Biaxially oriented polypropylene film

Abstract

To provide a biaxially-oriented polypropylene film that is thin but has excellent heat resistance and withstand voltage properties.SOLUTION: A biaxially-oriented polypropylene film has a polypropylene resin A with a melt strength of 0.1 g or more and less than 1.0 g, and a polypropylene resin B with a melt strength of 1.1 g or more and less than 4.0 g. With the melt strength of the polypropylene resin A defined as MSA(g) and the melt strength of the polypropylene resin B defined as MSB(g), the following formula 1 is satisfied: 1.0≤MSB-MSA≤3.9 (formula 1).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a biaxially oriented polypropylene film having excellent voltage resistance characteristics in a high-temperature environment. [Background technology]

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

[0003] In particular, its excellent high withstand voltage and low loss characteristics make it particularly suitable for use as a capacitor dielectric. Recently, various electrical equipment has been converted to inverters, which has led to an even stronger demand for smaller capacitors with larger capacities. Furthermore, as the operating environments become increasingly hot (85°C to 135°C), particularly in automotive applications (including hybrid and electric vehicles), solar power generation, and wind power generation, the demand for heat resistance in capacitors used in these applications is also increasing.

[0004] Therefore, there is a demand for biaxially oriented polypropylene film, which is the dielectric of capacitors, to be thinner, more heat-resistant, and have improved voltage resistance per thickness. To achieve both thinner film and higher voltage resistance, the properties and composition of the resin raw materials are being investigated.

[0005] For example, Patent Documents 1 to 5 describe a method for improving the withstand voltage characteristics by adding polypropylene having a long-chain branched structure or a crosslinked structure to make the surface of a biaxially oriented polypropylene film dense and rough, and Patent Documents 2 to 5 describe a method for similarly improving the withstand voltage characteristics by adding a branched polypropylene resin whose melt tension and melt flow index at a specific temperature satisfy a specific relationship. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-84813 [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-290380 [Patent Document 3] Japanese Patent Application Laid-Open No. 2011-122142 [Patent Document 4] Japanese Patent Application Laid-Open No. 2014-231584 [Patent Document 5] WO2017 / 221985 publication Summary of the Invention [Problem to be solved by the invention]

[0007] The above method improves the voltage resistance of biaxially oriented polypropylene films in high-temperature environments. However, in light of the trend toward smaller capacitors and higher temperatures in their use environments, it has been an issue to further improve heat resistance and voltage resistance without increasing the thickness. That is, an issue of the present invention is to provide a thin biaxially oriented polypropylene film that has excellent heat resistance and voltage resistance. [Means for solving the problem]

[0008] In order to solve the above-mentioned problems, the biaxially oriented polypropylene film of the present invention has the following constitution: That is, it contains polypropylene resin A having a melt tension of 0.1 g or more and less than 1.0 g, and polypropylene resin B having a melt tension of 1.1 g or more and less than 4.0 g, and the melt tension of the polypropylene resin A is MS A (g) The melt tension of the polypropylene resin B is MS B The biaxially oriented polypropylene film is characterized in that when (g) satisfies the following formula 1: Formula 1: 1.0≦MS B -MS A ≦3.9 [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a biaxially oriented polypropylene film which is thin but has excellent heat resistance and voltage resistance properties. DETAILED DESCRIPTION OF THE INVENTION

[0010] The biaxially oriented polypropylene film of the present invention will be described in more detail below. Note that, in the following numerical ranges expressed using "to", the upper and lower limits are included in the range, and the units of the upper and lower limits are the same.

[0011] The biaxially oriented polypropylene film of the present invention is a biaxially oriented polypropylene film obtained by stretching a cast sheet primarily composed of polypropylene resin in two perpendicular directions (usually the longitudinal and width directions). In other words, biaxial orientation as used herein means stretching in two perpendicular directions. Furthermore, in the present invention, polypropylene resin refers to a resin containing more than 50 mol% but not more than 100 mol% of propylene units, where the total structural units constituting the resin are taken as 100 mol%. Here, the longitudinal direction refers to the direction in which the film runs during the film production process (corresponding to the winding direction in the case of a film roll), and the width direction refers to the direction perpendicular to the longitudinal direction within the plane of the film.

