JP2022114442A - Biaxially oriented polypropylene film, metal film-laminated film, and film capacitor - Google Patents

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 The present invention relates to a biaxially oriented polypropylene film, a metal film laminated film, and a film capacitor, which have high voltage resistance in a high-temperature, high-voltage environment when used as a dielectric of a capacitor.

Biaxially oriented polypropylene films are excellent in transparency, mechanical properties, electrical properties, and the like, and are used in various applications such as packaging, tape, cable wrapping, and electrical applications such as capacitors.

Among them, in capacitor applications, it is particularly preferably used as a capacitor dielectric because of its excellent high withstand voltage characteristics and low loss characteristics. In recent years, various electrical equipment is being converted to inverters, and along with this, the demand for smaller size capacitors and larger capacity capacitors is becoming stronger. Furthermore, the use environment is becoming hotter (above 85°C and below 125°C), especially in automotive applications (including hybrid cars and electric vehicles), photovoltaic power generation, and wind power generation applications, and the demand for heat resistance of capacitors is increasing. there is

Therefore, it is required to make the biaxially oriented polypropylene film, which is a dielectric, thinner, more heat resistant, and to improve the withstand voltage per thickness, and also to improve the security of the capacitor. Here, the safety of the capacitor is the function of maintaining insulation by scattering the vapor-deposited metal due to the discharge energy during abnormal discharge in the metal vapor-deposited capacitor that uses the metal vapor-deposited film formed on the dielectric film as the electrode. , is an important function to prevent short circuits and destruction of capacitors. In order to improve the safety of capacitors, it is known that controlling the amount of air and the gap distance between the film layers that make up the capacitor is important. It is a problem to control the amount of air and the gap distance uniformly in the longitudinal direction and the width direction of the capacitor.

In order to improve the withstand voltage per thickness of the film and improve the security of the capacitor, mainly controlling the surface properties of the film has been studied. As a method for controlling the surface properties of a film, a method utilizing crystal transition from β-crystal to α-crystal of polypropylene (hereinafter referred to as the β-crystal method) is known. The method using this crystal transition does not require the mixing of impurities such as additives that may deteriorate the withstand voltage. , see Patent Documents 1 and 2).

In addition, as a technique that focuses on the balance between the longitudinal direction and the width direction of the slipperiness of the film, a method of isotropically controlling the in-plane slipperiness by adjusting the casting temperature and stretching temperature has been proposed (for example, patent Reference 3). In this method, the roll transportability of the film is improved, so that the workability into capacitors is improved, and uneven adhesion between layers and residual stress when processed into capacitors can be reduced.

JP-A-2008-133446 JP 2014-077057 A WO2016/158590

When the β-crystal method described in Patent Documents 1 and 2 is applied using a general linear polypropylene film, crater-like steep protrusions and recesses are formed at a low density, especially in the longitudinal and width directions. glossiness unevenness tends to increase. 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 and high-voltage environments, is sufficient. In addition, although the method described in Patent Document 3 can reduce unevenness in interlayer adhesion and residual stress when processed into a capacitor, it is not enough to control slipperiness, which is affected by unevenness in film thickness and electrification state. , it is insufficient to make uniform the amount of air and the gap distance between film layers when processed into a capacitor. Therefore, it cannot be said that the safety of the capacitor functions properly in recent high-temperature and high-voltage environments.

Therefore, the object of the present invention is to have high productivity, workability, and voltage resistance, and to obtain appropriate safety mainly in large-capacity capacitors. An object of the present invention is to provide a biaxially oriented polypropylene film having surface properties that can be uniformly controlled in the longitudinal and width directions.

The above problems can be achieved by the following. That is, in the polypropylene film of the present invention, at least one of the maximum gloss unevenness (RMD) in the longitudinal direction and the maximum gloss unevenness (RTD) in the width direction is 0.1% or more and 3.0% or less. It is a biaxially oriented polypropylene film.

When the biaxially oriented polypropylene film of the present invention is used as the dielectric of a capacitor, it is excellent in workability and voltage resistance, and the amount of air between film layers and the interlayer distance are uniform in the longitudinal direction and the width direction of the film during capacitor processing. can be controlled to Therefore, when it is made into a capacitor, it functions with high safety even in a high-temperature and high-voltage environment, and the life of the capacitor is also improved.

The biaxially oriented polypropylene film, metal film laminated film and film capacitor of the present invention are described in more detail below. It should be noted that, in the numerical ranges indicated below using "-", the upper limit and lower limit are included in the range, and the units of the upper limit and lower limit 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 two directions, the longitudinal direction and the width direction. In other words, biaxially oriented here means that the film is stretched in the longitudinal direction and the width direction. In the present invention, the polypropylene resin refers to a resin containing more than 50 mol % and 100 mol % or less of propylene units when the total structural units constituting the resin are taken as 100 mol %.

The biaxially oriented polypropylene film of the present invention contains a polypropylene resin as a main component. The polypropylene resin may include not only a propylene homopolymer but also a polypropylene copolymer and a branched chain polypropylene, which will be described later. In the present invention, the "main component" means that the proportion of the total components of the film 100% by mass is more than 50% by mass and 100% by mass or less, more preferably 80% by mass or more and 100% by mass. Below, more preferably 90% by mass or more and 100% by mass or less, particularly preferably 95% by mass or more and 100% by mass or less. Components other than the polypropylene resin in the film include the resins described later and additives such as antioxidants and lubricating agents. In the case where the film contains a plurality of types of polypropylene resin, if the total content of all polypropylene resins exceeds 50% by mass, it can be interpreted that the polypropylene resin is the main component.

