JP2012209541A - Biaxial stretching polypropylene film for capacitor, metalized film and film capacitor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a biaxial stretching polypropylene film for a capacitor with reduced deviation of dielectric strength characteristics at room temperature from those at high temperature, and also provide a metalized film and a film capacitor.SOLUTION: If VC denotes the dielectric breakdown voltage at 25°C and VC denotes the dielectric breakdown voltage at 100°C, the biaxial stretching polypropylene film for a capacitor has 350-450 V/μm of VC, and -2.0 to -0.5 of the slope of the dielectric breakdown voltages, (VC-VC)/75.

Description

  The present invention relates to a biaxially oriented polypropylene film for a capacitor suitable for packaging, industrial use, and the like, and more specifically, as a dielectric for a capacitor, a biaxially oriented polypropylene for a capacitor having stable withstand voltage characteristics from room temperature to high temperature. It relates to film.

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

  Among these, capacitors are preferably used for capacitors because of their excellent withstand voltage characteristics and low loss characteristics, not limited to DC applications and AC applications.

  Recently, there is an increasing demand for environmental and green energy-related applications such as hybrid cars, electric vehicles, wind power generation and solar power generation, especially for inverter applications.

  These applications are mainly those that have a larger capacity and a higher voltage, have a longer service life than conventional home appliances that have been used, and have a large environmental change. For this reason, in particular, there are increasing demands for small fluctuations in characteristics over a long period of time, such as usage environment and service life, and high reliability, and the design has been designed to reduce fluctuations in the electrical characteristics of capacitors as much as possible from the design stage.

  Further, particularly in automobile applications, there has been a strong demand for reducing the weight, size and heat resistance of capacitors in order to improve fuel efficiency, secure interior space, and increase the degree of freedom of member layout. Especially in an electric vehicle, since the motor voltage depending on the battery voltage is lowered, a large amount of current is passed to increase the output of the motor. For this reason, a large current flows through the capacitor to be used, and there is a demand for higher heat resistance than ever before. Further, in an electric vehicle without an engine, the usage environment as a capacitor is becoming lower and the temperature range of use is wider than ever, and the electric characteristics of the capacitor independent of temperature are being strongly demanded.

  In response to these requirements, various approaches have been made in the past, particularly with respect to increasing the heat resistance of films (reference documents 1 to 6).

  For example, Patent Document 1 describes a method of increasing the high-temperature withstand voltage by suppressing electron carriers by setting an appropriate amount of antioxidant in the film. Patent Document 2 describes a method of increasing the high-temperature withstand voltage by increasing crystallinity by adding high melt tension polypropylene.

  All of these methods are intended to meet market demands by increasing the withstand voltage at both normal temperature and high temperature. However, withstand voltage characteristics at room temperature are often too high for market demand, and there is insufficient demand for minimizing fluctuations in film performance during use at the inverter design stage. It was something.

  Further, Patent Documents 3 to 6 propose a method for improving a voltage resistance by reducing a low molecular component that becomes an insulation defect by using a raw material having a narrow molecular weight distribution such as a metallocene raw material.

  These are all intended for oil-immersed capacitors, and have been intended to improve the withstand voltage characteristics as a capacitor by suppressing the amount of low molecular components to suppress the components eluted in the oil. Although these methods are effective for oil-immersed capacitors, they have not been able to improve the withstand voltage characteristics at high temperatures of the film itself, and the market for increasing the current withstand voltage characteristics of the film itself has not been realized. The request was insufficient.

JP 2009-231705 A JP 2007-084813 A JP 2006-063186 A JP 2000-243655 A Japanese Patent Laid-Open No. 11-268145 Japanese Patent No. 08-156118

  An object of this invention is to provide the biaxially-stretched polypropylene film for capacitors, the metallized film, and the film capacitor which reduced the withstand voltage characteristic difference in normal temperature and high temperature.

  The present invention for solving the above problems has the following features.

(1) When the dielectric breakdown voltage at ^{25 ° C.} is V ^{25 ° C.} and the dielectric breakdown voltage at ^{100 ° C.} is V ^{100 ° C.} , V ^{100 ° C.} is 350 to 450 V / μm, and the dielectric breakdown voltage gradient (V ^{100 ° C.} A biaxially oriented polypropylene film for a capacitor having a value of −V ^{25 ° C.} ) / 75 of −2.0 to −0.5.

  (2) The biaxially stretched polypropylene film for capacitors as described in (1) above, wherein the mesopentad fraction of the polypropylene resin is 0.90 to 0.98.

  (3) Biaxial stretching for capacitors as described in (1) or (2) above, wherein the ratio (Mw / Mn) of the number average molecular weight (Mn) and the weight average molecular weight (Mw) of the polypropylene resin is 2-5. Polypropylene film.

  (4) Biaxial stretching for capacitors according to any one of (1) to (3) above, wherein the polypropylene resin is isotactic polypropylene obtained by polymerization using a catalyst having a metallocene compound supported on a polymer carrier. Polypropylene film.

  (5) The biaxially stretched polypropylene film for capacitors according to any one of the above (1) to (4), wherein the center line surface roughness (SRa) of at least one surface is also 10 nm to 50 nm.

  (6) A metallized film in which a metal film is provided on at least one surface of the biaxially stretched polypropylene film for capacitors according to any one of (1) to (5).

  (7) A film capacitor using the metallized film according to (6) above.

  The present invention can provide a polypropylene film having a reduced difference between a withstand voltage characteristic at normal temperature and a withstand voltage at high temperature, thereby making it possible to create a capacitor having stable characteristics from a low temperature to a high temperature and to obtain high long-term reliability. .

It is the schematic which shows the melting curve of the polypropylene observed by DSC. It is the schematic which shows the differential curve of a melting curve for WTm explaining.

