Biaxially stretched polypropylene film, method for producing same, metallized film, and capacitor

Abstract

Provided are: a biaxially stretched polypropylene film which exhibits high volume resistivity even at high temperatures; a metallized film which uses the biaxially stretched polypropylene film; and a long-life capacitor which exhibits little variation in capacitance even at high temperatures and which has excellent heat resistance. Specifically, the present invention provides a biaxially stretched polypropylene film containing a straight-chain polypropylene resin A and a branched polypropylene resin B. The biaxially stretched polypropylene film is characterized in that: the slope of an approximate straight line determined by a viscoelastic scatter diagram is-0.222 or more; the viscoelastic scatter diagram is obtained by drawing four-point coordinates of (logE ", logE ') under each of the conditions of vibration frequencies of 1 Hz, 2 Hz, 5 Hz, and 10 Hz, with the logarithm (logE") of the loss modulus (E ") at a temperature of 70 DEG C as the x-axis and the logarithm (logE') of the storage modulus (E ') at a temperature of 70 DEG C under each of the conditions as the y-axis.

Description

Technical Field

[0001] This invention relates to biaxially stretched polypropylene films and their manufacturing methods, metallized films, and capacitors. Background Technology

[0002] Stretch films with polypropylene as the main component have moisture-proof properties, as well as rigidity and heat resistance, and are therefore used in various industrial applications, primarily for packaging.

[0003] Furthermore, stretched films with polypropylene as the main component are used in film capacitors due to their excellent electrical properties. Film capacitors made of stretched films with polypropylene as the main component are used in electronic and electrical equipment, such as as high-voltage capacitors; filter capacitors and smoothing capacitors in various switching power supplies, converters, and inverters. In the increasingly demanding electric and hybrid vehicles, film capacitors are used, for example, in inverters and converters that control drive motors.

[0004] The use of film capacitors, especially automotive film capacitors, in high-temperature environments is increasing. For example, in devices that control automotive drive motors (inverters, converters, etc.), the use of heat-resistant semiconductors (such as silicon carbide semiconductors) has been increasing in recent years. Consequently, capacitors used in these devices require high heat resistance, such as above 120°C, preferably above 130°C. The upper limit of the operating temperature for conventional polypropylene film capacitors is said to be around 110°C, and maintaining stable electrical insulation (volume resistivity) at temperatures exceeding this is extremely difficult.

[0005] Among the resin films with high heat resistance, a film with excellent heat resistance, especially excellent water vapor impermeability, is disclosed (Patent Document 1) consisting of a resin composition comprising a polyolefin with polypropylene as the main component and a hydrogenated block copolymer.

[0006] In addition, as one of the resin films with high heat resistance, a film of a resin having an alicyclic structure on the main chain of a polymer obtained by polymerizing a polypropylene resin and a cyclic olefin monomer has been disclosed (Patent Document 2).

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent Application Publication No. 2014-37532

[0010] Patent Document 2: International Publication No. 2022 / 270577 Summary of the Invention

[0011] The problem the invention aims to solve

[0012] However, the polypropylene film described in Patent Document 1 has the problem of insufficient heat resistance.

[0013] Because such capacitors are used in environments with rising engine compartment temperatures and self-heating, they require high heat resistance at around 120°C (e.g., 100°C to 125°C). In other words, the capacitors must not short-circuit even at high temperatures, maintaining electrical insulation (volume resistivity) and electrostatic capacitance.

[0014] Therefore, there is a need to develop biaxially stretched polypropylene films that exhibit high volume resistivity even at high temperatures, as well as metallized films using these biaxially stretched polypropylene films. Furthermore, there is a need to develop long-life capacitors that exhibit small changes in electrostatic capacitance even at high temperatures and have excellent heat resistance.

[0015] The purpose of this invention is to provide a biaxially stretched polypropylene film that exhibits high volume resistivity even at high temperatures, a metallized film using the biaxially stretched polypropylene film, and a long-life capacitor that exhibits small changes in electrostatic capacitance even at high temperatures and excellent heat resistance.

[0016] Solution for solving the problem

[0017] To solve the aforementioned problems, the inventors conducted repeated and in-depth research, resulting in the discovery of a biaxially stretched polypropylene film containing linear polypropylene resin A and branched polypropylene resin B. When the slope of the approximate straight line (linear function) determined by the viscoelastic scatter plot of the biaxially stretched polypropylene film exceeds a specific value, the aforementioned objective can be achieved, thus completing this invention. The viscoelastic scatter plot is obtained by plotting the four coordinates of (logE”) at 70°C under vibration frequencies of 1Hz, 2Hz, 5Hz, and 10Hz as the x-axis and the logarithm of the storage modulus (E’) at 70°C under the aforementioned conditions as the y-axis. This approximate straight line will also be referred to as the "70°C viscoelastic approximate straight line".

[0018] That is, the present invention relates to the following biaxially stretched polypropylene film and its manufacturing method, metallized film, and capacitor.

[0019] Item 1. A biaxially oriented polypropylene film, characterized in that it contains linear polypropylene resin A and branched polypropylene resin B.

[0020] The slope of the approximate straight line determined by the viscoelastic scatter plot of the aforementioned biaxially stretched polypropylene film is greater than -0.222. The viscoelastic scatter plot is obtained by plotting the coordinates of four points (logE”) of the loss modulus (E”) at 70°C under the aforementioned conditions with vibration frequencies of 1Hz, 2Hz, 5Hz and 10Hz as the x-axis and the logarithm of the storage modulus (E’) at 70°C under the aforementioned conditions as the y-axis.

[0021] Item 2. The biaxially stretched polypropylene film according to Item 1 above, wherein the aforementioned biaxially stretched polypropylene film has a roughened surface and an unroughened surface, and the surface roughness (Vmc) of the aforementioned roughened surface, measured using a white light interference microscope, is 0.025 ml / m. 2 above.

[0022] Item 3. The biaxially stretched polypropylene film according to item 1 or 2 above, wherein the aforementioned biaxially stretched polypropylene film has α-crystals, the size of which is 129 Å or more.

[0023] Item 4. A biaxially stretched polypropylene film according to any one of items 1 to 3 above, used in a capacitor.

[0024] Item 5. A metallized film having a metal layer on at least one side of the biaxially stretched polypropylene film described in any one of Items 1 to 4 above.

[0025] Item 6. A capacitor comprising the metallized thin film described in Item 5 above.

[0026] Item 7. A method for manufacturing a biaxially stretched polypropylene film, characterized in that it includes a step of biaxially stretching a cast sheet containing linear polypropylene resin A and branched polypropylene resin B before biaxial stretching.

[0027] The slope of the approximate straight line determined by the viscoelastic scatter plot of the aforementioned biaxially stretched polypropylene film is greater than -0.222. The viscoelastic scatter plot is obtained by plotting the coordinates of four points (logE”) of the loss modulus (E”) at 70°C under the aforementioned conditions with vibration frequencies of 1Hz, 2Hz, 5Hz and 10Hz as the x-axis and the logarithm of the storage modulus (E’) at 70°C under the aforementioned conditions as the y-axis.

[0028] Item 8. The manufacturing method according to Item 7 above, wherein, in the aforementioned biaxial stretching process, the longitudinal stretching ratio is set to 5.0 times or less.

[0029] The effects of the invention

[0030] The biaxially stretched polypropylene film of the present invention exhibits high volume resistivity even at high temperatures of around 120°C (e.g., 100°C to 125°C). Furthermore, the present invention can provide metallized films using this biaxially stretched polypropylene film, as well as long-life capacitors exhibiting small changes in electrostatic capacitance even at high temperatures and excellent heat resistance. Attached Figure Description

[0031] Figure 1 The diagram shows a viscoelastic scatter plot obtained by plotting the coordinates of four points (logE”, logE”) at 70°C under vibration frequencies of 1Hz, 2Hz, 5Hz, and 10Hz for the biaxially stretched polypropylene film prepared in Example 1, with the logarithm (logE”) of the storage modulus (E’) at 70°C under the aforementioned conditions set as the x-axis and the logarithm (logE’) of the storage modulus (E’) at 70°C under the aforementioned conditions set as the y-axis; and a graph showing the slope of the approximate straight line determined by the viscoelastic scatter plot as -0.197 (rounded to the fourth decimal place).

[0032] Figure 2 The graph shows the relationship between the increase in volume resistivity of each biaxially stretched polypropylene film prepared in Examples 1-15 and Comparative Examples 2-6 and the slope of the approximately linear viscoelasticity at 70°C, with the volume resistivity of the biaxially stretched polypropylene film prepared in Comparative Example 1 as a reference (100%). Detailed Implementation

[0033] In this specification, the "~" in numerical range refers to above and below. That is, the expression α~β means α above and β below, or β above and α below, and the range includes α and β. In addition, when multiple lower limit values ​​and multiple upper limit values ​​are respectively recorded, any lower limit value and upper limit value can be selected and connected by "~".

[0034] In this specification, the expressions “containing” and “including” include the concepts of “containing”, “comprising”, “substantially composed of” and “composed of only”.

[0035] In this specification, the term "capacitor" includes concepts such as "capacitor", "capacitor element" and "film capacitor".

[0036] In this specification, the orientation of the biaxially stretched polypropylene film is described as follows. First, the mechanical direction of the film is the Machine Direction (hereinafter also referred to as the "MD direction"). The MD direction is sometimes referred to as the length direction or the flow direction. Second, the transverse direction of the film is the Transverse Direction (hereinafter also referred to as the "TD direction"). The TD direction is sometimes referred to as the width direction.