As the polypropylene copolymer, a polypropylene copolymer obtained by copolymerizing other unsaturated hydrocarbons can be preferably used. Copolymerization components of the polypropylene copolymer include, for example, 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, 5 -methyl-2-norbornene and the like. From the viewpoint of withstand voltage 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 100 mol %.

In addition, a polymer (resin) other than polypropylene resin may be added to the homopolymer of propylene. 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 withstand voltage characteristics and dimensional stability, the content of the other polymer is preferably 20% by mass or less when all components of the biaxially oriented polypropylene film are 100% by mass.

The polypropylene resin that is the main component of the biaxially oriented polypropylene film of the present invention preferably has a cold xylene soluble portion (hereinafter referred to as CXS) of 5% by mass or less. Here, CXS refers to a polypropylene component dissolved in xylene when a biaxially oriented polypropylene film is completely dissolved in xylene at 135°C and then precipitated at 20°C. It is considered that it corresponds to a component that is difficult to crystallize due to its low molecular weight and the like. The CXS of the polypropylene resin, which is 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. When the CXS is 5% by mass or less, the withstand voltage characteristics and dimensional stability of the biaxially oriented polypropylene film are improved. In order to make the CXS of the polypropylene resin within the above range, there are a method of increasing the catalytic activity in obtaining the resin, a method of washing the obtained resin with a solvent or the propylene monomer itself, and the like. Here, the "main component polypropylene resin" means the entire polypropylene resin that constitutes the film, and hereinafter, the "main component polypropylene resin" is similarly interpreted.

The mesopentad fraction of the polypropylene resin, which is 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 showing the stereoregularity of the crystal phase of the polypropylene resin measured by the nuclear magnetic resonance method (NMR method). Since the crystallinity and melting point of the polypropylene resin, which is the main component, increases as the numerical value increases, it is particularly preferable from the viewpoint of vapor deposition processability at high temperature when formed into a film. In order to obtain such a polypropylene resin with high stereoregularity, a method of washing the resin powder obtained with a solvent such as n-heptane, a method of selecting a catalyst and / or co-catalyst, a method of appropriately selecting a composition, etc. is preferably adopted. When the mesopentad fraction of the polypropylene resin, which is the main component, is within the above preferred range, the resulting film is excellent in withstand voltage characteristics and dimensional stability.

When the melt flow rate (hereinafter referred to as MFR) of the polypropylene resin that is the main component of the biaxially oriented polypropylene film of the present invention is measured according to JIS K 7210 (1995) Condition M (230 ° C., 2.16 kg) , preferably 1.0 to 10 g/10 minutes, more preferably 1.5 to 8 g/10 minutes, and even more preferably 2.0 to 5 g/10 minutes. When the MFR of the polypropylene resin is within the above preferable range, the biaxially oriented polypropylene film can be stably obtained due to excellent film formability, and the biaxially oriented polypropylene film also has excellent withstand voltage characteristics. In order to keep the MFR of the polypropylene resin, which is the main component, within the above range, a method of controlling the average molecular weight and the molecular weight distribution is preferably adopted. More specifically, the MFR can be lowered by increasing the average molecular weight or by reducing the variation in the molecular weight distribution.

The polypropylene resin, which is the main component of the biaxially oriented polypropylene film of the present invention, may contain branched chain polypropylene for the purpose of improving the film formability. In this case, the branched polypropylene has a melt tension (MS) and a melt flow rate (MFR) when measured at 230 ° C., log (MS) > -0.56 log (MFR) + 0.74. Branched polypropylene is preferred. Melt tension (MS) and melt flow rate (MFR) when measured at 230 ° C. To obtain a branched polypropylene that satisfies the relational expression log (MS) > -0.56 log (MFR) + 0.74, A method of blending polypropylene containing many high molecular weight components, a method of blending an oligomer or polymer having a branched structure, and a method of introducing a long-chain branched structure into the polypropylene molecule as described in JP-A-62-121704. A method or a method as described in Japanese Patent No. 2869606 is preferably used.

The branched polypropylene in the biaxially oriented polypropylene film of the present invention is not particularly limited, but a resin obtained by an electron beam crosslinking method is preferably used because the resin contains less gel component. Specific examples of the branched polypropylene 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) manufactured by Borealis. Examples include HMS-PP (WB130HMS, WB135HMS, etc.).

The term "branched polypropylene" as used herein refers to a polypropylene having 1 to 5 internal trisubstituted olefins with respect to ^10,000 carbon atoms. It can be confirmed by the proton ratio of the NMR spectrum. The branched-chain polypropylene has an action as an α-crystal nucleating agent, and can form a rough surface due to the crystal morphology if the addition amount is within a certain range. More specifically, by containing branched polypropylene, it is possible to control the spherulite size of the polypropylene generated in the cooling process of the melt-extruded resin sheet to be small, so the occurrence of insulation defects generated in the stretching process is suppressed, and the withstand voltage is improved. A polypropylene film having excellent properties can be obtained.

