Biaxially-oriented polyolefin film

Abstract

This biaxially-oriented polyolefin film is characterized in that: the proportion S of the melting heat amount in the range of 175-190°C to the melting heat amount in the range of 30-190°C obtained by differential scanning calorimetry is 10-70%; the biaxially-oriented polyolefin film has two layers having different contained amounts of a 4-methyl-1-pentene polymer; when, of the two layers, a layer having a smaller contained amount of the 4-methyl-1-pentene polymer is referred to as a layer A and the other layer having a greater contained amount thereof is referred to as a layer B, the layer B is positioned on at least one surface of the film; and the layer B contains the 4-methyl-1-pentene polymer as the main component. Provided is a biaxially-oriented polyolefin film that can be suitably used even in a high temperature environment in which conventional biaxially-oriented polyolefin films cannot be used as a release film or a process film.

Description

Biaxially oriented polyolefin film

[0001] The present invention relates to a biaxially oriented polyolefin film having excellent heat resistance.

[0002] Biaxially oriented polyolefin films are films obtained by biaxially stretching an unstretched polyolefin sheet, and because of their excellent productivity and film thickness accuracy, they are used in a variety of applications such as packaging, tapes, etc. Furthermore, because biaxially oriented polyolefin films also have excellent mechanical properties and releasability, they are also suitably used as release films and processing films for various components such as plastic products, building materials, and optical components.

[0003] The required properties of the release film and processing film are appropriately determined depending on the intended use. However, as the performance of materials improves and productivity improves, molding temperatures and processing temperatures tend to rise year by year, making heat resistance increasingly important. In addition, processing films also have applications where the generation of outgassing, such as water vapor, is undesirable, such as in electrodes for secondary batteries and processing films for metal sputtering. For these applications, biaxially oriented polyolefin films with relatively low moisture content are expected to be suitable.

[0004] However, for example, when processing a component formed on a release film or when manufacturing a component using a processing film, a process of heating at temperatures exceeding 160°C for at least several minutes is often required. Under such conditions, even polypropylene, which has relatively high heat resistance among polyolefins, is prone to fusion and deformation, which can cause problems such as impairing the quality of the component formed on the film. Therefore, in such cases, it has been very difficult to use biaxially oriented polyolefin films as release films or processing films.

[0005] One indicator of heat resistance is fusion resistance, and to improve this, studies have been conducted to form a layer containing a 4-methyl-1-pentene polymer or its copolymer, which is a resin with a relatively high melting point, on the surface of a film. However, when these resins are laminated with other polyolefins, the difference in the melting points between the two makes the appropriate stretching temperature different, which can easily cause incomplete layer formation or film rupture. In addition, these resins have poor adhesion to other polyolefins, making them prone to interlayer delamination between the substrate and the surface layer, which is another issue, making it difficult to demonstrate the properties required for release films and process films.

[0006] In the above circumstances, for example, Patent Documents 1 and 2 describe examples in which a layer made of a mixture of polypropylene and a 4-methyl-1-pentene polymer or a copolymer thereof is laminated as a surface layer on a polypropylene layer to improve interlayer adhesion and mold releasability at room temperature. Also, Patent Document 3 describes an example in which heat sealability is improved by laminating a matte layer of polypropylene containing a propylene block copolymer and adjusting the molecular weight of the inner polypropylene layer.

[0007] International Publication No. 2018 / 097161 Japanese Patent Application Laid-Open No. 2014-30974 Japanese Patent Application Laid-Open No. 2015-44406

[0008] However, the films obtained by the methods described in Patent Documents 1 and 2 are not intended for use in high-temperature environments, and fusion and deformation are problems in high-temperature environments. Furthermore, the films obtained by the method described in Patent Document 3 are intended for exposure to high temperatures only for extremely short periods of time, and therefore may not sufficiently suppress thermal shrinkage and fusion. In other words, polyolefin films obtained by these methods are difficult to use as release films or process films in high-temperature environments.

[0009] Therefore, an object of the present invention is to solve the above problems, that is, to provide a biaxially oriented polyolefin film that can be suitably used even in high-temperature environments, which has conventionally made it impossible to use it as a release film or a processing film.

[0010] In order to solve the above-mentioned problems, the biaxially oriented polyolefin film of the present invention has the following configuration: a biaxially oriented polyolefin film in which the proportion S of the heat of fusion in the range of 175 to 190°C to the heat of fusion in the range of 30 to 190°C as measured by differential scanning calorimetry is 10 to 70%, the biaxially oriented polyolefin film has two layers each having a different content of 4-methyl-1-pentene polymer, the layer with a relatively lower content of 4-methyl-1-pentene polymer being designated as layer A and the layer with a higher content of 4-methyl-1-pentene polymer being designated as layer B, the layer B being located on at least one of the film surfaces, and the layer B containing the 4-methyl-1-pentene polymer as a main component.

[0011] The present invention makes it possible to provide a biaxially oriented polyolefin film that can be suitably used even in high-temperature environments, where conventional biaxially oriented polyolefin films have been unable to be used as release films or process films.

[0012] FIG. 1 is a schematic diagram illustrating a pressurizing method used in evaluating heat resistance characteristics.