[0037] 1. Biaxially stretched polypropylene film

[0038] The biaxially oriented polypropylene film of the present invention is characterized in that it is a biaxially oriented polypropylene film containing linear polypropylene resin A and branched polypropylene resin B.

[0039] The slope of the approximate straight line determined by the viscoelastic scatter plot of the aforementioned biaxially stretched polypropylene film is greater than -0.222. The viscoelastic scatter plot is obtained by plotting the coordinates of four points (logE”) of the loss modulus (E”) at 70°C under the aforementioned conditions with vibration frequencies of 1Hz, 2Hz, 5Hz and 10Hz as the x-axis and the logarithm of the storage modulus (E’) at 70°C under the aforementioned conditions as the y-axis.

[0040] The biaxially oriented polypropylene film of the present invention, possessing the above-described characteristics, exhibits high volume resistivity even at high temperatures of around 120°C (e.g., 100°C to 125°C). Hereinafter, this temperature range will be abbreviated as "at high temperatures." Furthermore, by using a biaxially oriented polypropylene film that exhibits high volume resistivity even at high temperatures, it is possible to obtain a long-life capacitor with minimal change in electrostatic capacitance even at high temperatures and excellent heat resistance.

[0041] The exact function of this invention is not yet clear, but it is speculated that the biaxially stretched polypropylene film of this invention, containing linear polypropylene resin A and branched polypropylene resin B, has an approximate straight line whose slope is -0.222 or higher. This is achieved by plotting a viscoelastic scatter plot of the four coordinates of (logE”) at 70°C under vibrational frequencies of 1Hz, 2Hz, 5Hz, and 10Hz, with the x-axis representing the logarithm of the storage modulus (E’) at 70°C under the aforementioned conditions and the y-axis representing the logarithm of the storage modulus (E’) at 70°C under the aforementioned conditions. This optimization of the relationship between the storage modulus (E’) and the loss modulus (E”), which are indicators of the thermal motion state of the polypropylene molecular chains constituting the biaxially stretched polypropylene film, particularly the temperature-frequency dependence of molecular motion, has influenced the function of this invention.

[0042] Branched polypropylene resin B acts as a nucleating agent, promoting the "entanglement effect (pseudo-crosslinking effect)" and "crystallization effect" of polypropylene molecular chains. Specifically, the presence of branched polypropylene resin B leads to the formation of numerous β-crystals in the cast sheet before biaxial stretching. Stretching the cast sheet containing these β-crystals transforms them into α-crystals, resulting in highly crystallizable biaxially stretched polypropylene films with larger α-crystal crystal sizes. This is presumably a prerequisite for easily optimizing the relationship between the storage modulus (E') and loss modulus (E").

[0043] Furthermore, a large number of β crystals are formed on the cast sheet before biaxial stretching. By stretching the cast sheet containing β crystals, the β crystals are transformed into α crystals. Therefore, due to the density difference between β and α crystals, (approximately) arc-shaped irregularities are formed in the polypropylene film obtained by stretching, which can appropriately roughen the surface. This results in an increase in the surface roughness (Vmc) of the roughened surface and an increase in the solid volume of the core of the roughened surface. From the above viewpoint, the biaxially stretched polypropylene film of the present invention is also suitable as a material for dielectric films for capacitors.

[0044] On the other hand, if the branched polypropylene resin B is over-blended, crystallization and physical cross-linking are promoted, while "crystallization" is hindered. Therefore, it is preferable to adjust the mass ratio of linear polypropylene resin A to branched polypropylene resin B to increase the percentage of β crystals (the ratio of β crystals to the total of α and β crystals) in the cast film. This results in a synergistic effect of the "entanglement effect" and "crystallization effect" of the polypropylene molecular chains, exhibiting the characteristic of a predetermined slope in the approximately linear viscoelasticity of biaxially stretched polypropylene film at 70°C, and obtaining high volume resistivity at high temperatures. It should be noted that, as detailed later, the branched polypropylene resin B used in this invention preferably has a narrow molecular weight distribution within a specific range. Furthermore, from the viewpoint of the manufacturing method, it is preferable to use a propylene polymer polymerized using a metallocene catalyst.

[0045] The biaxially stretched polypropylene film of the present invention will now be described in detail.

[0046] The biaxially stretched polypropylene film of the present invention contains linear polypropylene resin A and branched polypropylene resin B.

[0047] The slope of the approximate straight line determined by the viscoelastic scatter plot of the biaxially stretched polypropylene film is greater than -0.222. The viscoelastic scatter plot is obtained by plotting the four coordinates of (logE”) at 70°C under the conditions of vibration frequencies of 1Hz, 2Hz, 5Hz and 10Hz as the x-axis and the logarithm of the storage modulus (E’) at 70°C under the aforementioned conditions as the y-axis.

[0048] Regarding the thickness of the biaxially stretched polypropylene film of the present invention, from the viewpoint of further improving the miniaturization and high capacitance of the capacitor when used in capacitors, the upper limit is preferably 6.0 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. Furthermore, from a manufacturing viewpoint, the lower limit is preferably 0.8 μm or more, more preferably 1.0 μm or more, further preferably 1.8 μm or more, and particularly preferably 2.0 μm or more. The method for measuring the thickness of the biaxially stretched polypropylene film in this specification is based on the method described in the examples.

[0049] The density of biaxially stretched polypropylene film is not particularly limited, but from the viewpoint of easier use in capacitors, 919 kg / m³ is preferred. 3 Above and 930kg / m 3 the following.

[0050] The biaxially stretched polypropylene film of the present invention contains linear polypropylene resin A and branched polypropylene resin B. Here, in the relationship between the aforementioned linear polypropylene resin A and the aforementioned branched polypropylene resin B, the aforementioned linear polypropylene resin A, which has a larger content, is referred to as the "main component polypropylene resin" or "base resin." The aforementioned branched polypropylene resin B, which has a relatively smaller content, is referred to as the "blended resin."

[0051] It should be noted that linear polypropylene resin A can be a single resin or a mixture of two or more (especially two) resins. For example, linear polypropylene resin A can be one of the linear polypropylene resins A1 described later, or it can be a mixture of linear polypropylene resin A1 and linear polypropylene resin A2 described later. In this case, the linear polypropylene resin A1, which has a higher content, is called the "base resin," and the linear polypropylene resin A2, which has a relatively lower content, is called the "blended resin."

[0052] (Linear polypropylene resin A1)

[0053] In the biaxially stretched polypropylene film of the present invention, the linear polypropylene resin A can be composed of one type of linear polypropylene resin A1 described in this project. The linear polypropylene resin A1 can be a crystalline polypropylene such as isotactic polypropylene or syndiotactic polypropylene.

[0054] The weight-average molecular weight (Mw) of the linear polypropylene resin A1 is preferably 250,000 or more and 360,000 or less, more preferably 280,000 or more and 350,000 or less, further preferably 300,000 or more and 350,000 or less, and particularly preferably 300,000 or more and 350,000 or less. When the weight-average molecular weight (Mw) of the linear polypropylene resin A1 is within the above range, it is easier to control the thickness of the cast sheet before biaxial stretching in the manufacturing process of biaxially stretched polypropylene film, and it is less likely to produce uneven thickness.

[0055] It should be noted that the weight-average molecular weight Mw of linear polypropylene resin A1 is a characteristic of linear polypropylene resin A1 used as a raw material resin. Furthermore, the number-average molecular weight Mn, Z-average molecular weight Mz, molecular weight distribution of weight-average molecular weight Mw and number-average molecular weight Mn (Mw / Mn), molecular weight distribution of Z-average molecular weight Mz and number-average molecular weight Mn (Mz / Mn), melt flow rate at 230°C (MFRA1), melt tension at 230°C, meso-five-unit component ratio ([mmmm]), and heptane-insoluble component (HI) of linear polypropylene resin A1, which will be described later, are also characteristics of linear polypropylene resin A1 used as a raw material resin.

[0056] The number average molecular weight Mn of the linear polypropylene resin A1 is preferably 30,000 or more and 54,000 or less, more preferably 33,000 or more and 52,000 or less, and even more preferably 33,000 or more and 47,000 or less. When the number average molecular weight Mn of the linear polypropylene resin A1 is within the above range, the change in electrostatic capacitance of the manufactured capacitor at high temperature is smaller, and the heat resistance is further improved.

[0057] The Z-average molecular weight Mz of the linear polypropylene resin A1 is preferably 1 million or more and 2 million or less, more preferably 1.2 million or more and 1.8 million or less. When the Z-average molecular weight Mz of the linear polypropylene resin A1 is within the above range, the volume resistivity of the biaxially stretched polypropylene film at high temperature is further improved.

[0058] The linear polypropylene resin A1 has a molecular weight distribution (Mw / Mn) of 7.0 or higher and 9.3 or lower. The lower limit is preferably 7.2 or higher, more preferably 7.3 or higher, and even more preferably 8.1 or higher. The upper limit is preferably 9.0 or lower, and more preferably 8.2 or lower. By achieving the above-mentioned Mw / Mn range, the tensile strength of the polypropylene film is further improved, thereby enabling the manufacture of thinner biaxially oriented polypropylene films. Furthermore, the volume resistivity of the biaxially oriented polypropylene film at high temperatures is further improved.

[0059] The molecular weight distribution (Mz / Mn) of the linear polypropylene resin A1 is preferably 10 or more and 100 or less, more preferably 15 or more and 70 or less, and even more preferably 15 or more and 60 or less. When Mz / Mn is within the above range, the tensile properties of the polypropylene film are further improved, and thinner biaxially stretched polypropylene films can be manufactured.

