Polyester film and method for producing the same

Abstract

To provide a new polyester film as a polyester film for a capacitor which has good heat resistance while maintaining electric characteristics equivalent to a conventional film, and can correspond to the conversion into the thin film, and a method for producing the same.SOLUTION: There is provided a method for producing a polyester film, in which when a polyester resin (X) is melt-extruded in a film shape together with a polypropylene resin (Y) using a bent type twin screw extruder having a raw material supply port and a vent opening in a cylinder, a content of the polypropylene resin (Y) is 1 mass% or more with respect to the total extrusion quantity Q (kg / h) per unit time, and the number N (rpm) of revolutions of a screw with respect to an inner diameter D (mm) of the cylinder and a total extrusion quantity (kg / h) per unit time satisfies the following expression (1): 0.05×10-6×D2.8≤Q / N≤6.0×10-6×D2.8.

Description

[Technical Field]

[0001] The present invention relates to a polyester film and a method for producing the same. [Background technology]

[0002] Polyethylene terephthalate (PET) film, a representative polyester film, especially biaxially oriented PET film, is widely used in a variety of fields, including industrial materials, optical materials, electronic component materials, and battery packaging, due to its excellent transparency, mechanical strength, heat resistance, and flexibility.

[0003] Furthermore, due to their excellent electrical properties such as low dielectric loss and high moisture resistance, resin films such as biaxially oriented polypropylene films are used as dielectric films for capacitors such as filter capacitors and smoothing capacitors for high-voltage capacitors, various switching power supplies, converters, inverters, etc. In recent years, there has been a growing need for further miniaturization and higher capacity of capacitors. For example, when resin films are used in capacitors for inverter power supply devices that control drive motors of electric vehicles, hybrid vehicles, and the like, small size, light weight, and high capacity are required (see Patent Document 1). As capacitors become higher in capacity, there is a trend toward requiring high voltage resistance characteristics (stable capacitance) over long periods of time, for example, in temperature ranges exceeding 120°C. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-231584 Summary of the Invention [Problem to be solved by the invention]

[0005] However, for example, while general-purpose biaxially oriented polypropylene film has good electrical properties, when used in temperatures above 120°C, the heat resistance of the film itself reaches its limit, making it difficult to use. Therefore, the present invention has been made in consideration of the above problems, and proposes a polyester film useful for capacitors, which maintains electrical properties comparable to those of conventional films, yet has good heat resistance and can be made thinner, and a method for producing the same. [Means for solving the problem]

[0006] As a result of extensive investigations, the present inventors have found that the above-mentioned problems can be solved by employing a polyester film production method in which, when a polyester resin (X) is melt-extruded into a film together with a polypropylene resin (Y) using a vented twin-screw extruder, the polypropylene resin (Y) is used in a specific amount, and the total extrusion rate per unit time Q (kg / h), the cylinder inner diameter D (mm), and the screw rotation speed N (rpm) satisfy a specific relationship, and have completed the present invention as described below. That is, the present invention provides the following [1] to

[13] . [1] A method for producing a polyester film, wherein a polyester resin (X) is melt-extruded together with a polypropylene resin (Y) into a film shape using a vented twin-screw extruder having a raw material supply port and a vent opening in the cylinder, the content of the polypropylene resin (Y) being 1 mass% or more relative to the total extrusion rate per unit time Q (kg / hour), and the screw rotation speed N (rpm) relative to the inner diameter D (mm) of the cylinder and the total extrusion rate per unit time Q (kg / hour) satisfies the following formula (1): 0.05×10 -6 ×D 2.8 ≦Q / N≦6.0×10 -6 ×D 2.8 (1) [2] The method for producing a polyester film according to the above [1], wherein the polyester film contains 1 to 30 parts by mass of polypropylene resin (Y) per 100 parts by mass of polyester resin (X). [3] The method for producing a polyester film according to the above [1] or [2], wherein the polyester resin (X) is at least one selected from polyethylene terephthalate and polyethylene-2,6-naphthalate. [4] The method for producing a polyester film according to any one of the above [1] to [3], wherein the polycondensation catalyst for the polyester resin (X) is a Ti-based or Sb-based catalyst. [5] The method for producing a polyester film according to any one of the above [1] to [4], wherein the film has a thickness of 0.5 to 12.0 μm. [6] A polyester film produced by the method for producing a polyester film according to any one of [1] to [5] above. [7] The polyester film according to [6] above, which has a polypropylene resin layer (Yt) whose cross-sectional structure has a layered structure and whose ratio (y / x) of the length in the thickness direction (x) to the length in the width direction (TD) (y) is 17 or more. [8] The polypropylene resin layer (Yt) is 0.08 pieces / μm 2 The polyester film according to [7] above. [9] The polyester film according to the above [7] or [8], wherein the value obtained by dividing the ratio (y / x) by the film thickness (t) is 1.5 or more.

