WO2025028584A1 - Biaxially stretched polypropylene film and method for producing same, metallized film, and capacitor - Google Patents

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

The present invention relates to a biaxially oriented polypropylene film and its manufacturing method, a metallized film, and a capacitor.

Stretched films made primarily of polypropylene are moisture-proof, rigid, heat-resistant, and other properties that make them suitable for packaging and other industrial applications.

In particular, stretched films whose main component is polypropylene are used for film capacitor applications due to their excellent electrical properties. Film capacitors made of stretched films whose main component is polypropylene are used in electronic devices, electrical equipment, etc., for example as high-voltage capacitors; various switching power supplies; filter capacitors for converters, inverters, etc., and smoothing capacitors. Film capacitors are used, for example, in inverters and converters that control drive motors in automobiles such as electric vehicles and hybrid vehicles, for which demand has been increasing in recent years.

Film capacitors, particularly automotive film capacitors, are increasingly being used in high-temperature environments. For example, in equipment (inverters, converters, etc.) that control automobile 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, of 120°C or higher, and preferably 130°C or higher. Capacitors that use conventional polypropylene film are said to have an upper limit of operating temperature of approximately 110°C, and it is extremely difficult to stably maintain electrical insulation (volume resistivity) in high-temperature environments that exceed this limit.

As one type of resin film with high heat resistance, a film made of a resin composition containing a polyolefin whose main component is polypropylene and a hydrogenated block copolymer has been disclosed as a film with excellent heat resistance and particularly excellent water vapor impermeability (Patent Document 1).

Furthermore, as one of the resin films with 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).

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

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

Since capacitors such as those described above are used in environments where temperatures rise in the engine compartment and where the capacitors generate heat themselves, they are required to have high heat resistance at high temperatures of around 120°C (e.g., 100°C to 125°C). In other words, the capacitors are required to maintain their electrical insulation (volume resistivity) and capacitance without shorting even at high temperatures.

Therefore, there is a need to develop a biaxially oriented polypropylene film that exhibits high volume resistivity even at high temperatures, and a metallized film that uses this biaxially oriented polypropylene film, as well as a capacitor that has a long life, excellent heat resistance, and small change in capacitance even at high temperatures.

The present invention aims to provide a biaxially oriented polypropylene film that exhibits high volume resistivity even at high temperatures, a metallized film using said 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.

As a result of intensive research conducted by the inventors to solve the above problems, the inventors have found that the above object can be achieved by a biaxially oriented polypropylene film containing linear polypropylene resin A and branched polypropylene resin B, in which 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 is taken as the x-axis, 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 slope of the approximation line (linear function) specified from a viscoelasticity scatter diagram in which the coordinates of the four points (log E'', log E') under each of the above conditions are plotted is equal to or greater than a specific value, and the inventors have completed the present invention. Hereinafter, this approximation line will also be referred to as the "70°C viscoelasticity approximation line".

That is, the present invention relates to the following biaxially oriented polypropylene film and its production method, metallized film, and capacitor.
Item 1. A biaxially oriented polypropylene film containing a linear polypropylene resin A and a branched polypropylene resin B,
The biaxially stretched 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.
A biaxially oriented polypropylene film.
Item 2. The biaxially oriented polypropylene film according to item 1, wherein 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/ ^m2 or more.
Item 3. The biaxially oriented polypropylene film according to item 1 or 2, 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 for use in a capacitor.
Item 5. A metallized film having a metal layer on at least one surface of the biaxially oriented polypropylene film according to any one of items 1 to 4 above.
Item 6. A capacitor comprising the metallized film according to item 5 above.
Item 7. A method for producing a biaxially oriented polypropylene film, comprising a step of biaxially stretching a cast sheet containing a linear polypropylene resin A and a branched polypropylene resin B before biaxial stretching,
The biaxially stretched 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.
A method for producing a biaxially oriented polypropylene film.
Item 8. The method according to item 7, wherein in the biaxial stretching step, a longitudinal stretching ratio is 5.0 times or less.

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 can also provide a metallized film using the biaxially oriented polypropylene film, and a capacitor with a long life, excellent heat resistance, and small capacitance change even at high temperatures.

FIG. 1 shows a viscoelasticity scatter diagram in which 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 of the biaxially stretched polypropylene film produced in Example 1 is plotted 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 is plotted on the y-axis, and the coordinates of four points (log E", log E') under each of the conditions are plotted, and the slope of the approximation line determined from the viscoelasticity scatter diagram is -0.197 (rounded off to the fourth decimal place). 1 is a graph showing the correlation between the improvement rate of the volume resistivity of each of the biaxially oriented 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 oriented polypropylene film produced in Comparative Example 1 is taken as the reference (100%).

In this specification, "to" in a numerical range means greater than or equal to and less than or equal to. In other words, 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. In addition, when multiple lower limit values and multiple upper limit values are listed separately, any lower limit value and upper limit value can be selected and connected with "to".

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

In this specification, the term "capacitor" includes the concepts of "capacitor," "capacitor element," and "film capacitor."

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 the "MD direction"). The MD direction is sometimes called the length direction or flow direction. Next, the horizontal direction of the film is the Transverse Direction (hereinafter also referred to as the "TD direction"). The TD direction is sometimes called the width direction.