[0060] The melt flow rate (MFRA1) of linear polypropylene resin A1 at 230°C is preferably 3.0 g / 10 min or more, more preferably 3.5 g / 10 min or more. Furthermore, the melt flow rate (MFRA1) of linear polypropylene resin A1 at 230°C is preferably 10.0 g / 10 min or less, more preferably 8.0 g / 10 min or less, further preferably 6.0 g / 10 min or less, and particularly preferably 5.0 g / 10 min or less. By achieving the above-mentioned melt flow rate (MFRA1) of linear polypropylene resin A1 at 230°C, the volume resistivity of the biaxially stretched polypropylene film at high temperatures is further improved. It should be noted that the method for measuring the melt flow rate (MFR) of the resin at 230°C in this specification is based on the method described in the examples.

[0061] The melt tension of the linear polypropylene resin A1 at 230°C is preferably 1.0 g or less, and more preferably less than 1.0 g. By maintaining the melt tension of the linear polypropylene resin A1 at 230°C within the above range, excellent flow characteristics in the molten state are achieved, thus reducing the likelihood of unstable flow such as melt fracture. Consequently, good film thickness uniformity is observed, resulting in the advantage of minimizing the formation of thin-walled portions prone to dielectric breakdown. It should be noted that the method for measuring the melt tension of the resin at 230°C in this specification is based on the method described in the examples.

[0062] The meso-pentameric component ratio ([mmmm]) of linear polypropylene resin A1 is preferably 99.8% or less, more preferably 99.5% or less, further preferably 99.0% or less, and most preferably 98.0% or less. Furthermore, the above-mentioned meso-pentameric component ratio is preferably 94.0% or more, more preferably 94.5% or more, and further preferably 95.0% or more. When the meso-pentameric component ratio is within the above range, the crystallinity of the polypropylene resin is improved due to moderately high stereoregularity, the volume resistivity of the biaxially stretched polypropylene film at high temperatures is further improved, and the curing (crystallization) speed during cast sheet molding becomes moderate, exhibiting moderate stretchability. It should be noted that the method for determining the meso-pentameric component ratio ([mmmm]) of the resin in this specification is based on the method described in the examples.

[0063] The heptane-insoluble component (HI) of the linear polypropylene resin A1 is preferably 97.0% or more, more preferably 97.5% or more, and even more preferably 98.0% or more. Furthermore, the heptane-insoluble component is preferably 99.5% or less, more preferably 99.0% or less. It should be noted that a higher heptane-insoluble component indicates higher stereoregularity of the resin. When the heptane-insoluble component is within the above range, the moderately high stereoregularity moderately increases the crystallinity of the polypropylene resin in the polypropylene film, further increasing the volume resistivity at high temperatures. Consequently, in the manufacturing process of the polypropylene film, the curing (crystallization) speed during the casting process before biaxial stretching becomes moderate, resulting in moderate stretchability. It should be noted that the method for determining the heptane-insoluble component (HI) of the resin in this specification is based on the method described in the examples.

[0064] Representative commercially available products of the aforementioned linear polypropylene resin A1 include, for example, HC300BF manufactured by Borealis and HPT-1 manufactured by Daehan Oil & Chemical Co.

[0065] (Linear polypropylene resin A2)

[0066] In the biaxially stretched polypropylene film of the present invention, as another method, the linear polypropylene resin A can be composed of a mixed resin with the aforementioned linear polypropylene resin A1 as the base resin and linear polypropylene resin A2 as the blending resin. The aforementioned linear polypropylene resin A2, like the aforementioned linear polypropylene resin A1, can be a crystalline polypropylene such as isotactic polypropylene or syndiotactic polypropylene.

[0067] In linear polypropylene resin A, the mixing ratio of the aforementioned linear polypropylene resin A1 (base resin) and the aforementioned linear polypropylene resin A2 (blended resin) is not limited as long as the content of the blended resin is low. If linear polypropylene resin A is set to 100% by mass, the content of the aforementioned linear polypropylene resin A2 is preferably less than 50% by mass, more preferably less than 49% by mass, and even more preferably less than 40% by mass. Furthermore, if linear polypropylene resin A is set to 100% by mass, the content of the aforementioned linear polypropylene resin A2 is preferably 10% by mass or more, more preferably 15% by mass or more, even more preferably 25% by mass or more, and particularly preferably 30% by mass or more.

[0068] The weight-average molecular weight (Mw) of the linear polypropylene resin A2 is preferably 300,000 or more, more preferably 350,000 or more, further preferably 360,000 or more, and particularly preferably more than 360,000. Furthermore, the weight-average molecular weight (Mw) of the linear polypropylene resin A2 is preferably 550,000 or less, more preferably 450,000 or less, and further preferably 420,000 or less. When the weight-average molecular weight (Mw) of the linear polypropylene resin A2 is within the above range, it is easier to control the thickness of the cast sheet before biaxial stretching in the manufacturing process of biaxially stretched polypropylene film, and it is less likely to produce thickness unevenness.

[0069] It should be noted that the weight-average molecular weight Mw of the linear polypropylene resin A2 mentioned above is a characteristic of the linear polypropylene resin A2 used as a raw material resin. Furthermore, the number-average molecular weight Mn, Z-average molecular weight Mz, molecular weight distribution (Mw / Mn), molecular weight distribution (Mz / Mn), melt flow rate at 230°C (MFRA2), melt tension at 230°C, MFRA1-MFRA2, meso-pentameric component ratio ([mmmm]), and heptane-insoluble component (HI) of the linear polypropylene resin A2 described later are also characteristics of the linear polypropylene resin A2 used as a raw material resin.

[0070] The number average molecular weight Mn of the linear polypropylene resin A2 is preferably 35,000 or more and 54,000 or less, more preferably 37,000 or more and 50,000 or less, and even more preferably 38,000 or more and 48,000 or less. When the number average molecular weight Mn of polypropylene resin A is within the above range, the change in electrostatic capacitance of the manufactured capacitor at high temperature is smaller, and the heat resistance is further improved.

[0071] The Z-average molecular weight Mz of the linear polypropylene resin A2 is preferably greater than 1.35 million and less than 2 million, more preferably greater than 1.4 million and less than 1.9 million. When the Z-average molecular weight Mz of the linear polypropylene resin A2 is within the above range, the volume resistivity of the biaxially stretched polypropylene film at high temperature is further improved.

[0072] The linear polypropylene resin A2 has a molecular weight distribution (Mw / Mn) of 7.0 or higher and 9.3 or lower. The lower limit is preferably 8.1 or higher, more preferably 8.2 or higher, and even more preferably 8.3 or higher. The upper limit is preferably 9.2 or lower, and more preferably 9.1 or lower. By achieving the above-mentioned Mw / Mn range, the tensile strength of the polypropylene film is further improved, thereby enabling the manufacture of thinner biaxially oriented polypropylene films. Furthermore, the volume resistivity of the biaxially oriented polypropylene film at high temperatures is further improved.

[0073] The molecular weight distribution (Mz / Mn) of the linear polypropylene resin A2 is preferably 30 or more and 40 or less, more preferably 33 or more and 36 or less. When Mz / Mn is within the above range, the tensile properties of the polypropylene film are further improved, and thinner biaxially stretched polypropylene films can be manufactured.

[0074] The melt flow rate (MFRA2) of the linear polypropylene resin A2 at 230°C is preferably 4.0 g / 10 min or less, more preferably 3.5 g / 10 min or less, even more preferably 3.2 g / 10 min or less, and particularly preferably less than 3.0 g / 10 min. Furthermore, the melt flow rate (MFRA2) of the linear polypropylene resin A2 at 230°C is preferably 0.1 g / 10 min or more, more preferably 0.5 g / 10 min or more, and even more preferably 1.5 g / 10 min or more.

[0075] It should be noted that the difference between the MFR (MFRA1) of the linear polypropylene resin A1 as the base resin and the MFR (MFRA2) of the linear polypropylene resin A2 as the blending resin, MFRA1-MFRA2, is preferably 0.8 g / 10 min or more. That is, it is preferable that MFRA1 is greater than MFRA2. The aforementioned MFRA1-MFRA2 is preferably 1.0 g / 10 min or more, more preferably 1.5 g / 10 min or more, and even more preferably 1.7 g / 10 min or more. When the aforementioned MFRA1-MFRA2 is less than 0.5 g / 10 min (this less than 0.5 g / 10 min also includes negative values), in the polypropylene film manufacturing process, a sea-island phase separation structure is not formed at the casting stage before biaxial stretching, or even if a sea-island phase separation structure is formed, the size of the islands is very small. Therefore, it may be difficult to obtain a polypropylene film with excellent volume resistivity at high temperatures. In particular, even if the difference between MFRA1 and MFRA2 is large, the size of the islands in the sea-island phase separation structure becomes very small when MFRA2 is large (when MFRA1-MFRA2 is negative).

[0076] The melt tension of the linear polypropylene resin A2 at 230°C is preferably 1.0 g or less, and more preferably less than 1.0 g. By maintaining the melt tension of the linear polypropylene resin A2 within the above range at 230°C, excellent flow characteristics in the molten state are achieved, thus reducing the likelihood of unstable flow such as melt fracture. Therefore, due to good film thickness uniformity, it has the advantage of being less prone to forming thin-walled portions that are susceptible to dielectric breakdown. It should be noted that the method for measuring the melt tension of the resin at 230°C in this specification is based on the method described in the examples.

