WO2024135552A1

WO2024135552A1 - Polypropylene film

Info

Publication number: WO2024135552A1
Application number: PCT/JP2023/044988
Authority: WO
Inventors: 利海辰喜; 正寿大倉; 聡士藤原
Original assignee: 東レ株式会社
Priority date: 2022-12-19
Filing date: 2023-12-15
Publication date: 2024-06-27

Abstract

Disclosed is a polypropylene film which has a layer A that contains a cyclic olefin resin and a polypropylene resin, wherein if a cross-section X is a cross-section obtained by cutting the polypropylene film at a plane that is parallel to the main orientation axis direction and the thickness direction, the orientation parameter I810/I840 of the cross-section X as determined by Raman spectrometry is 2.2 to 20.　The present invention addresses the problem of providing a polypropylene film which has excellent processability in a high temperature environment and high withstand voltage characteristics regardless of operating temperature, while exhibiting excellent reliability if used as a film capacitor.

Description

Polypropylene Film

The present invention relates to a polypropylene film that is particularly suitable for use in film capacitors.

In recent years, the majority of electrical equipment has been converted to inverters, which has led to an even stronger demand for smaller, larger capacity film capacitors. In particular, in fields such as automobiles (including electric vehicles and hybrid cars), electric aircraft, solar power generation, and wind power generation, these demands have led to demands for films for film capacitors that not only have improved voltage resistance and productivity and maintain suitability for processing in the manufacture of film capacitor elements, but also require thinner films and improved heat resistance.

Among polyolefin-based films, polypropylene film is said to have excellent heat resistance and voltage resistance. When applying to the above fields, it is important that the film has excellent dimensional stability at the operating temperature and stable electrical performance (voltage resistance, etc.) in a temperature range 10°C to 20°C higher than the operating temperature. From the perspective of heat resistance, it is said that the operating temperature will become even higher in the future when considering power semiconductor applications using silicon carbide (SiC).

In light of this, there is a demand for further improvements in the heat resistance and voltage resistance of film capacitors, and for films for film capacitors there is a demand for improvements in the dielectric breakdown voltage in high-temperature environments exceeding 110°C. However, as described in Non-Patent Document 1, the upper limit of the usable temperature for polypropylene film is said to be approximately 110°C, and it has been extremely difficult for polypropylene film to stably maintain voltage resistance in such a temperature environment.

In order to miniaturize film capacitors and improve their heat resistance, it is possible to use thinner films, films with higher dielectric constants, or films with glass transition temperatures that exceed the operating temperature range of the film capacitor.

For example, a laminate has been proposed in which two layers with different dielectric constants are alternately stacked, one layer being a cyclic olefin resin with a glass transition temperature exceeding 130°C, and the other being a polypropylene layer, thereby providing heat resistance and voltage resistance while maintaining a large electrostatic capacity (e.g., Patent Document 1). Also, a film has been proposed in which the processability is improved by co-extrusion and co-stretching when forming a laminate of a cyclic olefin resin and polypropylene (e.g., Patent Documents 2 and 3). Furthermore, a film has been proposed in which the thermal dimensional stability in a high-temperature environment is improved by blending a cyclic olefin resin and a polypropylene resin, forming a film, and biaxially stretching the film (e.g., Patent Document 4).

JP 2015-012076 A International Publication No. 2017/022706 JP 2018-034510 A Special Publication No. 2020-521867

However, the film of Patent Document 1 is not a laminate by coextrusion, but a laminate in which a cyclic olefin resin layer is formed on a polypropylene film by a coating method. Therefore, the cyclic olefin resin layer is easily peeled off, and the processability in a high-temperature environment and the performance and reliability when used as a film capacitor are not sufficient. The base layer of the laminated structure of the film of Patent Document 2 is also a single cyclic olefin resin. Therefore, it is difficult to increase the areal stretch ratio, and the voltage resistance in a high-temperature environment is insufficient, so the performance and reliability when used as a film capacitor are not sufficient. The base layer of the laminated structure of the film of Patent Document 3 is also a cyclic olefin resin, and an elastomer is contained to improve the stretchability to increase the areal stretch ratio, but the voltage resistance in a high-temperature environment is not satisfactory, and the performance and reliability when used as a film capacitor are not sufficient. The film of Patent Document 4 is a film simply made by blending a cyclic olefin resin and a polypropylene resin, so it is difficult to increase the areal stretch ratio. As a result, the voltage resistance in high-temperature environments was insufficient, and the performance and reliability of the film capacitors were not sufficient. In addition, although it is possible to stretch at a high areal stretch ratio by increasing the preheating temperature and stretching temperature during width-direction stretching, there was also the issue that the film stretched at a high temperature had a large decrease in voltage resistance when going from room temperature to high temperatures, resulting in unstable characteristics when used as a film capacitor.

The present invention aims to provide a polypropylene film that has excellent processability in high-temperature environments, high voltage resistance regardless of the operating temperature, and excellent reliability when used as a film capacitor.

The inventors conducted extensive research to solve the above problems, and came to invent a polypropylene film in which, when a layer containing a cyclic olefin resin and a polypropylene resin is designated as layer A and a cross section obtained by cutting the polypropylene film along a plane parallel to the main orientation axis direction and the thickness direction is designated as cross section X, the polypropylene film has layer A and the orientation parameter I810/I840 measured by Raman spectroscopic analysis of cross section X is 2.2 or more and 20 or less.

The present invention provides a polypropylene film that has excellent processability in high-temperature environments, high voltage resistance regardless of the operating temperature, and excellent reliability when used as a film capacitor.

FIG. 1 is a schematic diagram showing a rectangle of 1 μm×2 μm size defined so that a pair of short sides are parallel to the thickness direction within the cross section X of a polypropylene film of the present invention, and a domain of a cyclic olefin resin passing through a pair of sides of the rectangle parallel to the thickness direction. FIG. 1 shows TEM images (magnification: 20,000 times) of the layer A portion of the cross section X of a polypropylene film according to an embodiment of the present invention (Example 5), in which the treatment described up to ii) of (16) was performed (left) and the treatment described up to v) of (16) was performed (right).

The inventors conducted extensive research to solve the above-mentioned problems, and came to the following conclusions about why the films described in Patent Documents 1 to 4 above do not have sufficient dielectric breakdown voltage in high-temperature environments, sufficient voltage resistance and reliability in high-temperature environments when used as a film capacitor, and sufficient processability in high-temperature environments.

The film of Patent Document 1 is an unstretched film laminated by a coating method, and therefore it is considered to have problems such as peeling between layers in a high-temperature environment, insufficient mechanical properties, particularly breaking elongation, which makes it prone to breaking during processing of the film capacitor element, and reduced voltage resistance in a high-temperature environment. Assuming that the films of

Patent Documents

2 and 3 have problems with voltage resistance in a high-temperature environment, the ratio of stretching in the vertical direction during film production is insufficient in both cases, and the film has a large amount of mobile amorphous components, resulting in a low breakdown voltage at high temperatures. Assuming that the film of Patent Document 4 has problems with voltage resistance in a high-temperature environment, the mixing of the cyclic olefin resin and the polypropylene resin is insufficient, making it difficult to sufficiently increase the ratio of stretching in the vertical direction during film production, and the film has a large amount of mobile amorphous components, resulting in a low breakdown voltage at high temperatures. In addition, by increasing the preheating temperature and stretching temperature during widthwise stretching, it becomes possible to stretch at a high areal stretch ratio, but it was thought that there would be a problem in that a film stretched at a high temperature would experience a large decrease in voltage resistance from room temperature to high temperatures, and the characteristics would be unstable when used as a film capacitor.

In light of the above considerations, the inventors of the present invention have further studied the matter and have come up with the invention of a polypropylene film that solves the above problems. The polypropylene film of the present invention is a polypropylene film in which, when a layer containing a cyclic olefin resin and a polypropylene resin is designated as layer A and a cross section obtained by cutting the polypropylene film along a plane parallel to the main orientation axis direction and the thickness direction is designated as cross section X, the polypropylene film has layer A and has an orientation parameter I810/I840 of 2.2 or more and 20 or less as measured by Raman spectroscopic analysis of cross section X.

The polypropylene film of the present invention will be described in detail below. Note that when the upper and lower limits of the preferred ranges are stated separately below, the combination can be arbitrary. Here, polypropylene film refers to a sheet-like molded product whose main component is polypropylene resin, and the main component refers to a component that is contained in an amount of more than 50% by mass and not more than 100% by mass when all components constituting the film are taken as 100% by mass. Note that when the film contains multiple components equivalent to polypropylene resin, even if each component is less than 50% by mass, as long as the total of these components exceeds 50% by mass, it is considered that the main component is polypropylene resin.

Polypropylene resin is a resin that contains more than 50 mol% and up to 100 mol% of propylene units when the total constituent units of the resin are taken as 100 mol%, and does not fall under the category of cyclic olefin resin.

Cyclic olefin resin refers to a polyolefin resin that contains 10 mol% to 100 mol% of cyclic olefin units when all the constituent units constituting the resin are taken as 100 mol%. Note that for resins that contain multiple types of constituent units equivalent to cyclic olefin units, even if each cyclic olefin unit is less than 10 mol%, as long as the total of these constituent units is 10 mol% or more, it is considered to be a cyclic olefin resin.

In the following, polypropylene film may be simply referred to as film. The polypropylene film of the present invention is not a microporous film and does not have many pores. In other words, the polypropylene film of the present invention means a polypropylene film other than a microporous film. Here, a microporous film is defined as a film that has a pore structure penetrating both surfaces of the film and has a degree of air permeability of 5,000 seconds/100 ml or less when measured at a temperature of 23°C and a relative humidity of 65% using a type B Gurley tester according to JIS P 8117 (1998).

The main orientation direction in the polypropylene film of the present invention is described below. The main orientation direction refers to the direction in which the molecular chain orientation of the polypropylene resin is greatest within the film plane. When biaxial stretching is performed in the production of polypropylene film, stretching is usually performed in the longitudinal and width directions, and generally, the direction of the larger stretching ratio becomes the main orientation direction. If the stretching directions (longitudinal and width directions) are specified but the ratio is unknown, the maximum load until breakage is measured for each direction in a tensile test at 23°C described below, and the direction with the larger measured value can be determined as the main orientation direction.

As mentioned above, if there is a certain degree of difference in the stretch ratio, the main orientation axis direction can be easily identified if the stretch direction and stretch ratio are known, but in the case of a film whose stretch direction and stretch ratio are unknown or a film whose stretch ratios in two directions are almost equal, the main orientation axis direction can be identified by the following method. Specifically, a rectangle measuring 50 mm in length and 10 mm in width is cut out to form sample <1>, and the direction of the long side of sample <1> is defined as 0°. Next, a rectangular sample <2> of the same size is taken so that the long side direction is rotated 15° to the right from the 0° direction, and similarly, rectangular samples <3> to <12> are taken by rotating the long side direction of the rectangular samples by 15° each. Next, each rectangular sample is set in a tensile tester with an initial chuck distance of 20 mm so that the long side direction is the pulling direction (measurement direction), and a tensile test is performed at a tensile speed of 300 mm/min in a room temperature atmosphere. At this time, the maximum load until the sample breaks is read, and the maximum point strength stress is calculated by dividing the load by the cross-sectional area of the sample before the test (film thickness x width). The direction of the long side of the sample where this value was maximum is defined as the main orientation axis of the polypropylene film, and the direction perpendicular to this in the film plane is defined as the direction perpendicular to the main orientation axis of the polypropylene film.

If the sample width is less than 50 mm and the above tensile test cannot be performed, the crystal orientation of the α crystal (110) plane can be measured using wide-angle X-rays as follows, and the main orientation axis direction can be determined based on the following criteria. That is, X-rays (CuKα rays) are incident perpendicularly to the film surface, and the crystal peak at 2θ = approximately 14° (α crystal (110) plane) is scanned in the circumferential direction, and the direction with the highest diffraction intensity in the obtained diffraction intensity distribution is determined as the main orientation axis direction, and the direction perpendicular to this in the film plane can also be determined as the direction perpendicular to the main orientation axis direction.

