Biaxially oriented polypropylene film, metal film laminated film, and film capacitor

Abstract

To provide a biaxially oriented polypropylene film which has high productivity, processability and voltage resistance and which, in order to secure proper security mainly in a high capacity capacitor, has surface properties to uniformly regulate air volumes and gap distances between film layers of the capacitor in a longitudinal direction and a width direction.SOLUTION: Provided is a biaxially oriented polypropylene film characterized in that at least one of maximum gloss unevenness in a longitudinal direction (RMD) and maximum gloss unevenness in a width direction (RTD) is 0.1% or more and 3.0% or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a biaxially oriented polypropylene film, a metal film laminated film, and a film capacitor that have high voltage resistance when used as a dielectric in a capacitor under high temperature and high voltage environments. [Background technology]

[0002] Biaxially oriented polypropylene film has excellent transparency, mechanical properties, and electrical properties, and is therefore used in a variety of applications, including packaging, tape, cable wrapping, and electrical applications such as capacitors.

[0003] In particular, its excellent high withstand voltage and low loss characteristics make it particularly suitable for use as a capacitor dielectric. Recently, various electrical equipment has been converted to inverters, which has led to an even stronger demand for smaller capacitors with larger capacities. Furthermore, the operating environments are becoming increasingly hot (85°C to 125°C), particularly in automotive applications (including hybrid and electric vehicles), solar power generation, and wind power generation, and the demand for heat resistance in capacitors is increasing.

[0004] Therefore, there is a demand for thinner, more heat-resistant, and higher voltage resistance per unit thickness biaxially oriented polypropylene (BIPP) film, which serves as the dielectric, as well as for improved capacitor safety. Here, capacitor safety refers to the ability of metal vapor deposition capacitors, which use metal vapor deposition films formed on dielectric films as electrodes, to maintain insulation by scattering the vapor deposition metal due to the energy of discharge during abnormal discharges. This is an important function for preventing short circuits and breakdowns of the capacitor. To improve capacitor safety, it is known that controlling the amount of air and gap distance between the film layers that make up the capacitor is important. Especially for large-capacity capacitors with capacities of 100 μF or more, uniform control of the amount of air and gap distance in both the length and width directions of the capacitor is a challenge.

[0005] In order to improve the withstand voltage per film thickness and the safety of capacitors, studies have been conducted mainly on controlling the surface properties of the film. One known method for controlling the surface properties of a film is a method that utilizes the crystal transition from β-crystal to α-crystal of polypropylene (hereinafter referred to as the β-crystal method). This method utilizing crystal transition does not require the incorporation of impurities such as additives that may deteriorate the withstand voltage, and is therefore preferably used as a method for roughening the surface of biaxially oriented polypropylene films for capacitors (see, for example, Patent Documents 1 and 2).

[0006] Furthermore, as a technology that focuses on the balance between the longitudinal and transverse directions of the film's slipperiness, a method has been proposed in which the casting temperature and stretching temperature are adjusted to control the in-plane slipperiness isotropically (for example, Patent Document 3). This method improves the roll transportability of the film, thereby improving its processability into capacitors and reducing unevenness in interlayer adhesion and residual stress when processed into capacitors. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-133446 [Patent Document 2] Japanese Patent Application Laid-Open No. 2014-077057 [Patent Document 3] WO2016 / 158590 publication Summary of the Invention [Problem to be solved by the invention]

[0008] When a typical linear polypropylene film is used and the β-crystallization method described in Patent Documents 1 and 2 is applied, steep crater-like convex and concave portions are formed at a low density, tending to result in significant gloss unevenness, particularly in the longitudinal and transverse directions. Therefore, it cannot be said that the control of the amount of air between film layers, which is related to voltage resistance and safety in recent high-temperature, high-voltage environments, is sufficient. Furthermore, while the method described in Patent Document 3 can reduce unevenness in interlayer adhesion and residual stress when processed into a capacitor, simply controlling the slipperiness, which is affected by uneven film thickness and the electrostatic charge state, is insufficient to achieve uniform air amount and gap distance between film layers when processed into a capacitor. Therefore, it cannot be said that the safety of capacitors necessarily functions properly in recent high-temperature, high-voltage environments.

[0009] Therefore, the object of the present invention is to provide a biaxially oriented polypropylene film that has high productivity, processability, and voltage resistance, and also has surface properties that enable the amount of air and gap distance between the film layers of a capacitor to be uniformly controlled in the longitudinal and transverse directions of the film, in order to obtain appropriate safety, mainly in large-capacity capacitors. [Means for solving the problem]

[0010] The above-mentioned object can be achieved by the following: The polypropylene film of the present invention is a biaxially oriented polypropylene film characterized in that at least one of the maximum longitudinal gloss variation (RMD) and the maximum transverse gloss variation (RTD) is 0.1% or more and 3.0% or less. [Effects of the Invention]

[0011] When the biaxially oriented polypropylene film of the present invention is used as a dielectric for a capacitor, it has excellent processability and voltage resistance, and furthermore, the amount of air between the film layers and the interlayer distance can be controlled uniformly in the longitudinal and transverse directions during capacitor processing. Therefore, when the film is used as a capacitor, it functions with high safety even under high temperature and high voltage environments, and the life of the capacitor is also improved. DETAILED DESCRIPTION OF THE INVENTION

[0012] The biaxially oriented polypropylene film, metal film laminated film, and film capacitor of the present invention will be described in more detail below. Note that, in the following numerical ranges expressed using "to", the upper and lower limits are included in the range, and the upper and lower limits are expressed in the same units.

[0013] The biaxially oriented polypropylene film of the present invention is a biaxially oriented polypropylene film obtained by stretching a cast sheet in both the longitudinal and transverse directions. In other words, "biaxially oriented" as used herein means stretched in both the longitudinal and transverse directions. Furthermore, in the present invention, the term "polypropylene resin" refers to a resin containing more than 50 mol% but not more than 100 mol% of propylene units, where the total structural units constituting the resin are taken as 100 mol%.