[0012] The biaxially oriented polypropylene film of the present invention is primarily composed of a polypropylene resin. The polypropylene resin may include not only a propylene homopolymer but also a polypropylene copolymer or a long-chain branched polypropylene, as described below. In the present invention, the term "primary component" refers to a component that accounts for 85% to 100% by mass of the total components of the film (100% by mass), preferably 90% to 100% by mass, more preferably 95% to 100% by mass, and even more preferably 97% to 100% by mass. Components other than the polypropylene resin in the film include the resins described below, as well as additives such as antioxidants and lubricants. When multiple polypropylene resins are contained in a film, the film can be considered to be primarily composed of polypropylene resins if the combined content of all polypropylene resins is 85% by mass or greater.

[0013] As the polypropylene copolymer, for example, a polypropylene copolymer copolymerized with other unsaturated hydrocarbons can be preferably used. Examples of copolymerization components of the polypropylene copolymer include ethylene, 1-butene, 1-pentene, 3-methylpentene-1, 3-methylbutene-1, 1-hexene, 4-methylpentene-1, 5-ethylhexene-1, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-heptadecene, 1-octadecene, 1-eicosene, vinylcyclohexene, styrene, allylbenzene, cyclopentene, norbornene, and 5-methyl-2-norbornene. From the viewpoint of voltage resistance 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%.

[0014] Furthermore, other polymers (resins) than polypropylene resin may be added to the propylene homopolymer. Examples of other polymers that can be added to the propylene homopolymer include homopolymers of unsaturated hydrocarbons other than propylene and copolymers of unsaturated hydrocarbons containing propylene units. From the viewpoint of voltage resistance and dimensional stability, the content of other polymers is preferably 10% by mass or less, and more preferably 3% by mass or less, when the total components of the biaxially oriented polypropylene film are taken as 100% by mass.

[0015] [Polypropylene resin A] From the viewpoint of mechanical strength when formed into a film, it is important that the biaxially oriented polypropylene film of the present invention contains polypropylene resin A having a melt tension of 0.1 g or more and less than 1.0 g. Melt tension is the tension that occurs when a heated and molten resin is pulled, and is one indicator of film formability. Melt tension can be measured by extruding a heated and molten resin in the form of a strand and taking up the strand; actual measurement conditions are shown in the examples.

[0016] When the melt tension of polypropylene resin A is 0.1 g or more, good moldability is achieved, resulting in a film with excellent mechanical strength. If a resin with a melt tension of less than 0.1 g is used instead of polypropylene resin A, the mechanical strength during stretching decreases, making melt fracture during molding more likely to occur. In addition, it is extremely difficult to obtain a polypropylene resin with a melt tension of less than 0.1 g. From the above perspective, the melt tension of polypropylene resin A is preferably 0.2 g or more but less than 1.0 g, more preferably 0.3 g or more but less than 1.0 g, and particularly preferably 0.4 g or more but less than 1.0 g.

[0017] The melt tension can be increased by increasing the amount of long-chain branches in the molecular chain of the polypropylene resin, and can be decreased by making the molecular structure of the polypropylene resin closer to a linear chain. Methods for increasing the amount of long-chain branches include, for example, applying high-energy radiation or using organic peroxides to generate radicals and introduce long-chain branches. A melt tension of 0.1 g or more but less than 1.0 g means that the molecular chain contains no long-chain branches or only very little, if any. Therefore, a linear polypropylene resin can be suitably used as polypropylene resin A having a melt tension of 0.1 g or more but less than 1.0 g.

[0018] From the viewpoint of the withstand voltage characteristics and dimensional stability of the biaxially oriented polypropylene film, the polypropylene resin A preferably has a weight average molecular weight (Mw) of 250,000 or more and 450,000 or less. From the same viewpoint, the molecular weight distribution (Mw / Mn) is preferably 5.0 or more and 8.0 or less, and furthermore, taking into consideration film formation stability, it is more preferably 5.5 or more and 8.0 or less. Here, Mn represents the number average molecular weight.