  Hereinafter, the biaxially stretched polypropylene film, metallized film and film capacitor for capacitors of the present invention will be described in more detail.

  The biaxially stretched polypropylene film for capacitors of the present invention is preferably biaxially oriented, and is preferably stretched in the biaxial direction. Stretching is preferably performed by a stenter sequential biaxial stretching method.

In addition, the biaxially stretched polypropylene film for capacitors of the present invention has a dielectric breakdown voltage gradient (V ^{100 ° C.} −V) when the dielectric breakdown voltage at ^{25 ° C.} is V ^{25 ° C.} and the dielectric breakdown voltage at ^{100 ° C.} is V ^{100 ° C. 25 ° C.} ) / 75 is −2.0 to −0.5. Thus, by using a film with a controlled breakdown voltage, reliability in long-term use can be ensured. That is, by conducting a test at room temperature in the design stage, the capacitor characteristics at high temperatures can be estimated well, so that a more reliable capacitor can be made in the design stage. The calculation formula of (V ^{100 ° C.−} V ^{25 ° C.} ) / 75 representing the breakdown voltage gradient is obtained by dividing the breakdown voltage at each temperature by the temperature difference of 75 ° C., and the change in the breakdown voltage with temperature. Represents the width.

  In particular, in automotive applications, it is expected to be used in a wide variety of environments from extremely high temperatures such as the Middle East to cold regions such as North America. In addition, in the environment / green energy related applications represented by wind power generation and solar power generation, since long life per product is long, long-term reliability is particularly important. In such an application, it is desired that the withstand voltage change with respect to the temperature change is small, instead of requiring a high withstand voltage at room temperature.

In such a situation, if the value of (V ^{100 ° C.−} V ^{25 ° C.} ) / 75 is ^less than −2.0, the temperature change at a high temperature is large. I had to take voltage characteristics. For example, when the value of (V ^{100 ° C.−} V ^{25 ° C.} ) / 75 is −2.5, the dielectric breakdown voltage changes by 12.5 V / μm or more just by changing the ambient temperature by 5 ° C. In order to increase the reliability with respect to the temperature change, it was necessary to set an excessively high voltage from the normal temperature withstand voltage.

The value of ^{(V 100 ℃ -V 25 ℃)} / 75 of the present invention is preferably -1.7～-0.5, more preferably -1.2～-0.5, more preferably -0. 8 to -0.5. The higher the value of (V ^{100 ° C.−} V ^{25 ° C.} ) / 75, the better. However, the upper limit is substantially about −0.5.

For example, when the value of (V ^{100 ° C.−} V ^{25 ° C.} ) / 75 is −0.5, the change in dielectric breakdown voltage is 2.5V / μm even if the ambient temperature changes by 5 ° C. Even if the operating environment temperature changes several degrees, it is possible to reduce the change in the characteristics of the capacitor.

Further, in the film of the present invention, the dielectric breakdown voltage at 85 ° C. when the ^{V 85 ° C.,} the value of the breakdown voltage gradient ^{(V 85 ℃ -V 25 ℃)} / 60 is at -1.0 0.3 Preferably there is. More preferably, it is -0.85 to -0.3. The higher the value of ( ^{V85 ° C-} V25 ^{° C} ) / 60, the better, but the upper limit is substantially about -0.3. The calculation formula of dielectric breakdown voltage gradient (V ^{85 ° C.−} V ^{25 ° C.} ) / 60 is obtained by dividing the dielectric breakdown voltage at each temperature by the temperature difference of 60 ° C. Represents.

Furthermore, in the film of the present invention, V ^{100 ° C.} is preferably 350 to 450 V / μm. When V ^{100 ° C.} is lower than 350 V / μm, the absolute value itself is too low, so even if the change width of the dielectric breakdown voltage due to temperature is low, for the capacitor application assumed in the present invention, In addition, the withstand voltage characteristics are insufficient. V ^{100 ° C.} is more preferably 380 to 450 V / μm, and further preferably 400 to 450 V / μm. The higher the value of V ^{100 ° C.,} the better. However, the upper limit is substantially about 450 V / μm.

  In order to obtain the above breakdown voltage characteristics, it is important to generate a crystal structure uniformly in the cast sheet before stretching. This is because it is important to suppress the molecular motion that becomes an electron carrier at a high temperature, and thus it is possible to suppress the molecular motion by generating the crystal structure uniformly. Details will be described below.

  The glass transition point, which is the point at which the molecular motion of the amorphous part of the polypropylene resin changes, is known to be around 0 ° C., and the temperature at which the crystal part relaxes is also around 70 ° C. to 80 ° C. Are known. That is, at 100 ° C., which is the usage environment of the capacitor assumed in the present invention, a temperature at which molecular relaxation occurs in both the amorphous part and the crystalline part, and the molecular motion is active in both the amorphous part and the crystalline part. It is. Therefore, in order to maintain a high withstand voltage holding ratio at high temperatures, it is preferable to strengthen the molecular restraint state in both the crystalline part and the amorphous part. That is, it is preferable to increase the crystal / amorphous orientation.

  Semicrystalline polymers such as polypropylene are composites of amorphous and plate crystals as described in the polypropylene handbook (edited by Edward P. Moore, Jr., published by Kogyo Kenkyukai, 1998). A higher order structure is formed by the spherulite structure. A biaxially stretched film is obtained by destroying the spherulite structure, which is an aggregate of plate crystals, and by subdividing the crystals by breaking the plate crystals in the drawing process. It is comprised by.

  Therefore, by making the crystal structures of the spherulites and plate crystals uniform, the crystal and amorphous orientation states in the final film can be made uniform.

