Biaxially oriented polypropylene film

Abstract

To provide a biaxially-oriented polypropylene film that has high productivity, processability, and voltage resistance and also has a surface shape that allows uniformly controlling the air volume and clearance distance between film layers of a capacitor in the longitudinal and width directions of the film for proper security, mainly in a large-capacitance capacitor.SOLUTION: A biaxially-oriented polypropylene film has front and back faces, at least one of which is an X face, which has a maximum point height Sxp of 30 nm or more and 90 nm or less.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a biaxially oriented polypropylene film having high breakdown voltage resistance in a high temperature and high voltage environment when used as a dielectric of a capacitor.

Background Art

[0002] Biaxially oriented polypropylene films are excellent in transparency, mechanical properties, electrical properties, etc., and are used in various applications such as packaging, tape, electrical applications including cable wrapping and capacitors.

[0003] Among them, in capacitor applications, due to their excellent high breakdown voltage characteristics and low loss characteristics, they are particularly preferably used as dielectrics of capacitors. Recently, various electrical equipment is being inverterized, and accordingly, the requirements for miniaturization and large capacity of capacitors have been further strengthened. Furthermore, especially in automotive applications (including hybrid cars and electric cars), solar power generation, and wind power generation applications, the use environment is getting hotter (showing 85°C or higher and 125°C or lower), and the heat resistance requirements for capacitors are increasing.

[0004] Therefore, there is a demand for thinning, heat resistance improvement, and improvement of breakdown voltage per thickness of the biaxially oriented polypropylene film as a dielectric, and there is also a demand for improvement of the safety of capacitors. Here, the safety of a capacitor is a function of maintaining insulation by scattering the deposited metal by the discharge energy during abnormal discharge in a metal vapor deposition capacitor having a metal vapor deposition film formed on a dielectric film as an electrode, and is an important function for preventing short circuits and breakdowns of the capacitor. In order to enhance the safety of capacitors, it is known that control of the amount of air and the gap distance between film layers constituting the capacitor is important. In particular, when the size is large as in the case of a large-capacity capacitor of 100 μF or more, it is an issue to uniformly control the amount of air and the gap distance in the longitudinal and width directions of the capacitor.

[0005] To improve the voltage withstand capability per unit thickness of the film and enhance the safety of the capacitor, research has primarily focused on controlling the surface properties of the film. One known method for controlling the surface properties of a film is to utilize the crystal transition from β-crystal to α-crystal of polypropylene (hereinafter referred to as the β-crystal method). This method utilizing the crystal transition does not require the addition of impurities such as additives that may worsen the voltage withstand capability, and is therefore preferred as a method for roughening biaxially oriented polypropylene films for capacitors (see, for example, Patent Documents 1 and 2).

[0006] As techniques that focus on the density of surface roughness and the uniformity of protrusions, methods such as adding branched polypropylene (see, for example, Patent Documents 3 and 4) and mixing polypropylenes with different molecular weights and molecular weight distributions (see, for example, Patent Document 5) have been proposed. With these methods, the spherulite size can be controlled to be small, so it is possible to form high-density protrusions with uniform height.

[0007] Furthermore, as technologies that focus on the convex and concave parts of surface roughness, methods have been proposed such as annealing an unstretched sheet (see, for example, Patent Document 6) and high-temperature and high-pressure treatment of a longitudinally stretched sheet (see, for example, Patent Document 7). These methods allow for uniform control of the height of the convex and concave parts. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2008-133446 [Patent Document 2] Japanese Patent Publication No. 2014-077057 [Patent Document 3] WO2007 / 094072 publication [Patent Document 4] WO2012 / 121256 publication [Patent Document 5] Japanese Patent Publication No. 2014-231584 [Patent Document 6] Japanese Patent Publication No. 2019-172972 [Patent Document 7] Japanese Patent Publication No. 2019-172973 [Overview of the project] [Problems that the invention aims to solve]

[0009] When applying the β-crystallization method described in Patent Documents 1 and 2 to a general linear polypropylene film, steep, crater-like protrusions and depressions are formed at a low density, and dielectric breakdown tends to occur particularly easily in the depressions. Furthermore, when applying the methods described in Patent Documents 3, 4, and 5, which form protrusions of uniform height at high density, or when applying the methods described in Patent Documents 6 and 7, which uniformly control the height of the protrusions and depressions in the surface roughness, localized coarse protrusions cannot be suppressed, and the control of the amount of air between film layers, which relates to dielectric strength and safety in recent high-temperature and high-voltage environments, has not been sufficient.

[0010] Therefore, the object of the present invention is to provide a biaxially oriented polypropylene film that has high processability and voltage resistance, and in order to obtain appropriate safety mainly in large-capacity capacitors, suppresses the formation of localized coarse protrusions on the film surface and has a densely roughened surface. [Means for solving the problem]

[0011] The above-mentioned problems can be solved as follows: The biaxially oriented polypropylene film of the present invention is characterized in that, when the plane with a maximum point height Sxp of 30 nm or more and 90 nm or less is defined as the X plane, at least one of the planes is the X plane. [Effects of the Invention]

[0012] The present invention provides a biaxially oriented polypropylene film that has high processability and voltage resistance, and suppresses the formation of localized coarse protrusions on the film surface, while also providing a densely roughened surface, in order to obtain appropriate safety mainly in large-capacity capacitors. When the biaxially oriented polypropylene film of the present invention is used as a dielectric in a capacitor, it exhibits excellent processability and voltage resistance, and furthermore, the amount of air between film layers and the interlayer distance can be uniformly controlled in the longitudinal and width directions of the film during capacitor processing. As a result, when used as a capacitor, it functions with high safety even in high-temperature and high-voltage environments, and the lifespan of the capacitor is also improved. [Modes for carrying out the invention]

[0013] The biaxially oriented polypropylene film of the present invention will be described in more detail below. In the numerical ranges expressed using "~" below, the upper and lower limits are included within that range, and the units of the upper and lower limits are the same.

[0014] The biaxially oriented polypropylene film of the present invention is a biaxially oriented polypropylene film obtained by stretching a cast sheet in two directions: the longitudinal direction and the width direction. In other words, biaxial orientation as used here means stretching in the longitudinal direction and the width direction. Furthermore, in the present invention, polypropylene resin refers to a resin in which, when the total constituent units of the resin are set to 100 mol%, propylene units are contained in amounts exceeding 50 mol% and not exceeding 100 mol%.

[0015] The biaxially oriented polypropylene film of the present invention mainly comprises a polypropylene resin. The polypropylene resin may include not only a homopolymer of propylene, but also polypropylene copolymers and branched-chain polypropylenes described later. In the present invention, "main component" means that the proportion of the main component in 100% by mass of the total components of the film is more than 50% by mass and 100% by mass or less, more preferably 80% by mass or more and 100% by mass or less, even more preferably 90% by mass or more and 100% by mass or less, and particularly preferably 95% by mass or more and 100% by mass or less. Components other than the polypropylene resin in the film include resins described later, as well as additives such as antioxidants and lubricants. If the film contains multiple types of polypropylene resins, it can be interpreted as mainly comprising polypropylene resin if the total content of all polypropylene resins exceeds 50% by mass.

