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

Abstract

The present invention provides: a biaxially stretched polypropylene film which has a high volume resistivity even at high temperatures; a metallized film which uses the biaxially stretched polypropylene film; and a capacitor which has excellent heat resistance, low variation width in the electrostatic capacity even at high temperatures, and a long-life.　Specifically, the present invention provides a biaxially stretched polypropylene film which contains a linear polypropylene resin A and a branched polypropylene resin B. The biaxially stretched polypropylene film is characterized in that if the logarithm (logE") of the loss elastic modulus (E") at the temperature of 70°C at each one of the vibration frequencies 1 Hz, 2 Hz, 5 Hz, and 10 Hz is taken on the x-axis, and the logarithm (logE') of the storage elastic modulus (E') at the temperature of 70°C at each one of the above-described vibration frequencies is taken on the y-axis, the inclination of the approximate straight line that is specified from a scatter diagram of viscoelasticities obtained by plotting the coordinates of four points of (logE", logE') at the above-described vibration frequencies is -0.222 or more.

Description

Biaxially oriented polypropylene film and its manufacturing method, metallized film, and capacitor

[0001] The present invention relates to a biaxially oriented polypropylene film and a method for producing the same, a metallized film, and a capacitor.

[0002] Stretched films containing polypropylene as a main component are moisture-proof, rigid, heat-resistant, and so on, and are therefore used in a variety of industrial applications including packaging.

[0003] In particular, stretched films primarily composed of polypropylene are used in film capacitor applications due to their excellent electrical properties. Film capacitors made of stretched films primarily composed of polypropylene are used in electronic devices, electrical equipment, and the like, for example, as high-voltage capacitors; various switching power supplies; filter capacitors and smoothing capacitors for converters, inverters, and the like. Film capacitors are used, for example, in inverters and converters that control drive motors in automobiles such as electric vehicles and hybrid vehicles, which have seen increasing demand in recent years.

[0004] Film capacitors, particularly automotive film capacitors, are increasingly being used in high-temperature environments. For example, in devices (such as inverters and converters) that control automotive drive motors, the use of highly heat-resistant semiconductors (such as silicon carbide semiconductors) has increased in recent years. Accordingly, capacitors used in these devices are also required to have high heat resistance, for example, at temperatures of 120°C or higher, preferably 130°C or higher. Conventional capacitors using polypropylene film are said to have an upper limit of operating temperatures of approximately 110°C, and it is extremely difficult to stably maintain electrical insulation (volume resistivity) in high-temperature environments exceeding this limit.

[0005] As one of the resin films with high heat resistance, a film made of a resin composition containing a polyolefin mainly composed of polypropylene and a hydrogenated block copolymer has been disclosed as a film having excellent heat resistance and particularly excellent water vapor impermeability (Patent Document 1).

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

[0007] JP 2014-37532 A International Publication No. 2022 / 270577

[0008] However, the polypropylene film described in Patent Document 1 has a problem in that it does not have sufficient heat resistance.

[0009] The capacitors described above are used in environments where temperatures rise in an engine compartment or where the capacitors generate heat themselves, and therefore are required to have high heat resistance at high temperatures of around 120°C (e.g., 100°C to 125°C). That is, the capacitors are required to maintain electrical insulation (volume resistivity) and capacitance without short-circuiting even at high temperatures.

[0010] Therefore, there is a need for the development of biaxially oriented polypropylene films that exhibit high volume resistivity even at high temperatures, and metallized films using such biaxially oriented polypropylene films, as well as for the development of capacitors that have a small change in capacitance even at high temperatures, have excellent heat resistance, and have a long life.

[0011] The present invention aims to provide a biaxially oriented polypropylene film that exhibits high volume resistivity even at high temperatures, a metallized film using the biaxially oriented polypropylene film, and a capacitor that exhibits a small change in capacitance even at high temperatures, has excellent heat resistance, and has a long life.

[0012] As a result of extensive research aimed at solving the above-mentioned problems, the present inventors have found that the above-mentioned object can be achieved by a biaxially stretched polypropylene film containing a linear polypropylene resin A and a branched polypropylene resin B, wherein the slope of an approximated straight line (linear function) specified from a viscoelasticity scatter diagram obtained by plotting the coordinates of four points (log E'', log E') under each of the conditions, where the logarithm (log E'') of the loss modulus (E'') at a temperature of 70°C under vibration frequencies of 1 Hz, 2 Hz, 5 Hz, and 10 Hz is taken as the x-axis and the logarithm (log E') of the storage modulus (E') at a temperature of 70°C under each of the conditions is taken as the y-axis, is a specific value or greater. Based on this finding, the present invention has been completed. Hereinafter, this approximated straight line will also be referred to as the "70°C viscoelasticity approximated straight line."

[0013] That is, the present invention relates to the following biaxially oriented polypropylene film, its manufacturing method, metallized film, and capacitor: Item 1. A biaxially oriented polypropylene film containing a linear polypropylene resin A and a branched polypropylene resin B, wherein the biaxially oriented polypropylene film has a slope of -0.222 or more of an approximate straight line specified from a viscoelasticity scatter diagram in which the logarithm (log E'') of the loss modulus (E'') at a temperature of 70°C under vibration frequencies of 1 Hz, 2 Hz, 5 Hz, and 10 Hz is taken as the x-axis and the logarithm (log E') of the storage modulus (E') at a temperature of 70°C under each of the above conditions is taken as the y-axis, and the coordinates of four points (log E'', log E') under each of the above conditions are plotted. Item 2. The biaxially stretched polypropylene film has a roughened surface and a non-roughened surface, and the surface roughness (Vmc) of the roughened surface measured by a white light interference microscope is 0.025 ml / m ２Item 3. The biaxially oriented polypropylene film according to item 1, wherein the biaxially oriented polypropylene film has α crystallites, and the size of the α crystallites is 129 Å or more. Item 4. The biaxially oriented polypropylene film according to any one of items 1 to 3, which is used for a capacitor. Item 5. A metallized film having a metal layer on at least one side of the biaxially oriented polypropylene film according to any one of items 1 to 4. Item 6. A capacitor comprising the metallized film according to item 5. Item 7. Item 8. A method for producing a biaxially stretched polypropylene film, comprising a step of biaxially stretching a pre-biaxially stretched cast sheet containing a linear polypropylene resin A and a branched polypropylene resin B, wherein the biaxially stretched polypropylene film has a slope of -0.222 or more of an approximate straight line specified from a viscoelasticity scatter diagram in which the logarithm (log E'') of the loss modulus (E'') at a temperature of 70°C under each of vibration frequencies of 1 Hz, 2 Hz, 5 Hz, and 10 Hz is taken as the x-axis and the logarithm (log E') of the storage modulus (E') at a temperature of 70°C under each of the conditions is taken as the y-axis.

[0014] The biaxially oriented polypropylene film of the present invention can exhibit high volume resistivity even at high temperatures of about 120° C. (e.g., 100° C. to 125° C.) The present invention also provides a metallized film using the biaxially oriented polypropylene film, and a capacitor that exhibits a small change in capacitance even at high temperatures, has excellent heat resistance, and has a long life.

[0015] 1 is a graph showing a viscoelasticity scatter diagram in which the coordinates of four points (log E″, log E′) under each of the conditions are plotted, with the logarithm (log E″) of the loss modulus (E″) at a temperature of 70°C for the biaxially stretched polypropylene film produced in Example 1 at vibration frequencies of 1 Hz, 2 Hz, 5 Hz, and 10 Hz on the x-axis and the logarithm (log E′) of the storage modulus (E′) at a temperature of 70°C under each of the conditions on the y-axis, and showing that the slope of the approximation line specified from the viscoelasticity scatter diagram is −0.197 (rounded to the fourth decimal place). This graph shows the correlation between the improvement rate of the volume resistivity of each of the biaxially stretched polypropylene films produced in Examples 1 to 15 and Comparative Examples 2 to 6 and the slope of the 70°C viscoelasticity approximation line, when the volume resistivity of the biaxially stretched polypropylene film produced in Comparative Example 1 is taken as the reference (100%).

[0016] In this specification, the term "to" in a numerical range means "greater than or equal to" or "less than or equal to." That is, the notation "α to β" means "greater than or equal to α and less than or equal to β," or "greater than or equal to β and less than or equal to α," and includes α and β as a range. Furthermore, when multiple lower limit values ​​and multiple upper limit values ​​are listed separately, any lower limit value and any upper limit value can be selected and connected with "to."

[0017] In this specification, the expressions "contain" and "comprise" include the concepts of "contain", "include", "consist essentially of" and "consist only of".

[0018] In this specification, the expression "capacitor" includes the concepts of "capacitor," "capacitor element," and "film capacitor."

