JP7779129B2 - Biaxially oriented polypropylene film, metal film laminated film, and film capacitor - Google Patents

Description

The present invention relates to a biaxially oriented polypropylene film, a metal film laminated film, and a film capacitor that exhibit high voltage resistance in high-temperature, high-voltage environments when used as a dielectric in a capacitor.

Biaxially oriented polypropylene film has excellent transparency, mechanical properties, and electrical properties, making it suitable for a variety of applications, including packaging, tape, cable wrapping, and electrical applications such as capacitors.

In particular, its excellent high voltage resistance 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 increased demand for smaller capacitors with larger capacities. Furthermore, 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 there is a growing demand for heat-resistant capacitors.

As a result, there is a demand for thinner, more heat-resistant, and higher voltage resistance per thickness biaxially oriented polypropylene film, which serves as a dielectric, as well as improved capacitor safety. Here, capacitor safety refers to the ability of metal vapor deposition capacitors, whose electrodes are metal vapor deposition films formed on dielectric films, to maintain insulation by scattering the vapor-deposited metal using discharge energy during abnormal discharges. This is an important function for preventing capacitor short circuits and breakdown. It is known that controlling the amount of air and gap distance between the film layers that make up the capacitor is important to improving capacitor safety. Particularly for large-sized capacitors, such as those with a capacity of 100 μF or more, it is important to uniformly control the amount of air and gap distance in both the length and width directions of the capacitor.

In order to improve the withstand voltage per film thickness and the safety of capacitors, studies are primarily being conducted to control the surface texture of the film. One known method for controlling the surface texture of a film is to utilize the crystal transition from β to α crystals of polypropylene (hereinafter referred to as the β crystal method). This method of utilizing crystal transition does not require the incorporation of impurities such as additives, which could potentially worsen 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).

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

Japanese Patent Application Laid-Open No. 2008-133446 Japanese Patent Application Laid-Open No. 2014-077057 WO2016/158590 publication

When the β-crystallization method described in Patent Documents 1 and 2 is applied to a typical linear polypropylene film, steep crater-like convex and concave portions are formed at a low density, and gloss unevenness tends to increase, particularly in the longitudinal and transverse directions. As a result, it cannot be said that the amount of air between film layers, which is related to voltage resistance and safety in modern high-temperature, high-voltage environments, is adequately controlled. 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 ensure uniform air volume 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 modern high-temperature, high-voltage environments.

The objective 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 film layers of a capacitor to be uniformly controlled in the longitudinal and transverse directions of the film, in order to achieve appropriate safety, primarily in large-capacity capacitors.

The above-mentioned objectives can be achieved as follows. Specifically, 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.

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. Furthermore, during capacitor processing, the amount of air between the film layers and the interlayer distance can be uniformly controlled in the longitudinal and transverse directions of the film. As a result, when used as a capacitor, it provides high safety even in high-temperature, high-voltage environments, and improves the capacitor's lifespan.

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

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" here means stretched in both the longitudinal and transverse directions. Furthermore, in the present invention, the term "polypropylene resin" refers to a resin that contains more than 50 mol% but not more than 100 mol% of propylene units, assuming that the total structural units constituting the resin are 100 mol%.

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

As the polypropylene copolymer, polypropylene copolymers copolymerized with other unsaturated hydrocarbons can be suitably 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 standpoint of voltage resistance characteristics 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 %.

In addition, other polymers (resins) besides 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 standpoint of voltage resistance characteristics and dimensional stability, the content of other polymers is preferably 20% by mass or less when the total components of the biaxially oriented polypropylene film are taken as 100% by mass.

The polypropylene resin that constitutes 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 that remains dissolved in xylene when the biaxially oriented polypropylene film is completely dissolved in xylene at 135°C and then precipitated at 20°C. It is believed to be a component that is difficult to crystallize due to its low stereoregularity and low molecular weight. The CXS of the main polypropylene resin 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. Methods for achieving a CXS of the polypropylene resin within the above range include increasing the catalytic activity during resin production and washing the resulting resin with a solvent or propylene monomer itself. Note that the term "main polypropylene resin" refers to the entire polypropylene resin that constitutes the film, and the same applies hereinafter to the term "main polypropylene resin."