That is, when the crystal structure is non-uniform, the stress is concentrated on the fragile portion, and the orientation state is non-uniform. In other words, even if there is a portion with a high degree of orientation, there is a portion with a weak orientation, so the film may break down in the weak portion. By making the crystal structure uniform, it is possible to make the stretching stress applied to the crystal and the amorphous uniform in the subsequent longitudinal stretching step and lateral stretching step. That is, since the entire film is in a uniformly oriented state, it is possible to suppress molecular motion at a high temperature, and the withstand pressure retention ratio is improved, and the value of (V ^{100 ° C.−} V ^{25 ° C.} ) / 75 is − It can be set to 2.0-0.5.

  As a method for making the crystal structure uniform, it is desirable to optimize both the raw materials and the cooling conditions during casting. Hereinafter, the production method will be described while describing preferable raw material compositions and the like in order.

  First, as a polypropylene resin as a raw material, the ratio (Mw / Mn) of the number average molecular weight (Mn) to the weight average molecular weight (Mw) is preferably 2 to 5, more preferably 2 to 4.5. Preferably there is. Mw / Mn is a parameter indicating the molecular weight distribution of the polypropylene resin, and is obtained by a GPC (gel permeation chromatography) method. This means that the smaller the Mw / Mn, the smaller the molecular weight distribution. However, when the Mw / Mn is small, the uniformity of the sheet thickness when the molten polymer is formed into a sheet shape decreases or the biaxial stretchability decreases. Sometimes. However, it is preferable that Mw / Mn is small because the molecular weight of the polypropylene resin is uniform, so that the crystal size can be made uniform in film formation. If Mw / Mn exceeds 5, the crystal size may be non-uniform. The lower limit of Mw / Mn is theoretically 1 but this means a single molecular weight resin, which is difficult to obtain industrially with current catalyst technology. In practice, Mw / Mn is 2 or more.

  As a method for obtaining a resin having Mw / Mn of 2 to 5, it is possible to optimize the catalyst structure, such as a known Ziegler-Natta catalyst, a catalyst having a metallocene compound supported on a polymer carrier (metallocene catalyst), etc. This can be achieved by optimizing the catalyst configuration. Another method is to polymerize a polypropylene resin having a high molecular weight to some extent and heat-reducing it with peroxide or the like at the time of pelletization. The predetermined Mw / Mn can be obtained by a method of washing with, for example.

  Here, as specific examples of the metallocene catalyst comprising a metallocene compound and a co-catalyst, for example, the metallocene compound includes ethylene bisindenyl zirconium dichloride, ethylene bis (tetrahydroindenyl) zirconium dichloride, dimethylsilylene bis (dimethylcyclopentadiene). Enyl) zirconium dichloride, dimethylsilylenebis (trimethylcyclopentadienyl) zirconium dichloride, dimethylsilylenebis (indenyl) zirconium dichloride, dimethylsilylenebis (2-methylindenyl) zirconium dichloride, dimethylsilylenebis (2-methyl-4-) Isopropylindenyl) zirconium dichloride, isopropylidene (cyclopentadienyl) (9-fluorenyl) zirconium dichloride, Rohexylidene (cyclopentadienyl) (9-fluorenyl) zirconium dichloride, diphenylmethylene (cyclopentadienyl) (9-fluorenyl) zirconium dichloride, methylphenylmethylene (cyclopentadienyl) (9-fluorenyl) zirconium dichloride, isopropylidene (Cyclopentadienyl) (2,7-di-t-butyl-9-fluorenyl) zirconium dichloride, cyclohexylidene (cyclopentadienyl) (2,7-di-t-butyl-9-fluorenyl) zirconium dichloride, diphenyl Methylene (cyclopentadienyl) (2,7-di-t-butyl-9-fluorenyl) zirconium dichloride, methylphenylmethylene (cyclopentadienyl) (2,7-di-t-butyl-9-fluorenyl) ) And zirconium dichloride can be mentioned.

  In addition to aluminoxane, examples of the cocatalyst include dimethylanilinium tetrakis (pentafluorophenyl) borate and the like, and triphenylcarbenium tetrakis (pentafluorophenyl) borate and triphenylcarbenium. Tetrakis (pentafluorophenyl) aluminate, magnesium chloride, alumina, tris (pentafluorophenyl) boron can be used.

  In addition, when using a resin having Mw / Mn exceeding 5, by adding a crystal nucleating agent, crystallization starts from a large number of nuclei almost simultaneously, and the expansion of crystal growth is generally suppressed. The crystal size can be made uniform. As a kind of nucleating agent, it is preferable to add an α crystal nucleating agent (dibenzylidene sorbitols, sodium benzoate, etc.). However, when these crystal nucleating agents are added, the degree of crystallinity tends to increase, so that the withstand voltage at room temperature tends to be high, and the difference in withstand voltage between 25 ° C. and 100 ° C., which is the purpose of this application, should be reduced. However, it is preferable not to use as much as possible.

  In addition, various additives such as a crystal nucleating agent, an antioxidant, a thermal stabilizer, a slipping agent, an antistatic agent, an antiblocking agent, a filler, and a viscosity adjuster may be added to the polypropylene resin as long as the object of the present invention is not impaired. An agent, an anti-coloring agent and the like can be contained.

  Among these, the selection of the type and content of the antioxidant may be important when improving long-term heat resistance. That is, the antioxidant is a phenolic compound having steric hindrance, and at least one of them is preferably a high molecular weight type having a molecular weight of 500 or more. Specific examples thereof include various ones. For example, 1,3,5-trimethyl-2,4,6-, together with 2,6-di-t-butyl-p-cresol (BHT: molecular weight 220.4). Tris (3,5-di-t-butyl-4-hydroxybenzyl) benzene (for example, Irganox (registered trademark) 1330: molecular weight 775.2 manufactured by Ciba Geigy) or tetrakis [methylene-3 (3,5-di-t- (Butyl-4-hydroxyphenyl) propionate] methane (for example, Irganox 1010 manufactured by Ciba Geigy Inc .: molecular weight 1,177.7) is preferably used in combination.