When the branched polypropylene constituting the biaxially oriented polypropylene film of the present invention is contained, the content is preferably 0.05 to 3.0% by mass based on 100% by mass of the total components of the biaxially oriented polypropylene film. , more preferably 0.1 to 2.0% by mass, more preferably 0.3 to 1.5% by mass, and particularly preferably 0.5 to 1.0% by mass. When the content of the branched-chain polypropylene is within the above preferable range, the effect of improving the film formability is obtained, while the stereoregularity as a biaxially oriented polypropylene film is not lowered, so that the withstand voltage characteristics are excellent. In addition, when the branched-chain polypropylene in a biaxially-oriented polypropylene film is multiple types, content is calculated by totaling all the branched-chain polypropylenes.

The biaxially oriented polypropylene film of the present invention contains various additives, such as crystal nucleating agents, antioxidants, heat stabilizers, lubricants, antistatic agents, antiblocking agents, fillers, as long as the objects of the present invention are not impaired. It is also preferable to contain an agent, a viscosity modifier, an anti-coloring agent, and the like.

Among the above additives, the selection of the type and amount of antioxidant to be added is important from the viewpoint of long-term heat resistance. That is, the antioxidant is preferably a sterically hindered phenolic antioxidant, at least one of which is of a high molecular weight type having a molecular weight of 500 or more. Specifically, for example, 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 (for example, BASF "Irganox" (registered trademark) 1330: molecular weight 775.2), tetrakis [methylene-3 (3,5-di-t-butyl-4- Hydroxyphenyl)propionate]methane (for example, "Irganox" (registered trademark) 1010 manufactured by BASF, molecular weight 1177.7) and the like are preferably used alone or in combination. The total content of these antioxidants is preferably 0.03 to 1.0 parts by mass, more preferably 0.1 to 0.9 parts by mass, based on the total amount of the polypropylene resin. When the content of the antioxidant in the polypropylene resin composition is 0.03 parts by mass or more, it is easy to obtain the effect of preventing oxidation, and it is easy to maintain long-term heat resistance. On the other hand, when the antioxidant content in the polypropylene resin composition is 1.0 parts by mass or less, it is easy to maintain high-temperature withstand voltage 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, and even more preferably 39 to 48 mN/m. 40 to 47 mN/m is particularly preferred. When the surface wetting tension is equal to or higher than the above preferred lower limit, sufficient adhesion to the metal is achieved during metal vapor deposition. A method for adjusting the surface wetting tension of at least one side to 37 to 50 mN/m or the above preferred range includes, for example, a method of subjecting the film to surface treatment after biaxial stretching during film formation. Specific examples include corona discharge treatment, plasma treatment, glow treatment, and flame treatment, and these may be used alone or in combination.

The biaxially oriented polypropylene film of the present invention preferably has a glossiness of 125% or more and 150% or less, more preferably 125 to 145%, even more preferably 125 to 140%, on at least one surface. 128-138% is particularly preferred. A state in which at least one surface has a glossiness of 125% or more is a state in which an increase in light scattering density on the surface is suppressed. In other words, if the glossiness is 125% or more on at least one surface, it means that there are few unevennesses on the surface. is reduced. On the other hand, a glossiness of 150% or less on at least one surface means that the surface has unevenness to the extent that slipperiness can be ensured. Therefore, by adopting such a mode, the occurrence of transport wrinkles in the film transport process during film formation and processing is suppressed, the winding shape of the film roll is improved, and the breakage of the film is reduced. By setting the glossiness of both surfaces to 125% or more and 150% or less or the above preferable range, the above effect is improved as compared with the case where the glossiness of only one surface is set to the same range.

As a method for making the glossiness of at least one surface 125% or more and 150% or less or the above preferred range, the above-described polypropylene resin is used, and the casting process and longitudinal stretching process during film formation are specified as described later. A method of setting the condition of More specifically, in order to increase the glossiness, it is effective to lower the surface temperature of the casting drum in the casting process and to lower the longitudinal stretching temperature in the longitudinal stretching process.

In the biaxially oriented polypropylene film of the present invention, at least one of the maximum gloss unevenness (RMD) in the longitudinal direction and the maximum gloss unevenness (RTD) in the width direction is 0.1% or more and 3.0% or less. It is important, it is preferably 0.1% or more and 2.0% or less, more preferably 0.1% or more and 1.0% or less. RMD and RTD, which are uneven glossiness, are preferably as small as possible, but are set to 0.1% or more as a feasible range. By setting at least one of RMD and RTD within the above range, unevenness in the amount of air and gap distance between film layers is suppressed when films are laminated in the process of forming a capacitor. Therefore, by using such a biaxially oriented polypropylene film in a capacitor, it is possible to prevent the security from becoming too effective or less effective when the capacitor is used, extend the life of the capacitor, or cause short-circuit breakdown. can be made difficult. It is also possible to prevent meandering of the film and deterioration of the winding shape of the film roll in the film transport process during film formation and processing. In addition, the above effect is further improved by setting the RMD and RTD to 0.1% or more and 3.0% or less, or in the above preferable range.

In the biaxially oriented polypropylene film of the present invention, if at least one of RMD and RTD is 0.1% or more and 3.0% or less on at least one side, "at least one of RMD and RTD is 0.1% or more and 3. 0% or less." The same applies to the preferred ranges and modes described above.

As a method for adjusting at least one of RMD and RTD to 0.1% or more and 3.0% or less or the above preferred range, a method of making the surface temperature of the cast drum more uniform (described later) can be mentioned.

