Biaxially oriented polypropylene film

Abstract

The present invention provides a polypropylene film with which it is possible to obtain a film capacitor having better dielectric strength at high temperatures, and with which problems such as wrinkling and winding deviation in a capacitor winding process are reduced. This biaxially oriented polypropylene film has a first surface and a second surface, wherein at least the first surface has a density of peaks Spd of 2000 / mm2-8000 / mm2 inclusive and a developed interfacial area ratio Sdr of 0.001%-0.009% inclusive.

Description

Biaxially oriented polypropylene film

[0001] The present invention relates to a biaxially oriented polypropylene film, a metal layer-integrated polypropylene film, a film capacitor, a film roll, etc.

[0002] Polypropylene films have excellent electrical properties, such as high voltage resistance and low dielectric loss, as well as high moisture resistance. Therefore, they are widely used in electronic and electrical devices. Specifically, they are used as films for high-voltage capacitors, various switching power supplies, filter capacitors (e.g., converters, inverters, etc.), smoothing capacitors, etc.

[0003] In particular, in recent years, polypropylene films have begun to be widely used as capacitors for inverter power supplies that control drive motors of electric vehicles, hybrid vehicles, etc. Capacitors for inverter power supplies used in automobiles, etc., are required to be small, lightweight, high-capacity, and highly reliable over a long period of time.

[0004] Japanese Patent Application Laid-Open No. 2023-82646

[0005] From the viewpoint of the long-term reliability of the capacitors described above, capacitors that are used under high temperatures, such as in automobiles, are required to have voltage resistance at high temperatures.

[0006] Polypropylene films used in capacitors and other devices are turned into capacitor elements through processes such as element winding. In each of these processes, very thin and long polypropylene films are prone to wrinkles and misalignment during transportation and winding.

[0007] An object of the present invention is to provide a polypropylene film that can provide a film capacitor with excellent voltage resistance at high temperatures and that suppresses problems such as wrinkling and misalignment during the element winding process.

[0008] Patent Document 1 discloses a biaxially oriented polypropylene film in which the developed surface area ratio Sdr of the interface is adjusted to a certain range, and describes that adjusting the Sdr can improve voltage resistance, safety, and the like.

[0009] The inventors focused on adjusting not only the developed surface area ratio Sdr but also the peak density Spd. They found that these two roughness parameters are not correlated, and that the above-mentioned problems can be solved by adjusting both of these roughness parameters to fall within a predetermined range. That is, the present invention encompasses the following aspects.

[0010] Item 1. A biaxially oriented polypropylene film having a first surface and a second surface, wherein the peak density Spd on at least the first surface is 2000 / mm ２ More than 8000 / mm ２ and a developed area ratio Sdr of the interface is 0.001% or more and 0.009% or less.

[0011] Item 2. The biaxially stretched polypropylene film according to Item 1, wherein the bias Ssk on the first surface is −1.5 or more and 1.5 or less.

[0012] Item 3. The biaxially oriented polypropylene film according to Item 1 or 2, wherein the first surface has a kurtosis Sku of 0 or more and 100 or less.

[0013] Item 4. The biaxially stretched polypropylene film according to any one of Items 1 to 3, wherein the ratio of the quincunx region height S5p to the ten-point region height S10z on the first surface is 40% or more and 60% or less.

[0014] Item 5. The biaxially stretched polypropylene film according to any one of Items 1 to 4, wherein the ratio of the five-point valley region depth S5v to the ten-point region height S10z on the first surface is 40% or more and 60% or less.

[0015] Item 6. The biaxially oriented polypropylene film according to any one of Items 1 to 5, comprising a linear polypropylene resin A and a linear polypropylene resin B having different melt flow rates at 230°C.

[0016] Item 7. The biaxially oriented polypropylene film according to Item 6, wherein the linear polypropylene resin A and the linear polypropylene resin B have a number average molecular weight Mn of 34,000 or more and a molecular weight distribution Mw / Mn of 9.3 or less.

[0017] Item 8. The biaxially oriented polypropylene film according to Item 6 or 7, wherein the linear polypropylene resin A and the linear polypropylene resin B have a heptane insoluble content (HI) of 97.5% or more.

[0018] Item 9. The biaxially oriented polypropylene film according to any one of Items 6 to 8, containing long-chain branched polypropylene resin C.

[0019] Item 10. The biaxially oriented polypropylene film according to any one of Items 1 to 9, having a thickness of 1.4 to 6.0 μm.

[0020] Item 11. The biaxially oriented polypropylene film according to any one of Items 1 to 10, which is for use in a capacitor.

[0021] Item 12. A metal layer-integrated polypropylene film comprising the biaxially oriented polypropylene film according to any one of items 1 to 11, and a metal layer laminated on one or both sides of the biaxially oriented polypropylene film.

[0022] Item 13. A film capacitor comprising the metal layer-integrated polypropylene film according to Item 12.

[0023] Item 14. A film roll obtained by winding the biaxially oriented polypropylene film according to any one of items 1 to 11 into a roll.

[0024] According to the present invention, a film capacitor having excellent voltage resistance at high temperatures can be obtained, and a polypropylene film can be provided in which problems of wrinkling and misalignment during the element winding process are suppressed.

[0025] In this specification, the expressions "contain" and "comprise" include the concepts of "contain," "comprise," "consist essentially of," and "consist only of."

[0026] In this specification, the term "to" in a numerical range means "greater than or equal to" or "less than or equal to." That is, the expression "α to β" means "greater than or equal to α and less than or equal to β," or "greater than or equal to β and less than or equal to α," and includes both α and β as a range.

[0027] In this specification, the terms "element," "capacitor," "capacitor element," and "film capacitor" mean the same thing.

[0028] The polypropylene film of the present invention is not a microporous film and does not have a large number of pores.

[0029] The polypropylene film of the present invention may be composed of two or more layers, but is preferably composed of a single layer.

[0030] The polypropylene film of the present invention is a biaxially oriented polypropylene film having a first surface and a second surface, and has a peak density Spd of 2000 / mm on at least the first surface. ２ More than 8000 / mm ２ and the developed area ratio Sdr of the interface is 0.001% or more and 0.009% or less.

[0031] The peak density (Spd) represents the number of peaks per unit area, and a higher peak density indicates a higher number of contact points with other objects. Spd tends to increase as the number of coarse protrusions decreases. The developed interface area ratio (Sdr) represents the degree to which the developed area (surface area) of a defined region increases relative to the area of ​​the defined region. Sdr tends to decrease as the number of elliptical β-crystal roughening marks decreases. By appropriately controlling the coarse protrusions and elliptical β-crystal roughening marks, the interlayer distance and air volume can be uniformly controlled when the front and back surfaces of the film are superimposed. This improves the voltage resistance at high temperatures when used as a capacitor and extends the capacitor's lifespan. Furthermore, by appropriately controlling the coarse protrusions and elliptical β-crystal roughening marks, an appropriate dynamic friction force is generated, thereby suppressing problems such as wrinkles and misalignment during the element winding process.

[0032] Spd is preferably 3000 / mm ２ More than 8000 / mm２ or less, more preferably 3500 / mm ２ More than 8000 / mm ２ More preferably, 3500 / mm or less ２ More than 7000 / mm ２ More preferably, 3500 / mm or less ２ More than 6000 / mm ２ Below 3500 / mm, particularly preferably ２ More than 5000 / mm ２ or less, particularly preferably 4000 / mm ２ More than 5000 / mm ２ Particularly preferably 4500 / mm ２ More than 5000 / mm ２ The following is the result.

[0033] Sdr is preferably 0.001% or more and 0.008% or less, more preferably 0.001% or more and 0.007% or less, even more preferably 0.001% or more and 0.006% or less, still more preferably 0.001% or more and 0.005% or less, particularly preferably 0.001% or more and 0.004% or less, especially preferably 0.001% or more and 0.003% or less, and particularly preferably 0.0015% or more and 0.0025% or less.

[0034] In a preferred embodiment of the present invention, the first surface is preferably a smoother surface (smooth surface). Specifically, the smooth surface is a surface having a smaller root mean square height Sq. Typically, the smooth surface is the surface that does not come into contact with the metal drum immediately after extrusion from the die.

[0035] In order to more effectively exhibit the effects of the present invention, the polypropylene film of the present invention preferably has a bias Ssk of −1.5 or more and 1.5 or less on the first surface. Ssk is preferably −1.0 or more and 1.5 or less, more preferably −0.5 or more and 1.5 or less, even more preferably −0.5 or more and 1.0 or less, still more preferably 0.0 or more and 1.0 or less, and particularly preferably 0.0 or more and 0.8 or less.

[0036] From the viewpoint of more effectively exhibiting the effects of the present invention, the polypropylene film of the present invention preferably has a kurtosis Sku of 0 or more and 100 or less on the first surface. Sku is preferably 0 or more and 80 or less, more preferably 0 or more and 60 or less, even more preferably 0 or more and 40 or less, still more preferably 0 or more and 20 or less, particularly preferably 0 or more and 10 or less, especially more preferably 0 or more and 8 or less, especially more preferably 1 or more and 8 or less, and particularly preferably 2 or more and 6 or less.

