JP5224568B2 - Polypropylene film for capacitors - Google Patents

Description

  The present invention relates to a biaxially stretched polypropylene film used as a dielectric of a capacitor, and more particularly to a polypropylene film suitable as a vapor deposition capacitor having an average film thickness of 10 μm or less.

  Polypropylene has a high dielectric breakdown voltage and low dielectric loss among plastic films, and is therefore excellent as a dielectric for capacitors, and is particularly preferably used as a high-voltage capacitor and an AC capacitor. In recent years, there has been a growing demand for higher rated capacitor temperatures and larger capacitor capacities, and it has been proposed to use biaxially stretched polypropylene films made of highly stereoregular polypropylene resin as dielectrics (patents). Literatures 1-3).

However, when such a method is taken, the crystallinity of the polypropylene resin is unnecessarily increased, and biaxial stretching becomes difficult, or even if the stretching can be performed, the film thickness becomes inferior. In many cases, voids occur frequently, resulting in a film having a low dielectric breakdown voltage, resulting in a practical problem. As countermeasures against such problems, proposals have been made to use a specific propylene-1-butene copolymer as a polypropylene resin having excellent heat resistance and stretchability (Patent Documents 4 and 5). When such a resin composition is selected, the stretchability is certainly good, but the heat resistance necessary for a capacitor cannot always be obtained.
Japanese Patent Laid-Open No. 10-156938 (Claims) JP-A-10-156939 (Claims) JP-A-10-156940 (Claims) JP 59-149909 A (Claims) JP 2002-128825 A (Claim 1, Claim 3)

  The present invention provides a polypropylene film for a capacitor having a small heat shrinkage ratio, excellent thickness uniformity, processing suitability, and excellent long-term reliability.

In order to solve the above-described problems, the present invention has the following configuration.
(1) A copolymer of propylene and α-olefin, which comprises a polypropylene resin having a melting point of 155 to 164 ° C. and a cold xylene soluble content (CXS) of 1.5% by weight or less, and is measured with a micrometer. The average film thickness (MMV) is 2.0 to 8.0 μm, the ten-point average roughness (Rz) of any film surface is 0.05 to 1.8 μm, and the weight average thickness (WMV) The difference Δd (= MMV−WMV) from the average film thickness (MMV) is 0.03 to 0.2 μm, and the melting point of the polypropylene film is the melting point of the polypropylene resin constituting the film +2 to 15 ° C. Polypropylene film for capacitors characterized by
(2) The polypropylene film for capacitors according to (1), wherein the surface roughness of at least one film is 0.08 to 0.8 μm.
(3) The polypropylene film for capacitors according to either (1) or (2), wherein the α-olefin is 1-butene .

  The film of the present invention has excellent withstand voltage characteristics in a high temperature region and small thermal shrinkage, and thus exhibits particularly excellent characteristics when used at a high potential gradient (= applied voltage per unit thickness) under high temperature conditions. It becomes possible to do. The dielectric film has an average film thickness of 2 to 8 [mu] m, particularly preferably 2.5 to 6 [mu] m, for winding type capacitors and multilayer capacitors. In particular, since the film characteristics and thickness uniformity are excellent, it is possible to exhibit excellent characteristics as a dielectric film of a large-capacity metallized film capacitor having a capacitance per capacitor element of 50 to 500 μF.

  Hereinafter, the present invention will be described in detail together with preferred embodiments.

  The polypropylene resin constituting the polypropylene film for a capacitor of the present invention (hereinafter referred to as the film of the present invention) needs to be a copolymer of propylene and α-olefin. When there is no or very little copolymerization component, stretchability may be impaired, and the copolymerization component is preferably 0.1 mol% or more, more preferably 0.3 to 2 mol%. . Examples of the monomer that can be copolymerized with polypropylene include vinyl monomers such as ethylene and α-olefins, but it is necessary to be α-olefins. In the case of ethylene, stretchability is certainly good, but there is a problem that it is difficult to reduce the cold xylene soluble matter (CXS) described later, and the pressure resistance characteristics at high temperatures are likely to be lowered. This is thought to be due to the fact that the ethylene chains are not successfully incorporated into the polypropylene crystals. Examples of α-olefins include 1-butene, 1-pentene, 1-hexene, and 4-methyl-1-pentene. Among these, 1-butene is particularly easy to reduce CXS and has excellent pressure resistance characteristics, and This is preferable because the stretchability becomes good. In addition, examples of the copolymerization method include binary or multi-component random copolymerization, block copolymerization, graft polymerization, and the like. Preferred is that the configuration of binary random copolymerization has good stretchability. This is preferable. Furthermore, in order to obtain a predetermined α-olefin copolymerization amount, a high copolymerization concentration propylene-α-olefin copolymer may be polymerized in advance and melt-kneaded with a polypropylene homopolymer.