[0014] The biaxially oriented polypropylene film of the present invention is primarily composed of a polypropylene resin. The polypropylene resin may include not only a propylene homopolymer but also a polypropylene copolymer or a branched polypropylene, as described below. In the present invention, the term "primary component" refers to a component that accounts for more than 50% by mass and up to 100% by mass of the total components of the film (100% by mass), more preferably 80% by mass to 100% by mass, even more preferably 90% by mass to 100% by mass, and particularly preferably 95% by mass to 100% by mass. Components other than the polypropylene resin in the film include the resins described below, as well as additives such as antioxidants and lubricants. When multiple polypropylene resins are contained in a film, the film can be considered to be primarily composed of a polypropylene resin if the combined content of all polypropylene resins exceeds 50% by mass.

[0015] As the polypropylene copolymer, a polypropylene copolymer copolymerized with other unsaturated hydrocarbons can be preferably used. Examples of copolymerization components of the 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 voltage resistance and dimensional stability, the copolymerization amount of the polypropylene copolymer is preferably 1 mol% or less when the total structural units constituting the resin are taken as 100 mol%.

[0016] Furthermore, other polymers (resins) than polypropylene resin may be added to the propylene homopolymer. Examples of other polymers that can be added to the propylene homopolymer include homopolymers of unsaturated hydrocarbons other than propylene and copolymers of unsaturated hydrocarbons containing propylene units. From the viewpoint of voltage resistance and dimensional stability, the content of other polymers is preferably 20% by mass or less when the total amount of all components of the biaxially oriented polypropylene film is taken as 100% by mass.

[0017] The polypropylene resin serving as the main component of the biaxially oriented polypropylene film of the present invention preferably has a cold xylene soluble fraction (CXS) of 5% by mass or less. Here, CXS refers to the polypropylene component dissolved in xylene when the biaxially oriented polypropylene film is completely dissolved in xylene at 135°C and then precipitated at 20°C. This component is considered to be difficult to crystallize due to its low stereoregularity and low molecular weight. The CXS of the polypropylene resin serving as the main component is more preferably 5% by mass or less, even more preferably 3% by mass or less, and particularly preferably 1% by mass or less. A CXS of 5% by mass or less improves the voltage resistance and dimensional stability of the biaxially oriented polypropylene film. The CXS of the polypropylene resin can be adjusted to fall within the above range by, for example, increasing the catalytic activity during resin preparation or washing the resulting resin with a solvent or propylene monomer itself. Note that the term "main polypropylene resin" refers to the entire polypropylene resin constituting the film, and the same applies hereinafter to the term "main polypropylene resin."

[0018] The mesopentad fraction of the polypropylene resin serving as the main component of the biaxially oriented polypropylene film of the present invention is preferably 95% or more, more preferably 97% or more, from the viewpoint of heat shrinkage properties at high temperatures. The mesopentad fraction is an index of the stereoregularity of the crystalline phase of a polypropylene resin, as measured by nuclear magnetic resonance (NMR). The higher the mesopentad fraction value of the polypropylene resin serving as the main component, the higher the crystallinity and melting point, which is preferable, particularly from the viewpoint of high-temperature vapor deposition processability when formed into a film. To obtain such a polypropylene resin with high stereoregularity, methods such as washing the obtained resin powder with a solvent such as n-heptane, or appropriately selecting a catalyst and / or co-catalyst and composition are preferably employed. When the mesopentad fraction of the polypropylene resin serving as the main component is within the above-mentioned preferred range, the formed film exhibits excellent voltage resistance and dimensional stability.

[0019] The melt flow rate (hereinafter, MFR) of the polypropylene resin, which is the main component of the biaxially oriented polypropylene film of the present invention, is preferably 1.0 to 10 g / 10 min, more preferably 1.5 to 8 g / 10 min, and even more preferably 2.0 to 5 g / 10 min, when measured in accordance with JIS K 7210 (1995) condition M (230°C, 2.16 kg). When the MFR of the polypropylene resin is within the above-mentioned preferred range, excellent film-forming properties are achieved, resulting in stable biaxially oriented polypropylene film production, and the resulting biaxially oriented polypropylene film also exhibits excellent voltage resistance characteristics. To achieve the MFR of the main polypropylene resin within the above-mentioned range, methods such as controlling the average molecular weight and molecular weight distribution are preferably employed. More specifically, the MFR can be reduced by increasing the average molecular weight or reducing the variation in molecular weight distribution.

[0020] The polypropylene resin, which is the main component of the biaxially oriented polypropylene film of the present invention, may contain branched polypropylene to improve film formability. In this case, the branched polypropylene is preferably a branched polypropylene whose melt tension (MS) and melt flow rate (MFR) measured at 230°C satisfy the relationship log(MS) > -0.56log(MFR) + 0.74. To obtain a branched polypropylene whose melt tension (MS) and melt flow rate (MFR) measured at 230°C satisfy the relationship log(MS) > -0.56log(MFR) + 0.74, methods such as blending polypropylenes containing a large amount of high molecular weight components, blending oligomers or polymers with branched structures, introducing long-chain branched structures into polypropylene molecules as described in JP-A-62-121704, or methods such as those described in JP-A-2869606 are preferably used.

[0021] The branched polypropylene in the biaxially oriented polypropylene film of the present invention is not particularly limited, but resins obtained by electron beam crosslinking are preferably used because they contain less gel components. Specific examples of branched polypropylenes that can be used in the biaxially oriented polypropylene film of the present invention include "PRO-FAX" (registered trademark) PF-814 manufactured by LyondellBasell and "Daploy" (trademark) HMS-PP (WB130HMS, WB135HMS, etc.) manufactured by Borealis.

[0022] The branched polypropylene referred to here is a polypropylene having 1 to 5 internal tri-substituted olefins per 10,000 carbon atoms, and the presence of the internal tri-substituted olefins is 1 This can be confirmed by the proton ratio in the H-NMR spectrum. Branched-chain polypropylene acts as an α-crystal nucleating agent, and when added in a certain amount, it also makes it possible to form a rough surface due to the crystalline morphology. More specifically, by including branched-chain polypropylene, the size of the polypropylene spherulites formed during the cooling process of the melt-extruded resin sheet can be controlled to be small, thereby suppressing the occurrence of insulation defects formed during the stretching process, and allowing for the production of a polypropylene film with excellent voltage resistance characteristics.