[0019] In this specification, the directions of the biaxially oriented polypropylene film are as follows. First, the machine direction of the film is the Machine Direction (hereinafter also referred to as "MD direction"). The MD direction may also be called the length direction or flow direction. Next, the transverse direction of the film is the Transverse Direction (hereinafter also referred to as "TD direction"). The TD direction may also be called the width direction.

[0020] 1. Biaxially oriented polypropylene film The biaxially oriented polypropylene film of the present invention is a biaxially oriented polypropylene film containing a linear polypropylene resin A and a branched polypropylene resin B, characterized in that the biaxially oriented polypropylene film has a slope of -0.222 or more of an approximate straight line determined from a viscoelasticity scatter diagram in which the logarithm (log E'') of the loss modulus (E'') at a temperature of 70°C under vibration frequencies of 1 Hz, 2 Hz, 5 Hz, and 10 Hz is taken as the x-axis and the logarithm (log E') of the storage modulus (E') at a temperature of 70°C under each of the above conditions is taken as the y-axis, and the coordinates of four points (log E'', log E') under each of the above conditions are plotted.

[0021] The biaxially oriented polypropylene film of the present invention having the above characteristics can exhibit high volume resistivity even at high temperatures of about 120°C (e.g., 100°C to 125°C). Hereinafter, this temperature range will be abbreviated as "high temperature" in this specification. Furthermore, by using a biaxially oriented polypropylene film that exhibits high volume resistivity even at high temperatures, a capacitor with a small change in capacitance even at high temperatures, excellent heat resistance, and long life can be obtained.

[0022] Although the function of the present invention is not clear, the biaxially oriented polypropylene film of the present invention contains a linear polypropylene resin A and a branched polypropylene resin B, and a viscoelastic scatter diagram is obtained by plotting the coordinates of four points (log E'', log E') under each of the conditions, with the logarithm (log E'') of the loss modulus (E'') at a temperature of 70°C under vibration frequencies of 1 Hz, 2 Hz, 5 Hz, and 10 Hz as the x-axis and the logarithm (log E') of the storage modulus (E') at a temperature of 70°C under each of the conditions as the y-axis. The slope of the approximate straight line specified from the scatter diagram is -0.222 or more, and this optimizes the relationship between the storage modulus (E') and the loss modulus (E''), which is an index of the thermal motion state of the molecular chains of the polypropylene constituting the biaxially oriented polypropylene film (particularly the temperature and frequency dependence of molecular motion), and is presumed to affect the function of the present invention.

[0023] The branched polypropylene resin B acts as a crystal nucleating agent, and the inclusion of the branched polypropylene resin B promotes the "entanglement effect (pseudo-crosslinking effect)" and "crystallization effect" of the polypropylene molecular chains. Specifically, the inclusion of the branched polypropylene resin B results in the formation of a large amount of β crystals in the cast sheet before biaxial stretching. Stretching the cast sheet containing the β crystals transforms the β crystals into α crystals, resulting in a biaxially stretched polypropylene film with high crystallinity and large α crystallite size. This is also presumably a prerequisite for easily optimizing the relationship between the storage modulus (E') and loss modulus (E'') described above.

[0024] Furthermore, a large amount of β crystals is formed in the cast sheet before biaxial stretching. By stretching the cast sheet containing these β crystals, the β crystals transition to α crystals. Therefore, due to the difference in density between the β crystals and the α crystals, (approximately) arc-shaped irregularities are formed in the polypropylene film obtained by stretching, allowing for suitable surface roughening. This increases the surface roughness (Vmc) of the roughened surface, leading to an increase in the actual volume of the core portion of the roughened surface. From this perspective, the biaxially stretched polypropylene film of the present invention is suitable as a material for dielectric films for capacitors.

[0025] On the other hand, if too much branched polypropylene resin B is blended, crystallization and physical crosslinking are promoted, but "crystallinity" is inhibited. Therefore, it is preferable to increase the β crystal fraction (the ratio of β crystals to the total of α crystals and β crystals) in the cast sheet, thereby achieving a synergistic effect of the "entanglement effect" and "crystallization effect" of the polypropylene molecular chains, thereby satisfying the requirement of a predetermined slope in the 70 ° C viscoelasticity approximation line of the biaxially oriented polypropylene film, and adjusting the mass ratio of linear polypropylene resin A to branched polypropylene resin B so as to obtain high volume resistivity at high temperatures. Note that, as will be described in detail later, the branched polypropylene resin B used in the present invention preferably has a molecular weight distribution that is narrow within a specific range, and from the viewpoint of the production method, it is preferably a propylene polymer polymerized using a metallocene catalyst.

[0026] The biaxially oriented polypropylene film of the present invention will be described in detail below.

[0027] The biaxially oriented polypropylene film of the present invention contains a linear polypropylene resin A and a branched polypropylene resin B, and has a slope of an approximated straight line specified from a viscoelasticity scatter diagram in which the logarithm (log E'') of the loss modulus (E'') at a temperature of 70°C under vibration frequencies of 1 Hz, 2 Hz, 5 Hz, and 10 Hz is taken as the x-axis and the logarithm (log E') of the storage modulus (E') at a temperature of 70°C under each of the above conditions is taken as the y-axis, and the coordinates of four points (log E'', log E') under each of the above conditions are plotted.

[0028] The thickness of the biaxially oriented polypropylene film of the present invention is preferably 6.0 μm or less, more preferably 5.5 μm or less, even more preferably 3.5 μm or less, particularly preferably 3.0 μm or less, and most preferably 2.8 μm or less, from the viewpoint of further improving the miniaturization and high capacity of the capacitor when used in a capacitor. Furthermore, from the viewpoint of production, the lower limit is preferably 0.8 μm or more, more preferably 1.0 μm or more, even more preferably 1.8 μm or more, and particularly preferably 2.0 μm or more. The method for measuring the thickness of the biaxially oriented polypropylene film in this specification is the method described in the Examples.

[0029] The density of the biaxially oriented polypropylene film is not particularly limited, and from the viewpoint of making it easier to use in a capacitor, it is set to 919 kg / m ３ More than 930kg / m ３ The following is preferred:

[0030] The biaxially stretched polypropylene film of the present invention contains a linear polypropylene resin A and a branched polypropylene resin B. Here, in relation to the linear polypropylene resin A and the branched polypropylene resin B, the linear polypropylene resin A having a higher content is referred to as the "main polypropylene resin" or "base resin." Meanwhile, the branched polypropylene resin B having a relatively lower content is referred to as the "blend resin."

[0031] The linear polypropylene resin A may be a single resin or a mixed resin of two or more (particularly two) types. For example, the linear polypropylene resin A may be a type of linear polypropylene resin A1 described below, or a mixed resin of linear polypropylene resin A1 and linear polypropylene resin A2 described below. In this case, the linear polypropylene resin A1 with a higher content is referred to as the "base resin," and the linear polypropylene resin A2 with a relatively lower content is referred to as the "blend resin."

[0032] (Linear Polypropylene Resin A1) In the biaxially stretched polypropylene film of the present invention, the linear polypropylene resin A can be composed of one of the linear polypropylene resins A1 described in this section. The linear polypropylene resin A1 can be a crystalline polypropylene such as isotactic polypropylene or syndiotactic polypropylene.

[0033] The weight average molecular weight Mw of the linear polypropylene resin A1 is preferably 250,000 to 360,000, more preferably 280,000 to 350,000, even more preferably 300,000 to 350,000, and particularly preferably 300,000 to 350,000. 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 production process of the biaxially stretched polypropylene film, and thickness unevenness is less likely to occur.

[0034] The weight-average molecular weight Mw of the linear polypropylene resin A1 is a characteristic of the linear polypropylene resin A1 as a raw material resin. Similarly, the number-average molecular weight Mn, Z-average molecular weight Mz, molecular weight distribution (Mw / Mn) between the weight-average molecular weight Mw and the number-average molecular weight Mn, molecular weight distribution (Mz / Mn) between the Z-average molecular weight Mz and the number-average molecular weight Mn, melt flow rate at 230°C (MFRA1), melt tension at 230°C, mesopentad fraction ([mmmm]), and heptane-insoluble matter (HI) of the linear polypropylene resin A1, which will be described later, are also characteristics of the linear polypropylene resin A1 as a raw material resin.

[0035] The number-average molecular weight Mn of the linear polypropylene resin A1 is preferably from 30,000 to 54,000, more preferably from 33,000 to 52,000, and even more preferably from 33,000 to 47,000. When the number-average molecular weight Mn of the linear polypropylene resin A1 is within the above range, the capacitance change range of the produced capacitor at high temperatures is small, and the heat resistance is further improved.

