JP7524840B2 - Biaxially oriented polypropylene film for capacitors, polypropylene film for metal layer integrated capacitors, film capacitors, and film rolls - Google Patents

Description

The present invention relates to a biaxially oriented polypropylene film for capacitors, a polypropylene film for metal layer integrated capacitors, a film capacitor, and a film roll.

Polypropylene film can be used as a capacitor dielectric. For example, it can be used as a capacitor dielectric in inverters that make up the power control units of hybrid and electric vehicles.

JP 2019-44171 A JP 2001-48998 A International Publication No. 2015-012324

Conventionally, techniques for improving the voltage resistance of film capacitors at high temperatures have been known to increase crystallinity through the molecular design and composition of raw materials, as well as through the adjustment of film-forming conditions.

For example, Patent Documents 1 and 2 focus on the melting point of the main peak and the total heat of fusion in differential scanning calorimetry (DSC), and claim that the inclusion of a large amount of stable crystalline components is effective in improving voltage resistance at high temperatures. However, there is a limit to how high the melting point of the main peak can be, and if too many crystalline components are included, it reduces stretchability and deteriorates element processability, resulting in frequent occurrence of defective element shapes.

In addition, in Patent Document 3, it is said that in response to this issue, stretchability and element processability are ensured by blending a specific amount of polypropylene resin with low stereoregularity. However, since the main resin is highly crystallized, the stretchability is not necessarily sufficient.

In this situation, the main objective of the present invention is to provide a biaxially oriented polypropylene film for capacitors that has high voltage resistance at high temperatures and excellent stretchability and device processability. In addition, the present invention also aims to provide a metal layer-integrated polypropylene film, a film capacitor, and a film roll that utilize the biaxially oriented polypropylene film for capacitors.

The present inventors conducted intensive research to solve the above problems. As a result, they discovered that in differential scanning calorimetry (DSC) of a biaxially oriented polypropylene film for capacitors, by setting the heat of fusion of the entire film as 100%, and by setting the percentage (%) of the total heat of fusion of the sub-peaks on the lower temperature side of the main peak among two or more melting peaks and the percentage (%) of the heat of fusion at 150°C or less within a specific range, a biaxially oriented polypropylene film for capacitors having high voltage resistance at high temperatures and excellent stretchability and element processability can be obtained. The present invention was completed through further research based on this knowledge.

That is, the present invention includes the following.
Item 1. A biaxially oriented polypropylene film for a capacitor,
When the biaxially oriented polypropylene film for a capacitor was subjected to differential scanning calorimetry (DSC) at a heating rate of 30° C./min,
At least two melting peaks are observed,
The ratio of the total heat of fusion of the sub-peaks on the lower temperature side of the main peak of the fusion peak to the total heat of fusion (100%) is in the range of 60% to 70%;
The ratio of the heat of fusion at 150° C. or less to the total heat of fusion (100%) is in the range of 12% to 20%.
Biaxially oriented polypropylene film for capacitors.
Item 2. Measured by tensile testing in a 25°C environment.
The tensile breaking strength in the MD direction is 140 MPa or more and 170 MPa or less,
The tensile elongation at break in the MD direction is 130% or more and 160% or less,
The tensile breaking strength in the TD direction is 330 MPa or more and 370 MPa or less,
Item 2. The biaxially oriented polypropylene film for a capacitor according to item 1, having a tensile elongation at break in the TD direction of 50% or more and 70% or less.
Item 3. The biaxially oriented polypropylene film for a capacitor according to item 1 or 2, wherein when the biaxially oriented polypropylene film for a capacitor is heat-treated at 120° C. for 15 minutes, the sum of the heat shrinkage rate in the MD direction, the heat shrinkage rate in the TD direction, and the heat shrinkage rate in the 45° direction is 10% or less.
Item 4. The biaxially oriented polypropylene film for a capacitor according to any one of Items 1 to 3, wherein the polypropylene resin constituting the biaxially oriented polypropylene film for a capacitor has a Z-average molecular weight Mz of 650,000 or more and 945,000 or less.
Item 5. The biaxially oriented polypropylene film for a capacitor according to any one of Items 1 to 4, having a thickness of 0.8 μm or more and 6 μm or less.
Item 6. The biaxially oriented polypropylene film for a capacitor according to any one of items 1 to 5,
A metal layer-integrated polypropylene film for capacitors, comprising a metal layer laminated on one or both sides of the biaxially oriented polypropylene film for capacitors.
Item 7. A film capacitor having the metal layer-integrated polypropylene film according to Item 6 which is wound, or having a configuration in which a plurality of the metal layer-integrated polypropylene films for capacitors according to Item 6 are laminated.
Item 8. A film roll, in which the biaxially oriented polypropylene film according to any one of items 1 to 5 is wound into a roll shape.

According to the present invention, it is possible to provide a biaxially oriented polypropylene film for capacitors that has high voltage resistance at high temperatures and excellent stretchability and device processability. In addition, according to the present invention, it is also possible to provide a polypropylene film for metal layer-integrated capacitors, a film capacitor, and a film roll that utilize the biaxially oriented polypropylene film.

FIG. 1 is a schematic diagram of a melting curve (vertical axis: heat flux (W/g), horizontal axis: temperature (°C)) obtained by differential scanning calorimetry (DSC) of a biaxially oriented polypropylene film for a capacitor, and is a schematic diagram for explaining the proportion (%) of the total heat of fusion of sub-peaks on the lower temperature side than the main peak among two or more melting peaks, when the total heat of fusion is taken as 100%. FIG. 1 is a schematic diagram of a melting curve (vertical axis: heat flux (W/g), horizontal axis: temperature (°C)) obtained by differential scanning calorimetry (DSC) of a biaxially oriented polypropylene film for a capacitor, and is a schematic diagram for explaining the proportion (%) of the heat of fusion at 150°C or less when the total heat of fusion is taken as 100%.

The biaxially oriented polypropylene film for capacitors, the polypropylene film for metal layer-integrated capacitors, and the film capacitors of the present invention, as well as their manufacturing methods, are described in detail below.

In this specification, polypropylene may be abbreviated as PP, and polypropylene resin may be abbreviated as PP resin.

In this specification, the expressions "contain" and "comprise" include the concepts of "contain," "include," "consist essentially of," and "consist only of."

In this specification, the term "capacitor" includes the concepts of "capacitor", "capacitor element" and "film capacitor". In addition, the term "biaxially oriented polypropylene film for capacitors" may be abbreviated to "biaxially oriented polypropylene film", "film", "polypropylene film roll" may be abbreviated to "film roll", "metal layer integrated polypropylene film" may be abbreviated to "metal layer integrated polypropylene film", "metal layer integrated film", and "metal layer integrated polypropylene film roll" may be abbreviated to "metal layer integrated film roll". In addition, the biaxially oriented polypropylene film according to this embodiment is not a microporous film, so it does not have many pores. In addition, the biaxially oriented polypropylene film may be composed of two or more layers, but is preferably composed of a single layer.

In this specification, the directions of the biaxially oriented polypropylene film are as follows. First, the machine direction of the film is the same as the Machine Direction (hereinafter referred to as the "MD direction"). The MD direction is sometimes called the length direction or flow direction. Next, the transverse direction of the film is the same as the Transverse Direction (hereinafter referred to as the "TD direction"). The TD direction is sometimes called the width direction.

Biaxially oriented polypropylene film The biaxially oriented polypropylene film for capacitors according to this embodiment is characterized in that, when differential scanning calorimetry (DSC) is performed at a heating rate of 30°C/min, at least two melting peaks are observed, the proportion of the total heat of fusion formed by the sub-peaks on the lower temperature side of the main peak of the melting peaks to the total heat of fusion (100%) is in the range of 60% to 70%, and the proportion of the heat of fusion at 150°C or less to the total heat of fusion (100%) is in the range of 12% to 20%.

The biaxially oriented polypropylene film according to this embodiment has the above-mentioned structure, and thus exhibits high voltage resistance at high temperatures, as well as excellent stretchability and device processability. The mechanism behind this can be considered as follows.

That is, in the biaxially stretched polypropylene film, the low-temperature sub-peak of the DSC is believed to be derived from thermally metastable microcrystals with thin lamellar thickness. It is presumed that metastable microcrystals have characteristics in the state of molecular chain packing in the unit lattice that forms the crystals, and that their higher-order structure form is thought to hinder the flow of current when a voltage is applied to the biaxially stretched polypropylene film. In the biaxially stretched polypropylene film according to this embodiment, the ratio of the total heat of fusion of the sub-peaks on the lower temperature side of the main peak of the melting peak to the total heat of fusion (100%) is set to a specific range of 60% to 70%, which is considered to be the specific range that the current inhibition by the microcrystals becomes appropriate and contributes to high voltage resistance at high temperatures. Furthermore, in the biaxially stretched polypropylene film according to this embodiment, by controlling the ratio of the heat of fusion at 150°C or less to the total heat of fusion, it is considered that the thermally metastable crystals melt preferentially during stretching, the stretching stress does not increase unnecessarily, and the stretching property is improved. In addition, the fine crystals not only contribute to improving the voltage resistance at high temperatures, but also increase the strength of the biaxially oriented polypropylene film and provide moderate elongation, which is thought to improve the device processability. As described later, in the biaxially oriented polypropylene film of this embodiment, the ratio of the total heat of fusion formed by the subpeaks to the total heat of fusion and the ratio of the heat of fusion at 150°C or less can be suitably set within the above-mentioned specified range by appropriately controlling, for example, the transverse stretching temperature, heat setting temperature, and cooling temperature in the stretching process in the production of the biaxially oriented polypropylene film.