The mesopentad fraction of the polypropylene resin that constitutes 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 perspective of heat shrinkage characteristics at high temperatures. The mesopentad fraction is an index of the stereoregularity of the crystalline phase of the polypropylene resin, measured by nuclear magnetic resonance (NMR). A higher mesopentad fraction value for the main component polypropylene resin results in higher crystallinity and melting point, making it particularly preferable from the perspective 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 resulting 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 main component polypropylene resin is within the above preferred range, the formed film exhibits excellent voltage resistance and dimensional stability.

The melt flow rate (MFR) of the polypropylene resin that constitutes the main component of the biaxially oriented polypropylene film of the present invention, when measured in accordance with JIS K 7210 (1995) Condition M (230°C, 2.16 kg), 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 the MFR of the polypropylene resin is within the above 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 ensure that the MFR of the main component polypropylene resin falls within the above range, methods such as controlling the average molecular weight and molecular weight distribution are preferably employed. More specifically, the MFR can be lowered by increasing the average molecular weight or reducing the variation in molecular weight distribution.

The polypropylene resin that 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) when measured at 230°C satisfy the relationship log(MS) > -0.56 log(MFR) + 0.74. To obtain a branched polypropylene whose melt tension (MS) and melt flow rate (MFR) when measured at 230°C satisfy the relationship log(MS) > -0.56 log(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 Japanese Patent No. 2,869,606 are preferably used.

The branched polypropylene used 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 little 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.

The branched polypropylene referred to here is a polypropylene having from one to five internally trisubstituted olefins per 10,000 carbon atoms, and the presence of this internally trisubstituted olefin can be confirmed by the proton ratio in the ¹ H-NMR spectrum. Branched polypropylene acts as an α-crystal nucleating agent, and also enables the formation of a rough surface due to the crystalline form if added in a certain amount. More specifically, the inclusion of branched polypropylene makes it possible to control the size of polypropylene spherulites produced during the cooling process of the melt-extruded resin sheet to a small size, thereby suppressing the occurrence of insulation defects produced during the stretching process and enabling the production of a polypropylene film with excellent voltage resistance.

When 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 branched polypropylene is within the above preferred range, improved film formability is obtained, while the stereoregularity of the biaxially oriented polypropylene film is not reduced, resulting in excellent voltage resistance characteristics. Note that when the biaxially oriented polypropylene film contains multiple types of branched polypropylene, the content is calculated by adding together all of the branched polypropylenes.

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 color inhibitors, as long as the additives do not impair the objectives of the present invention.

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

The biaxially oriented polypropylene film of the present invention preferably has a surface wetting 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 wetting tension is at or above the preferred lower limit, sufficient adhesion with the metal is achieved during metal vapor deposition. One method for achieving a surface wetting tension of 37 to 50 mN/m on at least one side or within the preferred range above is to perform 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.

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 125-145%, even more preferably 125-140%, and particularly preferably 128-138%. 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 that surface, and by adopting this configuration, the deterioration of voltage resistance characteristics caused by such irregularities is mitigated. 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, by adopting this configuration, the occurrence of conveyance wrinkles during film conveyance during film production and processing is suppressed, improving the winding shape of the film roll and reducing film breakage. Furthermore, by setting the glossiness on both sides to 125% or more and 150% or less, or within the above preferred range, the above effects are improved compared to when the glossiness on only one side is set in the same range.

One method for achieving a gloss level of 125% or more and 150% or less, or within the above preferred range, on at least one surface is to use the polypropylene resin described above and set specific conditions for the casting and longitudinal stretching processes during film production, as described below. More specifically, reducing the surface temperature of the casting drum during the casting process and the longitudinal stretching temperature during the longitudinal stretching process are effective ways to increase gloss.

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 greater is used. By maintaining at least one of the RMD and RTD within the above range, variations in the amount of air between film layers and gap distances are suppressed when the film is laminated during processing into a capacitor. Therefore, using this type of biaxially oriented polypropylene film in a capacitor can prevent excessive or ineffective safety during use, extend the capacitor's life, and reduce the risk of short circuit damage. Furthermore, it can also prevent film meandering and deterioration of the film roll shape during the film transport process during film formation and processing. Furthermore, the above effects are further improved by ensuring that the RMD and RTD are 0.1% or more and 3.0% or less, or within the preferred ranges mentioned above.