The polyolefin film of the present invention has a layer A containing a cyclic olefin resin and a polypropylene resin, and when a cross section X is a cross section obtained by cutting the polypropylene film in a plane parallel to the main orientation axis direction and the thickness direction, the polyolefin film has a layer A, and the orientation parameter I810/I840 measured by Raman spectroscopy analysis of the cross section X is 2.2 or more and 20 or less. Here, the thickness direction means the direction perpendicular to the film surface. By keeping the orientation parameter I810/I840 of the cross section X within this range, a polypropylene film can be obtained that exhibits a small decrease in withstand voltage from room temperature to high temperatures. Note that hereinafter, the orientation parameter I810/I840 of the polypropylene film may be simply referred to as I810/I840.

I810/I840 is a parameter that reflects the tension state of the polypropylene chains in the polypropylene film, and the higher it is, the more the polypropylene chains can be relaxed in a high-temperature environment and the lowering of the voltage resistance can be suppressed. If I810/I840 is less than 2.2, the polypropylene chains in the polypropylene film will relax when heated, causing a problem of a decrease in the voltage resistance when the film is used as a film capacitor in a high-temperature environment. From the viewpoint of suppressing the decrease in the voltage resistance of the polypropylene film from room temperature to high temperatures, I810/I840 is preferably 2.8 or more, more preferably 3.5 or more, even more preferably 4.2 or more, particularly preferably 4.7 or more, and most preferably 5.0 or more. On the other hand, as mentioned above, the higher I810/I840 is the better, but from the viewpoint of feasibility, I810/I840 is 20 or less, preferably 15 or less, more preferably 10 or less, and even more preferably 7.0 or less.

I810/I840 can be measured by Raman spectroscopy, using polarized Raman measurement (beam diameter 1 μm, measured with polarized light parallel to the main orientation axis direction) from the surface of the polypropylene film. The measurement position is the center position in the thickness direction of the polypropylene film, and details of the measurement device and method will be described later.

The method for making I810/I840 2.2 or more and 20 or less in a polypropylene film containing a cyclic olefin resin is not particularly limited, but for example, it is effective to manufacture the polypropylene film by a sequential biaxial stretching method, set the stretching ratio in the width direction to 8.5 times or more, and perform width direction stretching by the following two-stage stretching method (I). It is also effective to use a polypropylene resin with high stereoregularity and melting point (for example, a polypropylene resin with a high mesopentad fraction and a high melting point). These methods may be used in combination as appropriate.

The two-stage drawing method (I) refers to a drawing method in which the following second-stage drawing is performed after the following first-stage drawing. That is, in the two-stage drawing method (I), a × b times drawing is performed in total in the first-stage drawing and the second-stage drawing (both of which are described below).
Two-stage stretching method (I):
First-stage stretching: When the final stretching ratio in the width direction is a × b times, the film is stretched in the width direction at a temperature of 160°C or higher and 185°C or lower (the lower limit is preferably 170°C, and the upper limit is preferably 180°C) and a stretching ratio of a times.
Second-stage drawing: The film is further drawn in the width direction at a draw ratio of b times at a temperature 1° C. to 20° C. lower than the temperature in the first-stage drawing.

The difference between the temperature of the second stretching stage and the temperature of the first stretching stage is preferably 5°C or more from the viewpoint of increasing I810/I840, and is preferably 10°C or less from the viewpoint of improving film formability. Note that the "final stretching ratio in the width direction" refers to the stretching ratio immediately after the second stretching stage is completed (i.e., the variation in stretching ratio due to the relaxation treatment is not taken into consideration).

In the polypropylene film of the present invention, the length in the main axis direction of the cyclic olefin resin domain in the cross section X is preferably 5.0 μm or more and 1 mm or less. By making the length in the main axis direction of the cyclic olefin resin domain in the cross section X 5.0 μm or more, the shape stability when used as a film capacitor at high temperatures for a long time can be improved. The length in the main axis direction of the cyclic olefin resin domain in the cross section X is preferably as long as possible from the viewpoint of shape stability when used as a film capacitor at high temperatures for a long time, but is 1 mm or less, preferably 30 μm or less from the viewpoint of realization. The length in the main axis direction of the cyclic olefin resin domain in the cross section X is more preferably 7.0 μm or more, even more preferably 10.0 μm or more, and particularly preferably 10.5 μm or more, from the viewpoint of improving shape stability (reliability) and improving life when used as a film capacitor at high temperatures for a long time.

The method for keeping the length of the main axis direction of the domain of the cyclic olefin resin in the cross section X between 5.0 μm and 1 mm is not particularly limited, but a method of stretching by a sequential biaxial stretching method so that the stretch ratio in the width direction is 8.0 times or more, preferably 8.5 times or more, is effective. Here, the domain length can be measured from an image of the cross section X obtained using a transmission electron microscope (TEM), and the details will be described later.

The polypropylene film of the present invention preferably has a structure in which the cyclic olefin resin is diffused into the sea portion of the polypropylene resin in the A layer portion of the cross section X (hereinafter, sometimes referred to as a diffusion structure). When observing the A layer portion of the cross section X with a TEM, such a diffusion structure is observed as a brightness intermediate between the cyclic olefin resin, which is observed as black, and the polypropylene resin, which is observed as white. Herein, the "diffusion structure" in the A layer portion of the cross section X means that when the observed image of the A layer portion of the cross section X is binarized using the method described below, the proportion of pixels with a brightness of 255 out of all pixels is 0.01% or more. Hereinafter, "pixels with a brightness of 255 when the observed image of the A layer portion of the cross section X is binarized using the method described below" may simply be referred to as white pixels in the A layer portion after binarization.

When the polypropylene film of the present invention has such a diffusion structure, the relaxation of the molecular chains of the polypropylene resin that occurs during long-term use in a high-temperature, high-voltage environment is suppressed. Therefore, when such a polypropylene film is used as a dielectric for a film capacitor, it is possible to easily reduce the deterioration of the reliability of the film capacitor. From the above perspective, the proportion of white pixels in the A layer portion after binarization is preferably 5% or more of all pixels, and more preferably 15% or more. On the other hand, from the perspective of improving thermal stability and voltage resistance by the domain structure and from the perspective of feasibility, it is preferable that the proportion of white pixels in the A layer portion after binarization is 50% or less.

The method of forming such a diffusion structure in the A layer of the polypropylene film of the present invention is not particularly limited, but includes a method in which the ratio of cyclic olefin resin to polypropylene resin in the A layer is set within the range described below, and the film is stretched at a suitable areal stretch ratio described below using a sequential biaxial stretching method, and then stretched in the width direction (transverse stretching) at a temperature at least 5°C higher than the glass transition temperature of the cyclic olefin resin used.

The polypropylene film of the present invention preferably has a dielectric loss tangent of 3×10 ⁻⁶ or more and 1×10 ⁻² or less. From the viewpoint of suppressing heat generation when the polypropylene film is used as a dielectric of a film capacitor and improving the reliability of the film capacitor, the dielectric loss tangent is preferably 1×10 ⁻² or less, more preferably 1×10 ⁻³ or less, even more preferably 5×10 ⁻⁴ or less, and particularly preferably 3×10 ⁻⁴ or less. On the other hand, from the above viewpoint, the lower the dielectric loss tangent, the more preferable it is, but from the viewpoint of feasibility, it is preferably 3×10 ⁻⁶ or more, more preferably 1×10 ⁻⁵ or more. The dielectric loss tangent of the polypropylene film can be measured in accordance with JIS C2138-2007, and the details thereof will be described later.

The method for keeping the dielectric loss tangent of the polypropylene film of the present invention within the range of 3×10 ⁻⁶ to 1×10 ⁻² is not particularly limited, but it is effective to reduce the content of components other than polypropylene contained in the polypropylene film. In particular, when the mass of the entire film is taken as 100 mass%, the content of such components is preferably 40 mass% or less, more preferably 30 mass% or less, even more preferably 20 mass% or less, particularly preferably 9.5 mass% or less, and most preferably 9.0 mass% or less.

In the polypropylene film of the present invention, it is preferable that in a rectangle of 1 μm×2 μm size defined in the A layer portion of the cross section X so that a pair of short sides are parallel to the thickness direction, there are 2.0 to 1000 domains of cyclic olefin resin passing through a pair of short sides. Hereinafter, a method for defining a rectangle of 1 μm×2 μm size in the cross section X of the polypropylene film of the present invention so that a pair of short sides are parallel to the thickness direction, and a method for determining the number of domains of cyclic olefin resin passing through a pair of short sides of the rectangle will be described with reference to the drawings (details of the observation conditions with TEM, etc. will be described later).

Figure 1 is a schematic diagram showing a 1 μm x 2 μm rectangle with a pair of sides parallel to the thickness direction in the cross section X of the polypropylene film of the present invention, and a domain of a cyclic olefin resin passing through a pair of short sides of the rectangle. References 1 to 5 in Figure 1 respectively represent a part of the cross section X, a sea part, an island part (domain), a 1 μm x 2 μm rectangle with a pair of sides parallel to the thickness direction in the cross section X, and a pair of short sides of the rectangle. The left side of Figure 1 is a part of the cross section X, and the right side is an enlarged view of a 1 μm x 2 μm rectangle indicated by a dashed line in the cross section X with a pair of short sides parallel to the thickness direction. In the polypropylene film of the present invention, the sea part is polypropylene resin, and the island part is cyclic olefin resin.

When defining a rectangle of 1 μm x 2 μm size within cross section X so that a pair of short sides are parallel to the thickness direction, the base of the rectangle is set in the sea portion, and if a domain is located on the side opposite the base, it is considered not to exist and is not counted in the number (in the example of Figure 1, no such domain exists).

Here, "domains of cyclic olefin resin passing through a pair of short sides parallel to the thickness direction" refers to domains of cyclic olefin resin passing through both a pair of short sides parallel to the thickness direction. That is, in the example of Figure 1 (right), the first, fifth, and seventh domains from the top fall into this category, but the second to fourth and sixth domains from the top do not, so there are three "domains of cyclic olefin resin passing through a pair of short sides parallel to the thickness direction" in this example.

By making the number of domains of such cyclic olefin resin 2.0 or more, the cyclic olefin resin is finely dispersed in a flatter shape within the plane. As a result, the high thermal stability of the cyclic olefin resin and the high voltage resistance of the polypropylene resin can be reflected in the polypropylene film, and the dielectric breakdown voltage of the polypropylene film in a high-temperature environment can be increased. Furthermore, when such a polypropylene film is used in a film capacitor, short circuit breakdown is less likely to occur, even when used for long periods of time, especially in high-temperature environments, and the voltage resistance of the film capacitor is maintained, resulting in high reliability.

The method of keeping the number of domains of such cyclic olefin resin within a suitable range is not particularly limited, but for example, when the mass of the entire film is taken as 100 mass%, it is effective to set the content of cyclic olefin resin to 1.0 mass% or more and 30 mass% or less, set the temperature of the filter lower than the extrusion temperature in the process of extruding a resin composition containing polypropylene resin and cyclic olefin resin, and perform biaxial stretching so that the areal stretching ratio is 40 times or more. The areal stretching ratio is more preferably 45 times or more, even more preferably 50 times or more, particularly preferably 54 times or more, and most preferably 60 times or more. It is also effective to compound the polypropylene resin and the cyclic olefin resin in advance before melting in the extruder to increase dispersibility. In the polypropylene film of the present invention, the number of domains of such cyclic olefin resin is preferably 4.0 or more, more preferably 6.0 or more, and even more preferably 7.0 or more from the above viewpoints. On the other hand, the upper limit of the number of domains of the cyclic olefin resin is 1000 from the viewpoint of feasibility, and more preferably 20.

In the polypropylene film of the present invention, when the mass of the entire film is taken as 100% by mass, the content of the cyclic olefin resin is preferably 1.0% by mass or more and 40% by mass or less. From the viewpoint of suppressing film rupture during stretching and increasing productivity, and from the viewpoint of appropriately controlling the number of domains of the cyclic olefin resin passing through the pair of short sides, the content of the cyclic olefin resin in the polypropylene film is more preferably 30% by mass or less, even more preferably 20% by mass or less, particularly preferably 9.5% by mass or less, and most preferably 9.0% by mass or less.

Generally, cyclic olefin resins are more expensive than polypropylene, so it is industrially preferable to obtain the desired effect with a smaller content, particularly 9.5 mass% or less. On the other hand, from the viewpoint of improving heat resistance when used as a film capacitor, it is more preferable that the content of cyclic olefin resin in the polypropylene film is 5.0 mass% or more. By keeping the content of cyclic olefin resin in the polypropylene film between 1.0 mass% and 40 mass% or within the above preferred range, it becomes easy to obtain a polypropylene film that has high dimensional stability when used for long periods of time under high temperature and high voltage, has high reliability when used as a film capacitor, and has excellent productivity.

