the casting drum and the cast sheet is properly discharged, and even if the cast sheet is thinned, close contact conveyance becomes easy, and a biaxially stretched polypropylene film that is extremely thinned can be easily obtained. In addition, even if the forming speed of the thinned cast sheet is increased, a phenomenon such as thickness fluctuation of the cast sheet, so-called stretching resonance (surging) phenomenon, does not easily occur, and a biaxially stretched polypropylene film that is extremely thinned and has a uniform thickness in the length direction can be easily obtained.

[0128] The method of close contact with the casting drum can use any of an air knife method, a contact roll method, an electrostatic application method, a water-cooled casting method, and the like, and the air knife method is preferable because adjustment related to sheet close contact is easy and the operation is simple.

[0129] In the case of using an air knife, the air blowing speed is preferably 70 m / sec or more and 130 m / sec or less, more preferably 80 m / sec or more and 120 m / sec or less, and further preferably 90 m / sec or more and 110 m / sec or less. In addition, the distance from the air knife blowing port to the cast sheet is preferably 2 mm or more and 5 mm or less, and more preferably 2 mm or more and 4 mm or less. If the air knife is used in such a range, the film-shaped thinned resin extruded from the T die can be properly close contacted to the casting drum, and the film vibration of the resin in a molten state from the T die outlet to the close contact to the casting drum can be suppressed. As a result, the above-mentioned film properties can be easily obtained.

[0130] 2-2. Biaxial stretching treatment

[0131] The biaxially stretched polypropylene film of the present application is obtained by performing biaxial stretching of the above-mentioned cast sheet in the longitudinal direction and the transverse direction, and as the stretching method, a simultaneous biaxial stretching method or a sequential biaxial stretching method can be mentioned, and from the viewpoint of stabilizing the thickness uniformization and improving the mechanical strength of the film, the sequential biaxial stretching method is preferable.

[0132] As the sequential biaxial stretching method, it is preferable that the cast sheet is first preheated by passing through a conveyance roll maintained at 70°C or higher and 135°C or lower, preferably 80°C or higher and 130°C or lower, and then heated to 130°C or higher and 155°C or lower, preferably 140°C or higher and 150°C or lower, immediately before the lengthwise stretching. By thus heating the cast sheet, excessive thermal expansion of the cast sheet is suppressed, the planarity of the sheet is easily maintained before the subsequent lengthwise stretching, and the sheet becomes easily uniform in adhesion to the conveyance roll.

[0133] From the viewpoint of easily obtaining the biaxially stretched polypropylene film of the present application, it is preferable that the method of simultaneously heating both surfaces of the cast sheet immediately before the lengthwise stretching is employed. The method is not particularly limited, and from the viewpoint of suppressing the difference in thermal history between the surfaces of the cast sheet, the method of heating the stretching nip roll or the method of heating using electromagnetic radiation is preferable. From the viewpoint of being able to simultaneously perform the fixation of the cast sheet immediately before the stretching and the temperature adjustment of the sheet, the method of heating the stretching nip roll is more preferable.

[0134] In the case of heating the stretching nip roll, from the viewpoint of suppressing the difference in thermal history between the 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.

[0135] In the case of heating the stretching nip roll, in order to make the shape of the roll surface follow the shape of the cast sheet surface, the rubber hardness of the roll is preferably 40° or higher and 80° or lower, more preferably 50° or higher and 70° or lower.

[0136] Subsequently, the lengthwise stretching is performed to 3 times or more and 7 times or less, preferably 4 times or more and 6 times or less, and immediately cooled to room temperature.

[0137] After the lengthwise stretching, the stretched film is introduced into a tenter, the both ends are gripped with a jig heated to 80°C or higher and 140°C or lower, preheated at a temperature of 140°C or higher and 185°C or lower, preferably 150°C or higher and 175°C or lower, and then stretched in the widthwise direction at a temperature of 140°C or higher and 170°C or lower, preferably 150°C or higher and 160°C or lower, to 6 times or more and 12 times or less, preferably 8 times or more and 11 times or less.

[0138] Subsequently, relaxation, heat setting, and winding are performed. The film after the winding can be subjected to aging treatment in an atmosphere of 20°C or higher and 45°C or lower, and then cut to the desired product width.