  The total content of these antioxidants is preferably in the range of 0.03 to 1% by mass relative to the total amount of polypropylene. If the amount of the antioxidant is too small, the long-term heat resistance may be poor. If the amount of the antioxidant is too large, the capacitor element may be adversely affected by blocking at a high temperature due to bleeding out of these antioxidants. The more preferable content is 0.1 to 0.9% by mass, and particularly preferably 0.2 to 0.8% by mass.

  Furthermore, it is preferable that the cold xylene soluble part (henceforth CXS) of a polypropylene resin is 4 mass% or less. Here, the cold xylene-soluble part (CXS) is a polypropylene component which is dissolved in xylene after being completely dissolved in xylene and precipitated at room temperature, and has low stereoregularity, low molecular weight, etc. For this reason, it is considered that it corresponds to a component that is difficult to crystallize. If many such components are contained in the resin, problems such as inferior thermal dimensional stability of the film and reduction in dielectric breakdown voltage at high temperatures may occur. Therefore, CXS is preferably 4% by mass or less, more preferably 3% by mass or less, and particularly preferably 2% by mass or less. The above range is preferably satisfied for the polypropylene resin to be used, but it is also preferable that the entire film containing the resin as a constituent component is satisfied. In addition, although it is so preferable that there is little CXS, a substantial lower limit is about 1 mass%.

  In order to obtain a CXS-containing resin or polypropylene film as described above, a method for increasing the catalytic activity in obtaining a polymer, a method for washing the obtained polymer with a solvent or the propylene monomer itself, and the like can be used.

  The mesopentad fraction of the polypropylene resin is preferably 0.90 to 0.98, more preferably 0.92 to 0.96. The mesopentad fraction is an index indicating the stereoregularity of the crystal phase of polypropylene measured by a nuclear magnetic resonance method (NMR method). The higher the numerical value, the higher the crystallinity, the higher the melting point, and the higher the temperature. This is preferable because the dielectric breakdown voltage is increased. On the other hand, if the mesopentad fraction is too high, the crystallinity becomes too high, and voids may be generated in the crystal gaps when produced by the production method of the present invention.

  Further, as the polypropylene resin, the melt flow index (MFR) is more preferably 1 to 10 g / 10 minutes (230 ° C., 21.18 N load), particularly preferably 2 to 5 g / 10 minutes (230 ° C., 21.18 N load). The thing of this range is preferable from the point of film forming property.

  Next, as a method of molding the polypropylene cast sheet of the present invention, after passing the raw polymer through a filtration filter, it is extruded from a slit die (flat die) at a temperature of 220 to 280 ° C. and solidified on a cooling drum. There is. Here, in order to make the crystal size uniform, it is important to first uniform the spherulites, which are composites of plate crystals. For this purpose, it is preferable to stop the spherulite growth after growing the spherulites in as short a time as possible. Specifically, it is preferable to immediately cool to 40 to 50 ° C. after holding for about 1 to 2 seconds at a resin temperature of 115 to 125 ° C. If the holding time is too short, spherulites may not be generated sufficiently. In addition, if the holding time is too long, spherulite size non-uniformity may become remarkable due to a difference in crystallization speed of components having different molecular weights in the polypropylene resin. In order to realize the holding and cooling at the above temperature, it is preferable to use at least two cooling drums. That is, after the spherulite is generated in the first drum, the spherulite growth can be stopped in the second drum.

  In order to realize these conditions, the process may be appropriately determined according to the resin temperature, the extrusion amount, the take-up speed, etc., but from the viewpoint of productivity, the diameter of the cooling drum greatly affects the holding time. The diameter of the first drum is preferably at least 1 m.

  Furthermore, in order to obtain the film of the present invention, it is important to select the cooling drum temperature in accordance with these holding times. Although the cooling drum temperature to be selected includes a certain degree of optionality because other factors influence as described above, the first drum temperature is preferably 50 to 100 ° C., more preferably 60 to 60 ° C. It is 80 degreeC, Most preferably, it is the range of 60-70 degreeC. If the cooling drum temperature is too high, the size of the spherulites may be uneven as described above. The second drum temperature is preferably 0 to 50 ° C, more preferably 5 to 40 ° C, and particularly preferably 10 to 30 ° C. If the temperature is too high, the spherulite growth does not stop, so the spherulite size may be non-uniform.

  The first cooling drum (casting drum) can be brought into close contact with an electrostatic application method, a close contact method using surface tension of water, an air knife method, a press roll method, an underwater casting method, or the like. However, an air knife method that has good flatness and can control the surface roughness is preferable.

  Next, the obtained cast sheet is preheated through a plurality of preheating rolls maintained at 120 to 150 ° C. At this time, the sheet is preferably in contact with the roll for 10 to 40 seconds, more preferably 15 to 35 seconds, and particularly preferably 20 to 30 seconds.

  By contacting the preheating roll for an appropriate time, it is possible to grow the thickness of the plate-like crystal inside the spherulite produced by the cast sheet, thereby increasing the overall crystallinity. At this time, since the spherulite growth is once stopped in the initial cooling step, the spherulite size is kept uniform even when heated again on the preheating roll, and the entire crystal size becomes uniform.

  As a method for evaluating the uniformity of the crystal size of the cast sheet obtained in the present invention, there is a method for evaluating a melting curve by DSC. It is known that the melting curve is determined by the thickness of the crystal as described in the polypropylene handbook (edited by Edward P. Moore, Jr., published by Industrial Research Council, Inc., 1998). That is, the narrower the peak width of the melting curve, the more uniform the crystal thickness.