  The melting point of the polypropylene resin constituting the film of the present invention is required to be 155 to 164 ° C, preferably 157 to 162 ° C. If the melting point is too low, there will be a problem with heat resistance. On the other hand, if it is too high, stretch spots are likely to occur, resulting in poor withstand voltage. Next, the cold xylene soluble content (CXS) of the polypropylene resin constituting the film of the present invention is required to be 1.5% by weight or less, preferably 1.0% by weight or less. The CXS component is considered to correspond to a low molecular weight component and / or a component having a random structure that is not incorporated into the crystal when the polypropylene is crystallized. The CXS component in the film is considered to be dissolved in the amorphous phase that connects the crystalline phase to the crystalline phase, but it does not contribute to the structural strength of the film because it is not incorporated into the crystal. Then, the mobility of the amorphous part or the crystal part is increased. That is, when CXS is too high, not only the thermal shrinkage rate is increased, but also the mobility of impurities (ionic components, etc.) in the film in the high temperature region is likely to increase, resulting in a large reduction in withstand voltage. On the other hand, the lower limit of CXS is not particularly specified, but from the viewpoint of the current technical level, it is considered industrially appropriate to be 0.1% by weight or more, more preferably 0.2% by weight.

  Further, since it is preferable in terms of heat resistance and voltage resistance characteristics that there are few low molecular weight components that are relatively difficult to crystallize without reaching the CXS component, it is preferable that the molecular weight distribution of the polypropylene resin constituting the film of the present invention is narrow. The polydispersity (Mw / Mn) defined by the ratio of the number average molecular weight (Mn) to the weight average molecular weight (Mw) is preferably 6 or less, more preferably 5 or less. Usually, when the polydispersity becomes small, there is a problem that the stretchability is deteriorated. However, in the present invention, the α olefin is copolymerized in the polypropylene resin so that the stretchability is hardly impaired. As a method of obtaining such a resin having a low polydispersity, a method of reducing the polydispersity by producing a high molecular weight resin once and reducing the molecular weight by melt extrusion with a peroxide, Examples thereof include a method of obtaining a resin with a single site catalyst such as a metallocene catalyst.

  Moreover, it is preferable that the intrinsic viscosity of the polypropylene resin which comprises this invention film is 1.4-2.0 dl / g, More preferably, it is 1.6-1.9 dl / g. If the intrinsic viscosity is lower than the range constituting the film of the present invention, the mechanical properties are inferior, causing problems in the capacitor manufacturing process and increasing the insulation defects. On the other hand, if the intrinsic viscosity is too high, not only the heat shrinkage but also the thickness unevenness may worsen, and the withstand voltage may decrease as a result.

  In order to obtain a polypropylene resin, the method described in JP-A No. 2002-128825 can be adopted. However, as a method for reducing CXS and ash content described later, the polypropylene powder obtained by the solvent extraction method can be washed. can give. From this viewpoint, the polymerization process is exemplified by a bulk polymerization method using propylene itself as a solvent, and the obtained resin powder is washed with propylene, but the resin powder obtained by the other polymerization method is exemplified by heptane or the like. A method of washing with a solvent is also possible. Of course, increasing the catalytic activity by utilizing a cocatalyst is also effective in reducing CXS and ash content. Further, the ash content resulting from the catalyst residue contained in the polypropylene resin constituting the film of the present invention is preferably 50 ppm or less, and more preferably 30 ppm or less because the withstand voltage characteristics are improved. Such ash is evaluated by measuring the residue remaining after burning a polypropylene film or resin. For example, a film of weight W (g) is put in a platinum crucible, sufficiently burned with a gas burner, and further heated at 750 ° C. It is treated with an electric furnace for 1 hour for complete ashing, and the weight W ′ (g) of the obtained ash is measured and defined as (W ′ / W) × 1,000,000 (ppm).