In the biaxially oriented polypropylene film of the present invention, if at least one of the RMD and 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 RTD is 0.1% or more and 3.0% or less." The same applies to the above-mentioned preferred ranges and aspects.

One method for keeping at least one of the RMD and RTD within 0.1% or more and 3.0% or less, or within the above preferred range, is to make the surface temperature of the casting drum more uniform (described below).

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 of 100 mm (longitudinal direction) x width direction length (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 direction 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 direction and the width direction 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 the 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 gloss value in the transverse direction - Minimum gloss value in the transverse direction.

The biaxially oriented polypropylene film of the present invention preferably has a thickness of 1.0 to 3.0 μm. A thickness of 1.2 to 2.8 μm is more preferable, and a thickness of 1.5 to 2.5 μm is even more preferable. A thickness of 1.0 μm or more can provide the biaxially oriented polypropylene film with excellent mechanical strength and high-temperature voltage resistance characteristics, 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 the biaxially oriented polypropylene film is 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, etc.

Next, the method for producing the biaxially oriented polypropylene film of the present invention will be described below, but the method is not necessarily limited to this.

First, the preferred polypropylene resin described above is fed into a single-screw melt extruder and melt-extruded at 200-260°C. Next, foreign matter and modified polymers 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 on a cooling roll.

From the perspective of properly generating β crystals and spherulites, 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. By maintaining a cast drum temperature of 60°C or higher, it is possible to prevent excessive β crystals from forming in the cast sheet and maintain the smoothness of the film obtained after biaxial stretching. This prevents the occurrence of conveyance wrinkles during the film conveyance process during film production and processing, and deterioration of the film roll shape. On the other hand, by maintaining a cast drum temperature of 100°C or lower, it is possible to prevent excessive formation of β crystals and spherulites in the cast sheet, which makes it easier to prevent meandering during the film conveyance process during film production and processing, and deterioration of the film roll shape.

Specific examples of methods for controlling the surface temperature of a casting drum more uniformly are described below, taking into account the mechanism by which the surface temperature of the casting drum becomes uneven. Casting drums typically have piping installed inside them parallel to the width direction or in a spiral configuration. The surface temperature of the casting drum can be controlled by introducing a temperature-controlled cooling medium into one end of the piping and allowing it to flow out the other end. However, the helical portion reduces the heat transfer coefficient, and the amount of heat generated during film cooling on the surface of the casting drum creates a temperature difference between the inflow and outflow of the cooling medium, resulting in uneven surface temperature of the casting drum. Temperature unevenness on the casting drum caused by this mechanism can be reduced, for example, by increasing the flow rate of the cooling medium.

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 cooling drum's piping for an extended period of time, these foreign matter may adhere to and accumulate on the inside surface 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 cooling drum surface. Therefore, by periodically cleaning the piping, etc., these deposits and agglomerates can be removed early, allowing for more uniform control of the cooling drum's surface temperature.

Another method for uniformly controlling the surface temperature of a casting drum is to use a casting drum with multiple jacket chambers within the thickness of the casting drum surface. A two-phase gas-liquid heat transfer medium is sealed within 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 cooled, it liquefies and changes phase back to a liquid-phase heat transfer medium. In this way, the heat transfer medium vaporizes when it comes into contact with the hot casting drum surface, removing latent heat, and then vaporizes when it comes into contact with the cold casting drum surface, providing latent heat, thereby uniformly controlling the temperature of the casting drum surface. Note that the "thickness of the casting drum surface" refers to the cylindrical portion that forms the side of the casting drum when the casting drum is imagined as a cylinder with a central axis of rotation.

Furthermore, the surface temperature variation 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 this configuration, the β crystals formed in the cast sheet are uniform in the width direction, making it easier to prevent the surface shape of the film from varying longitudinally and widthwise.