[0139] 3. Metal layer-integrated polypropylene film

[0140] The present application in one embodiment thereof also provides a metal layer-integrated polypropylene film (in the present specification, sometimes also referred to as "metal layer-integrated polypropylene film of the present application") comprising: the biaxially-stretched polypropylene film of the present application; and, a metal layer disposed on one or both faces of the aforementioned biaxially-stretched polypropylene film. Hereinafter, the metal layer-integrated polypropylene film of the present application is described in detail. The initial withstand voltage property and long-term durability under high temperature and high voltage of a capacitor obtained by winding the metal layer-integrated polypropylene film of the present application are excellent.

[0141] The polypropylene film of the present application can have an electrode installed on one or both faces thereof for processing into a capacitor. Such an electrode is not particularly limited as long as the present embodiment can obtain a target capacitor, and an electrode commonly used for manufacturing a capacitor can be used. As the electrode, for example, a metal foil, paper and a plastic film, etc. at least one face of which is metallized can be exemplified.

[0142] Capacitors are further required to be small and light, and thus it is preferable to directly metallize one or both faces of the polypropylene film of the present application to form an electrode. The metal used can be, for example, a simple substance of zinc, lead, silver, chromium, aluminum, copper, nickel, etc., a mixture of a plurality of them, and an alloy thereof, etc., but in consideration of the environment, economy, and capacitor performance, etc., zinc and aluminum are preferable.

[0143] As a method of directly metallizing the surface of the polypropylene film, for example, a vacuum evaporation method and a sputtering method can be exemplified, and the present embodiment is not particularly limited as long as a target capacitor can be obtained. From the viewpoints of productivity and economy, etc., a vacuum evaporation method is preferable. As the vacuum evaporation method, a crucible method, a wire method, etc. can be generally exemplified, but the present embodiment is not particularly limited as long as a target capacitor can be obtained, and the best one can be appropriately selected.

[0144] From the viewpoint of the electrical properties of the capacitor, the sheet resistance of the metal evaporation film is preferably 1 Ω / D or more and 100 Ω / D or less. From the viewpoint of the self-healing property, even within this range, a higher value is desirable, and if further based on the viewpoint of safety, the sheet resistance is more preferably 5 Ω / D or more and 50 Ω / D or less, and further preferably 10 Ω / D or more and 30 Ω / D or less. The sheet resistance of the metal evaporation film can be measured in the metal evaporation, for example, by a four-terminal method known to those skilled in the art. The sheet resistance of the metal evaporation film can be adjusted, for example, by adjusting the evaporation amount by adjusting the power of the evaporation source.

[0145] When the metal evaporation film is formed on one side of the film, in order to make the film into a capacitor when it is wound, an insulating edge is formed by not evaporating a certain width from one end of the film. Furthermore, in order to make the metal layer-integrated polypropylene film of the present application firmly engage with the metal-sprayed electrode, a heavy edge structure is preferably formed on the end opposite to the insulating edge. The film resistance of the heavy edge is generally 1 Ω / D or more and 8 Ω / D or less, and preferably 1 Ω / D or more and 5 Ω / D or less. The thickness of the metal film is not particularly limited, and is preferably 1 nm or more and 200 nm or less.

[0146] The edge pattern of the formed metal evaporation film is not particularly limited, but from the viewpoint of improving the safety and the like of the capacitor, a pattern including a so-called special edge such as a fishnet pattern or a T-edge pattern is preferable. When a metal evaporation film is formed on one side of the polypropylene film in a pattern including a special edge, the safety of the resulting capacitor is improved, which is effective from the viewpoint of capacitor breakdown, short circuit suppression, and the like, and is preferable.

[0147] As a method of forming the edge, a known method such as a tape method in which masking is performed using a tape at the time of evaporation, an oil method in which masking is performed by applying oil, and the like can be used without any limitation.

[0148] The metal layer-integrated polypropylene film of the present application is subjected to winding processing in which the film is wound along the length direction of the film, and can be processed into the capacitor of the present application described later. That is, two pieces of the metal layer-integrated polypropylene film of the present application are made into a pair, and are overlapped and wound in a manner in which the metal layer and the polypropylene film are alternately laminated. Thereafter, a procedure in which a pair of metal-sprayed electrodes is formed on both end surfaces by metal thermal spraying is performed, and a film capacitor is produced, and a capacitor is obtained.

[0149] 4. Capacitor

[0150] The present application provides, in one mode thereof, a capacitor (in the present specification, sometimes referred to as "the capacitor of the present application") including the metal layer-integrated polypropylene film of the present application. Hereinafter, the capacitor of the present application will be described in detail.