[0036] The Z-average molecular weight Mz of the linear polypropylene resin A1 is preferably 1,000,000 or more and 2,000,000 or less, and more preferably 1,200,000 or more and 1,800,000 or less. When the Z-average molecular weight Mz of the linear polypropylene resin A1 is in the above range, the volume resistivity of the biaxially oriented polypropylene film at high temperatures is further improved.

[0037] The molecular weight distribution (Mw / Mn) of the linear polypropylene resin A1 is 7.0 or more and 9.3 or less. The lower limit can be set preferably to 7.2 or more, more preferably 7.3 or more, and even more preferably 8.1 or more. The upper limit can be set preferably to 9.0 or less, more preferably 8.2 or less. By setting the Mw / Mn within the above range, the stretchability of the polypropylene film is further improved, and a thinner biaxially oriented polypropylene film can be produced. Furthermore, the volume resistivity of the biaxially oriented polypropylene film at high temperatures is further improved.

[0038] The molecular weight distribution (Mz / Mn) of the linear polypropylene resin A1 is preferably from 10 to 100, more preferably from 15 to 70, and even more preferably from 15 to 60. When Mz / Mn is within the above range, the stretchability of the polypropylene film is further improved, and a thinner biaxially oriented polypropylene film can be produced.

[0039] The melt flow rate (MFRA1) of the 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. The melt flow rate (MFRA1) of the 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, even more preferably 6.0 g / 10 min or less, and particularly preferably 5.0 g / 10 min or less. When the melt flow rate (MFRA1) of the linear polypropylene resin A1 at 230°C is within the above range, the volume resistivity of the biaxially oriented polypropylene film at high temperatures is further improved. The melt flow rate (MFR) of the resin at 230°C in this specification is measured according to the method described in the Examples.

[0040] The melt tension of linear polypropylene resin A1 at 230°C is preferably 1.0 g or less, and more preferably less than 1.0 g. When the melt tension of linear polypropylene resin A1 at 230°C is within the above range, the resin has excellent flow characteristics in the molten state, making it less likely to experience unstable flow such as melt fracture. This has the advantage of providing good film thickness uniformity, making it less likely to form thin-walled sections prone to dielectric breakdown. The melt tension of the resin at 230°C measured in this specification is the same as that described in the Examples.

[0041] The mesopentad fraction ([mmmm]) of the linear polypropylene resin A1 is preferably 99.8% or less, more preferably 99.5% or less, even more preferably 99.0% or less, and most preferably 98.0% or less. The mesopentad fraction is preferably 94.0% or more, more preferably 94.5% or more, and even more preferably 95.0% or more. When the mesopentad fraction is within the above range, the moderately high stereoregularity improves the crystallinity of the polypropylene resin, further improving the volume resistivity of the biaxially oriented polypropylene film at high temperatures. Furthermore, the solidification (crystallization) rate during cast sheet molding becomes moderate, allowing the film to exhibit moderate stretchability. The method for measuring the mesopentad fraction ([mmmm]) of the resin in this specification is the same as that described in the Examples.

[0042] The heptane insoluble fraction (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. The heptane insoluble fraction is preferably 99.5% or less, more preferably 99.0% or less. The higher the heptane insoluble fraction, the higher the stereoregularity of the resin. When the heptane insoluble fraction is within the above range, the moderately high stereoregularity improves the crystallinity of the polypropylene resin in the polypropylene film, thereby further improving the volume resistivity at high temperatures. Furthermore, in the polypropylene film production process, the solidification (crystallization) rate during cast sheet molding before biaxial stretching becomes moderate, resulting in moderate stretchability. The heptane insoluble fraction (HI) of the resin in this specification is measured according to the method described in the Examples.

[0043] Representative commercially available products of the linear polypropylene resin A1 include, for example, HC300BF manufactured by Borealis and HPT-1 manufactured by Taihan Yuhka.

[0044] (Linear Polypropylene Resin A2) In the biaxially stretched polypropylene film of the present invention, the linear polypropylene resin A may be composed of a mixed resin containing the linear polypropylene resin A1 as a base resin and the linear polypropylene resin A2 as a blend resin. As with the linear polypropylene resin A1, the linear polypropylene resin A2 may be a crystalline polypropylene such as isotactic polypropylene or syndiotactic polypropylene.

[0045] In the linear polypropylene resin A, the mixing ratio of the linear polypropylene resin A1 (base resin) to the linear polypropylene resin A2 (blend resin) is not limited as long as the blend resin content is lower. The content of the linear polypropylene resin A2 is preferably less than 50% by mass, more preferably 49% by mass or less, and even more preferably 40% by mass or less, based on 100% by mass of the linear polypropylene resin A. Furthermore, the content of the 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, based on 100% by mass of the linear polypropylene resin A.

[0046] The weight average molecular weight Mw of the linear polypropylene resin A2 is preferably 300,000 or more, more preferably 350,000 or more, even more 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 even more 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 biaxially stretched polypropylene film production process, and thickness unevenness is less likely to occur.

[0047] The weight-average molecular weight Mw of the linear polypropylene resin A2 is a characteristic of the linear polypropylene resin A2 as a raw material resin. Similarly, 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, mesopentad fraction ([mmmm]), and heptane-insoluble matter (HI) of the linear polypropylene resin A2, which will be described later, are also characteristics of the linear polypropylene resin A2 as a raw material resin.

[0048] 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 5.0,000 or less, and even more preferably 38,000 or more and 48,000 or less. When the number-average molecular weight Mn of the polypropylene resin A is within the above range, the capacitance change range of the produced capacitor at high temperatures is small, and the heat resistance is further improved.

[0049] The Z-average molecular weight Mz of the linear polypropylene resin A2 is preferably more than 1.35 million and not more than 2 million, and more preferably from 1.4 million to 1.9 million. When the Z-average molecular weight Mz of the linear polypropylene resin A2 is in the above range, the volume resistivity of the biaxially oriented polypropylene film at high temperatures is further improved.

[0050] The molecular weight distribution (Mw / Mn) of the linear polypropylene resin A2 is 7.0 or more and 9.3 or less. Among these, the lower limit can be set preferably to 8.1 or more, more preferably 8.2 or more, and even more preferably 8.3 or more. The upper limit can be set preferably to 9.2 or less, more preferably 9.1 or less. By setting the Mw / Mn within the above range, the stretchability of the polypropylene film is further improved, and a thinner biaxially oriented polypropylene film can be produced. Furthermore, the volume resistivity of the biaxially oriented polypropylene film at high temperatures is further improved.

[0051] The molecular weight distribution (Mz / Mn) of the linear polypropylene resin A2 is preferably from 30 to 40, more preferably from 33 to 36. When Mz / Mn is within the above range, the stretchability of the polypropylene film is further improved, and a thinner biaxially oriented polypropylene film can be produced.

[0052] 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. 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.

[0053] The difference MFRA1 - MFRA2 between the MFR (MFRA1) of the linear polypropylene resin A1 serving as the base resin and the MFR (MFRA2) of the linear polypropylene resin A2 serving as the blend resin is preferably 0.8 g / 10 min or more. That is, MFRA1 is preferably greater than MFRA2. 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. If MFRA1 - MFRA2 is less than 0.5 g / 10 min (this less than 0.5 g / 10 min includes negative values), a sea-island phase-separated structure may not be formed at the time of cast sheet molding prior to biaxial stretching in the polypropylene film production process, or even if it is formed, the island size may be very small, making it difficult to finally obtain a polypropylene film with excellent volume resistivity at high temperatures. In particular, even if the difference between MFRA1 and MFRA2 is large, when MFRA2 is larger (when the above MFRA1-MFRA2 is negative), the size of the islands in the sea-island phase separation structure becomes very small.

[0054] The melt tension of linear polypropylene resin A2 at 230°C is preferably 1.0 g or less, and more preferably less than 1.0 g. When the melt tension of linear polypropylene resin A2 at 230°C is within the above range, the resin has excellent flow characteristics in the molten state, making it less likely to experience unstable flow such as melt fracture. This has the advantage of providing good film thickness uniformity, making it less likely to form thin-walled sections prone to dielectric breakdown. The melt tension of the resin at 230°C measured in this specification is the same as that described in the Examples.

[0055] The mesopentad fraction ([mmmm]) of the linear polypropylene resin A2 is preferably 99.8% or less, more preferably 99.5% or less, and even more preferably 99.0% or less. The mesopentad fraction 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 mesopentad fraction is within the above range, the crystallinity of the polypropylene resin is improved due to the moderately high stereoregularity, and the volume resistivity of the biaxially oriented polypropylene film at high temperatures is further improved. Furthermore, the solidification (crystallization) rate during cast sheet molding becomes moderate, allowing the film to exhibit moderate stretchability. In addition, when the linear polypropylene resin A contains two types of linear polypropylene resin A1 and linear polypropylene resin A2 (preferably a mixture of linear polypropylene resin A1 and linear polypropylene resin A2), the mesopentad fraction of the linear polypropylene resin A1 can be set to 98.0% or less, and the mesopentad fraction of the linear polypropylene resin A2 can be set to 98.0% or more.