[0023] When a branched polypropylene is included in the biaxially oriented polypropylene film of the present invention, the content is preferably 0.05 to 3.0% by mass, more preferably 0.1 to 2.0% by mass, even more preferably 0.3 to 1.5% by mass, and particularly preferably 0.5 to 1.0% by mass, based on 100% by mass of all components of the biaxially oriented polypropylene film. When the content of the branched polypropylene is within the above preferred range, the film formability is improved while the stereoregularity of the biaxially oriented polypropylene film is not reduced, resulting in excellent voltage resistance characteristics. When the biaxially oriented polypropylene film contains multiple types of branched polypropylene, the content is calculated by adding together all the branched polypropylenes.

[0024] The biaxially oriented polypropylene film of the present invention may preferably contain various additives, such as nucleating agents, antioxidants, heat stabilizers, lubricants, antistatic agents, antiblocking agents, fillers, viscosity modifiers, and color inhibitors, within the scope of the present invention.

[0025] Among the additives mentioned above, the type and amount of antioxidant selected is important from the viewpoint of long-term heat resistance. Specifically, antioxidants are preferably sterically hindered phenolic antioxidants, with at least one of them being a high-molecular-weight type with a molecular weight of 500 or more. Specifically, it is preferable to use, alone or in combination, 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), or tetrakis[methylene-3(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane (e.g., BASF's "Irganox"® 1010: molecular weight 1177.7). The total content of these antioxidants is preferably 0.03 to 1.0 part by mass, and more preferably 0.1 to 0.9 part by mass, based on the total amount of polypropylene resin. When the content of the antioxidant in the polypropylene resin composition is 0.03 part by mass or more, the antioxidant effect is easily obtained and long-term heat resistance is easily maintained. On the other hand, when the content of the antioxidant in the polypropylene resin composition is 1.0 part by mass or less, high-temperature voltage resistance characteristics are easily maintained.

[0026] The biaxially oriented polypropylene film of the present invention preferably has a surface wet tension of 37 to 50 mN / m on at least one side, more preferably 38 to 49 mN / m, even more preferably 39 to 48 mN / m, and particularly preferably 40 to 47 mN / m. When the surface wet tension is equal to or greater than the above-mentioned preferred lower limit, sufficient adhesion with metal is achieved during metal vapor deposition. Examples of methods for achieving a surface wet tension of 37 to 50 mN / m on at least one side or within the above-mentioned preferred range include, for example, a method of performing a surface treatment after biaxial stretching during film formation. Specific examples include corona discharge treatment, plasma treatment, glow discharge treatment, and flame treatment, which may be used alone or in combination.

[0027] The biaxially oriented polypropylene film of the present invention preferably has a gloss of 125% or more and 150% or less on at least one side, more preferably 125 to 145%, even more preferably 125 to 140%, and particularly preferably 128 to 138%. A gloss of 125% or more on at least one side means that the increase in light scattering density on that surface is suppressed. In other words, a gloss of 125% or more on at least one side means that there are few irregularities on the surface, and this embodiment reduces the deterioration of voltage resistance characteristics caused by these irregularities. On the other hand, a gloss of 150% or less on at least one side means that there are irregularities on the surface to the extent that slipperiness is ensured. Therefore, this embodiment reduces the occurrence of conveyance wrinkles during film conveyance during film production and processing, improves the winding appearance of the film roll, and reduces film breakage. By setting the glossiness of both surfaces to 125% or more and 150% or less, or within the above-mentioned preferred range, the above-mentioned effects are improved compared to when the glossiness of only one surface is set in the same range.

[0028] A method for achieving a gloss of 125% or more and 150% or less, or within the above preferred range, on at least one surface includes using the above-mentioned polypropylene resin and setting specific conditions for the casting step and longitudinal stretching step during film formation, as described below. More specifically, in order to increase the gloss, it is effective to lower the surface temperature of the casting drum in the casting step or the longitudinal stretching temperature in the longitudinal stretching step.

[0029] It is important that the biaxially oriented polypropylene film of the present invention has at least one of the maximum longitudinal gloss variation (RMD) and maximum transverse gloss variation (RTD) of 0.1% to 3.0%, preferably 0.1% to 2.0%, and more preferably 0.1% to 1.0%. While smaller RMD and RTD gloss variations are preferable, a feasible range of 0.1% or more is set. By maintaining at least one of the RMD and RTD within the above range, unevenness in the amount of air between film layers and gap distance when the film is laminated during processing into a capacitor is suppressed. Therefore, using a biaxially oriented polypropylene film of this type in a capacitor can prevent the capacitor from becoming too or too ineffective during use, extending the capacitor's lifespan and reducing the risk of short circuit damage. Furthermore, meandering of the film and deterioration of the film roll shape can be prevented during the film transport process during film formation and processing. The above effects are further improved by making the RMD and RTD to be 0.1% or more and 3.0% or less, or within the above-mentioned preferred ranges.

[0030] In the biaxially oriented polypropylene film of the present invention, if at least one of the RMD and the RTD is 0.1% or more and 3.0% or less on at least one side, it can be considered that "at least one of the RMD and the RTD is 0.1% or more and 3.0% or less." The same applies to the above-mentioned preferred ranges and embodiments.

[0031] A method for making at least one of RMD and RTD 0.1% to 3.0% or within the above-mentioned preferable range includes a method for making the surface temperature of the casting drum more uniform (described later).