[0019] The melt flow rate (hereinafter, MFR) of the polypropylene resin A of the present invention, measured according to the conditions of JIS K 7210-1 (2014) (230°C, 2.16 kg), is preferably 1.0 to 10 g / 10 min, more preferably 1.5 to 8.0 g / 10 min, and even more preferably 2.0 to 5.0 g / 10 min. When the MFR of the polypropylene resin A is within the above-mentioned preferred range, excellent film-forming properties are achieved, resulting in stable production of biaxially oriented polypropylene films, and the resulting biaxially oriented polypropylene films also exhibit excellent voltage resistance characteristics. To achieve the MFR of the polypropylene resin A within the above-mentioned range, methods such as controlling the number-average molecular weight (Mn), weight-average molecular weight (Mw), and molecular weight distribution (Mw / Mn) are preferably employed. More specifically, the MFR can be lowered by reducing the number-average molecular weight and weight-average molecular weight and then narrowing the molecular weight distribution.

[0020] [Polypropylene resin B] From the viewpoint of achieving both film formation stability and the voltage resistance characteristics of the resulting biaxially oriented polypropylene film, the biaxially oriented polypropylene film of the present invention preferably contains polypropylene resin B having a melt tension of 1.1 g or more and less than 4.0 g. From the above viewpoints, the melt tension of polypropylene resin B is more preferably 1.1 g or more and less than 3.5 g, even more preferably 1.1 g or more and less than 3.0 g, particularly preferably 1.1 g or more and less than 2.5 g, and most preferably 1.3 g or more and less than 2.5 g. If the melt tension of polypropylene resin B is less than 1.1 g, the mechanical strength during stretching decreases, making melt fracture during molding more likely. On the other hand, if the melt tension of polypropylene resin B is 4.0 g or more, molecular orientation in high-temperature environments is likely to be relaxed, and the breakdown voltage during high-temperature use may decrease.

[0021] Melt tension can be increased by generating radicals to introduce long-chain branches, and can be adjusted by adjusting the number of long-chain branches and MFR. More specific methods for adjusting the number of long-chain branches include applying high-energy radiation, using organic peroxides, and adjusting the catalyst production method. Furthermore, by changing the amount of hydrogen used during polymerization, the molecular weight distribution of the resulting polymer, its bias toward both polymer ends, and branching can be controlled, thereby adjusting the MFR.

[0022] In addition, in the biaxially oriented polypropylene film of the present invention, it is preferable that polypropylene resin B is a long-chain branched polypropylene resin. Long-chain branched polypropylene resins act as α-crystal nucleating agents, and also enable the formation of a rough surface due to the crystalline morphology if added in a certain range. In other words, by including long-chain branched polypropylene, the size of polypropylene spherulites formed in the cooling process of the melt-extruded resin sheet can be controlled to be small, thereby suppressing the occurrence of insulation defects formed in the stretching process, and a polypropylene film with excellent voltage resistance characteristics can be obtained.

[0023] To obtain a long-chain branched polypropylene resin, a method of using high-energy ionizing radiation on a polypropylene resin (e.g., JP 62-121704 A), a method of reacting a specific organic peroxide with a polypropylene resin (e.g., JP 2869606 A), a method of reacting a thermally decomposable radical former and an ethylenically polyfunctional unsaturated monomer with a polypropylene resin (e.g., JP 10-330436 A), a method of using a specific catalyst during polymerization of a polypropylene resin (e.g., JP 2009-057542 A), or the like is preferably used.

[0024] The long-chain branched polypropylene resin preferably has an MFR of 4.0 g / 10 min or more and 6.5 g / 10 min or less, and more preferably has an MFR of 4.5 g / 10 min or more and 6.0 g / 10 min or less. Commercially available long-chain branched polypropylene resins that can be suitably used include "Profax" (registered trademark) (PF-814, etc.) manufactured by Lyondell Basell, "Daploy" (trademark) (WB130HMS, WB135HMS, etc.) manufactured by Borealis, and "WAYMAX" (registered trademark) (MFX8, EX4000, EX6000, etc.) manufactured by Japan Polypropylene Corporation.

[0025] The long-chain branched polypropylene resin referred to here is a polypropylene resin having 1 to 5 internal tri-substituted olefins per 10,000 carbon atoms, and the presence of these internal tri-substituted olefins is 1 This can be confirmed by the proton ratio in the H-NMR spectrum.