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,
The biaxially stretched 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.
It is characterized by:

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, in this specification, this temperature range will be abbreviated as "at high temperature." 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 a long life can be obtained.

The action of the present invention is not clear, but the biaxially oriented polypropylene film of the present invention contains linear polypropylene resin A and branched polypropylene resin B, and 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 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. The slope of the approximate straight line specified from the viscoelasticity scatter diagram in which the coordinates of the four points (log E'', log E') under each of the above conditions are plotted is -0.222 or more. 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 that constitutes the biaxially oriented polypropylene film (particularly the temperature and frequency dependence of molecular motion), and is presumed to affect the action of the present invention.

Branched polypropylene resin B acts as a crystal nucleating agent, and the inclusion of branched polypropylene resin B promotes the "entanglement effect (pseudo-crosslinking effect)" of the molecular chains of polypropylene and the "crystallization effect." In detail, the inclusion of branched polypropylene resin B causes a large amount of β crystals to form in the cast sheet before biaxial stretching. By stretching the cast sheet containing the β crystals, the β crystals transition to α crystals, so the biaxially stretched polypropylene film has high crystallinity and the crystallite size of the α crystals becomes large. This is also presumably a premise for making it easier to optimize the relationship between the storage modulus (E') and loss modulus (E'') mentioned above.

In addition, 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, 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 part 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.

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 adjust the mass ratio of linear polypropylene resin A to branched polypropylene resin B so that the β crystal fraction (the ratio of β crystals to the total of α crystals and β crystals) in the cast sheet is increased, thereby achieving a synergistic effect of the "entanglement effect" and "crystallization effect" of the polypropylene molecular chains, and that the biaxially oriented polypropylene film has a predetermined slope requirement in the 70°C viscoelasticity approximation line and a high volume resistivity at high temperatures. As will be described in detail later, the branched polypropylene resin B used in the present invention preferably has a narrow molecular weight distribution within a specific range, and from the viewpoint of the manufacturing method, is preferably a propylene polymer polymerized using a metallocene catalyst.

The biaxially oriented polypropylene film of the present invention is described in detail below.

The biaxially oriented polypropylene film of the present invention contains a linear polypropylene resin A and a branched polypropylene resin B,
The 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, is plotted, and the slope of the approximation line specified from the viscoelasticity scatter diagram in which the coordinates of four points (log E'', log E') under each of the above conditions are plotted is -0.222 or more.

The thickness of the biaxially oriented polypropylene film of the present invention is preferably 6.0 μm or less in terms of further improving the miniaturization and high capacity of a capacitor when used in a capacitor, 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 in terms of manufacturing. 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 in terms of manufacturing. The method for measuring the thickness of the biaxially oriented polypropylene film in this specification is the method described in the Examples.

The density of the biaxially oriented polypropylene film is not particularly limited, and is preferably 919 kg/m ³ or more and 930 kg/m ³ or less from the viewpoint of easier use in a capacitor.

The biaxially stretched polypropylene film of the present invention contains linear polypropylene resin A and branched polypropylene resin B. In this regard, in terms of the relationship between the linear polypropylene resin A and the branched polypropylene resin B, the linear polypropylene resin A, which has a higher content, is referred to as the "main polypropylene resin" or "base resin." In addition, the branched polypropylene resin B, which has a relatively lower content, is referred to as the "blend resin."

In addition, the linear polypropylene resin A may be one type of resin, or a mixed resin of two or more types (particularly two types). For example, the linear polypropylene resin A may be one 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 high content is called the "base resin", and the linear polypropylene resin A2 with a relatively low content is called the "blend resin".

(Linear polypropylene resin A1)
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 section. The linear polypropylene resin A1 can be a crystalline polypropylene such as isotactic polypropylene or syndiotactic polypropylene.

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, even more 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 in the above range, it is easier to control the thickness of the cast sheet before biaxial stretching in the manufacturing process of the biaxially stretched polypropylene film, and thickness unevenness is less likely to occur.

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

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 in the above range, the change in capacitance of the produced capacitor at high temperatures is small, and the heat resistance is further improved.

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

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

The molecular weight distribution (Mz/Mn) of the linear polypropylene resin A1 is preferably 10 to 100, more preferably 15 to 70, and even more preferably 15 to 60. When Mz/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.

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 in the above range, the volume resistivity of the biaxially oriented polypropylene film at high temperatures is further improved. In this specification, the melt flow rate (MFR) of the resin at 230°C is measured according to the method described in the Examples.

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, it has excellent flow characteristics in the molten state, and unstable flow such as melt fracture is unlikely to occur. Therefore, there is an advantage that the film thickness uniformity is good, and thin-walled parts that are prone to insulation breakdown are unlikely to be formed. Note that the method for measuring the melt tension of the resin at 230°C in this specification is the method described in the Examples.

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 in the above range, the crystallinity of the polypropylene resin is improved due to the moderately high stereoregularity, the volume resistivity of the biaxially oriented polypropylene film at high temperatures is further improved, and the solidification (crystallization) speed during cast sheet molding becomes moderate, allowing the resin to exhibit moderate stretchability. The method for measuring the mesopentad fraction ([mmmm]) of the resin in this specification is the method described in the Examples.