[0151] In the procedure of producing the capacitor, winding processing of the film is performed. For example, two pieces of the metal layer-integrated polypropylene film of the present application are overlapped and wound in a manner in which the metal layer and the polypropylene film in the metal layer-integrated polypropylene film of the present application are alternately laminated, and furthermore, in a manner in which the insulating edge portions become opposite sides. At this time, it is preferable to laminate the two pieces of the metal layer-integrated polypropylene film of the present application in a manner in which they are shifted by 1 to 2 mm. The winding machine used is not particularly limited, and for example, an automatic winding machine 3KAW-N2 manufactured by Kabushiki Kaisha Minato Seizosho or the like can be used.

[0152] When the flat capacitor is manufactured, the obtained winding is usually subjected to pressing after winding. The winding and the element formation of the capacitor are facilitated by the pressing. From the viewpoint of the control and stabilization of the interlayer gap, the optimum value of the applied pressure varies depending on the thickness of the polypropylene film and the like, and is, for example, 2 to 20 kg / cm 2 .

[0153] Next, metal is sprayed on both end surfaces of the winding to provide metal-sprayed electrodes, thereby manufacturing the capacitor. The capacitor is further subjected to a prescribed heat treatment. That is, in the present embodiment, a process of subjecting the capacitor to heat treatment at a temperature of 80 to 125°C under vacuum for 1 hour or more (hereinafter, sometimes referred to as "heat maturation") is included.

[0154] In the above process of subjecting the capacitor to heat treatment, the temperature of the heat treatment is 80°C or higher and 130°C or lower, and is preferably 90°C or higher and 125°C or lower. By subjecting the heat treatment at the aforementioned temperature, the effect of heat maturation can be obtained. Specifically, the pores between the films of the capacitor constituted of the metal layer-integrated polypropylene film according to the present application are reduced, the corona discharge is suppressed, and the internal structure of the metal layer-integrated polypropylene film according to the present application is changed, and the crystallization is promoted. As a result, it is considered that the withstand voltage is improved. In the case where the temperature of the heat treatment is lower than the prescribed temperature, the above effect by heat maturation cannot be sufficiently obtained. On the other hand, in the case where the temperature of the heat treatment is higher than the prescribed temperature, thermal decomposition, oxidative deterioration, and the like are sometimes generated in the polypropylene film.

[0155] As a method of subjecting the capacitor to heat treatment, for example, a publicly known method including a method using a thermostat bath under a vacuum atmosphere, a method using high-frequency induction heating, and the like can be appropriately selected. Specifically, the method using the thermostat bath is preferably adopted.

[0156] Regarding the time of the heat treatment, 1 hour or more is preferably set in terms of obtaining mechanical stability and thermal stability, and 10 hours or more is more preferably set, but 20 hours or less is more preferably set in terms of preventing heat wrinkle, molding failure, and the like. A lead is usually welded to the metal-sprayed electrode of the capacitor subjected to heat maturation. In addition, in order to impart weather resistance, and particularly in order to prevent humidity deterioration, the capacitor is preferably sealed into a case and is potted with an epoxy resin. The capacitor according to the present application is a small and large-capacity capacitor based on the metal layer-integrated polypropylene film according to the present application, and has initial withstand voltage and long-term durability at high temperature and high voltage.

[0157] The capacitor of the present application using the polypropylene film of the present application is suitable for use in a high temperature environment, and can be made into a small capacitor, and further, a high capacity capacitor (for example, an electrostatic capacity of 5 μF or more, preferably 10 μF or more, further preferably 20 μF or more, more further preferably 30 μF or more, and particularly preferably 40 μF or more. The upper limit of the electrostatic capacity is not particularly limited, and for example, 100 μF, 80 μF, 70 μF, or 60 μF). Thus, the capacitor of the present application can be used as a high voltage capacitor used in electronic devices, electric devices, and the like, a filter capacitor and a smoothing capacitor for various switching power supplies, converters, inverters, and the like. In addition, the capacitor of the present application can also be suitably used as an inverter capacitor, a converter capacitor, and the like for controlling a drive motor of an electric automobile, a hybrid automobile, and the like, which have been in high demand in recent years.

[0158] [Examples]

[0159] Hereinafter, the present application will be described in detail based on examples, but the present application is not limited to these examples.

[0160] (1) Measurement of properties of resin

[0161] (1-1) Measurement of weight average molecular weight (Mw), number average molecular weight (Mn), and molecular weight distribution (Mw / Mn) of polypropylene resin (1-2) Measurement of heptane-insoluble component (HI)

[0162] The weight average molecular weight (Mw), the number average molecular weight (Mn), and the molecular weight distribution (Mw / Mn) of the polypropylene resin used as a raw material in the examples and comparative examples were measured by GPC (gel permeation chromatography) under the following conditions.