[0016] As the polypropylene copolymer, polypropylene copolymers copolymerized with other unsaturated hydrocarbons can be suitably used. Examples of copolymer components of the above polypropylene copolymer include ethylene, 1-butene, 1-pentene, 3-methylpentene-1, 3-methylbutene-1, 1-hexene, 4-methylpentene-1, 5-ethylhexene-1, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-heptadecene, 1-octadecene, 1-eicosene, vinylcyclohexene, styrene, allylbenzene, cyclopentene, norbornene, and 5-methyl-2-norbornene. From the viewpoint of dielectric strength and dimensional stability, the copolymerization amount of the polypropylene copolymer is preferably 1 mol% or less when the total constituent units of the resin are considered to be 100 mol%.

[0017] In addition, other polymers (resins) other than polypropylene resin may be added to the homopolymer of propylene. As other polymers that can be added to the homopolymer of propylene, homopolymers of unsaturated hydrocarbons other than propylene, copolymers of unsaturated hydrocarbons containing propylene units, etc. can be used. From the viewpoints of withstand voltage characteristics and dimensional stability, the content of other polymers is preferably 20% by mass or less when the total resin components of the biaxially oriented polypropylene film are 100% by mass.

[0018] Furthermore, the biaxially oriented polypropylene film of the present invention preferably contains a highly stereoregular polypropylene resin A as a main component and a low stereoregular polypropylene resin B and a high melt tension polypropylene resin H. The highly stereoregular polypropylene resin A, the low stereoregular polypropylene resin B, and the high melt tension polypropylene resin H will be described later.

[0019] The biaxially oriented polypropylene film of the present invention is characterized in that at least one surface is an X surface when a surface having a maximum peak height Sxp of 30 nm or more and 90 nm or less is defined as the X surface. The maximum peak height Sxp is a kind of functional parameter defined in ISO25178 (2012), and is the difference in height between the load area ratio p% and the load area ratio q% in the bearing curve of height data (the frequency of a certain height is accumulated from the higher side and shown as a percentage with the total number of all height data being 100%. The load area ratio at a certain height C is Smr(C)). In the present invention, p = 2.5 (%) and q = 五十(%)). The maximum peak height Sxp is more preferably 35 nm or more and 85 nm or less, further preferably 40 nm or more and 80 nm or less, and particularly preferably 45 nm or more and 75 nm or less.

[0020] It should be noted that there is an error in the original text where "五十" in the description of "q = 五十(%)" should be "50". The above translation has been corrected accordingly.By setting the maximum point height Sxp to 30 nm or more, it is possible to suppress the generation of conveyance wrinkles in the film conveyance process during film formation and capacitor element processing, and it is possible to reduce the deterioration of the winding shape of the film roll and the appearance and internal shape defects of the capacitor element. Further, it is possible to prevent the interlayer distance between the film layers in the capacitor element from becoming too short, suppress the short-circuit breakdown of the capacitor element, and improve the life of the capacitor. On the other hand, by setting the maximum point height Sxp to 90 nm or less, it is possible to suppress the occurrence of film meandering in the film conveyance process during film formation and capacitor element processing, and prevent the deterioration of the winding shape of the film roll and the appearance and internal shape defects of the capacitor element. Furthermore, it is possible to prevent the interlayer distance between the film layers in the capacitor element from becoming too long, suppress the excessive operation of the safety function of the capacitor element, and improve the life of the capacitor. As a method of setting the maximum point height Sxp to 30 nm or more and 90 nm or less or the above preferred range, a method is preferably used in which a highly stereoregular polypropylene resin A is contained with a low stereoregular polypropylene resin B and a high melt tension polypropylene resin H and formed into a cast sheet, and then heat treatment is performed by passing the cast sheet through a temperature-controlled conveyance roll while nipping it with a temperature-controlled nip roll before the longitudinal stretching process. When the above method of performing heat treatment on the cast sheet is used, if the heat treatment temperature is high or the heat treatment time is long, the maximum point height Sxp is likely to decrease. On the other hand, if the heat treatment temperature is low or the heat treatment time is short, the maximum point height Sxp is likely to increase.

[0021] In the biaxially oriented polypropylene film of the present invention, when the film thickness is t (μm), it is preferable that Sxp / t is 10.0 nm / μm or more and 90.0 nm / μm or less on at least one X plane. More preferably, Sxp / t is 10.0 nm / μm or more and 40.0 nm / μm or less, still more preferably 10.0 nm / μm or more and 35.0 nm or less, and particularly preferably 20.0 nm / μm or more and 35.0 nm or less.

[0022] When Sxp / t is 10.0 nm / μm or higher, the occurrence of transport wrinkles in the film transport process during film formation and capacitor element processing can be suppressed, reducing deterioration of the film roll's winding shape and defects in the appearance and internal shape of the capacitor elements. On the other hand, when Sxp / t is 90.0 nm / μm or lower, it prevents the thickness of the thicker parts from becoming too thin relative to the overall thickness of the film including the protrusions, and can suppress a decrease in dielectric strength, especially in ultra-thin films of 3.0 μm or less. As a method to bring Sxp / t into the above preferred range, a method of controlling the heat treatment temperature and heat treatment time according to the film thickness in the heat treatment process before longitudinal stretching of the cast sheet is preferably used. When the film is thick, t becomes large, so Sxp / t decreases, but Sxp / t tends to increase by lowering the heat treatment temperature or shortening the heat treatment time. On the other hand, when the film is thin, t becomes small, so Sxp / t increases, but Sxp / t tends to decrease by raising the heat treatment temperature or lengthening the heat treatment time. Furthermore, the film thickness t can be controlled to be thicker by increasing the amount of resin extruded during the resin extrusion process, and thinner by decreasing the amount of resin extruded.

[0023] The biaxially oriented polypropylene film of the present invention preferably has a load area ratio Smr1 that separates the protruding peaks and the core portion of at least one of the X-planes from 7.0% to 13.0%. The load area ratio Smr1 that separates the protruding peaks and the core portion is a type of functional parameter defined in ISO 25178 (2012), and represents the load area ratio at the intersection of the height of the top of the core portion and the bearing curve, when the intersection point with the load area ratio of 0% is defined as the height of the top of the core portion. This is the equivalent straight line of the bearing curve of the height data (the straight line with the smallest slope among the straight lines where the difference in load area ratio is 40%).

[0024] The load area ratio Smr1 separating the protruding peaks and the core is more preferably 7.5% to 12.5%, and even more preferably 9.0% to 12.0%. When the load area ratio Smr1 separating the protruding peaks and the core is 7.0% or more, it prevents the number of protrusions on the film surface from becoming too small, improving the processability of the element. On the other hand, when the load area ratio Smr1 separating the protruding peaks and the core is 13.0% or less, it prevents the number of protrusions on the film surface from increasing and the variation in the height of the protrusions from becoming too large, and allows for uniform control of the interlayer distance between film layers in the width direction and longitudinal direction during capacitor element fabrication. Therefore, the safety of the capacitor element is improved, and the lifespan of the capacitor is easily improved. As a method for setting the load area ratio Smr1 separating the protruding peaks and the core within the above preferred range, a method of incorporating a branched-chain polypropylene resin (high-melt-tension polypropylene resin H) having appropriate melt tension is preferably used. When using a method that incorporates branched polypropylene resin, lowering the melt tension of the branched polypropylene resin tends to reduce the load area ratio Smr1 that separates the protruding peaks from the core.