[0032] Gloss, RMD, and RTD can be measured by the following method (if measurement of both sides is required, measure each side in the same way). First, a rectangular biaxially oriented polypropylene film sample measuring 100 mm (longitudinal direction) x width (width direction) is taken from a film roll, and the sample is divided into 60 equal parts parallel to the longitudinal direction to obtain 60 test pieces each measuring 100 mm x 1 / 60 mm of the width length. Next, the glossiness of the center of the obtained test piece is measured five times using a digital variable-angle glossmeter under conditions of an incident angle of 60° and an acceptance angle of 60°, and the average value is taken as the glossiness of the test piece. Thereafter, the same measurement is performed on all test pieces, and the maximum and minimum values ​​of the obtained glossiness are taken as the "maximum glossiness in the width direction" and the "minimum glossiness in the width direction," respectively. Furthermore, the longitudinal and width directions are reversed and the same measurement is performed to obtain the "maximum glossiness in the longitudinal direction" and the "minimum glossiness in the longitudinal direction." After the measurement is completed, the longitudinal gloss variation (RMD) and transverse gloss variation (RTD) are calculated using the following formula. The average gloss of all the test pieces is calculated and this is used as the gloss of the biaxially oriented polypropylene film. The digital variable-angle gloss meter used to measure the gloss is not particularly limited as long as it is capable of measuring, and for example, a digital variable-angle gloss meter UGV-5D manufactured by Suga Test Instruments Co., Ltd. can be used. Longitudinal gloss variation (RMD) = Maximum gloss value in the longitudinal direction - Minimum gloss value in the longitudinal direction Transverse gloss variation (RTD) = maximum transverse gloss - minimum transverse gloss.

[0033] The biaxially oriented polypropylene film of the present invention preferably has a thickness of 1.0 to 3.0 μm. The thickness is more preferably 1.2 to 2.8 μm, and even more preferably 1.5 to 2.5 μm. A thickness of 1.0 μm or more can provide the biaxially oriented polypropylene film with excellent mechanical strength and high-temperature voltage resistance, and can also prevent film breakage during film formation and processing. On the other hand, a thickness of 3.0 μm or less can increase the capacitance per volume when used as a capacitor dielectric. The thickness can be measured by the micrometer method in accordance with JIS C 2330 (2014), and can be adjusted by adjusting the discharge rate from the die, the die slit width, the rotation speed of the casting drum, and the stretch ratio.

[0034] Next, the method for producing the biaxially oriented polypropylene film of the present invention will be explained below, but the method is not necessarily limited thereto.

[0035] First, the above-mentioned preferred polypropylene resin is fed into a single-screw melt extruder and melt-extruded at 200 to 260°C. Next, foreign matter, modified polymers, etc. are removed using a filter installed midway through the polymer pipe. The resin is then extruded onto a casting drum through a T-die to form a cast sheet, which is then cooled with a cooling roll.

[0036] The surface temperature of the cast drum is preferably 60 to 100°C, more preferably 65 to 95°C, even more preferably 70 to 95°C, and particularly preferably 75 to 95°C, from the viewpoint of appropriate formation of β crystals and spherulites. By maintaining a cast drum temperature of 60°C or higher, the formation of too few β crystals in the cast sheet can be prevented, and the smoothness of the film obtained after biaxial stretching can be maintained. This prevents the occurrence of conveyance wrinkles during the film conveyance process during film production and processing, and deterioration of the winding shape of the film roll. On the other hand, by maintaining a cast drum temperature of 100°C or lower, the formation of excessive β crystals and spherulites in the cast sheet can be prevented, and the occurrence of meandering during the film conveyance process during film production and processing, and deterioration of the winding shape of the film roll can be easily prevented.

[0037] A specific example of a method for controlling the surface temperature of a casting drum more uniformly is described below, taking into account the mechanism by which the temperature of the casting drum surface becomes uneven. A casting drum typically has pipes installed inside it, either parallel to the width direction or spirally. The surface temperature of the casting drum can be controlled by introducing a temperature-controlled cooling medium into one end of the pipe and letting it flow out the other end. However, the spiral section reduces the heat transfer coefficient, and the heat generated during film cooling on the casting drum surface creates a temperature difference between the inflow and outflow of the cooling medium, resulting in uneven surface temperature of the casting drum. The temperature unevenness of the casting drum caused by this mechanism can be reduced, for example, by increasing the flow rate of the cooling medium.

[0038] In addition, industrial water is generally used as the cooling medium. This industrial water contains metal components such as iron and manganese, as well as foreign matter such as evaporation residues. If industrial water is allowed to flow through the piping of the cooling drum for a long period of time, these foreign matter may adhere to and accumulate on the inner side of the cooling drum, or may react with the metal on the side of the piping to form agglomerates. The formation and fall of these deposits and agglomerates causes localized temperature variations on the surface of the cooling drum. Therefore, by periodically cleaning the piping, etc., these deposits and agglomerates can be removed early, allowing for more uniform control of the surface temperature of the cooling drum.

[0039] Another method for uniformly controlling the surface temperature of a casting drum is to use a casting drum having multiple jacket chambers within the thickness of the casting drum surface. A two-phase gas-liquid heat transfer medium is sealed in the jacket chamber, and when the liquid-phase heat transfer medium comes into contact with the casting drum surface, which is locally heated by heat conduction, the liquid-phase heat transfer medium vaporizes and changes phase to a gas-phase heat transfer medium. When this gas-phase heat transfer medium comes into contact with the casting drum surface, which is still locally at a low temperature, it liquefies and changes phase back to a liquid-phase heat transfer medium. In this way, the heat transfer medium vaporizes upon contact with the hot casting drum surface, removing latent heat, and then vaporizes upon contact with the cold casting drum surface, providing latent heat, thereby uniformly controlling the temperature of the casting drum surface. The "thickness of the casting drum surface" refers to the cylindrical portion that forms the side surface when the casting drum is imagined as a cylinder with a central axis of rotation.

[0040] Furthermore, the surface temperature unevenness across the entire width of the cast drum immediately before the molten sheet discharged from the T-die comes into close contact with the cast drum is preferably 3.0° C. or less, more preferably 2.0° C. or less, and even more preferably less than 1.0° C. By adopting such an embodiment, the β crystals formed in the cast sheet are uniformly formed in the width direction, and it is easy to prevent the surface shape of the film from varying in the longitudinal and width directions.

[0041] The molten sheet discharged from the T-die lands on the casting drum and preferably remains in close contact with the drum for 1 to 3 seconds. A close contact time of 1 second or longer facilitates solidification of the molten sheet, making it easier to prevent breakage during the subsequent stretching process. On the other hand, a close contact time of 3 seconds or shorter can prevent excessive formation of β crystals in the cast sheet, making it easier to prevent meandering during the film transport process during film formation and processing, and deterioration of the film roll shape.