[0056] The heptane insoluble fraction (HI) of the linear polypropylene resin A2 is preferably 97.5% or more, more preferably 98.0% or more, even more preferably 98.5% or more, and particularly preferably 98.6% or more. The heptane insoluble fraction is preferably 99.5% or less, more preferably 99.0% or less. The heptane insoluble fraction (HI) of both the linear polypropylene resin A1 and the linear polypropylene resin A2 can also be set to 98.5% or more.

[0057] A typical commercially available product of the linear polypropylene resin A2 is S802M manufactured by Taihan Yuhka Co., Ltd.

[0058] (Branched Polypropylene Resin B) The biaxially stretched polypropylene film of the present invention contains a branched polypropylene resin B in addition to the linear polypropylene resin A.

[0059] Among the branched polypropylene resins B, branched polypropylene resins B obtained by polymerizing propylene using a metallocene catalyst are preferred. When a biaxially stretched polypropylene film contains the branched polypropylene resin B in addition to the linear polypropylene resin A, a large amount of β crystals are formed in the cast sheet before biaxial stretching. By stretching a cast sheet containing the β crystals, the β crystals transition to α crystals, and due to the difference in density between the β crystals and the α crystals, (approximately) arc-shaped irregularities are formed in the polypropylene film obtained by stretching, allowing the surface to be suitably roughened. This increases the surface roughness (Vmc) of the roughened surface, leading to an increase in the actual volume of the core portion of the roughened surface. As will be described in detail later, the biaxially stretched polypropylene film of the present invention 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 From this viewpoint, the biaxially oriented polypropylene film of the present invention is suitable as a material for a dielectric film for a capacitor.

[0060] When a branched polypropylene resin B obtained by crosslinking modification with peroxide is used as the branched polypropylene resin B, rather than a branched polypropylene resin B polymerized using a metallocene catalyst, the α-crystal nucleating effect of the branched polypropylene resin B obtained by crosslinking modification with peroxide promotes the formation of α-crystals in the cast sheet before biaxial stretching, while suppressing the formation of β-crystals. Even when a cast sheet containing α-crystals is stretched, crystallite transition is unlikely to occur, and unevenness is unlikely to form. Therefore, from the perspective of roughening the surface of a biaxially stretched polypropylene film, a branched polypropylene resin B polymerized using a metallocene catalyst can be preferably used.

[0061] In this regard, when a branched polypropylene resin B polymerized using a metallocene catalyst is used, the β-crystal fraction in the cast sheet before biaxial stretching is preferably 10% or more, more preferably 13% or more, even more preferably 15% or more, still more preferably 17% or more, and most preferably 19% or more. The β-crystal fraction in this specification is measured by the method described in the Examples.

[0062] The metallocene catalyst is generally a metallocene compound that forms a polymerization catalyst that produces an olefin macromer. Branched polypropylene resin B obtained by polymerizing propylene using a metallocene catalyst is preferred because it has an appropriate branch chain length and branch chain spacing, improves compatibility with linear polypropylene, and is more likely to have a more uniform composition and a more uniform surface shape.

[0063] Representative commercially available examples of the branched polypropylene resin B include 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.

[0064] The melt tension of the branched polypropylene resin B at 230° C. is preferably 3 g or more and 25 g or less, more preferably 5 g or more and 20 g or less, even more preferably 9 g or more and 20 g or less, and particularly preferably 9 g or more and 17 g or less. When the melt tension is within the above range, the volume resistivity of the biaxially oriented polypropylene film at high temperatures is further improved.

[0065] The melt flow rate of the branched polypropylene resin B at 230°C is preferably 0.1 to 12.0 g / 10 min, more preferably 1.0 to 6.0 g / 10 min, more preferably 1.5 to 4.0 g / 10 min, and even more preferably 2.0 to 3.5 g / 10 min. Therefore, the melt flow rate of the branched polypropylene resin B at 230°C can be set to 2.0 g / 10 min or more. Having a melt flow rate at 230°C within the above range results in excellent flow characteristics in the molten state, making unstable flow such as melt fracture less likely to occur, and further suppressing breakage during stretching. Therefore, better film thickness uniformity is achieved, 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.

[0066] 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 fluidity becomes more appropriate, the thickness of the cast sheet becomes easier to control, and it becomes easier to produce a thin stretched film. In addition, unevenness in the thickness of the cast sheet and stretched film is less likely to occur, and more appropriate stretchability can be obtained.

[0067] The number average molecular weight Mn of the branched-chain polypropylene resin B is preferably from 100,000 to 300,000, more preferably from 100,000 to 250,000, and even more preferably from 100,000 to 200,000. When the number average molecular weight Mn of the branched-chain polypropylene resin B is within the above range, the capacitance of the produced capacitor changes less at high temperatures, and the heat resistance is further improved.

[0068] The molecular weight distribution (Mw / Mn) of the branched polypropylene resin B is preferably 1.5 to 4.5, more preferably 1.8 to 4.5, and even more preferably 2.0 to 4.2. When Mw / Mn is in the above range, the stretchability of the polypropylene film is further improved, and a thinner biaxially oriented polypropylene film can be produced.

[0069] The Z-average molecular weight Mz of the branched-chain polypropylene resin B is preferably 600,000 or more and 2,000,000 or less, and more preferably 800,000 or more and 1,700,000 or less. When the Z-average molecular weight Mz of the branched-chain polypropylene resin B is within the above range, the volume resistivity of the biaxially oriented polypropylene film at high temperatures is further improved.

[0070] The molecular weight distribution (Mz / Mn) of the branched polypropylene resin B is preferably from 4 to 30, more preferably from 5 to 20. When Mz / Mn is within the above range, the stretchability of the polypropylene film is further improved, and a thinner biaxially oriented polypropylene film can be produced.

[0071] The molecular weight, molecular weight distribution, etc. of the branched polypropylene resin B can be controlled by adjusting the catalyst and polymerization conditions.

[0072] The content of the branched polypropylene resin B is preferably such that the mass ratio of the linear polypropylene resin A to the branched polypropylene resin B is such that the linear polypropylene resin A:the branched polypropylene resin B=99.0:1.0 to 85.0:15.0, more preferably the linear polypropylene resin A:the branched polypropylene resin B=98.0:2.0 to 92.0:8.0, and even more preferably the linear polypropylene resin A:the branched polypropylene resin B=97.0:3.0 to 93.0:7.0. In such a mass ratio, for example, when the total amount of linear polypropylene resin A and branched polypropylene resin B is taken as 100 mass%, and linear polypropylene resin A is a two-kind mixture of resin A1 and resin A2, it is preferable to set the content of linear polypropylene resin A1 to 55 mass% or more and 65 mass% or less, the content of linear polypropylene resin A2 to 30 mass% or more and 40 mass% or less, and the content of branched polypropylene resin B to 2 mass% or more and 10 mass% or less. By setting the blending ratio in such a manner, it becomes easier to obtain a high volume resistivity even at high temperatures compared to conventional products.

[0073] (Other Resins) The biaxially oriented polypropylene film of the present invention may contain other resins (hereinafter also referred to as "other resins") other than the linear polypropylene resin A and the branched polypropylene resin B. The "other resins" are resins other than the linear polypropylene resin A and the branched polypropylene resin B, and are not particularly limited as long as the desired biaxially oriented polypropylene film can be obtained. Examples of such other resins include polyolefins other than polypropylene, such as polyethylene, poly(1-butene), polyisobutene, 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. When other resins are contained, the total amount of the linear polypropylene resin A and the branched polypropylene resin B is preferably 95% by mass or more, and more preferably 98% by mass or more, based on 100% by mass of the entire resin components. In other words, the content of other resins is preferably 5% by mass or less, and more preferably 2% by mass or less, based on 100% by mass of the entire resin components.

[0074] (Additives) The biaxially stretched polypropylene film of the present invention may further contain additives, such as antioxidants, chlorine absorbers, ultraviolet absorbers, lubricants, plasticizers, flame retardants, antistatic agents, and colorants.

[0075] (Properties of Biaxially Stretched Polypropylene Film) The biaxially oriented polypropylene film of the present invention has a slope of an approximated straight line specified from a viscoelasticity scatter diagram in which the logarithm (log E'') of the loss modulus (E'') at a temperature of 70°C under each of vibration frequencies of 1 Hz, 2 Hz, 5 Hz and 10 Hz is taken as the x-axis and the logarithm (log E') of the storage modulus (E') at a temperature of 70°C under each of the above conditions is taken as the y-axis, and the coordinates of four points (log E'', log E') under each of the above conditions are plotted.