[0025] The biaxially oriented polypropylene film of the present invention preferably has a load area ratio Smr2 that separates the protruding valley portion and the core portion on at least one X-plane of 85.0% or more and 95.0% or less. The load area ratio Smr2 that separates the protruding valley portion and the core portion is a type of functional parameter defined in ISO25178(2012), and represents the load area ratio at the intersection of the height of the lower part of the core portion and the bearing curve, when the intersection point of the equivalent straight line of the bearing curve of the height data (the straight line with the smallest slope among the straight lines where the difference in load area ratio is 40%) with a load area ratio of 100% is defined as the height of the lower part of the core portion. The load area ratio Smr2 that separates the protruding valley portion and the core portion is more preferably 86.0% or more and 94.0%, more preferably 87.0% or more and 92.0%, even more preferably 87.0% or more and 90.5%, and particularly preferably 89.0% or more and 90.5%.

[0026] When the load area ratio Smr2 separating the protruding valley portion and the core portion is 85.0% or higher, it prevents the number of recesses on the film surface from increasing and the variation in recess depth from becoming too large, allowing for uniform control of the interlayer distance between film layers in the width and longitudinal directions during capacitor element fabrication, improving the safety of the capacitor element and making it easier to improve the lifespan of the capacitor. On the other hand, when the load area ratio Smr2 separating the protruding valley portion and the core portion is 95.0% or lower, it prevents the number of recesses on the film surface from becoming too small, improving the processability of the element. As a method for setting the load area ratio Smr2 separating the protruding valley portion and the core portion within the above preferred range, a method of incorporating a branched-chain polypropylene resin having appropriate melt tension is suitably used. By incorporating a branched-chain polypropylene resin, Smr2 can be increased.

[0027] The biaxially oriented polypropylene film of the present invention preferably has X-planes on both sides. If one side of the film is the A-plane and the other side is the B-plane, then in a capacitor element, the A-plane and B-plane sides are usually in contact, and air exists in the gap between the layers. Therefore, by making both sides of the film X-planes, the amount of interlayer air and the unevenness of the gap distance during capacitor element formation can be controlled more appropriately, and the lifespan of the capacitor tends to improve, especially at high temperatures. One method for making both sides of the film X-planes is to heat-treat a cast sheet containing all of the high stereoregularity polypropylene resin A, low stereoregularity polypropylene resin B, and high melt tension polypropylene resin H before longitudinal stretching, and it is also effective to appropriately control the temperature of the conveyor roll and nip roll during the heat treatment.

[0028] In the biaxially oriented polypropylene film of the present invention, it is preferable that the Sxp values ​​on both sides are different. When the Sxp values ​​on both sides are different, that is, when the Sxp value on one side is large and the Sxp value on the other side is small, it is easier to achieve both processability and dielectric strength of the film, and especially in ultrathin films of 3.0 μm or less, it is easier to achieve both high device processability and high dielectric strength. One means of adjusting the Sxp values ​​on both sides to be different is to apply heat treatment to the cast sheet before longitudinal stretching for a short time (e.g., less than 1.0 second). Here, "the Sxp values ​​on both sides are different" means that the difference in Sxp values ​​on both sides is 2 nm or more.

[0029] The biaxially oriented polypropylene film of the present invention is preferably subjected to surface treatments such as corona discharge treatment, plasma treatment, glow treatment, or flame treatment after biaxial stretching in order to improve adhesion to metal during metal deposition. Polypropylene films typically have low surface energy, and their surface wetting tension, an indicator of this energy, is about 30 mN / m, which presents challenges in adhesion to metal. These surface treatments increase the surface energy of the polypropylene film, and by raising the surface wetting tension to 42 mN / m or more and 48 mN / m or less through these surface treatments, adhesion to metal during metal deposition is improved. These surface treatments may be used individually or in combination.

[0030] Next, the polypropylene resin used in the biaxially oriented polypropylene film of the present invention will be described. The biaxially oriented polypropylene film of the present invention preferably has a high stereoregularity polypropylene resin A as its main component and contains a low stereoregularity polypropylene resin B and a high melt tension polypropylene resin H. By containing the low stereoregularity polypropylene resin B and the high melt tension polypropylene resin H, the melting point of the cast sheet can be appropriately controlled, and partial melting of the recesses can be efficiently carried out in the heat treatment process of the cast sheet described later. In addition, it is possible to prevent the size of the β crystals formed on the cast sheet from becoming excessively large and the density of the β crystals from becoming excessively high. Therefore, the surface shape (X surface) of the present invention can be formed more easily.

[0031] The highly stereoregular polypropylene A used in the biaxially oriented polypropylene film of the present invention refers to isotactic polypropylene resin. This isotactic polypropylene resin is known as a polypropylene resin commonly used in capacitor applications. The highly stereoregular polypropylene A used in the present invention is a linear polypropylene resin, and it is preferable that the mesopentade fraction (hereinafter sometimes referred to as mmmm) is 96.0% or more, the MFR is 0.5 g / 10 min to 5.0 g / 10 min, and the MS is 1.0 g or less. Specific examples of highly stereoregular polypropylene resin A include Borclean (trademark) (HC300BF, HC318BF, etc.) manufactured by Borealis.

[0032] The mmmm of highly stereoregular polypropylene resin A is preferably 96.0% or higher, and more preferably 97.0% or higher, from the viewpoint of dielectric strength in high-temperature environments. mmmm is an index indicating the stereoregularity of the crystalline phase of the polypropylene resin, measured by nuclear magnetic resonance (NMR) spectroscopy. The higher the mmmm value of the polypropylene resin, the higher the degree of crystallinity and melting point, and in particular, the improved dielectric strength at high temperatures. To obtain such a highly stereoregular polypropylene resin, methods such as washing the resin powder obtained with a solvent such as n-heptane, selecting a catalyst and / or co-catalyst, and selecting the composition as appropriate are preferably employed.

[0033] The melt flow rate (MFR) of highly stereoregular polypropylene resin A is preferably 0.5 g / 10 min to 5.0 g / 10 min, more preferably 1.0 g / 10 min to 4.5 g / 10 min, and even more preferably 1.5 g / 10 min to 4.0 g / 10 min, when measured in accordance with JIS K 7210-1 (2014). When the MFR of highly stereoregular polypropylene resin A is within the above preferred range, it exhibits excellent film-forming properties, resulting in a stable biaxially oriented polypropylene film, as well as excellent dielectric strength when used as a biaxially oriented polypropylene film. To achieve the above range for the MFR of the polypropylene resin, methods such as appropriately selecting the type and composition of the catalyst and the polymerization temperature, or polymerization in the presence of hydrogen, are preferably employed.