[0026] [Combination of polypropylene resin A and polypropylene resin B] In the biaxially oriented polypropylene film of the present invention, the mass ratio of polypropylene resin A to polypropylene resin B (polypropylene resin A:polypropylene resin B) is preferably 70:30 to 96:4, more preferably 82:18 to 96:4, even more preferably 87:13 to 96:4, and particularly preferably 92:8 to 96:4. Note that when a numerical range is indicated with "to", both the numerical values on either side are included in the range.

[0027] When the total of polypropylene resin A and polypropylene resin B is 100% by mass, if polypropylene resin B is 4% by mass or more, the spherulite size control effect of the long-chain branched polypropylene is fully exerted, and the insulation defects generated during the stretching process can be kept low, thereby improving the breakdown voltage in high-temperature environments. On the other hand, if polypropylene resin B is 30% by mass or less, the decrease in mechanical strength caused by excessive long-chain branched polypropylene and the decrease in breakdown voltage in high-temperature environments are reduced.

[0028] The biaxially oriented polypropylene film of the present invention is produced by subjecting polypropylene resin A to a melt tension of MS A (g) The melt tension of the polypropylene resin B is MS B When (g) is used, it is important to satisfy the following formula 1. Formula 1: 1.0≦MS B -MS A ≦3.9 Preferably, the melt tension of the polypropylene resin A is A (g) The melt tension of the polypropylene resin B is MS B When (g) is used, the following formula 2 is satisfied. Formula 2: 1.0≦MS B -MS A ≦2.9 More preferably, the melt tension of the polypropylene resin A is A (g) The melt tension of the polypropylene resin B is MS B When (g) is used, the following formula 3 is satisfied. Formula 3: 1.0≦MS B -MS A ≦2.0.

[0029] MS A and MS B By satisfying formula 1, preferably formula 2, and more preferably formula 3, the film-forming properties can be improved, and relaxation of molecular orientation in a high-temperature environment as a biaxially oriented polypropylene film can be suppressed, so that the resulting biaxially oriented polypropylene film has excellent voltage resistance properties.

[0030] [Additives, etc.] The biaxially oriented polypropylene film of the present invention may preferably contain various additives, such as nucleating agents, antioxidants, heat stabilizers, lubricants, antistatic agents, antiblocking agents, fillers, viscosity modifiers, and color inhibitors, within the scope of the present invention.

[0031] Among the above-mentioned additives, the type and amount of antioxidant to be added are important from the viewpoint of long-term heat resistance. That is, the antioxidant should be a sterically hindered phenolic one, and at least one of these should preferably be 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 (e.g., BASF's "Irganox" (registered trademark) 1330: molecular weight 775.2), tetrakis[methylene-3(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane (e.g., BASF's "Irganox" (registered trademark) 1010: molecular weight 1177.7), tris(2,4-di-t-butylphenyl)phosphite (e.g., BASF's "IRGAFOS" (registered trademark)), and the like are preferably used alone or in combination.

[0032] The total content of these antioxidants is preferably 0.03 to 1.0 mass% of all components constituting the biaxially oriented polypropylene film, and more preferably 0.1 to 0.9 mass%. When the antioxidant content in the polypropylene resin composition is 0.03 mass% or more, the antioxidant effect is easily obtained and long-term heat resistance is easily maintained. On the other hand, when the antioxidant content in the polypropylene resin composition is 1.0 mass% or less, high-temperature voltage resistance characteristics are easily maintained.

[0033] [Surface characteristics and thickness] The biaxially oriented polypropylene film of the present invention preferably has a surface wetting tension of 37 mN / m to 50 mN / m, more preferably 38 mN / m to 49 mN / m, even more preferably 39 mN / m to 48 mN / m, and particularly preferably 40 mN / m to 47 mN / m. When the surface wetting tension is above the preferred lower limit, sufficient adhesion to the metal is achieved during metal vapor deposition. Examples of methods for achieving a surface wetting tension of 37 mN / m to 50 mN / m or within the preferred range above on at least one side include, for example, performing a surface treatment after biaxial stretching during film formation. Specific examples include corona discharge treatment, plasma treatment, glow discharge treatment, and flame treatment, which may be used alone or in combination.