The content of the cyclic olefin resin in layer A of the polypropylene film of the present invention is preferably 0.5% by mass or more, more preferably 1% by mass or more, even more preferably 4% by mass or more, and particularly preferably 7% by mass or more, when the total components of layer A are taken as 100% by mass, from the viewpoint of increasing the thermal dimensional stability at high temperatures and increasing the reliability of the film capacitor when used as a dielectric of the film capacitor. On the other hand, the content of the cyclic olefin resin in layer A is preferably 38% by mass or less, more preferably 34% by mass or less, even more preferably 25% by mass or less, particularly preferably 19.5% by mass or less, and most preferably 9.8% by mass or less, when the total components of layer A are taken as 100% by mass, from the viewpoint of making it difficult for the film to break when the areal stretch ratio is increased during stretching. When the polypropylene film of the present invention is formed by laminating layers having different compositions of cyclic olefin resins, the layer with the highest content of cyclic olefin resin is taken as layer A.

In order to prevent film rupture when the areal stretch ratio is increased, the polypropylene film of the present invention preferably has a laminated structure in which a layer having a lower cyclic olefin resin content than layer A (hereinafter referred to as layer B) is laminated on at least one side of the outermost layer, and more preferably has layer B laminated on both sides. When the total mass of layer B is taken as 100 mass%, the cyclic olefin resin content of layer B is less than that of layer A, preferably 3 mass% or less, more preferably 1 mass% or less, and most preferably layer B does not contain cyclic olefin resin. When layer B is laminated on both sides, the composition of layer B may be the same or different.

As the cyclic olefin resin to be contained in the polypropylene film of the present invention, a cyclic olefin polymer or a cyclic olefin copolymer can be preferably used. Among them, from the viewpoint of increasing compatibility with polypropylene resin, it is more preferable to use a cyclic olefin copolymer in which a chain olefin monomer such as ethylene or propylene is copolymerized with a cyclic olefin such as norbornene, norbornadiene, tetracyclododecene, or a derivative thereof.

The details of the cyclic olefin resin contained in the polypropylene film of the present invention will be described. Cyclic olefin monomers that can be used to manufacture the cyclic olefin resin used in the polypropylene film of the present invention include monocyclic olefins such as cyclobutene, cyclopentene, cycloheptene, cyclooctene, cyclopentadiene, and 1,3-cyclohexadiene, bicyclo[2,2,1]hept-2-ene, 5-methyl-bicyclo[2,2,1]hept-2-ene, and 5,5-dimethyl-bicyclo[2,2,1]hept-2-ene. 5-ethyl-bicyclo[2,2,1]hept-2-ene, 5-butyl-bicyclo[2,2,1]hept-2-ene, 5-ethylidene-bicyclo[2,2,1]hept-2-ene, 5-hexyl-bicyclo[2,2,1]hept-2-ene, 5-octyl-bicyclo[2,2,1]hept-2-ene, 5-octadecyl-bicyclo[2,2,1]hept-2-ene, 5-methylidene-bicyclo[2,2,1]hept bicyclic olefins such as 5-vinyl-bicyclo[2,2,1]hept-2-ene, 5-vinyl-bicyclo[2,2,1]hept-2-ene, and 5-propenyl-bicyclo[2,2,1]hept-2-ene; tricyclo[4,3,0,12.5]deca-3,7-diene, tricyclo[4,3,0,12.5]deca-3-ene, tricyclo[4,3,0,12.5]undeca-3,7-diene, tricyclo[4,3,0,12.5]undeca-3,8-diene, tricyclo[ tricyclic olefins such as tetracyclo[4,4,0,12.5,17.10]dodec-3-ene, 8-methyltetracyclo[4,3,0,12.5]undec-3-ene, 5-cyclopentyl-bicyclo[2,2,1]hept-2-ene, 5-cyclohexyl-bicyclo[2,2,1]hept-2-ene, 5-cyclohexenyl-bicyclo[2,2,1]hept-2-ene, and 5-phenyl-bicyclo[2,2,1]hept-2-ene; b) [4,4,0,12.5,17.10] dodec-3-ene, 8-ethyltetracyclo [4,4,0,12.5,17.10] dodec-3-ene, 8-methylidenetetracyclo [4,4,0,12.5,17.10] dodec-3-ene, 8-ethylidenetetracyclo [4,4,0,12.5,17.10] dodec-3-ene, 8-vinyltetracyclo [4,4,0,12.5,17.10] dodec-3-ene, 8-propenyl tetracyclic olefins such as tetracyclo[4,4,0,12.5,17.10]dodec-3-ene, 8-cyclopentyl-tetracyclo[4,4,0,12.5,17.10]dodec-3-ene, 8-cyclohexyl-tetracyclo[4,4,0,12.5,17.10]dodec-3-ene, 8-cyclohexenyl-tetracyclo[4,4,0,12.5,17.10]dodec-3-ene, 8-phenyl-cyclopentyl- Tetracyclo[4,4,0,12.5,17.10]dodec-3-ene, tetracyclo[7,4,13.6,01.9,02.7]tetradeca-4,9,11,13-tetraene, tetracyclo[8,4,14.7,01.10,03.8]pentadeca-5,10,12,14-tetraene, pentacyclo[6,6,13.6,02.7,09.14]-4-hexadecene, pentacyclo[6,5,1,13.6,02.7,0 9.13]-4-pentadecene, pentacyclo[7,4,0,02.7,13.6,110.13]-4-pentadecene, heptacyclo[8,7,0,12.9,14.7,111.17,03.8,012.16]-5-eicosene, heptacyclo[8,7,0,12.9,03.8,14.7,012.17,113.16]-14-eicosene, and polycyclic olefins such as tetramers such as cyclopentadiene. These cyclic olefin monomers can be used alone or in combination of two or more.

　Among the above, from the viewpoints of productivity and surface properties, the following cyclic olefin monomers are preferably used: tricyclic olefins having 10 carbon atoms, such as bicyclo[2,2,1]hept-2-ene (hereinafter referred to as norbornene), tricyclo[4,3,0,12.5]dec-3-ene (hereinafter referred to as tricyclodecene), tetracyclic olefins having 12 carbon atoms, such as tetracyclo[4,4,0,12.5,17.10]dodec-3-ene (hereinafter referred to as tetracyclododecene), cyclopentadiene, or 1,3-cyclohexadiene.

As long as the cyclic olefin resin satisfies the above definition, it may be either a resin obtained by polymerizing only the above cyclic olefin monomer (hereinafter sometimes referred to as COP) or a resin obtained by copolymerizing the above cyclic olefin monomer with a chain olefin monomer (hereinafter sometimes referred to as COC).

　COP can be produced by known methods such as addition polymerization or ring-opening polymerization of cyclic olefin monomers. For example, there is a method of subjecting norbornene, tricyclodecene, tetracyclodecene, and their derivatives to ring-opening metathesis polymerization followed by hydrogenation, a method of addition polymerization of norbornene and its derivatives, and a method of subjecting cyclopentadiene and cyclohexadiene to 1,2-, 1,4-addition polymerization followed by hydrogenation. Among these, from the standpoint of productivity and moldability, the method of subjecting norbornene, tricyclodecene, tetracyclodecene, and their derivatives to ring-opening metathesis polymerization followed by hydrogenation is more preferable.

In the case of COC, preferred chain olefin monomers include ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 3-methyl-1-butene, 3-methyl-1-pentene, 3-ethyl-1-pentene, 4-methyl-1-pentene, 4-methyl-1-hexene, 4,4-dimethyl-1-hexene, 4,4-dimethyl-1-pentene, 4-ethyl-1-hexene, 3-ethyl-1-hexene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene, 1-eicosene, etc. Among these, ethylene or both ethylene and propylene can be particularly preferably used from the viewpoints of productivity and cost. In addition, methods for producing resins obtained by copolymerizing cyclic olefin monomers with chain olefin monomers include known methods such as addition polymerization of cyclic olefin monomers with chain olefin monomers, such as a method of addition polymerization of norbornene or its derivatives with ethylene.

In particular, from the viewpoint of productivity and moldability, a method of binary or terpolymerization of norbornene, tetracyclododecene, or a derivative thereof with ethylene and/or propylene can be used as a preferred method. That is, for example, binary polymerization of a tetracyclododecene derivative with ethylene, or terpolymerization of norbornene, ethylene, and propylene can be used as a preferred method.

The chain olefin monomer and the cyclic olefin monomer for obtaining the cyclic olefin copolymer may each be one type, or one or both may be two or more types. Among them, the use of a cyclic olefin copolymer having ethylene and propylene as structural units is particularly preferred from the viewpoint of increasing the compatibility between the cyclic olefin resin and the polypropylene resin. As the cyclic olefin monomer, it is preferred to use a norbornene derivative, norbornadiene, or a derivative thereof, from the viewpoint of increasing the heat resistance when made into a capacitor.

The polypropylene film of the present invention preferably has a crystallization temperature (Tmc) during the cooling process measured by differential scanning calorimetry of 110°C or more and 150°C or less, and more preferably a Tmc of 112°C or more and 150°C or less. By setting the Tmc to 110°C or more, the film before stretching becomes a structure having a fine crystal structure, so that after stretching, especially after biaxial stretching, dense protrusions are formed on the surface of the polypropylene film, and productivity is also improved. Such protrusions give the polypropylene film excellent voltage resistance at high temperatures. The upper limit of Tmc is 150°C or less from the viewpoint of feasibility. Tmc can be measured in accordance with JIS K7121-1987, and details will be described later.

The method of increasing the Tmc to 110°C or higher is not particularly limited, but includes a method of incorporating a crystal nucleating agent that promotes the crystallization of the polypropylene resin into the polypropylene film. Examples of crystal nucleating agents that can be used in the polypropylene film of the present invention include sorbitol-based nucleating agents, nonitol-based nucleating agents, amide-based nucleating agents, metal salts of aromatic carboxylic acids, metal salts of phosphates, polypropylene resins having a crosslinked structure, and branched polypropylene resins. From the viewpoint of suppressing film rupture during production due to foreign matter, it is preferable to use branched polypropylene resin.

Here, the branched polypropylene resin refers to a polypropylene resin containing a polypropylene molecular chain having at least one side chain with 6 or more carbon atoms in the molecular chain, and the presence of the branched structure can be confirmed by ¹³ C-NMR. The branched polypropylene resin acts as a nucleating agent to accelerate crystallization when solidifying the molten polypropylene resin. Therefore, by incorporating the branched polypropylene resin, more uniform crystallization of the film is promoted, and it becomes easy to stretch at a high ratio while suppressing the formation of film breakage and voids during stretching.

From the viewpoint of obtaining a nucleating agent effect, the content of the branched polypropylene resin is preferably 0.01% by mass or more, more preferably 0.1% by mass or more, and even more preferably 0.3% by mass or more, when the mass of the film is taken as 100% by mass. On the other hand, from the viewpoint of reducing film breakage during extrusion due to an increase in melt tension, the content of the branched polypropylene resin is preferably 50% by mass or less, more preferably 30% by mass or less, even more preferably 10% by mass or less, and particularly preferably 7.0% by mass or less, when the mass of the film is taken as 100% by mass.

Examples of branched polypropylene resins that can be used in the polypropylene film of the present invention include "Daploy" (registered trademark) WB135HMS manufactured by Borealis AG and "WAYMAX" (registered trademark) MFX6 manufactured by Japan Polypropylene Corporation.

The thickness of the polypropylene film of the present invention is preferably 0.5 μm or more and 15 μm or less from the viewpoint of improving heat resistance and miniaturizing the film capacitor. From the above viewpoint, the thickness of the polypropylene film is more preferably 6.0 μm or less, even more preferably 5.0 μm or less, particularly preferably 3.5 μm or less, and most preferably 3.0 μm or less. By making the thickness of the polypropylene film 15 μm or less, the effect of increasing the heat resistance of the cyclic olefin resin can be increased, the voltage resistance in a high-temperature environment can be increased, and the element size can be reduced when made into a film capacitor. The thickness of the polypropylene film can be measured with a known electronic micrometer, and details will be described later.