Glossiness, RMD, and RTD can be measured by the following methods (when both sides need to be measured, each side is similarly measured). First, from a film roll, a biaxially oriented polypropylene film sample in a rectangular shape of 100 mm (longitudinal direction) x width direction length (width direction) is taken, and the sample is divided into 60 equal parts in parallel with the longitudinal direction to obtain 100 mm x width. 60 specimens of 1/60 mm of the directional length are obtained. Then, the glossiness of the central part of the obtained test piece was measured five times under the conditions of an incident angle of 60 ° and a light receiving angle of 60 ° using a digital variable angle gloss meter, and the average value was the glossiness of the test piece. and Thereafter, the same measurement is performed for all test pieces, and the obtained maximum and minimum values of glossiness are defined as "maximum value of glossiness in the width direction" and "minimum value of glossiness in the width direction", respectively. Further, the same measurement is performed with the longitudinal direction and the width direction interchanged to obtain the “longitudinal direction maximum glossiness value” and the “longitudinal direction glossiness minimum value”. After the measurement is completed, the gloss unevenness (RMD) in the longitudinal direction and the gloss unevenness (RTD) in the width direction are obtained from the following equations. Also, the average value of the glossiness of all the test pieces is obtained, and this value is defined as the glossiness of the biaxially oriented polypropylene film. The digital variable angle gloss meter used for measuring the glossiness is not particularly limited as long as it can be measured.
Longitudinal gloss unevenness (RMD)=Longitudinal maximum glossiness−Longitudinal minimum glossiness Unevenness of widthwise glossiness (RTD)=Maximum glossiness in the width direction−Minimum glossiness in the width 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. By setting the thickness to 1.0 μm or more, the biaxially oriented polypropylene film can be made excellent in mechanical strength and high-temperature withstand voltage characteristics, and film breakage during film formation and processing can also be prevented. . On the other hand, by setting the thickness to 3.0 μm or less, the capacitance per volume can be increased when the biaxially oriented polypropylene film is used as a capacitor dielectric. The thickness can be measured by the micrometer method according to JIS C 2330 (2014), and the thickness is adjusted by adjusting the discharge amount from the nozzle, the slit width of the nozzle, the rotational speed of the cast drum, and the stretching 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 supplied to a single-screw melt extruder and melt extruded at 200 to 260°C. Next, a filter installed in the middle of the polymer tube removes foreign matter, modified polymer, and the like. Then, it is discharged from a T-die onto a cast drum to form a cast sheet, which is cooled by a cooling roll.

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., from the viewpoint of appropriately forming β crystals and spherulites. ~95°C is particularly preferred. By setting the cast drum temperature to 60° C. or higher, it is possible to prevent the amount of β crystals formed in the cast sheet from becoming too small, and to maintain the lubricity of the film obtained after biaxial stretching. Therefore, it is possible to prevent the occurrence of conveyance wrinkles and the deterioration of the winding shape of the film roll in the film conveyance process during film formation and processing. On the other hand, by setting the cast drum temperature to 100° C. or less, it is possible to prevent excessive formation of β crystals and spherulites in the cast sheet, and the occurrence of meandering in the film transport process during film formation and processing. It becomes easy to prevent deterioration of the winding shape of the film roll.

A specific example of a method for controlling the surface temperature of the cast drum more uniformly will be described below based on the mechanism by which the surface temperature of the cast drum becomes non-uniform. A cast drum is usually provided with a pipe inside in parallel with the width direction or spirally. The surface temperature of the cast drum can be controlled by inflowing a temperature-controlled cooling medium from one end of the pipe and flowing it out from the other end. However, the heat transfer rate of the helical portion is low, and the heat generated when cooling the film on the surface of the cast drum creates a temperature difference between when the cooling medium flows in and out, which causes the surface temperature of the cast drum to become uneven. Become. The temperature unevenness of the cast drum caused by such a mechanism can be reduced, for example, by increasing the flow rate of the cooling medium.

Industrial water is generally used as a cooling medium. This industrial water contains metal components such as iron and manganese as well as foreign matter such as evaporation residue. The above-mentioned foreign matter may adhere to the side surface and be partially deposited, or the foreign matter may react with the metal on the side surface of the pipe and form an agglomerate. Due to the occurrence and falling of these deposits and aggregates, temperature unevenness on the surface of the cooling drum occurs locally. By removing it, the temperature of the surface of the cooling drum can be controlled more uniformly.

Further, as a method of controlling the surface temperature of the cast drum uniformly, there is a method of using a cast drum having a plurality of jacket chambers within the wall thickness of the cast drum surface. By enclosing a gas-liquid two-phase heat transfer medium in the jacket chamber, when the liquid-phase heat transfer medium comes into contact with the surface of the cast drum, which is locally heated by heat conduction, the liquid-phase heat transfer medium is vaporized to produce gas-phase heat. Phase change to medium. When this gas-phase heat medium comes into contact with the surface of the cast drum, which is still locally at a low temperature, it is liquefied and phase-converted into a liquid-phase heat medium again. In this way, the heat medium is vaporized by contacting the high-temperature cast drum surface to take away latent heat, and then vaporized by contacting the low-temperature cast drum surface to provide latent heat, thereby uniformly controlling the temperature of the cast drum surface. can do. The term "thickness of the surface of the cast drum" refers to a cylindrical portion forming a side surface when the cast drum is likened to a column whose center axis is the rotation axis.