[0077] The meso-pentameric component percentage ([mmmm]) of linear polypropylene resin A2 is preferably 99.8% or less, more preferably 99.5% or less, and even more preferably 99.0% or less. Furthermore, the aforementioned meso-pentameric component percentage is preferably 94.0% or more, more preferably 94.5% or more, even more preferably 95.0% or more, and most preferably 98.0% or more. When the meso-pentameric component percentage is within the above range, the crystallinity of the polypropylene resin is improved due to the moderately high stereoregularity, the volume resistivity of the biaxially stretched polypropylene film at high temperatures is further improved, and the curing (crystallization) speed during cast sheet forming becomes moderate, exhibiting moderate stretchability. It should be noted that when the linear polypropylene resin A contains both linear polypropylene resin A1 and linear polypropylene resin A2 (preferably a mixture of linear polypropylene resin A1 and linear polypropylene resin A2), the meso-five-unit component ratio of linear polypropylene resin A1 can be set to 98.0% or less, and the meso-five-unit component ratio of linear polypropylene resin A2 can be set to 98.0% or more.

[0078] The heptane-insoluble component (HI) of linear polypropylene resin A2 is preferably 97.5% or more, more preferably 98.0% or more, further preferably 98.5% or more, and particularly preferably 98.6% or more. Furthermore, the heptane-insoluble component is preferably 99.5% or less, more preferably 99.0% or less. It should be noted that the heptane-insoluble component (HI) of both linear polypropylene resin A1 and linear polypropylene resin A2 may also be set to 98.5% or more.

[0079] As a representative commercially available product of the aforementioned linear polypropylene resin A2, one example is S802M manufactured by Daehan Oil & Chemical Co., Ltd.

[0080] (Branched polypropylene resin B)

[0081] In addition to the aforementioned linear polypropylene resin A, the biaxially stretched polypropylene film of the present invention also contains branched polypropylene resin B.

[0082] Of the aforementioned branched polypropylene resin B, branched polypropylene resin B obtained by polymerizing propylene using a metallocene catalyst is preferred. By making the biaxially stretched polypropylene film contain the aforementioned branched polypropylene resin B in addition to the aforementioned linear polypropylene resin A, a large number of β crystals are formed in the cast sheet before biaxial stretching. By stretching the cast sheet containing β crystals, the β crystals are transformed into α crystals. Therefore, due to the density difference between β crystals and α crystals, (approximately) arc-shaped irregularities are formed in the polypropylene film obtained by stretching, which can appropriately roughen the surface. This results in an increase in the surface roughness (Vmc) of the roughened surface and an increase in the solid volume of the core of the roughened surface. The biaxially stretched polypropylene film of the present invention preferably has a roughened surface and a non-roughened surface, and the surface roughness (Vmc) of the aforementioned roughened surface, measured using a white light interference microscope, is 0.025 ml / m. 2 The above will be discussed in detail later. From this perspective, the biaxially stretched polypropylene film of the present invention is also suitable as a material for dielectric films for capacitors.

[0083] When using branched polypropylene resin B obtained through crosslinking modification with peroxides instead of branched polypropylene resin B obtained through polymerization with a metallocene catalyst, the α-crystal nucleation effect of the branched polypropylene resin B obtained through crosslinking modification with peroxides promotes the formation of α-crystals and inhibits the formation of β-crystals in the cast film before biaxial stretching. Even when stretching the cast film containing α-crystals, it is difficult for microcrystal transformation to occur, and it is difficult to form unevenness. Therefore, from the viewpoint of roughening the biaxially stretched polypropylene film, branched polypropylene resin B polymerized with a metallocene catalyst can also be suitably used.

[0084] Regarding this, when using branched polypropylene resin B polymerized with a metallocene catalyst, the percentage of β-crystals in the cast sheet before biaxial stretching is preferably 10% or more, more preferably 13% or more, further preferably 15% or more, even more preferably 17% or more, and most preferably 19% or more. It should be noted that the method for determining the percentage of β-crystals in this specification is based on the method described in the examples.

[0085] Furthermore, metallocene catalysts are typically metallocene compounds that form catalysts for polymerization that generate olefin macromonomers. The branched polypropylene resin B obtained by polymerizing propylene using a metallocene catalyst exhibits moderate branch length and spacing, further improved compatibility with linear polypropylene, and readily yields a more uniform composition and surface shape, thus making it preferred.

[0086] Representative commercially available products of the aforementioned branched polypropylene resin B include, for example, WAYMAX-EX6000 manufactured by Japan Polypropylene Corporation, WAYMAX-MFX8 manufactured by Japan Polypropylene Corporation, WAYMAX-MFX6 manufactured by Japan Polypropylene Corporation, WAYMAX-MFX3 manufactured by Japan Polypropylene Corporation, and WAYMAX-EX4000 manufactured by Japan Polypropylene Corporation.

[0087] The melt tension of the aforementioned branched polypropylene resin B at 230°C is preferably 3g or more and 25g or less, more preferably 5g or more and 20g or less, even more preferably 9g or more and 20g or less, and particularly preferably 9g or more and 17g or less. By setting the melt tension to the above range, the volume resistivity of the biaxially stretched polypropylene film at high temperatures is further improved.

[0088] The melt flow rate of branched polypropylene resin B at 230°C is preferably 0.1–12.0 g / 10 min, more preferably 1.0–6.0 g / 10 min, even more preferably 1.5–4.0 g / 10 min, and even more preferably 2.0–3.5 g / 10 min. Therefore, the melt flow rate of branched polypropylene resin B at 230°C can also be set to 2.0 g / 10 min or more. By setting the melt flow rate at 230°C to the above range, the flow characteristics in the molten state are excellent, thus reducing the likelihood of unstable flow such as melt fracture, and further suppressing breakage during stretching. Therefore, the film thickness uniformity is better, thus suppressing the formation of thin-walled portions prone to dielectric breakdown, and further improving the volume resistivity of the biaxially stretched polypropylene film at high temperatures.

[0089] The weight-average molecular weight (Mw) of the branched polypropylene resin B is preferably 150,000 or more and 600,000 or less, more preferably 200,000 or more and 500,000 or less, even more preferably 250,000 or more and 500,000 or less, and particularly preferably 350,000 or more and 480,000 or less. When the weight-average molecular weight (Mw) of the branched polypropylene resin B is within the above range, the resin flowability becomes more suitable, the thickness of the cast sheet becomes easier to control, and the production of thin stretched films becomes easier. In addition, uneven thickness of the cast sheet and the stretched film is less likely to occur, and more suitable stretchability can be obtained.

[0090] The number average molecular weight Mn of the branched polypropylene resin B is preferably 100,000 or more and 300,000 or less, more preferably 100,000 or more and 250,000 or less, and even more preferably 100,000 or more and 200,000 or less. When the number average molecular weight Mn of the branched polypropylene resin B is within the above range, the change in electrostatic capacitance of the manufactured capacitor at high temperature is smaller, and the heat resistance is further improved.

[0091] The molecular weight distribution (Mw / Mn) of the branched polypropylene resin B is preferably 1.5 or more and 4.5 or less, more preferably 1.8 or more and 4.5 or less, and even more preferably 2.0 or more and 4.2 or less. By setting the Mw / Mn to the above range, the tensile strength of the polypropylene film is further improved, and thinner biaxially stretched polypropylene films can be manufactured.

[0092] The Z-average molecular weight Mz of the branched polypropylene resin B is preferably 600,000 or more and 2,000,000 or less, more preferably 800,000 or more and 1,700,000 or less. When the Z-average molecular weight Mz of the branched polypropylene resin B is within the above range, the volume resistivity of the biaxially stretched polypropylene film at high temperatures is further improved.

[0093] The molecular weight distribution (Mz / Mn) of the branched polypropylene resin B is preferably 4 or more and 30 or less, more preferably 5 or more and 20 or less. When Mz / Mn is within the above range, the tensile strength of the polypropylene film is further improved, and thinner biaxially stretched polypropylene films can be manufactured.

[0094] The molecular weight and molecular weight distribution of the aforementioned branched polypropylene resin B can be controlled by adjusting the catalyst and polymerization conditions.

[0095] Regarding the content of branched polypropylene resin B, the preferred mass ratio of the aforementioned linear polypropylene resin A to the aforementioned branched polypropylene resin B is 99.0:1.0 to 85.0:15.0, more preferably 98.0:2.0 to 92.0:8.0, and even more preferably 97.0:3.0 to 93.0:7.0. In such a mass ratio, for example, the total mass of linear polypropylene resin A and branched polypropylene resin B is set to 100% by mass. When the linear polypropylene resin A is a mixture of the aforementioned resin A1 and resin A2, it is preferable to set the content of the aforementioned linear polypropylene resin A1 to 55% by mass or more and 65% by mass or less, the content of the aforementioned linear polypropylene resin A2 to 30% by mass or more and 40% by mass or less, and the content of the aforementioned branched polypropylene resin B to 2% by mass or more and 10% by mass or less. By setting such a mixing ratio, a high volume resistivity can be easily obtained even at high temperatures compared to existing products.

[0096] (Other resins)

[0097] The biaxially oriented polypropylene film of the present invention may also contain other resins (hereinafter referred to as "other resins") besides the aforementioned linear polypropylene resin A and branched polypropylene resin B. "Other resins" refers to resins other than the aforementioned linear polypropylene resin A and branched polypropylene resin B, and there are no particular limitations as long as the target biaxially oriented polypropylene film can be obtained. Examples of such other resins include, for instance, polyolefins other than polypropylene such as polyethylene, poly(1-butene), polyisobutylene, poly(1-pentene), and poly(1-methylpentene); copolymers of α-olefins such as ethylene-propylene copolymers, propylene-butene copolymers, and ethylene-butene copolymers; vinyl monomer-diene monomer random copolymers such as styrene-butadiene random copolymers; and vinyl monomer-diene monomer-vinyl monomer random copolymers such as styrene-butadiene-styrene block copolymers. It should be noted that, when other resins are included, the total amount of the aforementioned linear polypropylene resin A and the aforementioned branched polypropylene resin B is preferably 95% by mass or more, and more preferably 98% by mass or more, in 100% by mass of the total resin composition. That is, in 100% by mass of the total resin composition, the content of other resins is preferably 5% by mass or less, more preferably 2% by mass or less.