[0076] FIG. 1 is a viscoelasticity scatter diagram in which the coordinates of four points (log E″, log E′) under each of the conditions are plotted, with the logarithm (log E″) of the loss modulus (E″) at a temperature of 70°C for the biaxially stretched polypropylene film produced in Example 1 under vibration frequencies of 1 Hz, 2 Hz, 5 Hz, and 10 Hz on the x-axis and the logarithm (log E′) of the storage modulus (E′) at a temperature of 70°C under each of the conditions on the y-axis. The diagram also illustrates that the slope of the approximation line determined from the viscoelasticity scatter diagram is −0.197 (rounded to the fourth decimal place). The storage modulus (E′) and the loss modulus (E″) under each of the conditions can be calculated using a dynamic viscoelasticity measuring device in accordance with JIS-K7244 (1999 edition), specifically by the method described in the Examples. Furthermore, as a method for determining the slope of the approximate line (linear function) from the coordinates of the four points under each of the above conditions, the slope of the approximate line (linear function) can be determined by using the statistical function "SLOPE" in calculation software (Microsoft's "Excel").

[0077] In the present invention, the slope of the approximate line is preferably −0.222 or more and −0.180 or less, more preferably −0.210 or more and −0.190 or less, and even more preferably −0.205 or more and −0.195 or less. By setting the slope of the approximate line to such a value, it becomes easier to obtain a high volume resistivity even at high temperatures compared to conventional products. If the slope of the approximate line is greater than −0.180 (if the film elastic modulus is high), it becomes easier to obtain the desired volume resistivity, but the risk of film breakage during the film production process increases.

[0078] The biaxially stretched polypropylene film of the present invention has α-crystallites, and the size of the α-crystallites is preferably 129 Å or more. Such a relatively large α-crystallite size can be obtained by increasing the β-crystal fraction in the cast sheet before biaxial stretching. The size of the α-crystallites is preferably 130 Å or more, more preferably 131 Å or more, such as 132 Å or more, 133 Å or more, 134 Å or more, 135 Å or more, 136 Å or more, etc. The practical upper limit of the size of the α-crystal crystallites is about 140 Å. The size of the α-crystal crystallites 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, and in detail by the method described in the Examples.

[0079] The biaxially stretched polypropylene film of the present invention has a roughened surface and a non-roughened surface, and the surface roughness (Vmc) of the roughened surface measured by a white light interference microscope is 0.025 ml / m ２ Of these, 0.030 ml / m or more is preferable. ２ More than 0.035 ml / m is preferable. ２ More preferably, 0.040 ml / m or more ２ Above, 0.045ml / m ２ The practical upper limit of the surface roughness (Vmc) of the roughened surface is 0.100 ml / m ２ In this way, the surface roughness (Vmc) of the roughened surface is about 0.025 ml / m ２ The surface roughness (Vmc) is a value measured using a white light interference microscope (VeatScan 2.0 manufactured by Ryoka Systems Co., Ltd.), and in detail, is measured by the method described in the Examples.

[0080] The biaxially stretched polypropylene film of the present invention has a high volume resistivity even at high temperatures. Specifically, the volume resistivity (ρV) measured in accordance with JIS C 2139-3-1:2018 under conditions of 120°C and 143 V / μm is 7.50 × 10 １４ It is preferable that the resistance is Ω cm or more, and 8.00 × 10 １４ It is more preferable that the resistivity is 8.50×10 Ω cm or more. １４ Ω・cm or more, 9.00×10１４ Ω・cm or more, 9.50×10 １４ Ω・cm or more, 1.00×10 １５ Ω・cm or more, 1.50×10 １５ The practical upper limit of the volume resistivity (ρV) is 9.0 × 10 １５ The volume resistivity (ρV) is a value measured in accordance with JIS C 2139-3-1:2018, and in detail by the method described in the examples.

[0081] 2. Manufacturing Method of Biaxially Stretched Polypropylene Film The manufacturing method of the biaxially oriented polypropylene film of the present invention is not particularly limited, and can be specified, for example, as "a manufacturing method of a biaxially oriented polypropylene film, comprising a step of biaxially stretching a pre-biaxially stretched cast sheet containing a linear polypropylene resin A and a branched polypropylene resin B, wherein the biaxially oriented polypropylene film has a gradient of -0.222 or more of an approximate straight line specified from a viscoelasticity scatter diagram obtained by plotting the coordinates of four points (log E'', log E') under each of the conditions, with the logarithm (log E'') of the loss modulus (E'') at a temperature of 70°C under each of the conditions of vibration frequencies of 1 Hz, 2 Hz, 5 Hz, and 10 Hz as the x-axis and the logarithm (log E') of the storage modulus (E') at a temperature of 70°C under each of the conditions as the y-axis."

[0082] Among them, from the viewpoint of the details of the production process, a polypropylene resin composition containing at least a linear polypropylene resin A and a branched polypropylene resin B is preferably produced at a temperature of 225° C. or higher and 270° C. or lower and a shear rate of 2000 s －１ More than 15000s －１ The manufacturing method can be described below by a manufacturing method including a melting step. The manufacturing method will be described below by way of example.

[0083] The above-described production method can provide a biaxially oriented polypropylene film that exhibits high volume resistivity even at high temperatures. The reason for this is believed to be due to the sea-island phase separation structure (particularly, the appropriate island size) of the cast sheet, which is achieved by using two specific, different polypropylene resins.

[0084] The method of mixing the resins to be applied to the above-mentioned production method is not particularly limited, and examples thereof include a method of dry-blending polymer powders or pellets of the base resin and the blend resin using a mixer or the like, and a method of supplying polymer powders or pellets of the base resin and the blend resin to a kneader and melt-kneading them to obtain a kneaded product.

[0085] The mixer and kneader are not particularly limited. The kneader may be any of a single-screw type, a twin-screw type, and a multi-screw type having three or more screws. In the case of a twin or more screw type, the screws may rotate in the same direction or in opposite directions.

[0086] In the case of melt-kneading, the kneading temperature is not particularly limited as long as a good kneaded product is obtained. Generally, the temperature is in the range of 200°C or higher and 300°C or lower, and from the viewpoint of suppressing resin deterioration, 230°C or higher and 270°C or lower is preferred. In addition, in order to suppress deterioration during kneading and mixing of the resin, an inert gas such as nitrogen may be purged into the kneader. The melt-kneaded resin may be pelletized to an appropriate size using a commonly known granulator. This allows the preparation of mixed polypropylene raw material resin pellets.

[0087] The polypropylene resin composition may contain an additive. The additive may be the same as the additive described for the biaxially oriented polypropylene film of the present invention. The polypropylene resin composition may contain the additive in an amount that does not adversely affect the biaxially oriented polypropylene film.

[0088] In the method for producing the biaxially stretched polypropylene film, first, polypropylene resin pellets, dry-mixed polypropylene resin pellets, or mixed polypropylene resin pellets previously prepared by melt-kneading are fed into an extruder and heated to melt.

[0089] The polypropylene resin composition is preferably melted at 170° C. or higher and 320° C. or lower. Specifically, the extruder temperature during heating and melting of the polypropylene resin composition is set to 225° C. or higher and 270° C. or lower. This allows a sea-island phase-separated structure to be formed at the time of forming into a cast sheet, as described below, and makes it possible to produce a biaxially oriented polypropylene film that exhibits high volume resistivity even at high temperatures.

[0090] The polypropylene resin composition was heated at a temperature of 225°C or higher and 270°C or lower at a shear rate of 2000 s －１ More than 15000s －１ It is preferable to melt the polypropylene at a shear rate of 2000 s or less. This allows a sea-island phase separation structure to be formed at the time of forming the cast sheet, which will be described later, and enables the production of a polypropylene film that exhibits high volume resistivity even at high temperatures. －１ If the shear rate is less than 15,000 s, the extrusion rate may not be constant, and the shape and dimensions of the raw sheet may become irregular or may fluctuate regularly, which may cause the raw sheet to break during transport or stretching. －１ If the shear rate exceeds this, a phenomenon called breakup may occur in the extruder, causing unmelted material to be extruded, making it impossible to obtain a uniform raw sheet, which may make it more likely to break during stretching, or excessive heat may be generated when passing through the tip clearance, causing significant deterioration of the polypropylene resin composition, which may result in a decrease in the volume resistivity of the film obtained by stretching, even if a uniform raw sheet is obtained. The shear rate can be adjusted by the cylinder diameter, screw rotation speed, and screw groove depth of the extruder.