[10] The polyester film according to any one of the above [7] to [9], wherein the polypropylene resin layer (Yt) has a structure that is continuous from end to end in the width direction (TD) of an observation field of 19 μm×25 μm.

[11] The polyester film according to any one of the above [7] to

[10] , which has a structure in which a plurality of the polypropylene resin layers (Yt) are provided and a polyester resin layer (Xt) is interposed between the polypropylene resin layers (Yt) adjacent to each other in the thickness direction.

[12] The polyester film according to any one of the above [6] to

[11] , which has a dielectric loss tangent (tan δ) at 1 kHz of 0.54 or less.

[13] A metal laminated film comprising the polyester film according to any one of the above [6] to

[12] and a metal layer provided on at least one surface thereof. [Effects of the Invention]

[0007] According to the present invention, it is possible to provide a polyester film that maintains electrical properties comparable to those of conventional polyester films, yet has good heat resistance, and is useful for capacitors, and a method for producing the same. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a cross-sectional photograph of the polyester film (sample film) obtained in Example 1 (magnification: 5000 times). [Figure 2] 1 is a cross-sectional photograph of the polyester film (sample film) obtained in Comparative Example 1 (magnification: 5000 times). DETAILED DESCRIPTION OF THE INVENTION

[0009] Next, an example of an embodiment of the present invention will be described, but the present invention is not limited to the embodiment described below. [Polyester film manufacturing method] The method for producing the polyester film of the present invention (hereinafter referred to as "the present production method") involves melt-extruding the polyester resin (X) together with the polypropylene resin (Y) into a film using a twin-screw extruder.

[0010] <Polyester film> The polyester film will be described in more detail below. The polyester film produced by this production method (hereinafter referred to as "the polyester film") is particularly preferable because it has excellent physical properties such as heat resistance, flatness, optical properties, and strength. The polyester film may be a single layer or a multilayer film (i.e., a laminate film) having two or more layers with different properties. The polyester film may be a non-stretched film (sheet) or a stretched film. Among these, a stretched film stretched uniaxially or biaxially is preferred. Among these, a biaxially stretched film is more preferred from the viewpoint of balance of mechanical properties and flatness. Therefore, a biaxially stretched polyester film is even more preferred.

[0011] (Polyester resin (X)) The polyester resin (X) that is the main component resin of the present polyester film may be a homopolyester or a copolymer polyester. The main component resin means the resin that has the largest mass ratio among the resins that make up the polyester film, and may account for 50 mass% or more, 75 mass% or more, 90 mass% or more, or 100 mass% of the resins that make up the polyester film.

[0012] The homopolyester is preferably one obtained by polycondensation of an aromatic dicarboxylic acid and an aliphatic glycol, such as terephthalic acid or 2,6-naphthalenedicarboxylic acid, or an aliphatic glycol, such as ethylene glycol, diethylene glycol, 1,4-butanediol, or 1,4-cyclohexanedimethanol. Typical examples of homopolyesters include polyethylene terephthalate (PET), polybutylene terephthalate (PBT), and polyethylene-2,6-naphthalate (PEN). In the present invention, polyethylene terephthalate (PET) and polyethylene-2,6-naphthalate (PEN) are particularly preferred, and these can also be used in combination.

[0013] On the other hand, when the polyester is a copolymer polyester, it is preferable that the copolymer contains 30 mol % or less of a third component. Examples of the dicarboxylic acid component of the copolymer polyester include one or more of isophthalic acid, phthalic acid, terephthalic acid, 2,6-naphthalenedicarboxylic acid, adipic acid, sebacic acid, etc., and examples of the glycol component include one or more of ethylene glycol, diethylene glycol, propylene glycol, 1,4-butanediol, 1,4-cyclohexanedimethanol, neopentyl glycol, etc. Among these, the polyester film is preferably a polyethylene terephthalate film containing 60 mol % or more, preferably 80 mol % or more of ethylene terephthalate units, or a polyethylene-2,6-naphthalate film containing 60 mol % or more, preferably 80 mol % or more of ethylene-2,6-naphthalate units.

[0014] (Polyester polycondensation catalyst) Examples of polycondensation catalysts used in obtaining the polyester by polycondensation include antimony compounds, germanium compounds, aluminum compounds, titanium compounds, etc. Among these, at least one of antimony compounds and titanium compounds is preferred, and it is particularly preferred to use a polyester obtained using a titanium compound. Therefore, the polyester film preferably contains at least one of an antimony compound (Sb-based) and a titanium compound (Ti-based), and more preferably contains a titanium compound (Ti-based). By using the titanium compound, it is possible to reduce the number of metal-containing aggregates, so-called coarse foreign matter, that originate from the titanium compound in the film.