The biaxially oriented polypropylene film according to this embodiment exhibits high voltage resistance at high temperatures, as well as excellent stretchability and device processability, making it suitable as a biaxially oriented polypropylene film for film capacitor applications.

When the biaxially oriented polypropylene film according to this embodiment is subjected to differential scanning calorimetry (DSC, DSC measurement, etc.) at a heating rate of 30°C/min, at least two melting peaks are observed. Among the melting peaks observed by DSC, the peak with the highest intensity is called the main peak, and the other peaks are called subpeaks.

When the biaxially oriented polypropylene film according to the present embodiment is subjected to DSC measurement under the condition of a temperature rise rate of 30°C/min, the ratio of the total heat of fusion of the sub-peaks on the lower temperature side of the main peak of the fusion peak (the heat of fusion fraction of the sub-peak) to the total heat of fusion (100%) is in the range of 60% to 70%. For example, as shown in the schematic diagram of FIG. 1, in the melting curve (vertical axis is heat flux (W/g), horizontal axis is temperature (°C)) obtained by DSC measurement of the biaxially oriented polypropylene film, the total heat of fusion of the sub-peaks on the lower temperature side of the main peak (peak on the highest temperature side) among two or more melting peaks corresponds to the area of the shaded region. The total heat of fusion corresponds to the total area of the region under the melting curve, and the ratio (%) of the total heat of fusion of the sub-peaks (the heat of fusion fraction of the sub-peak) can be calculated by calculating the total area and the area of the shaded region. The boundary between the main peak and the sub-peak adjacent to the main peak on the low temperature side corresponds to the valley of the melting curve, and the boundary line is drawn to determine the area. DSC measurement was performed according to the method described in the Examples.

The heat of fusion fraction of the sub-peak is preferably 62% or more, more preferably 63% or more, even more preferably 65% or more, even more preferably 66% or more, and particularly preferably 67% or more. On the other hand, the heat of fusion fraction of the sub-peak is preferably 69% or less, more preferably 68% or less.

When the biaxially oriented polypropylene film according to the present embodiment is subjected to DSC measurement under the condition of a heating rate of 30°C/min, the ratio of the heat of fusion at 150°C or less (heat of fusion fraction at 150°C or less) to the total heat of fusion (100%) is in the range of 12% to 20%. For example, as shown in the schematic diagram of FIG. 2, in the melting curve obtained by DSC measurement of the biaxially oriented polypropylene film, the heat of fusion at 150°C or less corresponds to the area of the shaded region. The total heat of fusion corresponds to the total area of the region under the melting curve, and the ratio (%) of the heat of fusion at 150°C or less (heat of fusion fraction at 150°C or less) can be calculated by calculating the total area and the area of the shaded region.

The heat fraction of fusion at 150°C or less is preferably 13% or more, more preferably 14% or more, and even more preferably 15% or more, while the heat fraction of fusion at 150°C or less is preferably 19% or less, more preferably 18% or less, even more preferably 17% or less, and even more preferably 16% or less.

In addition, the biaxially stretched polypropylene film according to this embodiment has a tensile breaking strength in the MD direction measured by a tensile test in a 25°C environment of preferably 140 to 170 MPa, more preferably 145 to 168 MPa, and even more preferably 150 to 165 MPa. In addition, the tensile breaking elongation in the MD direction measured by a tensile test in a 25°C environment is preferably 130 to 160%, more preferably 135 to 157%, and even more preferably 140 to 155%. In addition, the tensile breaking strength in the TD direction measured by a tensile test in a 25°C environment is preferably 330 to 370 MPa, more preferably 333 to 368 MPa, and even more preferably 335 to 365 MPa. In addition, the tensile breaking elongation in the TD direction measured by a tensile test in a 25°C environment is preferably 50 to 70%, more preferably 52 to 70%, and even more preferably 55 to 70%. The tensile breaking strength and tensile breaking elongation are measured according to the method described in the Examples.

Furthermore, when the biaxially oriented polypropylene film of this embodiment is heat-treated at 120° C. for 15 minutes, the sum of the heat shrinkage rate in the MD direction HS _MD , the heat shrinkage rate in the TD direction HS _TD , and the heat shrinkage rate in the 45° direction HS ₄₅ ° (HS _MD + HS _TD + HS ₄₅ °) is preferably 10% or less, more preferably 8.0% or less, and even more preferably 7.0% or less. The lower limit of the sum is, for example, 5.0%. The heat shrinkage rate is measured by the method described in the examples.

The two surfaces of the biaxially oriented polypropylene film of this embodiment can be defined as a first surface and a second surface. The first surface can be a rough surface. If the first surface is a rough surface, wrinkles are less likely to occur when winding the element in the capacitor production. The second surface can also be a rough surface.

The thickness of the biaxially oriented polypropylene film is preferably 0.8 μm or more and 6.0 μm or less from the viewpoint of ensuring the miniaturization and high capacity of the capacitor when used in a capacitor. Specifically, it is preferably 5.5 μm or less, more preferably 3.5 μm or less, even more preferably 3.0 μm or less, and particularly preferably 2.4 μm or less. Furthermore, from the viewpoint of manufacturing, the thickness of the biaxially oriented polypropylene film is preferably 1.0 μm or more, more preferably 1.8 μm or more, and even more preferably 2.2 μm or more. The method for measuring the thickness of the biaxially oriented polypropylene film in this specification is the method described in the Examples.

The density of the biaxially oriented polypropylene film is not limited, but in consideration of its use as a capacitor, it is preferable to set it to, for example, 919 g/cm ³ or more and 925 g/cm ³ or less. The method for measuring the density of the biaxially oriented polypropylene film in this specification is the method described in the Examples.

In the biaxially oriented polypropylene film of this embodiment, the molecular weight distribution (weight average molecular weight Mw/number average molecular weight Mn) of the polypropylene resin constituting the biaxially oriented polypropylene film (after mixing, if the polypropylene resin is composed of a mixture of multiple resins) is preferably 5.0 or more and 6.9 or less.

The lower limit of the molecular weight distribution (Mw/Mn) is more preferably 5.2 or more, even more preferably 6.0 or more, even more preferably 6.2 or more, and even more preferably 6.3 or more. The upper limit is more preferably 6.8 or less, even more preferably 6.7 or less, and even more preferably 6.6 or less. By having the molecular weight distribution (Mw/Mn) within this range, in combination with other requirements, a biaxially oriented polypropylene film that has excellent dielectric breakdown strength at high temperatures and suppressed thermal shrinkage in the machine direction (MD) can be suitably obtained.

In the biaxially oriented polypropylene film of this embodiment, the z-average molecular weight Mz of the polypropylene resin constituting the biaxially oriented polypropylene film (after mixing, if the polypropylene resin is composed of a mixture of multiple resins) is preferably 650,000 or more and 945,000 or less.

The lower limit of Mz is more preferably more than 700,000, even more preferably 710,000 or more, and even more preferably 720,000 or more. The upper limit is more preferably 920,000 or less, even more preferably 850,000 or less, and even more preferably 790,000 or less. By having Mz within this range, in combination with other requirements, a biaxially oriented polypropylene film is obtained that has excellent dielectric breakdown strength at high temperatures and in particular suppresses thermal shrinkage in the machine direction (MD). Note that if Mz exceeds 945,000, the thermal shrinkage of the biaxially oriented polypropylene film at high temperatures is likely to increase.

In the biaxially oriented polypropylene film of this embodiment, it is preferable that the weight fraction w at logarithmic molecular weight Log(M)=4.0 in the integral molecular weight distribution curve of the polypropylene resin constituting the biaxially oriented polypropylene film (after mixing, if the polypropylene resin is composed of a mixture of multiple resins) is 2.6% or more and 4.2% or less.

The lower limit of the weight fraction w is more preferably 2.8% or more, more preferably 3.0% or more, even more preferably 3.2% or more, and even more preferably 3.4% or more. The upper limit is more preferably 4.1% or less, even more preferably 4.0% or less, even more preferably 3.7% or less, and even more preferably 3.6% or less. By having the weight fraction w within this range, as an effect of combination with other requirements, a biaxially oriented polypropylene film having excellent dielectric breakdown strength at high temperatures and suppressed thermal shrinkage in the machine direction (MD) is obtained. In addition, by having the weight fraction w within this range, the film capacitor of this embodiment using the biaxially oriented polypropylene film as the capacitor dielectric has excellent thermal shock resistance in that the thermal compaction (deformation) of the capacitor is suppressed during repeated use between high and low temperatures.