In addition, the surface temperature unevenness of the entire width of the cast drum immediately before the molten sheet discharged from the T die is in close contact with the cast drum is preferably 3.0 ° C. or less, more preferably 2.0 ° C. or less. It is more preferably less than 0°C. By adopting such a mode, the β crystal formed in the cast sheet is formed uniformly in the width direction, and it is easy to prevent the surface shape of the film from varying in the longitudinal direction and the width direction.

It is preferable that the molten sheet ejected from the T-die lands on the casting drum and is in close contact with the drum for 1 to 3 seconds. When the contact time is 1 second or more, the melted sheet is easily solidified and easily prevented from being broken in the subsequent stretching step. On the other hand, when the contact time is set to 3 seconds or less, it is possible to prevent excessive formation of β crystals in the cast sheet, causing meandering in the film transport process during film production and processing, and the film roll. It becomes easier to prevent deterioration of the winding appearance.

As a method for bringing the molten sheet into close contact with the cast drum, methods such as the electrostatic application method, the air knife method, the nip roll method, and the underwater casting method can be adopted. From the viewpoint of property control, the air knife method is preferred. The air temperature of the air knife is preferably 60-120°C. By setting the air knife temperature to 60 ° C. or higher, it is possible to prevent the formation of β crystals in the cast sheet from becoming too small, to maintain the slipperiness of the film obtained after biaxial stretching, and to transport the film during film formation and processing. It becomes easier to prevent the occurrence of wrinkles during transportation and the deterioration of the winding shape of the film roll. On the other hand, by setting the air knife temperature to 120°C or less, in order to prevent excessive formation of β crystals in the cast sheet, it is possible to prevent meandering in the film transport process during film production and processing, and the winding of the film roll. Helps prevent deterioration of appearance.

The temperature difference between the cast drum temperature and the air knife temperature is preferably 20° C. or less, more preferably 10° C. or less, from the viewpoint of forming equivalent β crystals on both sides of the cast sheet. When the temperature difference between the cast drum temperature and the air knife temperature is 20° C. or less, it is possible to prevent different irregularities from being formed on the front and back sides of the film, and the front and back sides of the film tend to have the same slipperiness. Therefore, the amount of entrapped air is stabilized in the winding process during capacitor processing, and the inter-layer gap of the film and the amount of air tend to become uniform after the heat treatment process. As a result, the safety of the resulting capacitor during use is suppressed to a level that is just right, and the life of the capacitor is less likely to be shortened.

The cooling temperature of the cast sheet (surface temperature of the cooling roll) is preferably 10 to 50°C. When the cooling temperature is 10° C. or higher, it is easy to raise the temperature of the film to a desired temperature in the subsequent longitudinal stretching process, and it is easy to prevent the film from breaking in the longitudinal stretching process. On the other hand, when the cooling temperature is 50° C. or less, the formation of crystals in the cast sheet is likely to be stopped, and the surface properties of the film obtained after the stretching process become more uniform in the longitudinal direction.

In order 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 step. The crater-shaped surface irregularities that are characteristic of biaxially oriented polypropylene films that generally use the β-crystal method are first formed by the formation of depressions (volume reduction due to transition) due to α-crystal transition of β-crystals in the longitudinal stretching process. At the same time, mechanical deformation in the direction of longitudinal stretching occurs, and wedge-shaped recesses are formed on the surface of the longitudinally-stretched sheet. Next, in the horizontal stretching step, the wedge-shaped recesses are stretched, and the edges of the recesses are deformed into projections to form crater-shaped surface irregularities.

In order to obtain the unevenness on the surface of the biaxially oriented polypropylene film of the present invention, it is preferable to cause the formation of recesses due to β crystal to α crystal transition in the longitudinal stretching step and the mechanical deformation of the recesses in the longitudinal stretching direction in separate steps. . Furthermore, it is more preferable to apply an excessive amount of heat when transforming the β-crystal into the α-crystal so as to partially melt the depressions caused by the α-crystal transformation.

Specifically, it is preferable to heat-treat the cast sheet by passing it through temperature-controlled transport rolls while nipping the cast sheet with temperature-controlled nip rolls before the longitudinal stretching step. The diameter and number of the transport rolls may be adjusted according to the heat treatment time. Further, it is more preferable to cool the cast sheet after the heat treatment to stop partial melting.

The transfer roll temperature during heat treatment is preferably 160 to 170°C, more preferably 160 to 165°C. By setting the transfer roll temperature to 160° C. or higher, not only the transition from the β-crystal to the α-crystal, but also the partial melting of the depressions formed by the α-crystal transition tends to occur. Therefore, it is easy to form unevenness on the surface of the biaxially oriented polypropylene film of the present invention. On the other hand, by setting the transfer roll temperature to 170° C. or lower, it is possible to prevent the entire cast sheet from being melted and broken, and it is easy to maintain mass productivity.

The nip roll temperature is preferably 160-170°C, more preferably 160-165°C. By setting the nip roll temperature to 160° C. or higher, curling of the cast sheet on the nip rolls caused by the temperature difference between the front and back sides is reduced, so it is easy to reduce breakage in the stretching process and ensure mass productivity. On the other hand, by setting the nip roll temperature to 170° C. or less, breakage due to melting of the entire cast sheet is reduced, and mass productivity can be easily maintained.

The pressure of the nip rolls is preferably 0.30-0.60 MPa, more preferably 0.35-0.55 MPa. By setting the nip roll pressure to 0.30 MPa or more, the occurrence of wrinkles due to expansion of the film during heat treatment is suppressed, which facilitates 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, the deformation of the film due to the pressure is suppressed, so it becomes easy to reduce breakage in the stretching process and to ensure mass productivity.