[0015] When the present film is a laminated film, it is preferable that the polyester constituting its outermost layer (also called the "surface layer", for example, the surface layer onto which the cured resin layer described below is laminated) uses a titanium compound as its polycondensation catalyst, and for example, it is preferable that the surface layer described below contains a titanium compound. The content of titanium element derived from the titanium compound in the outermost layer is preferably 3 ppm by mass or more and 40 ppm by mass or less, and more preferably 4 ppm by mass or more and 35 ppm by mass or less. Within the above range, the catalyst-induced foreign matter can be reduced without reducing the production efficiency of polyester. From the same viewpoint, the content of antimony compounds in the outermost layer of the present film is preferably 100 ppm by mass or less.

[0016] (particle) Particles can be blended into the polyester resin (X) primarily for the purposes of imparting lubricity and preventing scratches during each process. When particles are blended, the type of particles to be blended is not particularly limited as long as they are capable of imparting lubricity. Specific examples include inorganic particles such as silica, calcium carbonate, magnesium carbonate, barium carbonate, calcium sulfate, calcium phosphate, magnesium phosphate, kaolin, aluminum oxide, and titanium oxide, and organic particles such as acrylic resin, styrene resin, urea resin, phenolic resin, epoxy resin, and benzoguanamine resin. Furthermore, precipitated particles obtained by precipitating and finely dispersing a portion of a metal compound such as a catalyst during the polyester production process can also be used.

[0017] On the other hand, the shape of the particles to be used is not particularly limited, and any of spherical, blocky, rod-like, flat, etc. may be used. Furthermore, there are no particular limitations on the hardness, specific gravity, color, etc. Two or more types of these particles may be used in combination as needed. The average particle size of the particles used is preferably 3 μm or less, more preferably 0.1 to 2 μm, and particularly preferably in the range of 0.1 to 1 μm. By using particles having an average particle size in the above range, an appropriate surface roughness can be imparted to the polyester film, ensuring good slipperiness and smoothness. When particles are blended, for example, a surface layer and an intermediate layer may be provided, and the particles may be contained in the surface layer. In this case, a multilayer structure may be formed having a particle-containing surface layer, an intermediate layer, and another particle-containing surface layer in this order.

[0018] In addition, an embodiment in which the polyester resin (X) does not substantially contain particles is also preferred. Here, "substantially not containing particles" means that particles are not intentionally contained, and specifically refers to a particle content (particle concentration) of 200 ppm by mass or less, more preferably 150 ppm by mass or less. The polyester film of the present invention contains a polypropylene resin (Y) that is incompatible with the polyester resin (X) constituting the polyester, and thus the polypropylene resin (Y) can form fine irregularities on the film surface, which allows the particle amount in the film to be kept below the upper limit, making it easier to impart slipperiness to the film while ensuring the transparency of the film. If the polyester film does not contain particles or contains only a small amount of particles, the transparency of the base film will be high and a polyester film with a good appearance will be obtained, but the slipperiness may be insufficient. In such cases, it is advisable to improve the slipperiness by incorporating particles into the cured resin layer, as described below.

[0019] (Polypropylene resin (Y)) The polypropylene resin (Y) may be a homopolymer of propylene, or a copolymer of propylene with ethylene and / or an α-olefin having 4 to 20 carbon atoms. In the present invention, one type of copolymerization component may be used, or two or more types may be used in combination, if necessary.

[0020] The method for producing the polypropylene resin (Y) is not particularly limited, and any conventionally known method, such as a method of polymerizing propylene and an optional comonomer using a Ziegler-Natta catalyst, can be used. This method is also described in Section 2.3.1 (pages 20-57) of "Polypropylene Handbook," edited by Edward P. Moore Jr., translated and supervised by Tetsuo Yasuda and Nobu Sakuma, published by the Industrial Research Institute (1998).

[0021] (MFR (Melt Flow Rate)) In the present invention, the MFR is preferably in the range of 0.1 to 10 g / 10 min, more preferably 0.5 to 8 g / 10 min, and even more preferably 1 to 8 g / 10 min. By satisfying the above range, good fluidity can be ensured during film forming processing. The MFR was measured in accordance with ISO 1133:1997.