[0034] The biaxially oriented polypropylene film of the present invention preferably has a thickness of 1.0 μm or more and 3.0 μm or less. The thickness is more preferably 1.2 μm or more and 2.8 μm or less, and even more preferably 1.5 μm or more and 2.5 μm or less. A thickness of 1.0 μm or more can provide the biaxially oriented polypropylene film with excellent mechanical strength and high-temperature voltage resistance, and can also prevent film breakage during film formation and processing. On the other hand, a thickness of 3.0 μm or less can increase the capacitance per volume when used as a capacitor dielectric. The thickness can be measured by the micrometer method in accordance with JIS C 2330 (2014). The thickness 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.

[0035] Next, the method for producing the biaxially oriented polypropylene film of the present invention will be explained below, but the method is not necessarily limited thereto.

[0036] First, the above-mentioned preferred polypropylene resin is fed into a single-screw melt extruder and melt-extruded at 200 to 260°C. Next, foreign matter, modified polymers, etc. are removed using a filter installed midway through the polymer pipe. The resin is then extruded onto a casting drum through a T-die to form a cast sheet, which is then cooled with a cooling roll.

[0037] The surface temperature of the cast drum is preferably 60 to 100°C, more preferably 65 to 95°C, even more preferably 70 to 95°C, and particularly preferably 75 to 95°C, from the viewpoint of appropriate formation of β crystals and spherulites. By maintaining a cast drum temperature of 60°C or higher, the formation of too few β crystals in the cast sheet can be prevented, and the smoothness of the film obtained after biaxial stretching can be maintained. This prevents the occurrence of conveyance wrinkles during the film conveyance process during film production and processing, and deterioration of the winding shape of the film roll. On the other hand, by maintaining a cast drum temperature of 100°C or lower, the formation of excessive β crystals and spherulites in the cast sheet can be prevented, and the occurrence of meandering during the film conveyance process during film production and processing, and deterioration of the winding shape of the film roll can be easily prevented.

[0038] The molten sheet discharged from the T-die lands on the casting drum and preferably remains in close contact with the drum for 1 to 3 seconds. A close contact time of 1 second or longer facilitates solidification of the molten sheet, making it easier to prevent breakage during the subsequent stretching process. On the other hand, a close contact time of 3 seconds or shorter can prevent excessive formation of β crystals in the cast sheet, making it easier to prevent meandering during the film transport process during film formation and processing, and deterioration of the film roll shape.

[0039] The cooling temperature of the cast sheet (surface temperature of the cooling roll) is preferably 10 to 60°C, more preferably 10 to 50°C. If the cooling temperature is 10°C or higher, the film can be easily heated to the desired temperature in the subsequent longitudinal stretching step, and breakage of the film in the longitudinal stretching step can be easily prevented. On the other hand, if the cooling temperature is 60°C or lower, crystal formation in the cast sheet can be easily stopped, and the surface properties of the film obtained after the stretching step become more uniform in the longitudinal direction.

[0040] Next, the cast sheet is stretched in the machine direction (machine stretching) in a machine stretching step to obtain a uniaxially oriented film. In the machine stretching step, the cast sheet is preferably passed through rolls controlled at a temperature of 125 to 145°C, and stretched in the machine direction at a predetermined stretching speed and draw ratio by controlling the difference in peripheral speed between the rolls.

[0041] The longitudinal stretching ratio is preferably 4.0 to 7.0 times, and more preferably 5.0 to 7.0 times. By setting the longitudinal stretching ratio to 4.0 times or more, the surface properties of the film become more uniform and the high-temperature voltage resistance characteristics become excellent. By setting the longitudinal stretching ratio to 7.0 times or less, film breakage during the longitudinal stretching step and film breakage during the subsequent transverse stretching step can be easily reduced.

[0042] Next, both widthwise ends of the uniaxially oriented film are held with clips and stretched in the width direction 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 form a biaxially oriented film. Note that, from the viewpoint of improving dimensional stability in a high-temperature environment, after stretching in the width direction, the film may be heat-treated at 150 to 170° C. and simultaneously relaxed by 10 to 20% in the width direction.

[0043] The biaxially oriented film is then subjected to corona discharge treatment in air, nitrogen, carbon dioxide, or a mixture of these gases, and the two 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 winding machine. Corona discharge treatment is carried out at a rate of 20 to 30 W·min / m to provide adequate wettability. 2Finally, the biaxially oriented film unwound from the intermediate product is slit to a specific width using 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.