[0037] In order to more effectively exhibit the effects of the present invention, the polypropylene film of the present invention preferably has, on the first side, a ratio of the quincunx region height S5p to the ten-point region height S10z of 40% or more and 60% or less, preferably 40% or more and 55% or less, more preferably 44% or more and 55% or less, and even more preferably 44% or more and 52% or less.

[0038] In order to more effectively exhibit the effects of the present invention, the polypropylene film of the present invention preferably has, on the first surface, a ratio of the five-point valley region depth S5v to the ten-point region height S10z of 40% or more and 60% or less, preferably 45% or more and 60% or less, more preferably 48% or more and 60% or less, and even more preferably 48% or more and 56% or less.

[0039] In one embodiment, the polypropylene film of the present invention has a quincunx region height S5p of, for example, 0.06 μm or more and 0.19 μm or less, preferably 0.06 μm or more and 0.14 μm or less, and more preferably 0.06 μm or more and 0.12 μm or less.

[0040] In one embodiment, the polypropylene film of the present invention has a five-point valley region depth S5v of, for example, 0.06 μm or more and 0.16 μm or less, preferably 0.06 μm or more and 0.14 μm or less, and more preferably 0.06 μm or more and 0.13 μm or less.

[0041] In one embodiment, the polypropylene film of the present invention has a ten-point region height S10z of, for example, 0.12 μm or more and 0.35 μm or less, preferably 0.12 μm or more and 0.28 μm or less, and more preferably 0.12 μm or more and 0.25 μm or less.

[0042] Spd, Sdr, Ssk, Sku, S5p, S5v, S10z, and Sq are all surface roughness parameters defined in ISO 25178, and are values ​​measured according to the method in (4-2) of the Examples described below.

[0043] The polypropylene film of the present invention preferably has a thickness of 9.5 μm or less, more preferably 6.0 μm or less, even more preferably 3.0 μm or less, even more preferably 2.9 μm or less, particularly preferably 2.8 μm or less, and particularly preferably 2.5 μm or less. The thickness of the polypropylene film of the present invention is preferably 0.8 μm or more, more preferably 1.0 μm or more, even more preferably 1.4 μm or more, even more preferably 1.5 μm or more, and particularly preferably 1.8 μm or more. A thickness within the range of 1.4 to 6.0 μm, 1.5 to 3.0 μm, or 1.5 to 2.9 μm, etc., is particularly preferred because, despite being very thin, the polypropylene film exhibits excellent processability in the slitting process, suppression of blocking during the vapor deposition process, and processability for winding elements.

[0044] When the thickness is 9.5 μm or less, the capacitance can be increased, making it suitable for use in a capacitor. From the viewpoint of manufacturing, the thickness can be set to 0.8 μm or more.

[0045] The thickness is a value measured according to the method of (4-1) in the Examples described later.

[0046] The polypropylene film and metal layer-integrated polypropylene film of the present invention are each wound into a roll, preferably in the form of a film roll. The film roll may or may not have a winding core. The film roll preferably has a winding core. The material of the winding core of the film roll is not particularly limited. Examples of the material include paper (paper tube), resin, fiber-reinforced plastic (FRP), metal, etc. Examples of the resin include polyvinyl chloride, polyethylene, polypropylene, phenolic resin, epoxy resin, acrylonitrile-butadiene-styrene copolymer, etc. Examples of plastics that constitute the fiber-reinforced plastic include polyester resin, epoxy resin, vinyl ester resin, phenolic resin, thermoplastic resin, etc. Examples of fibers that constitute the fiber-reinforced plastic include glass fiber, aramid fiber (Kevlar® fiber), carbon fiber, polyparaphenylenebenzoxazole fiber (Zylon® fiber), polyethylene fiber, boron fiber, etc. Examples of the metal include iron, aluminum, stainless steel, etc. The winding core of the film roll also includes a winding core formed by impregnating a paper tube with the resin. In this case, the material of the winding core is classified as a resin.

[0047] The polypropylene film of the present invention contains a polypropylene resin as a main component. In this specification, "containing a polypropylene resin as a main component" means that the polypropylene resin is contained in an amount of 50% by mass or more relative to the entire polypropylene film (when the entire polypropylene film is taken as 100% by mass). The content of the polypropylene resin relative to the entire polypropylene film is preferably 75% by mass or more, more preferably 90% by mass or more. The upper limit of the polypropylene resin content relative to the entire polypropylene film is, for example, 100% by mass, 98% by mass, etc.

[0048] The polypropylene resin is not particularly limited, and one type may be used alone, or two or more types may be used in combination. Among them, polypropylene resins that form β-type spherulites when made into a cast sheet are preferred.

[0049] Examples of polypropylene resins include linear polypropylene resins. The linear polypropylene resins can be used alone or in combination of two or more. In particular, from the viewpoint of easily adjusting Spd and Sdr to a predetermined range, it is preferable to use linear polypropylene resin A and linear polypropylene resin B having different melt flow rates at 230°C. Linear polypropylene resin A and linear polypropylene resin B are preferably homopolypropylene resins. However, the polypropylene resin in the present invention is not limited to the following resins.

[0050] The weight average molecular weight Mw of the linear polypropylene resin A is preferably 250,000 or more. Furthermore, the weight average molecular weight Mw of the linear polypropylene resin A is preferably 450,000 or less, more preferably 400,000 or less, even more preferably 350,000 or less, and particularly preferably 340,000 or less. When the weight average molecular weight Mw of the linear polypropylene resin A is 250,000 or more and 450,000 or less, the resin fluidity is appropriate. As a result, it is easy to control the thickness of the cast sheet, and it is easy to produce a thin stretched film. Furthermore, this is preferable because it reduces the occurrence of unevenness in the thickness of the cast sheet and stretched film, and appropriate stretchability can be obtained. The weight average molecular weight Mw of the linear polypropylene resin A is preferably 320,000 or more, more preferably 330,000 or more, from the viewpoint of easily adjusting Spd and Sdr within the specified range.

[0051] The number average molecular weight Mn of the linear polypropylene resin A is, for example, 30,000 or more and 52,000 or less. From the viewpoint of facilitating adjustment of Spd and Sdr within predetermined ranges, the number average molecular weight Mn of the linear polypropylene resin A is preferably 34,000 or more, more preferably 36,000 or more, even more preferably 38,000 or more, still more preferably 40,000 or more, particularly preferably 41,000 or more, and is also preferably 50,000 or less, more preferably 48,000 or less, more preferably 46,000 or less.

[0052] The z-average molecular weight Mz of the linear polypropylene resin A is, for example, 750,000 or more and 1,800,000 or less, preferably 900,000 or more and 1,700,000 or less, and more preferably 1,000,000 or more and 1,600,000 or less.

[0053] The molecular weight distribution [(weight average molecular weight Mw) / (number average molecular weight Mn)] of the linear polypropylene resin A is, for example, 5.0 to 11.0. From the viewpoint of facilitating adjustment of Spd and Sdr within predetermined ranges, Mw / Mn is preferably 9.3 or less, more preferably 9.0 or less, even more preferably 8.7 or less, still more preferably 8.5 or less, and particularly preferably 8.3 or less, and is also preferably 6.0 or more, more preferably 6.5 or more, and even more preferably 7.0 or more.

[0054] The molecular weight distribution [(z-average molecular weight Mz) / (number-average molecular weight Mn)] of the linear polypropylene resin A is, for example, 12.0 or more and 60.0 or less. From the viewpoint of facilitating adjustment of Spd and Sdr within predetermined ranges, Mz / Mn is preferably 50 or less, more preferably 40 or less, even more preferably 38 or less, and is preferably 20 or more, more preferably 25 or more.

[0055] When the molecular weight distribution of the linear polypropylene resin A is within the above-mentioned preferred range, unevenness in the thickness of the cast sheet and stretched film is unlikely to occur, and appropriate stretchability is obtained, which is preferable.

[0056] The melt flow rate (MFR) of the linear polypropylene resin A at 230°C is, for example, 2.8 g / 10 min or more and 10.0 g / 10 min or less, preferably 2.8 g / 10 min or more and 6.0 g / 10 min or less. Within this range, the resin has excellent flow characteristics in the molten state, making it less susceptible to unstable flow such as melt fracture, and also suppressing breakage during stretching. Therefore, the resin has good film thickness uniformity, which has the advantage of suppressing the formation of thin-walled portions prone to dielectric breakdown. From the viewpoint of easily adjusting Spd and Sdr within a predetermined range, the MFR is preferably 3.0 g / min or more, more preferably 3.2 g / min or more, even more preferably 3.4 g / min or more, and preferably 5.5 g / min or less, more preferably 5.0 g / min or less, even more preferably 4.5 g / min or less, and even more preferably 4.2 g / min or less.