[0022] (Melting Point) The melting point of the polypropylene resin (Y) is preferably 150°C or higher, more preferably 160°C or higher. When the melting point is equal to or higher than the lower limit, the desired heat resistance can be ensured. The upper limit of the melting point is not particularly limited, but is usually 170°C. The melting point in the present invention can be determined by differential scanning calorimetry (DSC). Specifically, the temperature is first raised from room temperature to 200°C, the thermal history is erased, and then the temperature is lowered to 40°C at a rate of 10°C / min, and the temperature is again measured at a rate of 10°C / min, and the melting point is defined as the temperature at the top of the endothermic peak.

[0023] (Containing amount) The amount of polypropylene resin (Y) blended relative to 100 parts by mass of polyester resin (X) is preferably 1 to 30 parts by mass, more preferably 5 to 30 parts by mass, even more preferably 10 to 30 parts by mass, and particularly preferably 10 to 25 parts by mass. By satisfying the above range, for example, the polyester film can have good electrical properties when used as a capacitor film. When the polyester film is a multilayer film, the above-mentioned blending amount refers to the blending amount in the entire polyester film. Therefore, even if the multilayer polyester film has layers with different blending amounts of polypropylene resin, for example, the blending amount refers to the blending amount of polypropylene resin in the entire laminate film. However, when only some of the layers contain the polypropylene resin (Y), and only some of the layers have, for example, a polypropylene resin layer (Yt) as described below, resulting in a layered structure, the amount of polypropylene resin (Y) in that layer may be as described above, and the overall amount does not necessarily need to be within the above range.

[0024] <Method for forming a cured resin layer> The method for forming the cured resin layer will be described in detail below. The cured resin layer may be formed by in-line coating or off-line coating. In-line coating is a method in which a coating liquid of a cured resin layer composition is applied to the surface of a polyester film on the production line in which the polyester film is produced. Off-line coating is a method in which a coating liquid is applied to a polyester film that has already been produced outside the system (outside the production line). From the viewpoint of ease of processing, the cured resin layer is preferably formed by in-line coating.

[0025] Specifically, in-line coating is a method of applying a coating liquid of a cured resin layer composition to a polyester film at any stage from melt-extrusion of the polyester to stretching, heat-setting, and winding up. Usually, the coating liquid is applied to the polyester film in any of the following stages: an unstretched sheet obtained by melting and quenching, a stretched uniaxially stretched film, a biaxially stretched film before heat-setting, and a film after heat-setting and before winding up.

[0026] Examples of the method for applying the coating liquid include air doctor coating, blade coating, rod coating, bar coating, knife coating, squeeze coating, impregnation coating, reverse roll coating, transfer roll coating, gravure coating, kiss roll coating, cast coating, spray coating, curtain coating, calendar coating, and extrusion coating.

[0027] Furthermore, although not particularly limited, for example, in sequential biaxial stretching, a method in which a coating solution is applied to a uniaxially stretched film that has been stretched in the longitudinal direction (machine direction) and then stretched in the transverse direction is particularly preferred. This method has the advantage of reducing production costs because it allows the polyester film and the cured resin layer to be formed simultaneously. Furthermore, since stretching is performed after coating, the thickness of the cured resin layer can be changed by adjusting the stretch ratio, making thin-film coating easier than offline coating. Furthermore, the thickness of the cured resin layer can be made more uniform.

[0028] Furthermore, by applying a coating liquid of the cured resin layer composition onto the polyester film before stretching, the cured resin can be stretched together with the polyester film, thereby allowing the cured resin layer to adhere firmly to the polyester film. Furthermore, in the production of biaxially stretched films, stretching while gripping the film edges with clips or the like allows the polyester film to be restrained in both the longitudinal and transverse directions, and in the heat setting step, the film can be heated to a high temperature while maintaining its flatness and without wrinkles, etc. Therefore, the heat treatment performed after application of the cured resin layer composition can be performed at a high temperature that cannot be achieved by other methods, allowing for stronger adhesion between the cured resin layer and the polyester film.

[0029] In addition, the coating solution of the cured resin layer composition applied to the polyester film may be subjected to either heat treatment or irradiation with active energy rays such as ultraviolet light, or both, whether by offline coating or inline coating, but at least heat treatment is preferably performed. The cured resin layer composition may be cured by one or both of heat treatment and irradiation with active energy rays. The heat treatment may be performed by heating, for example, in a heat setting step as described above, but may also be performed by other methods. When the coating solution of the cured resin layer composition contains a solvent, it is dried as appropriate, but it is preferably dried by the heat treatment described above. Furthermore, in order to improve the applicability of the coating liquid for forming the cured resin layer to the polyester film and the adhesion of the cured resin layer to the polyester film, the surface of the polyester film on which the cured resin layer is to be formed may be subjected to a surface treatment such as chemical treatment, corona discharge treatment, plasma treatment, ozone treatment, chemical treatment, or solvent treatment before application of the coating liquid.