[0091] The shear rate is 2000 s －１ More than 10000s －１ Preferably less than 2000s －１ More than 2300s －１The following is more preferable: When the shear rate is within this range, a biaxially oriented polypropylene film having a weight fraction w of 2.6% or more and 4.0% or less can be easily obtained, and the heat resistance of a film capacitor using the biaxially oriented polypropylene film as a capacitor dielectric can be further improved.

[0092] Next, the molten polypropylene resin composition is extruded into a sheet using a T-die and cooled and solidified on at least one metal drum, thereby forming an unstretched cast sheet. The surface temperature of the metal drum (the temperature of the metal drum that first comes into contact with the extruded 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 depending on the physical properties of the polypropylene resin used. If the surface temperature of the metal drum is lower than 10°C, it is difficult to obtain good sheet formability of the raw sheet, which may result in uneven stretching or breakage during stretching film formation.

[0093] The thickness of the cast sheet 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.

[0094] The biaxially oriented polypropylene film can be produced by subjecting the cast sheet to a biaxial stretching treatment. The stretching is preferably biaxial stretching, which orients the sheet biaxially in both the longitudinal and transverse directions, and a sequential biaxial stretching method is preferred. In the sequential biaxial stretching method, for example, the cast sheet is first maintained at a temperature of 100°C to 180°C (preferably 120°C to 170°C) and stretched in the machine direction by passing it between rolls with a speed difference. The stretching ratio in the machine direction is preferably 3.0 to 5.0 times, more preferably 4.0 to 4.9 times, and even more preferably 4.6 to 4.8 times. By setting the stretching ratio in the machine direction to 5.0 times or less, even when a branched polypropylene resin B is contained, the occurrence of stretching unevenness and film breakage can be easily suppressed when a width direction stretching treatment is performed following the machine direction stretching treatment. The sheet is then introduced into a tenter and stretched in the transverse direction. The temperature during transverse stretching is preferably 160° C. or higher and 180° C. or lower, and the stretching ratio in the transverse direction is preferably 3.0 times or higher and 11.0 times or lower. After transverse stretching, the film is then relaxed and heat-set, and wound up.

[0095] By the above-described manufacturing method, a biaxially oriented polypropylene film can be manufactured.

[0096] The biaxially stretched polypropylene film may be subjected to corona discharge treatment online or offline after the stretching and heat setting steps in order to further improve adhesive properties in subsequent steps such as metal deposition. Corona discharge treatment can be carried out using a known method. It is preferable to use air, carbon dioxide, nitrogen, or a mixture of these gases as the atmospheric gas.

[0097] The biaxially oriented polypropylene film of the present invention produced by the above-described production method can exhibit high volume resistivity even at high temperatures of about 120°C (e.g., 100°C to 125°C). This also makes it possible to provide a metallized film using the biaxially oriented polypropylene film, and a capacitor with a long life, excellent heat resistance, and a small capacitance change even at high temperatures. Therefore, the biaxially oriented polypropylene film of the present invention is suitable for use in capacitors, and can preferably be used as a dielectric in capacitors that constitute inverters in hybrid and electric vehicles.

[0098] 3. Metallized Film, Capacitor, and Methods for Producing the Same The metallized film of the present invention is a metallized film having a metal layer on at least one surface of the biaxially oriented polypropylene film described above.

[0099] The metal layer functions as an electrode. Examples of metals that can be used for the metal layer include simple metals such as zinc, lead, silver, chromium, aluminum, copper, and nickel, mixtures of multiple metals, and alloys of multiple metals. Among these, zinc and aluminum are preferred in terms of environmental impact, economy, and capacitor performance.

[0100] The method for laminating a metal layer on at least one side (one side or both sides) of the biaxially oriented polypropylene film is not particularly limited, and examples thereof include vacuum deposition, sputtering, etc. Vacuum deposition is preferred from the viewpoint of excellent productivity and economy. Examples of vacuum deposition methods include a crucible method and a wire method, and the most suitable method can be selected as appropriate.

[0101] The margin pattern when laminating a metal layer by vapor deposition is not particularly limited, but from the viewpoint of further improving the safety of the capacitor and further suppressing capacitor breakdown and short circuits, it is preferable to apply a pattern including a so-called special margin, such as a fishnet pattern or T-margin pattern, to one side of the biaxially oriented polypropylene film.

[0102] The method for forming the margin is not particularly limited, and it may be formed by a known method such as a tape method or an oil method.

[0103] The thickness of the metallized 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.

[0104] The capacitor of the present invention is a capacitor including the metallized film. The metallized film of the present disclosure can be laminated or wound by a conventionally known method to form a film capacitor.

[0105] The film capacitor may have a structure in which multiple metallized films are laminated, or may have a wound metallized film. Such a film capacitor can be suitably used as a capacitor for inverter power supplies that control drive motors of electric vehicles, hybrid vehicles, etc. It can also be suitably used in railroad vehicles, wind power generation, solar power generation, general home appliances, etc.

[0106] The present invention will be specifically described below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0107] The raw materials used in the examples and comparative examples are shown in Table 1.

[0108]

[0109] [Examples 1 to 14 and Comparative Examples 1 to 6] <Production of Biaxially Stretched Polypropylene Film> The raw materials shown in Table 1 were dry-blended at the mixing ratios (mass ratios) shown in Table 2. The mixture was then fed into an extruder and melted at a resin temperature of 230°C, extruded using a T-die, and solidified by wrapping around a metal drum whose surface temperature was maintained at 23°C to produce an unstretched cast sheet having a thickness of approximately 300 μm. The unstretched cast sheet was then stretched 4.6 times in the machine direction and then 9.4 times in the transverse direction at a temperature of 165°C using a Brückner KARO IV batch-type biaxial stretching machine to produce a biaxially oriented polypropylene film having a thickness of 7.0 μm.

[0110] [Example 15] The raw materials shown in Table 1 were dry-blended at the mixing ratios (mass ratios) shown in Table 2. The mixture was then fed into an extruder and melted at a resin temperature of 230°C, extruded using a T-die, and solidified by wrapping around a metal drum whose surface temperature was maintained at 23°C to produce an unstretched cast sheet having a thickness of approximately 300 μm. The unstretched cast sheet was then stretched 5.2 times in the machine direction and then 9.4 times in the transverse direction at a temperature of 165°C using a Brückner batch-type biaxial stretching machine KARO IV to produce a biaxially oriented polypropylene film having a thickness of 6.0 μm.

[0111] <Measurement Method> Various characteristics of each of the Examples and Comparative Examples were evaluated under the following measurement conditions.

[0112] (1) Evaluation of Resin Properties (Measurement 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) under the following conditions, 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 measured.

[0113] A Tosoh HLC-8121GPC-HT model GPC system with a built-in differential refractometer (RI) was used. Three Tosoh TSKgel GMHHR-H(20)HT columns were connected together, and one TSKgel guard column HHR(30) was used. Measurements were performed at a column temperature of 140°C using 1,2,4-trichlorobenzene with 0.05 wt% 2,6-di-tert-butyl-para-cresol (common name: BHT) as the eluent at a flow rate of 1.0 ml / min to obtain the number-average molecular weight (Mn), weight-average molecular weight (Mw), and z-average molecular weight (Mz). The molecular weight distribution (Mz / Mn) was calculated using the Mz and Mn values, and the molecular weight distribution (Mw / Mn) was calculated using the Mw and Mn values.

[0114] The measurement conditions are as follows. GPC apparatus: HLC-8121GPC / HT (manufactured by Tosoh) Light scattering detector: DAWN EOS (Wyatt Technology) Column: TSKgel guard column HHR (30) (7.8 mm ID x 7.5 cm) x 1 + TSKgel GMHHR-H (20) HT (7.8 mm ID x 30 cm) x 3 (manufactured by Tosoh) Eluent: 0.05 wt% BHT in 1,2,4-trichlorobenzene Flow rate: 1.0 mL / min Sample concentration: 2 mg / mL Injection amount: 300 μL Column temperature: 140°C System temperature: 40°C Pretreatment: The sample was accurately weighed, the eluent was added, and the mixture was dissolved by shaking at 140°C for 1 hour, followed by hot filtration using a 0.5 μm sintered metal filter. Calibration curve: A calibration curve of a quintic approximation curve was prepared using standard polystyrene manufactured by Tosoh Corporation. However, the molecular weight was converted to the molecular weight of polypropylene using the Q-factor.

[0115] From the obtained calibration curve and SEC chromatogram, the molecular weight (logarithmic value) was plotted on the horizontal axis and the integral value of the concentration fraction on the vertical axis using analysis software for the measuring device to obtain an integrated molecular weight distribution curve. In addition, the differential value (slope of the integral molecular weight distribution curve) of the integrated molecular weight distribution curve at each molecular weight was calculated, and the molecular weight (logarithmic value) was plotted on the horizontal axis and the differential value on the vertical axis to obtain a differential molecular weight distribution curve.