[0057] The heptane insoluble fraction (HI) of the linear polypropylene resin A is, for example, 96.0% or more and 99.5% or less. Within this range, the moderately high stereoregularity moderately improves the crystallinity of the resin, improving its voltage resistance at high temperatures. On the other hand, the solidification (crystallization) rate during cast sheet molding becomes moderate, resulting in moderate extensibility. From the viewpoint of easily adjusting Spd and Sdr within a predetermined range, HI is preferably 97.0% or more, more preferably 97.5% or more, even more preferably 97.8% or more, and also preferably 99.0% or less, more preferably 98.8% or less.

[0058] The content of linear polypropylene resin A is preferably 55% by mass or more, more preferably 60% by mass or more, and is preferably 90% by mass or less, more preferably 85% by mass or less, even more preferably 80% by mass or less, still more preferably 75% by mass or less, particularly preferably 70% by mass or less, and particularly preferably 65% ​​by mass or less, relative to 100% by mass of the total polypropylene resin in the polypropylene film of the present invention.

[0059] The weight average molecular weight Mw of the linear polypropylene resin B is, for example, 300,000 or more and 400,000 or less. From the viewpoint of facilitating adjustment of Spd and Sdr within predetermined ranges, the weight average molecular weight Mw of the linear polypropylene resin B is preferably 330,000 or more, more preferably 350,000 or more, and even more preferably 370,000 or more.

[0060] The number average molecular weight Mn of the linear polypropylene resin B is, for example, 30,000 or more and 52,000 or less. From the viewpoint of facilitating adjustment of Spd and Sdr within predetermined ranges, the number average molecular weight Mn of the linear polypropylene resin B is preferably 34,000 or more, more preferably 38,000 or more, even more preferably 42,000 or more, still more preferably 44,000 or more, and particularly preferably 45,500 or more, and is also preferably 50,000 or less, more preferably 48,000 or less, and more preferably 47,000 or less.

[0061] The z-average molecular weight Mz of the linear polypropylene resin B is, for example, 900,000 or more and 2,000,000 or less, preferably 1,200,000 or more and 1,800,000 or less, and more preferably 1,500,000 or more and 1,700,000 or less.

[0062] The molecular weight distribution [(weight average molecular weight Mw) / (number average molecular weight Mn)] of the linear polypropylene resin B is, for example, 7.0 to 10.0. From the viewpoint of facilitating adjustment of Spd and Sdr within predetermined ranges, Mw / Mn is preferably 9.3 or less, more preferably 9.0 or less, even more preferably 8.7 or less, still more preferably 8.5 or less, and particularly preferably 8.4 or less, and is also preferably 7.5 or more, more preferably 8.0 or more, and even more preferably 8.2 or more.

[0063] The molecular weight distribution [(z-average molecular weight Mz) / (number-average molecular weight Mn)] of the linear polypropylene resin B is, for example, 20.0 or more and 70.0 or less. From the viewpoint of facilitating adjustment of Spd and Sdr within predetermined ranges, Mz / Mn is preferably 60 or less, more preferably 50 or less, even more preferably 40 or less, and is preferably 25 or more, more preferably 30 or more.

[0064] When the molecular weight distribution of the linear polypropylene resin B is within the above-mentioned preferred range, unevenness in the thickness of the cast sheet and stretched film is unlikely to occur, and appropriate stretchability is obtained, which is preferable.

[0065] The melt flow rate (MFR) of the linear polypropylene resin B at 230° C. is, for example, 0.1 g / 10 min or more and less than 4.0 g / 10 min. From the viewpoint of facilitating adjustment of Spd and Sdr within predetermined ranges, the MFR is preferably 3.5 g / min or less, more preferably 3.0 g / min or less, and even more preferably 2.5 g / min or less, and is also preferably 1.0 g / min or more, more preferably 1.5 g / min or more, and even more preferably 2.0 g / min or more.

[0066] The heptane insoluble matter (HI) of the linear polypropylene resin B is, for example, 97.5% or more and 99.5% or less. From the viewpoint of facilitating adjustment of Spd and Sdr within a predetermined range, HI is preferably 98.0% or more, more preferably 98.5% or more, even more preferably 98.7% or more, and is preferably 99.2% or less, more preferably 99.0% or less.

[0067] When linear polypropylene resin B is used as the polypropylene resin, the content of linear polypropylene resin B is preferably 10% by mass or more, more preferably 15% by mass or more, even more preferably 20% by mass or more, still more preferably 25% by mass or more, particularly preferably 30% by mass or more, relative to 100% by mass of the total polypropylene resin in the polypropylene film of the present invention, and is preferably 45% by mass or less, more preferably 40% by mass or less, and even more preferably 36% by mass or less.

[0068] When linear polypropylene resin A and linear polypropylene resin B are used in combination as the polypropylene resin, the polypropylene resin preferably contains 55 to 90% by weight of linear polypropylene resin A and 45 to 10% by weight of linear polypropylene resin B, when the total polypropylene resin is taken as 100% by mass, more preferably contains 60 to 85% by weight of linear polypropylene resin A and 40 to 15% by weight of linear polypropylene resin B, and particularly preferably contains 60 to 80% by weight of linear polypropylene resin A and 40 to 20% by weight of linear polypropylene resin B.

[0069] When the polypropylene resin contains linear polypropylene resin A and linear polypropylene resin B, the polypropylene film of the present invention is in a finely mixed state (phase-separated state) of linear polypropylene resin A and linear polypropylene resin B, thereby improving the voltage resistance at high temperatures.

[0070] The linear polypropylene resin can be produced by a generally known polymerization method. There are no particular limitations as long as a linear polypropylene resin that can be used for polypropylene films can be produced. Examples of such polymerization methods include gas phase polymerization, bulk polymerization, and slurry polymerization.

[0071] The polymerization may be a single-stage polymerization using one polymerization reactor, or a multi-stage polymerization using at least two polymerization reactors. Furthermore, hydrogen or a comonomer may be added to the reactor as a molecular weight modifier.

[0072] The catalyst used in the polymerization may be any commonly known Ziegler-Natta catalyst, and is not particularly limited as long as it can produce a linear polypropylene resin. The catalyst may contain a co-catalyst component or a donor. By adjusting the catalyst and polymerization conditions, the molecular weight, molecular weight distribution, stereoregularity, etc. can be controlled.

[0073] The molecular weight, molecular weight distribution, differential distribution value difference, etc. of the linear polypropylene resin can be adjusted by appropriately selecting, for example, (i) the polymerization method and the conditions during polymerization, such as temperature and pressure, (ii) the shape of the reactor during polymerization, (iii) the presence or absence, type and amount of additives used, and (iv) the type and amount of catalyst used.

[0074] Specifically, the molecular weight, molecular weight distribution, and differential distribution value difference D of the linear polypropylene resin Ｍ The above adjustments can be carried out, for example, by a multi-stage polymerization reaction. Examples of the multi-stage polymerization reaction include the following methods.

[0075] First, in the first polymerization step, propylene and a catalyst are supplied to a first polymerization reactor. Hydrogen, acting as a molecular weight regulator, is mixed with these components in an amount necessary to achieve the desired polymer molecular weight. For example, in the case of slurry polymerization, the reaction temperature is approximately 70 to 100°C, and the residence time is approximately 20 to 100 minutes. Multiple reactors can be used, for example, in series. In this case, the polymerization product of the first step is continuously sent to the next reactor together with additional propylene, catalyst, and molecular weight regulator, followed by a second polymerization in which the molecular weight is adjusted to a lower or higher molecular weight than in the first polymerization step. By adjusting the yields (production amounts) of the first and second reactors, it is possible to adjust the composition (configuration) of the high-molecular-weight and low-molecular-weight components.

[0076] The molecular weight, molecular weight distribution, differential distribution value difference, etc. of the linear polypropylene resin can also be adjusted by peroxidation decomposition, for example, by a peroxidation treatment using a decomposing agent such as hydrogen peroxide or an organic peroxide.

[0077] When peroxide is added to a degradable polymer such as polypropylene, a hydrogen abstraction reaction occurs from the polymer, and some of the resulting polymer radicals recombine and undergo crosslinking reactions, but most of the radicals undergo secondary decomposition (beta scission), splitting into two polymers with lower molecular weights. In other words, the higher the molecular weight component, the higher the probability of decomposition. This increases the amount of low molecular weight components, allowing the molecular weight distribution to be adjusted.

[0078] When adjusting the content of low molecular weight components by blending (resin mixing), it is preferable to dry mix or melt mix at least two or more resins with different molecular weights. Generally, a two-polypropylene blend system in which a main resin is mixed with an additive resin having a higher or lower average molecular weight in an amount of about 1 to 40% by mass is preferably used because it is easy to adjust the amount of low molecular weight components.

[0079] In this case, the melt flow rate (MFR) may be used as a guide for the average molecular weight. In this case, the difference in MFR between the main resin and the additive resin is preferably set to about 1 to 30 g / 10 min from the viewpoint of convenience during the adjustment.