[0042] Methods for adhering the molten sheet to the casting drum include electrostatic application, air knife, nip roll, and underwater casting. The air knife method is preferred from the viewpoints of suppressing thickness unevenness, achieving high-speed film production, and controlling the film's surface properties. The air temperature of the air knife is preferably 60 to 120°C. By maintaining an air knife temperature of 60°C or higher, excessive β-crystal formation in the cast sheet is prevented, the smoothness of the film obtained after biaxial stretching is maintained, and the occurrence of conveyance wrinkles and deterioration of the film roll shape during the film conveyance process during film production and processing are easily prevented. On the other hand, by maintaining an air knife temperature of 120°C or lower, the formation of excessive β-crystals in the cast sheet is prevented, which makes it easier to prevent meandering and deterioration of the film roll shape during the film conveyance process during film production and processing.

[0043] The temperature difference between the cast drum temperature and the air knife temperature is preferably 20°C or less, and more preferably 10°C or less, from the viewpoint of forming equivalent β crystals on both sides of the cast sheet. Keeping the temperature difference between the cast drum temperature and the air knife temperature at 20°C or less prevents the formation of different irregularities on the front and back of the film, making it easier to achieve equivalent slipperiness on both sides of the film. This stabilizes the amount of air entrained during the winding process during capacitor processing, making it easier to achieve uniform interlayer gaps and air volume in the film after the heat treatment process. As a result, the resulting capacitor maintains just the right level of safety during use and is less likely to have a shortened lifespan.

[0044] The cooling temperature of the cast sheet (surface temperature of the cooling roll) is preferably 10 to 50°C. If the cooling temperature is 10°C or higher, the film can be easily heated to the desired temperature in the subsequent longitudinal stretching step, and breakage of the film in the longitudinal stretching step can be easily prevented. On the other hand, if the cooling temperature is 50°C or lower, crystal formation in the cast sheet can be easily stopped, and the surface properties of the film obtained after the stretching step become more uniform in the longitudinal direction.

[0045] To obtain the biaxially oriented polypropylene film of the present invention, it is preferable to subject the cast sheet to high-temperature heat treatment before the longitudinal stretching process. The crater-like surface irregularities characteristic of biaxially oriented polypropylene films generally produced using the β-crystal method are first formed during the longitudinal stretching process by the formation of depressions (volume reduction due to the transition) due to the transition from β-crystal to α-crystal, and the mechanical deformation of these depressions in the longitudinal stretching direction simultaneously occurs, resulting in the formation of wedge-shaped depressions on the surface of the longitudinally stretched sheet. Next, during the transverse stretching process, the wedge-shaped depressions are stretched, and the edges of the depressions are deformed into protrusions, resulting in the formation of crater-like surface irregularities.

[0046] In order to obtain the surface irregularities of the biaxially oriented polypropylene film of the present invention, it is preferable to cause the formation of depressions by the transformation of β crystals to α crystals in the longitudinal stretching step and the mechanical deformation of the depressions in the longitudinal stretching direction in separate steps. Furthermore, it is more preferable to apply an excess amount of heat when transforming β crystals to α crystals to partially melt the depressions formed during the α crystal transformation.

[0047] Specifically, before the longitudinal stretching step, the cast sheet is preferably nipped with temperature-controlled nip rolls and passed through temperature-controlled transport rolls to be heat-treated. The diameter and number of transport rolls may be adjusted depending on the heat treatment time. After the heat treatment, the cast sheet is preferably cooled to stop partial melting.

[0048] The temperature of the conveying roll during the heat treatment is preferably 160 to 170°C, more preferably 160 to 165°C. By setting the conveying roll temperature to 160°C or higher, not only the transition from β crystal to α crystal occurs but also the recesses formed by the α crystal transition tend to partially melt. This makes it easier to form surface irregularities in the biaxially oriented polypropylene film of the present invention. On the other hand, by setting the conveying roll temperature to 170°C or lower, it is possible to prevent the entire cast sheet from melting and breaking, making it easier to maintain mass productivity.

[0049] The nip roll temperature is preferably 160 to 170°C, and more preferably 160 to 165°C. By setting the nip roll temperature to 160°C or higher, curling of the cast sheet on the nip roll caused by the temperature difference between the front and back sides is reduced, thereby reducing breakage during the stretching step and ensuring mass productivity. On the other hand, by setting the nip roll temperature to 170°C or lower, breakage due to melting of the entire cast sheet is reduced, making it easier to maintain mass productivity.

[0050] The pressure of the nip rolls is preferably 0.30 to 0.60 MPa, and more preferably 0.35 to 0.55 MPa. By setting the nip roll pressure to 0.30 MPa or more, the occurrence of wrinkles due to film expansion during heat treatment is suppressed, thereby reducing breakage in the longitudinal stretching step and ensuring mass productivity. On the other hand, by setting the nip roll pressure to 0.60 MPa or less, deformation of the film due to pressure is suppressed, thereby reducing breakage in the stretching step and ensuring mass productivity.

[0051] The heat treatment time for the cast sheet is preferably 1 to 10 seconds. By setting the heat treatment time to 1 second or longer, not only the transition from β crystals to α crystals but also partial melting of the recesses due to volume reduction occurs easily, making it easier to obtain the surface irregularities of the biaxially oriented polypropylene film of the present invention. On the other hand, by setting the heat treatment time to 10 seconds or shorter, melting of the entire film and the occurrence of wrinkles due to film expansion are suppressed, making it easier to ensure mass productivity. Furthermore, since excessive partial melting of the recesses is not caused, it is easy to prevent the film from becoming excessively smooth after stretching.

[0052] The cooling temperature of the heat-treated cast sheet is preferably 80 to 120°C. By setting the cooling temperature to 80°C or higher, it is easier to raise the film temperature to the desired temperature in the subsequent longitudinal stretching step, reducing breakage in the longitudinal stretching step. On the other hand, if the cooling temperature is 120°C or lower, partial melting of the cast sheet surface is suppressed, and the surface properties of the film obtained after the stretching step become more uniform in the longitudinal direction.