It is preferable that the molten sheet discharged from the T-die lands on the casting drum and remains in close contact with the drum for 1 to 3 seconds. If the close contact time is 1 second or more, the molten sheet is more likely to solidify, making it easier to prevent breakage during the subsequent stretching process. On the other hand, if the close contact time is 3 seconds or less, it is possible to prevent excessive formation of β crystals in the cast sheet, making it easier to prevent meandering during the film transport process during film production and processing, and deterioration of the winding shape of the film roll.

Methods for adhering the molten sheet to the casting drum include electrostatic application, air knife, nip roll, and underwater casting. However, the air knife method is preferred from the standpoints of suppressing thickness unevenness, enabling 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 setting the air knife temperature to 60°C or higher, it is possible to prevent excessive formation of β crystals in the cast sheet, maintain the slipperiness of the film obtained after biaxial stretching, and prevent the occurrence of conveyance wrinkles and deterioration of the film roll shape during the film conveyance process during film production and processing. On the other hand, by setting the air knife temperature to 120°C or lower, it is possible to prevent excessive formation of β crystals in the cast sheet, thereby preventing the occurrence of meandering and deterioration of the film roll shape during the film conveyance process during film production and processing.

From the perspective of forming equivalent β crystals on both sides of the cast sheet, 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. Keeping the temperature difference between the cast drum temperature and the air knife temperature 20°C or less prevents the formation of different irregularities on the front and back of the film, making it easier to achieve equivalent slip properties on both sides of the film. This stabilizes the amount of air entrained during the winding process during capacitor processing, and makes 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.

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, it is easier to raise the film to the desired temperature in the subsequent longitudinal stretching process, making it easier to prevent the film from breaking during the longitudinal stretching process. On the other hand, if the cooling temperature is 50°C or lower, it is easier to stop crystal formation in the cast sheet, making the surface properties of the film obtained after the stretching process more uniform in the longitudinal direction.

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 transformation of β crystals into α crystals (volume reduction due to the transformation), and these depressions are mechanically deformed in the longitudinal stretching direction simultaneously, 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.

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 during the longitudinal stretching process, and the mechanical deformation of the depressions in the longitudinal stretching direction, to occur in separate processes. Furthermore, it is even more preferable to apply an excess amount of heat when transforming β crystals to α crystals, thereby partially melting the depressions formed during the α crystal transition.

Specifically, before the longitudinal stretching process, it is preferable to nip the cast sheet with temperature-controlled nip rolls and pass it through temperature-controlled transport rolls to perform the heat treatment. The diameter and number of transport rolls can be adjusted depending on the heat treatment time. It is also more preferable to cool the cast sheet after the heat treatment to stop partial melting.

The transport roll temperature during heat treatment is preferably 160 to 170°C, and more preferably 160 to 165°C. By setting the transport roll temperature to 160°C or higher, not only is the transition from β crystals to α crystals more likely to occur, but the recesses formed by the α crystal transition are also more likely to melt partially. 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 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 productivity.

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, reducing breakage during the stretching process and making it easier to ensure mass productivity. On the other hand, by setting the nip roll temperature to 170°C or lower, breakage caused by melting of the entire cast sheet is reduced, making it easier to maintain mass productivity.

The nip roll pressure 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, making it easier to reduce breakage during the longitudinal stretching process and ensure mass productivity. On the other hand, by setting the nip roll pressure to 0.60 MPa or less, film deformation due to pressure is suppressed, making it easier to reduce breakage during the stretching process and ensure mass productivity.

The heat treatment time for the cast sheet is preferably 1 to 10 seconds. By setting the heat treatment time to 1 second or more, not only is the transition from β crystals to α crystals more likely to occur, but partial melting of the recesses due to volume reduction also occurs more 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 less, melting of the entire film and the occurrence of wrinkles due to film expansion are suppressed, making it easier to ensure mass productivity. Furthermore, because excessive partial melting of the recesses is not caused, it is easier to prevent the film from becoming excessively smooth after stretching.

The cooling temperature of the cast sheet after heat treatment is preferably 80 to 120°C. By setting the cooling temperature to 80°C or higher, it becomes easier to raise the film temperature to the desired temperature in the subsequent longitudinal stretching process, reducing breakage during the 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.