[0080] As the linear polypropylene resin, commercially available products can also be used.

[0081] The polypropylene resin preferably contains a long-chain branched polypropylene resin from the viewpoint of easily adjusting Spd and Sdr within a predetermined range. Among long-chain branched polypropylene resins, long-chain branched polypropylene resin C (hereinafter also referred to as "long-chain branched polypropylene resin C") obtained by polymerizing propylene using a metallocene catalyst is preferred. Specifically, when long-chain branched polypropylene resin C is contained in the polypropylene resin, a large amount of β crystals is formed in the cast sheet. Furthermore, since the β crystals are transformed into α crystals by stretching a cast sheet containing β crystals, (approximately) arc-shaped irregularities are formed in the polypropylene film obtained by stretching due to the difference in density between the β crystals and the α crystals, which is preferable in that the surface can be suitably roughened.

[0082] In particular, it is more preferable that the polypropylene resin contains a linear polypropylene resin A and a long-chain branched polypropylene resin C.

[0083] Furthermore, it is more preferable that the polypropylene resin contains linear polypropylene resin A and linear polypropylene resin B, and also contains long-chain branched polypropylene resin C. Linear polypropylene resin A and linear polypropylene resin B can be in a finely mixed state (phase-separated state), and stretching such an unstretched polypropylene film complicates the arrangement of the resin components constituting the film. Therefore, by further including long-chain branched polypropylene resin C, the arrangement of the resin components constituting the film becomes more complex, improving the voltage resistance of the stretched film, and forming fine (approximately) arc-shaped irregularities, making it possible to achieve more suitable surface roughening.

[0084] In addition, if a long-chain branched polypropylene resin obtained by crosslinking modification with a peroxide is used instead of the long-chain branched polypropylene resin C polymerized using a metallocene catalyst, the α-crystal nucleating effect of the long-chain branched polypropylene resin obtained by crosslinking modification with a peroxide will promote the formation of α-crystals in the cast sheet and significantly suppress the formation of β-crystals. Even if a cast sheet containing α-crystals is stretched, crystallite transition does not occur, so unevenness is unlikely to form. Therefore, in order to roughen the surface of a polypropylene film, long-chain branched polypropylene resin C polymerized using a metallocene catalyst is suitable.

[0085] The metallocene catalyst is generally a metallocene compound that forms a polymerization catalyst that produces an olefin macromer. A long-chain branched polypropylene resin C obtained by polymerizing propylene using a metallocene catalyst is preferred because the polypropylene has an appropriate branch chain length and branch chain spacing, resulting in excellent compatibility with linear polypropylene. It is also preferred because it provides a uniform composition and a uniform surface shape. In the production of the long-chain branched polypropylene resin C, other conditions besides the type and amount of catalyst used, such as (i) the polymerization method and conditions such as temperature and pressure during polymerization, (ii) the shape of the reactor during polymerization, and (iii) the presence or absence, type, and amount of additives, can be the same as those described in the section on the method for producing a linear polypropylene resin, taking into consideration the molecular weight, molecular weight distribution, differential distribution value difference, etc., of the long-chain branched polypropylene resin C to be produced.

[0086] The melt tension of the long-chain branched polypropylene resin C is, for example, 5 or more and 30 or less. From the viewpoint of facilitating adjustment of Spd and Sdr within predetermined ranges, the melt tension is preferably 10 or more and 24 or less, more preferably 14 or more and 20 or less.

[0087] The weight-average molecular weight Mw of the long-chain branched polypropylene resin C is preferably 150,000 or more and 600,000 or less, more preferably 200,000 or more and 500,000 or less, even more preferably 250,000 or more and 450,000 or less, and particularly preferably 350,000 or more and 420,000 or less. When the weight-average molecular weight Mw of the long-chain branched polypropylene resin C is 150,000 or more and 600,000 or less, the resin fluidity becomes appropriate. As a result, it is easy to control the thickness of the cast sheet, and it is easy to produce a thin stretched film. In addition, unevenness in the thickness of the cast sheet and stretched film is less likely to occur, and appropriate stretchability is obtained, which is preferable.

[0088] The number average molecular weight Mn of the long-chain branched polypropylene resin C is, for example, 100,000 or more and 300,000 or less, preferably 120,000 or more and 200,000 or less.

[0089] The z-average molecular weight Mz of the long-chain branched polypropylene resin C is, for example, 500,000 or more and 1,200,000 or less, preferably 700,000 or more and 1,000,000 or less.

[0090] The molecular weight distribution [(weight average molecular weight Mw) / (number average molecular weight Mn)] of the long-chain branched polypropylene resin C is preferably 1.5 or more and 4.5 or less, more preferably 1.8 or more and 4.2 or less, even more preferably 2.0 or more and 4.0 or less, particularly preferably 2.1 or more and 3.9 or less, and particularly preferably 2.2 or more and 3.0 or less.

[0091] The long-chain branched polypropylene resin C has a ratio of (z-average molecular weight Mz) / (number-average molecular weight Mn) of preferably 4.0 or more and 9.0 or less, more preferably 4.2 or more and 8.0 or less, even more preferably 4.5 or more and 7.0 or less, and particularly preferably 5.0 or more and 6.0 or less.

[0092] The molecular weight, molecular weight distribution, etc. of the long-chain branched polypropylene resin C can be controlled by adjusting the catalyst and polymerization conditions, as described above.

[0093] The melt flow rate (MFR) of the long-chain branched polypropylene resin C at 230°C is preferably 0.1 to 12 g / 10 min, more preferably 0.5 to 5 g / 10 min, even more preferably 0.7 to 3.5 g / 10 min, and particularly preferably 1.0 to 2.5 g / 10 min. When the MFR of the long-chain branched polypropylene resin C at 230°C is within the above range, the resin has excellent flow properties in the molten state, making it less likely to experience unstable flow such as melt fracture, and also reducing breakage during stretching. Therefore, the resin has good film thickness uniformity, which has the advantage of preventing the formation of thin-walled portions prone to dielectric breakdown.

[0094] The heptane insoluble matter (HI) of the long-chain branched polypropylene resin C is preferably 98.0% or more, more preferably 98.5% or more, and even more preferably 98.8% or more. The heptane insoluble matter (HI) of the long-chain branched polypropylene resin C is preferably 99.5% or less, more preferably 99.3% or less. When the HI of the long-chain branched polypropylene resin C is within the above-mentioned preferred range, β crystals are more suitably formed in the cast sheet, and as a result, the surface of the polypropylene film can be suitably roughened.

[0095] The content of the long-chain branched polypropylene resin C is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, even more preferably 1.0% by mass or more, and particularly preferably 1.5% by mass or more, based on 100% by mass of the total polypropylene resin in the polypropylene film. The content of the long-chain branched polypropylene resin C is preferably 20% by mass or less, more preferably 10% by mass or less, even more preferably 7.0% by mass or less, even more preferably 5.0% by mass or less, and particularly preferably 4.0% by mass or less, based on 100% by mass of the total polypropylene resin in the polypropylene film. The polypropylene film of the present invention can contain one or more long-chain branched polypropylene resins C.

[0096] Representative commercially available long-chain branched polypropylene resins C include, for example, MFX3 and MFX6 manufactured by Japan Polypropylene Corporation, and MFX8 manufactured by Japan Polypropylene Corporation.

[0097] The above-mentioned physical properties of the polypropylene resin are values ​​measured according to the method of (2) in the Examples described later.

[0098] The polypropylene film of the present invention may contain a resin other than polypropylene resin (hereinafter also referred to as "other resin"). The "other resin" refers to a resin other than polypropylene resin, which is generally considered to be the main component resin, and is not particularly limited as long as the desired polypropylene film can be obtained. Examples of other resins include polyolefins other than polypropylene, such as polyethylene, poly(1-butene), polyisobutene, poly(1-pentene), and poly(1-methylpentene); copolymers of α-olefins, such as ethylene-propylene copolymers, propylene-butene copolymers, and ethylene-butene copolymers; random copolymers of vinyl monomers and diene monomers, such as styrene-butadiene random copolymers; and random copolymers of vinyl monomers, diene monomers, and vinyl monomers, such as styrene-butadiene-styrene block copolymers. The polypropylene film of the present invention can contain an amount that does not adversely affect the desired polypropylene film. The polypropylene film of the present invention may contain preferably 10 parts by mass or less, more preferably 5 parts by mass or less, of the other resin per 100 parts by mass of the polypropylene resin. The polypropylene film of the present invention may also contain other resins in an amount of preferably 0.1 part by mass or more, more preferably 1 part by mass or more, per 100 parts by mass of the polypropylene resin.