[0163] Specifically, a differential refractometer (RI) built-in high temperature GPC device, HLC-8121 GPC-HT manufactured by Tosoh Corporation, was used. As columns, three TSKgel GMHHR-H (20) HT manufactured by Tosoh Corporation were connected. The measurement was performed at a column temperature of 140°C, while flowing trichlorobenzene as an eluent at a flow rate of 1.0 ml / min. A standard curve was prepared using a 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 the values of Mw and Mn.

[0164] (1-3) Measurement of melt flow rate (MFR)

[0165] The polypropylene resin used as a raw material in the examples and comparative examples was compression molded into 10 mm x 35 mm x 0.3 mm to prepare a measurement sample of about 3 g. Next, about 150 mL of heptane was added, and Soxhlet extraction was performed for 8 hours. The heptane-insoluble component was calculated from the mass of the sample before and after extraction.

[0166] (2) Preparation of casting drum

[0167] For the polypropylene resin used as a raw material in the examples and comparative examples, the melt flow rate (MFR) of the raw material resin pellets in the form of a pellet was measured using a melt indexer of Toyo Seiki Co., Ltd. in accordance with the conditions M of JIS K 7210. Specifically, first, a sample weighing 4 g was inserted into a cylinder having a test temperature of 230°C, and was preheated for 3.5 minutes under a load of 2.16 kg. Thereafter, the weight of the sample extruded from the bottom hole in 30 seconds was measured, and the MFR (g / 10 minutes) was calculated. The above measurement was repeated three times, and the average value thereof was taken as the measured value of the MFR.

[0168] (3) Production of biaxially stretched polypropylene film

[0169] As a metal drum (casting drum) used when manufacturing a sheet for biaxial stretching processing (casting sheet) in the process of manufacturing a biaxially stretched polypropylene film, casting drums of A to E were prepared.

[0170] At an arbitrary position on the surface of the casting drum, the number of grooves intersecting an imaginary line of 0.1 mm in length and the average value of the groove width were measured when the imaginary line was set at an arbitrary position in the width direction. Specifically, the measurement was performed as follows. At 4 positions in the circumferential direction of the casting drum (0 degrees, 90 degrees, 180 degrees, and 270 degrees on the clock) and 3 positions in the width direction (10 (one end), 50 (center), and 90 (one end) with respect to the face length 100), a total of 12 positions, an imaginary line of 0.1 mm in the width direction was drawn in the center of the screen, and the number of intersecting grooves and the groove width were measured, and the average value was calculated.

[0171] The number of grooves and the groove width are shown in Table 1.

[0172] [Table 1]

[0173]

[0174] (4) Measurement of properties of biaxially stretched polypropylene film

[0175] A biaxially stretched polypropylene film was manufactured in accordance with the manufacturing conditions of Table 2. The thickness of the biaxially stretched polypropylene film was measured using a micrometer (JIS-B7502) in accordance with JIS-C2330.

[0176] [Example 1]

[0177] Polypropylene resin A (Mw = 270,000, Mw / Mn = 5.7, heptane insoluble component = 97.8%, MFR = 5.6 g / 10 minutes, Prime Polymer) and polypropylene resin B (Mw = 380,000, Mw / Mn = 8.3, heptane insoluble component = 98.8%, MFR = 2.3 g / 10 minutes, Hanwha Oil & Chemical) were supplied to an extruder at a mass ratio of A:B = 65:35, and melted at a resin temperature of 230°C. Thereafter, after removing foreign matter, modified polymers, and the like with a filter provided in the middle of a polymer tube, extrusion was performed using a T-die, wound on a casting drum maintained at a surface temperature of 92°C, and allowed to solidify to produce a cast sheet having a thickness of 0.1 mm at a speed of 60 m / minute. Note that the casting drum used was A of Table 1.

[0178] In addition, as a method of closely adhering to the casting drum, an air knife was used, the air speed of the air blown out was set to 110 m / second, and the distance between the air outlet of the air knife and the cast sheet was set to 3 mm.

[0179] The obtained cast sheet was preheated at a temperature of 130°C, the cast sheet was clamped with a conveyer roller heated to 145°C and a stretching clamp roller of rubber hardness 70° heated to 110°C, stretched 5 times in the length direction, and immediately returned to room temperature.

[0180] Thereafter, the stretched film was introduced into a tenter, the both ends were held with clamps at 110°C, preheated at 170°C, stretched to 10 times in the width direction at a temperature of 155°C, and relaxation and heat setting were performed, and a biaxially stretched polypropylene film having a thickness of 2.3 μm was wound into a roll shape.