In a preferred embodiment of the biaxially oriented polypropylene film of the present embodiment, the polypropylene resin constituting the biaxially oriented polypropylene film (in the case where the polypropylene resin is a mixture of a plurality of resins, after mixing) is:
The molecular weight distribution (Mw/Mn) is 5.0 or more and 6.6 or less,
The Mz is 720,000 or more and 790,000 or less,
The weight fraction w is 3.5% or more and 3.7% or less.
By using a polypropylene resin having such physical properties, it is easy to obtain a biaxially oriented polypropylene film that is excellent in dielectric breakdown strength at high temperatures and has reduced thermal shrinkage in the machine direction (MD).

The Mw, Mn, Mz, molecular weight distribution (Mw/Mn), and weight fraction w of the polypropylene resin constituting the biaxially stretched polypropylene film of this specification, as well as the Mw, Mn, Mz, molecular weight distribution (Mw/Mn), molecular weight distribution (Mz/Mn), differential distribution value when logarithmic molecular weight Log(M)=4.5, differential distribution value when logarithmic molecular weight Log(M)=6.0, molecular weight differential distribution value difference (DM), and weight fraction w of polypropylene resin A and polypropylene resin B when the polypropylene resin is composed of multiple resins, are measured according to the methods described in the Examples.

The biaxially oriented polypropylene film of this embodiment contains polypropylene resin. The content of polypropylene resin is preferably 90% by weight or more, more preferably 95% by weight or more, based on the entire biaxially oriented polypropylene film (when the entire biaxially oriented polypropylene film is taken as 100% by weight). The upper limit of the content of polypropylene resin is, for example, 100% by weight, 98% by weight, etc., based on the entire biaxially oriented polypropylene film.

The lower the total ash content of polypropylene resin, the better for electrical properties. The total ash content is preferably 50 ppm or less, more preferably 40 ppm or less, and even more preferably 30 ppm or less, based on the polypropylene resin. The lower limit of the total ash content is, for example, 2 ppm or 5 ppm. The lower the total ash content, the fewer impurities such as polymerization catalyst residues there are.

The polypropylene resin may contain one type of polypropylene resin alone, or may contain two or more types of polypropylene resins.

When the biaxially oriented polypropylene film of this embodiment contains two or more types of polypropylene resin, the polypropylene resin with the greatest content is positioned as the main component in this specification, and is referred to as the "main polypropylene resin" or "base resin" in this specification. Also, when the biaxially oriented polypropylene film contains only one type of polypropylene resin, this polypropylene resin is also positioned as the main component in this specification, and is referred to as the "main polypropylene resin" in this specification.

When the biaxially stretched polypropylene film of this embodiment contains two or more types of polypropylene resin (particularly two types), it can contain, for example, the following polypropylene resin A (base resin, which is the main component) and the following polypropylene resin B (blend resin). Below, an example is described in which two types of polypropylene resin, polypropylene resin A (base resin) and polypropylene resin B (blend resin), are used.

The content of polypropylene resin A is preferably more than 50% by weight, more preferably 55% by weight or more, even more preferably 60% by weight or more, and even more preferably 65% by weight or more, based on 100% by weight of polypropylene resin. The upper limit of the content of polypropylene resin A is less than 100% by weight, preferably 95% by weight or less, more preferably 90% by weight or less, even more preferably 80% by weight or less, and even more preferably 75% by weight or less, based on 100% by weight of polypropylene resin. An example of polypropylene resin A is isotactic polypropylene.

The weight average molecular weight Mw of polypropylene resin A is preferably 250,000 or more and less than 350,000, more preferably 280,000 or more and less than 350,000, and even more preferably 280,000 or more and less than 340,000. When Mw is 250,000 or more and less than 350,000, it is easy to control the thickness of the cast raw sheet in the manufacturing process of the biaxially oriented polypropylene film, and thickness unevenness is unlikely to occur.

The number average molecular weight Mn of polypropylene resin A is preferably 30,000 or more and 54,000 or less, more preferably 33,000 or more and 52,000 or less, and even more preferably 33,000 or more and 47,000 or less. When Mn is 30,000 or more and 54,000 or less, it is easy to obtain a capacitor element with small thermal shrinkage and good thermal shock resistance.

The z-average molecular weight Mz of polypropylene resin A is preferably 700,000 or more and 1,550,000 or less, more preferably 750,000 or more and 1,500,000 or less. When Mz is 700,000 or more and 1,550,000 or less, a film with high dielectric breakdown strength at high temperatures is easily obtained.

The molecular weight distribution (Mw/Mn) of polypropylene resin A is preferably 5.0 or more, more preferably 5.5 or more, and even more preferably 6.0 or more. The Mw/Mn of polypropylene resin A is preferably 10.0 or less, and more preferably 9.5 or less. When Mw/Mn is 5.0 or more and 10.0 or less, the stretchability is improved and it is easy to obtain a thin film.

The molecular weight distribution (Mz/Mn) of polypropylene resin A is preferably 10 to 70, more preferably 15 to 60, and even more preferably 15 to 50. When Mz/Mn is 10 to 70, stretchability is improved and a thin film can be easily obtained.

In the molecular weight distribution curve of polypropylene resin A, the differential distribution value when the logarithmic molecular weight Log(M) is 4.5 is preferably 28.0 or more. The upper limit is preferably 32.0 or less. The differential distribution value when the logarithmic molecular weight Log(M) is 6.0 is preferably 17.0 or more, more preferably 20.0 or more. The upper limit is preferably 24.0 or less, more preferably 22.0 or less. Furthermore, the difference (molecular weight differential distribution value difference (DM)) obtained by subtracting the differential distribution value when the logarithmic molecular weight Log(M) is 6.0 from the differential distribution value when the logarithmic molecular weight Log(M) is 4.5 is preferably 8.0 or more and 18.0 or less, more preferably 8.0 or more and 11.0 or less, even more preferably 8.2 or more and 10.0 or less, and even more preferably 8.4 or more and 8.8 or less.

From the Mw value of polypropylene resin A (250,000 or more and less than 350,000), it can be seen that when comparing a component with a logarithmic molecular weight Log(M) of 4.5, which is a representative distribution value of components with a molecular weight of 10,000 to 100,000 on the low molecular weight side (hereinafter also referred to as "low molecular weight components"), with a component with a logarithmic molecular weight Log(M) of around 6.0, which is a representative distribution value of components with a molecular weight of around 1,000,000 on the high molecular weight side (hereinafter also referred to as "high molecular weight components"), the low molecular weight components are more prevalent by a ratio of 8.0% to 18.0%.

In other words, even if the molecular weight distribution Mw/Mn is 5.0 to 10.0, this merely indicates the width of the molecular weight distribution, and does not reveal the quantitative relationship between the high molecular weight and low molecular weight components. Therefore, it is preferable that the polypropylene resin A according to this embodiment has a wide molecular weight distribution and at the same time contains a large amount of components with a molecular weight of 10,000 to 100,000, in a ratio of 8.0% to 18.0%, compared to the component with a molecular weight of 1,000,000. This makes the crystallite size smaller, which is preferable because it makes it easier to obtain a roughened surface of the biaxially oriented polypropylene film.

The lower limit of the weight fraction w of polypropylene resin A is preferably 3.8% or more, more preferably 4.0% or more. The upper limit is preferably 4.4% or less, more preferably 4.2% or less. When the weight fraction w of polypropylene resin A is within this range, and when combined with the weight fraction w of polypropylene resin B described below, the weight fraction w after mixing of polypropylene resin A and polypropylene resin B is 2.6% or more and 4.2% or less, it becomes easier to obtain a biaxially oriented polypropylene film that has excellent dielectric breakdown strength at high temperatures and suppressed thermal shrinkage in the machine direction (MD).

The melt flow rate (MFR _A ) of polypropylene resin A at 230° C. is preferably 4.8 g/10 min or more, more preferably 5.0 g/10 min or more, and even more preferably 5.5 g/10 min or more. The upper limit of MFR _A is preferably 10.0 g/10 min or less, more preferably 8.0 g/10 min or less, and even more preferably 6.0 g/10 min or less. MFR _A can also be set to 4.8 g/10 min or more and 5.5 g/10 min or less. The method for measuring the melt flow rate (MFR) in this specification is the method described in the examples. The unit g/10 min of the melt flow rate is also called dg/min.

In a preferred embodiment of the biaxially oriented polypropylene film of the present embodiment, the polypropylene resin A is
The Mw of the polypropylene resin A is 275,000 or more and less than 350,000,
The molecular weight distribution (Mw/Mn) of the polypropylene resin A is 5.8 or more and 10.0 or less,
The melt flow rate (MFR _A ) of the polypropylene resin A is 4.8 g/10 min or more and 5.5 g/10 min or less;
By using polypropylene resin A having such physical properties, molding of a cast sheet (stretched precursor) by an extruder can be easily performed.