[0099] The polypropylene film of the present invention may further contain at least one additive in addition to the resin component. The "additive" refers to an additive generally used in polypropylene, and is not particularly limited as long as the desired polypropylene film can be obtained. Examples of additives include nucleating agents (α-crystal nucleating agents, β-crystal nucleating agents), antioxidants, necessary stabilizers such as chlorine absorbers and UV absorbers, lubricants, plasticizers, flame retardants, antistatic agents, inorganic fillers, organic fillers, etc. Examples of inorganic fillers include barium titanate, strontium titanate, and aluminum oxide. When using such additives, they can be included in an amount that does not adversely affect the desired polypropylene film.

[0100] The "nucleating agent" is not particularly limited as long as it is commonly used in polypropylene and can provide the desired polypropylene film.

[0101] Examples of the nucleating agent include an α-crystal nucleating agent that preferentially nucleates α-crystals and a β-crystal nucleating agent that preferentially nucleates β-crystals.

[0102] Among α-crystal nucleating agents, organic nucleating agents include dispersion-type nucleating agents and solution-type nucleating agents. Examples of dispersion-type nucleating agents include phosphate ester metal salt-based nucleating agents, carboxylate metal salt-based nucleating agents, and rosin metal salt-based nucleating agents. Examples of solution-type nucleating agents include sorbitol-based nucleating agents, nonitol-based nucleating agents, xylitol-based nucleating agents, and amide-based nucleating agents.

[0103] Examples of the β-crystal nucleating agent include amide-based nucleating agents, di- or polycarboxylic acid metal salt-based nucleating agents, quinacridone-based nucleating agents, aromatic sulfonic acid-based nucleating agents, phthalocyanine-based nucleating agents, and tetraoxaspiro compound-based nucleating agents.

[0104] The nucleating agent can be dry-blended or melt-blended with the polypropylene raw material and pelletized before use, or can be fed into an extruder together with polypropylene pellets. The surface roughness of the film can be adjusted to a desired level by using a nucleating agent. A typical example of a commercially available nucleating agent is NJSTAR NU-100 manufactured by New Japan Chemical Co., Ltd., which is a β-crystal nucleating agent. When the polypropylene film of the present invention contains a β-crystal nucleating agent, the content thereof is preferably 1 to 1000 ppm by mass, more preferably 50 to 600 ppm by mass, relative to the mass of the resin component (mass of the entire resin component).

[0105] The term "antioxidant" is generally used in polypropylene and is not particularly limited as long as it can produce the desired polypropylene film. Antioxidants are generally used for two purposes. One purpose is to suppress thermal and oxidative degradation in an extruder, and the other purpose is to contribute to suppressing degradation during long-term use as a capacitor film and improving capacitor performance. Antioxidants that suppress thermal and oxidative degradation in an extruder are also called "primary agents," and antioxidants that contribute to improving capacitor performance are also called "secondary agents."

[0106] Two types of antioxidants may be used for these two purposes, or one type of antioxidant may be used for both purposes.

[0107] An example of a primary agent is 2,6-di-tertiary-butyl-para-cresol (general name: BHT). The primary agent can usually be added for the purpose of suppressing thermal and oxidative degradation in an extruder during the preparation of a polypropylene resin composition, which will be described later in the description of the polypropylene film production method. The antioxidant added to the polypropylene resin composition for this purpose is mostly consumed during the molding process in the extruder, and almost no antioxidant remains in the film after film formation. Therefore, when the polypropylene film of the present invention contains a primary agent, the content thereof is usually less than 100 ppm by mass relative to the mass of the resin components (by mass when the resin components are taken as the whole).

[0108] The secondary agent may be a hindered phenol-based antioxidant having a carbonyl group.

[0109] The "hindered phenol antioxidant having a carbonyl group" is generally understood to be a hindered phenol antioxidant having a carbonyl group, and is not particularly limited as long as the desired polypropylene film can be obtained.

[0110] Examples of the hindered phenol antioxidant having a carbonyl group include triethylene glycol-bis[3-(3-tert-butyl-5-methyl-4-hydroxyphenyl)propionate] (trade name: Irganox 245), 1,6-hexanediol-bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (trade name: Irganox 259), pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (trade name: Irganox 1010), 2,2-thio-diethylenebis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (trade name: Irganox 1011), Examples of suitable hydroxyl groups include pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (trade name: Irganox 1035), octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate (trade name: Irganox 1076), and N,N'-hexamethylenebis(3,5-di-tert-butyl-4-hydroxy-hydrocinnamamide) (trade name: Irganox 1098). Of these, pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] is particularly preferred because it has a high molecular weight, is highly compatible with polypropylene, and has low volatility and excellent heat resistance.

[0111] The polypropylene film of the present invention may contain one or more hindered phenol antioxidants (secondary agents) having a carbonyl group for the purpose of suppressing deterioration that progresses over time during long-term use. When the polypropylene film of the present invention contains one or more hindered phenol antioxidants having a carbonyl group, the content thereof is preferably 4000 ppm by mass or more and 6000 ppm by mass or less, more preferably 4500 ppm by mass or more and 6000 ppm by mass or less, relative to the mass of the resin component (mass of the resin component as a whole). From the viewpoint of achieving an appropriate effect, it is preferable that the content of the hindered phenol antioxidant having a carbonyl group in the film is 4000 ppm by mass or more and 6000 ppm by mass or less.

[0112] A polypropylene film containing an optimum specific range of amount of a hindered phenol-based antioxidant having a carbonyl group, which has good compatibility with polypropylene at the molecular level, is preferred because it improves long-term durability.

[0113] The term "chlorine absorbent" is generally used in polypropylene and is not particularly limited as long as it can be used to obtain the desired polypropylene film. Examples of chlorine absorbents include metal soaps such as calcium stearate. When such a chlorine absorbent is used, it can be contained in an amount that does not adversely affect the desired polypropylene film.

[0114] The polypropylene film of the present invention can be produced by a commonly known method for producing a biaxially stretched polypropylene film. For example, the polypropylene film can be produced by preparing a cast sheet from a polypropylene resin composition obtained by mixing linear polypropylene resin A, linear polypropylene resin B, and long-chain branched polypropylene resin C, optionally together with other resins, additives, etc., and then biaxially stretching the cast sheet. Spd and Sdr can be adjusted within a predetermined range by adjusting the temperature during resin extrusion (melt temperature before extrusion), the air temperature of the air knife, the surface temperature of the metal drum immediately after extrusion, the longitudinal stretching temperature, etc.

[0115] <Preparation of Polypropylene Resin Composition> The method for preparing the polypropylene resin composition is not particularly limited, and examples thereof include a method in which polymer powders or pellets of linear polypropylene resin A, linear polypropylene resin B, and long-chain branched polypropylene resin C are dry-blended, if necessary, together with other resins, additives, etc., using a mixer or the like; and a method in which polymer powders or pellets of linear polypropylene resin A, linear polypropylene resin B, and long-chain branched polypropylene resin C are supplied, if necessary, together with other resins, additives, etc., to a kneader and melt-kneaded to obtain a melt blend resin composition.

[0116] The mixer or kneader is not particularly limited. The kneader may be of a single screw type, a twin screw type, or a multi-screw type having two or more screws. In the case of a twin or more screw type, the kneader may be of either a co-rotating or counter-rotating type.

[0117] In the case of blending by melt kneading, the kneading temperature is not particularly limited as long as good kneading can be achieved, but is preferably in the range of 170 to 320°C, more preferably in the range of 200 to 300°C, and even more preferably in the range of 230 to 270°C. 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 can be pelletized to an appropriate size using a commonly known granulator to obtain pellets of the melt blend resin composition.

[0118] When preparing the polypropylene resin composition, a primary agent as an antioxidant described in the section on additives above can be added for the purpose of suppressing thermal degradation and oxidative degradation in the extruder.

[0119] When the polypropylene resin composition contains a primary agent, the content thereof is preferably 1000 ppm by mass to 5000 ppm by mass relative to the mass of the resin components (mass of the resin components as a whole). Most of the antioxidant for this purpose is consumed in the molding process in the extruder, and almost no antioxidant remains in the film after film formation.

[0120] The hindered phenol antioxidant having a carbonyl group, which has been described above in the section on additives, can be added to the polypropylene resin composition as a secondary agent.

[0121] When the polypropylene resin composition contains a hindered phenol-based antioxidant having a carbonyl group, the content thereof is preferably 100 ppm by mass to 10,000 ppm by mass, more preferably 5,500 ppm by mass to 7,000 ppm by mass, relative to the mass of the resin components (mass of the resin components as a whole). A considerable amount of the hindered phenol-based antioxidant having a carbonyl group is also consumed in the extruder.

[0122] When the polypropylene resin composition does not contain a primary agent, a larger amount of the hindered phenol-based antioxidant having a carbonyl group can be used. This is because the consumption of the hindered phenol-based antioxidant having a carbonyl group increases in the extruder. When the polypropylene resin composition does not contain a primary agent but contains the hindered phenol-based antioxidant having a carbonyl group, the content thereof is 6,000 ppm by mass to 8,000 ppm by mass or less relative to the mass of the resin components (mass of the resin components as a whole).