[0044] The biaxially oriented polypropylene film of the present invention is preferably used as a dielectric for a capacitor, but is not limited to the type of capacitor. Specifically, from the viewpoint of electrode configuration, it may be either a foil-wound capacitor or a metal-vapor-deposited film capacitor, and is also preferably used in an oil-immersion type capacitor containing insulating oil or a dry capacitor that does not use insulating oil at all. In addition, from the viewpoint of shape, it may be a wound type or a laminated type. Due to the properties of the biaxially oriented polypropylene film of the present invention, it is particularly preferably used as a metal-vapor-deposited film capacitor. [Example]

[0045] The present invention will be described in detail below with reference to examples. The polypropylene resins used were as follows, and their properties were measured and evaluated by the following methods. However, Examples 3 to 6 are considered as reference examples.

[0046] [Polypropylene resin] The following polypropylene resins A1 to A2 and polypropylene resins B1 to B5 were used to produce the polypropylene films of the Examples and Comparative Examples. The melt tension, MFR, number average molecular weight (Mn), weight average molecular weight (Mw), and molecular weight distribution (Mw / Mn) of each resin are shown in Table 1. These values were measured in the form of raw material resin pellets according to the following measurement method. Note that hereinafter, polypropylene resins A1 to A2 and polypropylene resins B1 to B5 may be referred to as resins A1 to A2 and resins B1 to B5, respectively. Polypropylene resin A1: isotactic polypropylene, the production method of which and a composition containing the same will be described later. Polypropylene resin A2: Isotactic polypropylene (manufactured by Prime Polymer Co., Ltd.). Polypropylene resin B1: long-chain branched isotactic polypropylene, the production method of which and a composition containing the same will be described later. Polypropylene resin B2: Long-chain branched polypropylene resin (manufactured by Prime Polymer Co., Ltd.). Polypropylene resin B3: Long-chain branched polypropylene resin ("WAYMAX" (registered trademark) EX4000 manufactured by Japan Polypropylene Corporation). Polypropylene resin B4: Long-chain branched polypropylene resin ("WAYMAX" (registered trademark) MFX3 manufactured by Japan Polypropylene Corporation). Polypropylene resin B5: Long-chain branched polypropylene resin ("WAYMAX" (registered trademark) MFX6 manufactured by Japan Polypropylene Corporation).

[0047] [Table 1]

[0048] [Method for producing polypropylene resin A1] Anhydrous magnesium chloride, decane, and 2-ethylhexyl alcohol were mixed and heated at 130°C for 2 hours to form a homogeneous solution. Phthalic anhydride was added to the heated solution and stirred at 130°C for 1 hour to dissolve the phthalic anhydride. The resulting phthalic anhydride solution was cooled to 23°C and left for 10 hours, after which it was added dropwise to titanium tetrachloride cooled to -20°C. The resulting mixture was heated to 110°C over 4 hours, diisobutyl phthalate was added, stirred, and filtered to obtain a solid. The resulting solid was then washed with decane and hexane to obtain a titanium catalyst for propylene polymerization. Propylene polymerization was carried out using the resulting titanium catalyst, triethylaluminum as a cocatalyst, and hydrogen as a chain transfer agent. After the catalyst in the resulting product was deactivated, it was thoroughly washed with propylene monomer to obtain a polypropylene resin. Furthermore, BHT and BASF "Irganox" 1010 (registered trademark) were added as antioxidants to the obtained polypropylene resin so that the concentrations were 0.1% by mass and 0.2% by mass, respectively, and the mixture was kneaded at a temperature of 260°C and pelletized to obtain a polypropylene resin composition, which was designated as polypropylene resin A1.