[0013] The biaxially oriented polyolefin film of the present invention is characterized in that the proportion S of the heat of fusion in the range of 175 to 190°C to the heat of fusion in the range of 30 to 190°C as measured by differential scanning calorimetry is 10 to 70%, the film has two types of layers each having a different content of 4-methyl-1-pentene polymer, and when the layer with a relatively lower content of 4-methyl-1-pentene polymer is designated as layer A and the layer with a higher content is designated as layer B, layer B is located on at least one of the film surfaces, and layer B contains the 4-methyl-1-pentene polymer as a main component. The polyolefin film of the present invention will be described below.

[0014] In the present invention, when a numerical range is expressed as "a to b," the numerical range includes the values ​​of both ends a and b, and when a unit is written only after the numerical range a to b, the unit is the same throughout the entire numerical range.

[0015] Here, film refers to a sheet-like molded product whose main component is a thermoplastic resin. The main component refers to a component contained in an amount of more than 50% by mass but not more than 100% by mass, when the total components of the object (film, layer, etc.) are taken as 100% by mass. Biaxial orientation refers to molecular orientation in two perpendicular directions. A biaxially oriented film can be obtained by stretching a sheet in two perpendicular directions (usually the longitudinal direction and the width direction). The longitudinal direction refers to the direction in which the film runs during the manufacturing process (the winding direction in the case of a film roll), and the width direction refers to the direction perpendicular to the longitudinal direction within the plane of the film.

[0016] A polyolefin film refers to a film whose main component is an olefin resin. However, when multiple olefin resins are contained, even if the content of each individual olefin resin is 50% by mass or less of the entire film, the film is considered to be a polyolefin film if the total content of all olefin resins exceeds 50% by mass. Here, an olefin resin (e.g., polypropylene resin, 4-methyl-1-pentene polymer) refers to a resin that contains more than 50 mol% but not more than 100 mol% of structural units derived from olefin hydrocarbons, where the total structural units constituting the resin composition are taken as 100 mol%. Furthermore, it is preferable that the A layer and the B layer are in contact with each other. Here, "the A layer and the B layer are in contact with each other" refers to a configuration in which the B layer is laminated on one or both sides of the A layer without any other layer in between, and the same configuration is also used in the description below of a film having one side and a membrane "in contact with each other."

[0017] From the viewpoint of suppressing deformation at high temperatures, it is important that the biaxially oriented polyolefin film of the present invention has a ratio S of the heat of fusion in the range of 175 to 190°C to the heat of fusion in the range of 30 to 190°C obtained by differential scanning calorimetry (DSC measurement) of 10 to 70%.

[0018] The heat of fusion ratio S indicates the amount of polyolefin crystals that can remain in the biaxially oriented polyolefin film even at relatively high temperatures. From the above perspective, the lower limit of the heat of fusion ratio S is preferably 15%, more preferably 20%, and even more preferably 25%. If the heat of fusion ratio S of the biaxially oriented polyolefin film is less than 10%, the amount of crystals responsible for maintaining the structure in a high-temperature environment may be insufficient. Therefore, if the heat of fusion ratio S of the biaxially oriented polyolefin film is less than 10%, when used as a release film or processing film in a high-temperature environment, the film will experience increased thermal shrinkage and reduced rigidity, causing excessive embedding or sticking to the mating member. Such excessive embedding or sticking not only adversely affects the surface shape of the mating member, but also raises the risk of damage or deformation of the mating member when the biaxially oriented polyolefin film is peeled from the mating member. On the other hand, the upper limit of the heat of fusion ratio S is 70%, taking into account the thermal properties of the polyolefin that constitutes the biaxially oriented polyolefin film, and 50% is preferred from the viewpoint of compatibility with other physical properties. The method for measuring the proportion S of heat of fusion in DSC measurement will be described later in detail (the same applies to the heat of fusion H described later). The term "counterpart" as used herein refers to a part that is laminated on or adhered to the surface of the biaxially oriented polyolefin film when the biaxially oriented polyolefin film is used as a release film or a process film.

[0019] In order to set the ratio S of the heat of fusion of the biaxially oriented polyolefin film within the above range, a method of setting the raw material composition within the range described later or a method of setting the film production conditions within the range described later can be used. In terms of raw material composition, in order to promote the formation of high-melting-point crystals by film production at high temperatures, a polyolefin with a high melting point (e.g., polypropylene) is used as the raw material, and the half-crystallization time and angular frequency ω of the resin constituting the biaxially oriented polyolefin film are set. ２００ It is effective to properly adjust the temperature and relaxation ratio to control the relaxation characteristics. In terms of the process, it is also effective to set the preheating and stretching temperatures for longitudinal and transverse stretching within the ranges described below, and further to introduce relaxation steps after longitudinal and transverse stretching, and treat the film at a temperature and relaxation ratio within the ranges described below. These methods can also be used in combination as appropriate.