[0123] <Preparation of Cast Sheet> A cast sheet can be obtained by feeding pre-prepared pellets of a dry blend resin composition and / or a melt blend resin composition into an extruder, melting them, filtering them, and then melt-extruding them through a T-die at a temperature of preferably 220°C to 300°C, more preferably 250°C to 290°C, and even more preferably 265°C to 280°C. The melt-extruded resin composition is then cooled and solidified in at least one metal drum maintained at a temperature (casting temperature) of preferably 70°C to 120°C, more preferably 80°C to 110°C, even more preferably 90°C to 105°C, and particularly preferably 95°C to 100°C. In this process, the melt-extruded resin composition is preferably pressed against the metal drum with an air knife. The air temperature of the air knife is preferably 105°C to 160°C, more preferably 120°C to 150°C, even more preferably 130°C to 145°C, and even more preferably 135°C to 145°C. This makes it possible to suppress the generation of β crystals on the air knife surface side, and to easily control Spd and Sdr within the target ranges.

[0124] The thickness of the cast sheet is not particularly limited as long as the desired polypropylene film can be obtained, but is preferably 30 μm to 2000 μm, more preferably 50 μm to 1000 μm, and even more preferably 70 μm to 200 μm.

[0125] During the process of producing a cast sheet (particularly in an extruder), polypropylene undergoes considerable thermal degradation (oxidative degradation) and shear degradation. The degree of progression of such degradation, i.e., changes in molecular weight distribution and stereoregularity, can be suppressed by purging the extruder with nitrogen (oxidation suppression), the screw shape (shear force) in the extruder, the internal shape of the T-die (shear force) during casting, the amount of antioxidant added (oxidation suppression), the winding speed (extension force) during casting, etc.

[0126] <Stretching Treatment> The biaxially stretched polypropylene film can be produced by subjecting the cast sheet to a stretching treatment. Sequential biaxial stretching is preferred as the stretching method. In this sequential biaxial stretching method, the cast sheet is first maintained at a temperature of preferably 135 to 150°C, more preferably 140 to 146°C, passed through rolls with a speed difference, stretched 3 to 7 times, preferably 4 to 5 times, in the machine direction, and immediately cooled to room temperature. By appropriately adjusting the temperature in this longitudinal stretching step, Spd and Sdr can be easily adjusted to appropriate ranges. Subsequently, the stretched film is introduced into a tenter and transversely stretched 3 to 11 times in the width direction at a temperature of preferably 150°C or higher, more preferably 150 to 180°C, followed by relaxation, heat setting, and winding into a roll.

[0127] The film wound into a roll is subjected to an aging treatment in an atmosphere of about 20 to 45°C, and then, while being unwound (unwound), it is slit (cut) to the desired product width using a slitter or the like, and each piece is wound again.

[0128] Such a stretching step results in a biaxially stretched film having excellent mechanical strength and rigidity, and the surface irregularities are more clearly defined, resulting in a finely roughened film.

[0129] After the stretching and heat setting steps, the polypropylene film may be subjected to corona discharge treatment online or offline. Corona discharge treatment can improve adhesive properties in subsequent steps such as metal deposition processing. Corona discharge treatment can be performed using a known method. It is preferable to perform the treatment using air, carbon dioxide gas, nitrogen gas, or a mixture thereof as the atmospheric gas.

[0130] To process the polypropylene film into a capacitor, a metal layer may be laminated on one or both sides of the polypropylene film to form a metal layer-integrated polypropylene film. The metal layer functions as an electrode. Examples of metals that can be used for the metal layer include zinc, lead, silver, chromium, aluminum, copper, nickel, and other metals, as well as mixtures and alloys of these metals. However, considering environmental, economical, and capacitor performance considerations, zinc and aluminum are preferred.

[0131] Examples of methods for laminating a metal layer on one or both sides of the polypropylene film include vacuum deposition and sputtering. Vacuum deposition is preferred from the viewpoints of productivity and economy. Examples of vacuum deposition methods include the crucible method and the wire method, but the method is not particularly limited and an optimal method can be selected as appropriate.

[0132] 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 a T-margin pattern, to one side of the film. This is effective in terms of increasing safety and preventing capacitor damage and short circuits.

[0133] As a method for forming the margin, any generally known method such as a tape method or an oil method can be used without any limitation.

[0134] When forming a metal layer on the polypropylene film, the polypropylene film wound in a roll is unwound (unwound), a metal layer such as a vapor-deposited film is formed on one or both surfaces, and the film is then wound again.

[0135] The metal layer-integrated polypropylene film can be laminated in a plurality of layers by a conventionally known method, or can be wound around an element to form a film capacitor.

[0136] Specifically, a blade is used to slit the center of each margin of the metal layer-integrated polypropylene film, to prepare a take-up reel having a margin on one surface.

[0137] Next, using a winding reel with a left margin and a winding reel with a right margin, the two sheets are stacked and wound together so that the vapor-deposited area extends beyond the margin in the width direction (element winding process). Next, the core material is removed from the wound body and pressed. Next, external electrodes are formed on both end surfaces, and lead wires are attached to the external electrodes. This completes the wound film capacitor.

[0138] The present invention will be described in detail below based on examples, but the present invention is not limited to these examples.

[0139] (1) Preparation of Polypropylene Resin Table 1 shows the polypropylene resins used to produce the biaxially oriented polypropylene films of the Examples and Comparative Examples.

[0140] Resin A-1, Resin A-2, Resin A-3, Resin B-1, and Resin B-2 are all homopolypropylene resins. Branched-chain polypropylene resin C is a branched-chain polypropylene resin polymerized using a metallocene catalyst.

[0141] Table 1 also shows the 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), melt flow rate (MFR), heptane insoluble content (HI), and melt tension of each resin. These values ​​are those in the form of raw material resin pellets. The measurement methods are as follows.

[0142]

[0143] (2) Measurement of physical properties of polypropylene resin (2-1) Measurement of various average molecular weights and various molecular weight distributions of linear polypropylene resin Using SEC (size exclusion chromatography), various average molecular weights and various molecular weight distributions were measured under the following conditions. Apparatus: HLC-8321 GPC / HT (detector: differential refractometer (RI)) (manufactured by Tosoh Corporation) Column: TSKgel Guard column H ＨＲ (30) HT (7.5mm I.D. x 7.5cm) x 1 + TSKgel GMH ＨＲ -H(20)HT (7.8 mm ID x 30 cm) x 3 (Tosoh Corporation) Eluent: 1,2,4-trichlorobenzene (GPC grade, Fujifilm Wako Pure Chemical Industries, Ltd.) + dibutylhydroxytoluene (0.05%) Flow rate: 1.0 mL / min Detection conditions: polarity = (-) Injection volume: 300 μL Column temperature: 140°C System temperature: 40°C Sample concentration: 1 mg / mL Pretreatment: The sample was weighed, and a solvent (1,2,4-trichlorobenzene with 0.1% dibutylhydroxytoluene added) was added and the mixture was dissolved by shaking at 140°C for 1 hour. The mixture was then heated and filtered through a 0.5 μm sintered filter. Visual observation of the sample solution revealed no undissolved matter. Calibration curve: A quintic approximation calibration curve was created using standard polystyrene (Tosoh Corporation). The molecular weight was converted to the molecular weight of polypropylene using the Q-factor.

[0144] From the obtained calibration curve and SEC chromatogram, the number average molecular weight (Mn), weight average molecular weight (Mw), and Z average molecular weight (Mz) were obtained using analysis software for the measurement device. The Mw and Mn values ​​were used to obtain the molecular weight distribution (Mw / Mn). The Mz and Mn values ​​were also used to obtain the molecular weight distribution (Mz / Mn).

[0145] (2-2) Measurement of Various Average Molecular Weights and Various Molecular Weight Distributions of Branched Polypropylene Resins Using SEC-MALS (size exclusion chromatography-multiangle light scattering detector), various average molecular weights and various molecular weight distributions were measured under the following conditions. This is a method of measuring the average molecular weight by fractionating according to molecular size using SEC and measuring the absolute molecular weight using MALS. Apparatus: HLC-8321GPC / HT (with built-in differential refractometer (RI)) (manufactured by Tosoh Corporation) Light scattering detector: DAWN HELEOS (manufactured by Wyatt Technology) MALS laser wavelength: 664 nm Column: TSKgel guard column H ＨＲ (30) HT (7.5mm I.D. x 7.5cm) x 1 + TSKgel GMH ＨＲ -H(20)HT (7.8 mm I.D. x 30 cm) x 3 (Tosoh Corporation) Eluent: 1,2,4-trichlorobenzene (GPC grade, Fujifilm Wako Pure Chemical Industries, Ltd.) + dibutylhydroxytoluene (0.05%) Flow rate: 1.0 mL / min Injection volume: 300 μL Column temperature: 140°C System temperature: 40°C Sample concentration: 1 mg / mL Pretreatment: The sample was weighed, a solvent (1,2,4-trichlorobenzene with 0.1% dibutylhydroxytoluene added) was added, and the mixture was dissolved by shaking at 140°C for 1 hour. The mixture was then heated and filtered through a 0.5 μm sintered filter. Visual observation of the sample solution revealed no insoluble matter.