In addition, in a more preferred embodiment of the biaxially oriented polypropylene film of the present embodiment, the polypropylene resin A is
The Mw of the polypropylene resin A is 280,000 or more and 300,000 or less,
The molecular weight distribution (Mw/Mn) of the polypropylene resin A is 6.0 or more and 6.5 or less,
The melt flow rate (MFR _A ) of the polypropylene resin A is 5.0 g/10 min or more and 5.5 g/10 min or less;
This can be an embodiment.

The heptane insoluble content of polypropylene resin A is preferably 97.0% or more. The heptane insoluble content is preferably 98.5% or less. The higher the heptane insoluble content, the higher the stereoregularity of the resin. When the heptane insoluble content (HI) is 97.0% or more and 98.5% or less, the moderately high stereoregularity leads to a moderate improvement in the crystallinity of the polypropylene resin in the biaxially oriented polypropylene film, and the dielectric breakdown strength at high temperatures is improved. Furthermore, in the manufacturing process of the biaxially oriented polypropylene film, the solidification (crystallization) speed during cast raw sheet molding becomes moderate, and the film has moderate extensibility. The method for measuring the heptane insoluble content (HI) in this specification is the method described in the Examples.

The lower the total ash content of polypropylene resin A, the better for electrical properties. The total ash content is preferably 50 ppm or less, more preferably 40 ppm or less, and even more preferably 30 ppm or less, based on polypropylene resin A. The lower limit of the total ash content is, for example, 2 ppm or 5 ppm.

The content of polypropylene resin B is preferably less than 50% by weight, more preferably 49% by weight or less, even more preferably 40% by weight or less, and particularly preferably 35% by weight or less, relative to 100% by weight of polypropylene resin. The lower limit of the content of polypropylene resin B is, for example, preferably 10% by weight or more, more preferably 15% by weight or more, and even more preferably 25% by weight or more, relative to 100% by weight of polypropylene resin. An example of polypropylene resin B is isotactic polypropylene. In this embodiment, it is particularly preferable that the ratio of the mass of polypropylene resin A to the total mass of polypropylene resin A and polypropylene resin B is 65% by weight or more and 75% by weight or less.

The Mw of polypropylene resin B is preferably 350,000 or more, more preferably 390,000 or more. The Mw of polypropylene resin B is preferably 550,000 or less, more preferably 450,000 or less, and even more preferably 400,000 or less. When the Mw is 350,000 or more and 550,000 or less, it is easy to control the thickness of the cast raw sheet in the manufacturing process of the biaxially oriented polypropylene film, and thickness unevenness is unlikely to occur.

The Mn of polypropylene resin B is preferably 40,000 or more and 54,000 or less, more preferably 42,000 or more and 50,000 or less, and even more preferably 44,000 or more and 48,000 or less. When Mn is 40,000 or more and 54,000 or less, it is easy to obtain a capacitor element with small thermal shrinkage and good thermal shock resistance.

The Mz of polypropylene resin B is preferably more than 1.55 million and not more than 2 million, more preferably from 1.58 million to 1.7 million. When Mz is more than 1.55 million and not more than 2 million, a film with high dielectric breakdown strength at high temperatures is easily obtained.

The molecular weight distribution (Mw/Mn) of polypropylene resin B is preferably 5.0 or more, more preferably 5.5 or more, even more preferably 7.0 or more, and even more preferably 7.5 or more. The upper limit of Mw/Mn in polypropylene resin B is, for example, 11.0 or less, preferably 10.0 or less, and more preferably 8.5 or less. When Mw/Mn is 5.0 or more and 11.0 or less, stretchability is improved and a thin film can be easily obtained.

The molecular weight distribution (Mz/Mn) of polypropylene resin B is preferably 30 or more and 40 or less, more preferably 33 or more and 36 or less. When Mz/Mn is 30 or more and 40 or less, the stretchability is improved and it is easy to obtain a thin film.

In the molecular weight distribution curve of polypropylene resin B, the differential distribution value when the logarithmic molecular weight Log(M) is 4.5 is preferably 24.0 or more, more preferably 27.0 or more. The upper limit is preferably 35.0 or less, more preferably 32.0 or less. The differential distribution value when the logarithmic molecular weight Log(M) is 6.0 is preferably 28.0 or more, more preferably 30.0 or more. The upper limit is preferably 35.0 or less, more preferably 33.0 or less. Furthermore, the difference obtained by subtracting the differential distribution value when the logarithmic molecular weight Log(M) is 6.0 from the differential distribution value when the logarithmic molecular weight Log(M) is 4.5 (molecular weight differential distribution value difference (DM)) is preferably -11.0 or more and 7.0 or less, more preferably -6.0 or more and 0.0 or less, and even more preferably -4.0 or more and -2.0 or less.

When the polypropylene resin contains the above-mentioned polypropylene resins A and B, the difference in Mw, Mw/Mn, and differential distribution value between polypropylene resins A and B is different, that is, there is a difference in the molecular weight distribution composition, and therefore the biaxially stretched polypropylene film obtained by mixing and molding has a subtly different quantitative relationship between the high molecular weight component and the low molecular weight component, and therefore takes a kind of finely mixed (phase separated) state, which is considered to be preferable because the crystal size is easily finer. Furthermore, even with the same stretch ratio, there is a tendency for high orientation to be easily achieved, and the surface is also easily finely roughened, which is considered to be preferable. When the polypropylene resin contains polypropylene resins A and B, it is considered that this embodiment has excellent effects for the reasons described above, but this embodiment is not limited in any way by these reasons.

The lower limit of the weight fraction w of polypropylene resin B is preferably 2.0% or more, more preferably 3.0% or more. The upper limit is preferably 5.0% or less, more preferably 4.2% or less. The weight fraction w of polypropylene resin B is within this range, and by combining it with the weight fraction w of polypropylene resin A described above, the weight fraction w after mixing polypropylene resin A and polypropylene resin B is 2.6% or more and 4.2% or less, making it easier to obtain a biaxially oriented polypropylene film that has excellent dielectric breakdown strength at high temperatures and suppresses thermal shrinkage in the machine direction (MD).

The melt flow rate (MFR _B ) at 230° C. of polypropylene resin B is preferably 4.5 g/10 min or less, more preferably 4.0 g/10 min or less, even more preferably 3.0 g/10 min or less, and even more preferably 2.1 g/10 min or less. The lower limit of MFR _B is preferably 0.1 g/10 min or more, more preferably 0.5 g/10 min or more, and even more preferably 1.5 g/10 min or more.

The difference MFR _A -MFR _B between MFR _A of polypropylene resin A as the base resin of the main component and MFR B of polypropylene resin B as the blend resin is preferably set to 1.5 g/10 min or more. That is, MFR _A is larger than MFR _B. The difference MFR _A -MFR _{B is} preferably 1.6 g/10 min or more, more preferably 2.0 g/10 min or more, and even more preferably 3.0 g/10 min or more. When the difference MFR _A -MFR _B is less than 1.5 g/10 min (less than 1.5 g/10 min includes negative values), in the manufacturing process of the biaxially oriented polypropylene film, a sea-island phase separation structure is not formed at the time of casting raw sheet molding, or even if it is formed, the size of the islands is very small, so that it may be difficult to finally obtain a biaxially oriented polypropylene film having excellent dielectric breakdown strength at high temperatures. In particular, even if the difference between MFR _A and MFR _B is large, when MFR _B is larger (when the difference MFR _A - MFR _B is negative), the size of the islands in the sea-island phase separation structure will be very small.

In a preferred embodiment of the biaxially oriented polypropylene film of the present embodiment, the polypropylene resin B is
The Mw of the polypropylene resin B is 385,000 or more and 550,000 or less,
The molecular weight distribution (Mw/Mn) of the polypropylene resin B is 8.4 or more and 11.0 or less,
The melt flow rate (MFR _B ) of the polypropylene resin B is 0.1 g/10 min or more and 2.2 g/10 min or less;
By using polypropylene resin B having such physical properties, molding of a cast sheet (stretched precursor) by an extruder can be easily performed.

In a more preferred embodiment of the biaxially oriented polypropylene film of the present embodiment, the polypropylene resin B is
The Mw of the polypropylene resin B is 390,000 or more and 550,000 or less,
The molecular weight distribution (Mw/Mn) of the polypropylene resin B is 8.5 or more and 11.0 or less,
The melt flow rate (MFR _B ) of the polypropylene resin B is 1.0 g/10 min or more and 2.1 g/10 min or less;
This can be an embodiment.

The heptane insoluble content of polypropylene resin B is preferably 97.5% or more, more preferably 98.0% or more, even more preferably more than 98.5%, and particularly preferably 98.6% or more. The heptane insoluble content is preferably 99.5% or less, more preferably 99.0% or less.

The lower the total ash content of polypropylene resin B, the better for electrical properties. The total ash content is preferably 50 ppm or less, more preferably 40 ppm or less, and even more preferably 30 ppm or less, based on polypropylene resin B. The lower limit of the total ash content is, for example, 2 ppm or 5 ppm.

The above describes an example in which two types of resin, polypropylene resin A (base resin) and polypropylene resin B (blend resin), are used, but in this embodiment, the biaxially oriented polypropylene film can also contain a resin other than polypropylene resin. In that case, the total amount of polypropylene resin A and polypropylene resin B can be, for example, 90% by weight or more, 95% by weight or more, or 100% by weight, assuming that the entire resin is 100% by weight.