From the viewpoint of improving processability, the polypropylene film of the present invention preferably has a value of tear strength in the direction perpendicular to the main orientation axis/tear strength in the main orientation axis direction (hereinafter sometimes referred to as tear strength ratio) of 0.10 or more and 10.0 or less. From the above viewpoint, the tear strength ratio is more preferably 0.40 or more, even more preferably 0.50 or more, and particularly preferably 0.80 or more. By making the tear strength ratio 0.10 or more, it becomes easier to suppress breakage when the polypropylene film is slit into strips along the main orientation axis direction. The upper limit of the tear strength ratio is not particularly limited, but from the viewpoint of feasibility, it is preferably 10.0, more preferably 2.0.

There is no particular limitation on the method for achieving a tear strength of 0.10 or more, but it is effective to perform sequential biaxial stretching so that the stretch ratio in the width direction is 3.0 times or less, and preferably 2.0 times or less, relative to the stretch ratio in the longitudinal direction, and to keep the content of cyclic olefin resin within the preferred range mentioned above. The tear strength ratio can be measured using a known tear tester in accordance with JIS K 7128-2:1998, and details will be described later.

The polypropylene film of the present invention preferably has an ash content of 0.0 ppm or more and 1000 ppm or less when the total mass of the polypropylene film of the present invention is taken as 100% by mass. By controlling the ash content of the polypropylene film of the present invention within a suitable range, the reliability of the film capacitor can be improved when the film is used as a dielectric of the film capacitor. From the above viewpoint, the lower the ash content, the more preferable, more preferably 500 ppm or less, even more preferably 200 ppm or less, particularly preferably 100 ppm or less, and most preferably 55 ppm. Theoretically, the ash content is 0.0 ppm or more, and from the viewpoint of feasibility, it is preferably 10 ppm.

The ash content can be measured in accordance with JIS K 7250-1:2006, the details of which will be described later. In addition, the method for controlling the ash content to the preferred range is not particularly limited, but it is effective to reduce the ash content of the polypropylene resin, which is the main component for producing the polypropylene film of the present invention.

The polypropylene film of the present invention may contain various additives, such as organic particles, inorganic particles, crystal nucleating agents, antioxidants, heat stabilizers, chlorine scavengers, slipping agents, antistatic agents, antiblocking agents, fillers, viscosity modifiers, and color inhibitors, as long as the additives do not impair the object of the present invention. These additives may be used alone or in combination, and when multiple layers are present, they may be added to any of the layers.

When antioxidants are included among these additives, the type and amount of antioxidant added are important from the viewpoint of long-term heat resistance of polypropylene film. From the above viewpoint, it is preferable to use one or more sterically hindered phenolic antioxidants, and at least one of them is preferably a high molecular weight type with a molecular weight of 500 or more. Specific examples include various types, but it is preferable to use 2,6-di-t-butyl-p-cresol (BHT: molecular weight 220.4) in combination with 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene (e.g., BASF's "Irganox" (registered trademark) 1330: molecular weight 775.2) or tetrakis[methylene-3(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane (e.g., BASF's "Irganox" (registered trademark) 1010: molecular weight 1,177.7).

The total content of high molecular weight antioxidants with a molecular weight of 500 or more is preferably in the range of 0.1 to 1.0 parts by mass relative to the total amount of resin. If the amount of antioxidant is too small, the long-term heat resistance may be poor, and if the amount of antioxidant is too large, blocking at high temperatures due to bleed-out of these antioxidants may have an adverse effect on the film capacitor element. From the above perspective, the more preferred content is 0.2 to 0.7 parts by mass, and even more preferably 0.3 to 0.5 parts by mass, relative to 100 parts by mass of the total resin. If the polypropylene film has a laminated structure of two or more layers, it is preferable that each layer contains 0.3 to 0.5 parts by mass of high molecular weight antioxidants with a molecular weight of 500 or more, from the viewpoint of suppressing defects such as fish eyes and improving quality and voltage resistance performance.

The polypropylene film of the present invention may contain resins other than polypropylene resin and cyclic olefin resin, as long as the object of the present invention is not impaired. Specific resins include vinyl polymer resins including various polyolefin resins, polyester resins, polyamide resins, polyphenylene sulfide resins, polyimide resins, polycarbonate resins, etc., and particularly preferred examples include polymethylpentene and syndiotactic polystyrene. The content of these resins is preferably less than 3% by mass, more preferably 2% by mass or less, and even more preferably 1% by mass or less, assuming that the entire resin components constituting the polypropylene film are 100% by mass. By keeping the content of resins other than polypropylene resin to less than 3% by mass, the effect of the domain interface can be suppressed and the decrease in the dielectric breakdown voltage in a high-temperature environment can be reduced.

The polypropylene film of the present invention is preferably used as a dielectric for film capacitors, but the type of film capacitor is not limited. Specifically, from the viewpoint of electrode configuration, it may be either a laminated film capacitor of metal foil and film, or a metal vapor deposition film capacitor, and it is also preferably used in oil-immersed type film capacitors impregnated with insulating oil, and dry capacitors that do not use insulating oil at all. However, due to the characteristics of the polypropylene film of the present invention, it is particularly preferably used as a metal vapor deposition film capacitor. From the viewpoint of shape, it may be either a wound type or a laminated type (the film capacitor of the present invention will be described later).

Since polypropylene film usually has a low surface energy and it is difficult to stably apply metal vapor deposition to it, it is preferable to perform a surface treatment before vapor deposition in order to improve adhesion to the metal film. Specific examples of surface treatments include corona discharge treatment, plasma treatment, glow discharge treatment, and flame treatment.

The polypropylene film of the present invention can be obtained by obtaining a polypropylene resin sheet using a resin composition containing a polypropylene resin as the main component and a cyclic olefin resin, and then biaxially stretching, heat treating and relaxing the sheet. As a method of biaxial stretching, any of the inflation simultaneous biaxial stretching method, tenter simultaneous biaxial stretching method and tenter sequential biaxial stretching method can be used. Among these, the tenter sequential biaxial stretching method and the tenter simultaneous biaxial stretching method are preferred in terms of controlling the mechanical properties and thermal dimensional stability while increasing the film forming stability, crystalline/amorphous structure, surface properties, and especially the stretch ratio of the present invention, and the tenter sequential biaxial stretching method is more preferred in terms of bringing the I810/I840 of the polypropylene film of the present invention into the preferred range described above.

Next, a preferred method for producing the polypropylene film of the present invention will be described. The preferred method for producing the polypropylene film of the present invention comprises, in this order, a casting step in which a resin composition containing a polypropylene resin and a cyclic olefin resin is melt-extruded onto a support to form a polypropylene resin sheet, and a stretching step in which the polypropylene resin sheet is stretched in the longitudinal and width directions, and in the stretching step, stretching in the width direction is performed by a two-stage stretching method (I) described below. Incidentally, "having a casting step and a stretching step in this order" means that the casting step and the stretching step are present in this order, regardless of whether there are other steps upstream of the casting step, between the casting step and the stretching step, or downstream of the stretching step.

The manufacturing method includes a casting step in which a resin composition containing polypropylene resin and a cyclic olefin resin is melt-extruded onto a support to form a polypropylene resin sheet. The resin composition containing polypropylene resin and a cyclic olefin resin is not particularly limited as long as it contains polypropylene resin as the main component and the cyclic olefin resin, but in order to increase the dispersibility of the cyclic olefin resin, it is preferable to use a compound resin composition in which the cyclic olefin resin and the polypropylene resin are pre-kneaded in advance. The support is not particularly limited as long as it can cool and solidify the resin composition melt-extruded into a sheet to obtain a polypropylene resin sheet, and for example, a cooling drum or the like can be used.

In this manufacturing method, downstream of the casting process, there is a stretching process in which the polypropylene resin sheet is stretched in the longitudinal and width directions. "Stretching in the longitudinal and width directions" includes both a sequential biaxial stretching method in which stretching in the longitudinal direction is performed followed by stretching in the width direction, and a simultaneous biaxial stretching method in which stretching in the longitudinal and width directions is performed simultaneously, but the sequential biaxial stretching method is preferred from the viewpoint of individually and preferably controlling the temperature conditions in stretching in the longitudinal direction and stretching in the width direction.

In the stretching step in the production method, stretching in the width direction is performed by the following two-stage stretching method (I). By adopting such a stretching method, the orientation parameter I810/I840 measured by Raman spectroscopy of the cross section X can be easily controlled to a suitable range.
Two-stage stretching method (I):
First-stage stretching: When the final stretching ratio in the width direction is a × b times, the film is stretched in the width direction at a temperature of 160°C or higher and 185°C or lower (the lower limit is preferably 170°C, and the upper limit is preferably 180°C) and a stretching ratio of a times.
Second-stage drawing: The film is further drawn in the width direction at a draw ratio of b times at a temperature 1° C. to 20° C. lower than the temperature in the first-stage drawing.

When the two-stage stretching method (I) is used for simultaneous biaxial stretching, it is preferable to perform simultaneous biaxial stretching in the first stage such that the width direction stretching satisfies the conditions for the first stage stretching, and then perform width direction stretching in the second stage such that the conditions for the second stage stretching are satisfied. However, the preferred temperature range for the first stage stretching in simultaneous biaxial stretching is 150°C or higher and 180°C or lower, more preferably 165°C or higher and 180°C or lower.

In this manufacturing method, in a rectangle of 1 μm×2 μm size in the A layer portion of cross section X, a pair of short sides of which are parallel to the thickness direction, the area stretch ratio is preferably 40 times or more from the viewpoint of adjusting the number of domains of the cyclic olefin resin passing through a pair of short sides within a suitable range. Here, the area stretch ratio refers to the longitudinal stretch ratio×width stretch ratio.

The method for producing the polypropylene film of the present invention will be described in more detail below. However, the polypropylene film of the present invention is not limited to that obtained by the method below.

First, when producing the polypropylene film of the present invention, it is preferable to compound the cyclic olefin resin, polypropylene resin, and antioxidant in advance from the viewpoint of improving the dispersion state of the cyclic olefin resin and polypropylene resin and increasing the dielectric breakdown voltage of the resulting polypropylene film at high temperatures.

Although single-screw extruders, twin-screw extruders, etc. can be used for compounding, it is preferable to use a twin-screw extruder in particular from the viewpoint of achieving a good dispersion state. The resin temperature during compounding is preferably within the following temperature range from the viewpoint of improving the dispersion state of the cyclic olefin resin and the polypropylene resin and further increasing the dielectric breakdown voltage at high temperatures of the resulting polypropylene film. First, it is preferable that it is 300°C or less, and more preferably 280°C or less. On the other hand, it is preferable that it is 200°C or more, and more preferably 230°C or more.

The content of the cyclic olefin resin in the resin composition obtained by compounding is preferably 0.5% by mass or more, more preferably 1% by mass or more, even more preferably 4% by mass or more, and particularly preferably 7% by mass or more, when the total amount of the compounded components is 100% by mass. On the other hand, from the viewpoint of improving the dispersion state of the cyclic olefin resin, the content of the cyclic olefin resin in the compound resin is preferably 49% by mass or less, and more preferably 40% by mass or less.

The amount of antioxidant is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, and even more preferably 0.4% by mass or more, when the total components in the resin raw material obtained by compounding are taken as 100% by mass. The upper limit is 1.0 part by mass. In addition, by making the mesopentad fraction of the polypropylene resin 0.960 or more, the resulting polypropylene film has a high melting point and is suitable for use at high temperatures, which is preferable.

Then, the resin composition obtained by compounding with polypropylene resin is fed to a single screw extruder after adjusting the amount of cyclic olefin resin to the desired level, and after passing through a filter, is extruded into a sheet from a slit nozzle. At this time, the temperature of the filter is preferably within the range of extrusion temperature -40°C or more and extrusion temperature or less, and more preferably extrusion temperature -40°C or more and extrusion temperature -20°C or less. By keeping the temperature of the filter within this range, it becomes easy to keep the number of domains of the cyclic olefin resin passing through the pair of short sides within a suitable range. The molten sheet extruded from the slit nozzle is then solidified on a temperature-controlled casting drum (cooling drum) to obtain a polypropylene resin sheet.