[0034] The melt tension (MS) of the highly stereoregular polypropylene resin A is preferably 1.0 g or less. When the MS of the highly stereoregular polypropylene resin A is 1.0 g or less, the flow properties are improved when the resin is melted, and the occurrence of uneven film thickness and film tearing can be suppressed.

[0035] In the biaxially oriented polypropylene film of the present invention, the content of highly stereoregular polypropylene resin A is preferably more than 50% by mass and 98.5% by mass or less, more preferably 80% by mass or more and 97% by mass or less, and even more preferably 85% by mass or more and 95% by mass or less, when the total amount of polypropylene resin constituting the film is taken as 100% by mass. When the content of highly stereoregular polypropylene resin A is within the above preferred range, the stereoregularity of the biaxially oriented polypropylene film is increased, and it exhibits excellent dielectric strength.

[0036] Next, the low stereoregularity polypropylene resin B will be described. The low stereoregularity polypropylene resin B used in the biaxially oriented polypropylene film of the present invention refers to a polypropylene resin with low stereoregularity, more specifically, a polypropylene resin in which mmmm is 30% or more and 60% or less, and MFR is 200 g / 10 min or more and 600 g / 10 min or less. In order to obtain the low stereoregularity polypropylene resin, a method using a metallocene-based catalyst during the polymerization of the polypropylene resin (for example, the method described in WO2003 / 087172) is preferably used. As a specific example of the low stereoregularity polypropylene resin B, "L-MODU" (registered trademark) (S600, etc.) manufactured by Idemitsu Kosan Co., Ltd. is used.

[0037] The mmmm of the low stereoregularity polypropylene resin B is 30% to 60%, preferably 35% to 55%, and more preferably 40% to 50%. When the mmmm of the low stereoregularity polypropylene resin B is within the above preferred range, it prevents the melting point of the cast sheet from becoming too high, allows for efficient partial melting of the recesses in the heat treatment process of the cast sheet described later, and makes it easier to obtain the surface shape of the present invention (the X surface). It also prevents the stereoregularity of the film from decreasing too much, making it easier to maintain dielectric strength. In order to keep the mmmm of the polypropylene resin within the above range, a method of appropriately selecting a polymerization catalyst is preferably employed.

[0038] The MFR of low stereoregularity polypropylene resin B, when measured in accordance with JIS K 7210-1 (2014), is preferably 200 g / 10 min to 600 g / 10 min, more preferably 250 g / 10 min to 550 g / 10 min, and more preferably 300 g / 10 min to 500 g / 10 min. When the MFR of low stereoregularity polypropylene resin B is within the above range, it prevents the melting point of the cast sheet from becoming too high, and in the heat treatment process of the cast sheet described later, partial melting of the recesses proceeds efficiently, making it easier to obtain the surface shape of the present invention (the X surface). In order to keep the MFR of the polypropylene resin within the above range, methods such as appropriately selecting the type and composition of the catalyst and the polymerization temperature, or polymerization in the presence of hydrogen are preferably employed.

[0039] In the biaxially oriented polypropylene film of the present invention, the content of low stereoregularity polypropylene resin B is preferably 0.5% by mass or more and 5.0% by mass or less, more preferably 1.0% by mass or more and 4.5% by mass or less, and even more preferably 1.5% by mass or more and 4.0% by mass or less, when the total amount of polypropylene resin constituting the film is considered as 100% by mass. When the content of high stereoregularity polypropylene resin B is within the above preferred range, it prevents the melting point of the cast sheet from becoming too high, allows for efficient partial melting of the recesses in the heat treatment process of the cast sheet described later, and makes it easier to obtain the surface shape of the present invention (the X surface). Furthermore, it prevents the stereoregularity of the film from decreasing too much and makes it easier to maintain dielectric strength.

[0040] Next, the high melt-tension polypropylene resin H will be described. The high melt-tension polypropylene resin H used in the biaxially oriented polypropylene film of the present invention is a branched-chain polypropylene resin having an MFR of 4 g / 10 min or more and 12 g / 10 min or less, and an MS of 2.0 g or more and 40 g or less. To obtain high melt-tension polypropylene, methods such as using high-energy ionization radiation on the polypropylene resin (for example, Japanese Patent Publication No. 62-121704), reacting the polypropylene resin with a specific organic peroxide (for example, Japanese Patent No. 2869606), reacting the polypropylene resin with a pyrolytic radical-forming agent and an ethylene-based polyfunctional unsaturated monomer (for example, Japanese Patent Publication No. 10-330436), and using a specific catalyst during the polymerization of the polypropylene resin (for example, Japanese Patent Publication No. 2009-057542) are preferably used. Specific examples of high melt-tension polypropylene resin H include "WAYMAX" (registered trademark) (EX4000, MFX3, etc.) manufactured by Nippon Polypropylene Co., Ltd.

[0041] High melt tension polypropylene resin H has a branched structure in its molecular chain. Branched polypropylene resin is defined as a polypropylene resin having five or fewer internal trisubstituted olefins per 10,000 carbon atoms. The presence of these internal trisubstituted olefins can be confirmed by the proton ratio in the 1H-NMR spectrum. While the branched polypropylene resin acts as an α-nucleating agent, it can also form a rough surface due to its crystalline form within a certain range of addition amounts. That is, the size of the polypropylene spherulites generated during the cooling process of the melt-extruded resin sheet can be controlled to be small, resulting in a biaxially oriented polypropylene film with excellent voltage resistance.

[0042] The melting frame rate (MFR) of the high melt-tension polypropylene resin H is 4.0 g / 10 min to 12 g / 10 min, preferably 5.0 g / 10 min to 11 g / 10 min, and more preferably 6.0 g / 10 min to 10 g / 10 min. When the MFR of the high melt-tension polypropylene resin H is within the above range, it prevents the melting point of the unstretched sheet from becoming too high, allows for efficient partial melting of the recesses in the heat treatment process of the cast sheet described later, and makes it easier to obtain the surface shape of the present invention (the X surface). It also prevents the melting point of the film from becoming too low, making it easier to maintain dielectric strength, especially at high temperatures. In order to keep the MFR of the polypropylene resin within the above range, methods such as appropriately selecting the type and composition of the catalyst and the polymerization temperature, or polymerization in the presence of hydrogen are preferably employed.

[0043] The mass of the high melt-tension polypropylene resin H is 1.5 g to 40 g, preferably 2.0 g to 20 g, and more preferably 2.5 g to 8.0 g. When the mass of the high melt-tension polypropylene resin H is within the above range, it prevents the size of the β crystals formed on the cast sheet from becoming too large and the density of the β crystals from becoming too high, making it easier to obtain the surface shape of the present invention (the X surface). In order to keep the mass of the polypropylene resin within the above range, it is preferable to employ a method of appropriately selecting the type and composition of the catalyst.

[0044] In the biaxially oriented polypropylene film of the present invention, the content of high melt-tension polypropylene resin H is preferably 1.0% to 10% by mass, more preferably 2.0% to 9.0% by mass, and even more preferably 3.0% to 8.0% by mass, when the total polypropylene resin constituting the film is considered as 100% by mass. When the content of high melt-tension polypropylene resin H is within the above preferred range, the size and density of the β crystals can be appropriately controlled, and the melting point of the unstretched sheet can be prevented from becoming too high. Furthermore, partial melting of the recesses can proceed efficiently in the heat treatment process of the cast sheet described later, making it easier to obtain the surface shape of the present invention (the X surface).