The biaxially oriented polypropylene film of this embodiment may further contain additives. Examples of additives include antioxidants, chlorine absorbers, UV absorbers, lubricants, plasticizers, flame retardants, antistatic agents, and colorants.

The biaxially oriented polypropylene film of this embodiment is for use in capacitors, and specifically, can be suitably used as a dielectric for capacitors. As described below, the biaxially oriented polypropylene film of this embodiment can be a polypropylene film for metal layer-integrated capacitors having a metal layer laminated on one or both sides, and a film capacitor can be produced by winding this metal layer-integrated polypropylene film or by laminating multiple metal layer-integrated polypropylene films for capacitors.

Considering the above-mentioned use for capacitors, it is desirable for the biaxially oriented polypropylene film of this embodiment to have the following dielectric breakdown strength and heat shrinkage resistance properties.

The dielectric breakdown strength (VDC120°C) of the biaxially oriented polypropylene film of this embodiment at a DC voltage of 120°C is preferably 400 V/μm or more, more preferably 420 V/μm or more, even more preferably 450 V/μm or more, and particularly preferably 500 V/μm or more. The upper limit of the dielectric breakdown strength at a DC voltage of 120°C is preferably as high as possible, and is, for example, 550 V/μm or 570 V/μm. The dielectric breakdown strength at a DC voltage is measured by the method described in the Examples.

The dielectric breakdown strength (VDC100°C) of the biaxially oriented polypropylene film of this embodiment at a DC voltage of 100°C is preferably 450 V/μm or more, more preferably 500 V/μm or more, and even more preferably 530 V/μm or more. The higher the upper limit of the dielectric breakdown strength at a DC voltage of 100°C, the more preferable it is, and it is, for example, 580 V/μm or 585 V/μm.

The dielectric breakdown strength (VDC80°C) of the biaxially oriented polypropylene film of this embodiment at a DC voltage of 80°C is preferably 500 V/μm or more, more preferably 520 V/μm or more, even more preferably 530 V/μm or more, and particularly preferably 550 V/μm or more. The upper limit of the dielectric breakdown strength at a DC voltage of 80°C is preferably as high as possible, and is, for example, 580 V/μm or 585 V/μm.

The dielectric breakdown strength (VDC25°C) of the biaxially oriented polypropylene film of this embodiment at a DC voltage of 25°C is preferably 550 V/μm or more, more preferably 570 V/μm or more, even more preferably 580 V/μm or more, and particularly preferably 610 V/μm or more. The upper limit of the dielectric breakdown strength at a DC voltage of 25°C is preferably as high as possible, and is, for example, 635 V/μm or 640 V/μm.

Furthermore, the biaxially oriented polypropylene film of this embodiment has a ratio (VDC120°C/VDC25°C) of the dielectric breakdown strength at a DC voltage of 120°C (VDC120°C) to the dielectric breakdown strength at a DC voltage of 25°C (VDC25°C) is preferably 0.72 or more, more preferably 0.74 or more, even more preferably 0.75 or more, and even more preferably 0.80 or more. The higher the upper limit of this ratio, the better, and it is, for example, 1.00.

Furthermore, the biaxially oriented polypropylene film of this embodiment has a ratio (VDC100°C/VDC25°C) of the dielectric breakdown strength at a DC voltage of 100°C (VDC100°C) to the dielectric breakdown strength at a DC voltage of 25°C (VDC25°C) is preferably 0.77 or more, more preferably 0.80 or more, and even more preferably 0.85 or more. The upper limit of this ratio is preferably as high as possible, and is, for example, 1.00.

Furthermore, the biaxially oriented polypropylene film of this embodiment has a ratio (VDC80°C/VDC25°C) of the dielectric breakdown strength at a DC voltage of 80°C (VDC80°C) to the dielectric breakdown strength at a DC voltage of 25°C (VDC25°C) is preferably 0.83 or more, more preferably 0.85 or more, even more preferably 0.88 or more, and even more preferably 0.90 or more. The higher the upper limit of this ratio, the better, and it is, for example, 1.00.

Manufacturing method of biaxially oriented polypropylene film The manufacturing method of the biaxially oriented polypropylene film of this embodiment described above is not limited, but the biaxially oriented polypropylene film of this embodiment can be suitably manufactured, for example, by adopting the manufacturing method described below (hereinafter referred to as "the manufacturing method of the biaxially oriented polypropylene film of this embodiment").

The method for producing a polypropylene film of the present embodiment is a method for producing the polypropylene film of the present embodiment described above,
When the biaxially oriented polypropylene film for capacitors was subjected to differential scanning calorimetry (DSC) at a heating rate of 30° C./min,
At least two melting peaks are observed,
The ratio of the total heat of fusion of the sub-peaks on the lower temperature side of the main peak of the fusion peak to the total heat of fusion (100%) is in the range of 60% to 70%;
The ratio of the heat of fusion at 150° C. or less to the total heat of fusion (100%) is in the range of 12% to 20%.
It is characterized by:

In the method for producing a biaxially stretched polypropylene film of the present embodiment, it is preferable to use a polypropylene resin composition containing at least polypropylene resin A and polypropylene resin B. Here, the content of polypropylene resin A in the polypropylene resin composition being greater than the content of polypropylene resin B in the polypropylene resin composition means that, in the relationship between polypropylene resin A and polypropylene resin B, polypropylene resin A is the base resin that is the main component, and polypropylene resin B is a blend resin for the base resin. The terms "polypropylene resin A" and "polypropylene resin B" in the method for producing a biaxially oriented polypropylene film of this embodiment correspond to the terms "polypropylene resin A" and "polypropylene resin B" in the above-mentioned item for the biaxially oriented polypropylene film. The Mw, Mn, Mz, molecular weight distribution (Mw/Mn), molecular weight distribution (Mz/Mn), differential distribution value when logarithmic molecular weight Log(M)=4.5, differential distribution value when logarithmic molecular weight Log(M)=6.0, molecular weight differential distribution value difference (DM), weight fraction w, and MFR of each resin are as described above. In the method for producing a polypropylene resin of this embodiment, it is particularly preferable to use one in which the difference between MFR _A and MFR _B , MFR _A -MFR _B , is 1.5 g/10 min or more.

The method of mixing the resins used in the manufacturing method of this embodiment is not particularly limited, but examples include a method of dry blending the polymer powder or pellets of the base resin and the blend resin using a mixer or the like, and a method of feeding the polymer powder or pellets of the base resin and the blend resin to a kneader and melt-kneading them to obtain a kneaded product.

The mixer and the kneader are not particularly limited. The kneader may be a single screw type, a twin screw type, or a multi-screw type with more than one screw. In the case of a twin or more screw type, the kneader may be either a co-rotating or counter-rotating type.

In the case of melt kneading, the kneading temperature is not particularly limited as long as a good kneaded product is obtained. In general, the temperature is in the range of 200°C to 300°C, and from the viewpoint of suppressing deterioration of the resin, 230°C to 270°C is preferable. In addition, in order to suppress deterioration during kneading and mixing of the resin, an inert gas such as nitrogen may be purged into the kneader. The melt kneaded resin may be pelletized to an appropriate size using a commonly known granulator. In this way, mixed polypropylene raw material resin pellets can be obtained.

The total ash content due to polymerization catalyst residues and the like contained in the polypropylene raw material resin is preferably as low as possible in order to improve electrical properties. The total ash content is preferably 50 ppm or less, more preferably 40 ppm or less, and particularly preferably 30 ppm or less, based on 100 parts by weight of polypropylene resin.

The polypropylene resin may contain an additive. The "additive" is an additive generally used in polypropylene resins, and is not particularly limited as long as a biaxially oriented polypropylene film can be obtained. Examples of the additive include an antioxidant, a chlorine absorber, an ultraviolet absorber, a lubricant, a plasticizer, a flame retardant, and an antistatic agent. The polypropylene resin may contain the additive in an amount that does not adversely affect the biaxially oriented polypropylene film.

In the method for producing a biaxially stretched polypropylene film of this embodiment, first, polypropylene resin pellets, dry-mixed polypropylene resin pellets, or mixed polypropylene resin pellets prepared in advance by melt kneading are fed into an extruder and heated to melt.

The polypropylene resin composition is melted at 225°C or higher and 270°C or lower. Specifically, the extruder temperature setting when the polypropylene resin composition is heated and melted is 225°C or higher and 270°C or lower. As a result, assuming that the specific polypropylene resin composition is used, a sea-island phase separation structure is formed at the time of casting the raw sheet, as described below, and ultimately, a biaxially oriented polypropylene film with excellent dielectric breakdown strength at high temperatures is obtained.