[0098] (additive)

[0099] The biaxially oriented polypropylene film of the present invention may further contain additives. Examples of additives include antioxidants, chlorine absorbers, ultraviolet absorbers, lubricants, plasticizers, flame retardants, antistatic agents, and colorants.

[0100] (Properties of biaxially stretched polypropylene film)

[0101] The slope of the approximate straight line determined by the viscoelastic scatter plot of the biaxially stretched polypropylene film of the present invention is greater than -0.222. The viscoelastic scatter plot is obtained by plotting the coordinates of four points (logE”) of the loss modulus (E”) at 70°C under the conditions of vibration frequencies of 1Hz, 2Hz, 5Hz and 10Hz as the x-axis, and the logarithm (logE’) of the storage modulus (E’) at 70°C under the aforementioned conditions as the y-axis.

[0102] Figure 1 To obtain a viscoelastic scatter plot by plotting the four coordinates of (logE”) at 70°C under the vibration frequencies of 1Hz, 2Hz, 5Hz, and 10Hz of the biaxially stretched polypropylene film prepared in Example 1, with the logarithm (logE”) of the storage modulus (E’) at 70°C under the aforementioned conditions set as the x-axis and the logarithm (logE’) of the storage modulus (E’) at 70°C under the aforementioned conditions set as the y-axis; and to illustrate, a graph showing the slope of the approximate straight line determined by the viscoelastic scatter plot as -0.197 (rounded to the fourth decimal place). It should be noted that the storage modulus (E’) and loss modulus (E”) at 70°C under the aforementioned conditions can be calculated using a dynamic viscoelasticity measuring apparatus according to JIS-K7244 (1999 edition), specifically based on the method described in the examples. Alternatively, as a method to determine the slope of an approximate straight line (linear function) from the coordinates of the four points under the aforementioned conditions, the slope of the approximate straight line (linear function) can be determined by using the statistical function "SLOPE" in calculation software (Microsoft's "Excel").

[0103] In this invention, the slope of the approximate straight line is preferably -0.222 or higher and -0.180 or lower, more preferably -0.210 or higher and -0.190 or lower, and even more preferably -0.205 or higher and -0.195 or lower. By setting the slope of the approximate straight line in this way, a high volume resistivity can be easily obtained even at high temperatures compared to existing products. When the slope of the approximate straight line is greater than -0.180 (when the elastic modulus of the film increases), the desired volume resistivity is easily obtained, but the risk of film breakage during the film manufacturing process increases.

[0104] The biaxially stretched polypropylene film of the present invention preferably has α-crystals, the size of which is 129 Å or more. Such a relatively large α-crystal size can be obtained by increasing the percentage of β-crystals in the cast sheet before biaxial stretching. The size of the α-crystals is preferably 130 Å or more, more preferably 131 Å or more, and is 132 Å or more, 133 Å or more, 134 Å or more, 135 Å or more, 136 Å or more, etc. The actual upper limit of the size of the α-crystals is about 140 Å. The size of the α-crystals is calculated using the Scherrer formula from the half-width of the reflection peak of the α-crystal (040) plane measured by wide-angle X-ray diffraction, specifically based on the method described in the embodiments.

[0105] The biaxially stretched polypropylene film of the present invention preferably has a roughened surface and a non-roughened surface, and the surface roughness (Vmc) of the roughened surface, measured using a white light interference microscope, is 0.025 ml / m. 2 The above. Preferably, it is 0.030 ml / m 2 The above, more preferably 0.035 ml / m 2 The above is 0.040 ml / m 2 Above, 0.045ml / m 2 The above, etc. The actual upper limit of the surface roughness (Vmc) of the roughened surface is 0.100 ml / m. 2 Approximately. The surface roughness (Vmc) of such a roughened surface is 0.025 ml / m. 2 This means that the core of the roughened surface has a large solid volume. The aforementioned surface roughness (Vmc) is a value measured using a white light interferometer microscope (Mitaka Systems Co., Ltd. "VeatScan2.0"), specifically based on the method described in the examples.

[0106] The biaxially stretched polypropylene film of the present invention exhibits high volume resistivity even at high temperatures. Specifically, according to JIS C 2139-3-1:2018, the volume resistivity (ρV) measured at 120°C and 143 V / μm is preferably 7.50 × 10⁻⁶. 14 Ω·cm or higher, more preferably 8.00×10 14 Ω·cm or higher. Among them, is 8.50 × 10⁻⁶. 14 Ω·cm or more, 9.00×10 14 Ω·cm or more, 9.50×10 14 Ω·cm or more, 1.00×10 15 Ω·cm or more, 1.50×10 15 Ω·cm and above. The actual upper limit of volume resistivity (ρV) is 9.0 × 10⁻⁶. 15Approximately Ω·cm. The volume resistivity (ρV) is a value measured according to JIS C2139-3-1:2018, specifically based on the method described in the examples.

[0107] 2. Method for manufacturing biaxially stretched polypropylene film

[0108] The method for manufacturing the biaxially stretched polypropylene film of the present invention is not particularly limited. For example, it can be defined as: "A method for manufacturing a biaxially stretched polypropylene film, characterized in that it includes a step of biaxially stretching a cast sheet containing linear polypropylene resin A and branched polypropylene resin B before biaxial stretching."

[0109] The slope of the approximate straight line determined by the viscoelastic scatter plot of the aforementioned biaxially stretched polypropylene film is greater than -0.222. The viscoelastic scatter plot is obtained by plotting the four coordinates of (logE”) at 70°C under the aforementioned conditions, with the logarithm (logE”) of the loss modulus (E”) at 70°C under vibration frequencies of 1Hz, 2Hz, 5Hz, and 10Hz as the x-axis and the logarithm (logE’) of the storage modulus (E’) at 70°C under the aforementioned conditions as the y-axis.

[0110] From the perspective of manufacturing process details, it can be manufactured by the following method, which preferably involves a temperature of 225°C or higher and 270°C or lower and a shear rate of 2000 s. -1 Above and 15000s -1 The following describes the process of melting a polypropylene resin composition containing at least linear polypropylene resin A and branched polypropylene resin B. The manufacturing method described above will be explained exemplarily below.

[0111] According to the above manufacturing method, it is possible to provide a biaxially stretched polypropylene film that exhibits high volume resistivity even at high temperatures. The reason for being able to provide a biaxially stretched polypropylene film that exhibits high volume resistivity even at high temperatures is believed to be due to the sea-island phase separation structure (especially the appropriate island size) of the cast sheet produced by using two specific and different polypropylene resins.

[0112] There are no particular limitations on the method of mixing the resin used in the above manufacturing method. Examples include: a method of dry mixing the polymer powder or granules of the base resin and the blended resin using a mixer or the like; and a method of supplying the polymer powder or granules of the base resin and the blended resin to a mixer for melt mixing to obtain a compound.

[0113] The mixers and compounders mentioned above are not particularly limited. The compounders mentioned above can be any type of single-screw, twin-screw, or multi-screw (three or more screws). In the case of twin-screw or multi-screw types, they can be any type of co-rotating or anti-rotating.

[0114] When using melt mixing, there are no particular limitations on the mixing temperature as long as a good mixture can be obtained. The temperature is typically between 200°C and 300°C, and from the viewpoint of suppressing resin deterioration, it is preferably between 230°C and 270°C. Furthermore, to suppress resin deterioration during mixing, inactive gases such as nitrogen can be purged from the mixer. The melt-mixed resin can then be granulated to an appropriate size using a commonly known granulator. This allows the preparation of mixed polypropylene raw material resin granules.

[0115] The aforementioned polypropylene resin composition may also contain additives. As additives, the same additives described in the biaxially oriented polypropylene film of the present invention may be used. The aforementioned polypropylene resin composition may contain the above-mentioned additives in an amount that does not adversely affect the biaxially oriented polypropylene film.

[0116] In the above-mentioned method for manufacturing biaxially stretched polypropylene film, polypropylene resin granules, dry-mixed polypropylene resin granules, or pre-melted polypropylene resin granules are first fed into an extruder for heating and melting.

[0117] The aforementioned polypropylene resin composition is preferably melted at a temperature of 170°C or higher and 320°C or lower. Specifically, the extruder temperature for heating and melting the polypropylene resin composition is set to 225°C or higher and 270°C or lower. This results in a sea-island phase separation structure during the casting process, as described later, enabling the production of biaxially stretched polypropylene films that exhibit high volume resistivity even at high temperatures.

[0118] The polypropylene resin composition is preferably prepared at a temperature of 225°C or higher and 270°C with a shear rate of 2000 s. -1 Above and 15000s -1 The following melting process is performed. This results in the formation of the sea-island phase separation structure during the casting process, as described later, enabling the production of polypropylene films exhibiting high volume resistivity even at high temperatures. The shear rate is below 2000 s⁻¹. -1 At this time, the extrusion rate is not constant, raising concerns about irregular or regular changes in the shape and size of the billet, and the potential for breakage during billet transport and stretching. Additionally, shear rates exceeding 15000 s⁻¹... -1There are concerns that unmelted material may be extruded within the extruder due to a phenomenon known as breakup, resulting in an inability to obtain uniform preform sheets and a higher risk of breakage during stretching. Alternatively, there are concerns that excessive heat generation during tooth clearance can significantly degrade the polypropylene resin composition, leading to a decrease in the volume resistivity of the film obtained through stretching, even if a uniform preform sheet is obtained. The shear rate can be adjusted by the extruder barrel diameter, screw speed, and screw flute depth.