[0045] In this way, the biaxially oriented polypropylene film of the present invention preferably has the following characteristics: when a polypropylene resin having a mesopentad fraction (mmmm) of 30% or more and a melt flow rate (MFR) of 200 g / 10 min or more and 600 g / 10 min is designated as low stereoregularity polypropylene resin B, and a polypropylene resin having an MFR of 4 g / 10 min or more and 12 g / 10 min or less and a melt tension (MS) of 1.5 g or more and 40 g or less is designated as high melt tension polypropylene resin H, then of the total 100% by mass of the polypropylene resin constituting the film, the low stereoregularity polypropylene resin B is 0.5% by mass or more and 5.0% by mass or less, and the high melt tension polypropylene resin H is 1.0% by mass or more and 10% by mass or less. By adopting this configuration, the melting point of the cast sheet can be appropriately controlled, partial melting of the recesses can be efficiently carried out in the heat treatment process of the cast sheet described later, and the size of the β crystals formed on the cast sheet can be prevented from becoming too large and the density of the β crystals can be prevented from becoming too high, making it easier to obtain the surface shape of the present invention (the X surface).

[0046] The biaxially oriented polypropylene film of the present invention may also preferably contain various additives, such as nucleating agents, antioxidants, heat stabilizers, lubricants, antistatic agents, antiblocking agents, fillers, viscosity modifiers, and anticoloring agents, to the extent that it does not impair the objectives of the present invention.

[0047] Among the additives mentioned above, the selection of the type and amount of antioxidant is important from the viewpoint of long-term heat resistance. Specifically, the antioxidant should be a sterically hindered phenol type, and at least one of them should be a high molecular weight type with a molecular weight of 500 or more. Specifically, for example, it is preferable to use 2,6-di-t-butyl-p-cresol (BHT: molecular weight 220.4), 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene (e.g., BASF's "Irganox"® 1330: molecular weight 775.2), tetrakis[methylene-3(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane (e.g., BASF's "Irganox"® 1010: molecular weight 1177.7) alone or in combination. The total content of these antioxidants is preferably 0.03 parts by mass or more and 1.0 part by mass or less relative to the total amount of polypropylene resin, and more preferably 0.1 parts by mass or more and 0.9 parts by mass or less. When the antioxidant content in the polypropylene resin composition is 0.03 parts by mass or more, the antioxidant effect is easily obtained and long-term heat resistance is easily maintained. On the other hand, when the antioxidant content in the polypropylene resin composition is 1.0 part by mass or less, high-temperature voltage resistance characteristics are easily maintained.

[0048] The biaxially oriented polypropylene film of the present invention is preferably obtained by molding a polypropylene resin composition, mainly composed of the above-mentioned highly stereoregular polypropylene resin A, and containing a low stereoregular polypropylene resin B and a high melt tension polypropylene resin H, into a sheet, and then biaxially stretching it. The biaxial stretching can be performed by any of the following methods: simultaneous inflation biaxial stretching, simultaneous tenter biaxial stretching, or sequential tenter biaxial stretching, but sequential tenter biaxial stretching is preferred from the viewpoint of film formation stability and thickness uniformity. In particular, it is preferable to stretch in the longitudinal direction first, and then in the width direction.

[0049] Next, the method for producing the biaxially oriented polypropylene film of the present invention will be described below, but is not necessarily limited thereto.

[0050] First, the highly stereoregular polypropylene resin A, the low stereoregular polypropylene resin B, and the high melt tension polypropylene resin H are dry-blended in the preferred ratios described above and supplied to a single-screw melt extruder, where they are melt-extruded at a temperature of 200°C to 260°C. Next, foreign matter and modified polymers are removed using a filter installed in the middle of the polymer tube. Then, the molten sheet is discharged from the T-die onto a cast drum, cooled and solidified to form a cast sheet, and subsequently cooled with a cooling roll controlled to a low temperature.

[0051] The temperature of the casting drum is preferably 70°C to 110°C, more preferably 75°C to 105°C, and even more preferably 80°C to 100°C, from the viewpoint of appropriately generating β-crystals and spherulites. Setting the casting drum temperature to 70°C or higher prevents too few β-crystals from forming in the cast sheet and maintains the slipperiness of the film obtained after biaxial stretching, thereby preventing the occurrence of transport wrinkles in the film transport process during film formation and processing, and preventing deterioration of the winding shape of the film roll. On the other hand, setting the casting drum temperature to 110°C or lower prevents excessive formation of β-crystals in the cast sheet, making it easier to prevent meandering in the film transport process during film formation and processing, and preventing deterioration of the winding shape of the film roll.

[0052] Preferably, the molten sheet discharged from the T-die lands on the casting drum and remains in contact with the drum for 1 second to 3 seconds. A contact time of 1 second or more facilitates the solidification of the molten sheet and helps prevent breakage during the subsequent stretching process. On the other hand, a contact time of 3 seconds or less prevents the excessive formation of β crystals in the cast sheet, which helps prevent meandering during the film transport process in film formation and processing, and deterioration of the winding shape of the film roll.

[0053] While methods such as electrostatic application, air knife method, nip roll method, and underwater casting can be used to adhere the molten sheet to the casting drum, the air knife method is preferred from the viewpoint of suppressing thickness unevenness, high-speed film formation, and controlling the surface properties of the film.

[0054] The temperature of the cooling roll used to cool the cast sheet is preferably between 10°C and 50°C. Setting the cooling temperature above 10°C makes it easier to raise the film to the desired temperature in the subsequent high-temperature heat treatment process. On the other hand, setting the cooling temperature below 50°C reduces crystal formation in the cast sheet, making it easier to reduce the longitudinal variation in surface irregularities of the film obtained after biaxial stretching.

[0055] To obtain the biaxially oriented polypropylene film of the present invention, it is preferable to perform high-temperature heat treatment on the cast sheet before the longitudinal stretching process. Generally, the crater-like surface irregularities characteristic of biaxially oriented polypropylene films using the β-crystal method occur first, in the longitudinal stretching process, when the formation of depressions due to the α-crystal transition of the β-crystal (volume reduction due to the transition) and the mechanical deformation of these depressions in the longitudinal stretching direction occur simultaneously, forming wedge-shaped depressions on the surface of the longitudinally stretched sheet. Next, in the transverse stretching process, the wedge-shaped depressions are stretched, and the edges of the depressions deform into protrusions, thereby forming crater-like surface irregularities.

[0056] To obtain the surface irregularities of the biaxially oriented polypropylene film of the present invention, it is preferable to cause the formation of depressions due to the α-crystal transition of β-crystals and the mechanical deformation of the depressions in the longitudinal stretching direction in separate processes. Furthermore, it is even more preferable to apply an excessive amount of heat when the β-crystals are transitioned to α-crystals to partially melt the depressions formed during the α-crystal transition. Specifically, it is preferable to heat-treat the cast sheet by passing it through a temperature-controlled transport roll while nipping it with a temperature-controlled nip roll before the longitudinal stretching process. It is also more preferable to cool the cast sheet after the heat treatment to stop the partial melting.