[0053] Next, the cast sheet is stretched in the machine direction (machine stretching) in a machine stretching step to obtain a uniaxially oriented film. In the machine stretching step, the cast sheet is preferably passed through rolls controlled at a temperature of 125 to 145°C, and stretched in the machine direction at a predetermined stretching speed and draw ratio by controlling the difference in peripheral speed between the rolls.

[0054] The longitudinal stretching ratio is preferably 4.0 to 7.0 times, and more preferably 5.0 to 7.0 times. By setting the longitudinal stretching ratio to 4.0 times or more, the surface properties of the film become more uniform and the high-temperature voltage resistance characteristics become excellent. By setting the longitudinal stretching ratio to 7.0 times or less, film breakage during the longitudinal stretching step and film breakage during the subsequent transverse stretching step can be easily reduced.

[0055] Next, both widthwise ends of the uniaxially oriented film are held with clips and stretched in the widthwise direction at a stretching ratio of 8 to 15 times in a tenter-type stretching machine controlled at a temperature of 140 to 165°C to form a biaxially oriented film.

[0056] The biaxially oriented film is then subjected to a corona discharge treatment in air, nitrogen, carbon dioxide, or a mixture thereof, and the widthwise ends held by the clips are cut and removed, after which the biaxially oriented film is wound up as an intermediate product on a winder. Finally, the biaxially oriented film unwound from the intermediate product is slit to a specific width on a slitter and wound around a core as a film roll to obtain a roll of the biaxially oriented polypropylene film of the present invention.

[0057] The biaxially oriented polypropylene film of the present invention is preferably used as a dielectric for a capacitor, but is not limited to the type of capacitor. Specifically, from the viewpoint of electrode configuration, it may be either a foil-wound capacitor or a metal-vapor-deposited film capacitor, and is also preferably used in an oil-immersion type capacitor containing insulating oil or a dry capacitor that does not use insulating oil at all. In addition, from the viewpoint of shape, it may be 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-vapor-deposited film capacitor.

[0058] The metal film laminated film of the present invention will be described below. The metal film laminated film of the present invention has a metal film on at least one side of the biaxially oriented polypropylene film of the present invention. While the method for forming the metal film is not particularly limited, a preferred method is, for example, vapor-depositing a metal such as aluminum on at least one side of the biaxially oriented polypropylene film to form a metal film that will serve as an internal electrode of the film capacitor. In this case, other metal components such as nickel, copper, gold, silver, chromium, and zinc can also be vapor-deposited simultaneously with or sequentially with the aluminum. A protective layer such as oil can also be provided on the metal film.

[0059] The thickness of the metal film is preferably 20 to 100 nm from the viewpoint of the electrical characteristics and safety of the capacitor. For the same reason, the surface resistance of the metal film is preferably 1 to 20 Ω / sq. The surface resistance can be controlled by the type of metal used and the film thickness.

[0060] In the present invention, after forming the metal film, the metal film laminated film can be subjected to aging treatment or heat treatment at a specific temperature, if necessary. In addition, at least one side of the metal film laminated film can be coated with polyphenylene oxide or the like for insulation or other purposes.

[0061] The film capacitor of the present invention will be described below. The film capacitor of the present invention has a laminated or wound configuration of the metal film laminated film of the present invention. That is, the film capacitor of the present invention has the metal film laminated film of the present invention, and includes both laminated film capacitors obtained by laminating metal film laminated films and wound film capacitors obtained by winding metal film laminated films. A preferred method for producing a wound film capacitor will be described below, but the present invention is not necessarily limited to this.

[0062] First, aluminum is vacuum-deposited on one side of the biaxially oriented polypropylene film of the present invention. The aluminum is deposited in stripes with margins running longitudinally. Next, a blade is used to slit the surface at the center of each deposited area and at the center of each margin, producing a tape-like take-up reel with a margin on one side. Two tape-like take-up reels with left and right margins are stacked together and wound together so that the deposited area extends beyond the margin in the width direction to obtain a wound body. After heat-treating the wound body, metallicon is sprayed on both widthwise end faces 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 noise protection, automobiles (such as hybrid cars, power windows and winders), and power sources. The film capacitor of the present invention can be used effectively in these applications. [Example]

[0063] The present invention will be described in detail below with reference to examples. The properties were measured and evaluated by the following methods. However, hereinafter, Examples 1, 4 to 6, and 8 are considered as reference examples.

[0064] (1) Glossiness A biaxially oriented polypropylene film sample was taken from an intermediate product with a width of 6,000 mm, in the shape of a rectangle measuring 100 mm (longitudinal direction) x 6,000 mm (transverse direction). The sample was then divided into 60 equal parts parallel to the longitudinal direction to obtain 60 test pieces each measuring 100 mm x 100 mm. Next, the glossiness of the center of the obtained test piece was measured five times on the drum surface and the non-drum surface using a digital variable angle glossmeter UGV-5D manufactured by Suga Test Instruments Co., Ltd., under conditions of an incident angle of 60° and an incident angle of 60°, and the average value was taken as the glossiness of each surface of the test piece. The same measurement was then performed on all test pieces, and the maximum and minimum values ​​of the glossiness of each surface obtained were taken as the "maximum glossiness in the width direction" and "minimum glossiness in the width direction" for each surface, respectively. Furthermore, the same measurement was performed with the longitudinal and width directions reversed, and the "maximum glossiness in the longitudinal direction" and "minimum glossiness in the longitudinal direction" for each surface were obtained. After the measurement, the longitudinal gloss variation (RMD) and transverse gloss variation (RTD) on each surface were calculated using the following formula: The average gloss values ​​on each surface of all test pieces were calculated and used as the gloss values ​​on each surface of the biaxially oriented polypropylene film. Longitudinal gloss variation (RMD) = Maximum gloss value in the longitudinal direction - Minimum gloss value in the longitudinal direction Transverse gloss variation (RTD) = maximum transverse gloss - minimum transverse gloss.

[0065] (2) Thickness The thickness of the biaxially oriented polypropylene film was measured by the micrometer method in accordance with JIS C 2330 (2014).