[0146] Data processing was performed using analysis software Astra Ver. 5.3.4 manufactured by Wyatt Technology. The analysis was performed under the following conditions: Absolute molecular weight and radius of gyration were determined from the Zimm plot. The refractive index (1.501) of 1,2,4-trichlorobenzene at 140°C used in the calculation was determined by first approximation from the refractive index values ​​at 20°C and 135°C. The refractive index concentration increment (dn / dc) of the sample was determined from the literature value (-0.092 mL / g). Reference: K. Lederer and I. Mingozzi, Pure and Applied Chemistry, 69(5), 993-1006 (1997). - With GPC-MALS, there are regions where it is difficult to evaluate absolute molecular weight and radius of gyration for the following reasons, so these regions were excluded from the range of absolute molecular weight calculation and radius of gyration evaluation. - Low concentration (both high and low molecular weight sides): At the base of the peak in the chromatogram, the sample concentration is insufficient and sufficient sensitivity cannot be obtained. - Isotropic scattering (low molecular weight side only): When the molecular size is approximately 1 / 20 or less of the wavelength of the incident light, the scattered light does not exhibit anisotropy, and the radius of gyration cannot be obtained due to the measurement principle.

[0147] The above analysis yielded the radius of gyration (Rw), number average molecular weight (Mn), weight average molecular weight (Mw), and Z average molecular weight (Mz). The Mw and Mn values ​​were used to obtain the molecular weight distribution (Mw / Mn). The Mz and Mn values ​​were also used to obtain the molecular weight distribution (Mz / Mn).

[0148] (2-3) Measurement of Melt Flow Rate (MFR) The melt flow rate (MFR) of each resin in the form of raw resin pellets was measured using a melt indexer manufactured by Toyo Seiki Co., Ltd. in accordance with condition M of JIS K 7210. Specifically, a weighed 4 g sample was first inserted into a cylinder set to a test temperature of 230°C and preheated for 3.5 minutes under a load of 2.16 kg. Thereafter, the weight of the sample extruded from the bottom hole over 30 seconds was measured, and the MFR (g / 10 min) was calculated. The above measurement was repeated three times, and the average value was used as the measured MFR.

[0149] (2-4) Measurement of heptane insoluble matter [HI] A linear polypropylene resin was press-molded to a size of 10 mm x 35 mm x 0.3 mm to prepare a measurement sample of approximately 3 g. Next, approximately 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.

[0150] (2-5) Measurement of Melt Tension Using a Capillograph 1B manufactured by Toyo Seiki Seisakusho, the resin was extruded into a string shape under the following conditions, and the tension detected on the pulley when it was taken up around a roller was taken as the melt tension. Capillary: diameter 2.0 mm, length 40 mm Cylinder diameter: 9.55 mm Cylinder extrusion speed: 20 mm / min Take-up speed: 4.0 m / min Temperature: 230°C If the melt tension is extremely high, a take-up speed of 4.0 m / min may cause the resin to break. In such cases, the take-up speed is reduced, and the tension at the highest possible take-up speed is taken as the melt tension.

[0151] (3) Preparation of biaxially stretched polypropylene film (Example 1) Linear polypropylene resin A-1, linear polypropylene resin B-2, and branched polypropylene resin C were dry-blended in a ratio of linear polypropylene resin A-1 / linear polypropylene resin B-2 / branched polypropylene resin C = 63 / 35 / 2 (mass ratio). The resin was melted at a resin temperature of 270 ° C., extruded using a T-die, and solidified by pressing hot air at 140 ° C. against a metal drum maintained at a surface temperature of 97 ° C. with an air knife to produce a cast sheet with a thickness of approximately 90 μm. The obtained unstretched cast sheet was maintained at a temperature of 143 ° C., passed between rolls with a speed difference, stretched 4.6 times in the machine direction, and immediately cooled to room temperature. Subsequently, the stretched film was introduced into a tenter and stretched 9.8 times in the width direction at a temperature of 155 ° C. Next, the roughened surface side (the side in contact with the metal drum immediately after T-die extrusion) was stretched at 25 W · min / m 2 The film was subjected to a corona discharge treatment in the atmosphere at a treatment speed of 1000 rpm, thereby obtaining a biaxially oriented polypropylene film having a thickness of 2.0 μm.

[0152] (Example 2) A biaxially stretched polypropylene film having a thickness of 2.0 μm was obtained in the same manner as in Example 1, except that linear polypropylene resin A-1, linear polypropylene resin B-2, and branched polypropylene resin C were dry-blended in a mass ratio of linear polypropylene resin A-1 / linear polypropylene resin B-2 / branched polypropylene resin C = 63 / 34 / 3.

[0153] Example 3 A biaxially stretched film having a thickness of 2.3 μm was obtained in the same manner as in Example 1, except that the thickness of the cast sheet was 104 μm.

[0154] (Example 4) Linear polypropylene resin A-1, linear polypropylene resin B-2, and branched polypropylene resin C were dry-blended in a ratio of linear polypropylene resin A-1 / linear polypropylene resin B-2 / branched polypropylene resin C = 63 / 34 / 3 (mass ratio), and the thickness of the cast sheet was 104 μm. A biaxially stretched film having a thickness of 2.3 μm was obtained in the same manner as in Example 1.

[0155] (Example 5) Linear polypropylene resin A-2, linear polypropylene resin B-2, and branched polypropylene resin C were dry-blended in a ratio of linear polypropylene resin A-2 / linear polypropylene resin B-2 / branched polypropylene resin C = 63 / 35 / 2 (mass ratio), and the thickness of the cast sheet was 104 μm. A biaxially oriented polypropylene film having a thickness of 2.3 μm was obtained in the same manner as in Example 1.

[0156] (Example 6) Linear polypropylene resin A-2, linear polypropylene resin B-2, and branched polypropylene resin C were dry-blended in a ratio of linear polypropylene resin A-2 / linear polypropylene resin B-2 / branched polypropylene resin C = 63 / 34 / 3 (mass ratio), and the thickness of the cast sheet was 104 μm. Except for this, a biaxially oriented polypropylene film having a thickness of 2.3 μm was obtained in the same manner as in Example 1.

[0157] (Example 7) Linear polypropylene resin A-1, linear polypropylene resin B-2, and branched polypropylene resin C were dry-blended in a ratio of linear polypropylene resin A-1 / linear polypropylene resin B-2 / branched polypropylene resin C = 63 / 34 / 3 (mass ratio), and the resin was melted at a resin temperature of 260 ° C., then extruded using a T-die, and solidified by pressing hot air at 140 ° C. against a metal drum maintained at a surface temperature of 93 ° C. with an air knife to produce a cast sheet of about 130 μm. The obtained unstretched cast sheet was maintained at a temperature of 144 ° C., passed through rolls with a speed difference, stretched 4.8 times in the machine direction, and immediately cooled to room temperature. Subsequently, the stretched film was introduced into a tenter and stretched 9.7 times in the width direction at a temperature of 158 ° C. Next, the roughened surface side (the side in contact with the metal drum immediately after T-die extrusion) was stretched at 25 W · min / m 2 The film was subjected to a corona discharge treatment in the atmosphere at a treatment speed of 1000 kJ / min, thereby obtaining a biaxially oriented polypropylene film having a thickness of 2.8 μm.

[0158] (Example 8) Linear polypropylene resin A-2 and linear polypropylene resin B-2 were dry-blended at a linear polypropylene resin A-2 / linear polypropylene resin B-2=65 / 35 (mass ratio), and the thickness of the cast sheet was 104 μm. Except that, in the same manner as in Example 1, a biaxially oriented polypropylene film having a thickness of 2.3 μm was obtained.

[0159] (Comparative Example 1) A biaxially stretched polypropylene film having a thickness of 2.0 μm was obtained in the same manner as in Example 1, except that linear polypropylene resin A-3 and linear polypropylene resin B-1 were dry-blended in a mass ratio of linear polypropylene resin A-3 / linear polypropylene resin B-1 = 65 / 35.

[0160] (Comparative Example 2) Linear polypropylene resin A-3 and linear polypropylene resin B-1 were dry-blended at a ratio of linear polypropylene resin A-3 / linear polypropylene resin B-1 = 65 / 35 (mass ratio), and the resin was melted at a resin temperature of 260 ° C., then extruded using a T-die, and solidified by pressing hot air at 140 ° C. against a metal drum maintained at a surface temperature of 93 ° C. with an air knife to produce a cast sheet of about 130 μm. The obtained unstretched cast sheet was maintained at a temperature of 144 ° C., passed through rolls with a speed difference, stretched 4.8 times in the machine direction, and immediately cooled to room temperature. Subsequently, the stretched film was introduced into a tenter and stretched 9.7 times in the width direction at a temperature of 158 ° C. Next, the roughened surface side (the side in contact with the metal drum immediately after T-die extrusion) was stretched at 25 W · min / m 2 The film was subjected to a corona discharge treatment in the atmosphere at a treatment speed of 1000 kJ / min, thereby obtaining a biaxially oriented polypropylene film having a thickness of 2.8 μm.