As mentioned above, the polypropylene film of the present invention is preferably laminated in order to increase the areal stretch ratio. In this case, polypropylene resin and a resin composition obtained by compounding are mixed and fed to a single screw extruder as the raw material for layer A, and polypropylene resin is fed to another single screw extruder as the raw material for layer B. The molten resin is then laminated in a two-layer structure of layer A/layer B or a three-layer structure of layer B/layer A/layer B using a feed block method with melt co-extrusion, and this is extruded into a sheet from a slit-shaped die and solidified on a temperature-controlled cooling drum to obtain an unstretched polypropylene film.

Regarding whether the structure is a single layer or a laminated structure, from the viewpoint of cooling and solidifying the molten resin while appropriately controlling the crystal growth, the temperature of the cooling drum is preferably 10°C or higher and 110°C or lower, and more preferably 10°C or higher and 95°C or lower.

The molten sheet may be adhered to the cooling drum by any of the following methods: electrostatic application, adhesion using the surface tension of water, air knife method, press roll method, underwater casting method, air chamber method, etc. However, the air knife method is preferred because it provides good flatness and allows control of surface roughness. It is also preferable to appropriately adjust the position of the air knife so that air flows downstream of the film production in order to prevent vibration of the film. The air temperature of the air knife depends on the temperature of the cooling drum, but is preferably 5°C or higher and 130°C or lower. From the viewpoint of preventing a large difference in the surface properties of both sides of the resulting polypropylene film, it is more preferable that the absolute value of the difference with the temperature of the cooling drum does not exceed 50°C.

Next, the polypropylene resin sheet is biaxially stretched to obtain a biaxial orientation. During stretching, the polypropylene resin sheet is brought into contact with a roll set to a predetermined longitudinal stretching temperature, and stretched in the longitudinal direction at a predetermined ratio. The longitudinal stretching temperature is preferably 100°C or higher, more preferably 120°C or higher, and even more preferably 140°C or higher, from the viewpoint of suppressing film rupture. On the other hand, it is preferably 170°C or lower, more preferably 165°C or lower, and even more preferably 160°C or lower. In addition, the longitudinal stretching ratio is preferably 3.5 times or higher, more preferably 4.0 times or higher, even more preferably 5.0 times or higher, and particularly preferably 5.2 times or higher, from the viewpoint of increasing the areal stretching ratio and increasing the dielectric breakdown voltage at high temperatures. On the other hand, from the viewpoint of suppressing film rupture, the longitudinal stretching ratio is preferably 10 times or lower. After stretching the polypropylene resin sheet in the longitudinal direction in this manner, it is cooled to room temperature to obtain a uniaxially oriented polypropylene film.

The obtained uniaxially oriented polypropylene film is introduced into a tenter while both ends in the width direction are held by clips. From the viewpoint of increasing the transverse stretch ratio, it is preferable that the tenter atmosphere temperature (width direction preheating temperature) in the preheating process immediately before the width direction stretching is set to the width direction stretching temperature + 5°C or more (the width direction stretching temperature here is the first stage stretching temperature when the two-stage stretching method (I) described later is adopted). On the other hand, from the viewpoint of suppressing the decrease in withstand voltage at high temperatures, it is preferable that the width direction preheating temperature is set to the width direction stretching temperature + 15°C or less, more preferably +12°C or less, and even more preferably +10°C or less. By keeping the temperature of the preheating process within the above range, the fibril structure highly oriented in the length direction can be further strengthened by the longitudinal stretching, and the dielectric breakdown voltage of the obtained polypropylene film can be increased. In addition, it is preferable to stabilize the molecular chains that are insufficiently oriented by high-temperature preheating after the uniaxial stretching from the viewpoint of improving thermal dimensional stability.

Then, the preheated uniaxially oriented polypropylene film is stretched in the width direction while both width direction ends are held by clips. The tenter atmosphere temperature (width direction stretching temperature) at this time is preferably 150°C or higher, more preferably 155°C or higher, and even more preferably 160°C or higher, from the viewpoint of uniformly stretching the cyclic olefin resin with a high glass transition temperature and improving the reliability of the film capacitor when used as a dielectric of the film capacitor. On the other hand, from the above viewpoint, the width direction stretching temperature is preferably 190°C or lower, more preferably 185°C or lower. Note that when the two-stage stretching method (I) described later is adopted, it is preferable that the temperature conditions in the second stage of stretching are within the above range.

From the viewpoint of increasing the dielectric breakdown voltage of the resulting polypropylene film, the widthwise stretching ratio (the final stretching ratio when the widthwise stretching is performed in multiple stages, such as when the two-stage stretching method (I) is adopted) is preferably 8.5 times or more, more preferably 9.3 times or more, and even more preferably 10.0 times or more. On the other hand, from the viewpoint of stable film formation, the widthwise stretching ratio is preferably 20.0 times or less, more preferably 17.0 times or less, and even more preferably 15.0 times or less. By setting the widthwise stretching ratio to 8.5 times or more, it becomes easy to set the length in the main orientation axis direction of the domain of the cyclic olefin resin to 5 μm or more.

As described above, the stretching in the width direction is preferably performed at a temperature lower than the preheating temperature, and is preferably performed by the following two-stage stretching method (I) in which the first stage stretching and the second stage stretching are performed in sequence. By adopting such a stretching method, the orientation parameter I810/I840 measured by Raman spectroscopy of the cross section X can be easily controlled to a suitable range.
Two-stage stretching method (I):
First-stage stretching: When the final stretching ratio in the width direction is a × b times, the film is stretched in the width direction at a temperature of 160°C or higher and 185°C or lower (the lower limit is preferably 170°C, and the upper limit is preferably 180°C) and a stretching ratio of a times.
Second-stage drawing: The film is further drawn in the width direction at a draw ratio of b times at a temperature 1° C. to 20° C. lower than the temperature in the first-stage drawing.

The stretching ratio a in the first stretching stage is preferably 1.8 to 13.3 times, more preferably 1.8 to 9.0 times. The stretching ratio (b) in the second stretching stage is preferably 1.1 to 5.0 times, more preferably a>b, from the viewpoint of keeping I810/I840 within a suitable range and suppressing a decrease in voltage resistance when used at high temperatures.

With regard to the ratio of the stretch ratio in the width direction to the stretch ratio in the longitudinal direction, from the viewpoint of keeping I810/I840 within the above-mentioned range, the stretch ratio in the width direction is preferably 1.5 times or more relative to the stretch ratio in the longitudinal direction, and more preferably 1.7 times or more. On the other hand, performing sequential stretching so that the stretch ratio in the width direction is 3.0 times or less relative to the stretch ratio in the longitudinal direction is more effective in keeping the tear strength ratio within a suitable range.

The areal stretching ratio is preferably 40 times or more. By setting the areal stretching ratio to 40 times or more, it becomes easy to keep the number of domains of the cyclic olefin resin passing through a pair of short sides in the cross section X within a suitable range. As a result, the obtained film has a high dielectric breakdown voltage, especially at high temperatures. In the present invention, the areal stretching ratio is the longitudinal stretching ratio multiplied by the widthwise stretching ratio. In the present invention, the widthwise stretching ratio refers to the stretching ratio after widthwise stretching and before relaxation treatment. From the above viewpoint, the areal stretching ratio is more preferably 45 times or more, even more preferably 50 times or more, particularly preferably 54 times or more, and most preferably 60 times or more. There is no particular limit to the upper limit of the areal stretching ratio, but from the viewpoint of feasibility, it is 90 times in the case of sequential biaxial stretching and 150 times in the case of simultaneous biaxial stretching.

The important point about the polypropylene film of the present invention is to increase I810/I840 while incorporating a cyclic olefin resin. That is, in the production of the polypropylene film of the present invention, for example, the dispersibility of the cyclic olefin resin domains dispersed in the polypropylene resin can be increased by pre-mixing, and a high areal stretching ratio can be achieved by using the two-stage stretching method described above, thereby increasing the withstand voltage at 135°C.

In the manufacture of the polypropylene film of the present invention, it is preferable to provide a heat treatment and relaxation treatment process after biaxial stretching. In this process, the heat treatment is performed at a temperature of 150°C to 190°C in the tenter atmosphere while providing 2 to 30% relaxation in the width direction while holding the film tensely with clips, from the viewpoint of improving the reliability of the film capacitor when it is used as a film capacitor, and more preferably 150°C to 164°C, and even more preferably 150°C to 159°C. In addition, from the viewpoint of improving the reliability of the film capacitor when the polypropylene film is used as a film capacitor, the relaxation treatment rate is preferably 5% or more, more preferably 7% or more, even more preferably 9% or more, and particularly preferably 13% or more. On the other hand, from the viewpoint of increasing the dielectric breakdown voltage of the polypropylene film, the relaxation treatment is more preferably 25% or less, and even more preferably 18% or less.

After undergoing heat treatment and relaxation treatment, the polypropylene film is guided to the outside of the tenter, and the clips on both ends in the width direction are released in a room temperature atmosphere. After that, the film edges are slit in the winder process, and the polypropylene film is wound into a roll. Before winding the polypropylene film, it is preferable to perform a surface treatment such as a corona discharge treatment on at least one side in air, nitrogen, carbon dioxide gas, or a mixture of these gases in order to improve the adhesion of the evaporated metal.

Specific examples of the production conditions to be considered in order to obtain the polypropylene film of the present invention are as follows. It is preferable to satisfy all of these production conditions, but they do not necessarily have to be all present and may be combined as appropriate. For example, instead of "in the sequential biaxial stretching, the preheating temperature before the width direction stretching is the width direction stretching temperature +5°C or more and +15°C or less," simultaneous biaxial stretching may be adopted.
- The mesopentad fraction of the main component, polypropylene resin, is 0.960 or more.
- Contains a polypropylene resin having a crosslinked structure or a branched polypropylene resin.
- Pre-mixing the cyclic olefin resin and the polypropylene resin.
- The extruder filter temperature is set lower than the extrusion temperature.
The content of the cyclic olefin resin is from 1% by mass to 40% by mass.
The area stretch ratio of the biaxial stretching is 40 times or more.
The stretch ratio in the width direction is 8.5 times or more.
The stretching ratio in the width direction is 1.5 to 3.0 times, preferably 1.7 to 3.0 times, the stretching ratio in the longitudinal direction.
In the sequential biaxial stretching, the preheating temperature before stretching in the width direction is from the width direction stretching temperature + 5°C to the width direction stretching temperature + 15°C.
The transverse stretching is carried out by the two-stage stretching method (I).
- Heat treatment and relaxation treatment are performed after biaxial stretching.
The heat treatment temperature after biaxial stretching is 150°C or higher.

Next, we will explain the metal film laminated film made using the polypropylene film of the present invention, the film capacitor made using the same, and the manufacturing methods for the same.

The metal film laminated film of the present invention has a metal film on at least one side of the polypropylene film of the present invention. This metal film laminated film can be obtained by providing a metal film on at least one side of the polypropylene film of the present invention described above.

In the present invention, the method of applying the metal film is not particularly limited, but a preferred method is to deposit aluminum or an alloy of aluminum and zinc on at least one side of the polypropylene film to provide a metal film such as a vapor deposition film that will become the internal electrode of the film capacitor. At this time, other metal components such as nickel, copper, gold, silver, and chromium can also be deposited simultaneously with or successively to the aluminum. Also, a protective layer can be provided on the vapor deposition film using oil or the like. If the surface roughness of the polypropylene film differs between the front and back, it is preferable to provide a metal film on the front side, which has relatively less roughness, to form a metal film laminated film from the viewpoint of increasing voltage resistance.

In the present invention, if necessary, after forming the metal film, the metal film laminated film can be annealed or heat treated at a specific temperature. In addition, at least one side of the metal film laminated film can be coated with a resin such as polyphenylene oxide for insulation or other purposes.

The film capacitor of the present invention is made using the metal film laminated film of the present invention. In other words, the film capacitor of the present invention has the metal film laminated film of the present invention.

The film capacitor of the present invention preferably has a capacitance density of 1.1 μF/cm ³ or more and 18 μF/cm ³ or less. The higher the capacitance density of the film capacitor, the smaller the volume of a capacitor element of the same capacitance is when it is manufactured, and therefore miniaturization is possible, so that the capacitance density is more preferably 1.5 μF/cm ³ or more. The higher the capacitance density of the film capacitor, the more preferable it is, but from the viewpoint of feasibility, it is preferably 18 μF/cm ³ or less, and more preferably 10 μF/cm ³ or less. By using the polypropylene film of the present invention formed to a thickness of 4.5 μm or less, it is easy to set the capacitance density to 1.1 μF/cm ³ or more, and by using the polypropylene film formed to a thickness of 3.5 μm or less, it is easy to set the capacitance density to 1.5 μF/cm ³ or more.