  Here, in the crystal of polypropylene, α-type crystal (monoclinic system, melting point: 160 to 172 ° C.) and β-type crystal (hexagonal system, melting point: 140 to 155 ° C.) may coexist. When seed crystals are present together, there are two peaks in the melting curve (FIG. 1), so it is necessary to devise to define the width of the melting peak.

  Therefore, in the present invention, in order to evaluate the homogenization of the crystal, the melting curve obtained by DSC is differentiated, and the peak width on the high temperature side from the portion (A) corresponding to the inflection point in the original curve is changed to the crystal homogenization. It was used as an index (Fig. 2). By taking the differential value, the baseline can be efficiently removed. Specifically, the ratio [1] / [2] (hereinafter referred to as WTm) of the peak width [2] on the high temperature side from A shown in FIG. 2 and the peak width [1] at the point where the height of the differential value is halved. ) To evaluate homogenization.

  In this invention, it is preferable that WTm is 0.3-0.5 in a cast sheet, More preferably, it is 0.35 or more, More preferably, it is 0.4 or more. The larger the WTm, the more uniform the crystal size. Therefore, the higher the WTm, the better. However, the practical upper limit is about 0.50.

  Next, it extends | stretches 2-6 times in the longitudinal direction through this cast sheet between the rolls which provided the circumferential speed difference, and cools to room temperature. What is important here is the temperature during stretching. The roll temperature during stretching is preferably 100 to 130 ° C, more preferably 100 to 120 ° C, and particularly preferably 100 to 115 ° C. These temperature regions mean that the temperature is lower than usual, and high orientation can be imparted by stretching at a low temperature.

  Conventionally, it has been known that if Mw / Mn is small, the stretchability deteriorates. However, in the present invention, the crystal size inside the cast sheet is made uniform, so that the stress during stretching is made uniform and the stretchability of the film is improved. Can be greatly improved, and a film with excellent thickness unevenness can be obtained. For this reason, it is possible to stretch the film even under a low temperature condition in which voids are generated by stretching between non-uniform crystals in the stretched film in the past.

  Also, by stretching at a low temperature, the difference in stretchability between the crystalline part and the amorphous part becomes large, so the amorphous part is preferentially stretched and the surface can be appropriately roughened. Become.

  Subsequent to stretching in the longitudinal direction, the stretched film is guided to a stenter and stretched 5 to 15 times in the width direction at a temperature of 130 to 150 ° C., and then 120 to 150 ° C. while giving 2 to 20% relaxation in the width direction. In order to improve the adhesion of the deposited metal to the surface to be deposited, a corona discharge treatment is performed in air, nitrogen, carbon dioxide, or a mixed gas thereof to obtain a film. As an example of the corona discharge treatment, the discharge treatment is performed with an output of about 10 to 20 kW. Moreover, also about extending | stretching of the width direction, it is preferable to extend | stretch as low temperature as possible similarly to the longitudinal stretch.

  In the film thus obtained, since the crystals are uniform in the cast sheet, all the crystals are uniformly subjected to stretching stress in the longitudinal stretching and the transverse stretching, and the orientation in the final transverse stretching direction Sexuality becomes stronger. Therefore, it is possible to suppress the crystal mobility at a high temperature, and it is possible to suppress the difference between the high voltage and the withstand voltage at room temperature.

  For example, a polypropylene resin having a mesopentad fraction of 0.99 and Mw / Mn of 4.8 polymerized using a metallocene catalyst, etc., is melted at 275 ° C. and extruded into a sheet form from a T die as a first cooling drum When the second cooling drum is set at 93 ° C. and 60 ° C., the sheet on the cooling drum is held at 110 to 135 ° C. for more than 2 seconds. In this case, although Mw / Mn is low, the initial spherulite growth time is long, and the mesopentad fraction is too high, so that the crystal becomes non-uniform and the WTm is likely to be less than 0.3. As a result, it is estimated that the difference in withstand voltage between high temperature and normal temperature is large.

  The glossiness of the film surface of the present invention is preferably in the range of 110 to 150%, more preferably 120 to 130%. That is, reducing the gloss level means increasing the light scattering density on the film surface, that is, the film surface is rough. When the glossiness is lowered to less than 110%, the handleability of the film is improved, but the practical lower limit is 110% from the film forming conditions of the present invention. On the other hand, if the glossiness exceeds 150%, the surface becomes smooth and the interlayer between the films is difficult to slip, making it difficult to form a flat capacitor element.

  Furthermore, the centerline surface roughness (SRa) of at least one surface of the biaxially stretched polypropylene film for capacitors of the present invention is preferably 10 nm or more and 50 nm or less, more preferably 20 to 40 nm. The higher the Ra is, the better the handleability of the film is. However, from the film forming conditions of the present invention, the practical upper limit is about 50 nm. On the other hand, if Ra is less than 10 nm, it is difficult to form a flat capacitor element because the film layer is difficult to slip.

  Further, the ash content of the biaxially stretched polypropylene film for capacitors of the present invention is preferably 50 ppm or less (mass basis, the same shall apply hereinafter), more preferably 30 ppm or less, and particularly preferably 20 ppm or less. When the ash content exceeds 50 ppm, the dielectric breakdown resistance of the film is lowered, and the dielectric breakdown strength may be lowered when a capacitor is used. In order to make the ash content within this range, it is important to use a raw material with little catalyst residue, but a method of reducing contamination from the extrusion system at the time of film formation as much as possible, for example, bleed time (raw material before film formation) It is possible to adopt a method such as taking 1 hour or more to pass through the extrusion system and washing the inside of the piping. The smaller the ash content, the better. However, the practical lower limit is about 10 ppm.