[0030] As described above, the polyester film with a cured resin layer may have another layer between the polyester film and the cured resin layer, such as an antistatic layer, an easy-adhesion layer, or an oligomer sealing layer, which has various functions.

[0031] The thickness of the cured resin layer is preferably 0.005 to 1 μm. By making the thickness 1 μm or less, the components constituting the cured resin layer are prevented from migrating to the cured resin layer, and further, blocking during winding is more easily prevented. On the other hand, by making the thickness 0.005 μm or more, it is easier to suppress attenuation of the electrostatic potential. From these viewpoints, the thickness of the cured resin layer is more preferably 0.01 μm or more, even more preferably 0.02 μm or more, and is more preferably 0.2 μm or less, even more preferably 0.1 μm or less, and particularly preferably 0.06 μm or less.

[0032] <Method of manufacturing the present polyester film> As an example of the method for producing the present polyester film, a method for producing a biaxially stretched film will be described below, although the present polyester film is not limited to the method described here.

[0033] First, using a known method, raw materials, such as polyester chips, are fed into a melt extrusion device, heated to above the melting point of each polymer, the molten polymer is extruded through a die, and cooled and solidified on a rotating cooling drum to a temperature below the glass transition point of the polymer, thereby obtaining an unoriented sheet in a substantially amorphous state. In this case, it is advisable to improve dispersion by twin-screw kneading extrusion. The rotation speed and residence time of the extruder should satisfy the following. By satisfying the following ranges, the polypropylene resin is finely dispersed in the polyester resin, and a film with good electrical properties can be obtained.

[0034] The extruder used in the present invention is preferably a vented twin-screw extruder having a raw material supply port and a vent opening in the cylinder, and has at least one or more decompressed vents. The diameter (cylinder inner diameter) D (mm) of the twin-screw extruder used in the present invention is not particularly limited. The length L (mm) of the twin-screw extruder used in the present invention is preferably 20D (D is the cylinder inner diameter of the twin-screw extruder) to 50D, more preferably 25D to 45D. When L is 50D or less, the residence time in the extruder is not prolonged, and thermal degradation of the polyester and the incompatible polymer is not suppressed, which is preferable. On the other hand, when L is 20D or more, the polyester tends to be sufficiently plasticized, resulting in stable melt extrusion.

[0035] In the present invention, when the polyester resin (X) is melt-extruded together with the polypropylene resin (Y) into a film, the content of the polypropylene resin (Y) is 1% by mass or more relative to the total extrusion rate per unit time Q (kg / hour). When the content of the polypropylene resin (Y) is 1% by mass or more, the polyester film can have good electrical properties, for example, when used as a polyester film for capacitors. From the above viewpoints, the content of the polypropylene resin (Y) is preferably in the range of 1 to 30% by mass, more preferably 1 to 20% by mass, and particularly preferably 1 to 15% by mass, relative to the total extrusion rate per unit time Q (kg / hour). The content of the polypropylene resin (Y) is more preferably 1 to 30 parts by mass, further preferably 5 to 20 parts by mass, and particularly preferably 5 to 15 parts by mass, relative to 100 parts by mass of the polyester resin (X).

[0036] In the present invention, it is essential that the screw rotation speed N (rpm) satisfies the following formula (1) relative to the cylinder inner diameter D (mm) and the total extrusion rate Q (kg / hour) per unit time. In the region satisfying the range of formula (1), the rotation speed is appropriate for the extrusion rate, the degree of renewal of the molten resin surface under vacuum is moderate, and sufficient degassing is achieved, enabling the desired polyester to be molded. From the above viewpoints, it is more preferable to satisfy the following formula (2), and even more preferable to satisfy the following formula (3).

[0037] 0.05×10 -6 ×D 2.8 ≦Q / N≦6.0×10 -6 ×D 2.8 (1) 0.1×10 -6 ×D 2.8 ≦Q / N≦6.0×10 -6 ×D 2.8 (2) 0.1×10 -6 ×D 2.8 ≦Q / N≦5.5×10 -6 ×D 2.8 (3)

[0038] Next, the unoriented sheet is stretched in one direction using a roll or tenter type stretching machine, at a stretching temperature of usually 25 to 120°C, preferably 35 to 100°C, and at a stretching ratio of usually 2.5 to 7 times, preferably 2.8 to 6 times. Next, the film is stretched in a direction perpendicular to the first-stage stretching direction, at a stretching temperature of usually 50 to 140° C. and a stretching ratio of usually 3.0 to 7 times, preferably 3.5 to 6 times. In the above stretching, a method of performing unidirectional stretching in two or more stages can also be employed. In the present invention, as described above, it is preferable to increase the draw ratio in the transverse direction, and therefore it is preferable to set the draw ratio at 4.5 times or more in either the first or second stage.