The heat treatment time of the cast sheet is preferably 1 to 10 seconds. By setting the heat treatment time to 1 second or more, not only the transition from the β crystal to the α crystal, but also the partial melting of the recessed portions due to the volume reduction is likely to occur, so that the surface unevenness of the biaxially oriented polypropylene film of the present invention can be obtained. Cheap. On the other hand, by setting the heat treatment time to 10 seconds or less, the occurrence of wrinkles due to melting of the entire film and expansion of the film can be suppressed, and mass productivity can be easily ensured. Furthermore, since the partial melting of the concave portions does not become excessive, it is easy to prevent excessive flattening of the film 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 is easy to raise the film temperature to a desired temperature in the subsequent longitudinal stretching step, and breakage in the longitudinal stretching step is reduced. On the other hand, when the cooling temperature is 120° C. or less, partial melting of the surface of the cast sheet is suppressed, and the surface properties of the film obtained after the stretching process become more uniform in the longitudinal direction.

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

The longitudinal draw ratio is preferably 4.0 to 7.0 times, more preferably 5.0 to 7.0 times. By setting the longitudinal draw ratio to 4.0 times or more, the surface properties of the film become more uniform and the high-temperature withstand voltage characteristics are also excellent. By setting the longitudinal draw ratio to 7.0 times or less, it is easy to reduce film breakage in the longitudinal drawing process and film breakage in the next transverse drawing process.

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

After that, the biaxially oriented film is subjected to corona discharge treatment in air, nitrogen, carbon dioxide gas, or a mixture of these, and both widthwise ends held by clips are cut and removed, and then the biaxially oriented film is wound as an intermediate product by a winder. Finally, the biaxially oriented film unwound from the intermediate product is slit into a specific width by 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.

The biaxially oriented polypropylene film of the present invention is preferably used as a capacitor dielectric, but is not limited to the type of capacitor. Specifically, from the viewpoint of the electrode configuration, it may be either a foil-wrapped capacitor or a metal-evaporated film capacitor, and it is also preferable for an oil-immersed capacitor containing insulating oil and a dry capacitor that does not use insulating oil at all. Used. Moreover, from the viewpoint of the shape, it may be of a winding type or a layered type. Due to the properties of the biaxially oriented polypropylene film of the present invention, it is particularly preferably used as a metal-evaporated film capacitor.

The metal film laminate 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. Although the method of forming the metal film is not particularly limited, for example, a method of evaporating a metal such as aluminum on at least one side of the biaxially oriented polypropylene film to provide a metal film that serves as an internal electrode of a film capacitor is preferably used. At this time, simultaneously or sequentially with aluminum, other metal components such as, for example, nickel, copper, gold, silver, chromium, and zinc can also be deposited. A protective layer can also be provided on the metal film with oil or the like.

The thickness of the metal film is preferably 20 to 100 nm from the viewpoint of electrical characteristics and safety of the capacitor. Also, for the same reason, the metal film preferably has a surface resistance value of 1 to 20 Ω/sq. The surface resistance value can be controlled by the type of metal used and the film thickness.

In the present invention, after the metal film is formed, the metal film laminated film can be subjected to aging treatment or heat treatment at a specific temperature, if necessary. Also, for insulation or other purposes, at least one surface of the metal film laminated film can be coated with polyphenylene oxide or the like.

The film capacitor of the present invention will be described below. The film capacitor of the present invention has a structure in which the metal film laminated film of the present invention is laminated or wound. That is, the film capacitor of the present invention has the metal film laminated film of the present invention. including both film capacitors. A preferred method for manufacturing a wound film capacitor 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. At that time, aluminum is vapor-deposited in stripes having margins running in the longitudinal direction of the film. Next, a blade is inserted into the center of each vapor-deposited portion and the center of each margin portion on the surface to form a slit, thereby producing a tape-shaped take-up reel having a margin on one side of the surface. A tape-shaped take-up reel having a margin on the left or right is wound by stacking two reels, one each for the left margin and the right margin, so that the vapor-deposited portion protrudes from the margin in the width direction. get After heat-treating the wound body, metallikon is thermally sprayed on both end surfaces in the width direction to form external electrodes, and lead wires are welded to the metallikon to obtain a wound film capacitor. Film capacitors are used in a wide variety of applications, including vehicles, home appliances (televisions, refrigerators, etc.), general noise control, automobiles (hybrid cars, power windows, wipers, etc.), and power supplies. It can be used preferably.