[0119] The preferred shear rate is 2000 s. -1 Above and 10000s -1 The following is more preferably 2000s -1 Above and 2300s -1 Below, by maintaining a shear rate within the above range, biaxially oriented polypropylene films with a weight fraction w of 2.6% or more and 4.0% or less can be easily obtained, further improving the heat resistance of film capacitors using biaxially oriented polypropylene films as capacitor dielectrics.

[0120] Next, the molten polypropylene resin composition is extruded into a sheet using a T-die, and then cooled and cured using at least one metal drum, thereby forming an unstretched cast sheet. The surface temperature of the metal drum (the temperature of the metal drum initially in contact with the material after extrusion) is preferably 10°C or higher and 105°C or lower, more preferably 15°C or higher and 100°C or lower. The surface temperature of the metal drum can be determined based on the properties of the polypropylene resin used. When the surface temperature of the metal drum is below 10°C, it is difficult to obtain good sheet-forming properties of the preform sheet, and therefore uneven stretching and breakage may easily occur during stretching and film formation.

[0121] The thickness of the cast film is not particularly limited, but is preferably 0.05 mm or more and 2 mm or less, and more preferably 0.1 mm or more and 1 mm or less.

[0122] Biaxially stretched polypropylene film can be manufactured by biaxially stretching the aforementioned cast sheet. The stretching is preferably biaxially oriented in both the longitudinal and transverse directions, and a sequential biaxial stretching method is preferred. As a sequential biaxial stretching method, for example, firstly, the cast sheet is held at a temperature of 100°C or higher and 180°C or lower (preferably 120°C or higher and 170°C or lower) and stretched along the flow direction by passing it through rollers with a speed difference. The stretching ratio in the flow direction is preferably 3.0 times or higher and 5.0 times or lower, more preferably 4.0 times or higher and 4.9 times or lower, and even more preferably 4.6 times or higher and 4.8 times or lower. By setting the stretching ratio in the flow direction to 5.0 times or lower, even when branched polypropylene resin B is present, uneven stretching is suppressed when stretching in the width direction is performed after stretching in the flow direction, and film breakage is easily suppressed. Next, the sheet is fed into a tenter frame and stretched in the transverse direction. The temperature during transverse stretching is preferably above 160°C and below 180°C, and the transverse stretching ratio is preferably above 3.0 times and below 11.0 times. Following transverse stretching, relaxation and heat setting are performed, followed by winding.

[0123] The manufacturing method described above can be used to manufacture biaxially stretched polypropylene films.

[0124] For the aforementioned biaxially stretched polypropylene film, from the viewpoint of further improving adhesion properties in subsequent processes such as metal vapor deposition, corona discharge treatment can be performed online or offline after the stretching and heat setting processes. Corona discharge treatment can be performed using known methods. Air, carbon dioxide, nitrogen, or mixtures thereof are preferably used as the atmosphere gas.

[0125] The biaxially oriented polypropylene film of the present invention, manufactured by the above-described method, exhibits high volume resistivity even at high temperatures of around 120°C (e.g., 100°C to 125°C). Furthermore, this provides metallized films using this biaxially oriented polypropylene film, as well as capacitors with small changes in electrostatic capacitance even at high temperatures and excellent heat resistance and long lifespan. Therefore, the biaxially oriented polypropylene film of the present invention is suitable for use as a capacitor, and preferably suitable as a dielectric material for capacitors constituting inverters in hybrid electric vehicles / electric vehicles.

[0126] 3. Metallized thin films, capacitors, and their manufacturing methods

[0127] The metallized film of the present invention is a metallized film having a metal layer on at least one side of the above-mentioned biaxially stretched polypropylene film.

[0128] The metal layer functions as an electrode. The metals used in the metal layer can be, for example, elemental metals such as zinc, lead, silver, chromium, aluminum, copper, and nickel, as well as mixtures or alloys thereof. Among these, zinc and aluminum are preferred from the viewpoints of environmental impact, economic efficiency, and excellent capacitor performance.

[0129] There are no particular limitations on the method for laminating a metal layer on at least one side (one side or both sides) of a biaxially stretched polypropylene film; examples include vacuum evaporation and sputtering. From the viewpoint of superior productivity and economy, vacuum evaporation is preferred. Examples of vacuum evaporation methods include crucible methods and line methods; the optimal method can be selected appropriately.

[0130] There are no particular limitations on the margin pattern when stacking metal layers by vapor deposition. From the viewpoint of further improving the safety of capacitors and further suppressing capacitor damage and short circuits, it is preferable to implement patterns such as fishing net patterns and T-edge patterns containing so-called special edges on one side of the biaxially stretched polypropylene film.

[0131] There are no particular limitations on the method for forming the edge; it can be formed using known methods such as tape or oil.

[0132] The thickness of the metallized thin film of the present invention is not particularly limited, but is preferably 1.8 μm or more and 3.0 μm or less, and more preferably 2.0 μm or more and 2.8 μm or less.

[0133] The capacitor of the present invention is a capacitor comprising the aforementioned metallized thin film. The metallized thin film of this disclosure can be laminated or wound into a thin-film capacitor using existing known methods.

[0134] The aforementioned film capacitor can have a structure with multiple stacked metallized films, or it can have a wound metallized film. Such film capacitors are suitable for use as capacitors in inverter power supply equipment for controlling drive motors in electric vehicles, hybrid vehicles, etc. They are also suitable for applications in railway vehicles, wind power generation, solar power generation, and general household appliances.

[0135] Example

[0136] The following examples and comparative examples illustrate the present invention in detail. However, the present invention is not limited to the examples.

[0137] It should be noted that the raw materials used in the examples and comparative examples are shown in Table 1.

[0138] [Table 1]

[0139]

[0140] [Examples 1-14 and Comparative Examples 1-6]

[0141] <Manufacturing of Biaxially Stretched Polypropylene Film>

[0142] The raw materials shown in Table 1 were dry-blended according to the mixing ratios (by mass) shown in Table 2. Next, the mixture was fed to an extruder, melted at a resin temperature of 230°C, and extruded using a T-die. The mixture was then wound onto a metal drum with a surface temperature maintained at 23°C to solidify, producing an unstretched cast sheet with a thickness of approximately 300 μm. Then, using a Brueckner KARO IV intermittent biaxial stretching machine, the unstretched cast sheet was stretched 4.6 times in the flow direction at 165°C, followed by a 9.4-fold stretch in the transverse direction, to produce a biaxially stretched polypropylene film with a thickness of 7.0 μm.

[0143] [Example 15]

[0144] The raw materials shown in Table 1 were dry-blended according to the mixing ratios (by mass) shown in Table 2. Next, the mixture was fed to an extruder, melted at a resin temperature of 230°C, and extruded using a T-die. The mixture was then wound onto a metal drum with a surface temperature maintained at 23°C to solidify, producing an unstretched cast sheet approximately 300 μm thick. Then, using a Brueckner KARO IV intermittent biaxial stretching machine, the unstretched cast sheet was stretched 5.2 times in the flow direction at 165°C, followed by a 9.4-fold stretch in the transverse direction, to produce a biaxially stretched polypropylene film with a thickness of 6.0 μm.

[0145] <Determination Method>

[0146] For each embodiment and comparative example, various characteristics were evaluated under the following measurement conditions.

[0147] (1) Evaluation of resin properties

[0148] (Determination of number-average molecular weight (Mn), weight-average molecular weight (Mw), z-average molecular weight (Mz), molecular weight distribution (Mw / Mn), molecular weight distribution (Mz / Mn), and differential distribution value of polypropylene) Using GPC (gel permeation chromatography), the number-average molecular weight (Mn), weight-average molecular weight (Mw), z-average molecular weight (Mz), molecular weight distribution (Mw / Mn), molecular weight distribution (Mz / Mn), and differential distribution value of the distribution curve of polypropylene were determined under the following conditions.

[0149] A differential refractometer (RI) with built-in high-temperature GPC apparatus, namely HLC-8121GPC-HT type, manufactured by Tosoh Corporation, was used. Three Tosoh-manufactured TSKgel GMHHR-H(20)HT columns were connected, and one TSKgel guardcolumn HHR(30) column was used. At a column temperature of 140°C, 0.05 wt% of 2,6-di-tert-butyl-p-cresol (generic name: BHT) was added to 1,2,4-trichlorobenzene at a flow rate of 1.0 ml / min as the eluent, and the number-average molecular weight (Mn), weight-average molecular weight (Mw), and z-average molecular weight (Mz) were determined. The molecular weight distribution (Mz / Mn) was obtained using the values ​​of Mz and Mn, and the molecular weight distribution (Mw / Mn) was obtained using the values ​​of Mw and Mn.

[0150] The measurement conditions are as follows.

[0151] GPC Unit: HLC-8121GPC / HT (Tosoh Manufacturing)

[0152] Light scattering detector: DAWN EOS (Wyatt Technology)

[0153] Column: 1 TSKgel guardcolumnHHR(30)(7.8mmID×7.5cm) + 3 TSKgel GMHHR-H(20)HT(7.8mmID×30cm) (Tosoh manufacture)

[0154] Eluent: 0.05 wt% BHT in 1,2,4-trichlorobenzene

[0155] Flow rate: 1.0 mL / min

[0156] Sample concentration: 2 mg / mL

[0157] Injection volume: 300μL

[0158] Column temperature: 140℃

[0159] System temperature: 40℃

[0160] Pretreatment: Accurately weigh the sample, add the eluent, shake and dissolve at 140℃ for 1 hour, and then perform hot filtration using a 0.5μm sintered metal filter.