[0181] <Example 2>

[0182] In the production of the cast sheet, the distance between the air outlet of the air knife and the cast sheet was set to 5 mm, and otherwise, a biaxially stretched polypropylene film was obtained in the same manner as in Example 1.

[0183] <Example 3>

[0184] In the production of the cast sheet, the air speed of the air blown out of the air knife was set to 70 m / second, and otherwise, a biaxially stretched polypropylene film was obtained in the same manner as in Example 2.

[0185] <Example 4>

[0186] In the production of the cast sheet, the casting drum was made to be B of Table 1, and otherwise, a biaxially stretched polypropylene film was obtained in the same manner as in Example 1.

[0187] <Example 5>

[0188] In the production of the cast sheet, the casting drum was made to be C of Table 1, and otherwise, a biaxially stretched polypropylene film was obtained in the same manner as in Example 1.

[0189] <Example 6>

[0190] In the production of the cast sheet, the casting drum was made to be E of Table 1, the air-blowing air speed of the air knife was set to 130 m / sec, the distance of the air-blowing port of the air knife from the sheet was set to 2 mm, and in the lengthwise stretching treatment, the temperature of the stretching nip roll was set to 130°C, and otherwise, biaxially-stretched polypropylene film was obtained in the same manner as in Example 1.

[0191] <Example 7>

[0192] In the lengthwise stretching treatment, the rubber hardness of the stretching nip roll was set to 50°, and otherwise, biaxially-stretched polypropylene film was obtained in the same manner as in Example 6.

[0193] <Example 8>

[0194] In the production of the cast sheet, the casting drum was made to be D of Table 1, the air-blowing speed of the air knife was set to 70 m / sec, the distance of the air-blowing port of the air knife from the sheet was set to 5 mm, and otherwise, biaxially-stretched polypropylene film was obtained in the same manner as in Example 1.

[0195] <Example 9>

[0196] The thickness of the biaxially-stretched polypropylene film finally wound was set to 1.8 μm, and otherwise, biaxially-stretched polypropylene film was obtained in the same manner as in Example 1.

[0197] <Example 10>

[0198] The thickness of the biaxially-stretched polypropylene film finally wound was set to 6 μm, and otherwise, biaxially-stretched polypropylene film was obtained in the same manner as in Example 1.

[0199] <Example 11>

[0200] In the production of the cast sheet, the resin supplied to the extruder was made to be A, and otherwise, biaxially-stretched polypropylene film was obtained in the same manner as in Example 1.

[0201] <Comparative Example 1>

[0202] In the production of the cast sheet, the air-blowing air speed of the air knife was set to 140 m / sec, and otherwise, the operation was performed in the same manner as in Example 1. In this case, the air of the air knife entered between the resin extruded from the T die in a sheet shape and the casting drum, and the film vibration of the resin in a molten state from the T die outlet to the close contact with the casting drum became remarkable, and thus biaxially-stretched film could not be produced.

[0203] <Comparative Example 2>

[0204] The biaxially-stretched polypropylene film was obtained in the same manner as in Example 4, except that the air-blowing air wind speed of the air knife was set to 70 m / sec and the distance between the air-blowing outlet of the air knife and the sheet was set to 5 mm in the production of the cast sheet.

[0205] <Comparative Example 3>

[0206] The biaxially-stretched polypropylene film was obtained in the same manner as in Example 5, except that the air-blowing air wind speed of the air knife was set to 70 m / sec and the distance between the air-blowing outlet of the air knife and the sheet was set to 5 mm in the production of the cast sheet.

[0207] <Comparative Example 4>

[0208] The 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 in the lengthwise stretching treatment.

[0209] <Comparative Example 5>

[0210] The 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.

[0211] <Comparative Example 6>

[0212] The biaxially-stretched polypropylene film was obtained in the same manner as in Example 12, except that the temperature of the stretching nip roll was set to 90°C and the hardness of the stretching nip roll was set to 50° in the lengthwise stretching treatment.