[0020] By adopting the above-mentioned raw material composition and process conditions, stretching and relaxation treatments can be performed under high temperature conditions, promoting relaxation of the amorphous portion that causes heat shrinkage stress, while enabling the formation of high-melting-point crystals that were not previously expected in biaxially oriented polyolefin films. As a result, the resulting biaxially oriented polyolefin film is less likely to deform even in high-temperature environments, making it usable in such environments.

[0021] From the viewpoint of suppressing fusion in a high-temperature environment, the biaxially oriented polyolefin film of the present invention has two types of layers each having a different content of 4-methyl-1-pentene polymer. When the layer with a relatively low content of 4-methyl-1-pentene polymer is designated as Layer A and the layer with a relatively high content of 4-methyl-1-pentene polymer is designated as Layer B, Layer B is located on at least one surface, and Layer B contains a 4-methyl-1-pentene polymer as a main component.

[0022] Here, "having different contents of 4-methyl-1-pentene polymer" means that when comparing the amounts (% by mass) of 4-methyl-1-pentene polymer contained in two layers, the difference is 20% by mass or more. The term "major component" refers to a component contained in a layer at a ratio of more than 50% by mass and not more than 100% by mass, where 100% by mass is taken as the total of all components constituting the layer. When a layer contains multiple types of 4-methyl-1-pentene polymers, the layer is treated as containing a 4-methyl-1-pentene polymer as the major component if the total amount of all the polymers contained exceeds 50% by mass. The term "4-methyl-1-pentene polymer" refers to a polymer containing more than 90% by mol and not more than 100% by mol of structural units derived from 4-methyl-1-pentene, where 100% by mol is taken as the total of all structural units constituting the polymer. The content of 4-methyl-1-pentene polymer in the film can be determined, for example, by separating each layer by cutting or the like, separating and quantifying each polymer component by chromatography using a temperature rising elution fractionator or solvent extraction, and then １３ There is a method for calculating the structural units derived from 4-methyl-1-pentene by C-NMR.

[0023] The phrase "Layer B is located on at least one surface" means both an embodiment in which Layer B is located on one surface and an embodiment in which Layer B is located on both surfaces, regardless of the presence or absence of layers other than Layer A and Layer B. Layer B is a layer responsible for suppressing fusion, and from the viewpoint of achieving this effect on both surfaces, an embodiment in which Layer B is located on both surfaces is preferred.

[0024] When the biaxially oriented polyolefin film of the present invention has three or more layers and the contents (% by mass) of the 4-methyl-1-pentene polymer in the layers are all different, the layer located on the outermost surface and having the highest 4-methyl-1-pentene polymer content is referred to as layer B, and the layer having the lowest 4-methyl-1-pentene polymer content is referred to as layer A. However, when there are multiple layers with the lowest 4-methyl-1-pentene polymer content, the layer with the greatest thickness is referred to as layer A. Furthermore, when the contents of the 4-methyl-1-pentene polymer in the outermost layers on both sides are equal and highest, the outermost layers on both sides are referred to as layer B.

[0025] In the biaxially oriented polyolefin film of the present invention, the upper limit of the 4-methyl-1-pentene polymer content in Layer A is preferably 10% by mass, more preferably 5% by mass, even more preferably 3% by mass, particularly preferably 1% by mass, and most preferably 0% by mass (i.e., no 4-methyl-1-pentene polymer is contained), from the viewpoint of heat shrinkage properties. Furthermore, the 4-methyl-1-pentene polymer content in Layer B is preferably more than 80% by mass but not more than 100% by mass, from the viewpoint of suppressing fusion in high-temperature environments, with the lower limit being more preferably 90% by mass, even more preferably 95% by mass, particularly preferably 98% by mass, and most preferably 100% by mass (i.e., consisting solely of 4-methyl-1-pentene polymer). When Layer B is present on both surfaces, the compositions thereof may be the same or different. An example of an embodiment in which Layer B has a different composition is when the outermost layers on both sides have the same and highest 4-methyl-1-pentene polymer content (% by mass), but the types of 4-methyl-1-pentene polymer and other resins are different.

[0026] In order to prevent delamination that impairs the quality of a mating member when the biaxially oriented polyolefin film of the present invention is used as a release film or a processing film, it is preferable that the developed area ratio Sdr of at least one film surface is 0.2 to 10. Here, "at least one film surface" means one or both sides, and when the biaxially oriented polyolefin film is used as a release film or a processing film, it is preferable to laminate a mating member onto Layer B. That is, when Layer B is present on one surface, it is preferable that the Sdr of the Layer B surface is within the above range, and when Layer B is present on both surfaces, it is sufficient to set the Sdr of one or both surfaces within the above range depending on the surface to which the mating member is laminated.

[0027] Sdr is a surface parameter that indicates the degree to which the surface area is increased by the uneven structure of the film surface. Typically, a high value is observed when the surface contains a dense uneven structure. From the above perspective, the lower limit of Sdr is more preferably 0.5, and even more preferably 1.0. The upper limit of Sdr is not particularly limited, but from the viewpoint of film formability, it is substantially 10, preferably 5.0. When Sdr is 0.2 or more, the layer B containing the 4-methyl-1-pentene polymer on the surface of the biaxially oriented polyolefin film has a dense uneven structure, thereby suppressing localized surface damage caused by a coarse uneven structure. Therefore, when used with a release film, the releasability of the biaxially oriented polyolefin film and the mating member, as well as the fusion resistance of the biaxially oriented polyolefin film itself, are easily maintained even in high-temperature environments. Sdr can be measured using a known non-contact surface / layer cross-sectional shape measurement system (e.g., the "VertScan" (registered trademark) series from Ryoka Systems Co., Ltd.), details of which will be described later.