The heptane insoluble content (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 content is preferably 99.5% or less, and more preferably 99.0% or less. The higher the heptane insoluble content, the higher the stereoregularity of the resin. When the heptane insoluble content is within the above range, the moderately high stereoregularity improves the crystallinity of the polypropylene resin in the polypropylene film, and the volume resistivity at high temperatures is further improved. Furthermore, in the manufacturing process of the polypropylene film, the solidification (crystallization) speed during cast sheet molding before biaxial stretching becomes moderate, and the film has moderate stretchability. The method for measuring the heptane insoluble content (HI) of the resin in this specification is the method described in the examples.

Typical commercially available products of the linear polypropylene resin A1 include, for example, HC300BF manufactured by Borealis and HPT-1 manufactured by Taihan Petrochemicals.

(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 in which the linear polypropylene resin A1 is a base resin and the linear polypropylene resin A2 is a blend resin. The linear polypropylene resin A2 may be a crystalline polypropylene such as isotactic polypropylene or syndiotactic polypropylene, as in the linear polypropylene resin A1.

In the linear polypropylene resin A, the mixing ratio of the linear polypropylene resin A1 (base resin) and the linear polypropylene resin A2 (blend resin) is not limited as long as the blend resin has a lower content. 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 the linear polypropylene resin A being 100% by mass. 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 the linear polypropylene resin A being 100% by mass.

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. 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 in the above range, it is easier to control the thickness of the cast sheet before biaxial stretching in the manufacturing process of the biaxially stretched polypropylene film, and thickness unevenness is less likely to occur.

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 insolubles (HI) of the linear polypropylene resin A2 described below are also characteristics of the linear polypropylene resin A2 as a raw material resin.

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 in the above range, the change in capacitance of the produced capacitor at high temperatures is small, and the heat resistance is further improved.

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 1.4 million or more and not more than 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.

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 to preferably 8.1 or more, more preferably 8.2 or more, and even more preferably 8.3 or more. The upper limit can be set to preferably 9.2 or less, and more preferably 9.1 or less. By having Mw/Mn in the above range, the stretchability of the polypropylene film is further improved, and a thinner biaxially oriented polypropylene film can be produced. In addition, the volume resistivity of the biaxially oriented polypropylene film at high temperatures is further improved.

The molecular weight distribution (Mz/Mn) of the linear polypropylene resin A2 is preferably 30 to 40, more preferably 33 to 36. When Mz/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.

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.

The difference MFRA1-MFRA2 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 blend resin is preferably 0.8 g/10 min or more. In other words, MFRA1 is preferably larger than MFRA2. The above 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 the above MFRA1-MFRA2 is less than 0.5 g/10 min (less than 0.5 g/10 min includes negative values), in the polypropylene film manufacturing process, a sea-island phase separation structure is not formed at the time of cast sheet molding before biaxial stretching, or even if it is formed, the size of the islands is very small, so that it may be 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, if MFRA2 is larger (if the above MFRA1-MFRA2 is negative), the size of the islands in the sea-island phase separation structure will be very small.

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, it has excellent flow characteristics in the molten state, and unstable flow such as melt fracture is unlikely to occur. Therefore, there is an advantage that the film thickness uniformity is good, and thin-walled parts that are prone to insulation breakdown are unlikely to be formed. Note that the method for measuring the melt tension of the resin at 230°C in this specification is the method described in the Examples.

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 in the above range, the crystallinity of the polypropylene resin is improved due to the moderately high stereoregularity, the volume resistivity of the biaxially oriented polypropylene film at high temperatures is further improved, and the solidification (crystallization) speed during cast sheet molding becomes moderate, allowing the film to exhibit moderate extensibility. 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 linear polypropylene resin A1 can be set to 98.0% or less, and the mesopentad fraction of linear polypropylene resin A2 can be set to 98.0% or more.

The heptane insoluble fraction (HI) of 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 linear polypropylene resin A1 and linear polypropylene resin A2 can also be set to 98.5% or more.

A representative commercially available product of the linear polypropylene resin A2 is, for example, S802M manufactured by Taihan Oil & Chemical Co., Ltd.

(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.

Among the branched polypropylene resins B, the branched polypropylene resin B obtained by polymerizing propylene using a metallocene catalyst is preferred. By containing the branched polypropylene resin B in addition to the linear polypropylene resin A in the biaxially stretched polypropylene film, a large amount of β crystals are formed in the cast sheet before biaxial stretching. Since the β crystals are transferred to α crystals by stretching the cast sheet containing the β crystals, the polypropylene film obtained by stretching has (approximately) arc-shaped irregularities due to the difference in density between the β crystals and the α crystals, and the surface can be suitably roughened. This increases the surface roughness (Vmc) of the roughened surface, leading to an increase in the actual volume of the core part of the roughened surface. Although details will be described 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 by a white light interference microscope is preferably 0.025 ml/m ² or more. From this viewpoint, the biaxially stretched polypropylene film of the present invention is suitable as a material for a dielectric film for a capacitor.