[0213] [Table 2]

[0214]

[0215] (4-1) Measurement of spot

[0216] Figure 1

[0217] From the center position in the width direction of each of the biaxially-stretched polypropylene films of the examples and comparative examples, 50 mm x 50 mm measurement samples were cut out at a total of 10 places every 10 m in the length direction. Next, using a digital microscope (KEYENCE CORPORATION digital microscope VHX-2000), under lens magnification: 30 times, light quantity: 50% of the full range, measurement method: reflection measurement, field of view range: 11.5 mm x 8.6 mm, for one face (the face on which the metal layer was laminated when the capacitor was produced) of each measurement sample, the image of "halo removal + clear mode (processing of tone brightness, outline emphasis, and removal of glare)" was observed, and the "spots" (a representative spot is shown in FIG. 6) of approximately elliptical shape observed in the field of view range were measured. Figure 2The number of the spots in the field of view of each sample (total of 10) was added, and the total value was divided by 10 (= the number of the samples for measurement) to obtain the average number of the elliptical spots per field of view (a region of 11.5 mm x 8.6 mm).

[0218] In addition, for each of the spots measured above, the length in the longitudinal direction and the width in the transverse direction of the biaxially-stretched polypropylene film were measured, and the longer one (the major axis of the spot) was taken as the maximum length of the spot (the diameter in the longitudinal direction in the case of the representative example shown in FIG. 6). The maximum lengths of the respective spots were added, and the total value was divided by the number of the spots to obtain the average maximum length of the spots. (4-2) Measurement of protruding peak portion height Rpk and protruding valley portion depth Rvk

[0219] Figure 3 As the light-interference non-contact surface shape measuring machine, "VertScan 2.0 (Model: R5500GML)" manufactured by Ryoka Systems Inc. was used to measure the peak height Rpk and the valley depth Rvk of the biaxially-stretched polypropylene film of the Examples and the Comparative Examples. First, with the WAVE mode, a 530 white filter and a 1 x BODY lens barrel were applied, and a x 10 objective lens was used to perform measurement of 470.92 μm x 353.16 μm per field of view.

[0220] As for the inside of the spot, for all the spots measured in the above (4-1), measurement was performed at the five positions (the intersection of the major axis and the minor axis, the center of the line connecting one end of the major axis and the aforementioned intersection, the center of the line connecting the other end of the major axis and the aforementioned intersection, the center of the line connecting one end of the minor axis and the aforementioned intersection, and the center of the line connecting the other end of the minor axis and the aforementioned intersection) shown in FIG. 6.

[0221] (4-3) Measurement of number of insulation defects As for the outside of the spot, measurement was performed at the center of each of the samples for measurement.

[0222] The obtained data was subjected to noise removal processing based on a median filter (3 x 3), and then, Gaussian filter processing based on a cutoff value of 30 μm was performed to remove the waviness component. Thereby, the state in which the measurement surface state could be appropriately measured was obtained. Next, analysis was performed using the analysis software "VS-Viewer" of "VertScan 2.0", and the average value of each of the values obtained at the above positions was calculated with respect to the peak height Rpk and the valley depth Rvk of the lubricity evaluation parameter. The details are as follows.

[0223] The obtained data was subjected to noise removal processing based on a median filter (3 x 3), and then, Gaussian filter processing based on a cutoff value of 30 μm was performed to remove the waviness component. Thereby, the state in which the measurement surface state could be appropriately measured was obtained. Next, analysis was performed using the analysis software "VS-Viewer" of "VertScan 2.0", and the average value of each of the values obtained at the above positions was calculated with respect to the peak height Rpk and the valley depth Rvk of the lubricity evaluation parameter. The details are as follows.

[0224] ​The measured values of the protruding peak portion height Rpk in the spot are added together, and the resulting total value is divided by X (= the number of measured spots x 5 (the number of measurement sites in one spot)) to obtain the average protruding peak portion height Rpk in the spot.

[0225] The measured values of the protruding peak portion height Rpk outside the spot of each measurement sample are added together, and the resulting total value is divided by 10 (= the number of measurement samples) to obtain the average protruding peak portion height Rpk outside the spot.

[0226] The average protruding peak portion height Rpk outside the spot is subtracted from the average protruding peak portion height Rpk in the spot to obtain the difference between the two.

[0227] In addition, the average protruding peak portion height Rpk outside the spot is added to the average protruding peak portion height Rpk in the spot, and the resulting value is divided by 2, and the resulting value is taken as the average protruding peak portion height Rpk of the film. Note that in the case where the average number of spots is 0, the measurement average value of the center of each measurement sample is taken as the average protruding peak portion height Rpk of the film.

[0228] The measured values of the protruding valley portion depth Rvk in the spot are added together, and the resulting total value is divided by X (= the number of measured spots x 5 (the number of measurement sites in one spot)) to obtain the average protruding valley portion depth Rvk in the spot.

[0229] The measured values of the protruding valley portion depth Rvk outside the spot of each measurement sample are added together, and the resulting total value is divided by 10 (= the number of measurement samples) to obtain the average protruding valley portion depth Rvk outside the spot.