  In addition, it is preferable to add a thermal stabilizer and an antioxidant for imparting chemical stability of the polypropylene resin to the film of the present invention. Specifically, phenolic, hindered amine, phosphite, Lactone type and tocopherols are exemplified. Specifically, dibutylhydroxytoluene, pentaerythritol tetrakis [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] (Ciba Specialty Chemicals Co., Ltd.) ): Registered trademark “Irganox 1010”), 1,3,5-trimethyl-2,4,6-tris (3,5-di-tert-butyl-4-hydroxy) benzene (Ciba Specialty Chemicals): Registered trademark "Irganox 1330"), tris (2,4-di-t-butylphenyl) phosphor Doo (Ciba Specialty Chemicals Co., Ltd .: registered trademark "Irgafos168"), and the like. Among these, at least one selected from phenolic antioxidants or a combination thereof, a combination of phenolic and phosphite, and phenolic and lactone, phenolic and phosphite and lactone Is preferable from the viewpoint of improving the stability of polypropylene.

  The addition amount is preferably 1000 to 10,000 ppm in the polypropylene film constituting the film of the present invention as the total amount of the antioxidant.

  In order for the film of the present invention to stabilize the element winding property and self-healing property that are important as capacitor characteristics, the ten-point average roughness (Rz), which is an index of the surface roughness, is 0 at least on one film surface. It is necessary to be 0.05 to 1.8 μm. Preferably, it is 0.05 to 1.8 μm on any surface. Here, when Rz is large, it means that there are large protrusions on the film surface, and when Rz is too large, there are unnecessary large protrusions, so that the vicinity becomes an electrical singular point. Dielectric breakdown easily occurs and the withstand voltage decreases. On the other hand, if Rz is too small, the slipperiness of the film is deteriorated and the element winding property is deteriorated, or the self-recovery property when a metallized film capacitor is formed is difficult to function and the reliability is lowered. Here, the self-recovery property means a characteristic of maintaining insulation by minimizing a local discharge phenomenon or dielectric breakdown occurring in the capacitor. Specifically, in a capacitor in which an electrode is formed by metallizing the surface of a dielectric film, when a part of the dielectric breaks down by optimizing the film resistance of the electrode metal film, the surrounding metal Insulation is restored by discharging energy, or the electrode metal film is segmented and patterned so as to be electrically connected between the segments, and an appropriate resistance is given to the connecting portion between the segments. Thus, a technique such as providing a fuse function that cuts the connecting portion with Joule heat when an excessive current flows is applied. In other words, self-healing is composed of the metal scattering property around the dielectric breakdown and the fuse cutting property if necessary. In either case, the dielectric film is smooth and has no gap with the dielectric film of the counter electrode. If they are in close contact, it is difficult to ensure insulation, and in the worst case, the capacitor may be destroyed. From the above viewpoints, it is particularly preferable that the Rz of one of the film surfaces is in the range of 0.05 to 1.8 μm and the other film surface roughness is 0.08 to 0.8 μm. Preferably, it is 0.1-0.7 micrometer especially preferably.

  In particular, the film of the present invention has excellent thermal dimensional stability as a capacitor, and in the case of a large-capacity capacitor in which a thin film is wound for a long time, the element shape is stabilized when the element is molded. As a result, a highly reliable capacitor can be obtained. As the film thickness, the average film thickness (MMV) measured with a micrometer is preferably 2.0 to 8.0 μm, and more preferably 2.5 to 6.0 μm. In this case, as an index of surface roughness, when the difference Δd (MMV−WMV) between the average film thickness (MMV) and the weight average thickness (WMV) is 0.03 to 0.2 μm, This is preferable because the slipperiness and withstand voltage characteristics are improved.