The capacitance density (μF/cm ³ ) of a film capacitor can be calculated from the element capacitance (μF) and element volume (cm ³ ) by the following formula. The element capacitance can be measured in accordance with JIS C 4908:2007 at an ambient temperature of 23° C. The element volume refers to the volume of the part where the vapor deposition film is wound, not including the exterior material, metallicon, and reel, and can be measured by a known 3D scanner type three-dimensional measuring machine. The method for measuring the capacitance density (μF/cm ³ ) of a film capacitor will be described in detail later.
Capacitance density (μF/cm ³ )=element capacitance (μF)/element volume (cm ³ ).

The film capacitor of the present invention preferably has a withstand voltage of 0.60 kV or more at 135°C, more preferably 0.75 kV or more, and even more preferably 1.0 kV or more. Since the polypropylene film of the present invention has the above-mentioned characteristics and thus has a high withstand voltage at high temperatures, using this as the dielectric of a film capacitor makes it easy to achieve a withstand voltage of 0.60 kV or more at 135°C for the resulting film capacitor, and by using a preferred embodiment of the polypropylene film of the present invention, the withstand voltage at 135°C can be further increased.

For example, the film capacitor of the present invention can be obtained by laminating or winding the metal film laminate film of the present invention described above in various ways. Examples of preferred methods for manufacturing a wound film capacitor are as follows.

Aluminum is vapor-deposited under reduced pressure on one side of a polypropylene film. At this time, it is vapor-deposited in stripes with margins running in the longitudinal direction. Next, a blade is used to slit the center of each vapor-deposited section and the center of each margin on the surface, creating a tape-like take-up reel with a margin on one side of the surface. Two tape-like take-up reels with a left margin and one with a right margin are stacked together and wound so that the vapor-deposited section extends beyond the margin in the width direction, to obtain a wound body.

When vapor deposition is performed on both sides, one side is vapor deposited in stripes with a margin running in the longitudinal direction, and the other side is vapor deposited in stripes so that the longitudinal margin is located in the center of the vapor deposition area on the back side. Next, a blade is cut into the center of the margins on both sides to create a tape-like take-up reel with a margin on one side on each side (for example, if there is a margin on the right side of the front side, there will be a margin on the left side on the back side). The resulting reel and one unvapor-deposited laminated film are then overlapped and wound in two so that the metallized film extends beyond the laminated film in the width direction to obtain a wound body.

A method for obtaining the film capacitor of the present invention from the metal layer laminated film of the present invention includes, for example, removing the core material from the wound body produced as described above, pressing it, spraying metallicon on both end faces to form external electrodes, and welding lead wires to the metallicon to form a wound film capacitor. Film capacitors have a wide range of applications, including power control units for electric automobiles such as electric vehicles, hybrid vehicles, and fuel cell vehicles, electric aircraft such as drones, railway vehicles, solar power generation and wind power generation, and general home appliances, and the film capacitor of the present invention can also be suitably used for these applications. In addition, the polypropylene film of the present invention can be used for various applications such as packaging films, release films, process films, sanitary products, agricultural products, construction products, and medical products, and can be particularly preferably used for applications that include a heating process in film processing.

The power control unit, electric automobile, and electric aircraft of the present invention will be described below. The power control unit of the present invention has the film capacitor of the present invention. The power control unit is a system that manages power in electric automobiles, electric aircraft, and the like that have mechanisms that are driven by electricity. By installing the film capacitor of the present invention in the power control unit, it is possible to reduce the size of the power control unit itself, improve its heat resistance, and increase its efficiency, resulting in improved fuel efficiency.

The electric vehicle of the present invention has the power control unit of the present invention. Here, the electric vehicle refers to a vehicle that has a mechanism that is driven by electric power, such as an electric vehicle, a hybrid vehicle, or a fuel cell vehicle. As mentioned above, the power control unit of the present invention can be made compact, and also has excellent heat resistance and efficiency, so that equipping an electric vehicle with the power control unit of the present invention leads to improved fuel efficiency, etc.

The electric aircraft of the present invention has the power control unit of the present invention. Here, the electric aircraft refers to an aircraft having a mechanism that is driven by electricity, such as a manned electric aircraft or a drone. As mentioned above, the power control unit of the present invention can be made compact and has excellent heat resistance and efficiency, so that equipping an electric aircraft with the power control unit of the present invention leads to improved fuel efficiency, etc.

The packaging material of the present invention is characterized by using the polypropylene film of the present invention. The packaging material of the present invention has excellent structural stability against heat during deposition, and has good water vapor barrier properties and oxygen barrier properties, especially when a transparent deposition layer is laminated, so it can be suitably used for packaging items that are easily deteriorated by water vapor or oxygen.

[Measurement and evaluation methods]
(1) Film Thickness The thickness of ten randomly selected points on the polypropylene film was measured in an atmosphere of 23° C. and 65% RH using a contact-type electronic micrometer (K-312A type) manufactured by Anritsu Corp. The arithmetic average value of the thicknesses at the ten points was taken as the film thickness (unit: μm) of the polypropylene film.

(2) Direction of main orientation axis and direction perpendicular to main orientation axis of polypropylene film In each of the examples and comparative examples, the direction of the main orientation axis of the polypropylene film was determined as follows according to the following method (tensile test). In each of the examples and comparative examples, the direction perpendicular to the main orientation axis of the polypropylene film was the direction perpendicular to the main orientation axis in the film plane.
Examples 1 to 6, Comparative Examples 1 to 4, 7, and 8: The width direction was the main orientation axis direction of the polypropylene film.
Comparative Example 5: The long side direction of the sample was the main orientation axis direction of the polypropylene film.

<Tensile test>
First, a rectangular sample <1> with a length of 50 mm and a width of 10 mm was cut out, and the direction of the long side of the sample <1> was defined as 0°. Next, a rectangular sample <2> of the same size was taken so that the long side direction was rotated 15° to the right from the 0° direction, and rectangular samples <3> to <12> were taken by rotating the long side direction of the rectangular sample by 15° each in the same manner. Next, each rectangular sample was set in a tensile tester with an initial chuck distance of 20 mm so that the long side direction was the tensile direction (measurement direction), and a tensile test was performed at a tensile speed of 300 mm/min in an atmosphere at room temperature. At this time, the maximum load until the rectangular sample broke was read, and the value divided by the cross-sectional area (film thickness x width) of the sample before the test was calculated as the stress of the maximum point strength. The long side direction of the sample with the maximum value was determined as the main orientation axis direction of the polypropylene film.

(3) Crystallization temperature (Tmc) during cooling of polypropylene film
The crystallization temperature (Tmc) during the cooling process of the polypropylene film was measured in accordance with JIS K7121-1987. First, using a differential scanning calorimeter (EXSTAR DSC6220 manufactured by Seiko Instruments), 3 mg of the film was heated from 30 ° C. to 260 ° C. at 20 ° C./min in a nitrogen atmosphere. Next, after holding at 260 ° C. for 5 minutes, the temperature was lowered to 30 ° C. at 20 ° C./min, and the peak temperature of the exothermic peak obtained during the cooling process was measured. The same measurement was performed three times, and the average value of the obtained peak temperatures was taken as the cooling crystallization temperature (Tmc) of the polypropylene film. In addition, when multiple exothermic peaks were observed in one measurement, the peak temperature of the exothermic peak with the highest peak temperature was taken as the peak temperature of the measurement.

(4) Glass transition temperature (Tg) of cyclic olefin resin
Measurement was performed according to JIS K7121-1987. Using a differential scanning calorimeter (EXSTAR DSC6220 manufactured by Seiko Instruments), 3 mg of film or resin was heated from 30°C to 260°C at 20°C/min in a nitrogen atmosphere, then held at 260°C for 5 minutes, and cooled to 30°C at 20°C/min. After further holding at 20°C for 5 minutes, the temperature was raised again from 30°C to 260°C at 20°C/min. From the DSC curve obtained during the reheating process, the glass transition temperature (Tg) was calculated according to the following formula.
Glass transition temperature=(extrapolated glass transition onset temperature+extrapolated glass transition finish temperature)/2.

(5) I810/I840
Using the following equipment and conditions, a cross section X was cut out from a polypropylene film by a microtome method, and polarized Raman measurement (beam diameter 1 μm, parallel to the main orientation axis direction) was performed on the center position of the cross section X in the thickness direction by microscopic Raman spectroscopy. The Raman band intensities at 810 cm ⁻¹ and 840 cm ⁻¹ obtained by the measurement were taken as I810 and I840, respectively, and I810/I840 was calculated. The measurement was performed at five locations that were 1 cm or more away from the edge of the film and 1 cm or more away from the center of the cross section that was previously measured, and the average of the measurements obtained for each cross section was used as the I810/I840 of the polypropylene film. The polarized Raman spectrum was obtained by irradiating a linearly polarized light to the polypropylene film and detecting only the component of the scattered light parallel to the incident light. In order to eliminate this, a λ/4 plate was placed after the analyzer and before the grating, and the scattered light was introduced to the grating in a state where the polarization state of the scattered light was eliminated.
<Measurement Equipment>
Measuring device: inVia (manufactured by RENISHAW)
<Measurement conditions>
Measurement mode: Raman microscope objective lens: ×100
Beam diameter: 1 μm
Light source: Semiconductor laser / 532 nm
Laser power: 300mW
Diffraction grating: Single -3000 gr/mm
Slit: 65 μm
Detector: CCD/RENISHAW 1024x256.

(6) The number of cyclic olefin resin domains passing through a pair of sides parallel to the thickness direction in a 1 μm×2 μm rectangle in layer A (domains/2 μm ² )
Using a microtome method, a polypropylene film was cut in a plane parallel to the main orientation axis direction and the thickness direction to prepare a polypropylene film piece having a cut surface. The cut surface was stained with _RuO4 , and then the stained surface was cut to obtain an ultrathin section having a cross section X. The cross section X of the ultrathin section was observed and photographed using a transmission electron microscope (Hitachi, Ltd. Transmission Electron Microscope (TEM) HT7700) under the following observation conditions. At this time, the cyclic olefin resin was stained blacker than the polypropylene resin.
<Observation conditions>
Acceleration voltage: 100 kV
Observation magnification: 2,000 times. A rectangle was drawn on the collected cross-sectional X image, with a pair of sides of 1 μm in the thickness direction and ² μm in the direction perpendicular to the thickness direction (main orientation axis direction), and the number of domains of the cyclic olefin resin passing through a pair of sides parallel to the thickness direction in the rectangle was counted. The same measurement was performed a total of 10 times by changing the position of the rectangle in the image, and the average value of the obtained domain numbers was calculated, and this was defined as the number of domains of the cyclic olefin resin passing through a pair of sides parallel to the thickness direction in the A layer (pieces/2 μm ² ). In addition, when a rectangle with a pair of sides of 1 μm in the thickness direction and 2 μm in the direction perpendicular to the thickness direction is defined in the cross-sectional X, the base of the rectangle is set to the sea part, and if a domain is located on the side opposite to the base, this is considered to be absent and is not counted as the number. In addition, domains with constrictions were also treated as connected domains.

(7) Length of the main axis direction of the domain of the cyclic olefin resin in the cross section X The cross section X was observed under the conditions described in (6), and five domains were selected in the upward direction from the center of the field of view without moving the field of view, and then five domains were selected in the downward direction. The length of the main axis direction of the selected domains was measured, and the average value was taken as the length of the main axis direction of the domain of the cyclic olefin resin in the cross section X. In addition, when the selected domain has an end outside the field of view, the field of view was moved from one end to the other end to obtain multiple images, and the length of the main axis direction of the domain was determined from the images connected together. If 10 domains could not be selected in one field of view, the field of view was moved to another field of view, and the observation was continued until the measurement of 10 domains was completed.

(8) Dielectric tangent The dielectric tangent of the polypropylene film was measured according to JIS C2138-2007. First, the film was cut into a square shape of 50 mm x 50 mm, and a conductive paste was applied to one side with a diameter of Φ18 mm and the other side with a diameter of Φ28 mm to form an electrode. The electrode-formed sample was stored in an environment of 22 ° C. and 60% RH for 90 hours, and then the dielectric tangent was measured five times using a precision LCR meter HP-4284A (manufactured by Agilent Technologies) under conditions of 22 ° C., 60% RH, and a frequency of 10 kHz, and the average value of the obtained values was taken as the dielectric tangent of the polypropylene film.