[0028] In order to set Sdr within the above range, it is effective to set the raw material composition and film-forming conditions within the ranges described below. In particular, in terms of the raw material, in order to form a dense uneven structure in combination with stretching, the angular frequency ω of the 4-methyl-1-pentene polymer contained in layer B is set to 0.5. ２６０It is effective to set the speed within an appropriate range (for example, 200 rad / s or less) to strengthen the entanglement of molecular chains and improve the followability during stretching. In terms of process, it is also effective to set the preheating and stretching temperatures in the stretching step within preferred ranges. These methods can be used in combination as appropriate.

[0029] From the viewpoint of release properties at high temperatures, the biaxially oriented polyolefin film of the present invention preferably has a static contact angle I with diiodomethane of 60 to 90° on at least one film surface, and more preferably satisfies the above requirement on the surface of Layer B. The static contact angle I with diiodomethane tends to be high when the outermost surface of the biaxially oriented polyolefin film is uniformly coated with a 4-methyl-1-pentene polymer, and conversely, tends to be low when a polyolefin other than a 4-methyl-1-pentene polymer (e.g., polypropylene) is present in a large amount in the outermost layer. Hereinafter, the "static contact angle I with diiodomethane" may be referred to as the "contact angle I."

[0030] From the above viewpoints, the lower limit of the contact angle I is preferably 60°, more preferably 63°, even more preferably 66°, and particularly preferably 69°. There are no particular limitations on the upper limit of the contact angle I, but from the viewpoints of the raw material characteristics of the 4-methyl-1-pentene polymer and the film formability, it is preferably 90°, more preferably 80°. When the contact angle I is 60° or more, the surface of the biaxially oriented polyolefin film (mainly the surface of Layer B) is more uniformly covered with the 4-methyl-1-pentene polymer, and defects such as excessively dense exposure of polyolefins other than the 4-methyl-1-pentene polymer are greatly reduced. Therefore, when the biaxially oriented polyolefin film of the present invention is used as a release film so that a mating member is formed on the surface of the layer, the releasability of the mating member and the fusion resistance of the biaxially oriented polyolefin film itself are more likely to be maintained even in high-temperature environments.

[0031] In order to set the contact angle I within the above range, it is effective to set the layer structure and film formation conditions within the ranges described below. In particular, in order to uniformly stretch the 4-methyl-1-pentene polymer in the surface layer (mainly layer B) by stretching, the angular frequency ω ２６０It is effective to set the thickness of the layer B to a suitable range. It is also effective to adjust the thickness of the layer B to a suitable range. These methods can be used in combination as appropriate.

[0032] From the viewpoint of handling, the biaxially oriented polyolefin film of the present invention has a Young's modulus in the X direction and a Young's modulus in the Y direction, where the main orientation axis direction is the X direction and the direction perpendicular to the main orientation is the Y direction. Ｘ＋Ｙ From the above viewpoint, it is preferable that E Ｘ＋Ｙ The lower limit of E is more preferably 6.0 GPa. Ｘ＋Ｙ The upper limit of E is preferably 10 GPa. Ｘ＋Ｙ When the modulus is 5.2 GPa or more, the stiffness of the biaxially oriented polyolefin film tends to be high, and when used as a release film or a process film, the handling properties are improved, and the workability and transportability in a high-temperature environment are improved.

[0033] E of the biaxially oriented polyolefin film of the present invention Ｘ＋Ｙ In order to make the value of the raw materials and film-forming conditions fall within the above range, it is effective to make the values ​​of the raw materials and film-forming conditions fall within the ranges described below. In particular, in terms of raw materials, the melting point and angular frequency ω of the polyolefin resin (e.g., polypropylene resin) other than the 4-methyl-1-pentene polymer contained in the A layer are preferably within the ranges described below. ２００ In terms of the process, it is effective to set the longitudinal stretching ratio and the transverse stretching ratio within appropriate ranges. In terms of the layer structure, it is also effective to set the thickness ratio of Layer B within the range described below. These methods can be used in combination as appropriate.

[0034] Here, the main orientation axis direction (X direction) of the biaxially oriented polyolefin film of the present invention refers to the direction in which the Young's modulus is highest when the Young's modulus is measured in each of the directions forming angles of 0°, 15°, 30°, 45°, 60°, 75°, 90°, 105°, 120°, 135°, 150°, and 165° with respect to the longitudinal direction in the film plane, where the longitudinal direction is taken as 0°, under an environment of 30° C. If it is unclear from the appearance of the biaxially oriented polyolefin film which direction is the longitudinal direction, the X direction can be identified by drawing lines at 15° intervals based on an arbitrary straight line on the film plane, sampling slit-shaped film pieces parallel to each line, and similarly measuring the Young's modulus.