  Moreover, it is preferable that it becomes + 2-10 degreeC with respect to melting | fusing point of the polypropylene resin which comprises this invention film, More preferably, it is +4-+8 degreeC with respect to melting | fusing point of this invention film. Here, the melting point of the polypropylene resin is the melting point of the α-type crystal when the spherulites are formed by cooling from the molten state, and the melting point of the film is defined as the melting point after biaxially stretching the resin. That is, in the biaxial stretching process of polypropylene, the orientation of the molecular chain proceeds while the spherulite is broken and the lamella is partially melted and recrystallized by heating and stretching a non-oriented sheet composed of spherulites. It is considered that by promoting the thickening of the lamella thickness in the process of recrystallization of the lamella, the melting point is increased as a result, and the heat resistance of the film is improved. In the present invention, the preferred draw ratio is 40 to 100 times, more preferably 45 to 90 times in terms of area magnification.

  The film of the present invention is produced by a biaxial stretching method, but may be a tenter method or a tubular (bubble) method. Among these, the tenter method is preferable because the thickness unevenness and flatness are improved. The tenter method further includes a simultaneous biaxial stretching method and a sequential biaxial stretching method, and any method may be used. Hereinafter, a method for obtaining the film of the present invention by sequential biaxial stretching will be described, but the present invention is not limited thereto.

  Polypropylene resin is melt-kneaded at 230-270 ° C. using an extruder, melt-extruded into a sheet from a T-type slit die via a polymer filter while being metered by a gear pump. Next, the molten sheet is cooled and solidified while closely contacting with a metal drum controlled at 70 to 90 ° C. by air pressure. The sheet obtained here is preheated by a heated metal roll, the film temperature is raised to 130 to 155 ° C., and 4.5 to 8 times, preferably 5 to 7 times, between a pair of rolls provided with a peripheral speed difference. To a uniaxially stretched film. Next, both ends in the width direction of the uniaxially stretched film are held by clips, guided to a heating oven, preheated to 150 to 170 ° C., and then stretched 7 to 12 times, preferably 8 to 11 times in the width direction, to be a biaxially stretched film And annealing at 140 to 160 ° C. while allowing 0 to 20% relaxation in the width direction. After trimming both edge portions of the biaxially stretched film obtained in this way, the corona discharge treatment is performed, and then wound into a roll.

  The wound film is cut into a necessary product width after being subjected to an aging treatment in an atmosphere of 20 to 40 ° C.

  The electrode in the case of using the film of the present invention for a capacitor is not particularly limited. For example, even if it is a metal foil or at least one side of which is metallized paper or plastic film, one or both sides of the film of the present invention are formed. Direct metallization is also acceptable. For capacitor applications where a reduction in size and weight is desired, it is particularly suitable for directly metallizing a film. At this time, the type of metal used includes, but is not particularly limited to, simple substances such as zinc, tin, silver, chromium, aluminum, copper, nickel, and a mixture or alloy of plural kinds.

Moreover, examples of the method for directly metallizing the film include a vacuum deposition method and a sputtering method, and are not particularly limited, but the vacuum deposition method is more preferable from the viewpoint of productivity and economy. In general, examples of the vacuum deposition method include a crucible method and a wire feed method, but the method is not particularly limited and may be appropriately selected. The margin pattern in the case of metallization by vapor deposition is not particularly limited, and may be a normal pattern or a special margin pattern provided for the purpose of improving the security of the capacitor.
Further, the configuration method of those margins is not particularly limited, and for example, a tape method or an oil method may be used.

  The structure and form of the capacitor made of the film of the present invention are not particularly limited. For example, a dry type, a liquid impregnation type, a round type or a flat press type may be used. It is particularly suitable for a flat capacitor in which wrinkles are easily generated in the step) and shape stability of the element is required.

  Hereinafter, embodiments of the present invention will be described based on examples, but the present invention is not limited to the examples.

Next, measurement methods and evaluation methods used in the examples of the present invention will be described.
(1) Cold xylene solubles (CXS)
After dissolving 0.5 g of a polypropylene film sample in 100 ml of boiling xylene and allowing to cool, the polypropylene components dissolved in the filtrate after being recrystallized in a constant temperature water bath at 20 ° C. for 1 hour are obtained by liquid chromatography. Quantify (X (g)). Using the precision value (X0 (g)) of 0.5 g of the sample, the following formula is used.

CXS value (% by weight) = X / X0 × 100
(2) Intrinsic viscosity ([η])
0.1 mg of a sample was dissolved in 100 ml of tetralin at 135 ° C., and this solution was measured using a viscometer in a constant temperature bath at 135 ° C., and the intrinsic viscosity [η] was determined from the specific viscosity S according to the following formula (unit: : Dl / g).