  The biaxially stretched polypropylene film for capacitors of the present invention preferably has a film thickness by a micrometer method of 1.0 to 5.0 μm from the viewpoint of capacitor element size and film formation stability. The film thickness by the micrometer method is more preferably 1.2 to 4.0 μm, and particularly preferably 1.5 to 3.0 μm. If the film is too thin, the mechanical strength and dielectric breakdown strength may be inferior. Further, if the film is too thick, temperature unevenness inside the cast sheet at the time of cooling increases, and it may be difficult to generate uniform spherulites in the cast sheet.

  The biaxially stretched polypropylene film for capacitors of the present invention is preferably used as a dielectric film for capacitors, but is not limited to the type of capacitor. Specifically, from the viewpoint of electrode configuration, either a foil wound capacitor or a metal vapor deposition film capacitor may be used, and it is also preferably used for an oil immersion type capacitor impregnated with insulating oil or a dry type capacitor not using insulating oil at all. It is done. Further, from the viewpoint of shape, it may be a winding type or a laminated type. However, it is particularly preferably used as a metal vapor deposition film capacitor because of the characteristics of the film of the present invention.

  In addition, since a polypropylene film usually has a low surface energy and it is difficult to stably deposit metal, it is preferable to perform a surface treatment in advance for the purpose of improving metal adhesion. Specific examples of the surface treatment include corona discharge treatment, plasma treatment, glow treatment, and flame treatment. Usually, the surface wetting tension of a polypropylene film is about 30 mN / m, but it is possible to adjust the wetting tension to 37 to 50 mN / m, preferably about 39 to 48 mN / m by these surface treatments. It is preferable because it is excellent in safety and has good security.

  In the present invention, it is preferable to form a metallized film by providing a metal film on the surface of the biaxially stretched polypropylene film for capacitors. Although the method is not particularly limited, for example, a method in which aluminum is deposited on at least one surface of a polypropylene film to form a metal film such as an aluminum deposited film that becomes an internal electrode of a film capacitor is preferably used. At this time, other metal components such as nickel, copper, gold, silver, chromium, and zinc can be deposited simultaneously or sequentially with aluminum. In addition, a protective layer can be provided on the deposited film with oil or the like.

  The thickness of the metal film is preferably in the range of 20 to 100 nm from the viewpoint of electrical characteristics and self-heeling properties of the film capacitor. For the same reason, the surface electrical resistance value of the metal film is preferably in the range of 1 to 20Ω / □. The surface electrical resistance value can be controlled by the type of metal used and the film thickness. Here, the measurement of the surface electric resistance value is as follows. The metallized film is cut into a rectangle having a full width (50 mm) in the width direction and 10 mm in the length direction to obtain a sample. With respect to the obtained sample, the resistance of the metal film between 30 mm in the width direction was measured by the 4-terminal method, and the obtained measurement value was multiplied by the measurement width (10 mm) to divide the distance between the electrodes (30 mm) to obtain 10 mm. X Calculate the membrane resistance per 10 mm.

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

  The metallized film thus obtained can be laminated or wound by various methods to obtain a film capacitor. An example of a preferred method for producing a wound film capacitor is as follows.

  Aluminum is vacuum deposited on one side of the polypropylene film. In that case, it vapor-deposits in the stripe form which has the margin part which runs in a film longitudinal direction. Next, a tape-shaped take-up reel having a margin on one side is prepared by inserting a blade into the center of each vapor deposition section on the surface and the center of each margin section. Two tape-shaped take-up reels with margins on the left or right are wound on each other so that the vapor deposition part protrudes from the margin part in the width direction. Get. The core material is removed from the wound body and pressed, and the metallicon is sprayed on both end faces to form external electrodes, and a lead wire is welded to the metallicon to obtain a wound film capacitor.

  Hereinafter, various measurement methods used in the present invention will be described.

(1) Film thickness (μm)
The micrometer method thickness was measured according to 7.4.1.1 of JIS C-2330 (2001).

(2) Gloss (Glossiness)
According to JIS K-7105 (1981), the average value of five data measured under the conditions of an incident angle of 60 ° and a light receiving angle of 60 ° using a digital variable angle gloss meter UGV-5D manufactured by Suga Test Instruments Co., Ltd. It was.

(3) Melt flow index (MFR)
According to JIS-K7210 (1999), measurement was performed at a measurement temperature of 230 ° C. and a load of 21.18N.

(4) WTm
The measurement was performed under the following conditions using a Seiko RDC220 differential scanning calorimeter.

<Sample preparation>
A sample of 5 mg is enclosed in an aluminum pan for measurement.

<Measurement>
The film was melted from 30 ° C. to 280 ° C. at a heating rate of 20 ° C./min. WTm was calculated | required in the differential curve obtained by using the data analysis software IGOR Pro 6 by WaveMetrics for the obtained melting curve. Since the data differentiation process is a generally known technique, any software other than IGOR Pro 6 may be used as long as it has similar functions.

(5) Mesopentad fraction (mmmm)
The sample was dissolved in a solvent, and the mesopentad fraction (mmmm) was determined using ¹³ C-NMR under the following conditions (Reference: New edition of Polymer Analysis Handbook) Editorial 1995 P609-611).

A. Measurement conditions Apparatus: Bruker, DRX-500
Measurement nucleus: ¹³ C nucleus (resonance frequency: 125.8 MHz)
Measurement concentration: 10 wt%
Solvent: benzene / heavy orthodichlorobenzene = mass ratio 1: 3 mixed solution Measurement temperature: 130 ° C.
NMR sample tube: 5 mm tube Pulse width: 45 ° (4.5 μs)
Pulse repetition time: 10 seconds Conversion count: 10,000 times Measurement mode: complete decoupling
B. Analysis condition LB (line broadening factor) was set to 1.0, and Fourier transform was performed to set the mmmm peak to 21.86 ppm. Peak splitting is performed using WINFIT software (manufactured by Bruker). At that time, peak splitting is performed as follows from the peak on the high magnetic field side, soft automatic fitting is performed, peak splitting is optimized, and mmmm and ss (mmmm spinning sideband peak) The sum of the peak fractions is defined as the mesopentad fraction (mmmm).