[0057] The transport roll temperature during heat treatment is preferably 160°C to 170°C, and more preferably 160°C to 168°C. By setting the transport roll temperature to 160°C or higher, not only is the transition from β-crystal to α-crystal easier to occur, but partial melting of the depressions formed by the α-crystal transition is also easier to achieve, thus making it easier to obtain the surface shape (X-plane) of the biaxially oriented polypropylene film of the present invention. On the other hand, by setting the transport roll temperature to 170°C or lower, it is possible to prevent the entire cast sheet from melting and tearing, making it easier to maintain mass production.

[0058] The nip roll temperature is preferably between 160°C and 170°C, and more preferably between 160°C and 168°C. Setting the nip roll temperature to 160°C or higher reduces curling of the cast sheet on the nip roll caused by the temperature difference between the front and back sides, thereby reducing tearing during the stretching process and facilitating mass production. On the other hand, setting the nip roll temperature to 170°C or lower reduces breakage due to the melting of the entire cast sheet, making it easier to maintain mass production.

[0059] The nip roll pressure is preferably 0.30 MPa to 0.60 MPa, and more preferably 0.35 MPa to 0.55 MPa. Setting the nip roll pressure to 0.30 MPa or higher suppresses the occurrence of wrinkles due to film expansion during heat treatment, thereby reducing tearing in the longitudinal stretching process and facilitating mass production. On the other hand, setting the nip roll pressure to 0.60 MPa or lower suppresses film deformation due to pressure, thereby reducing breakage in the stretching process and facilitating mass production.

[0060] The heat treatment time for the cast sheet is preferably 0.1 seconds or more and 1.0 second or less, more preferably 0.2 seconds or more and 0.9 seconds or less, and even more preferably 0.3 seconds or more and 0.8 seconds or less. By setting the heat treatment time to 0.1 seconds or more, not only is the transition from β crystal to α crystal easier, but partial melting of the depressions due to volume reduction is also more likely to occur, making it easier to obtain the surface shape (X-plane) of the biaxially oriented polypropylene film of the present invention. On the other hand, by setting the heat treatment time to 1.0 second or less, melting of the entire film and the occurrence of wrinkles due to film expansion are suppressed, making it easier to ensure mass production. Furthermore, since the partial melting of the depressions is not excessive, it is easier to prevent the film from becoming excessively smooth after stretching.

[0061] The cooling temperature of the cast sheet after heat treatment is preferably between 80°C and 120°C. Setting the cooling temperature to 80°C or higher makes it easier to raise the film temperature to the desired temperature in the subsequent longitudinal stretching process, thereby reducing breakage during the longitudinal stretching process. On the other hand, setting the cooling temperature to 120°C or lower suppresses partial melting of the cast sheet surface, resulting in a more uniform surface texture in the longitudinal direction of the film obtained after the stretching process.

[0062] Next, in the longitudinal stretching process, the heat-treated cast sheet is stretched in the longitudinal direction (longitudinal stretching) to obtain a uniaxially oriented film. Specifically, it is preferable to pass the heat-treated cast sheet through a temperature-controlled longitudinal stretching roll and stretch it in the longitudinal direction at a predetermined stretching ratio by the difference in peripheral speed between the rolls.

[0063] The temperature of the longitudinal stretching roll is preferably between 135°C and 155°C. Setting the temperature of the longitudinal stretching roll above 135°C prevents insufficient heat during longitudinal stretching and reduces film breakage during the longitudinal stretching process. On the other hand, setting the temperature of the longitudinal stretching roll below 155°C increases the orientation of the uniaxially oriented film, which improves the dielectric strength of the biaxially oriented polypropylene film obtained after biaxial stretching.

[0064] The longitudinal stretching ratio is preferably 4.0 times or more and 7.0 times or less, and more preferably 5.0 times or more and 7.0 times or less. Setting the longitudinal stretching ratio to 4.0 times or more makes it easier to reduce variations in the longitudinal and lateral directions of the surface shape of the biaxially oriented polypropylene film obtained after biaxial stretching, and also improves the dielectric strength of the biaxially oriented polypropylene film by increasing its orientation. On the other hand, setting the longitudinal stretching ratio to 7.0 times or less makes it easier to reduce film breakage during the longitudinal stretching process and film breakage during the subsequent transverse stretching process.

[0065] Next, the uniaxially oriented film is gripped at both ends in the width direction with clips and stretched in the width direction at a stretching ratio of 8 to 15 times using a tenter-type stretcher controlled at a temperature of 155°C to 165°C. Then, it is heat-set at a temperature of 150°C to 170°C and a relaxation ratio of 5% to 15% to obtain a biaxially oriented polypropylene film.

[0066] Next, the biaxially oriented polypropylene film is subjected to corona discharge treatment in air, nitrogen, carbon dioxide, or a mixture thereof. After that, both ends in the width direction, held with clips, are cut and removed, and the film is wound up as an intermediate product using a winding machine. Finally, the biaxially oriented polypropylene film unwound from the intermediate product is slit to a specific width using a slitter, and the resulting film roll is wound onto a core to obtain a roll of the biaxially oriented polypropylene film of the present invention.

[0067] The biaxially oriented polypropylene film of the present invention is preferably used as a dielectric for capacitors, but is not limited to the type of capacitor. Specifically, in terms of electrode configuration, it may be either a foil-wound capacitor or a metal-deposited film capacitor, and it is also preferably used in oil-immersion type capacitors containing insulating oil or dry-type capacitors that do not use insulating oil at all. Furthermore, in terms of shape, it may be either a wound type or a laminated type. Due to the properties of the biaxially oriented polypropylene film of the present invention, it is particularly preferably used as a metal-deposited film capacitor.

[0068] Next, a metal film laminate film using the biaxially oriented polypropylene film of the present invention will be described. As a method for forming the metal film, a method of depositing a metal such as aluminum onto at least one side of the biaxially oriented polypropylene film to provide a metal film that will serve as the internal electrode of the film capacitor is preferably used. At this time, other metal components such as nickel, copper, gold, silver, chromium, and zinc can be deposited simultaneously or sequentially with the aluminum. In addition, a protective layer such as oil can be provided on the metal film. The thickness of the metal film is preferably 20 nm to 100 nm from the viewpoint of the electrical characteristics and safety of the capacitor. Also, for the same reason, the surface resistance of the metal film is preferably 1 Ω / sq to 20 Ω / sq. The surface resistance can be controlled by the type of metal used and the film thickness.