[Η] = (S / 0.1) × (1 + 0.22 × S)
(3) Copolymerization amount of 1-butene The IR spectrum method was used to determine the weight percent from the following formula and convert it to mol%.

1-butene copolymerization amount (% by weight) = 1.208 K ′
Absorbance at K ′ = 767 cm ⁻¹ Reference 1: P256-259 of Polymer Analysis Handbook (published by Asakura Shoten in 1985)
Reference 2: NJ Wegemer: J. Appl. Polym. Sci., 14, 573 (1970)
(4) Melting point (° C)
The measurement was performed under the following conditions using a Seiko RDC220 differential scanning calorimeter.
Preparation of sample: 10 mg of specimen is sealed in an aluminum pan for measurement.

Measurement conditions The endothermic peak observed when the temperature is increased from room temperature to 280 ° C. at a rate of 20 ° C./min is defined as 1stRun melting point (1st-Tm (° C.)). Next, it is rapidly cooled to room temperature at 320 ° C./min, and the endothermic peak observed when the temperature is increased again to 280 ° C. at a rate of 20 ° C./min is defined as 2ndRun melting point (2nd-Tm (° C.)). In any case, when multiple peak values are observed, the melting peak having the largest peak area is adopted. The above measurement was repeated 5 times, and the average value of the remaining 3 points, excluding 2 points of the maximum value and the minimum value, was defined as Tm (° C.).

The melting point of the film is 1stRun melting point, and the resin melting point is 2ndRun melting point.
(5) Centerline average roughness (Ra) and ten-point average roughness (Rz)
Measured according to JIS B-0601 (1982) using a “non-contact three-dimensional fine shape measuring instrument (ET-30HK)” and “three-dimensional roughness analyzer (MODEL SPA-11)” manufactured by Kosaka Laboratory Ltd. . The number of measurements was 3, and the average value was used.

  The detailed conditions are as follows.

    Measurement surface treatment: Aluminum was vacuum-deposited on the measurement surface to obtain a non-contact method.

Measurement length: 1mm
Horizontal magnification: 200x Vertical magnification: 20000x Cutoff: 0.25mm
Width direction feed speed: 0.1 mm / sec Length direction feed pitch: 10 μm
Number of feeds in length direction: 20 times Measurement direction: width direction of film (6) Δd (= MMV−WMV) (μm)
The average film thickness (MMV) was measured according to 7.4.1.1 of JIS C-2330 (2001).
Next, after measuring the weight average thickness (WMV) according to 7.4.1.2 of JIS C-2330 (2001), it was obtained by the following formula.
Δd (μm) = MMV−WMV
(7) 140 ° C. Thermal Shrinkage Rate In accordance with JIS-Z-1712, the dimensional change rate when the sample film is held in a hot air oven at 140 ° C. for 15 minutes under the following conditions is defined as the thermal shrinkage rate. The MD (longitudinal direction) of film formation is preferably 6% or less, and the TD (width direction) is preferably 3% or less.

(A) Sample width 10mm x length 200mm
(B) Oven conditions: 140 ° C., load 3 g
(C) The measurement length is obtained by the following equation using a carefully read value of the film length L1 (mm) before and after the processing, with L0 before processing = 100 mm as a reference.

Thermal contraction rate (%) = (L0−L1) / L0 × 100
(8) Dielectric breakdown voltage (V / μm)
According to JIS C2330 (2001 edition) 7.4.11.2 B method (plate electrode method).

The measurement was performed 20 times for each sample, and an average value Xmin of four values was obtained from 20 average values Xav and minimum values. Even if Xav is large and good, if Xmin is small, the variation is large, which may cause a problem. Therefore, Xmin is preferably 60% or more of Xav.
(9) Polydispersity of polypropylene resin (Mw / Mn)
It calculated | required by the monodisperse polystyrene reference | standard using the gel permeation chromatography (GPC).

  The number average molecular weight (Mn) and the weight average molecular weight (Mw) are defined by the following formulas based on the molecular number (Ni) of the molecular weight (Mi) at each elution position of the GPC curve obtained through the molecular weight calibration curve. Is done.