[0161] (Comparative Example 3) Linear polypropylene resin A-3 and linear polypropylene resin B-2 were dry-blended at a linear polypropylene resin A-3 / linear polypropylene resin B-2=65 / 35 (mass ratio), and the thickness of the cast sheet was 104 μm. A biaxially oriented polypropylene film having a thickness of 2.3 μm was obtained in the same manner as in Example 1.

[0162] (4) Evaluation of Physical Properties of Polypropylene Film (4-1) Measurement of Thickness Measurement was performed using a paper thickness measuring instrument MEI-11 (measurement pressure 100 kPa, descending speed 3 mm / sec, measurement terminal φ=16 mm, measurement force 20.1 N) manufactured by Citizen Seimitsu Co., Ltd. in an environment of a temperature of 23±2°C and a humidity of 50±5% RH. The sample was cut out from the roll with 10 or more sheets stacked, and handled so as not to wrinkle or trap air in the film when cutting out. Five measurements were performed on a 10-sheet stack sample, and the thickness was calculated by dividing the average of the five measurements by 10.

[0163] (4-2) Measurement of Roughness Parameters The smooth surfaces (the surfaces that do not come into contact with the metal drum immediately after extrusion through the T-die) of the biaxially oriented polypropylene films of the Examples and Comparative Examples were measured for various surface roughness parameters (peak density Spd, developed interface area ratio Sdr, deviation Ssk, sharpness Sku, five-point peak region height S5p, five-point valley region depth S5v, and ten-point region height S10z) specified in ISO 25178. Specifically, they are as follows.

[0164] The surface roughness of the smooth surface was measured using an optical interference type non-contact surface profiler (VertScan 2.0 (model: R5500GML) manufactured by Ryoka Systems Co., Ltd.). As a measurement sample, the film was cut into an arbitrary size of about 20 cm square, and after fully smoothing out any wrinkles, it was set on a measurement stage using an electrostatic contact plate or the like. Using WAVE mode, a 530 white filter and a 1x BODY lens barrel were applied, and a x10 objective lens was used to measure 470 μm x 353 μm per field of view.

[0165] The obtained data was then subjected to noise removal processing using a median filter (3 x 3), followed by Gaussian filtering with a cutoff value of 80 μm to remove waviness. This allowed for proper measurement of the smooth surface condition. Next, analysis was performed using the "ISO parameters" plug-in function "Bearing" in the "VS-Viewer" analysis software for "VertScan 2.0." Various surface roughness parameters specified in ISO 25178 (peak density Spd, interface developed area ratio Sdr, deviation Ssk, sharpness Sku, five-point peak region height S5p, five-point valley region depth S5v, and ten-point region height S10z) were determined, and the average values ​​obtained at the 10 locations were calculated.

[0166] Based on the above measurement results, the ratio of the five-point peak region height S5p to the ten-point region height S10z (= S5p / S10z × 100) and the ratio of the five-point valley region depth S5v to the ten-point region height S10z (= S5v / S10z × 100) were calculated.

[0167] (5) Performance Evaluation (5-1) Evaluation of Wrinkle Suppression in the Element Winding Process The biaxially oriented polypropylene films obtained in the Examples and Comparative Examples were slit to a width of 620 mm and wound up. The corona discharge-treated film surface (the roughened surface "the surface that comes into contact with the metal drum immediately after extrusion through the T-die") was subjected to aluminum deposition with a T-margin deposition pattern at a deposition resistance of 15 Ω / □ using a continuous vacuum deposition machine manufactured by ULVAC, Inc., to obtain a metal layer-integrated polypropylene film. The pattern deposition was performed by wire vacuum deposition at a deposition rate of 400 m / min, and heavy edge deposition was performed by crucible vacuum deposition. The film length after deposition was 50,000 m.

[0168] The resulting 620 mm wide metal layer-integrated polypropylene film was slit into a small roll of 30 mm wide and 10,000 m long at a slitting speed of 350 m / min using a blade at the center of each margin.

[0169] Two of the resulting small rolls were overlapped and wound using a winding reel with a left margin and a winding reel with a right margin, with the vapor-deposited portion extending beyond the margin in the width direction (element winding process). The winding was performed using an automatic winding machine, Model 3KAW-N2, manufactured by Kaito Seisakusho Co., Ltd., with a winding tension of 200 g and 1,360 turns. The winding was visually observed from the beginning to the end, and any rolls exhibiting wrinkles or misalignment were deemed unacceptable. The percentage of acceptable rolls relative to the total number of rolls manufactured was used as an index of processability (hereinafter referred to as element winding yield). A higher element winding yield is preferable. Based on the element winding yield, the degree of wrinkle suppression during the element winding process was evaluated as follows: A = Good: Element winding yield 95% or more; B = Acceptable: Element winding yield 85% or more but less than 95%; C = Poor: Element winding yield less than 85%.

[0170] (5-2) Evaluation of Capacitor Elements by Life Testing: The cylindrical element windings wound in the element winding process (5-1) above that passed were pressed into flat elements using the same automatic winding machine at a pressure of 0.35 MPa. The flat elements were then heat-treated at 120°C for 15 hours while being pressed at a pressure of 0.30 MPa. After that, zinc metal was sprayed onto the element end faces, lead wires were soldered to the sprayed end faces, and the elements were then sealed with epoxy resin to produce film capacitors. The capacitance of the resulting capacitors was 75 μF (±5 μF). Hereinafter, the capacitance of the capacitors was measured using a Hioki E.E. Corporation LCR HiTester 3522-50.

[0171] A DC voltage of 750 V was continuously applied to the obtained capacitor for 1000 hours in a high-temperature bath at 115°C. Based on the capacitance of the capacitor before and after the load, the rate of change in capacitance before and after the voltage load was calculated using the following formula. The test was performed on two samples, and the average value was used for evaluation.

[0172]

[0173] The rate of change in capacitance was evaluated according to the following evaluation criteria: (Evaluation Criteria) AA: Rate of change in capacitance is less than 0.5%. A: Rate of change in capacitance is 0.5% or more and less than 1.0%. B: Rate of change in capacitance is 1.0% or more and less than 3.0%. C: Rate of change in capacitance is 3.0% or more and less than 5.0%. D: Rate of change in capacitance is 5.0% or more.

[0174] (6) Results The measurement and evaluation results are shown in Table 2.

[0175]

Claims

1. A biaxially oriented polypropylene film having a first surface and a second surface, and a peak density Spd of at least 2000 / mm on at least the first surface. ２ More than 8000 / mm ２ and a developed area ratio Sdr of the interface is 0.001% or more and 0.009% or less.

2. The biaxially oriented polypropylene film according to claim 1, wherein the bias Ssk on the first surface is -1.5 or more and 1.5 or less.

3. The biaxially oriented polypropylene film according to claim 1, wherein the kurtosis Sku of the first surface is 0 or more and 100 or less.

4. A biaxially oriented polypropylene film as described in claim 1, wherein the ratio of the quincunx region height S5p to the ten-point region height S10z on the first surface is 40% or more and 60% or less.

5. A biaxially oriented polypropylene film according to claim 1, wherein the ratio of the five-point valley region depth S5v to the ten-point region height S10z on the first surface is 40% or more and 60% or less.

6. A biaxially oriented polypropylene film according to claim 1, which contains linear polypropylene resin A and linear polypropylene resin B having different melt flow rates at 230°C.

7. A biaxially oriented polypropylene film according to claim 6, wherein the linear polypropylene resin A and the linear polypropylene resin B have a number average molecular weight Mn of 34,000 or more and a molecular weight distribution Mw / Mn of 9.3 or less.

8. The biaxially oriented polypropylene film according to claim 6, wherein the heptane insoluble fraction (HI) of the linear polypropylene resin A and the linear polypropylene resin B is 97.5% or more.

9. The biaxially oriented polypropylene film according to claim 6, which contains long-chain branched polypropylene resin C.

10. The biaxially oriented polypropylene film according to any one of claims 1 to 9, having a thickness of 1.4 to 6.0 µm.

11. The biaxially oriented polypropylene film according to any one of claims 1 to 9, which is used for a capacitor.

12. A metal layer-integrated polypropylene film comprising the biaxially oriented polypropylene film according to any one of claims 1 to 9 and a metal layer laminated on one or both sides of the biaxially oriented polypropylene film.

13. A film capacitor comprising the metal layer-integrated polypropylene film according to claim 12.

14. A film roll comprising the biaxially oriented polypropylene film according to any one of claims 1 to 9 wound into a roll.

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