[0066] (3) Surface temperature and unevenness of the casting drum The surface temperature across the entire width of the casting drum just before the molten sheet discharged from the T-die came into contact with it was measured using a thermograph (FLUKE Ti29 industrial / commercial thermograph, measurable temperature range -20°C to 600°C), and the average, maximum, and minimum values ​​were read. The average value obtained was taken as the surface temperature of the casting drum, and the difference between the maximum and minimum values ​​obtained was taken as the surface temperature unevenness of the casting drum.

[0067] (4) Product roll (processing defect rate) The film was slit at a slitting speed of 450 m / min to a film roll width of 620 mm and a film roll length of 60,000 m, and the wound film roll was used as an evaluation sample to calculate the yield rate (processing defect rate) during film roll processing using the following formula. The obtained value was used to evaluate according to the following criteria. Processing defect rate (%) = (number of misaligned or wrinkled pieces) x 100 / total number of processed pieces 〇: Less than 2%. △: More than 2% and less than 5%. ×: Over 5%.

[0068] (5) Processability of the wound body in capacitor manufacturing Aluminum was vacuum-deposited on the corona-treated side of the biaxially oriented polypropylene film using a vacuum deposition machine manufactured by ULVAC Inc. to a surface resistance of 15 Ω / sq. The aluminum was deposited in stripes with longitudinal margins (79.0 mm wide strips, 1.0 mm wide strips). The film was then slit with a blade at the center of each deposited strip and at the center of each margin to produce tape-like take-up reels with a total width of 40 mm and 0.5 mm margins on either the left or right end. Two strips from the left and right margins of the resulting reels were overlapped and wound together so that the deposited portion extended 0.5 mm beyond the margins in the width direction, resulting in a wound body with a capacitance of 120 μF. A KAW-4NHB winding machine manufactured by Kaito Seisakusho Co., Ltd. was used for winding. Finally, the wound body was heat-treated at 140°C in a reduced-pressure atmosphere for 10 hours. The wound body was visually inspected, and those with external or internal wrinkles or distortions were deemed defective. 50 wound bodies were similarly produced and the same evaluation was repeated, and the workability of the wound bodies was evaluated according to the following criteria. 〇: Less than 1 defective product △: 2 or more but less than 3 defective items ×: 4 or more defective items.

[0069] (6) Evaluation of capacitor characteristics A wound body with a capacitance of 120 μF was obtained using the method described in (5). The wound body was then heat-treated for 10 hours in a reduced-pressure atmosphere at 140°C, and metallikon was sprayed onto both end faces in the width direction to form external electrodes. Lead wires were then welded to the metallikon to obtain a capacitor. Next, the capacitor characteristics were evaluated for 10 capacitors. First, the capacitance (C0) was measured at room temperature. Next, a voltage of 200 VDC / μm (400 V when the thickness was 2.0 μm) was applied to the capacitor at a high temperature of 125°C for 400 hours. Thereafter, the capacitance (C) was measured at room temperature, and the rate of change in capacitance (ΔC) before and after voltage application was calculated using the following formula. The capacitance was measured using an LCR HiTester 3522-50 manufactured by Hioki E.E. Corporation. ΔC=((C0-C) / C0)×100 The average value of the rate of change (ΔC) in capacitance before and after the application of voltage for 10 capacitors was taken as the rate of change in capacitance before and after the application of voltage for that sample, and was evaluated according to the following criteria. ○: ΔC is less than 3% △: ΔC is 3% or more and less than 5% ×: ΔC is 5% or more.

[0070] (7) Film forming properties The film formability was evaluated according to the following criteria: The time from when film production was stopped due to the occurrence of film breakage until film production was restarted was excluded from the observation time. ◯: No film tearing occurred for 48 hours or more. △: The film broke 1 to 3 times in 48 hours. ×: The film broke four or more times within 48 hours.

[0071] Example 1 Polypropylene resin (manufactured by Prime Polymer Co., Ltd., melting point: 166°C, MFR: 2.5g / 10min, mesopentad fraction: 0.991) was fed into a single-screw melt extruder, melt-extruded at 250°C, and then removed with a 25μm cut sintered filter. The molten resin was then melt-extruded into a sheet from a T-slit die. The molten sheet was then brought into close contact with the surface of a casting drum controlled to a surface temperature of 90°C and a temperature variation of 1.0°C using an air knife at an air temperature of 90°C, solidifying the molten sheet, and then cooled on a cooling roll maintained at a temperature of 30°C. The time the molten sheet was in close contact with the casting drum was 1.5 seconds. Here, the surface that contacted the casting drum was designated the drum surface (D surface), and the surface that did not contact the casting drum was designated the non-drum surface (non-D surface). The casting drum used had multiple jacket chambers within its wall thickness and contained a gas-liquid two-phase heat transfer medium. The surface temperature was controlled by controlling the temperature of the cooling water passed through the casting, and the temperature unevenness was controlled by controlling the amount of cooling water passed through the casting (the same applies to Examples 2 to 8).

[0072] The resulting cast sheet was placed on a conveyor roll at 165°C, nipped with a nip roll at 165°C under a pressure of 0.45 MPa for 5 seconds, and then cooled on a cooling roll at 100°C. It was then stretched 5.5 times in the machine direction with a longitudinal stretching roll at 145°C to obtain a uniaxially oriented film. The uniaxially oriented film was then gripped at both widthwise ends with clips and stretched 11 times in the widthwise direction at 160°C, followed by 12% relaxation in the widthwise direction at 158°C to obtain a biaxially oriented film. The biaxially oriented film was then gradually cooled to room temperature, and the drum surface (D-side) was stretched at 25 W·min / m 2 After corona discharge treatment at a treatment intensity of 1000 m / min, both widthwise ends of the film held by clips were cut off and the biaxially oriented film was wound up to obtain a 6,000 mm wide intermediate product. The biaxially oriented film was then unwound from the intermediate product and slit to a width of 620 mm using a slitter, and 60,000 m was wound longitudinally around a core to form a film roll, yielding a 2.0 μm thick biaxially oriented polypropylene film. The evaluation results of the resulting biaxially oriented polypropylene film are shown in Table 1.