Number average molecular weight: Mn = Σ (Ni · Mi) / ΣNi
Weight average molecular weight: Mw = Σ (Ni · Mi2) / Σ (Ni · Mi)
Molecular weight distribution: Mw / Mn
The measurement conditions were as follows (() indicates the manufacturer)
Apparatus: Gel permeation chromatograph GPC-150C (Waters)
Detector: Differential refractive index detector RI Sensitivity 32 ×, 20% (Waters)
Column: Shodex HT-806M (2) (Showa Denko)
Solvent: 1,2,4-trichlorobenzene (BHT 0.1 w / v% added) (Aldrich)
Flow rate: 1.0 ml / min
Temperature: 135 ° C
Sample: Dissolution condition 165 ± 5 ° C. × 10 minutes (stirring)
Concentration 0.20w / v%
Filtration Membrane filter pore size 0.45μm (Showa Denko)
Injection volume: 200 μl
Molecular weight calibration: The molecular weight of polypropylene was determined using the relationship between molecular weight and retention time obtained by measuring monodisperse polystyrene (Tosoh) under the same conditions as the specimen. Data processing which is a relative value based on polystyrene: According to a GPC data processing system manufactured by Toray Research Center.
(10) Evaluation of vapor deposition capacitor characteristics On the films obtained in Examples and Comparative Examples described later, aluminum was solid-deposited with a ULVAC vacuum vapor deposition machine so that the film resistance was 5 Ω / sq, and the film longitudinal direction was long. As the vertical direction, it was slit into a width of 50 mm and a length of 2000 m, and 24 (12 pairs) vapor deposition reels were collected.

  Next, using this reel, the capacitor element was wound up with an element winding machine manufactured by Minato Seisakusho, metallized, and then heat-treated at 120 ° C. for 16 hours in a vacuum. Potting with resin finished the capacitor element. The capacitor element capacity at this time was 10 μF.

Apply 5 capacitor elements arbitrarily from here, apply a voltage of 600 VDC to the capacitor element at room temperature for 10 minutes, then increase the applied voltage to 700 VDC and apply for 10 minutes, and so on every 10 minutes. A so-called step-up test in which the voltage was increased by 100 VDC was performed, and the voltage was increased to a maximum voltage gradient of 400 V / μm (2400 V if the film thickness was 6 μm), and the insulation resistance and breakdown characteristics of the device were confirmed. The case where the insulation resistance was remarkably reduced and the case where a short circuit breakage occurred was defined as NG. The breakdown condition was confirmed by disassembling the element after the step-up test.
The evaluation criteria are shown below. When the failure mode is difficult, the practical characteristics can be satisfied by improving the capacitor element manufacturing conditions, etc., but in the case of evaluation with problems, there are many practical problems.

No problem in breakdown mode: No decrease in insulation resistance, no dielectric breakdown over 4 layers of dielectric film observed at element disassembly Failure in failure mode: No decrease in insulation resistance, but penetrated over 4 layers Dielectric breakdown is observed.

        Alternatively, although there is a decrease in insulation resistance, there is no dielectric breakdown across four or more layers.

There is a problem in the breakdown mode: The insulation resistance is remarkably reduced, and through-type dielectric breakdown is observed over four or more layers.

Hereinafter, description will be made based on the implementation and comparative examples.
1. Preparation of polypropylene resin Polypropylene resin can be prepared according to JP-A-2002-128825.

  As a method for producing the propylene polymer used in the present invention, there is a method in which propylene is polymerized in the presence of a catalyst system comprising a solid catalyst component, an organoaluminum compound and an electron donor component, using a known polymerization method. Can be mentioned. Examples of the polymerization method include bulk polymerization, solution polymerization, slurry polymerization, and gas phase polymerization. Bulk polymerization is a method in which polymerization is performed using a liquid olefin as a medium at the polymerization temperature, and solution polymerization or slurry polymerization is in an inert hydrocarbon solvent such as propane, butane, isobutane, pentane, hexane, heptane, and octane. The gas phase polymerization is a method in which a gaseous monomer is used as a medium and the gaseous monomer is polymerized in the medium. These polymerization methods can be either batch type or continuous type, and these polymerization methods may be arbitrarily combined. From the industrial and economical viewpoint, a continuous gas phase polymerization method is preferred.