[0039] After stretching, the film is subsequently heat-set at a temperature of 130 to 270° C. under tension or under relaxation of 30% or less, thereby obtaining the present polyester film as a biaxially oriented film. The heat resistance and other properties of the polyester film can be improved by subjecting it to a heat setting treatment. When producing a laminated polyester film having a multilayer structure as the present polyester film, the resin compositions constituting each layer are prepared and co-extruded to obtain a multilayer film. When co-extruding, all layers may be extruded under the above conditions, but it is sufficient that at least one layer is produced under the above extrusion conditions. For example, in the case of a three-layer structure of surface layer / middle layer / surface layer, only the middle layer may be extruded under the above conditions, or all layers may be extruded under the above conditions. The subsequent stretching and the like may be carried out in the same manner as above.

[0040] (film thickness) The thickness of the polyester film of the present invention is preferably 0.5 to 12.0 μm. A thickness within this range is suitable for use, for example, in capacitors. From the above viewpoints, the thickness of the polyester film of the present invention is more preferably 0.5 to 10.0 μm, and particularly preferably 1.0 to 8.0 μm.

[0041] <Characteristics of this polyester film> (Cross-sectional structure of film) The polyester film of the present invention preferably has a polypropylene resin layer (Yt) having a layered cross-sectional structure and a ratio (y / x) of the length in the thickness direction (x) to the length in the width direction (TD) (y) of 17 or more. In addition to the above, it is preferable to simultaneously satisfy at least one of the following additional requirements 1) to 4). 1) The number of polypropylene resin layers (Yt) per unit area (pieces / μm) in which the ratio (y / x) of the length in the thickness direction (x) to the length in the width direction (TD) (y) is 17 or more 2 ) is 0.08 pieces / μm 2It is preferable that the number of particles is 0.10 particles / μm or more. 2 That's all. 2) The value obtained by dividing the ratio (y / x) by the film thickness (t) is preferably 1.5 or more, more preferably 2.0 or more, even more preferably 4.0 or more, and particularly preferably 5.0 or more. 3) It is preferable that the polypropylene resin layer (Yt) has a structure that is continuous from end to end in the width direction (TD) of the observation field. The size of the observation field is 19 μm×25 μm. 4) It is preferable that the film has a layered structure in which there are a plurality of polypropylene resin layers (Yt) and a polyester resin layer (Xt) is interposed between adjacent polypropylene resin layers (Yt) in the thickness direction. By simultaneously satisfying at least one, preferably two or more, more preferably three or more, and most preferably all of the above 1) to 4), it is possible to obtain a polyester film having a good dielectric loss tangent.

[0042] Specific means for forming the above structure include, for example, ensuring sufficient mixing time when the raw materials are mixed, setting the stretching ratio, particularly the transverse stretching ratio, to 4.0 times or more in terms of stretching conditions, selecting a polypropylene resin with an MFR of 0.1 to 10 g / 10 min, and using a compatibilizer in combination. These methods may be used alone or in combination.

[0043] In the present invention, in order to accommodate capacitor applications, the film thickness is different from normal, being mainly in the ultra-thin range (0.5 to 12 μm), which differs from other applications (for example, to provide cushioning as a paper replacement) in which voids are intentionally formed by using an incompatible polypropylene resin in the polyester resin. The reason for using polypropylene resin is, for example, to improve the electrical properties, particularly in the low frequency range, of a film for a capacitor. From this viewpoint, the present invention was completed by focusing on the relationship between the cross-sectional structure of the film and the electrical properties (dielectric loss tangent) as described above, based on the design concept that the electrical properties will be improved if the formation of voids derived from the polypropylene resin in the film after film molding processing is minimized.

[0044] Although the mechanism by which the layered structure is formed is unknown, it is presumed that when a film having a thickness in the ultrathin region (0.5 to 12 μm) is stretched, forces are applied from above and below the film in the thickness direction. The combined effect of the forces applied from above and below the film in the thickness direction and the degree of kneading improves the adhesion between the polyester resin layer (Xt) and the polypropylene resin layer (Yt). As a result, it is presumed that, for example, air present at the interface between the two is pushed out, resulting in a layered structure that is less likely to form voids.

[0045] (Dielectric tangent (tanδ)) The polyester film of the present invention preferably has a tan δ at 1 kHz of 0.54 or less, more preferably 0.50 or less. When the tan δ satisfies the above range, the electrical properties of the film become good, making it suitable for use in capacitors.