[0116] From these curves, the number average molecular weight Mn, weight average molecular weight Mw, and Z average molecular weight Mz were obtained. The values ​​of Mw and Mn were used to obtain the molecular weight distribution (Mw / Mn). The values ​​of Mz and Mn were also used to obtain the molecular weight distribution (Mz / Mn).

[0117] (Heptane-insoluble content (HI)) Each polypropylene resin raw material used in the Examples and Comparative Examples was press-molded into a 10 mm x 35 mm x 0.3 mm sample to prepare approximately 3 g of a measurement sample. Approximately 150 mL of heptane was then added, and Soxhlet extraction was performed for 8 hours. The heptane-insoluble content was calculated from the sample mass before and after extraction.

[0118] (Mesopentad Fraction) Each polypropylene resin used as a raw material in the Examples and Comparative Examples was dissolved in a solvent, and the mesopentad fraction was measured using a high-temperature Fourier transform nuclear magnetic resonance spectrometer (high-temperature FT-NMR) under the following conditions. High-temperature nuclear magnetic resonance (NMR) spectrometer: JEOL Ltd., high-temperature Fourier transform nuclear magnetic resonance spectrometer (high-temperature FT-NMR), model number: JNM-ECP500 Observation nucleus: 13C (125 MHz) Measurement temperature: 135°C Solvent: ortho-dichlorobenzene (ODCB: mixed solvent of ODCB and deuterated ODCB (mixing ratio = 4 / 1)) Measurement mode: single pulse proton broadband decoupling Pulse width: 9.1 μsec (45° pulse) Pulse interval: 5.5 sec Number of accumulations: 4,500 Shift reference: CH ３ (mmmm)=21.7ppm

[0119] The pentad fraction, which represents the degree of stereoregularity, was calculated as a percentage (%) from the integrated intensity of each signal derived from a combination of five pentads (pentads) consisting of meso (m) tangles arranged in the same direction and racemo (r) tangles arranged in the opposite direction (e.g., mmmm, mrrm, etc.). The assignment of each signal derived from mmmm, mrrm, etc. was based on the spectral descriptions in, for example, T. Hayashi et al., Polymer, Vol. 29, p. 138 (1988).

[0120] (Melt Tension) Using a Capillograph 1B manufactured by Toyo Seiki Seisakusho, the raw material resins used in the Examples and Comparative Examples were extruded into a string shape under the following conditions, and the tension detected on the pulley when the string was taken up around a roller was taken as the melt tension: Capillary: diameter 2.0 mm, length 40 mm, Cylinder diameter: 9.55 mm, Cylinder extrusion speed: 20 mm / min, Take-up speed: 4.0 m / min, Temperature: 230°C

[0121] If the melt tension is extremely high, the resin may break at a take-up speed of 4.0 m / min. In such cases, the take-up speed is reduced, and the tension at the highest possible take-up speed is taken as the melt tension.

[0122] (Melt Flow Rate (MFR)) The melt flow rate (MFR) of the raw material resin pellets used in the Examples and Comparative Examples was measured in accordance with Condition M of JIS K 7210 using a melt indexer manufactured by Toyo Seiki Seisakusho. Specifically, a 4 g sample was first inserted into a cylinder heated to a test temperature of 230°C and preheated for 3.5 minutes under a load of 2.16 kg. Thereafter, the weight of the sample extruded from the bottom hole over 30 seconds was measured, and the MFR (unit: g / 10 min or g / 10 min) was calculated. The above measurement was repeated three times, and the average value was used as the measured MFR.

[0123] (2) Evaluation of properties of biaxially stretched polypropylene film (dynamic viscoelasticity (storage modulus (E')) and loss modulus (E'')) The measurement conditions and procedures were as follows.

[0124] As the dynamic viscoelasticity measuring device, a "Viscoelasticity Measuring Device (Model: DMS6100)" manufactured by Seiko Instruments Inc. was used.

[0125] As a measurement sample, a cast sheet extruded at each blending ratio in Table 2 was further biaxially stretched to obtain a biaxially stretched polypropylene film, which was cut into a strip of 40 mm in the longitudinal direction and 8 mm in the transverse direction, and the temperature dependence of the biaxially stretched polypropylene film was measured under the measurement conditions shown below in accordance with JIS-K7244 (1999 edition). Test mode: Tensile mode Chuck distance: 20 mm Vibration frequency: 1 Hz, 2 Hz, 5 Hz, and 10 Hz Strain amplitude: 10 μm Minimum tension: 100 mN Tension gain: 1.2 Initial force amplitude: 100 mN Temperature range: -60 to 150°C Heating rate: 5°C / min Measurement atmosphere: in air Measurement thickness: 6.0 or 7.0 μm From these measurement results, the storage modulus (E') and loss modulus (E'') of the biaxially oriented polypropylene film at a temperature dispersion of 70°C (vibration frequencies of 1 Hz, 2 Hz, 5 Hz, and 10 Hz) were determined.

[0126] (Method for Determining the Slope of the Approximate Line (Linear Function)) The logarithm (log E'') of the loss modulus (E'') at a temperature of 70°C under each of the vibration frequencies of 1 Hz, 2 Hz, 5 Hz, and 10 Hz obtained from the dynamic viscoelasticity measurement was taken as the x-axis, and the logarithm (log E') of the storage modulus (E') at a temperature of 70°C under each of the conditions was taken as the y-axis. The slope of the approximate line (linear function) determined from a viscoelasticity scatter diagram was determined by plotting the coordinates of four points (log E'', log E') under each of the conditions. A specific method for determining the slope of the approximate line (linear function) was determined by using the statistical function "SLOPE" in calculation software ("Excel" manufactured by Microsoft Corporation).

[0127] (Film Thickness) The thickness of the biaxially oriented polypropylene film was measured using a paper thickness measuring instrument MEI-11 manufactured by Citizen Seimitsu Co., Ltd. (measurement pressure 100 kPa, descending speed 3 mm / sec, measurement probe φ=16 mm, measuring force 20.1 N) in an environment of temperature 23±2°C and humidity 50±5% RH. The sample was cut from the roll with 10 or more sheets stacked, and handled so as not to wrinkle or trap air in the film during cutting. Five measurements were taken for each 10-sheet stack sample, and the thickness was calculated by dividing the average of the five measurements by 10.

[0128] (Measurement of β-crystal fraction of cast sheet before biaxial stretching) The β-crystal fraction was evaluated using the K value determined by X-ray diffraction intensity measurement. Specifically, the K value was calculated by the method described in the non-patent document "A. Turner-Jones et al., Makromol. Chem., Vol. 75, p. 134 (1964)." The X-ray diffraction intensity measurement conditions were as follows.

[0129] Measurement equipment: Rigaku Corporation, X-ray diffraction equipment Mini-FLEX300 X-ray source: CuKα ray, monochromator filtered Irradiation output: 30 KV-10 mA Scattering slit: 1.25 deg Receiving slit: 1.25 deg Scanning axis: 2θ / θ Scanning speed: 2 deg / min

[0130] The K value was calculated from the obtained intensity curve using the following formula, as the ratio of the sum of the heights of three diffraction peaks derived from α crystals to one diffraction peak derived from β crystals.

[0131] K value (intensity ratio %) = Hβ / (Hβ + HαI + HαII + HαIII) × 100 (where Hβ is the intensity (height) of the peak corresponding to the crystalline diffraction of the β crystal (2θ = 16 deg), HαI is the intensity (height) of the peak corresponding to the crystalline diffraction of the α crystal (110) plane, HαII is the intensity (height) of the peak corresponding to the crystalline diffraction of the α crystal (040) plane, and HαIII is the intensity (height) of the peak corresponding to the crystalline diffraction of the α crystal (130) plane. However, in all cases, the intensities (heights) were used after subtracting amorphous scattering.)

[0132] (Measurement of crystallite size of α-crystals in biaxially stretched polypropylene films) The crystallite size of α-crystals in the biaxially stretched polypropylene films according to the examples and comparative examples was measured for each sheet of film using an X-ray diffractometer under the following conditions.

[0133] Measuring device: X-ray diffraction device Mini-FLEX300 manufactured by Rigaku Corporation X-ray source: CuKα ray, monochromator filtered Irradiation output: 30 KV-10 mA Scattering slit: 1.25 deg Receiving slit: 1.25 deg Scanning axis: 2θ / θ

[0134] From the obtained data, an analytical computer was used to optimize the data using the integrated powder X-ray analysis software PDXL (Ver. 2.1.3.4) provided as standard with the instrument, using a division-type pseudo-Voight function, and the half-width of the diffraction reflection peak of the α-crystal (040) plane was calculated. From the half-width, the crystallite size was calculated using Scherrer's formula (D = K × λ / (β × cos θ)).