When branched polypropylene resin B obtained by cross-linking modification with peroxide is used as branched polypropylene resin B, rather than branched polypropylene resin B polymerized using a metallocene catalyst, the α-crystal nucleating effect of branched polypropylene resin B obtained by cross-linking modification with peroxide promotes the formation of α-crystals and suppresses the formation of β-crystals in the cast sheet before biaxial stretching. Even if a cast sheet containing α-crystals is stretched, crystallite transfer is unlikely to occur and unevenness is unlikely to form. Therefore, branched polypropylene resin B polymerized using a metallocene catalyst can be preferably used from the viewpoint of roughening the surface of a biaxially stretched polypropylene film.

In this regard, when using branched polypropylene resin B polymerized using a metallocene catalyst, 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, even more preferably 17% or more, and most preferably 19% or more. The method for measuring the β crystal fraction in this specification is the method described in the Examples.

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 preferable because it has an appropriate branch chain length and branch chain spacing, has improved compatibility with linear polypropylene, and is more easily obtained with a more uniform composition and a more uniform surface shape.

Typical commercially available branched polypropylene resin B includes, for example, Japan Polypropylene WAYMAX-EX6000, Japan Polypropylene WAYMAX-MFX8, Japan Polypropylene WAYMAX-MFX6, Japan Polypropylene WAYMAX-MFX3, Japan Polypropylene WAYMAX-EX4000, etc.

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. By having the melt tension in the above range, the volume resistivity of the biaxially oriented polypropylene film at high temperatures is further improved.

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. By having a melt flow rate at 230°C within the above range, the flow characteristics in the molten state are excellent, so that unstable flow such as melt fracture is less likely to occur, and breakage during stretching is more suppressed. Therefore, the film thickness uniformity is better, so the formation of thin-walled parts where insulation breakdown is likely to occur is suppressed, and the volume resistivity of the biaxially stretched polypropylene film at high temperatures is further improved.

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 is 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.

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 in the above range, the change in capacitance of the produced capacitor at high temperatures is small, and the heat resistance is further improved.

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. By having Mw/Mn in the above range, the stretchability of the polypropylene film is further improved, and a thinner biaxially oriented polypropylene film can be produced.

The Z-average molecular weight Mz of the branched 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 polypropylene resin B is in the above range, the volume resistivity of the biaxially oriented polypropylene film at high temperatures is further improved.

The molecular weight distribution (Mz/Mn) of the branched polypropylene resin B is preferably 4 or more and 30 or less, and more preferably 5 or more and 20 or less. When Mz/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.

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

Regarding the content of branched polypropylene resin B, it is preferable that the mass ratio of the linear polypropylene resin A to the branched polypropylene resin B is linear polypropylene resin A: branched polypropylene resin B = 99.0: 1.0 to 85.0: 15.0, and among these, it is more preferable that the linear polypropylene resin A: branched polypropylene resin B = 98.0: 2.0 to 92.0: 8.0, and even more preferable that the linear polypropylene resin A: 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 the linear polypropylene resin A and the branched polypropylene resin B is 100 mass%, and the linear polypropylene resin A is a mixture of the resins A1 and A2, it is preferable to set the content of the linear polypropylene resin A1 to 55 mass% or more and 65 mass% or less, the content of the linear polypropylene resin A2 to 30 mass% or more and 40 mass% or less, and the content of the branched polypropylene resin B to 2 mass% or more and 10 mass% or less. By setting the blending ratio in this way, it becomes easier to obtain a high volume resistivity even at high temperatures compared to conventional products.

(Other resins)
The biaxially stretched 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 stretched polypropylene film can be obtained. Examples of the 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; random copolymers of vinyl monomers and diene monomers, such as styrene-butadiene random copolymers; and random copolymers of vinyl monomers and diene monomers and vinyl monomers, such as styrene-butadiene-styrene block copolymers. In addition, 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, in 100% by mass of the entire resin component. In other words, the content of the other resins is preferably 5% by mass or less, and more preferably 2% by mass or less, in 100% by mass of the entire resin component.

(Additives)
The biaxially oriented 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.

(Characteristics of biaxially oriented polypropylene film)
In the biaxially stretched polypropylene film of the present invention, the slope 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 the 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, is plotted, and the slope of the approximate straight line specified from the viscoelasticity scatter diagram in which the coordinates of four points (log E'', log E') under each of the above conditions are plotted is -0.222 or more.

1 is an exemplary diagram showing a viscoelasticity scatter diagram in which 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 of the biaxially stretched polypropylene film produced in Example 1 is plotted 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 is plotted on the y-axis, and the slope of the approximation line determined from the viscoelasticity scatter diagram is -0.197 (rounded off to the fourth decimal place). The storage modulus (E') at a temperature of 70°C and the loss modulus (E'') at a temperature of 70°C under each of the conditions can be calculated using a dynamic viscoelasticity measuring device in accordance with JIS-K7244 (1999 edition), and in detail by the method described in the Examples. In addition, the method for determining the slope of the approximate line (linear function) from the coordinates of the four points under each of the above conditions is to use the statistical function "SLOPE" in calculation software (Microsoft's "Excel") to determine the slope of the approximate line (linear function).