[0069] Next, a film capacitor using the biaxially oriented polypropylene film of the present invention will be described. The film capacitor has a structure in which metal film laminates are stacked or wound. Below, an example of a method for manufacturing a wound film capacitor will be described. First, aluminum is vacuum deposited on one side of the biaxially oriented polypropylene film. At that time, the aluminum is deposited in a stripe shape having a margin portion running in the longitudinal direction of the film. Next, a blade is made to slit the center of each deposited portion and the center of each margin portion on the surface, and a tape-shaped winding reel with a margin on one side of the surface is produced. Two tape-shaped winding reels, one with a left margin and one with a right margin, are stacked and wound together so that the deposited portion extends beyond the margin portion in the width direction to obtain a wound body. After heat treatment of the wound body, metallicon is sprayed onto both ends in the width direction to form external electrodes, and lead wires are welded to the metallicon to obtain a wound film capacitor. Film capacitors have a wide range of applications, including vehicles, home appliances (such as televisions and refrigerators), general dust protection, automobiles (hybrid cars, power windows, wipers, etc.), and power supplies. Film capacitors using the biaxially oriented polypropylene film of the present invention can also be suitably used in these applications. [Examples]

[0070] The present invention will be described in detail below with reference to examples. The characteristics were measured and evaluated by the following methods.

[0071] [Evaluation methods for each characteristic] (1) Mesopentad fraction (mmmm) Polypropylene resin samples were dissolved in a solvent, and the mesopentade fraction (mmmm) was determined using 13C-NMR under the following conditions (Reference: New Edition Polymer Analysis Handbook, edited by The Japan Society for Analytical Chemistry and the Polymer Analysis Research Group, 1995, pp. 609-611).

[0072] A. Measurement conditions Equipment: Bruker DRX-500 Measurement nucleus: 13C nucleus (resonance frequency: 125.8MHz) Measured concentration: 10wt% Solvent: Benzene / deuterium dichlorobenzene = mass ratio 1:3 mixed solution Measurement temperature: 130℃ Spin speed: 12Hz NMR sample tube: 5mm tube Pulse width: 45° (4.5μs) Pulse repetition time: 10 seconds Data points: 64K Number of conversions: 10,000 Measurement mode: complete decoupling B.Analysis conditions A Fourier transform was performed with a line broadening factor (LB) of 1.0, resulting in a mmmm peak of 21.86 ppm. Peak splitting was performed using WINFIT software (Bruker). During this process, the peak splitting was performed starting from the high-field side as follows, and then automatic fitting was performed using the accompanying software. After optimizing the peak splitting, the sum of the mmmm peak fractions was calculated. The above measurement was performed five times, and the average value was taken as the mesopentad fraction (mmmm) of this sample. peak (a)mrrm (b)(c)rrrm (split into two peaks) (d)rrrr (e)mrmr (f)mrmm+rmrr (g)mmrr (h)rmmr (i)mmmr (j)mmmm.

[0073] (2) Melt Flow Index (MFR) (Unit: g / 10min) Measurements were taken at 230°C and with a weight of 2.16 kg, in accordance with JIS K 7210-1 (2014).

[0074] (3) Melt tension (MS) (unit: g) Using a Capillograph 1B manufactured by Toyo Seiki Seisakusho Co., Ltd. (capillary diameter 2.0 mm, capillary length 40 mm, cylinder diameter 9.55 mm), polypropylene resin was heated to 230°C, and the molten polypropylene was extruded into strands at an extrusion speed of 20 mm / min. The tension was then measured when these strands were pulled back at a speed of 4.0 m / min.

[0075] (4) Maximum point height (Sxp), load area ratio separating the protruding peak and core (Smr1), load area ratio separating the protruding valley and core (Smr2) Measurements were performed using the non-contact surface and layer cross-sectional shape measurement system "VertScan" (registered trademark) 2.0 (model: R3300GL-Lite-AC) manufactured by Ryoka Systems Co., Ltd. Ten measurement points were randomly selected from the film roll corresponding to the center position in the width direction from the center position of the film roll, and the average of the measurements from these ten points was defined as the Sxp, Smr1, and Smr2 of the sample. The detailed conditions for each measurement were as follows. Note that one field of view (field of view area: 939 μm vertical × 1,252 μm horizontal = 1,175,628 μm) was used for each measurement. 2 Measurements were taken of the following:

[0076] A. Measurement conditions CCD camera: SONY HR-57 1 / 2” Objective lens: 10X Lens tube: 0.5X BODY Wavelength filter: 530 white Measurement mode: Wave Field of view size: 640 x 480 Scan range: (Start) 5μm, (Stop) -5μm B.Measurement method A dedicated sample holder was used for film measurement. The sample holder consists of two detachable metal plates with a circular hole in the center. The film is placed between these plates, ensuring it is wrinkle-free, and the measurement is taken on the central circular portion of the film. The film and sample holder were positioned so that the longitudinal direction of the film roll aligned with the vertical direction of the measurement field of view.

[0077] C. Analysis method The data obtained from the above measurements was analyzed using the image analysis software VS-Viewer of "VertScan" (registered trademark) 2.0. First, noise was removed using a median filter (5x5), and then undulation components were removed using a Gaussian filter with a cutoff value of 250 μm. Next, Sxp, Smr1, and Smr2, as defined in ISO25178 (2012), were calculated using the "ISOPara" function. In the "ISOPara" function, the S-Filter was set to 6.0 μm.

[0078] (5) Thickness (t) (Unit: μm) The thickness was measured using the micrometer method in accordance with JIS C 2330 (2014).

[0079] (6) Wetting tension (unit: mN / m) In accordance with JIS K 6768 (1999), the wetting tension of the corona-treated surface of a biaxially oriented polypropylene film was measured using a test mixture of ethylene glycol monoethyl ether, formamide, and methanol.

[0080] (7) Component processability in capacitor manufacturing Aluminum was vacuum-deposited onto the corona-treated side of a biaxially oriented polypropylene film using a vacuum deposition machine manufactured by ULVAC, Inc., to achieve a capacitance of 15 Ω / sq. The aluminum was deposited in a stripe pattern with a margin running along the longitudinal direction (a repeating pattern of a deposited section width of 79.0 mm and a margin section width of 1.0 mm). Next, slits were made by cutting into the center of each deposited section and each margin section to create a tape-like winding reel with a total width of 40 mm and a 0.5 mm margin at either the left or right end. Two of these reels, one with the left margin and one with the right margin, were overlapped and wound together so that the deposited portion extended 0.5 mm beyond the margin in the width direction, obtaining a winding body with a capacitance of 120 μF. KAW-4NHB, manufactured by Kaito Seisakusho Co., Ltd., was used for winding the element. Finally, the device was heat-treated in a reduced-pressure atmosphere at 130°C for 10 hours to obtain a capacitor element. Capacitor elements with external or internal wrinkles or distortions were rejected, and the processability of the elements was evaluated based on the number of rejected elements. The percentage of rejected elements relative to the total number manufactured was used as an indicator of processability. Fifty capacitor elements were manufactured and evaluated according to the following criteria. ◎: No defective products 〇: 1 defective item △: 2 or more defective items and 3 or less defective items ×: 4 or more defective items.