[0035] The "orthogonal direction to the main orientation" (Y direction) in the biaxially oriented polyolefin film of the present invention refers to the direction perpendicular to the main orientation axis direction in the film plane. The Y direction is the direction perpendicular to the X direction specified by the above method in the film plane, and is automatically determined once the X direction is determined. The Young's modulus (E Ｘ ) and Y-direction Young's modulus (E Ｙ The method for measuring Young's modulus including the Young's modulus will be described in detail later.

[0036] From the viewpoint of use as a release film or a processing film in a high-temperature environment, the biaxially oriented polyolefin film of the present invention preferably has a heat of fusion H of 0.1 to 20 J / g in the range of 190°C to 260°C as determined by differential scanning calorimetry. From the above viewpoint, the lower limit of the heat of fusion H is more preferably 1 J / g, and even more preferably 3 J / g. From the same viewpoint, the upper limit of the heat of fusion H is more preferably 15 J / g, and even more preferably 10 J / g. When the heat of fusion H is within the above range, the biaxially oriented polyolefin film contains an appropriate amount of high-melting point resin (such as a 4-methyl-1-pentene polymer) to improve heat resistance and achieve an appropriate surface shape. Therefore, when such a biaxially oriented polyolefin film is used as a release film or a processing film, it is possible to maintain high heat shrinkage properties, while also easily maintaining releasability from mating members and the fusion resistance of the biaxially oriented polyolefin film itself even in a high-temperature environment.

[0037] In order to ensure that the heat of fusion H of the biaxially oriented polyolefin film of the present invention falls within the above range, it is effective to set the raw materials and film-forming conditions within the ranges described below. In particular, it is effective to set the proportion of the 4-methyl-1-pentene polymer in each layer of the biaxially oriented polyolefin film within an appropriate range. It is also effective to control the discharge rate to set the proportion of layer B, which contains a large amount of 4-methyl-1-pentene polymer, within an appropriate range. These methods can be used in combination as appropriate.

[0038] From the viewpoint of heat shrinkage properties in a high temperature environment, the biaxially oriented polyolefin film of the present invention has a shrinkage stress of 160°C in the X direction and a shrinkage stress of 160°C in the Y direction, P Ｘ＋Ｙ From the above viewpoint, it is preferable that P Ｘ＋Ｙ The upper limit of P is more preferably 2.0, even more preferably 1.5, and particularly preferably 1.0. Ｘ＋Ｙ By making the P value 3.0 or less, when a biaxially oriented polyolefin film is overlaid or attached to a mating member and then treated in a molding press or a heating oven under a high temperature environment, the biaxially oriented polyolefin film is prevented from thermal shrinkage. Therefore, deformations such as wrinkles and curls are less likely to occur in the biaxially oriented polyolefin film, and as a result, the transfer of these deformations to the mating member to which it is attached is also reduced. Ｘ＋Ｙ The lower limit of is preferably -1.0 from the viewpoint of film formability of the biaxially oriented polyolefin film, and is preferably 0.0 in consideration of compatibility with rigidity at high temperatures. The method for measuring the 160°C shrinkage stress in the TMA measurement will be described later in detail.

[0039] P Ｘ＋Ｙ In order to make the above range, the raw material composition of the biaxially oriented polyolefin film is set within the range described below, and the film-forming conditions are set within the range described below. In particular, in order to appropriately control amorphous relaxation under high-temperature film-forming conditions, a polyolefin resin raw material other than the 4-methyl-1-pentene polymer is used that has a melting point and a frequency ω ２００It is effective to use a polyolefin resin in which the temperature is in the preferred range described later. In terms of process, it is also effective to set the preheating and stretching temperatures for longitudinal stretching and transverse stretching in the preferred range described later, and further to introduce a relaxation step after longitudinal stretching and transverse stretching, and to treat at a temperature and total area relaxation rate in the preferred range described later. These methods can also be used in combination as appropriate.

[0040] In the biaxially oriented polyolefin film of the present invention, the content of components whose logarithm of molecular weight M, Log M, is 5.0 or less in a molecular weight distribution curve measured by gel permeation chromatography is preferably 30.0 to 39.0 mass%. The upper limit of the content of components whose Log M is 5.0 or less is more preferably 37.0 mass%, even more preferably 36.0 mass%, and particularly preferably 35.0 mass%. The lower limit of the content of components whose Log M is 5.0 or less is more preferably 31.0 mass%, even more preferably 33.0 mass%.

[0041] Furthermore, the content of components whose logarithm of molecular weight M, Log M, is 6.0 or more is preferably 3.0 to 10.0% by mass. The upper limit of the content of components whose Log M is 6.0 or more is more preferably 8.0% by mass, even more preferably 6.0% by mass, and particularly preferably 5.0% by mass. The lower limit of the content of components whose Log M is 6.0 or more is more preferably 3.5% by mass, even more preferably 4.0% by mass.