In the present invention, the slope of the approximation 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 approximation line in this manner, it becomes easier to obtain a high volume resistivity even at high temperatures compared to conventional products. If the slope of the approximation line is greater than -0.180 (if the film elastic modulus becomes high), it becomes easier to obtain the desired volume resistivity, but there is a higher risk of the film breaking during the film manufacturing process.

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 crystallites is about 140 Å. The size of the α crystallites is calculated using Scherrer's 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.

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 preferably 0.025 ml/m ² or more. Among these, 0.030 ml/m ² or more is preferable, 0.035 ml/m ² or more is more preferable, 0.040 ml/m ² or more, 0.045 ml/m ² or more, etc. The practical upper limit of the surface roughness (Vmc) of the roughened surface is about 0.100 ml/m ^2. Thus, the surface roughness (Vmc) of the roughened surface being 0.025 ml/m ² or more means that the actual volume of the core part of the roughened surface is large. The surface roughness (Vmc) is a value measured by a white light interference microscope ("VeatScan 2.0" manufactured by Ryoka Systems Co., Ltd.), and in detail, according to the method described in the examples.

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 143V/μm is preferably 7.50×10 ¹⁴ Ω·cm or more, more preferably 8.00×10 ¹⁴ Ω·cm or more. Among these, 8.50×10 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 or more, etc. The practical upper limit of the volume resistivity (ρV) is about 9.0×10 ¹⁵ Ω·cm. 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.

2. Method for Producing Biaxially Stretched Polypropylene Film The method for producing the biaxially oriented polypropylene film of the present invention is not particularly limited, and for example, there is provided a method for producing a biaxially oriented polypropylene film, comprising the step of biaxially stretching a cast sheet containing a linear polypropylene resin A and a branched polypropylene resin B before biaxial stretching,
The biaxially stretched 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.
A method for producing a biaxially oriented polypropylene film.
It can be identified as follows.

Among them, from the viewpoint of the details of the production process, the polypropylene resin composition containing at least a linear polypropylene resin A and a branched polypropylene resin B can be produced by a production method including a step of melting the polypropylene resin composition at a temperature of 225° C. or more and 270° C. or less and at a shear rate of 2000 ^{s −1} or more and 15000 s ⁻¹ or less. The above production method will be described below by way of example.

The above manufacturing method can provide a biaxially oriented polypropylene film that exhibits high volume resistivity even at high temperatures. The reason that a biaxially oriented polypropylene film that exhibits high volume resistivity even at high temperatures can be provided 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 types of polypropylene resins.

The method of mixing the resins used in the above manufacturing method is not particularly limited, but examples include a method of dry blending the polymer powder or pellets of the base resin and the blend resin using a mixer, or a method of feeding the polymer powder or pellets of the base resin and the blend resin to a kneader and melt-kneading them to obtain a kneaded product.

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

When mixing by melt kneading, there are no particular restrictions on the mixing temperature as long as a good mixture is obtained. In general, the temperature is in the range of 200°C to 300°C, and from the viewpoint of suppressing deterioration of the resin, 230°C to 270°C is preferable. In addition, in order to suppress deterioration during the kneading and mixing of the resin, an inert gas such as nitrogen may be purged into the kneader. The melt kneaded resin may be pelletized to an appropriate size using a commonly known granulator. In this way, mixed polypropylene raw material resin pellets can be prepared.

The polypropylene resin composition may contain additives. The additives may be the same as those described in the biaxially oriented polypropylene film of the present invention. The polypropylene resin composition may contain the additives in an amount that does not adversely affect the biaxially oriented polypropylene film.

In the manufacturing method of the biaxially stretched polypropylene film, first, polypropylene resin pellets, dry-mixed polypropylene resin pellets, or mixed polypropylene resin pellets prepared in advance by melt kneading are fed into an extruder and heated to melt.

The polypropylene resin composition is preferably melted at 170°C or higher and 320°C or lower. Specifically, the extruder temperature setting when the polypropylene resin composition is heated and melted is 225°C or higher and 270°C or lower. This forms a sea-island phase separation structure at the time of forming the cast sheet, which will be described later, and allows the production of a biaxially oriented polypropylene film that exhibits high volume resistivity even at high temperatures.

The polypropylene resin composition is preferably melted at a shear rate of 2000 s ^-1 to 15000 s ^-1 at a temperature of 225°C to 270°C. This forms a sea-island phase separation structure at the time of forming the cast sheet, which will be described later, and a polypropylene film showing high volume resistivity even at high temperatures can be produced. If the shear rate is less than 2000 s ^-1 , the extrusion amount is not constant, and the shape and dimensions of the raw sheet may become irregular or may fluctuate regularly, which may cause breakage during transport of the raw sheet or breakage during stretching. If the shear rate exceeds 15000 s ^-1 , unmelted material is extruded due to a phenomenon called break-up in the extruder, and a uniform raw sheet cannot be obtained, which may cause breakage during stretching, or excessive heat generation when passing through the tip clearance may cause significant deterioration of the polypropylene resin composition, which may cause 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, the screw rotation speed, and the groove depth of the screw of the extruder.

The shear rate is preferably 2000 s ^-1 or more and 10000 s ^-1 or less, more preferably 2000 s ^-1 or more and 2300 s ^-1 or less. By keeping the shear rate within this range, it becomes easier to obtain a biaxially oriented polypropylene film having a weight fraction w of 2.6% or more and 4.0% or less, and the heat resistance of a film capacitor using the biaxially oriented polypropylene film as a capacitor dielectric is further improved.