[0230] The average protruding valley portion depth Rvk outside the spot is subtracted from the average protruding valley portion depth Rvk in the spot to obtain the difference between the two.

[0231] In addition, the average protruding valley portion depth Rvk outside the spot is added to the average protruding valley portion depth Rvk in the spot, and the resulting value is divided by 2, and the resulting value is taken as the average protruding valley portion depth Rvk of the film. Note that in the case where the average number of spots is 0, the measurement average value of the center of each measurement sample is taken as the average protruding valley portion depth Rvk of the film.

[0232] (4-4) Measurement of slow axis angle and variation amplitude of slow axis angle

[0233] The number of insulation defects (pieces / 10 m of film) of the biaxially stretched polypropylene films of the examples and the comparative examples was measured using an insulation defect inspection device having an unwinding-winding mechanism. 2). The test subject polypropylene film was passed between a high voltage electrode and a grounded metal roller, and the film was caused to travel while a direct current voltage was applied. The number of discharges generated at the insulation defect portion was measured using a counter attached to the device. The measured number was divided by the measurement area, and the number of insulation defects of the film (number / 10 m 2 ). The measurement conditions are described below.

[0234] • The interval between the high voltage electrode and the grounded metal roller: 50 μm

[0235] • The wrap angle at which the film contacted the grounded metal roller: 120°

[0236] • The shape of the high voltage electrode: a metal plate having a thickness of 4 mm and the same width as the metal roller

[0237] • The unwinding speed: 20 m / min

[0238] • The direct current voltage: 600 V / μm

[0239] • The measurement area: 472 m 2

[0240] • The test environment temperature: 20°C.

[0241] (4-5) Measurement results

[0242] From the center of each of the biaxially-stretched polypropylene films of the examples and comparative examples, 50 mm x 50 mm measurement samples were cut out every 10 m in the length direction at a total of 10 points (at positions of 0 m, 10 m, 20 m, 30 m, 40 m, 50 m, 60 m, 70 m, 80 m, and 90 m). Next, the width direction of the aforementioned measurement sample was set to 0°, and the angle of the acute angle that the width direction of the aforementioned measurement sample made with the slow axis was measured as the slow axis angle. In the 10 measurement samples, the difference between the maximum value and the minimum value of the slow axis angle was taken as the "variation amplitude". The measurement device and measurement conditions are described below.

[0243] Measurement device: retardation measurement device RE-100 manufactured by Otsuka Electronics Co., Ltd.

[0244] Light source: laser light emitting diode (LED)

[0245] Band pass filter: 550 nm (measurement wavelength)

[0246] Measurement interval: 0.1 seconds

[0247] Cumulative number of times: 10 times

[0248] Number of measurement points: 15 points

[0249] Gain: 10 dB

[0250] Measurement environment: temperature 23°C, humidity 60%.

[0251] (5) Evaluation of process passability

[0252] The results of the measurement of the properties of the biaxially stretched polypropylene film are shown in Table 3.

[0253] [Table 3]

[0254]

[0255] (6) Evaluation of voltage resistance

[0256] The production of the biaxially stretched film of the starting examples and the comparative examples was started, and the time from the time when the thickness of the obtained film reached the target thickness (Table 2) ± 2% to the time when the film was stretched to break was measured (the time during which the film could be continuously produced). Note that the time when the thickness reached the target thickness ± 2% was measured and confirmed according to the thickness measurement method described above (3). 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.

[0257] A++: The film could be produced without being stretched to break even if more than 48 hours had passed.

[0258] A+: The film could be produced without being stretched to break within more than 32 hours and less than 48 hours.

[0259] A: The film could be produced without being stretched to break within more than 24 hours and less than 32 hours.

[0260] B: The film could be produced without being stretched to break within more than 16 hours and less than 24 hours.

[0261] C: Stretching to break occurred within more than 8 hours and less than 16 hours.

[0262] D: Stretching to break occurred within less than 8 hours.

[0263] E: The film could not be produced.

[0264] (6-1) Production of capacitor

[0265] A capacitor was produced using the biaxially stretched film of the examples and the comparative examples, and the voltage resistance of the capacitor was evaluated.

[0266] (6-2) Evaluation of initial voltage resistance

[0267] The T-edge evaporation pattern aluminum was evaporated to the measurement sample obtained in the above (4-1) using a vacuum evaporation machine manufactured by ULVAC, Inc. to evaporate resistance 20 Ω / D, thereby obtaining a metallized film in which a metal film was contained on one side of a biaxially stretched polypropylene film.