[0049] [Method for producing polypropylene resin B1] Montmorillonite was added as a layered silicate to a solution of distilled water and 96% sulfuric acid to form a slurry, which was then heated at 90°C for 210 minutes. Distilled water was added to the heated slurry and filtered to obtain Solid 1, which was then poured into an aqueous lithium sulfate solution and reacted at room temperature for 120 minutes, after which distilled water was added and the mixture was further filtered. The remaining solid was then washed with distilled water until the pH reached 5-6 and filtered to obtain Solid 2. Solid 2 was pre-dried overnight at 100°C under a nitrogen stream, and then further dried under reduced pressure at 200°C for 2 hours to obtain chemically treated smectite. Heptane was added to the obtained chemically treated smectite to form a slurry, to which triisobutylaluminum was added and stirred for 1 hour, followed by washing with heptane. The washed chemically treated smectite and triisobutylaluminum were mixed, and Solution 1 was added. The mixture was stirred at room temperature for 20 minutes. Then, triisobutylaluminum and Solution 2 were added and stirred at room temperature for 1 hour. (Solution 1: rac-dichloro[1,1'-dimethylsilylenebis{2-(5-methyl-2-furyl)-4-(4-i-propylphenyl)indenyl}]hafnium dissolved in toluene. Solution 2: rac-dichloro[1,1'-dimethylsilylenebis{2-methyl-4-(4-chlorophenyl)-4-hydroazulenyl}]hafnium dissolved in toluene.) Heptane was then added to the solution, which was then introduced into the autoclave. The internal temperature of the autoclave was raised to 40°C, and propylene was fed at a rate of 10 g / hour. Prepolymerization was carried out for 4 hours while maintaining the temperature at 40°C. The propylene feed was then stopped, and the residual polymerization was carried out for 1 hour. The supernatant of the catalyst slurry thus obtained was removed by decantation, and then triisobutylaluminum was added to the remaining part, followed by stirring for 5 minutes and further drying under reduced pressure for 1 hour to obtain a prepolymerized catalyst.

[0050] The atmosphere in the stirred autoclave was then thoroughly purged with propylene, and thoroughly dehydrated liquefied propylene was introduced. Hydrogen triisobutylaluminum was then added, and the internal temperature was raised to 70°C. Next, a prepolymerization catalyst was introduced under pressure with argon to initiate polymerization, and the internal temperature was maintained at 70°C. After two hours, ethanol was introduced under pressure, unreacted propylene was purged, and the autoclave was purged with nitrogen to terminate the polymerization. The resulting polymer was dried at 90°C under a nitrogen stream for one hour to obtain a powdery polymer. The resulting powdery polymer was blended with antioxidants "Irganox" (registered trademark) 1010 (manufactured by BASF) and "IRGAFOS" (registered trademark) 168 (manufactured by BASF), and mixed for three minutes at room temperature using a high-speed agitator mixer. The mixture was then melt-kneaded and pelletized in a twin-screw extruder to obtain a long-chain branched polypropylene resin composition. This was designated polypropylene resin B1.

[0051] [Evaluation of characteristics] The evaluation of each characteristic was carried out by the following methods.

[0052] (1) Melt flow rate (MFR) (unit: g / 10 min) Measurements were performed at 230°C and 2.16 kg in accordance with JIS K 7210-1 (2014).

[0053] (2) Melt tension (MS) (unit: g) Measurements were performed using a melt tension tester (capillary diameter 2.1 mm, cylinder diameter 9.55 mm) manufactured by Toyo Seiki Seisakusho, Ltd., according to the following procedure. First, the polypropylene resin was heated to 230°C to melt it. Next, the molten polypropylene resin was extruded at an extrusion speed of 15 mm / min to form a strand, and the tension was measured while this strand was taken up at a speed of 6.5 m / min, and the obtained value was designated as MS (g).

[0054] (3) Thickness The thickness of the biaxially oriented polypropylene film was measured by the micrometer method in accordance with JIS C 2330 (2014).

[0055] (4) Breakdown voltage In accordance with JIS C 2330 (2001), electrodes were placed in a hot air oven controlled at 135°C, and the dielectric breakdown voltage (BDV) of the biaxially oriented polypropylene film was measured. This measurement was performed 50 times, and the top 30 breakdown voltage values were determined and divided by the film thickness determined in (3) above to determine the dielectric breakdown voltage per μm (V / μm). The high-temperature withstand voltage characteristics were evaluated using the top 30 breakdown voltage values in the above dielectric breakdown voltage test according to the following criteria. ○:400V / μm or more △: 380V / μm or more, less than 400V / μm ×: Less than 380V / μm.