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

The longitudinal stretching ratio is preferably 4.0 to 7.0 times, and more preferably 5.0 to 7.0 times. A longitudinal stretching ratio of 4.0 times or more results in more uniform film surface properties and excellent high-temperature voltage resistance characteristics. A longitudinal stretching ratio of 7.0 times or less helps to reduce film breakage during the longitudinal stretching process and the subsequent transverse stretching process.

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

The biaxially oriented film is then subjected to a corona discharge treatment in air, nitrogen, carbon dioxide, or a mixture of these gases, and both 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, yielding a roll of biaxially oriented polypropylene film of the present invention.

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, from the perspective of electrode configuration, it may be either a foil-wound capacitor or a metal-vapor-deposited film capacitor, and is also preferably used in oil-immersed capacitors that contain insulating oil, or dry capacitors that do not use insulating oil at all. Furthermore, from the perspective 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-vapor-deposited film capacitor.

The metal film laminated film of the present invention is 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 there are no particular limitations on the method for forming the metal film, a preferred method is, for example, to vapor-deposit a metal such as aluminum onto at least one side of the biaxially oriented polypropylene film to form a metal film that will serve as an internal electrode for the film capacitor. At this time, 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 formed on the metal film.

The thickness of the metal film is preferably 20 to 100 nm from the perspective of the capacitor's electrical characteristics and safety. 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.

In the present invention, if necessary, after forming the metal film, the metal film laminated film can be subjected to aging treatment or heat treatment at a specific temperature. Furthermore, for insulation or other purposes, a coating such as polyphenylene oxide can be applied to at least one side of the metal film laminated film.

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. In other words, 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 manufacturing wound film capacitors will be described below, but the method is not necessarily limited to this.

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 inserted into the center of each deposited area and the center of each margin on the surface to create a tape-like take-up reel with a margin on one side. Two tape-like take-up reels with a left margin and one with a right margin are stacked 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 onto 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 wipers), and power sources. The film capacitor of the present invention can be used effectively in these applications.

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

(1) Gloss A biaxially oriented polypropylene film sample was taken from an intermediate product having a width of 6,000 mm in a rectangular shape of 100 mm (longitudinal direction) x 6,000 mm (width direction), and the sample was divided into 60 equal parts parallel to the longitudinal direction to obtain 60 test pieces of 100 mm x 100 mm. Next, the gloss at the center of the obtained test piece was measured five times on the drum surface and 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 acceptance angle of 60 °, and the average value was taken as the gloss of each surface of the test piece. Thereafter, the same measurement was performed on all test pieces, and the maximum and minimum values of the gloss of each surface obtained were defined as the "maximum gloss in the width direction" and "minimum gloss in the width direction" on each surface, respectively. Furthermore, the same measurement was performed with the longitudinal direction and width direction reversed, and the "maximum gloss in the longitudinal direction" and "minimum gloss in the longitudinal direction" on 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 gloss value in the transverse direction - Minimum gloss value in the transverse direction.

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

(3) Surface temperature and unevenness of the cast drum The surface temperature of the entire width of the cast drum immediately before the molten sheet discharged from the T-die came into close contact with the drum was measured using a thermograph (Ti29 industrial/commercial thermograph manufactured by Fluke, measurable temperature range -20°C to 600°C), and the average, maximum, and minimum values were read. The average value was taken as the surface temperature of the cast drum, and the difference between the maximum and minimum values was taken as the surface temperature unevenness of the cast drum.

(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 pieces with misalignment or wrinkles) x 100 / total number of pieces processed. 〇: 2% or less.
△: More than 2% and 5% or less.
×: More than 5%.

(5) Processability of Wound Body in Capacitor Manufacturing: Aluminum was vacuum-deposited on the corona-treated side of 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, repeated). The film was then slit with a blade at the center of each deposited strip and the center of each margin, producing 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 of the resulting 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, yielding a wound body with a capacitance of 120 μF. A KAW-4NHB manufactured by Kaito Seisakusho, Inc., was used for winding. Finally, the wound body was heat-treated for 10 hours in a reduced-pressure atmosphere at 140°C. The wound body was visually inspected, and those with wrinkles or distortions in appearance or inside were deemed defective. 50 wound bodies were similarly produced and the same evaluation was repeated, and the processability of the wound bodies was evaluated according to the following criteria.
◯: 1 or less defective items △: 2 to 3 defective items ×: 4 or more defective items

(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 of 10 capacitors before and after the application of voltage was taken as the rate of change in capacitance of that sample before and after the application of voltage, and was evaluated according to the following criteria.
◯: ΔC is less than 3% △: ΔC is 3% or more but less than 5% ×: ΔC is 5% or more.