[0135] In the Scherrer formula, D is the crystallite size (nm), K is a constant (shape factor: 0.94 was used in this example), λ is the X-ray wavelength (nm), β is the calculated half-width, and θ is the diffraction Bragg angle. λ was 0.15418 nm.

[0136] (Measurement of Surface Roughness (Vmc) of Roughened Side) Biaxially stretched polypropylene films have a roughened side and a non-roughened side due to the stretching treatment. Here, the roughened side means the side with greater surface roughness, and the non-roughened side means the side with less surface roughness.

[0137] Specifically, the surface roughness (Vmc) of the roughened surface was measured using a white light interference microscope, "VertScan 2.0 (Model: R5500GML)" manufactured by Ryoka Systems Co., Ltd., as an optical interference non-contact surface profiler. As a measurement sample, the film was cut into an arbitrary size of approximately 20 cm square, and after fully smoothing out any wrinkles, it was set on a measurement stage using an electrostatic contact plate or the like. The measurement method is described in detail below.

[0138] First, using WAVE mode, a 530 white filter and a 1×BODY lens barrel are applied, and a ×10 objective lens is used to measure 470.92 μm × 353.16 μm per field of view. This operation is performed at 10 locations at 1 cm intervals in the machine direction from the center in both the machine direction and width direction of the target sample (biaxially stretched polypropylene film).

[0139] Next, the obtained data is subjected to noise removal processing using a median filter (3 × 3), and then subjected to Gaussian filtering processing with a cutoff value of 30 μm to remove waviness components, thereby enabling the state of the roughened surface to be properly measured.

[0140] Next, analysis is performed using the "ISO parameters" in the "bearing" plug-in function of the "VS-Viewer" analysis software of "VertScan 2.0".

[0141] Finally, the average value of Vmc obtained at the 10 locations is calculated, thereby obtaining the Vmc value of the roughened surface.

[0142] (Measurement of Volume Resistivity (ρV)) The volume resistivity (ρV) of the biaxially stretched polypropylene film was measured under conditions of 120° C. and 143 V / μm in accordance with JIS C 2139-3-1:2018.

[0143] First, a volume resistivity measuring jig (hereinafter also referred to as "jig") was placed in a thermostatic chamber at 120° C. The jig had the following configuration: A DC power supply and a DC ammeter were connected to the jig.

[0144] <Volume resistivity measurement jig> Main electrode (diameter 50 mm) Counter electrode (diameter 85 mm) Annular guard electrode surrounding the main electrode (outer diameter 80 mm, inner diameter 70 mm) Each electrode was made of gold-plated copper, and conductive rubber was attached to the surface that came into contact with the sample. The conductive rubber used was EC-60BL (W300) manufactured by Shin-Etsu Silicones, and the glossy side of the conductive rubber was attached so that it came into contact with the gold-plated copper.

[0145] Next, a biaxially stretched polypropylene film (hereinafter also referred to as "sample") was set in a jig in a thermostatic chamber. Specifically, a main electrode and a guard electrode were attached to one side of the sample, and a counter electrode was attached to the other side, and the sample and each electrode were attached to each other under a load of 5 kgf. The sample was then left to stand for 30 minutes.

[0146] Next, a voltage was applied to the sample using a DC power supply so that the potential gradient was 143 V / μm.

[0147] The current value was read one minute after the voltage was applied, and the volume resistivity was calculated using the following formula. A Keithley 2290-10 (DC power supply) was used to apply the voltage, and a Keithley 2635B (DC ammeter) was used to measure the current value. Volume resistivity = [(effective electrode area) x (applied voltage)] / [(sample thickness) x (current value)] Here, the effective electrode area was calculated using the following formula: (effective electrode area) = pi x [[[(diameter of main electrode) + (inner diameter of guard electrode)] / 2] / 2] ２ This was repeated three times, and the arithmetic mean value calculated to one significant digit was taken as the volume resistivity (Ω·cm).

[0148] (3) Characterization of Capacitor Element (Lifetime Test Based on Capacitance Change Rate) A 620 mm wide biaxially stretched polypropylene film was wound and metal was vapor-deposited using an ULVAC winding vacuum vapor deposition machine (model number: EWE-060). Al metal was vapor-deposited at 20 Ω / □ on the active area, and Zn metal was vapor-deposited at 5 Ω / □ on the heavy edge area, producing a metallized film with insulating margins and divided electrodes. The metallized film was then slit into 30 mm wide strips using a Nuintek slitter (model number: NT-750) to produce small rolls of metallized film. Next, two 30 mm wide metallized films were stacked on top of each other using an automatic winder (model number: 3KAW-N2) manufactured by Kaito Manufacturing Co., Ltd., with a winding speed of 4 m / sec, a winding tension of 180 g, and a contact roller pressure of 260 g. The number of turns was set to achieve a capacitance of 50 μF, and the metallized film was wound. The wound element was then flattened by pressing, and zinc metal was sprayed onto the element end faces while the press load was still applied to form electrode leads. The element was then heat-treated at 120°C for 15 hours to thermally cure. This produced a capacitor element.

[0149] First, the capacitance (C) of the element before the test was measured, and then a DC voltage of 750 V was applied to the element in an environment of 115°C. The capacitance (C') of the element after 1000 hours had elapsed was measured, and the capacitance change rate (ΔC / C) was calculated using the following formula: ΔC / C (%) = (C' - C) / C x 100 The evaluation criteria for the life test were as follows: AA: Capacity change rate (capacity decrease rate) is 0.5% or less A: Capacity change rate (capacity decrease rate) is more than 0.5% and less than 1% B: Capacity change rate (capacity decrease rate) is more than 1% and less than 3% C: Capacity change rate (capacity decrease rate) is more than 3% and less than 5% D: Capacity change rate (capacity decrease rate) is more than 5%, or the element ruptured during the test A rating of C or higher is considered pass (usable), and A or higher is more preferable.

[0150] The results of the evaluation of the various characteristics of the examples and comparative examples are shown in Table 2.

[0151]

[0152] As is clear from the results in Table 2, the biaxially oriented polypropylene films of Examples 1 to 15, which meet the requirements of the present invention and have a slope of the 70°C viscoelasticity approximation line of -0.222 or more, exhibit high volume resistivity even at high temperatures of around 120°C (e.g., 100°C to 125°C). The present invention can provide metallized films using such biaxially oriented polypropylene films, and capacitors with long life, excellent heat resistance, and small capacitance changes even at high temperatures. Specifically, the biaxially oriented polypropylene films of Examples 1 to 15 exhibit a volume resistivity increase of 120% or more compared to the biaxially oriented polypropylene film of Comparative Example 1, which does not contain branched polypropylene resin B and has a slope of the 70°C viscoelasticity approximation line of less than -0.222.

Claims

1. A biaxially oriented polypropylene film containing a linear polypropylene resin A and a branched polypropylene resin B, wherein the biaxially oriented polypropylene film has a slope of an approximation line specified from a viscoelasticity scatter diagram in which the logarithm (log E'') of the loss modulus (E'') at a temperature of 70°C under each of vibration frequencies of 1 Hz, 2 Hz, 5 Hz and 10 Hz is taken as the x-axis and the logarithm (log E') of the storage modulus (E') at a temperature of 70°C under each of the above conditions is taken as the y-axis, and the coordinates of four points (log E'', log E') under each of the above conditions are plotted, and the slope of the approximation line specified from the viscoelasticity scatter diagram is -0.222 or more.

2. The biaxially oriented polypropylene film has a roughened surface and a non-roughened surface, and the surface roughness (Vmc) of the roughened surface measured by a white light interference microscope is 0.025 ml / m ２ The biaxially oriented polypropylene film according to claim 1 .

3. The biaxially oriented polypropylene film according to claim 1, wherein the biaxially oriented polypropylene film has α crystallites, and the size of the α crystallites is 129 Å or more.

4. The biaxially oriented polypropylene film according to claim 1, which is for use in a capacitor.

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

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

7. A method for producing a biaxially oriented polypropylene film, comprising a step of biaxially stretching a pre-biaxially stretched cast sheet containing a linear polypropylene resin A and a branched polypropylene resin B, wherein the biaxially oriented polypropylene film has a gradient of an approximation line specified from a viscoelasticity scatter diagram in which the logarithm (log E'') of the loss modulus (E'') at a temperature of 70°C under each of vibration frequencies of 1 Hz, 2 Hz, 5 Hz and 10 Hz is taken as the x-axis, and the logarithm (log E') of the storage modulus (E') at a temperature of 70°C under each of the above conditions is taken as the y-axis, and the coordinates of four points (log E'', log E') under each of the above conditions are plotted.

8. The method according to claim 7, wherein in the biaxial stretching step, the longitudinal stretching ratio is 5.0 times or less.

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