[0042] When the content ratio of the component with Log M of the logarithm of the molecular weight M of the biaxially oriented polyolefin film, Log M, is 5.0 or less and the component with Log M of 6.0 or more, is within the above range, the biaxially oriented polyolefin film will have appropriate relaxation characteristics, will be excellent in both achieving a high crystalline melting point and amorphous relaxation during film formation, and will likely exhibit excellent heat resistance. In order to achieve the content ratio of the component with Log M of 5.0 or less and the component with Log M of 6.0 or more within the above range, it is effective to set the raw material composition of the biaxially oriented polyolefin film within the range described below and to set the film formation conditions within the range described below. In particular, the angular frequency ω of the polyolefin resin other than the 4-methyl-1-pentene polymer used in layer A ２００(details will be described later) is set within the preferred range described later, and it is effective to adjust it by the pre-mixing temperature and the mixing temperature during film formation.

[0043] From the viewpoint of heat resistance, etc., the biaxially oriented polyolefin film of the present invention has a melting point Tm with the highest peak intensity obtained in the second run of DSC measurement at a temperature rise rate of 20°C / min. １ From the above viewpoint, it is preferable that Tm １ The lower limit of Tm is more preferably 166.0°C, further preferably 166.5°C, and particularly preferably 167.0°C. １ The upper limit of Tm is 170.0°C, taking into consideration the properties of polypropylene resin, as will be described later. １ The method for measuring this will be described in detail later.

[0044] Tm １ is an index showing the melting point of the raw material of the polyolefin resin other than the 4-methyl-1-pentene polymer contained in the biaxially oriented polyolefin film of the present invention, and when this is within the above range, film formation under higher temperature conditions becomes possible. Therefore, it is easy to improve the heat shrinkage characteristics, and it is also advantageous in terms of the formation of high melting point crystals, and as a result, the heat resistance of the obtained biaxially oriented polyolefin film can be improved. １ In order to achieve the above range, it is effective to set the raw material composition of the biaxially oriented polyolefin film within the range described below, in particular to use a high-melting point raw material and minimize mixtures other than the high-melting point raw material, or to increase the proportion of layer A in the entire biaxially oriented polyolefin film. These methods may be used in combination as appropriate.

[0045] From the viewpoint of suppressing fusion at high temperatures, the biaxially oriented polyolefin film of the present invention has a melting point Tm observed at 190°C or higher obtained in the second run of DSC measurement at a temperature rise rate of 20°C / min. ２ From the above viewpoint, it is preferable that Tm ２ The lower limit of Tm is more preferably 220°C, and even more preferably 225°C. ２ The upper limit of Tm is more preferably 240°C. ２ The method for measuring this will be described in detail later.

[0046] Tm ２ is an index showing the melting point of the 4-methyl-1-pentene polymer contained in the biaxially oriented polyolefin film of the present invention, and when this is within the above range, it is possible to achieve both film formability and resistance to fusion at high temperatures. ２ In order to achieve the above range, it is effective to set the raw material composition of the biaxially oriented polyolefin film within the range described below, and in particular to use a 4-methyl-1-pentene polymer having a melting point within an appropriate range and to set the addition ratio thereof within an appropriate range. These methods may be used in combination as appropriate.

[0047] The thickness of the biaxially oriented polyolefin film of the present invention is not particularly limited and is adjusted appropriately depending on the application, but is preferably 0.5 to 100 μm from the viewpoint of handleability. From the above viewpoints, the thickness of the biaxially oriented polyolefin film is more preferably 1 to 70 μm, and even more preferably 1 to 55 μm. The thickness of the biaxially oriented polyolefin film can be adjusted by adjusting the screw rotation speed of the extruder, the width of the unstretched sheet, the film-forming speed, the stretching ratio, etc., within a range that does not deteriorate other physical properties. The thickness of the biaxially oriented polyolefin film can be measured using a known micro thickness meter, the details of which will be described later.

[0048] Next, raw materials that can be used in the production of the biaxially oriented polyolefin film of the present invention will be described, but the raw materials are not necessarily limited to these.

[0049] The biaxially oriented polyolefin film of the present invention has two types of layers each having a different content of 4-methyl-1-pentene polymer. Of the two types of layers, the layer with a relatively low content of 4-methyl-1-pentene polymer is referred to as Layer A, and the layer with a relatively high content of 4-methyl-1-pentene polymer is referred to as Layer B. The content of 4-methyl-1-pentene polymer referred to here refers to the content (% by mass) of 4-methyl-1-pentene polymer when the entire layer is taken as 100% by mass.

[0050] From the viewpoint of improving releasability and reducing thermal shrinkage, the proportion of layer A in the biaxially oriented polyolefin film of the present invention is preferably 70 to 99.5% on a thickness basis. The lower limit of the proportion of layer A is more preferably 80%, even more preferably 90%, and particularly preferably 95%. On the other hand, the upper limit of layer A is more preferably 99%, even more preferably 98%, and particularly preferably 97%. Note that "thickness basis" refers to the proportion of the thickness of layer A when the total thickness of the biaxially oriented polyolefin film is taken as 100%. By having the proportion of layer A be 70% or more on a thickness basis, it is possible to maintain favorable thermal shrinkage properties. On the other hand, by having the proportion of layer A be 99.5% or less on a thickness basis, it is possible to ensure a thickness of layer B sufficient to exhibit releasability.