[0046] <Metal laminated film> The polyester film of the present invention can be formed into a metal-laminated film by providing a metal layer on at least one surface thereof, and the metal-laminated film is useful for capacitors. Examples of metals include copper, silver, chromium, aluminum, nickel, and zinc, among which aluminum and zinc are preferred from the standpoints of cost and environmental friendliness. The thickness of the metal layer is preferably in the range of 10 to 5000 Å, more preferably in the range of 100 to 4000 Å, and even more preferably in the range of 100 to 2000 Å. A thickness within the above range is advantageous in terms of electrical properties.

[0047] <Explanation of terms, etc.> In the present invention, the term "film" includes the term "sheet", and the term "sheet" includes the term "film". In the present invention, when it is stated that "X to Y" (X and Y are any numbers), unless otherwise specified, it means "X or more and Y or less", and also means "preferably larger than X" or "preferably smaller than Y". Furthermore, when it is stated that the amount is "X or more" (X is any number), it also means that the amount is "preferably greater than X" unless otherwise specified, and when it is stated that the amount is "Y or less" (Y is any number), it also means that the amount is "preferably smaller than Y" unless otherwise specified. [Example]

[0048] The present invention will now be described in more detail with reference to examples, although the present invention is not limited to the examples described below.

[0049] <Evaluation method> The methods for measuring and evaluating various physical properties and characteristics are as follows.

[0050] (1) Intrinsic viscosity (IV) 1 g of polyester was precisely weighed, dissolved in 100 ml of a mixed solvent of phenol / tetrachloroethane = 50 / 50 (weight ratio), and measured at 30°C.

[0051] (2) Film thickness After fixing a small piece of film in an epoxy resin, it was cut with a microtome and the cross section of the film was observed using a transmission electron microscope. Two interfaces were observed in the cross section, roughly parallel to the film surface, as light and dark areas. The distance from the two interfaces to the film surface was measured from 10 photographs, and the average value was taken as the film thickness.

[0052] (3) Melting point of polypropylene resin (Y) Differential scanning calorimetry (DSC) is performed by heating from room temperature to 200°C, erasing the thermal history, then lowering the temperature to 40°C at a rate of 10°C / min, and measuring again at a heating rate of 10°C / min. The endothermic peak temperature is defined as the temperature at which the endothermic peak is reached.

[0053] (4) Ratio (y / x) of the length (x) in the thickness direction of the polypropylene resin layer (Yt) to the length (y) in the width direction (TD) The cross-sectional structure of the sample film was observed using an SEM (Hitachi High-Tech Corporation, Model: FE-SEM SU8220) (observation field: 19 μm × 25 μm, magnification: 5000 times). Then, using the photographed cross-sectional image, the ratio (y / x) of the length (x) in the thickness direction of the polypropylene resin layer (Yt) to the length (y) in the width direction (TD) was measured. In addition, the number of polypropylene resin layers (Yt) per unit area in the observation field where the ratio (y / x) was 17 or more (number / μm 2 ), the number of particles in the observation field (particles / 19 μm×25 μm), and the number of particles in the observation field / film thickness (t) were calculated.

[0054] (5) Dielectric tangent (tanδ) A sample film with circular Al vapor-deposited on both sides was placed on an apparatus (HP (HEWLETT PACKARD) Model: 4284A), and electrodes were placed on top and bottom of the sample film. The current frequency was set to 1 kHz, and tan δ was measured. Specifically, when an AC voltage is applied to a capacitor, power loss occurs, and the loss angle at this time is defined as δ, and tan δ is the dielectric tangent. The smaller the value of tan δ, the better the capacitor.

[0055] The raw materials of the polyester films in the examples and comparative examples are as follows. (polyester film) (a) Polyester resin (X1): Polyethylene terephthalate homopolymer having an intrinsic viscosity of 0.63 (polycondensation catalyst: antimony). (b) Polyester resin (X2): polyethylene terephthalate homopolymer (polycondensation catalyst: antimony) containing 0.5% by mass of silica particles with an average particle size of 0.8 μm and having an intrinsic viscosity of 0.65. (c) Polypropylene resin (Y): melting point 163°C, MFR = 7.5 (Sumitomo Chemical Co., Ltd.: FLX80E4)

[0056] [Example 1] 70% by mass of polyester X1 having an intrinsic viscosity of 0.63 (dL / g), 20% by mass of polyester X2 having an intrinsic viscosity of 0.65 (dL / g), and 10% by mass of polypropylene Y1 having an MFR of 0.75 were melt-extruded into a film at 285°C using a vented twin-screw extruder (cylinder diameter D = 47 mm, screw rotation direction = co-rotation), and then quenched and solidified using an electrostatic adhesion method on a rotating cooling drum at 25°C to obtain an amorphous film. The total extrusion rate (Q) was 20 (kg / h), and the screw rotation speed (N) was 100 (rpm). Next, the film was stretched 3.2 times in the machine direction (MD) at a film temperature of 85°C using the difference in roll peripheral speed. It was introduced into a tenter and stretched 4.7 times in the transverse direction (TD) at 100°C. After that, it was heat-treated at 220°C and then relaxed 0.5% in the transverse direction to obtain a polyester film with a thickness of 8.1 μm. 1 shows a cross-sectional photograph (magnification: 5000 times) of the polyester film (sample film) obtained in Example 1. Table 1 shows the physical properties measured by the above-mentioned methods.