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 to form an unstretched cast sheet. The surface temperature of the metal drum (the temperature of the metal drum that first comes into contact after extrusion) is preferably 10°C or higher and 105°C or lower, and more preferably 15°C or higher and 100°C or lower. The surface temperature of the metal drum can be determined according to the physical properties of the polypropylene resin used. If the surface temperature of the metal drum is less than 10°C, it is difficult to obtain good sheet formability of the raw sheet, and there is a risk that uneven stretching or breakage may occur easily during stretching film formation.

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.

The biaxially oriented polypropylene film can be produced by subjecting the cast sheet to a biaxial stretching process. The stretching is preferably biaxial stretching in which the sheet is oriented biaxially in the longitudinal and transverse directions, and the stretching method is preferably a sequential biaxial stretching method. In the sequential biaxial stretching method, for example, the cast sheet is first kept at a temperature of 100°C to 180°C (preferably 120°C to 170°C) and stretched in the flow direction by passing it between rolls with a speed difference. The stretching ratio in the flow direction is preferably 3.0 times to 5.0 times, more preferably 4.0 times to 4.9 times, and even more preferably 4.6 times to 4.8 times. By setting the stretching ratio in the flow direction to 5.0 times or less, even when the sheet contains branched polypropylene resin B, the occurrence of stretching unevenness is suppressed when performing a stretching process in the width direction following the stretching process in the flow direction, and film breakage is easily suppressed. Next, the sheet is guided to 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 transverse stretch ratio is preferably 3.0 times or higher and 11.0 times or lower. After transverse stretching, the film is then relaxed, heat-set, and wound up.

　By using the manufacturing method described above, biaxially oriented polypropylene film can be produced.

The biaxially oriented polypropylene film may be subjected to corona discharge treatment online or offline after the stretching and heat setting processes are completed, in order to further improve the adhesive properties in subsequent processes such as the metal deposition processing process. The corona discharge treatment can be performed using a known method. It is preferable to use air, carbon dioxide gas, nitrogen gas, or a mixture of these gases as the atmospheric gas.

The biaxially oriented polypropylene film of the present invention produced by the above-mentioned 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 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 be preferably used as a dielectric for a capacitor that constitutes an inverter in a hybrid vehicle or electric vehicle.

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 above-mentioned biaxially oriented polypropylene film.

The metal layer functions as an electrode. Metals that can be used for the metal layer include, for example, single metals such as zinc, lead, silver, chromium, aluminum, copper, and nickel, mixtures of multiple types of these, and alloys of these. Among these, zinc and aluminum are preferred in terms of their environmental impact, economic efficiency, and excellent capacitor performance.

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 include vacuum deposition and sputtering. From the viewpoint of excellent productivity and economic efficiency, vacuum deposition is preferred. Examples of vacuum deposition methods include the crucible method and wire method, and the most suitable method can be selected as appropriate.

The margin pattern used when laminating the metal layer by vapor deposition is not particularly limited, but from the viewpoint of further improving the safety of the capacitor and further suppressing damage and short circuits of the capacitor, 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.

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

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.

The capacitor of the present invention is a capacitor that includes the above-mentioned metallized film. The metallized film of the present disclosure can be laminated or rolled by a conventionally known method to form a film capacitor.

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 devices that control the drive motors of electric vehicles, hybrid vehicles, etc. It can also be suitably used in applications such as railway vehicles, wind power generation, solar power generation, and general home appliances.

The present invention will be specifically explained below with reference to examples and comparative examples. However, the present invention is not limited to the examples.

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

[Examples 1 to 14 and Comparative Examples 1 to 6]
<Production of biaxially oriented polypropylene film>
The raw materials shown in Table 1 were dry-blended at the mixing ratio (mass ratio) shown in Table 2. The mixture was then fed to 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 kept at 23° C. to produce an unstretched cast sheet having a thickness of about 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 batch-type biaxial stretching machine KARO IV to produce a biaxially stretched polypropylene film having a thickness of 7.0 μm.

[Example 15]
The raw materials shown in Table 1 were dry-blended at the mixing ratio (mass ratio) shown in Table 2. The mixture was then fed to 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 kept at 23° C. to produce an unstretched cast sheet having a thickness of about 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 stretched polypropylene film having a thickness of 6.0 μm.

<Measurement method>
For each of the examples and comparative examples, various characteristics were evaluated under the following measurement conditions.

(1) Evaluation of Resin Characteristics (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), 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 under the following conditions.

　A Tosoh Corporation HLC-8121GPC-HT type high-temperature GPC device with built-in differential refractometer (RI) was used. Three Tosoh Corporation 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, with 1,2,4-trichlorobenzene containing 0.05 wt% 2,6-di-tertiary-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 obtained using the values of Mz and Mn, and the molecular weight distribution (Mw/Mn) was obtained using the values of Mw and Mn.