(9) Tear strength in the direction perpendicular to the main orientation axis/tear strength in the direction of the main orientation axis Measured according to JIS K 7128-2:1998. Specifically, three rectangular test pieces of 63.5 mm x 50 mm were taken from the polypropylene film so that the measurement direction was the long side. The tear strength of each test piece was measured using a light-load tear tester manufactured by Toyo Seiki, and the tear strength was calculated by dividing the tear strength by the film thickness measured by the method described in (1). The average value of the tear strengths of the three test pieces was taken as the tear strength in that direction. The measurement was performed in each of the main orientation axis direction and the direction perpendicular to the main orientation axis, and the tear strength in the direction perpendicular to the main orientation axis/tear strength in the direction of the main orientation axis of the film was calculated from the tear strength in each direction.

(10) Dielectric breakdown voltage of polypropylene film (V/μm)
After heating the polypropylene film for 1 minute in an oven kept at the measurement temperature, a breakdown voltage test was performed in that atmosphere in accordance with JIS C2330 (2001) 7.4.11.2 B method (flat electrode method) to measure the breakdown voltage. However, for the lower electrode, a metal plate described in JIS C2330 (2001) 7.4.11.2 B method was used, and "Conductive Rubber E-100 <65>" manufactured by Togawa Rubber Co., Ltd. of the same dimensions was placed on top of it. The breakdown voltage test was performed 30 times, and the obtained value was divided by the thickness of the polypropylene film (measured in (1) above) to convert it to V/μm. The average value of 20 points, excluding the 5 points from the largest value and the 5 points from the smallest value out of the total 30 measured values (calculated values), was taken as the breakdown voltage of the polypropylene film at that temperature. Three types of measurement temperatures were used: 23°C, 105°C, and 135°C. In the case of 23° C., the dielectric breakdown voltage test was performed without heating in an oven, with the ambient temperature in the room kept at 23° C.

(11) Evaluation of film capacitor characteristics The wet tension of each surface of a polypropylene film was measured in accordance with JIS K 6768-1995. On the surface with higher wet tension, aluminum was deposited using a vacuum deposition machine manufactured by ULVAC, Inc., so that the film resistance was 10 Ω/sq. During deposition, a deposition film A was provided with a deposition pattern having a so-called T-shaped margin (longitudinal pitch (period) of 17 mm, fuse width of 0.5 mm) in which a margin was provided in a direction perpendicular to the longitudinal direction using masking oil, and a deposition film B was not provided with a deposition pattern having a T-shaped margin. The obtained deposition films A and B were each slit to obtain deposition reels A and B with a film width of 50 mm (edge margin width of 2 mm). Next, the evaporation reels A and B were alternately stacked, and the capacitor elements were wound using a device winding machine (KAW-4NHB) manufactured by Kaito Manufacturing Co., Ltd., so that the element capacitance after finishing as a capacitor element would be 10 μF, and after metallicon processing, they were subjected to heat treatment for 12 hours while reducing the pressure in an atmosphere at 135°C, and lead wires were attached to finish the capacitor element. Using 10 capacitor elements thus obtained, a so-called step-up test was performed in which a voltage of 150 VDC was applied to the capacitor elements at an evaluation temperature, and after 10 minutes had elapsed at that voltage, the applied voltage was gradually increased in steps at 50 VDC/minute. The evaluation temperatures were 23°C and 135°C, and the test was performed at each temperature.

<Voltage resistance evaluation>
In the step-up test, the capacitance was measured and plotted on a graph, and the voltage at which the capacitance became 75% of the initial value was divided by the thickness of the polypropylene film (the value measured in (1) above) to determine the withstand voltage at each temperature. Similar measurements were performed on 10 capacitor elements, and the average value of the obtained values was calculated, and the withstand voltage reduction rate due to temperature change and the withstand voltage at 135°C were evaluated according to the following criteria.

<Voltage withstand voltage drop rate due to temperature change>
The withstand voltage at 23° C. was rated as B (23), and the withstand voltage at 135° C. was rated as B (135), and the evaluation was based on the following evaluation criteria: "A" in the evaluation criteria means usable, "B" and "C" means usable under certain conditions, and "D" means poor practical performance and difficult to use.
A:B(135)/B(23) was greater than 0.70.
B: B(135)/B(23) was greater than 0.60 and not greater than 0.70.
C: B(135)/B(23) was greater than 0.50 and equal to or less than 0.60.
D: B(135)/B(23) was smaller than 0.5.

<Voltage resistance at 135°C>
The evaluation was performed according to the following criteria, with B (135) as the standard. In the evaluation criteria, "A" means usable, "B" and "C" mean usable under certain conditions, and "D" means poor practical performance and difficult to use.
A:B(135) was greater than 410 V/μm.
B: B(135) was greater than 360 V/μm and equal to or less than 410 V/μm.
C: B(135) was greater than 330 V/μm and equal to or less than 360 V/μm.
D: B(135) was 330 V/μm or less.

<Reliability evaluation>
The voltage was increased for the 10 capacitor elements until the capacitance was reduced to 18% or less of the initial value, and then the capacitor element with the highest withstand voltage was disassembled to check the state of breakdown and evaluate its reliability using the following evaluation criteria: "S" in the evaluation criteria means that it can be used, "A", "B", and "C" mean that it can be used depending on the conditions, and "D" means that it has poor practical performance and is difficult to use.
A: Neither change in element shape nor breakage in the form of penetration was observed.
B: There was no change in the shape of the element, and penetration damage was observed in 1 to 5 layers of the polypropylene film.
C: The shape of the element was not changed, and penetration damage was observed in 6 to 10 layers of the polypropylene film.
D: Change in element shape was observed, or breakage of 11 or more layers through the polypropylene film was observed.

<Processability evaluation>
The processability was evaluated according to the frequency with which the polypropylene film broke when slitting after the deposition, using the following evaluation criteria: "A" means usable, "B" and "C" mean usable depending on the conditions, and "D" means poor practical performance and difficult to use.
A: The number of breaks per 20,000 m of slit length was 1 or less.
B: The number of breaks per 20,000 m of slit length was more than 1 and 3 or less.
C: The number of breaks per 20,000 m of slit length was more than 3 and 10 or less.
D: The number of breaks per 20,000 m of slit length was more than 10.

(12) Capacitance Density of Film Capacitor Element The capacitance of 10 capacitor elements was measured using a Keysight Technologies E4980A precision LCR meter at a frequency of 1 kHz and an atmospheric temperature of 23°C in accordance with JIS C 4908:2007 to determine the element capacitance of each element. Next, the volume of each element was measured using a Keyence VL-500 3D scanner-type three-dimensional measuring machine to determine the element volume. From the obtained values, the capacitance density (μF/cm ³ ) of each element was calculated according to the following formula, and the average value of the 10 capacitor elements was used as the measured value.
Capacitance density (μF/cm ³ )=element capacitance (μF)/element volume (cm ³ ).

(13) Withstand voltage of film capacitor at 135°C A so-called step-up test was performed using 10 capacitor elements, in which a voltage of 150 VDC was applied to the capacitor elements at a high temperature of 135°C, and after 10 minutes at that voltage, the applied voltage was gradually increased in steps of 50 VDC/minute, and this was repeated. In the step-up test, the change in capacitance was measured and plotted on a graph, and the voltage at which the capacitance became 75% of the initial value was recorded as the withstand voltage of the film capacitor at 135°C.

(14) Film capacitor performance evaluation <Life evaluation>
Ten capacitor elements were used, and a voltage of 750 VDC was applied to the capacitor elements at a high temperature of 135° C., and the capacitor elements were removed every 100 hours to measure the capacitance. The time at which the capacitance became 90% or less of the initial value was determined, and the average value of the 10 capacitor elements was calculated as the lifespan, which was evaluated according to the following criteria. A means that it can be used favorably, and B means that it is difficult to use.
A: The life span is 2000 hours or more.
B: The life is less than 2000 hours.

<Element size evaluation (index for miniaturization)>
The average value of the volumes of 10 capacitor elements measured by the method in (12) was taken as the element volume, and evaluation was performed according to the following criteria: A and B indicate that the capacitor element can be miniaturized, and C indicates that it is difficult to miniaturize the capacitor element.
A: The element volume is 80 ^cm3 or less.
B: The element volume is greater than 80 cm ³ and less than or equal to 160 cm ³ .
C: The element volume is greater than 160 ^cm3 .

(15) Ash Content The weighed resin or polypropylene film was burned in an electric furnace to evaluate the amount of ash content in accordance with JIS K 7250-1: 2006. For the weighing, an electronic balance XP26 manufactured by Mettler Toledo was used, and an electric furnace FO510 manufactured by Yamato Scientific was used. The electric furnace was heated to 600°C for burning.

(16) Diffusion structure in layer A of cross section X TEM observation of layer A of cross section X of a polypropylene film was performed under the same conditions as those described in (6) except that the observation magnification was changed to 20,000 times. The obtained image was processed in ImageJ (version 1.53t) as follows to determine the proportion of pixels of the diffusion structure.
i) From the image obtained by TEM observation, a 250 pixel x 250 pixel piece was cut out so that the center of gravity was the center of the layer A portion when viewed in the thickness direction.
ii) Converted into an 8-bit grayscale image.
iii) The brightness was adjusted using Auto in the Brightness/Contrast function of the Adjust tab in the Image tab.
iv) The image was binarized so that pixels with a brightness between 60 and 100 had a brightness of 255, and other pixels had a brightness of 0.
v) Median filter processing was performed with a setting of 10 pixels.
vi) The number of pixels with a brightness of 255 was counted, divided by the total number of pixels of the cut-out image, 62,500 pixels, and multiplied by 100 to obtain the percentage of pixels with a diffuse structure.
The same measurement was performed five times so as not to include the same pixel, and the average value of the obtained percentage of the number of pixels of the diffusion structure was regarded as the percentage (%) of the number of pixels of the diffusion structure in the layer A part of the cross section X of the polypropylene film.

[Resin, etc.]
The following resins and the like were used in the production of the polypropylene films in each of the Examples and Comparative Examples.

<Polypropylene resin>
Polypropylene resin 1:
Homopolypropylene having a mesopentad fraction of 0.970, a melting point of 166° C., a melt flow rate (MFR) of 3.3 g/10 min, and an ash content of 20 ppm (Borealis AG's “Borclean”® HC300BF).
Polypropylene resin 2:
A homopolypropylene having a mesopentad fraction of 0.982, a melting point of 168° C., a melt flow rate (MFR) of 2.2 g/10 min, and an ash content of 15 ppm.
Branched polypropylene (B1): A branched polypropylene resin having a melt flow rate (MFR) of 2.4 g/10 min ("Daploy" (registered trademark) WB135HMS from Borealis AG).

<Components other than polypropylene resin>
Cyclic olefin resin (C1):
Polyplastics "TOPAS" (registered trademark) 6013F-04 (a resin made by copolymerizing ethylene and norbornene (COC), with a glass transition temperature of 138°C and non-crystalline)
Cyclic olefin resin (C2):
Polyplastics "TOPAS" (registered trademark) 6017S-04 (a resin made by copolymerizing ethylene and norbornene (COC), with a glass transition temperature of 178°C and non-crystalline)
Cyclic olefin resin (C3):
Mitsui Chemicals'"APEL" (registered trademark) 5014CL (a resin (COC) made by copolymerizing ethylene and norbornadiene derivatives, with a glass transition temperature of 136°C and amorphous nature)
Antioxidant:
"IRGANOX" (registered trademark) 1010 manufactured by Ciba Specialty Chemicals.

<Cyclic olefin resin pre-mixed raw material>
Raw material (A1):
The components were mixed so that the polypropylene resin 1 was 59.5 parts by mass, the cyclic olefin resin (C1) was 40 parts by mass, and the antioxidant was 0.5 parts by mass, and the components were kneaded and extruded in a twin-screw extruder set at 260°C. The strands were then water-cooled and chipped.
Raw material (A2):
The components were mixed so that the polypropylene resin 2 was 59.5 parts by mass, the cyclic olefin resin (C3) was 40 parts by mass, and the antioxidant was 0.5 parts by mass, and the components were kneaded and extruded in a twin-screw extruder set at 260°C. The strands were then water-cooled and chipped.
Raw material (A3):
The components were mixed so that the polypropylene resin 2 was 59.5 parts by mass, the cyclic olefin resin (C2) was 40 parts by mass, and the antioxidant was 0.5 parts by mass, and the mixture was kneaded and extruded in a twin-screw extruder set at 260°C. The strands were then water-cooled and chipped.