[0073] Example 2 A biaxially oriented polypropylene film having a thickness of 2.0 μm was obtained in the same manner as in Example 1, except that the surface temperature unevenness of the casting drum was controlled to 0.5° C. The evaluation results of the obtained biaxially oriented polypropylene film are shown in Table 1.

[0074] Example 3 A biaxially oriented polypropylene film having a thickness of 2.0 μm was obtained in the same manner as in Example 1, except that the surface temperature of the casting drum was controlled to 92° C. and the surface temperature unevenness was controlled to 0.5° C. The evaluation results of the obtained biaxially oriented polypropylene film are shown in Table 1.

[0075] Example 4 A biaxially oriented polypropylene film having a thickness of 2.0 μm was obtained in the same manner as in Example 1, except that the surface temperature of the casting drum was controlled to 70° C. and the surface temperature unevenness was controlled to 0.5° C. The evaluation results of the obtained biaxially oriented polypropylene film are shown in Table 1.

[0076] Example 5 A biaxially oriented polypropylene film having a thickness of 2.0 μm was obtained in the same manner as in Example 1, except that the temperatures of the conveying roll for the cast sheet and the nip roll were controlled to 162° C. The evaluation results of the obtained biaxially oriented polypropylene film are shown in Table 1.

[0077] Example 6 A biaxially oriented polypropylene film having a thickness of 2.0 μm was obtained in the same manner as in Example 1, except that the temperatures of the conveying roll for the cast sheet and the nip roll were controlled to 168° C. The evaluation results of the obtained biaxially oriented polypropylene film are shown in Table 1. Example 7 A biaxially oriented polypropylene film having a thickness of 2.0 μm was obtained in the same manner as in Example 1, except that the surface temperature unevenness of the cast drum was controlled to 0.1° C. The evaluation results of the obtained biaxially oriented polypropylene film are shown in Table 1. Example 8 A biaxially oriented polypropylene film having a thickness of 2.0 μm was obtained in the same manner as in Example 1, except that the surface temperature unevenness of the cast drum was controlled to 2.5° C. The evaluation results of the obtained biaxially oriented polypropylene film are shown in Table 1.

[0078] (Comparative Example 1) A biaxially oriented polypropylene film having a thickness of 2.0 μm was obtained in the same manner as in Example 1, except that a casting drum having no jacket chamber on the surface of the casting drum was used and the temperature unevenness of the casting surface was controlled to 3.5° C. The evaluation results of the obtained biaxially oriented polypropylene film are shown in Table 1. The surface temperature of the casting drum was controlled by controlling the temperature of the cooling water passed inside the casting drum, and the temperature unevenness was controlled by controlling the amount of cooling water passed inside the casting drum (the same applies to Comparative Examples 2 to 4).

[0079] (Comparative Example 2) A biaxially oriented polypropylene film having a thickness of 2.0 μm was obtained in the same manner as in Example 1, except that a casting drum having no jacket chamber on the casting drum surface thickness was used and the casting surface temperature unevenness was controlled to 10.0° C. The evaluation results of the obtained biaxially oriented polypropylene film are shown in Table 1.

[0080] (Comparative Example 3) A biaxially oriented polypropylene film having a thickness of 2.0 μm was obtained in the same manner as in Example 1, except that a cast drum without a jacket chamber on the cast drum surface was used, the cast surface temperature unevenness was controlled to 3.5° C., and the temperatures of the cast sheet conveying roll and nip roll were controlled to 155° C. The evaluation results of the obtained biaxially oriented polypropylene film are shown in Table 1.

[0081] Comparative Example 4 A biaxially oriented polypropylene film having a thickness of 2.0 μm was obtained in the same manner as in Example 1, except that a cast drum without a jacket chamber on the cast drum surface was used, the cast surface temperature unevenness was controlled to 3.5° C., and the temperatures of the cast sheet conveying roll and nip roll were controlled to 175° C. The evaluation results of the obtained biaxially oriented polypropylene film are shown in Table 1.

[0082] [Table 1] [Industrial Applicability]

[0083] The biaxially oriented polypropylene film of the present invention has high productivity, processability and voltage resistance, and can therefore be suitably used as a dielectric for film capacitors.

Claims

1. A biaxially oriented polypropylene film characterized in that at least one of the maximum longitudinal gloss variation (RMD) and the maximum transverse gloss variation (RTD) is 0.1% or more and 0.5% or less, and the gloss on at least one surface is 125% or more and 145% or less. However, the RMD and the RTD are measured by the following measurement method. Measurement method: First, a rectangular biaxially oriented polypropylene film sample measuring 100 mm (longitudinal direction) x width direction length (width direction) was collected from a film roll, and the biaxially oriented polypropylene film sample was divided into 60 equal parts parallel to the longitudinal direction to obtain 60 test pieces each measuring 100 mm x 1 / 60 mm of the width direction length. Next, the glossiness of the center of the obtained test piece was measured five times using a digital variable-angle glossmeter under conditions of an incident angle of 60° and an acceptance angle of 60°, and the average value was taken as the glossiness of the test piece. Thereafter, the same measurement was performed on all the test pieces, and the maximum and minimum glossiness values ​​obtained were taken as the maximum glossiness in the width direction and the minimum glossiness in the width direction, respectively. Furthermore, the longitudinal and width directions were reversed and the same measurement was performed to obtain the maximum glossiness in the longitudinal direction and the minimum glossiness in the longitudinal direction. After the measurement is completed, the longitudinal gloss unevenness (RMD) and the transverse gloss unevenness (RTD) are calculated using the following formulas. Longitudinal gloss unevenness (RMD) = Maximum gloss value in the longitudinal direction - Minimum gloss value in the longitudinal direction Transverse gloss variation (RTD) = maximum gloss value in the transverse direction - minimum gloss value in the transverse direction.

2. A biaxially oriented polypropylene film as described in claim 1, wherein the RMD and the RTD are 0.1% or more and 3.0% or less.

3. A metal film laminated film comprising the biaxially oriented polypropylene film according to claim 1 or 2, and a metal film on at least one surface of the biaxially oriented polypropylene film.

4. A film capacitor having a laminated or wound configuration of the metal film laminated film according to claim 3.

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