The measurement conditions are as follows.
GPC device: 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: 1,2,4-trichlorobenzene containing 0.05 wt% BHT
Flow rate: 1.0mL/min
Sample concentration: 2 mg/mL
Injection volume: 300 μL
Column temperature: 140°C
System temperature: 40°C
Pretreatment: The sample was precisely weighed, and the eluent was added thereto, followed by dissolution by shaking at 140°C for 1 hour, and hot filtration was performed using a 0.5 µm sintered metal filter.
Calibration curve: A calibration curve of a quintic approximation curve was prepared using standard polystyrene from Tosoh Corp. However, the molecular weight was converted to the molecular weight of polypropylene using the Q-factor.

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 the analysis software for the measurement device to obtain an integral molecular weight distribution curve. In addition, the differential value of the integral molecular weight distribution curve for each molecular weight (slope of the integral molecular weight distribution curve) 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.

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).

(Heptane insolubles (HI))
Each polypropylene resin used in the examples and comparative examples was press-molded to 10 mm x 35 mm x 0.3 mm to prepare a measurement sample of about 3 g. Then, about 150 mL of heptane was added and Soxhlet extraction was performed for 8 hours. The heptane insoluble matter was calculated from the sample mass before and after extraction.

(Mesopentad fraction)
Each of the raw polypropylene resins used in the examples and comparative examples was dissolved in a solvent, and the mesopentad fraction was measured under the following conditions using a high-temperature Fourier transform nuclear magnetic resonance spectrometer (high-temperature FT-NMR).
High-temperature nuclear magnetic resonance (NMR) apparatus: JEOL Ltd., high-temperature Fourier transform nuclear magnetic resonance apparatus (high-temperature FT-NMR), model number: JNM-ECP500
Observed nucleus: 13C (125MHz)
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 integrations: 4,500 Shift reference: _CH3 (mmmm) = 21.7 ppm

The pentad fraction, which indicates the degree of stereoregularity, was calculated as a percentage (%) from the integral intensity of each signal derived from a combination of five pentads (pentads) consisting of meso (m) quintuplets arranged in the same direction and racemo (r) quintuplets arranged in the opposite direction (mmmm, mrrm, etc.). The attribution 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)".

(Melt tension)
Using a Capillograph 1B manufactured by Toyo Seiki Seisakusho, the raw material resin used in the examples and comparative examples was extruded into a string shape under the following conditions, and the tension detected at the pulley when the resin was wound 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. Winding speed: 4.0 m/min. Temperature: 230° C.

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.

(Melt Flow Rate (MFR))
The melt flow rate (MFR) of the raw resin pellets used in the examples and comparative examples was measured using a melt indexer manufactured by Toyo Seiki Seisakusho Co., Ltd. in accordance with condition M of JIS K 7210. Specifically, a sample weighed to 4 g was inserted into a cylinder at a test temperature of 230° C. and preheated for 3.5 minutes under a load of 2.16 kg. The weight of the sample extruded from the bottom hole in 30 seconds was then measured to determine the MFR (unit: g/10 min or g/10 min). The above measurement was repeated three times, and the average value was taken as the measured MFR.

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

The dynamic viscoelasticity measuring device used was a Seiko Instruments "Viscoelasticity Measuring Device (Model: DMS6100)."

As a measurement sample, the 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 rectangular shape of 40 mm in the longitudinal direction and 8 mm in the transverse direction, and the temperature dependency of the biaxially stretched polypropylene film was measured under the measurement conditions shown below in accordance with JIS-K7244 (1999 edition).
Test mode: tension mode Chuck distance: 20 mm
Vibration frequency: 1 Hz, 2 Hz, 5 Hz, and 10 Hz
Strain amplitude: 10 μm
Minimum tension: 100mN
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 the measurement results, the storage modulus (E') and loss modulus (E'') of the biaxially oriented polypropylene film at a temperature distribution of 70° C. (vibration frequencies of 1 Hz, 2 Hz, 5 Hz, and 10 Hz) were determined.

(How to determine the slope of an 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 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. The slope of the approximate straight line (linear function) specified from the viscoelasticity scatter diagram in which the coordinates of four points (log E'', log E') under each of the conditions are plotted was specified. As a specific method for specifying the slope of the approximate straight line (linear function), it was specified by using the statistical function "SLOPE" of calculation software ("Excel" manufactured by Microsoft).

(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 terminal φ=16 mm, measurement 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 was handled so as not to wrinkle or trap air in the film when cutting. Five measurements were taken for a 10-sheet stack sample, and the thickness was calculated by dividing the average of the five measurements by 10.

(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. In detail, 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.

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

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

K value (intensity ratio %) = Hβ/(Hβ+HαI+HαII+HαIII) x 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 intensity (height) was used after subtracting amorphous scattering.)

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

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

The data obtained was used to calculate the half-width of the diffraction reflection peak of the α crystal (040) plane using an analytical computer and the integrated powder X-ray analysis software PDXL (Ver. 2.1.3.4) that comes standard with the instrument, after optimization using a split pseudo-Voight function. The crystallite size was calculated from the half-width using Scherrer's formula (D = K x λ/(β x cosθ)).

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

(Measurement of surface roughness (Vmc) of roughened surface)
A biaxially oriented polypropylene film has 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.