<Branched chain polypropylene pre-mixed raw material>
Raw material (D1):
The components were mixed so that the polypropylene resin 1 was 89.5 parts by mass, the branched polypropylene (B1) was 10 parts by mass, and the antioxidant was 0.5 parts by mass, and the mixture was kneaded and extruded in a twin-screw extruder set at 260°C. The strands were then water-cooled and chipped.
Raw material (D2):
The components were mixed so that the polypropylene resin 2 was 89.5 parts by mass, the branched polypropylene (B1) was 10 parts by mass, and the antioxidant was 0.5 parts by mass, and the mixture was kneaded and extruded in a twin-screw extruder set at 260°C. The strands were then water-cooled and chipped.

Example 1
A resin composition in which the raw material (A1) was mixed to 80.0 parts by mass, the polypropylene resin 1 was mixed to 19.7 parts by mass, and the antioxidant was mixed to 0.3 parts by mass was fed to a single-screw extruder for the A layer. In the single-screw extruder, the resin composition was melted at a temperature of 250°C, and foreign matter was removed using a sintered filter with a cut of 80 μm and adjusted to 250°C. The molten resin composition was then discharged from a T-die in the form of a sheet. Thereafter, the molten sheet was adhered to a casting drum whose surface temperature was kept at 30°C by an air knife (air temperature: 23°C) and cooled and solidified to obtain an unstretched polypropylene film. The unstretched polypropylene film was heated to a temperature of 155°C by a group of multiple rolls, and stretched 3.5 times in the longitudinal direction between rolls with a difference in peripheral speed to obtain a uniaxially oriented polypropylene film. Subsequently, the uniaxially oriented polypropylene film was guided to a tenter by holding both ends in the width direction with multiple clips, and preheated at 185°C. Next, the uniaxially oriented polypropylene film was introduced into a transverse stretching chamber consisting of two chambers, the first chamber and the second chamber, and stretched 3.2 times in the width direction at 180°C in the first chamber, and then stretched 3.7 times in the width direction at 175°C in the second chamber (11.8 times stretched in the width direction in total in the two chambers). Further, as heat treatment and relaxation treatment, heat treatment was performed at 165°C while giving 12% relaxation in the width direction, and the clip was released by leading to the outside of the tenter. Furthermore, the surface of the film after heat treatment (the side contacting the casting drum) was subjected to corona discharge treatment in the atmosphere at a treatment intensity of 25 W·min/ ^m2 to obtain a polypropylene film. The evaluation results are shown in Table 1-1.

Example 2
The resin composition obtained by mixing the raw material (A1) at 48.0 parts by mass, the polypropylene resin 1 at 51.7 parts by mass, and the antioxidant at 0.3 parts by mass was fed to a single screw extruder for the A layer, and the polypropylene resin 1 was fed to a single screw extruder for the B layer. In each single screw extruder, the resin composition and the polypropylene resin 1 were melted at 260°C, and foreign matter was removed using a sintered filter with a cutoff of 80 μm and adjusted to 230°C. The resin composition (for the A layer) and the polypropylene resin 1 (for the B layer) were laminated using a feed block so that the layer thickness ratio was 1/10/1 in a three-layer structure of the B layer/A layer/B layer. The obtained molten laminate was discharged from a T die into a sheet shape, and the sheet was brought into close contact with a casting drum with a surface temperature maintained at 30°C by an air knife, and cooled and solidified to obtain an unstretched polypropylene film. Thereafter, a polypropylene film was obtained in the same manner as in Example 1, except that the film-forming conditions were as shown in Table 1-1. The evaluation results are shown in Table 1-1.

(Examples 3 to 5, Comparative Examples 3, 4, and 6)
A polypropylene film was obtained in the same manner as in Example 2, except that the raw material formulation and film-forming conditions were as shown in Tables 1-1 and 1-2. The evaluation results are shown in Tables 1-1 and 1-2. The film thickness was adjusted by increasing or decreasing the discharge rate of the extruder, and the lamination ratio was adjusted by the feed block. In Comparative Examples 3, 4, and 6, the tenter had only the first transverse stretching chamber, and stretching was also performed in one stage (hereinafter, the same applies to examples in which a tenter having only the first transverse stretching chamber was used). In Comparative Example 6, when an attempt was made to obtain a 5.5 μm polypropylene film, film formation was not possible due to film breakage (since a polypropylene film was not obtained, the evaluation in Table 1-2 is indicated by diagonal lines).

(Example 6, Comparative Examples 1, 5, 7, and 9)
A polypropylene film was obtained in the same manner as in Example 1, except that the raw material formulation and film-forming conditions were as shown in Table 1. The evaluation results are shown in Tables 1-1 and 1-2. Note that Comparative Example 5 was an unstretched film, and no steps after stretching were performed. In Comparative Example 9, an attempt was made to obtain a 4.6 μm polypropylene film, but film formation was not possible due to film breakage (since a polypropylene film could not be obtained, the evaluation in Table 1-2 is indicated by diagonal lines).

(Comparative Example 2)
Each component was mixed so that polypropylene resin 2 was 79.7 parts by mass, cyclic olefin resin (C1) was 20 parts by mass, and antioxidant was 0.3 parts by mass, and fed to a single-screw extruder set at 260 ° C., melted at a temperature of 260 ° C., and then foreign matter was removed with a sintered filter with a cutoff of 80 μm, and the molten single-layer polymer was extruded from a T-die. This was adhered to a casting drum maintained at 90 ° C. by an air knife and cooled and solidified to obtain an unstretched polypropylene film. The obtained unstretched polypropylene film was guided to a simultaneous biaxial stretching machine with both ends in the width direction held by multiple clips, preheated at 163 ° C. while holding, and then simultaneously biaxially stretched at the same temperature at a magnification of 3.6 times in the longitudinal direction and 8.2 times in the width direction. Next, the film was guided to the outside of the simultaneous biaxial stretching machine without heat treatment and relaxation treatment, and the clips at both ends in the width direction were released, and then corona discharge treatment was performed in the same manner as in Example 1 to obtain a polypropylene film. The evaluation results are shown in Table 1-2.

(Comparative Example 8)
A polypropylene film was obtained in the same manner as in Comparative Example 2, except that the raw material formulation and film-forming conditions were as shown in Table 1-2 and the film thickness after stretching was 4.5 μm. The evaluation results are shown in Table 1-2.

(Example C1)
On the corona discharge-treated surface of the polypropylene film obtained in Example 5, aluminum was vapor-deposited at a film resistance of 20 Ω/sq using a vacuum vapor deposition machine manufactured by ULVAC, Inc. During vapor deposition, a vapor deposition pattern having a so-called T-shaped margin (longitudinal pitch (period) of 17 mm, fuse width of 0.5 mm) in which a margin was provided in a direction perpendicular to the longitudinal direction using masking oil was produced as a vapor deposition film C1A, and a vapor deposition film C1B without a vapor deposition pattern having a T-shaped margin was produced. The vapor deposition films C1A and C1B were each slit to obtain vapor deposition reels C1A and C1B with a film width of 50 mm (edge margin width of 2 mm). Next, the evaporation reels C1A and C1B were alternately stacked and the capacitor elements were wound on a device winding machine (KAW-4NHB) manufactured by Kaito Seisakusho Co., Ltd. so that the capacitance of the capacitor elements after finishing was 10 μF, and after metallicon treatment, the capacitor elements were subjected to heat treatment for 12 hours under reduced pressure at 135°C, and lead wires were attached to finish the capacitor elements. The evaluation results of the obtained capacitor elements are shown in Table 2.

(Example C2, Comparative Examples C1 and C2)
A capacitor element was obtained in the same manner as in Example C1, except that the polypropylene film used was a polypropylene film shown in Table 2. The evaluation results are shown in Table 2.

Note that none of the polypropylene films in each example and in Table 1-2 are microporous films.

The polypropylene film of the present invention can be widely used for industrial purposes such as film capacitors, packaging, release agents, and tapes, and is particularly suitable for use in film capacitors that are used at high temperatures and voltages because of its excellent voltage resistance properties in high-temperature environments.

1: Part of cross section X 2: Sea part 3: Island part (domain)
4: A rectangle of 1 μm×2 μm size defined in the cross section X so that a pair of sides are parallel to the thickness direction. 5: A pair of short sides of a rectangle of 1 μm×2 μm size defined in the cross section X so that a pair of sides are parallel to the thickness direction.

Claims

A polypropylene film having a layer A containing a cyclic olefin resin and a polypropylene resin, and a cross section X of the polypropylene film cut along a plane parallel to the main orientation axis direction and the thickness direction, the polypropylene film having the layer A, and an orientation parameter I810/I840 of the cross section X measured by Raman spectroscopic analysis of the cross section X being 2.2 or more and 20 or less.
The polypropylene film according to claim 1, wherein the length in the main orientation axis direction of the domain of the cyclic olefin resin in the cross section X is 5.0 μm or more and 1 mm or less.
The polypropylene film according to claim 1 or 2, having a dielectric loss tangent of 3×10 −6 or more and 1×10 −2 or less.
The polypropylene film according to any one of claims 1 to 3, in which a pair of short sides of a rectangle of 1 μm x 2 μm in size is defined in the layer A portion of the cross section X so that the pair of short sides are parallel to the thickness direction, there are 2.0 to 1,000 domains of the cyclic olefin resin passing through the pair of short sides.
The polypropylene film according to any one of claims 1 to 4, in which the content of the cyclic olefin resin is 1.0% by mass or more and 40% by mass or less when the mass of the entire film is 100% by mass.
The polypropylene film according to any one of claims 1 to 5, in which the crystallization temperature (Tmc) during the cooling process measured by differential scanning calorimetry is 110°C or higher and 150°C or lower.
The polypropylene film according to any one of claims 1 to 6, having a thickness of 0.5 μm or more and 15 μm or less.
The polypropylene film according to any one of claims 1 to 7, in which the value of tear strength in the direction perpendicular to the main orientation axis/tear strength in the main orientation axis direction is 0.10 or more and 10.0 or less.
The polypropylene film according to any one of claims 1 to 8, in which the ash content is 0.0 ppm or more and 1000 ppm or less when the mass of the entire polypropylene film is taken as 100 mass %.
The polypropylene film according to any one of claims 1 to 9, wherein the A layer portion of the cross section X has a structure in which a cyclic olefin resin is diffused into the sea portion of the polypropylene resin.
A metal film laminated film having a metal film on at least one side of the polypropylene film described in any one of claims 1 to 10.
A film capacitor using the metal film laminate film described in claim 11.
The film capacitor according to claim 12, having a capacitance density of 1.1 μF/cm 3 or more and 18 μF/cm 3 or less.
The film capacitor according to claim 12 or 13, having a withstand voltage of 0.60 kV or more at 135°C.
A power control unit having a film capacitor according to any one of claims 12 to 14.
An electric vehicle having the power control unit according to claim 15.
An electric aircraft having the power control unit according to claim 15.
A packaging material made using the polypropylene film described in any one of claims 1 to 10.
A method for producing a polypropylene film according to any one of claims 1 to 18, comprising the steps of: a casting step of melt-extruding a resin composition containing a polypropylene resin and a cyclic olefin resin onto a support to form a polypropylene resin sheet; and a stretching step of stretching the polypropylene resin sheet in the longitudinal direction and the width direction, in that order, wherein in the stretching step, stretching in the width direction is performed by the following two-stage stretching method (I).
Two-stage stretching method (I):
First-stage stretching: Stretching in the width direction is performed at a temperature of 160° C. to 185° C. and a stretching ratio of a, where the final stretching ratio in the width direction is a×b.
Second-stage stretching: The film is further stretched in the width direction at a stretch ratio of b times at a temperature 1° C. to 20° C. lower than the temperature in the first-stage stretching.
The method for producing a polypropylene film according to claim 19, wherein the areal stretching ratio is 40 times or more.
The method for producing a polypropylene film according to claim 19 or 20, wherein the stretching in the stretching step is performed by a sequential biaxial stretching method in which stretching in the longitudinal direction is followed by stretching in the width direction.