[0057] [Comparative Example 1] 90% by mass of polyester X1 having an intrinsic viscosity of 0.63 (dL / g) and 10% by mass of polypropylene Y1 having an MFR of 0.75 were melt-extruded into a film at 285°C using a vented single-screw extruder (cylinder diameter D = 150 mm, screw rotation direction = co-rotation), and then quenched and solidified using an electrostatic adhesion method on a rotating cooling drum at 25°C to obtain an amorphous film. The total extrusion rate (Q) was 445.7 (kg / h), and the screw rotation speed (N) was 41.42 (rpm). Next, the film was stretched 3.8 times in the machine direction (MD) at a film temperature of 86° C. using the difference in roll peripheral speed. The film was introduced into a tenter and stretched 4.0 times in the transverse direction (TD) at 85° C. After that, it was heat-treated at 220° C. to obtain a polyester film with a thickness of 10.9 μm. 2 shows a cross-sectional photograph (magnification: 5000 times) of the polyester film (sample film) obtained in Comparative Example 1. Table 1 shows the physical properties measured by the above-mentioned methods.

[0058] [Table 1]

[0059] It was found that Example 1, in which a twin-screw extruder was used, had a better dielectric loss tangent (tan δ) than Comparative Example 1, in which a single-screw extruder was used. This is thought to be because, when focusing on the cross-sectional structure of the film, although the polypropylene resin (Y) in both films has a flat layer structure, a larger number of layer structures were confirmed in the observation field of view in Example 1. In other words, in relation to the electrical properties (dielectric loss tangent), the electrical properties tended to be better when the number of layers of the layer structure present in the film thickness direction was greater. The mechanism by which the layered structure is formed is unknown, but it is presumed that when stretching a film having a thickness in the ultrathin region (0.5 to 12 μm), particularly when the stretching ratio in the transverse direction is 4.5 times or more, stronger forces are applied from above and below the film in the thickness direction. It is presumed that the synergistic effect of the forces applied from above and below in the thickness direction of the film and the degree of kneading further improves the adhesion between the polyester resin layer (Xt) and the polypropylene resin layer (Yt). As a result, for example, air present at the interface between the polyester resin layer (Xt) and the polypropylene resin layer (Yt) is pushed out, resulting in a layered structure that is less likely to form voids.

Claims

1. A vented twin-screw extruder having a raw material supply port and a vent opening in a cylinder is used to melt-extrude a polyester resin (X) together with a polypropylene resin (Y) into a film shape, the polypropylene resin (Y) being contained in an amount of 1 to 30 parts by mass per 100 parts by mass of the polyester resin (X), A method for producing a polyester film, the method comprising the steps of: (a) extruding a polypropylene resin (Y) in an amount of 1% by mass or more relative to the total extrusion rate per unit time Q (kg / hour); (b) extruding a cylinder having an inner diameter D (mm) and a screw rotation speed N (rpm) relative to the total extrusion rate per unit time Q (kg / hour) such that N satisfies the following formula (3); (c) extruding an unoriented sheet into a film shape; (d) stretching the unoriented sheet in one direction at a temperature range of 25 to 120°C in a tenter-type stretching machine at a draw ratio of 2.5 to 7; (e) stretching the unoriented sheet in a direction perpendicular to the first-stage stretching direction at a temperature range of 50 to 140°C in a direction perpendicular to the first-stage stretching direction at a draw ratio of 3.0 to 7; and (f) subsequently heat setting the unoriented sheet at a temperature of 130 to 270°C under tension or relaxation of 30% or less to obtain a biaxially oriented film having a thickness of 0.5 to 12.0 μm. 0.1×10 －６ ×D ２．８ ≦Q / N≦5.5×10 －６ ×D ２．８ (3)

2. 2. The method for producing a polyester film according to claim 1, wherein the polyester resin (X) is at least one selected from the group consisting of polyethylene terephthalate and polyethylene-2,6-naphthalate.

3. The method for producing a polyester film according to claim 1 or 2, wherein the polycondensation catalyst for the polyester resin (X) is a Ti-based or Sb-based catalyst.

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