The polypropylene resin composition is melted at a shear rate of 2000 s ^-1 to 15000 s ^-1 at a temperature of 225°C to 270°C. As a result, on the premise that the specific polypropylene resin composition is used, a sea-island phase separation structure is formed at the time of casting the raw sheet as described below, and finally, a biaxially stretched polypropylene film excellent in dielectric breakdown strength at high temperatures is obtained. If the shear rate is lower than 2000 s ^-1 , the extrusion rate is not constant, and the shape and dimensions of the raw sheet become irregular or fluctuate regularly, which makes the raw sheet more likely to break during transportation or stretching.

Furthermore, if the shear rate exceeds 15,000 s ^-1 , unmelted material is extruded in the extruder due to a phenomenon called break-up, making it impossible to obtain a uniform raw sheet, which makes it more likely to break during stretching, or excessive heat is generated when passing through the tip clearance, causing significant deterioration of the polypropylene resin composition, resulting in a decrease in the dielectric breakdown strength of the film obtained by stretching, even if a uniform raw sheet is obtained. The shear rate can be adjusted by the cylinder diameter and screw rotation speed of the extruder, and the groove depth of the screw.

The shear rate may be 2000 s ^-1 or more and 15000 s ^-1 or less, preferably 2000 s ^-1 or more and 10000 s ^-1 or less, more preferably 2000 s ^-1 or more and 2300 s ^-1 or less. By having the shear rate within this range, a biaxially oriented polypropylene film having a weight fraction w of 2.6% or more and 4.2% or less can be easily obtained, and in combination with other requirements, a film capacitor using the biaxially oriented polypropylene film as a capacitor dielectric can obtain excellent thermal shock resistance in that thermal compaction (deformation) of the capacitor is suppressed during repeated use between high and low temperatures.

Next, the molten resin composition is extruded into a sheet using a T-die, and cooled and solidified on at least one metal drum to form an unstretched cast raw sheet. The surface temperature of the metal drum (the temperature of the metal drum that first comes into contact after extrusion) is preferably 50°C or higher and 105°C or lower, and more preferably 60°C or higher and 100°C or lower. The surface temperature of the metal drum can be determined according to the physical properties of the polypropylene resin used. If the surface temperature of the metal drum is significantly lower than 50°C, it is difficult to obtain good sheet formability of the raw sheet, and it is difficult to obtain a good biaxially stretched polypropylene film without uneven stretching or breakage during stretching film formation.

The thickness of the cast raw sheet is not particularly limited as long as a biaxially oriented polypropylene film can be obtained, but is usually preferably 0.05 mm or more and 2 mm or less, and more preferably 0.1 mm or more and 1 mm or less.

The biaxially stretched polypropylene film can be produced by subjecting the polypropylene cast raw sheet to a stretching process. The stretching is preferably biaxial stretching in which the sheet is oriented biaxially in the longitudinal and transverse directions, and the stretching method is preferably a sequential biaxial stretching method. In the sequential biaxial stretching method, for example, the cast raw sheet is first kept at a temperature of 110°C to 170°C (preferably 135°C to 170°C) and stretched in the flow direction by passing it between rolls with a speed difference. The stretching ratio in the flow direction is preferably 3.5 times to 5.5 times, more preferably 4.2 times to 5.4 times. The sheet is then guided to a tenter and stretched in the transverse direction. The temperature during transverse stretching is preferably 150°C to 165°C, and the stretching ratio in the transverse direction is preferably 9 times to 11 times. Thereafter, a relaxation process, a heat setting process, and a cooling process are performed to 2 times to 10 times. As a result of the above, a biaxially stretched polypropylene film is obtained.

In the production of the biaxially stretched polypropylene film of the present disclosure, in order to suitably set the sub-peak fusion heat fraction and the fusion heat fraction below 150°C within the above-mentioned specific range, it is particularly desirable to appropriately control the transverse stretching temperature, heat setting temperature, and cooling temperature. More specifically, the transverse stretching temperature is preferably controlled to a range of 150°C or more and 165°C or less, and more preferably to a range of 150°C or more and 160°C or less. The heat setting temperature is preferably controlled to a range of 130°C or more and 160°C or less, and more preferably to a range of 130°C or more and 140°C or less. The cooling temperature is preferably controlled to a range of 80°C or more and 130°C or less, and more preferably to a range of 90°C or more and 100°C or less. The difference between the transverse stretching temperature and the heat setting temperature is preferably controlled to a range of 5°C or more and 35°C or less. The difference between the heat setting temperature and the cooling temperature is preferably controlled to a range of 30°C or more and 50°C or less, and more preferably to a range of 30°C or more and 40°C or less. The reason for this is as follows. That is, in the stretching process, the molecular chains aligned along the machine direction in the longitudinal stretching process are regularly rearranged perpendicular to the machine direction while undergoing crystal collapse in the transverse stretching process. In the subsequent heat setting and cooling processes, the rearranged molecular chains are recrystallized, and at the same time, the non-crystalline portion is relaxed. The transverse stretching process, heat setting and cooling processes are processes in which crystals are refined and recrystallized, and the crystal structure can be controlled by controlling the temperature of the process within the above-mentioned temperature range. If the heat setting temperature in the heat setting process of the stretching process exceeds 160°C, the polypropylene crystals become coarse, and in addition to the decrease in voltage resistance, the element winding suitability is likely to decrease. On the other hand, if the heat setting temperature is less than 130°C, breakage occurs frequently and stretching becomes difficult. Furthermore, if the difference between the transverse stretching temperature and the heat setting temperature in the stretching process is 35°C or more, stretching unevenness, which is a very localized unstretched portion, is likely to occur. If the difference between the heat setting temperature and the cooling temperature is 50°C or more, breakage occurs frequently and stretching becomes difficult. For these reasons, in the production of the biaxially stretched polypropylene film of this embodiment, it is particularly preferable to set the heat setting temperature, stretching temperature, cooling temperature, and the difference between these temperatures within the above ranges.

The thickness of the biaxially oriented polypropylene film is preferably 0.8 μm or more and 6.0 μm or less, as described above, from the viewpoint of ensuring the miniaturization and high capacity of the capacitor when used in a capacitor. Specifically, 5.5 μm or less is preferable, 3.5 μm or less is more preferable, 3.0 μm or less is even more preferable, and 2.4 μm or less is particularly preferable. Furthermore, from the viewpoint of manufacturing, the thickness of the biaxially oriented polypropylene film is preferably 1.0 μm or more, more preferably 1.8 μm or more, and even more preferably 2.2 μm or more.

In order to improve the adhesive properties of the biaxially oriented polypropylene film in a post-process such as a metal deposition process, a corona discharge treatment may be performed online or offline after the stretching and heat setting processes are completed. The corona discharge treatment can be performed using a known method. It is preferable to use air, carbon dioxide gas, nitrogen gas, or a mixture of these gases as the atmospheric gas.

The biaxially oriented polypropylene film of this embodiment obtained in this manner has excellent dielectric breakdown strength (dielectric breakdown strength) when a voltage is applied at a high temperature of about 120°C (100°C to 120°C) even when the film thickness is as thin as 6.0 μm or less, and also has excellent stretchability and element processability. In addition, the film capacitor of this embodiment using the biaxially oriented polypropylene film as a capacitor derivative has excellent heat resistance at a high temperature of about 120°C (100°C to 120°C), specifically, it has excellent life performance in that the decrease in the capacitance of the capacitor is suppressed even when used for a long time at the above high temperature, and also has excellent thermal shock resistance in that the thermal tightening (deformation) of the capacitor is suppressed when used repeatedly between the above high and low temperatures assumed in the engine room. Therefore, the biaxially oriented polypropylene film of this embodiment is suitable for film capacitor applications, and can be preferably used as a derivative of a capacitor that constitutes an inverter in a hybrid vehicle or electric vehicle.

Metal Layer-Integrated Polypropylene Film Capacitor and Its Manufacturing Method The biaxially oriented polypropylene film of this embodiment may be a metal layer-integrated polypropylene film having a biaxially oriented polypropylene film and a metal layer laminated on one or both sides of the biaxially oriented polypropylene film, taking into consideration processing into a capacitor.

The metal layer functions as an electrode. Examples of metals that can be used for the metal layer include simple metals such as zinc, lead, silver, chromium, aluminum, copper, and nickel, mixtures of multiple types of these, and alloys of these. However, taking into consideration the environment, economy, and capacitor performance, zinc and aluminum are preferred.

Examples of methods for laminating a metal layer on one or both sides of a biaxially oriented polypropylene film include vacuum deposition and sputtering. From the viewpoints of productivity and economy, vacuum deposition is preferred. Examples of vacuum deposition methods include the crucible method and the wire method, but there are no particular limitations and the most suitable method can be selected as appropriate.

The margin pattern used when laminating the metal layer by vapor deposition is not particularly limited, but in order to improve the safety and other properties of the capacitor, it is preferable to apply a pattern including a so-called special margin, such as a fishnet pattern or T-margin pattern, to one side of the biaxially oriented polypropylene film. This increases safety and is also effective in preventing capacitor destruction and short circuits.

The method for forming the margin can be any commonly known method, such as the tape method or oil method, without any restrictions.

The metal layer-integrated polypropylene film of this embodiment can be laminated or rolled by conventional methods to form a film capacitor.