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 shape measuring device. As a measurement sample, the film was cut into an arbitrary size of about 20 cm square, and after fully smoothing out any wrinkles, it was set on the measurement stage using an electrostatic contact plate or the like. The measurement method is explained in detail below.

First, using WAVE mode, a 530 white filter and a 1x BODY telescope, and a x10 objective lens, measurements are taken of 470.92 μm x 353.16 μm per field of view. This operation is performed on 10 locations at 1 cm intervals in the flow direction from the center in both the flow and width directions of the target sample (biaxially oriented polypropylene film).

Then, the obtained data is subjected to noise removal processing using a median filter (3 x 3), and then subjected to Gaussian filter processing with a cutoff value of 30 μm to remove waviness components. This makes it possible to properly measure the condition of the roughened surface.

Next, the analysis is performed using the "ISO parameters" in the "Bearing" plug-in function of the "VS-Viewer" analysis software for "VertScan 2.0".

Finally, calculate the average Vmc value obtained at the 10 locations. This gives the Vmc value of the roughened surface.

(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.

First, a volume resistivity measuring jig (hereinafter also referred to as the "jig") was placed in a thermostatic chamber in a 120°C environment. The jig was configured as follows. A DC power supply and a DC ammeter were also connected to the jig.

<Volume resistivity measuring tool>
Main electrode (50mm diameter)
Counter electrode (diameter 85 mm)
A ring-shaped guard electrode (outer diameter 80 mm, inner diameter 70 mm) surrounding the main electrode
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 Silicone Co., Ltd., and the glossy side of the conductive rubber was attached so as to come into contact with the gold-plated copper.

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

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

After the voltage was applied, the current value was read one minute later, and the volume resistivity was calculated by the following formula: Voltage was applied using a 2290-10 (DC power source) manufactured by Keithley, and the current value was measured using a 2635B (DC ammeter) manufactured by Keithley.
Volume resistivity = [(effective electrode area) x (applied voltage)] / [(sample thickness) x (current value)]
Here, the effective electrode area was calculated by the following formula.
(Effective electrode area) = Pi × [[[(diameter of main electrode) + (inner diameter of guard electrode)]/2]/2] ²
This was repeated three times, and the arithmetic average value calculated to one significant digit was taken as the volume resistivity (Ω·cm).

(3) Characterization of capacitor elements (life test based on capacitance change rate)
A 620 mm wide biaxially stretched polypropylene film was wound and metal was deposited using a winding vacuum deposition machine (model number: EWE-060) manufactured by ULVAC. Al metal was deposited at 20 Ω/□ on the active part, and Zn metal was deposited at 5 Ω/□ on the heavy edge part to produce a metallized film with an insulating margin and a divided electrode. The metallized film thus produced was then slit into a 30 mm wide strip by a Nuintek slitter (model number: NT-750) to produce a small roll of metallized film. Next, two 30 mm wide metallized films were stacked together using an automatic winder (model number: 3KAW-N2 type) manufactured by Kaito Seisakusho, and the number of turns was set so that the element capacitance was 50 μF under the conditions of a winding speed of 4 m/sec, a winding tension of 180 g, and a contact roller contact pressure of 260 g, and the metallized film was wound. The wound element was flattened by pressing, and then, while still applying a press load, zinc metal was sprayed onto the element end faces to form electrode lead-out portions, and the element was then heat-treated at 120°C for 15 hours to be thermally cured. This produced a capacitor element.

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 was measured, and the capacitance change rate (ΔC/C) was calculated using the following formula.
△C/C(%)=(C'-C)/C×100
The evaluation criteria for the life test were as follows:
AA: Capacity change rate (capacity decrease rate) is 0.5% or lessA: 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 explodes during the testC or above is pass (usable), and A or above is more preferable.

The results of the above-mentioned various characteristic evaluations of the examples and comparative examples are shown in Table 2.

As is clear from the results in Table 2, the biaxially oriented polypropylene films of Examples 1 to 15, which satisfy the requirements of the present invention and have a slope of the 70°C viscoelasticity approximation line of -0.222 or more, can exhibit high volume resistivity even at high temperatures of about 120°C (e.g., 100°C to 125°C), and the present invention can provide a metallized film using the biaxially oriented polypropylene film, and a capacitor with a long life, excellent heat resistance, and small capacitance change even at high temperatures. In detail, the biaxially oriented polypropylene films of Examples 1 to 15 do not contain branched polypropylene resin B, and are found to have a volume resistivity increased by 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 (8)

A biaxially oriented polypropylene film containing a linear polypropylene resin A and a branched polypropylene resin B,
The biaxially stretched 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.
A biaxially oriented polypropylene film. The biaxially oriented polypropylene film according to claim 1, wherein 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/m2 or ^more . 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. The biaxially oriented polypropylene film according to claim 1, which is for use in a capacitor. 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. A capacitor comprising the metallized film of claim 5. A method for producing a biaxially oriented polypropylene film, comprising a step of biaxially stretching a cast sheet containing a linear polypropylene resin A and a branched polypropylene resin B before biaxial stretching,
The biaxially stretched 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.
A method for producing a biaxially oriented polypropylene film. The manufacturing method according to claim 7, in which the longitudinal stretching ratio in the biaxial stretching step is 5.0 times or less.

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