[0268] After slitting into 50 mm in width, two metallized films were laminated, and winding of 840 turns was performed using a take-up machine 3KAW-N2 manufactured by ZOJI MFG. CO., LTD. at a take-up tension of 210 g. After heat treatment of the element wound element at 120°C for 15 hours while being pressed, zinc metal was thermally sprayed to the end surface of the element, thereby obtaining a flat capacitor. A lead was welded to the end surface of the flat capacitor, and then sealed with an epoxy resin. Note that the static capacitance of the obtained capacitor was 50 μF.

[0269] (6-3) Evaluation of long-term voltage resistance

[0270] The initial static capacitance (CO) of the capacitor before the test was measured using an LCR HITESTER 3522-50 manufactured by SIKI ELECTRIC CO., LTD. Next, a direct current voltage of 450 V / μm was applied to the capacitor for 10 seconds. The static capacitance (C1) of the capacitor after the application of the voltage was similarly measured, and the change rate of the capacitance before and after the application of the voltage was calculated by the following equation.

[0271]

[0272] The aforementioned change rate ΔC was measured for 100 elements, and evaluation was performed in accordance with the following criteria. The number of each capacitor of A+ to C was calculated, and the proportion of the capacitor of A+ and A (initial voltage resistance yield) was calculated. The initial voltage resistance yield of 95% or more was recorded as a pass.

[0273] A+: ΔC is less than -0.2%.

[0274] A: ΔC is more than -1% and is -0.5% or less.

[0275] B: ΔC is more than -2% and is -1% or less.

[0276] C: ΔC is -2% or less.

[0277] (7) Evaluation results

[0278] The initial static capacitance (CO) of the capacitor before the test was measured using an LCR HITESTER 3522-50 manufactured by SIKI ELECTRIC CO., LTD. Next, a direct current voltage of 320 V / μm was continuously applied to the capacitor in a high-temperature tank at 115°C for 1000 hours. The static capacitance (C 1000), the capacity change rate (ΔC before and after voltage application (ΔC 1000 ).

[0279]

[0280] The capacity change rate ΔC before and after voltage application (ΔC 1000 The number of capacitors of A+ to C was calculated, and the proportion of capacitors of A+ and A (long-term voltage resistance yield) was calculated. The long-term voltage resistance yield of 90% or more was evaluated as pass.

[0281] A+: ΔC 1000 less than 0.5%.

[0282] A: ΔC 1000 more than -5% and -0.5% or less.

[0283] B: ΔC 1000 more than -10% and -5% or less.

[0284] C: ΔC 1000 -10% or less.

[0285] ​

[0286] The results of the process pass and voltage resistance evaluation are shown in Table 4.

[0287] [Table 4]

[0288]

Claims

1. A biaxially stretched polypropylene film having a first side and a second side, wherein, The average maximum length of the approximately elliptical spots on the first surface is less than 3.0 mm, and The difference between the average height of the protruding peak outside the spot and the average height of the protruding peak inside the spot is less than 0.040 μm.

2. The biaxially stretched polypropylene film according to claim 1, wherein, The difference between the average depth of the protruding valley outside the spot and the average depth of the protruding valley inside the spot is less than 0.020 μm.

3. The biaxially stretched polypropylene film according to claim 1, wherein, The variation range of the slow axis angle is between 0.3° and 2.8°.

4. The biaxially stretched polypropylene film according to claim 1, wherein, The average number of approximately elliptical spots in each 11.5mm × 8.6mm area of ​​the first surface is less than 0.

6.

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

6. The biaxially stretched polypropylene film according to claim 1, wherein the thickness is 1.7 μm or more and 6.5 μm or less.

7. The biaxially stretched polypropylene film according to claim 1, wherein it is a single-layer film.

8. The biaxially stretched polypropylene film according to any one of claims 1 to 7, used in a capacitor.

9. A metal-integrated polypropylene film comprising: a biaxially oriented polypropylene film according to any one of claims 1 to 7; and a metal layer disposed on one or both sides of the biaxially oriented polypropylene film.

10. A capacitor comprising the integral polypropylene film with a metal layer as described in claim 9.

11. A method for manufacturing a biaxially oriented polypropylene film according to any one of claims 1 to 7, comprising: Cast sheets are obtained using a casting drum with a microcracked surface. When an imaginary line of 0.1 mm in length is set at any position along the width direction on the surface of the casting drum, this imaginary line intersects with one to fifteen grooves, and the groove width is 1 μm to 10 μm. The cast sheet is subjected to biaxial stretching treatment.

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