  The amount of 1-butene copolymerization was changed and adjusted. CXS obtained the target value by washing the obtained powder with propylene. In pelletization, 4500 ppm of Irganox 1010 manufactured by Ciba Specialty Chemicals and 50 ppm of calcium stearate manufactured by Shin Nippon Rika Co., Ltd. were melted and kneaded in an extruder at 250 ° C., and pelletized.

  Moreover, Borealis polypropylene resin HC300BF was prepared for comparative evaluation. The properties of the polypropylene resin are shown in Table 1.

2. Film-forming methods All of the films were biaxially stretched by the following film-forming methods to obtain film samples.

  The prepared polypropylene pellets were extruded into a sheet form from a T-shaped die having a slit width of 300 mm from an extruder having a screw diameter of 65 mmφ, and formed into a sheet on a chill roll having an appropriately set temperature.

  Next, the sheet was guided to a roll stretching apparatus and stretched in the longitudinal direction, and then stretched by a mechanical ratio of 7 times in the width direction with a tenter. The draw ratio and temperature were changed in each implementation and comparative example.

Example 1
PP-1 was used as a polypropylene resin, cooled and solidified to a chill roll temperature of 85 ° C., and then stretched 4.9 times at 140 ° C. in the longitudinal direction and 9 times at 156 ° C. in the width direction to obtain a biaxially stretched film.

  The Rz of the film thus obtained is 0.221 μm on the chill roll surface, 0.890 μm on the anti-chill roll surface, the heat shrinkage is 5.5% in the longitudinal direction and 2% in the width direction, and the variation in dielectric breakdown voltage is small. There was no problem with the destruction mode of the capacitor. The film forming conditions are shown in Table 2, and the film characteristics are shown in Table 3.

Example 2
PP-1 was used as a polypropylene resin, cooled and solidified at a chill roll temperature of 85 ° C., and stretched 5.3 times at 145 ° C. in the longitudinal direction and 10 times at 156 ° C. in the width direction to obtain a biaxially stretched film.

  The Rz of the film thus obtained is 0.255 μm on the chill roll surface, 1.050 μm on the anti-chill roll surface, the thermal shrinkage is 5% in the longitudinal direction, and 1.5% in the width direction, and the variation in breakdown voltage is small. Also, there was no problem with the breakdown mode of the capacitor. The film forming conditions are shown in Table 2, and the film characteristics are shown in Table 3.

Example 3
PP-2 is used as a polypropylene resin, cooled and solidified at a chill roll temperature of 90 ° C., stretched 5.5 times at 145 ° C. in the longitudinal direction, and stretched 10 times at 157 ° C. in the width direction to form a biaxially stretched film. .

  The Rz of the film thus obtained is 0.289 μm on the chill roll surface, 1.200 μm on the anti-chill roll surface, the thermal shrinkage is 4.5% in the longitudinal direction and 1.5% in the width direction, and the variation in dielectric breakdown voltage There was no problem with the breakdown mode of the capacitor. The film forming conditions are shown in Table 2, and the film characteristics are shown in Table 3.

Example 4
PP-2 is used as a polypropylene resin, and after cooling and solidifying at a chill roll temperature of 90 ° C., it is stretched 5 times at 140 ° C. in the longitudinal direction, and then stretched 10 times at 157 ° C. in the width direction. Obtained.
The Rz of the film thus obtained is 0.255 μm on the chill roll surface, 1.000 μm on the anti-chill roll surface, the thermal shrinkage is 4.8% in the longitudinal direction, and 2.5% in the width direction. The capacitor destruction mode was not a problem. The film forming conditions are shown in Table 2, and the film characteristics are shown in Table 3.

Example 5
PP-2 is used as the polypropylene resin, and after cooling and solidifying at a chill roll temperature of 90 ° C., the film is stretched 5 times at 135 ° C. in the longitudinal direction, and then stretched 10.5 times at 157 ° C. in the width direction. Got.