[0051] The proportion of Layer B in the biaxially oriented polyolefin film of the present invention is preferably 0.5 to 30% based on thickness, from the viewpoint of the stretchability and heat absorption characteristics of the 4-methyl-1-pentene polymer. The lower limit of Layer B is more preferably 1%, even more preferably 2%, and particularly preferably 3%. The upper limit of Layer B is more preferably 20%, even more preferably 10%, and particularly preferably 5%. Note that the thickness of Layer A or Layer B referred to here refers to the total thickness of each layer when multiple layers are present.

[0052] The biaxially oriented polyolefin film of the present invention may be formed by combining layers other than the A layer and the B layer, but is preferably formed by only the A layer and the B layer from the viewpoint of heat resistance.

[0053] From the viewpoint of biaxial stretchability and heat shrinkage properties, it is preferable that Layer A in the biaxially oriented polyolefin film of the present invention uses polypropylene composition A. In the present invention, polypropylene composition A is a composition containing 90 to 100 mass% of polypropylene resin when all constituent components are taken as 100 mass%, having a melting point of 165.0 to 170.0°C, and an angular frequency ω ２００The term "polypropylene composition A" refers to a composition having a melting point, half-crystallization time, and angular frequency ω of 10 to 70 rad / s and a melting point, half-crystallization time, and an angular frequency ω of 5 to 200 seconds. The lower limit of the amount of polypropylene resin in polypropylene composition A is more preferably 95% by mass, even more preferably 97% by mass, and particularly preferably 99% by mass, and most preferably 100% by mass, i.e., the polypropylene composition A is composed solely of polypropylene resin. When the composition contains multiple components equivalent to polypropylene resin, the total amount of these components exceeds 95% by mass of the entire resin, and the melting point, half-crystallization time, and angular frequency ω are all within the range of 10 to 200 seconds. ２００ is within the above range, the composition corresponds to Polypropylene Composition A. Furthermore, a polypropylene resin refers to a resin containing propylene units in a proportion of more than 50 mol % and not more than 100 mol %, when all structural units constituting the molecular chain of the resin are taken as 100 mol %.

[0054] The melting point of polypropylene composition A is 165.0 to 170.0°C from the viewpoint of the heat resistance of the resulting biaxially oriented polyolefin film. From the above viewpoint, the lower limit of the melting point of polypropylene composition A is preferably 166.0°C, more preferably 166.5°C, and even more preferably 167.0°C. When polypropylene composition A has a melting point of 165.0°C or higher, high-melting-point crystals are easily formed when the film is formed at high temperatures, improving the heat resistance of the resulting biaxially oriented polyolefin film. The melting points of polypropylene composition A and other components can be measured by DSC, the details of which will be described later. Even when multiple components are contained, the melting point of polypropylene composition A is determined to be the temperature with the highest peak intensity in the DSC melting curve.

[0055] From the viewpoint of the heat resistance of the resulting biaxially oriented polyolefin film, the polypropylene composition A is selected from the group consisting of a propylene copolymer having an angular frequency ω 1 at which the loss tangent obtained by melt viscoelasticity measurement at 200°C is 1. ２００ is 10 to 70 rad / s. Angular frequency ω ２００ is an index of the relaxation characteristics of the polypropylene composition. ２００When the angular frequency ω is small, the relaxation characteristics are low (i.e., relaxation is slow) due to the entanglement of molecular chains, but the entanglement of molecular chains is easily maintained even at high temperatures, which is advantageous for increasing the melting point of the crystal. ２００ When the angular frequency ω of the polypropylene composition A is large, the molecular chains are less entangled with each other, and therefore the relaxation characteristics are high (i.e., relaxation is fast), and the residual strain is efficiently reduced in the Relax step, and the amorphous portion is easily relaxed. ２００ From the above viewpoint, the lower limit of the angular frequency ω is more preferably 15 rad / s, and even more preferably 20 rad / s. ２００ From the above viewpoint, the upper limit of the angular frequency ω is more preferably 50 rad / s, and even more preferably 40 rad / s or less. ２００ When the angular frequency ω is within the above range, the amorphous portion of the polypropylene composition A is relaxed and the crystal melting point is increased, which makes it possible to further improve the heat resistance of the obtained biaxially oriented polyolefin film. ２００ can be measured by a known rotational rheometer, the details of which will be described later (angular frequency ω ２６０ also applies).

[0056] Angular frequency ω of polypropylene composition A ２００ In order to achieve the above range, it is effective to adjust the molecular weight distribution of the polypropylene resin that is the main component of polypropylene composition A to an appropriate range, adjust the molecular weight distribution of the polypropylene resin by adjusting the pre-mixing conditions when obtaining polypropylene composition A, or add a branched polypropylene within an appropriate range. These methods can be combined as appropriate.