[0161] Standard curve: A standard curve approximating the standard polystyrene manufactured by Tosoh Corporation was prepared using five orders of magnitude. The molecular weight was converted to the molecular weight of polypropylene using the Q factor.

[0162] Based on the obtained standard curve and SEC chromatogram, using the analytical software for the assay apparatus, the molecular weight (logarithmic value) is plotted on the horizontal axis, and the integral value of the concentration fraction is plotted on the vertical axis to obtain the integral molecular weight distribution curve. Additionally, the differential value of the integral molecular weight distribution curve at each molecular weight (the slope of the integral molecular weight distribution curve) is calculated, and the molecular weight (logarithmic value) is plotted on the horizontal axis, and the differential value is plotted on the vertical axis to obtain the differential molecular weight distribution curve.

[0163] These curves yield the number-average molecular weight (Mn), weight-average molecular weight (Mw), and Z-average molecular weight (Mz). The molecular weight distribution (Mw / Mn) is obtained using the values ​​of Mw and Mn. Similarly, the molecular weight distribution (Mz / Mn) is obtained using the values ​​of Mz and Mn.

[0164] (Heptane-insoluble component (HI))

[0165] Using the polypropylene resins from the raw materials used in the examples and comparative examples, approximately 3 g of sample for determination was prepared by compression molding to a size of 10 mm × 35 mm × 0.3 mm. Then, approximately 150 mL of heptane was added, and Soxhlet extraction was performed for 8 hours. The heptane-insoluble component was calculated from the sample mass before and after extraction.

[0166] (Ratio of meso-five-unit components)

[0167] The polypropylene resins of the raw materials used in the examples and comparative examples were dissolved in a solvent, and the proportions of the meso-five-unit components were determined using a high-temperature Fourier transform nuclear magnetic resonance (FT-NMR) device under the following conditions.

[0168] High-Temperature Nuclear Magnetic Resonance (NMR) Device: Manufactured by JEOL Ltd., High-Temperature Fourier Transform Nuclear Magnetic Resonance Device (High-Temperature FT-NMR), Model: JNM-ECP500

[0169] Observation core: 13C (125MHz)

[0170] Measurement temperature: 135℃

[0171] Solvent: o-dichlorobenzene (ODCB: a mixed solvent of ODCB and deuterated ODCB (mixing ratio = 4 / 1))

[0172] Measurement mode: Single-pulse proton broadband decoupling

[0173] Pulse amplitude: 9.1 μsec (45° pulse)

[0174] Pulse interval: 5.5 seconds

[0175] Total number of times: 4500

[0176] Displacement reference: CH3 (mm) = 21.7ppm

[0177] The percentage of the pentads representing stereoregularity is calculated as a percentage (%) of the intensity integral of each signal from combinations of pentads (mmmm, mrrm, etc.) derived from co-orienting pentads (monic (m)) and co-orienting pentads (r) (mmmm, mrrm, etc.). The attribution of each signal derived from mmmm, mrrm, etc., is referenced, for example, to the spectra described in “T. Hayashi et al., Polymer, Vol. 29, p. 138 (1988)”.

[0178] (melt tension)

[0179] Using a capillary rheometer 1B manufactured by Toyo Seiki Co., Ltd., the resin of the raw materials used in the examples and comparative examples was extruded into a strip under the following conditions, and the tension detected by the pulley when the strip was wound on the roller was taken as the melt tension.

[0180] Capillary tube: 2.0 mm in diameter, 40 mm in length

[0181] Barrel diameter: 9.55mm

[0182] Extrusion speed: 20mm / minute

[0183] Winding speed: 4.0 m / min

[0184] Temperature: 230℃

[0185] It should be noted that when the melt tension is extremely high, the resin may sometimes break at a traction speed of 4.0 m / min. In such cases, the traction speed should be reduced, and the tension at the highest traction speed should be taken as the melt tension.

[0186] (Mel flow rate (MFR))

[0187] The melt flow rate (MFR) of the resin granules of the raw materials used in the examples and comparative examples was determined using a melt index manufactured by Toyo Seiki Co., Ltd., according to condition M of JIS K 7210. Specifically, firstly, a 4g sample was inserted into a barrel at a test temperature of 230°C and preheated for 3.5 minutes under a load of 2.16kg. Then, the weight of the sample extruded from the bottom hole within 30 seconds was measured, and the MFR (unit: g / 10min or g / 10min) was calculated. The above measurement was repeated 3 times, and the average value was taken as the measured value of MFR.

[0188] (2) Evaluation of the properties of biaxially stretched polypropylene film

[0189] (Dynamic viscoelasticity (storage modulus (E') and loss modulus (E”))

[0190] The measurement conditions and procedures are as follows.

[0191] As a dynamic viscoelasticity testing machine, the "Viscoelasticity Testing Apparatus (Model: DMS6100)" manufactured by Seiko Instruments was used.

[0192] As the test samples, biaxially stretched polypropylene films obtained by further biaxially stretching the cast sheets extruded with each blending ratio in Table 2 were cut into strips with a longitudinal length of 40 mm and a transverse length of 8 mm. The temperature dependence of the biaxially stretched polypropylene films was determined according to JIS-K7244 (1999 edition) under the test conditions shown below.

[0193] Test mode: Tensile mode

[0194] Chuck pitch: 20mm

[0195] Vibration frequencies: 1Hz, 2Hz, 5Hz and 10Hz

[0196] Strain amplitude: 10 μm

[0197] Minimum tension: 100mN

[0198] Tension gain: 1.2

[0199] Initial force amplitude: 100mN

[0200] Temperature range: -60~150℃

[0201] Heating rate: 5℃ / minute

[0202] Atmosphere to be measured: air

[0203] Measured thickness: 6.0 or 7.0 μm

[0204] Based on the measurement results, the storage modulus (E') and loss modulus (E”) of the biaxially stretched polypropylene film at a temperature dispersion of 70℃ (vibration frequencies of 1Hz, 2Hz, 5Hz and 10Hz) were determined.

[0205] (Methods for determining the slope of an approximate straight line (linear function))

[0206] The slope of an approximate straight line (linear function) determined by a viscoelastic scatter plot was established. This viscoelastic scatter plot was obtained by plotting the coordinates of four points (logE”) at 70°C under the aforementioned conditions of vibration frequencies of 1Hz, 2Hz, 5Hz, and 10Hz obtained from dynamic viscoelastic measurements, with the logarithm (logE”) of the energy loss modulus (E”) at 70°C under the aforementioned conditions set as the x-axis, and the logarithm (logE’) of the energy storage modulus (E’) at 70°C under the aforementioned conditions set as the y-axis. The slope of the approximate straight line (linear function) was determined using the statistical function "SLOPE" in Microsoft Excel.

[0207] (Thin film thickness)

[0208] The thickness of biaxially stretched polypropylene film was measured using a CITIZEN SEIMITSUCO.,LTD. MEI-11 paper thickness gauge (measuring pressure 100 kPa, descent speed 3 mm / s, measuring terminal φ=16 mm, measuring force 20.1 N) at an environment of 23±2℃ and 50±5%RH. Samples were cut from the roll with at least 10 overlapping sheets, ensuring no wrinkles or air ingress during cutting. Five measurements were performed on the 10-sheet overlapping samples, and the average of the five measurements was divided by 10 to calculate the thickness.

[0209] (Determination of β crystal percentage in cast sheets before biaxial stretching)

[0210] The percentage of β crystals was evaluated using the K value obtained by X-ray diffraction intensity measurement. Specifically, the K value was calculated using the method described in the non-patent literature “A. Turner-Jones et al., Makromol. Chem., Vol. 75, p. 134 (1964)”. It should be noted that the X-ray diffraction intensity measurement conditions are as follows.

[0211] Measurement apparatus: Rigaku Corporation, Mini-FLEX300 X-ray diffraction apparatus

[0212] X-ray source: CuKα-ray monochromator filter

[0213] Irradiation output: 30KV-10mA

[0214] Scattering slit: 1.25 deg

[0215] Light-receiving slit: 1.25 deg

[0216] Scan axis: 2θ / θ

[0217] Scanning speed: 2deg / minute

[0218] Based on the obtained intensity curve, the K value is calculated using the following formula: the sum of the heights of the three diffraction peaks originating from the α crystal is given by the ratio of the height of the one diffraction peak originating from the β crystal.

[0219] K-value (intensity ratio %) = Hβ / (Hβ + HαI + HαII + HαIII) × 100 [Where Hβ is the intensity (height) of the peak corresponding to the diffraction of the β crystal (2θ = 16 degrees), HαI is the intensity (height) of the peak corresponding to the diffraction of the α crystal (110) plane, HαII is the intensity (height) of the peak corresponding to the diffraction of the α crystal (040) plane, and HαIII is the intensity (height) of the peak corresponding to the diffraction of the α crystal (130) plane. All values ​​are the intensity (height) after subtracting the amorphous scattering.]

[0220] (Determination of crystallite size of α-crystals in biaxially stretched polypropylene film)

[0221] Using an X-ray diffraction apparatus, the crystallite size of the α-crystals of the biaxially stretched polypropylene films of the Examples and Comparative Examples was determined under the following conditions in the form of a single film.