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

(1) Gloss From an intermediate product with a length in the width direction of 6,000 mm, a biaxially oriented polypropylene film sample in a rectangular shape of 100 mm (longitudinal direction) x 6,000 mm (width direction) was taken, and the sample was measured in the longitudinal direction. 60 test pieces of 100 mm×100 mm were obtained by dividing into 60 equal parts parallel to the direction. Next, the glossiness of the central part of the obtained test piece was measured using a digital variable angle gloss meter UGV-5D manufactured by Suga Test Instruments Co., Ltd. under the conditions of an incident angle of 60 ° and a light receiving angle of 60 °. Each measurement was performed 5 times, and the average value was taken as the glossiness of each surface of the test piece. After that, the same measurement was performed for all the test pieces, and the maximum and minimum glossiness values of each surface obtained were determined as the "maximum glossiness value in the width direction" and the "minimum glossiness value in the width direction" on each surface. " Furthermore, the same measurement was performed with the longitudinal direction and the width direction interchanged to obtain the “longitudinal gloss maximum value” and the “longitudinal gloss minimum value” for each surface. After the measurement, the longitudinal gloss unevenness (RMD) and the widthwise gloss unevenness (RTD) on each surface were obtained from the following equations. Also, the average value of the glossiness of each side of all the test pieces was obtained, and this was defined as the glossiness of each side of the biaxially oriented polypropylene film.
Longitudinal gloss unevenness (RMD)=Longitudinal maximum glossiness−Longitudinal minimum glossiness Unevenness of widthwise glossiness (RTD)=Maximum glossiness in the width direction−Minimum glossiness in the width direction.

(2) Thickness The thickness of the biaxially oriented polypropylene film was measured by a micrometer method according to JIS C 2330 (2014).

(3) Surface temperature of cast drum and unevenness The surface temperature of the entire width of the cast drum immediately before the molten sheet discharged from the T die comes into close contact with 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 obtained average value was defined as the surface temperature of the cast drum, and the difference between the obtained maximum and minimum values was defined as the uneven surface temperature of the cast drum.

(4) Product roll (processing defect rate)
Slit at a slit speed of 450 m / min so that the film roll width is 620 mm and the film roll winding length is 60,000 m, and the wound film roll is used as an evaluation sample. asked. Using the obtained values, evaluation was made according to the following criteria.
Machining defect rate (%) = (number of winding misalignment/wrinkles) x 100/total number of machining o: 2% or less.
(triangle|delta): More than 2% and 5% or less.
x: Exceeded 5%.

(5) Processability of wound body in capacitor production On the corona-treated side of the biaxially oriented polypropylene film, aluminum was applied using a vacuum deposition machine manufactured by ULVAC Co., Ltd. so that the surface resistance value was 15 Ω / sq. Vacuum evaporated. At that time, aluminum was vapor-deposited in stripes having a margin portion running in the longitudinal direction (a vapor-deposited portion width of 79.0 mm and a margin portion width of 1.0 mm were repeated). Then, a blade was inserted into the center of each vapor-deposited portion and the center of each margin to form a slit to prepare a tape-shaped take-up reel having a total width of 40 mm and a margin of 0.5 mm at either end. Two reels of the left margin and the right margin of the obtained reel were superimposed and wound so that the vapor-deposited part protruded 0.5 mm from the margin in the width direction. got KAW-4NHB manufactured by Kaito Seisakusho Co., Ltd. was used for the winding. Finally, the wound body was heat treated in a reduced pressure atmosphere at 140°C for 10 hours. This wound body was visually observed, and the one with wrinkles or distortion in the appearance or inside was regarded as a defective product. 50 wound bodies were produced in the same manner and the same evaluation was repeated, and the workability of the wound body was evaluated according to the following criteria.
◯: 1 or less defective products △: 2 or more defective products and less than 3 defective products ×: 4 or more defective products.

(6) Evaluation of Capacitor Characteristics A wound body having a capacitance of 120 μF was obtained by the method described in (5). After that, the wound body was heat-treated in a reduced pressure atmosphere at 140° C. for 10 hours, metallikon was thermally sprayed on both end surfaces in the width direction to form external electrodes, and lead wires were welded to the metallikon to obtain a capacitor. Next, capacitor characteristics were evaluated for 10 capacitors. First, the capacitance (C0) was measured at room temperature. Then, a voltage of 200 VDC/μm (applied voltage of 400 V when the thickness is 2.0 μm) was applied to the capacitor at a high temperature of 125° C. for 400 hours. After that, the capacitance (C) was measured at room temperature, and the rate of change (ΔC) in capacitance before and after voltage application was calculated from the following formula. The capacitance was measured by LCR Hitester 3522-50 manufactured by Hioki Electric Co., Ltd.
ΔC = ((C0-C)/C0) x 100
The average value of the capacitance change rate (ΔC) before and after voltage application of ten capacitors was defined as the capacitance change rate before and after voltage application of the sample, and evaluation was made according to the following criteria.
◯: ΔC is less than 3% △: ΔC is 3% or more and less than 5% ×: ΔC is 5% or more.

(7) Film formability The film formability of the film was evaluated according to the following criteria. Note that the time from stopping the film formation due to film tearing to resuming the film formation was excluded from the observation time.
Good: No tearing of the film occurred after 48 hours or longer.
Δ: Film tear occurred 1 to 3 times in 48 hours.
x: Film tear occurred 4 or more times in 48 hours.

(Example 1)
Polypropylene resin (manufactured by Prime Polymer Co., melting point: 166°C, MFR: 2.5 g/10 min, mesopentad fraction: 0.991) was supplied to a single-screw melt extruder and melt extruded at 250°C to obtain a 25 µm A cut sintered filter was used to remove foreign matter. The molten resin was then melt-extruded into a sheet through a T-shaped slit die. After that, the melted sheet is brought into close contact with the cast drum surface controlled to a surface temperature of 90°C and a temperature unevenness of 1.0°C with an air knife of an air temperature of 90°C and solidified, and then placed on a cooling roll maintained at a temperature of 30°C. cooled. The time during which the cast drum and the molten sheet were in close contact was 1.5 seconds each. Here, the surface on the side that contacts the cast drum is the drum surface (D surface), and the surface that does not contact the cast drum is the non-drum surface (non-D surface). In addition, the cast drum uses a cast drum that has multiple jacket chambers within the wall thickness and contains a gas-liquid two-phase heat transfer medium, and the surface temperature is controlled by the temperature control of the cooling water that flows inside the cast. was adjusted by controlling the amount of cooling water passed through the inside of the cast (the same applies to Examples 2 to 8).