That is, the film capacitor of this embodiment may have a configuration in which multiple metal layer-integrated polypropylene films are laminated, or may have a wound metal layer-integrated polypropylene film. Such a film capacitor can be suitably used as a capacitor for inverter power devices that control drive motors in electric vehicles, hybrid vehicles, etc. In addition, it can also be suitably used in railroad cars, wind power generation, solar power generation, general home appliances, etc.

The present invention will be described in detail below with reference to examples and comparative examples. However, the present invention is not limited to the examples. Unless otherwise specified, parts and percentages refer to "parts by mass" and "% by mass", respectively.

<Resin>
Details of the resins (PP resin A1 and PP resin B1) used in the examples and comparative examples are summarized in Table 1 below, and the measurement methods for each physical property are also described.

PP resin A1: manufactured by Prime Polymer Co., Ltd. PP resin B1: manufactured by Daehan Petrochemical Co., Ltd., S802M Type A

<Measurements of number average molecular weight (Mn), weight average molecular weight (Mw), z-average molecular weight (Mz), molecular weight distribution (Mw/Mn), molecular weight distribution (Mz/Mn), and weight fraction w of polypropylene resin>
First, the average molecular weight and molecular weight distribution of each polypropylene resin were measured using SEC (size exclusion chromatography) under the following conditions.
Apparatus: HLC-8321GPC/HT (detector: differential refractometer (RI)) (manufactured by Tosoh Corporation)
Column: TSKgel guard column HHR (30) HT (7.5 mm I.D. x 7.5 cm) x 1 + TSKgel GMHHR-H (20) HT (7.8 mm I.D. x 30 cm) x 3 (manufactured by Tosoh Corporation)
Eluent: 1,2,4-trichlorobenzene (Fujifilm Wako Pure Chemical Industries, Ltd., for GPC) + BHT (0.05%)
Flow rate: 1.0 mL/min Detection conditions: polarity-(-)
Injection volume: 0.3 mL
Column temperature: 140 ° C.
System temperature: 40°C
Sample concentration: 1 mg/mL
Sample pretreatment: The sample was weighed, and a solvent (1,2,4-trichlorobenzene with 0.1% BHT added) was added and the sample was dissolved by shaking at 140° C. for 1 hour. Then, the sample was heated and filtered through a 0.5 μm sintered filter.
Calibration curve: A calibration curve of a quintic approximation curve was prepared using standard polystyrene from Tosoh Corporation. However, the molecular weight was converted to the molecular weight of polypropylene using the Q-factor.

Using the obtained calibration curve and SEC chromatogram, the molecular weight (logarithmic value) was plotted on the horizontal axis and the integral value of the concentration fraction on the vertical axis using the analysis software for the measuring device to obtain an integral molecular weight distribution curve. The differential value (slope of the integral molecular weight distribution curve) of the integral molecular weight distribution curve for each molecular weight was calculated, and the molecular weight (logarithmic value) was plotted on the horizontal axis and the differential value on the vertical axis to obtain a differential molecular weight distribution curve.

From these curves, the number average molecular weight Mn, weight average molecular weight Mw, and Z average molecular weight Mz were obtained. The molecular weight distribution (Mw/Mn) was obtained using the values of Mw and Mn. Furthermore, the value when the logarithmic molecular weight Log(M) = 4.0 in the integrated molecular weight distribution curve was taken as the weight fraction w. This weight fraction w indicates the weight fraction of molecules with a logarithmic molecular weight Log(M) = 4.0, i.e., a molecular weight of 10,000 or less.

<<Measurement of differential distribution value when logarithmic molecular weight Log(M)=4.5, differential distribution value when logarithmic molecular weight Log(M)=6.0, and molecular weight differential distribution value difference (DM)>>
For each polypropylene resin, the differential distribution value when the logarithmic molecular weight Log(M)=4.5 and the differential distribution value when the logarithmic molecular weight Log(M)=6.0 were obtained by the following method. First, the time curve (elution curve) of the intensity distribution detected by the RI detector was converted into a distribution curve for the molecular weight M (Log(M)) of the standard polystyrene using a calibration curve prepared using the above-mentioned standard polystyrene. Next, an integral distribution curve for Log(M) was obtained when the total area of the distribution curve was 100%, and then the integral distribution curve was differentiated by Log(M) to obtain a differential distribution curve for Log(M). From this differential distribution curve, the differential distribution values when Log(M)=4.5 and Log(M)=6.0 were read. The difference between the differential distribution value when Log(M)=4.5 and the differential distribution value when Log(M)=6.0 was taken as the molecular weight differential distribution value difference (DM). The series of operations up to obtaining the differential distribution curve was carried out using analysis software built into the GPC measuring device used.

<Measurement of heptane insolubles (HI)>
Each polypropylene resin was press-molded to 10 mm x 35 mm x 0.3 mm to prepare a measurement sample of about 3 g. Next, about 150 mL of heptane was added and Soxhlet extraction was performed for 8 hours. The heptane insoluble matter was calculated from the sample mass before and after extraction.

<Melt flow rate (MFR) measurement>
The melt flow rate (MFR) of the raw resin pellets used in the examples and comparative examples was measured in accordance with JIS K 7210 condition M using a melt indexer manufactured by Toyo Seiki Co., Ltd. Specifically, a sample weighed to 4 g was inserted into a cylinder at a test temperature of 230° C. and preheated for 3.5 minutes under a load of 2.16 kg. The weight of the sample extruded from the bottom hole in 30 seconds was then measured to determine the MFR (unit: g/10 min or g/10 min). The above measurement was repeated three times, and the average value was taken as the measured MFR.

[Examples 1 to 6 and Comparative Examples 1 to 5]
[Production of biaxially oriented polypropylene film and evaluation of its properties]
Polypropylene resin A1 (75 parts by mass) and polypropylene resin B1 (25 parts by mass) were mixed to obtain a dry blend resin composition. Next, the dry blend resin composition was fed to an extruder and melted at a melting temperature of 250 ° C. and a shear rate of 2000 s ^-1 . This molten resin was extruded using a T-die, and solidified by wrapping it around a metal drum whose surface temperature was maintained at 95 ° C. to produce a cast raw sheet. The unstretched cast raw sheet was kept at a temperature of 140 ° C., passed between rolls with a speed difference, stretched 4.5 times in the flow direction, and immediately cooled to room temperature. Subsequently, the stretched film obtained by stretching in the flow direction was introduced into a tenter, and each was stretched 10 times in the width direction at the transverse stretching temperature shown in Table 2, and then relaxed at a relaxation rate of 12%. Subsequently, each was heat-set at the heat-setting temperature shown in Table 2, and then cooled at the cooling temperature shown in Table 2 to obtain a biaxially stretched polypropylene film having a width of about 5 m and a thickness of 2.3 μm. The obtained biaxially oriented polypropylene film was wound around an iron core having a diameter of 400 mm to form a jumbo roll of about 80,000 m. The wound biaxially oriented polypropylene film was subjected to an aging treatment in an atmosphere of 35° C. for 24 hours.

The thickness, differential scanning calorimetry, dielectric breakdown strength measurement, stretchability evaluation, and device processability evaluation methods for the biaxially oriented polypropylene films obtained in each Example and Comparative Example are shown below. The results of each measurement and evaluation are shown in Table 2 or Table 3.

<Extensibility evaluation>
The production of the film was started using a biaxial stretching device set to a predetermined thickness (2.3 μm), and the time during which the film could be continuously produced from the time when the obtained film thickness reached the target thickness ±2% until the film broke or the like (hereinafter, also referred to as "continuous film production time") was measured. The time when the thickness reached the target thickness ±2% was confirmed by cutting out the film and measuring the film thickness using a micrometer (JIS-B7502) in accordance with JIS-C2330. Based on the obtained continuous film production time, the stretchability was evaluated according to the following evaluation criteria.
(Evaluation Criteria for Stretchability)
◯: Film formation was possible without breakage due to stretching even after 8 hours.
x: Stretching broke within 8 hours.

<Measurement of polypropylene film thickness>
A paper thickness measuring instrument MEI-11 (measurement pressure 100 kPa, drop speed 3 mm/sec, measurement terminal φ=16 mm, measurement force 20.1 N) manufactured by Citizen Seimitsu Co., Ltd. was used in an environment of temperature 23±2°C and humidity 50±5% RH. The sample was cut from the roll with 10 or more sheets stacked, and was handled so as not to wrinkle or trap air in the film when cutting. Five measurements were taken for a 10-sheet stack sample, and the thickness was calculated by dividing the average of the five measurements by 10.

<Differential Scanning Calorimetry (DSC)>
The evaluation of the melting peak of the biaxially stretched polypropylene film and the partial heat of melting fraction relative to the total heat of melting (respectively, the ratio of the total heat of melting of the sub-peaks on the lower temperature side of the melting peak to the total heat of melting of the biaxially stretched polypropylene film (100%), and the ratio of the heat of melting at 150°C or less to the total heat of melting (100%) (heat of melting fraction at 150°C or less)) was calculated using a Perkin-Elmer Diamond DSC, an input compensation type DSC, according to the following procedure. First, about 3 mg of the polypropylene film was weighed out, packed in an aluminum sample holder, and set in the DSC device. The temperature was raised from 0°C to 260°C at a rate of 30°C/min under a nitrogen flow, and the melting curve was measured. As a result of the DSC measurement, at least two melting peaks were obtained between 100°C and 190°C.