[0056] (5) Number average molecular weight (Mn), weight average molecular weight (Mw), molecular weight distribution (Mw / Mn) The average molecular weight and molecular weight distribution of each resin constituting the film were measured using GPC. The measuring device used was an HLC-8321GPC / HT (detector: RI) (manufactured by Tosoh Corporation). The measuring conditions were as follows: Column: TSKgel GMHHR-H(20)HT (7.8 mm ID x 30 cm) x 3 (Tosoh Corporation) Eluent: 1,2,4-trichlorobenzene (Fujifilm Wako Pure Pharmaceuticals GPC grade) + BHT (0.05%) Flow rate: 1.0mL / min Injection volume: 0.3mL Column temperature: 140℃ Sample concentration: 1 mg / ml Calibration curve: A quintic approximation curve using standard polystyrene manufactured by Tosoh Corporation, and the molecular weight was converted into a polypropylene value using the Q-factor.

[0057] (6) Film formation stability The film formability was evaluated according to the following criteria: The time from when film production was stopped due to the occurrence of film breakage until film production was restarted was excluded from the observation time. ◯: No film tearing occurred for over 48 hours. △: The film broke 1 to 3 times in 48 hours. ×: The film broke four or more times within 48 hours.

[0058] Example 1 Resin A2 and Resin B2 were continuously metered and mixed at a mass ratio of 95 / 5 by weight to form a dry-blend resin composition, which was fed into a single-screw melt extruder and melt-extruded at 250°C. The mixture was then passed through a sintered filter with a 25 μm cutoff. The molten resin mixture was then melt-extruded into a sheet through a T-slit die. The molten sheet was then brought into close contact with the surface of a casting drum (controlled at a surface temperature of 95°C) using an air knife at an air temperature of 90°C, solidifying the sheet, and then cooled on a cooling roll maintained at 50°C. The molten sheet remained in close contact with the casting drum for 1.5 seconds. The resulting cast sheet was stretched 5.5 times in the longitudinal direction using a longitudinal stretching roll at 140°C to obtain a uniaxially oriented film. The uniaxially oriented film was then gripped at both widthwise ends with clips and stretched 11 times in the widthwise direction at 160°C, followed by 12% relaxation in the widthwise direction at 158°C to obtain a biaxially oriented film. Next, the biaxially oriented film was gradually cooled to room temperature, and the drum surface (surface D) was heated at 25 W·min / m 2 After subjecting the film to a corona discharge treatment at a treatment intensity of 1000 m / s, both widthwise ends of the film held by clips were cut off and the biaxially oriented film was wound up to obtain a 6,000 mm wide intermediate product. The biaxially oriented film was then unwound from the intermediate product and slit to a film width of 820 mm using a slitter, and 60,000 m was wound longitudinally around a core to form a film roll, yielding a roll of biaxially oriented polypropylene film with a thickness of 2.0 μm. The evaluation results of the resulting biaxially oriented polypropylene film are shown in Table 2.

[0059] (Examples 2 to 6, Comparative Examples 1 to 5) A biaxially oriented polypropylene film was obtained in the same manner as in Example 1, except that the resin composition was as shown in Table 2. The evaluation results of the obtained biaxially oriented polypropylene film are shown in Table 2.

[0060] [Table 2]

[0061] In the table, the content of polypropylene resin was calculated assuming the entire film to be 100% by mass, except for the antioxidant, which was present in a very small amount and was therefore not taken into consideration when calculating the content. [Industrial Applicability]

[0062] According to the present invention, it is possible to provide a biaxially oriented polypropylene film that is thin yet has excellent heat resistance and voltage resistance. Because the biaxially oriented polypropylene film of the present invention has the above-mentioned excellent properties, it can be suitably used for capacitor applications.

Claims

1. The polypropylene resin A has a melt tension of 0.1 g or more and less than 1.0 g, and the polypropylene resin B has a melt tension of 1.1 g or more and 3.5 g or less, The melt tension of the polypropylene resin A is MS Ａ (g) The melt tension of the polypropylene resin B is MS Ｂ A biaxially oriented polypropylene film characterized in that when (g) satisfies the following formula 1: Formula 1: 1.0 ≦ MS Ｂ -MS Ａ ≦2.9

2. The biaxially oriented polypropylene film according to claim 1, wherein the mass ratio of the polypropylene resin A to the polypropylene resin B (the polypropylene resin A: the polypropylene resin B) is 70:30 to 96:

4.

3. The biaxially oriented polypropylene film according to claim 1 or 2, wherein the polypropylene resin B is a long-chain branched polypropylene resin.

4. The biaxially oriented polypropylene film according to any one of claims 1 to 3, which is for use in a capacitor.

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