  The measurement is performed 5 times and the average value is obtained.

Peak (a) mrrm
(B) (c) rrrrm (divided as two peaks)
(D) rrrr
(E) mrmm + rmrr
(F) mmrr
(G) mmmr
(H) ss (mmmm spinning sideband peak)
(I) mmmm
(J) rmmr
(6) Cold xylene soluble part (CXS)
Dissolve 0.5 g of polypropylene film sample in 100 ml of boiling xylene, allow to cool, recrystallize in a constant temperature water bath at 20 ° C. for 1 hour, and then determine the polypropylene components dissolved in the filtrate by liquid chromatography. (X (g)). Using the precision value (X0 (g)) of 0.5 g of the sample, the following formula is used.

CXS (mass%) = (X / X0) × 100
(7) Ratio of number average molecular weight to weight average molecular weight (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 (the parentheses indicate 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) (Ardrich)
Flow rate: 1.0 ml / min
Temperature: 135 ° C
Sample: Dissolution condition 165 ± 5 ° C. × 10 minutes (stirring)
Concentration: 0.20 w / 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. Relative value based on polystyrene.

  Data processing: According to the GPC data processing system manufactured by Toray Research Center.

(8) Centerline average roughness (Ra)
Measured according to JIS-B-0601 (1982) using “Non-contact three-dimensional fine shape measuring instrument (ET-30HK)” and “Three-dimensional roughness analyzer (MODEL SPA-11)” manufactured by Kosaka Laboratory Ltd. did. The measurement was repeated 10 times in the longitudinal direction, and the centerline average roughness (Ra) was determined as the average value.

(9) Dielectric breakdown voltage (V ^{25 ° C} , V ^{85 ° C} , V ^{100 ° C} )
The dielectric breakdown voltage was measured according to JIS C2330 (2001) 7.4.11.2 method B (flat plate electrode method). However, for the lower electrode, “conductive rubber E-100 <65>” made by Togawa Rubber Co., Ltd. of the same size is placed on the metal plate described in the B method of JIS C2330 (2001) 7.4.11.2. Were used as electrodes.

  The average value obtained by the measurement was divided by the film thickness (μm) of the measured sample and expressed in V / μm.

The measurement temperatures were 25 ° C., 85 ° C., and 100 ° ^C., and expressed as V ^{25 ° C.} , V ^{85 ° C.} , and V ^{100 ° C.} , respectively.

  Hereinafter, an example is given and the effect of the present invention is further explained.

Example 1
The mesopentad fraction of polypropylene polymerized using dimethylsilylene bis- (2-methyl-4-isopropyl-indenyl) zirconium dichloride as the metallocene compound and methylaluminoxane (manufactured by Tosoh Corporation Akzo) as the cocatalyst is 0.95, Using a polypropylene resin having Mw / Mn of 3, it is supplied to an extruder having a temperature of 260 ° C., melt-extruded into a sheet form from a T-type slit die at a resin temperature of 255 ° C., and the molten sheet is used with two cooling drums. Cooled and solidified. The first cooling drum at this time is referred to as a cooling drum 1, and the second cooling drum is referred to as a cooling drum 2. Specifically, it was brought into contact with the cooling drum 1 held at 60 ° C. and then brought into contact with the cooling drum 2 held at 30 ° C. to be cooled and solidified. At this time, the holding time of 110 to 135 ° C. was 1.5 seconds as a result of measurement by the radiation thermometer.

Next, the sheet was held on a roll maintained at 130 ° C. for 30 seconds, and then passed through a roll provided with a peripheral speed difference maintained at 115 ° C. and stretched 4.7 times in the longitudinal direction. Subsequently, the film was guided to a tenter, stretched 10 times in the width direction at a temperature of 150 ° C., then heat treated at 140 ° C. while giving 6% relaxation in the width direction, and a biaxially stretched polypropylene having a film thickness of 4.0 μm. A film was obtained. Further, the surface of the film in contact with the cooling drum 1 was subjected to corona discharge treatment in the air at a treatment strength of 20 W · min / m ² . The properties of the biaxially stretched polypropylene film thus obtained were as shown in Table 1.

(Example 2)
A biaxially stretched polypropylene film having a film thickness of 4.0 μm was obtained in the same manner as in Example 1 except that the temperature of the cooling drum 1 was changed to 50 ° C. The retention time at 110 to 135 ° C. of the sheet on the cooling drum was 1.0 seconds as a result of measurement by a radiation thermometer.

(Example 3)
A biaxially stretched polypropylene film having a film thickness of 4.0 μm was obtained in the same manner as in Example 1 except that the temperature of the cooling drum 1 was 100 ° C. and the temperature of the cooling drum 2 was 10 ° C. The 110-135 ° C. holding time of the sheet on the cooling drum was 2.0 seconds as a result of measurement by a radiation thermometer.

(Example 4)
Example 1 with the exception of using a polypropylene resin (The Polyolefin Company (singapore) Pte “COSMOPLENE”) having a mesopentad fraction of 0.95 and Mw / Mn of 5 polymerized with a Ziegler-Natta catalyst. Similarly, a biaxially stretched polypropylene film having a film thickness of 4.0 μm was obtained.

(Example 5)
Polypropylene polymerized using a Ziegler-Natta catalyst has a mesopentad fraction of 0.95 and a Mw / Mn of 8 (polypropylene resin (5014L, manufactured by Korea Oil & Fats Co., Ltd.)). A biaxially stretched polypropylene film having a film thickness of 4.0 μm was obtained in the same manner as in Example 1 except that 0.5% by mass of a polypropylene resin (High melt tension polypropylene Profax PF-814, manufactured by Basell) was blended.