The total heat of fusion was calculated from the total area between the baseline and the melting curve (integrated endotherm with respect to temperature). Furthermore, the heat of fusion fraction of the subpeak was calculated from the area between the low-temperature peak curve and the baseline from the boundary line (integrated endotherm up to the boundary line) by setting a boundary line (the valley of the melting curve) between the main peak and the subpeak on the low-temperature side adjacent to the main peak, as shown in Figure 1. The fraction of the partial heat of fusion of the low-temperature melting peak relative to the total heat of fusion was defined as the heat of fusion fraction of the subpeak, and was evaluated as a percentage (%). The fraction of the heat of fusion below 150°C was calculated as the proportion of the heat of fusion in the region below 150°C of the total heat of fusion, as shown in Figure 2.

<Tensile strength and elongation at break>
In a measurement environment of 25°C, the tensile breaking strength (tensile breaking stress) and tensile breaking elongation in the MD and TD directions of the biaxially stretched polypropylene film were measured by the following procedure. In accordance with JIS K 7127:1999, a rectangular sample having a length of 150 mm and a width of 15 mm was cut out from the biaxially stretched polypropylene film. At this time, for the sample for measuring the breaking strength and breaking elongation in the MD direction, the sample was cut out so that the MD direction was the length direction, and for the sample for measuring the tensile breaking strength and tensile breaking elongation in the TD direction, the sample was cut out so that the TD direction was the length direction. Next, the sample was set in the chuck of an oven-equipped tensile tester (Tensilon universal testing machine RTG-1210 manufactured by A&D Co., Ltd.) with a chuck distance of 100 mm. Next, the sample was subjected to a tensile test at a test speed of 200 mm/min. Next, the tensile breaking strength and tensile breaking elongation were determined by automatic analysis using data processing software built into the testing machine.

<Heat shrinkage rate HS ( 120 °C)>
The biaxially oriented polypropylene film was cut into a rectangle having a width of 20 mm and a length of 130 mm to prepare a measurement sample. At this time, for the sample for measuring the heat shrinkage rate in the MD direction, the sample was cut out so that the length direction of the sample coincided with the MD direction. For the sample for measuring the heat shrinkage rate in the TD direction, the sample was cut out so that the length direction of the sample coincided with the TD direction. For the sample for measuring the heat shrinkage rate in the 45° direction (middle between the MD direction and the TD direction), the sample was cut out so that the length direction of the sample coincided with the 45° direction. Three measurement samples were prepared for each. Next, a point of 100 mm in length was measured with a ruler, and a mark was marked at that point. Then, the three measurement samples were hung without load in a hot air circulation type thermostatic chamber at 120° C. and held for 15 minutes. The sample was then cooled to room temperature (23°C), the distance between the marked lines was measured with a ruler, and the heat shrinkage in the MD direction HS _MD (%), the heat shrinkage in the TD direction HS _TD (%), and the total heat shrinkage in the 45° direction HS ₄₅ ° (%), HS _MD + HS _TD + HS ₄₅ °, were calculated using the following formula. The results are shown in Table 2.
Heat shrinkage rate HS (%) = [(marked line spacing before heating - marked line spacing after heating) / (marked line spacing before heating)] x 100
The average value of the measurements for three pieces was taken as the heat shrinkage rate (%).
The measurement conditions other than those described here were in accordance with "25. Dimensional change" of JIS C 2151:2019.

<Measurement of dielectric breakdown strength of polypropylene film: direct current (DC)>
The dielectric breakdown voltage (BDV) of the polypropylene films according to the examples and comparative examples was measured 12 times under the following test conditions with the electrode configuration described in JIS C2151 (2006) 17.2.2 (flat electrode method). The applied voltage at the time when the leakage current of the upper limit reference value described below was detected during voltage increase was taken as the BDV. The BDV was divided by the thickness (μm) of the film, and the average value of the 8 points excluding the top 2 points and the bottom 2 points out of the 12 measurement results was taken as the dielectric breakdown strength ES (VDC/μm). The results are shown in Table 3.
Test piece: approx. 150 mm x 150 mm
Conditioning of test piece: 30 minutes under atmospheric conditions Power supply: DC Atmosphere: In air, 80°C, 100°C, 120°C
Tester: Kikusui Electronics Co., Ltd. DC withstand voltage/insulation resistance tester TOS9213AS
Voltage rise rate: 100V/s
Current detection response speed: MID
Upper limit: 5mA

[Production of film capacitors and evaluation of their characteristics]
Using the biaxially stretched polypropylene films obtained in each of the Examples and Comparative Examples, film capacitors were produced according to the following procedure.

A special deposition pattern margin and insulating margin were formed on the biaxially stretched polypropylene film to provide film capacitor safety, and aluminum deposition was performed so that the surface resistivity of the metal film was 20 Ω/□, resulting in a metal layer-integrated polypropylene film. Next, the metal layer-integrated polypropylene film was slit to an arbitrary width, and two metal layer-integrated polypropylene films were combined and wound using a Kaito Seisakusho automatic winder 3KAW-N2 model at a winding speed of 4 m/sec, winding tension of 180 g, and contact roller contact pressure of 260 g, with the number of turns set so that the element capacitance was 50 μF.

The wound element was flattened by pressing, and then, with the press load still applied, zinc metal was sprayed onto the end faces of the element to form electrode outlets, and the element was then heat-treated at 120°C for 15 hours to thermally harden it.

After thermal curing, leads were soldered to the end faces of the elements and sealed with epoxy resin to obtain flat film capacitors. The capacitance of the resulting film capacitors was all 50 μF (± 3 μF).

The method for evaluating the element winding processability during the manufacturing process of the film capacitors obtained in each Example and Comparative Example is shown below. The evaluation results are shown in Table 3.

<Evaluation of element winding processability>
Of the small rolls obtained by deposition and slitting, two sheets were overlapped and wound using a winding reel with a left margin and a winding reel with a right margin so that the deposition portion protruded from the margin in the width direction (element winding process). The winding was performed using an automatic winding machine 3KAW-N2 type manufactured by Kaito Seisakusho Co., Ltd., with a winding tension of 200 g and 1360 turns. At that time, the winding was visually observed from the beginning to the end of the winding, and those that had wrinkles or shifts were rejected, and the ratio of the number of rejected ones to the total number of manufactured pieces was expressed as a percentage and used as an index of processability (hereinafter referred to as element winding yield). The higher the element winding yield, the better. 95% or more was evaluated as good "○", and less than 95% was evaluated as bad "×". The results are shown in Table 3.

Claims (8)

A biaxially oriented polypropylene film for a capacitor,
When the biaxially oriented polypropylene film for a capacitor was subjected to differential scanning calorimetry (DSC) at a heating rate of 30° C./min,
At least two melting peaks are observed,
The ratio of the total heat of fusion of the sub-peaks on the lower temperature side of the main peak of the fusion peak to the total heat of fusion (100%) is in the range of 60% to 70%;
The ratio of the heat of fusion at 150° C. or less to the total heat of fusion (100%) is in the range of 12% to 20%.
Biaxially oriented polypropylene film for capacitors. Measured by tensile testing in a 25°C environment.
The tensile breaking strength in the MD direction is 140 MPa or more and 170 MPa or less,
The tensile elongation at break in the MD direction is 130% or more and 160% or less,
The tensile breaking strength in the TD direction is 330 MPa or more and 370 MPa or less,
The biaxially oriented polypropylene film for capacitors according to claim 1, having a tensile elongation at break in the TD direction of 50% or more and 70% or less. The biaxially oriented polypropylene film for capacitors according to claim 1 or 2, in which the sum of the heat shrinkage rate in the MD direction, the heat shrinkage rate in the TD direction, and the heat shrinkage rate in the 45° direction is 10% or less when the biaxially oriented polypropylene film for capacitors is heat-treated at 120°C for 15 minutes. The biaxially oriented polypropylene film for capacitors according to any one of claims 1 to 3, wherein the polypropylene resin constituting the biaxially oriented polypropylene film for capacitors has a Z-average molecular weight Mz of 650,000 or more and 945,000 or less. The biaxially oriented polypropylene film for capacitors according to any one of claims 1 to 4, having a thickness of 0.8 μm or more and 6 μm or less. The biaxially oriented polypropylene film for a capacitor according to any one of claims 1 to 5,
A metal layer-integrated polypropylene film for capacitors, comprising a metal layer laminated on one or both sides of the biaxially oriented polypropylene film for capacitors. A film capacitor having a wound metal layer-integrated polypropylene film according to claim 6, or having a structure in which multiple metal layer-integrated capacitor polypropylene films according to claim 6 are laminated. A film roll in which the biaxially oriented polypropylene film according to any one of claims 1 to 5 is wound into a roll.