The Rz of the film thus obtained is 0.221 μm on the chill roll surface, 0.790 μm on the anti-chill roll surface, the thermal shrinkage is 5% in the longitudinal direction, and 2.5% in the width direction, and the variation in dielectric breakdown voltage is small. There was no problem with the destruction mode of the capacitor. The film forming conditions are shown in Table 2, and the film characteristics are shown in Table 3.
Example 6
PP-2 is used as a polypropylene resin, cooled and solidified at a chill roll temperature of 98 ° C., stretched 5 times at 135 ° C. in the longitudinal direction, and then stretched 10 times at 157 ° C. in the width direction to obtain a biaxially stretched film. It was. The Rz of the film thus obtained is 0.805 μm on the chill roll surface, 1.322 μm on the anti-chill roll surface, the thermal shrinkage is 5% in the longitudinal direction and 2.5% in the width direction, and the variation in dielectric breakdown voltage is small. There was no problem with the destruction mode of the capacitor. The film forming conditions are shown in Table 2, and the film characteristics are shown in Table 3.

Comparative Example 1
PP-3 is used as a polypropylene resin, and after cooling and solidifying at a chill roll temperature of 85 ° C., it is stretched 5 times at 135 ° C. in the longitudinal direction, and then stretched 8.5 times at 150 ° C. to obtain a biaxially stretched film. It was.

  The heat shrinkage rate of the film thus obtained was as large as 10% in the longitudinal direction and 4% in the width direction, causing problems such as whitening during aluminum deposition. The film forming conditions are shown in Table 2, and the film characteristics are shown in Table 3.

Comparative Example 2
Using PP-4 as a polypropylene resin, after cooling and solidifying at a chill roll temperature of 85 ° C., it was stretched 4.9 times at 145 ° C. in the longitudinal direction and then stretched 10.5 times at 156 ° C. in the width direction to be biaxial A stretched film was obtained.

The film thus obtained had a large variation in the ratio between the maximum value and the minimum value of the dielectric breakdown voltage of 0.58. The film forming conditions are shown in Table 2, and the film characteristics are shown in Table 3.
Example 7
PP-1 is used as a polypropylene resin, and after cooling and solidifying at a chill roll temperature of 60 ° C., it is stretched by 149 times in the longitudinal direction and then by 9 times at 156 ° C. in the width direction to obtain a biaxially stretched film. It was.

  The film thus obtained had a smooth Rz of 0.085 μm on the chill roll surface and 0.040 μm on the anti-chill roll surface. In the element evaluation after the step-up evaluation, penetration-like destruction over about 10 layers was observed and it was evaluated that the destruction mode was difficult, but the practical characteristics were satisfactory. The film forming conditions are shown in Table 2, and the film characteristics are shown in Table 3.

Comparative Example 3
PP-2 is used as a polypropylene resin, cooled and solidified at a chill roll temperature of 90 ° C, stretched 5 times at 147 ° C in the longitudinal direction, and then stretched 9.5 times at 156 ° C in the width direction. Got. However, a method of further heating the anti-chill roll surface side with a radiation heater when extending in the longitudinal direction was adopted.

  The film thus obtained had an Rz of 0.85 μm on the chill roll surface, an Rz of 1.95 μm on the anti-chill roll surface, a low dielectric breakdown voltage of 570 V / μm, and a large variation. The film forming conditions are shown in Table 2, and the film characteristics are shown in Table 3.

  The polypropylene film obtained according to the present invention is excellent in thermal dimensional stability and excellent in uniformity of thickness and film properties, and thus is particularly suitable for use as a vapor deposition capacitor, but it can also be used for other foil wound capacitors. Moreover, it can also be preferably used as a thermosetting member molding application (so-called mold release application), a protective film, a food packaging application, and an adhesive tape because of its excellent thermal dimensional stability in addition to the capacitor application.

Claims (3)

An average film measured by a micrometer, comprising a polypropylene resin having a melting point of 155 to 164 ° C. and a cold xylene solubles (CXS) of 1.5% by weight or less, which is a copolymer of propylene and α-olefin. The thickness (MMV) is 2.0 to 8.0 μm, the ten-point average roughness (Rz) of any film surface is 0.05 to 1.8 μm, and the weight average thickness (WMV) and the average film The difference Δd (= MMV−WMV) from the thickness (MMV) is 0.03 to 0.2 μm, and the melting point of the polypropylene film is the melting point of the polypropylene resin constituting the film +2 to 15 ° C. Polypropylene film for capacitors. 2. The polypropylene film for capacitors according to claim 1, wherein at least one film has a surface roughness of 0.08 to 0.8 [mu] m. The polypropylene film for a capacitor according to claim 1, wherein the α-olefin is 1-butene.