(Example 6)
Polypropylene polymerized using a Ziegler-Natta catalyst has a mesopentad fraction of 0.95 and a Mw / Mn of 8 (polypropylene resin (5014L, manufactured by Korea Oil & Fats Co., Ltd.)). A biaxially stretched polypropylene film having a film thickness of 3.0 μm was obtained in the same manner as in Example 1 except that 2.0% by mass of a polypropylene resin (high melt tension polypropylene Profax PF-814, manufactured by Basell) was blended.

(Example 7)
The mesopentad fraction of polypropylene polymerized using dimethylsilylene bis- (2-methyl-4-isopropyl-indenyl) zirconium dichloride as the metallocene compound and methylaluminoxane (manufactured by Tosoh Akzo) as the cocatalyst is 0.98, A biaxially stretched polypropylene film having a film thickness of 2.0 μm was obtained in the same manner as in Example 1 except that the temperature of the cooling drum 1 was changed to 100 ° C. using a polypropylene resin having Mw / Mn of 3. The 110-135 ° C. holding time of the sheet on the cooling drum was 1.8 seconds as a result of measurement by a radiation thermometer.

(Comparative Example 1)
A film thickness of 4.0 μm was obtained in the same manner as in Example 1 except that a polypropylene resin (“Borclean” manufactured by Borealis) having a mesopentad fraction of polypropylene of 0.95 and Mw / Mn of 6.0 was used. A biaxially stretched polypropylene film was obtained.

(Comparative Example 2)
In Example 1, a film was formed in the same manner as in Example 1 except that the temperature of the cooling drum 2 was set to 80 ° C. However, tearing occurred in the longitudinal stretching process, and the film could not be collected.

(Comparative Example 3)
In Example 1, a biaxially oriented polypropylene film having a film thickness of 4.0 μm was prepared in the same manner as in Example 1 except that the temperature of the cooling drum 2 was 80 ° C. and the roll temperature during longitudinal stretching was 150 ° C. Obtained.

(Comparative Example 4)
A polypropylene resin (“Borclean” manufactured by Borealis) having a mesopentad fraction of 0.98 and an Mw / Mn of 6.0 is added to a branched polypropylene resin (High melt tension polypropylene Profax manufactured by Basell) in 0.5. It was blended by mass%, supplied to an extruder having a temperature of 250 ° C., melt-extruded into a sheet form from a slit die, and cooled and solidified on a cooling drum 1 kept at a temperature of 90 ° C. and a cooling drum 2 kept at a temperature of 50 ° C. The holding time of the sheet on the cooling drum at 110 to 135 ° C. was 2.4 seconds as a result of measurement with a radiation thermometer.

  Next, the film was stretched 5 times in the length direction with a roll maintained at a temperature of 140 ° C. The time during this period was 15 seconds. Next, the film was stretched 10 times in the width direction at a temperature of 160 ° C., then heat-set at 150 ° C., and subjected to corona discharge treatment on one side to obtain a film having a thickness of 3.0 μm.

(Comparative Example 5)
A polypropylene resin having a mesopentad fraction of 0.985 and an Mw / Mn of 6.4 (“Borclean” manufactured by Borealis) and a branched polypropylene resin (high melt tension polypropylene Profax PF-814 manufactured by Basell) 0.2% by mass was blended, melted at 250 ° C., extruded into a sheet form from a T-die, and cooled and solidified on a cooling drum 1 at a temperature of 85 ° C. and a cooling drum 2 maintained at 50 ° C. The holding time of the sheet on the cooling drum at 110 to 135 ° C. was 2.2 seconds as a result of measurement with a radiation thermometer.

  Next, this sheet was stretched 4.7 times in the length direction at a temperature of 143 ° C. The time during this period was 15 seconds. Next, both ends were held with clips and guided into a hot air oven, preheated in an atmosphere of 166 ° C., stretched 9 times in the transverse direction at 157 ° C., and then heat-treated at a temperature of 160 ° C. Thereafter, a corona discharge treatment was applied to one surface of the film so that the wetting tension was 42 mN / m to obtain a film having a thickness of 3.0 μm.

(Comparative Example 6)
A film having a thickness of 3.0 μm was obtained in the same manner as in Comparative Example 4 except that the branched polypropylene resin was not used.

Claims (7)

When the dielectric breakdown voltage at ^{25 ° C.} is V ^{25 ° C.} and the dielectric breakdown voltage at ^{100 ° C.} is V ^{100 ° C.} , V ^{100 ° C.} is 350 to 450 V / μm, and the dielectric breakdown voltage gradient (V ^{100 ° C.−} V ^{25 C} )) A biaxially stretched polypropylene film for capacitors having a value of / 75 of -2.0 to -0.5. The biaxially stretched polypropylene film for capacitors according to claim 1, wherein the polypropylene resin has a mesopentad fraction of 0.90 to 0.98. The biaxially stretched polypropylene film for capacitors according to claim 1 or 2, wherein the ratio (Mw / Mn) of the number average molecular weight (Mn) and the weight average molecular weight (Mw) of the polypropylene resin is 2-5. The biaxially stretched polypropylene film for capacitors according to any one of claims 1 to 3, wherein the polypropylene resin is isotactic polypropylene obtained by polymerization using a catalyst in which a metallocene compound is supported on a polymer carrier. The biaxially stretched polypropylene film for capacitors according to any one of claims 1 to 4, wherein the center line surface roughness (SRa) of any surface is 10 nm or more and 50 nm or less. A metallized film in which a metal film is provided on at least one side of the biaxially stretched polypropylene film for capacitors according to any one of claims 1 to 5. A film capacitor using the metallized film according to claim 6.

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