[0222] Measurement apparatus: Rigaku Corporation, Mini-FLEX300 X-ray diffraction apparatus

[0223] X-ray source: CuKα-ray monochromator filter

[0224] Irradiation output: 30KV-10mA

[0225] Scattering slit: 1.25 deg

[0226] Light-receiving slit: 1.25 deg

[0227] Scan axis: 2θ / θ

[0228] Based on the obtained data, the half-width of the diffraction reflection peak of the α crystal (040) plane was calculated using an analytical computer and the comprehensive powder X-ray analysis software PDXL (Ver. 2.1.3.4) included with the device standard, after optimization using a segmented pseudo-Voight function. The crystallite size was then determined using the Scherrer formula (D = K × λ / (β × cosθ)).

[0229] It should be noted that in the Scherrer formula, D is the crystallite size (nm), K is a constant (shape factor: 0.94 in this embodiment), λ is the X-ray wavelength (nm), β is the calculated half-width, and θ is the diffraction Bragg angle. 0.15418 nm is used as λ.

[0230] (Determination of surface roughness (Vmc) of roughened surfaces)

[0231] Biaxially stretched polypropylene films have roughened and unroughened surfaces due to the stretching process. Here, a roughened surface refers to a surface with a higher surface roughness, and an unroughened surface refers to a surface with a lower surface roughness.

[0232] Specifically, the surface roughness (Vmc) of the roughened surface was measured using a white optical interferometer microscope, the "VertScan2.0 (model: R5500GML)" manufactured by Ryoka Systems, as a non-contact optical interferometer. As the sample for measurement, the film was cut into arbitrary sizes of approximately 20 cm square, and with the wrinkles fully stretched, it was placed on the measurement stage using an electrostatic sealing plate, etc. The measurement method is described in detail below.

[0233] First, using WAVE mode, a 530 white filter, and a 1×BODY lens barrel, with a ×10 objective lens, measurements were performed at 470.92 μm × 353.16 μm for each field of view. This operation was performed at 10 points along the flow direction, starting from the center of the sample (biaxially stretched polypropylene film) in both the flow direction and width direction.

[0234] Next, the obtained data is subjected to noise removal processing based on a median filter (3×3), followed by Gaussian filtering based on a cutoff value of 30μm to remove fluctuation components. This results in a data set that can be appropriately used to measure the roughened surface condition.

[0235] Next, we used the "ISO Parameters" function in the "Bearing" plugin of the "VS-Viewer" analysis software "VertScan2.0" to perform the analysis.

[0236] Finally, the average value of Vmc obtained at the above 10 locations is calculated. Based on the above, the Vmc value of the roughened surface is obtained.

[0237] (Determination of volume resistivity (ρV))

[0238] According to JIS C 2139-3-1:2018, the volume resistivity (ρV) of biaxially stretched polypropylene film was determined at 120℃ and 143V / μm.

[0239] First, a fixture for measuring volume resistivity (hereinafter also referred to as "the fixture") is set up in a constant temperature bath at 120°C. The structure of the fixture is described below. Additionally, a DC power supply and a DC ammeter are connected to the fixture.

[0240] <Jig for measuring volume resistivity>

[0241] Main electrode (50mm in diameter)

[0242] Counter electrode (85mm in diameter)

[0243] A ring-shaped protective electrode (outer diameter 80mm, inner diameter 70mm) surrounds the main electrode.

[0244] Each electrode is made of gold-plated copper, and conductive rubber is adhered to the surface that contacts the sample. The conductive rubber used is EC-60BL (W300) manufactured by Shin-Etsu Chemical Co., Ltd., and is adhered by making the glossy surface of the conductive rubber contact the gold-plated copper.

[0245] Next, the biaxially stretched polypropylene film (hereinafter also referred to as the "sample") is placed in a fixture within a constant temperature bath. Specifically, the main electrode and the guard electrode are brought into contact with one side of the sample, and the counter electrode is brought into contact with the other side, with a load of 5 kgf applied to ensure contact between the sample and each electrode. Then, the sample is allowed to stand for 30 minutes.

[0246] Next, a DC power supply was used to apply a voltage to the sample with a potential gradient of 143 V / μm.

[0247] After applying voltage, the current value is read after 1 minute, and the volume resistivity is calculated using the following formula. It should be noted that the voltage is applied using a Keithley 2290-10 (DC power supply), and the current value is measured using a Keithley 2635B (DC ammeter).

[0248] Volume resistivity = [(effective electrode area) × (applied voltage)] / [(sample thickness) × (current value)]

[0249] Here, the effective electrode area is calculated using the following formula.

[0250] (Effective electrode area) = Pi × [[[(Diameter of main electrode) + (Inner diameter of protective electrode)] / 2] / 2] 2

[0251] Repeat this process three times, and use the arithmetic mean of the results with one significant figure as the volume resistivity (Ω·cm).

[0252] (3) Evaluation of the characteristics of capacitor elements

[0253] (Life test based on electrostatic capacitance change rate)

[0254] A 620mm wide biaxially stretched polypropylene film was wound and metallized using a ULVAC roll-to-roll vacuum evaporation machine (model: EWE-060). 20Ω / □ Al metal was deposited on the active portion, and 5Ω / □ Zn metal was deposited on the heavy edge portion, creating a metallized film with insulating edges and dividing electrodes. Next, a Nuintek dicing machine (model: NT-750) was used to cut the metallized film into strips 30mm wide in the width direction, creating small rolls of the metallized film. Then, using a Kaito automatic winding machine (model: 3KAW-N2), at a winding speed of 4m / s, a winding tension of 180g, and a contact roller contact pressure of 260g, the number of turns was set to a component electrostatic capacitance of 50μF. The 30mm wide metallized film was wound in a double-overlapping configuration. After the wound element is flattened by pressing, zinc metal is sprayed onto the end face of the element under a pressing load to form an electrode extraction part. Then, it is heat-treated at 120°C for 15 hours to thermocure it. This process produces a capacitor element.

[0255] First, after measuring the component's electrostatic capacitance (C) before the test, a DC voltage of 750V is applied to the component at 115°C. The component's electrostatic capacitance (C') is measured after 1000 hours, and the rate of change of electrostatic capacitance (ΔC / C) is determined using the following formula.

[0256] △C / C(%)=(C'-C) / C×100

[0257] The evaluation criteria for life testing are as follows.

[0258] The AA capacity change rate (capacity reduction rate) is less than 0.5%.

[0259] A. The rate of change in capacity (rate of decrease in capacity) exceeds 0.5% but is less than 1%.

[0260] B. Capacity change rate (capacity reduction rate) exceeding 1% but less than 3%

[0261] C. Capacity change rate (capacity reduction rate) exceeding 3% but less than 5%

[0262] D. Capacity change rate (capacity reduction rate) exceeds 5%, or component breaks during the test.

[0263] If it is C or above, it is qualified (can be used), and it is more preferable to be A or above.

[0264] The results of the evaluation of the various characteristics of the embodiments and comparative examples are shown in Table 2.

[0265] [Table 2]

[0266]

[0267] As shown in Table 2, the biaxially oriented polypropylene films of Examples 1-15, which possess the characteristics of the present invention and have a slope of -0.222 or higher for the approximate straight line of viscoelasticity at 70°C, exhibit high volume resistivity even at high temperatures of around 120°C (e.g., 100°C to 125°C). The present invention provides metallized films using these biaxially oriented polypropylene films, as well as capacitors with small changes in electrostatic capacitance even at high temperatures and excellent heat resistance and long lifespan. Specifically, it can be seen that the biaxially oriented polypropylene films of Examples 1-15, compared to the biaxially oriented polypropylene film of Comparative Example 1, which does not contain branched polypropylene resin B and has a slope of less than -0.222 for the approximate straight line of viscoelasticity at 70°C, have a volume resistivity that increases to over 120%.

Claims

1. A biaxially stretched polypropylene film, characterized in that, It contains linear polypropylene resin A and branched polypropylene resin B. The slope of the approximate straight line determined by the viscoelastic scatter plot of the biaxially stretched polypropylene film is greater than -0.

222. The viscoelastic scatter plot is obtained by plotting the four coordinates of (logE”) at 70°C under the conditions of vibration frequencies of 1Hz, 2Hz, 5Hz and 10Hz as the x-axis and the logarithm of the storage modulus (E’) at 70°C under the conditions of the same conditions as the y-axis.

2. The biaxially stretched polypropylene film according to claim 1, wherein, The biaxially stretched polypropylene film has a roughened surface and an unroughened surface. The surface roughness (Vmc) of the roughened surface, measured using a white light interference microscope, is 0.025 ml / m. 2 above.

3. The biaxially stretched polypropylene film according to claim 1, wherein, The biaxially stretched polypropylene film has α-crystals with a size of 129 Å or larger.

4. The biaxially stretched polypropylene film according to claim 1, used in a capacitor.

5. A metallized film having a metal layer on at least one side of the biaxially stretched polypropylene film according to any one of claims 1 to 4.

6. A capacitor comprising the metallized thin film of claim 5.

7. A method for manufacturing a biaxially stretched polypropylene film, characterized in that, It has a process for biaxially stretching cast sheets containing linear polypropylene resin A and branched polypropylene resin B before biaxial stretching. The slope of the approximate straight line determined by the viscoelastic scatter plot of the biaxially stretched polypropylene film is greater than -0.

222. The viscoelastic scatter plot is obtained by plotting the four coordinates of (logE”) at 70°C under the conditions of vibration frequencies of 1Hz, 2Hz, 5Hz and 10Hz as the x-axis and the logarithm of the storage modulus (E’) at 70°C under the conditions of the same conditions as the y-axis.

8. The manufacturing method according to claim 7, wherein, In the biaxial stretching process, the longitudinal stretching ratio is set to 5.0 times or less.

Images