(7) Film-forming property The film-forming property of the film 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 resumed was excluded from the observation time.
◯: No film breakage occurred for 48 hours or more.
Δ: The film broke 1 to 3 times within 48 hours.
x: The film broke four or more times within 48 hours.

Example 1
Polypropylene resin (manufactured by Prime Polymer Co., Ltd., melting point: 166 ° C., MFR: 2.5 g / 10 min, mesopentad fraction: 0.991) was fed into a single-screw melt extruder, melt-extruded at 250 ° C., and foreign matter was removed using a sintered filter with a 25 μm cutoff. The molten resin was then melt-extruded into a sheet from a T-type slit die. Thereafter, the molten sheet was brought into close contact with the surface of a cast 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., solidified, and then cooled on a cooling roll maintained at a temperature of 30 ° C. The time during which the cast drum and the molten sheet were in close contact with each other was 1.5 seconds. Here, the surface that contacts the cast drum is referred to as the drum surface (D surface), and the surface that does not contact the cast drum is referred to as the non-drum surface (non-D surface). The casting drum used had a plurality of jacket chambers in 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).

The obtained cast sheet was placed on a conveying roll at a temperature of 165 ° C. and heat-treated for 5 seconds while being nipped with a nip roll at a temperature of 165 ° C. at a pressure of 0.45 MPa, and then cooled on a cooling roll at a temperature of 100 ° C. The sheet was then stretched 5.5 times in the longitudinal direction with a longitudinal stretching roll at a temperature of 145 ° C. to obtain a uniaxially oriented film. The uniaxially oriented film was then clamped 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 surface) side was subjected to a corona discharge treatment at a treatment intensity of 25 W min / m ^2. The biaxially oriented film was then wound up, with both widthwise ends of the film clamped by the clips cut off, to obtain a 6,000 mm wide intermediate product. The biaxially oriented film was then unwound from the intermediate product and slit using a slitter to a width of 620 mm, and 60,000 m of the film was wound around a core in the longitudinal direction to form a film roll, yielding a biaxially oriented polypropylene film with a thickness of 2.0 μm. The evaluation results of the obtained biaxially oriented polypropylene film are shown in Table 1.

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.

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.

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.

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 and nip roll for the cast sheet were controlled to 162° C. The evaluation results of the obtained biaxially oriented polypropylene film are shown in Table 1.

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 and nip roll for the cast sheet 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 casting 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 casting drum was controlled to 2.5° C. The evaluation results of the obtained biaxially oriented polypropylene film are shown in Table 1.

(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 on 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 through the inside of the casting drum, and the temperature unevenness was controlled by controlling the amount of cooling water passed through the inside of the casting drum (the same applies to Comparative Examples 2 to 4).

(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.

(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 having no jacket chamber on the cast drum surface thickness 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.

(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 having no jacket chamber on the cast drum surface thickness 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.

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 (4)

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 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 a receiving angle of 60°, and the average value was taken as the glossiness of the test piece. Subsequently, 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 same measurement was performed with the longitudinal and width directions reversed to obtain the maximum glossiness in the longitudinal direction and the minimum glossiness in the longitudinal direction. After the measurement was completed, the longitudinal gloss unevenness (RMD) and the width gloss unevenness (RTD) were calculated using the following formula.
Longitudinal gloss unevenness (RMD) = Maximum gloss value in the longitudinal direction - Minimum gloss value in the longitudinal direction
Transverse gloss unevenness (RTD) = maximum gloss value in the transverse direction - minimum gloss value in the transverse direction. 2. The biaxially oriented polypropylene film according to claim 1, wherein the RMD and the RTD are 0.1% or more and 3.0% or less . 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. A film capacitor having a laminated or wound configuration of the metal film laminated film according to claim 3 .