The obtained cast sheet was heat-treated for 5 seconds while being nipped with nip rolls at a temperature of 165°C under a pressure of 0.45 MPa on a transport roll at a temperature of 165°C, and then cooled on a cooling roll at a temperature of 100°C. Then, the film was stretched 5.5 times in the longitudinal direction with longitudinal stretching rolls at a temperature of 145° C. to obtain a uniaxially oriented film. After that, both ends of the uniaxially oriented film in the width direction were held with clips, stretched 11 times in the width direction at 160°C, and relaxed 12% in the width direction at 158°C to obtain a biaxially oriented film. Subsequently, the biaxially oriented film was slowly cooled to room temperature, and the drum surface (D surface) side thereof was subjected to corona discharge treatment at a treatment intensity of 25 W min/m ² , and both ends of the film in the width direction were held with clips. The cut biaxially oriented film was wound up to obtain an intermediate product with a width of 6,000 mm. Next, the biaxially oriented film was unwound from the intermediate product, slit with a slitter so that the film width was 620 mm, and a film roll of 60,000 m in the longitudinal direction was wound around a core to obtain a biaxial film having a thickness of 2.0 μm. An oriented polypropylene film was obtained. Table 1 shows the evaluation results of the obtained biaxially oriented polypropylene film.

(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 cast drum was controlled to 0.5°C. Table 1 shows the evaluation results of the obtained biaxially oriented polypropylene film.

(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 cast drum was controlled to 92°C and the surface temperature unevenness was controlled to 0.5°C. Table 1 shows the evaluation results of the obtained biaxially oriented polypropylene film.

(Example 4)
A biaxially oriented polypropylene film with a thickness of 2.0 μm was obtained in the same manner as in Example 1, except that the surface temperature of the cast drum was controlled at 70° C. and the surface temperature unevenness was controlled at 0.5° C. Table 1 shows the evaluation results of the obtained biaxially oriented polypropylene film.

(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 cast sheet transport roll and the nip roll were controlled at 162°C. Table 1 shows the evaluation results of the obtained biaxially oriented polypropylene film.

(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 cast sheet transport roll and the nip roll were controlled at 168°C. Table 1 shows the evaluation results of the obtained biaxially oriented polypropylene film.
(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. Table 1 shows the evaluation results of the obtained biaxially oriented polypropylene film.
(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. Table 1 shows the evaluation results of the obtained biaxially oriented polypropylene film.

(Comparative example 1)
A biaxially oriented polypropylene film having a thickness of 2.0 μm was prepared in the same manner as in Example 1 except that a cast drum having no jacket chamber was used for the cast drum surface thickness and the cast surface temperature unevenness was controlled to 3.5 ° C. got Table 1 shows the evaluation results of the obtained biaxially oriented polypropylene film. The surface temperature of the cast drum was adjusted by controlling the temperature of the cooling water passed through the inside of the cast, and the uneven temperature was adjusted by controlling the amount of cooling water passed through the inside of the cast (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 prepared in the same manner as in Example 1 except that a cast drum having no jacket chamber was used for the cast drum surface thickness, and the cast surface temperature unevenness was controlled to 10.0 ° C. got Table 1 shows the evaluation results of the obtained biaxially oriented polypropylene film.

(Comparative Example 3)
Same as Example 1 except that a cast drum without a jacket chamber was used in the cast drum surface thickness, the cast surface temperature unevenness was controlled at 3.5°C, and the temperature of the cast sheet transport roll and nip roll was controlled at 155°C. A biaxially oriented polypropylene film having a thickness of 2.0 μm was obtained. Table 1 shows the evaluation results of the obtained biaxially oriented polypropylene film.

(Comparative Example 4)
Same as Example 1 except that a cast drum without a jacket chamber was used in the cast drum surface thickness, the cast surface temperature unevenness was controlled at 3.5°C, and the temperature of the cast sheet transport roll and nip roll was controlled at 175°C. A biaxially oriented polypropylene film having a thickness of 2.0 μm was obtained. Table 1 shows the evaluation results of the obtained biaxially oriented polypropylene film.

INDUSTRIAL APPLICABILITY The biaxially oriented polypropylene film of the present invention has high productivity, workability, and voltage resistance, and therefore can be suitably used as a dielectric for film capacitors.

Claims (5)

A biaxially oriented polypropylene film, wherein at least one of the maximum gloss unevenness (RMD) in the longitudinal direction and the maximum gloss unevenness (RTD) in the width direction is 0.1% or more and 3.0% or less. 2. The biaxially oriented polypropylene film of claim 1, wherein said RMD and said RTD are between 0.1% and 3.0%. 3. The biaxially oriented polypropylene film according to claim 1, which has a glossiness of 125% or more and 150% or less on at least one side. A metal film laminated film comprising the biaxially oriented polypropylene film according to any one of claims 1 to 3 and a metal film on at least one side thereof. A film capacitor having a structure in which the metal film laminated film according to claim 4 is laminated or wound.