[0081] (8) Capacitor life test Aluminum was vacuum-deposited onto the corona-treated side of a biaxially oriented polypropylene film using a vacuum deposition machine manufactured by ULVAC, Inc., to achieve a capacitance of 15 Ω / sq. The aluminum was deposited in a stripe pattern with a margin running along the longitudinal direction (a repeating pattern of 79.0 mm width in the deposited area and 1.0 mm width in the margin). Next, slits were made by cutting into the center of each deposited area and each margin, creating a tape-like winding reel with a total width of 40 mm and a 0.5 mm margin at either the left or right end. Two of these reels, one from the left margin and one from the right margin, were overlapped and wound together so that the deposited portion extended 0.5 mm beyond the margin in the width direction, obtaining a winding body with a capacitance of 120 μF. KAW-4NHB, manufactured by Kaito Seisakusho Co., Ltd., was used for winding the element. Finally, the device was heat-treated in a reduced-pressure atmosphere at 130°C for 10 hours. Metallicon was sprayed onto both ends in the width direction to form external electrodes, and lead wires were welded to the Metallicon to obtain a capacitor element. Next, the capacitor characteristics of 10 capacitor elements were evaluated. First, the capacitance (C0) was measured at room temperature. Then, a voltage of 200VDC / μm (400V applied when the thickness (t) was 2.0μm) was applied to the capacitor elements at a high temperature of 125°C for 600 hours. After that, the capacitance (C) was measured at room temperature, and the rate of change of capacitance (ΔC) before and after voltage application was calculated using the following formula. ΔC = ((C0 - C) / C0) × 100 The average of the rate of change (ΔC) of capacitance before and after voltage application for 10 capacitor elements was used as the rate of change for each sample, and it was evaluated according to the following criteria. ◎: ΔC is less than 2% ○: ΔC is between 2% and less than 3% △: ΔC is between 3% and less than 5% ×: ΔC is 5% or more.

[0082] [Resins, etc., used in the manufacture of biaxially oriented polypropylene film] The resins and other materials used in the production of the biaxially oriented polypropylene films in each example and comparative example are as follows. High stereoregularity polypropylene resin A1: Polypropylene resin (Borealis "Borclean" (trademark) HC300BF) with 98% mmmm, MFR of 3.3g / 10min, and MS of 1.0g. Low stereoregularity polypropylene resin B1: Polypropylene resin with a mmmm content of 45% and an MFR of 350 g / 10 min (manufactured by Idemitsu Kosan Co., Ltd., "L-MODU" (registered trademark) S600). High melt-tension polypropylene resin H1: Polypropylene resin with an MFR of 6.2g / 10min and an MS of 4g (WAYMAX® EX4000, manufactured by Nippon Polypropylene Co., Ltd.) High melt-tension polypropylene resin H2: Polypropylene resin with an MFR of 2.5g / 10min and a melt tension MS of 32g (Borealis "Daploy" (trademark) WB135HMS).

[0083] (Example 1) High stereoregularity polypropylene resin A1, low stereoregularity polypropylene resin B1, and high melt tension polypropylene resin H1 were dry-blended in a ratio of 93:2:5 (by mass) and supplied to a single-screw melt extruder. At this time, 0.4 parts by mass of BASF Japan's "Irganox" (registered trademark) 1010 and 0.1 parts by mass of 2,6-di-t-butyl-p-cresol (BHT) were added as antioxidants. In the single-screw melt extruder, the polypropylene resin mixture was melted at a temperature of 250°C, and foreign matter was removed using a 25 μm cut sintered filter. Next, the molten polypropylene resin mixture was formed into a sheet using a T-type slit die, extruded, and solidified on a cast drum maintained at a temperature of 90°C using an air knife. After solidification, it was cooled on a cooling roll maintained at a temperature of 30°C. The contact time between the cast drum and the molten sheet was 1.5 seconds each. Here, the side in contact with the cast drum was designated as the drum surface (D surface), and the side not in contact was designated as the non-drum surface (non-D surface). The resulting cast sheet was then heat-treated on a conveyor roll at 165°C, nipping at a pressure of 0.45 MPa on a nip roll at 165°C for 0.5 seconds, and then cooled on a cooling roll at 100°C. Next, the cast sheet was stretched longitudinally at a stretching ratio of 6.0 times using a longitudinal stretching roll at 145°C. After that, the widthwise ends of the resulting longitudinally stretched sheet were gripped with clips and guided to a tenter, where it was stretched in the widthwise direction at a stretching ratio of 11 times at 160°C, and then relaxed by 12% in the widthwise direction at 158°C. After that, it was slowly cooled to room temperature, and the drum surface (D surface) side of the film was subjected to a temperature of 25 W·min / m 2 Corona discharge treatment was performed at the specified treatment strength, the edges of the film held with clips were cut and removed, and the film with the edges removed was wound on a winding machine. Next, the film was slit using a slitter to a width of 0.82 m, and 30,000 m in the longitudinal direction was wound onto a core as a film roll to obtain a biaxially oriented polypropylene film with a thickness of 2.0 μm. The evaluation results are shown in Table 1.

[0084] (Examples 2-6, Comparative Examples 1-8) The manufacturing process was the same as in Example 1, except that the polypropylene resin composition, the heat treatment time of the cast sheet, and the conveyor roll temperature during the heat treatment of the cast sheet were as shown in Table 1. In Examples 2-6 and Comparative Examples 1-6, biaxially oriented polypropylene films with a thickness of 2.0 μm were obtained, but in Comparative Examples 7 and 8, film breakage occurred during manufacturing, making it impossible to obtain biaxially oriented polypropylene films. The evaluation results are shown in Table 1.

[0085] [Table 1]

[0086] The polypropylene resin composition is given as a value representing 100% by mass of the total polypropylene resin. [Industrial applicability]

[0087] The biaxially oriented polypropylene film of the present invention has high productivity, processability, and dielectric strength, making it suitable for use as a dielectric in film capacitors.

Claims

1. A biaxially oriented polypropylene film characterized in that, when the surface with a maximum point height Sxp is defined as the X-plane, both surfaces are the X-plane, the load area ratio Smr2 separating the protruding valleys and core portions of both X-planes is 85.0% to 95.0%, and when the film thickness is t (μm), Sxp / t on both X-planes is 10.0 nm / μm to 90.0 nm / μm.

2. The biaxially oriented polypropylene film according to Claim 1, wherein the load area ratio Smr1 separating the protruding peaks and core portions of both X surfaces is 7.0% or more and 13.0% or less.

3. The biaxially oriented polypropylene film according to claim 1 or 2, wherein the SXp on both sides are different from each other.

4. A biaxially oriented polypropylene film according to any one of claims 1 to 3, wherein a polypropylene resin having a mesopentad fraction (mmmm) of 30% or more and 60% or less and a melt flow rate (MFR) of 200 g / 10 min or more and 600 g / 10 min is defined as low stereoregularity polypropylene resin B, and a polypropylene resin having an MFR of 4 g / 10 min or more and 12 g / 10 min or less and a melt tension (MS) of 1.5 g or more and 40 g or less is defined as high melt tension polypropylene resin H, wherein of 100% by mass of the total polypropylene resin constituting the film, the low stereoregularity polypropylene resin B accounts for 0.5% by mass or more and 5.0% by mass or less, and the high melt tension polypropylene resin H accounts for 1.0% by mass or more and 10% by mass or less.

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