[0057] The polypropylene composition A preferably has a crystallization half time of 5 to 200 seconds. The upper limit of the crystallization half time of the polypropylene composition A is more preferably 100 seconds, even more preferably 50 seconds, and particularly preferably 30 seconds. When the crystallization half time is within the above range, the polypropylene resin component in the composition is likely to recrystallize even during film formation, and high-melting-point crystals are likely to form. To achieve a crystallization half time of the polypropylene composition A within the above range, it is effective to appropriately adjust the crystallinity and molecular weight distribution of the polypropylene resin constituting the polypropylene composition A, or to add a branched polypropylene in an appropriate range. The crystallization half time is an index of the crystallization rate and can be measured by DSC (details will be described later). When the polypropylene composition A contains multiple components, the elapsed time at which the peak intensity is highest in the DSC melting curve is taken as the crystallization half time of the polypropylene composition A.

[0058] The polypropylene composition A is preferably composed of a homopolypropylene resin alone or a mixture of two or more homopolypropylene resins. Furthermore, as described below, components such as a branched polypropylene resin, which is a homopolypropylene resin useful for controlling crystallization rate, or a block copolymer polypropylene resin or a random copolymer polypropylene resin other than a homopolypropylene resin may be mixed into the polypropylene composition A by pre-kneading, as long as the properties are not impaired. When a component other than a branched polypropylene resin or a homopolypropylene resin is added to the polypropylene composition A, the amount added is preferably 1.2% by mass or less, more preferably 1.0% by mass or less, and even more preferably 0.5% by mass or less, of the total components of the polypropylene composition A. Here, the term "homopolypropylene resin" refers to a polypropylene resin in which the amount of propylene units is 99.9 to 100 mol %, where the total structural units constituting the molecular chain of the resin is 100 mol %.

[0059] The polypropylene resin used in polypropylene composition A preferably satisfies the above-described conditions for homopolypropylene resin A. Commercially available resins that satisfy these requirements include, for example, polypropylene resins F-704NP and F133A manufactured by Prime Polymer Co., Ltd., polypropylene resin HC310BF manufactured by Borealis, and polypropylene FY6H manufactured by Japan Polypropylene Corporation. Furthermore, commercially available branched polypropylenes that can be added to polypropylene composition A include, for example, "Daploy" WB130HMS, WB135HMS, and WB140HMS manufactured by Borealis, and "WAYMAX" (registered trademark) MFX8, MFX6, and MFX3 manufactured by Japan Polypropylene Corporation.

[0060] Layer B in the biaxially oriented polyolefin film of the present invention preferably contains the above-mentioned 4-methyl-1-pentene polymer in a proportion of 80 to 100%. The lower limit of the proportion of the 4-methyl-1-pentene polymer in Layer B is more preferably 90% or more, even more preferably 95% or more, particularly preferably 98% or more, and most preferably 100% by mass. If the proportion of the 4-methyl-1-pentene polymer in Layer B is less than 80%, the fusion resistance of the resulting biaxially oriented polyolefin film at high temperatures will be affected. Furthermore, a 4-methyl-1-pentene copolymer or the like may be added to Layer B for the purpose of improving adhesion to other layers, etc.

[0061] From the viewpoint of the structure formation by stretching, the 4-methyl-1-pentene polymer in the biaxially oriented polyolefin film of the present invention is selected from the group consisting of a 4-methyl-1-pentene polymer having an angular frequency ω at which the loss tangent obtained by melt viscoelasticity measurement at 260°C becomes 1. ２６０ It is preferable that the angular frequency ω is 1 to 500 rad / s. ２６０ is the angular frequency at which the loss tangent of the molten polypropylene resin becomes 1, and is an index of the relaxation characteristics of the polypropylene resin. ２６０ The lower limit of the angular frequency ω of the 4-methyl-1-pentene polymer is more preferably 5.0 rad / s, and even more preferably 30 rad / s. ２６０ The upper limit is more preferably 200 rad / s, and even more preferably 50 rad / s.

[0062] Angular frequency ω of 4-methyl-1-pentene polymer ２６０ By setting the stretching speed at 1.0 to 500 rad / s, the entanglement of molecular chains is not relaxed during stretching, and the stretching stress is easily transmitted uniformly, thereby reducing the formation of a coarse uneven structure on the surface and the occurrence of voids or holes in the layer. As a result, the obtained biaxially oriented polyolefin film has excellent heat resistance, such as resistance to fusion at high temperatures.

[0063] The melting point of the 4-methyl-1-pentene polymer in the biaxially oriented polyolefin film of the present invention is preferably 200 to 250°C. The lower limit of the melting point of the 4-methyl-1-pentene polymer is more preferably 210°C, even more preferably 220°C, and particularly preferably 225°C. On the other hand, the upper limit of the melting point of the 4-methyl-1-pentene polymer is more preferably 240°C, even more preferably 235°C. When the melting point of the 4-methyl-1-pentene polymer in the biaxially oriented polyolefin film of the present invention is within the above range, it is easy to co-stretch with another layer containing another polyolefin as a main component. Furthermore, when Layer B contains such a 4-methyl-1-pentene polymer, fusion between the surface of Layer B and a mating member is easily suppressed when the two come into contact in a high-temperature environment.

[0064] The entire resin used in the biaxially oriented polyolefin film of the present invention may contain various additives, such as inorganic and organic particles, crystal nucleating agents, antioxidants, heat stabilizers, slipping agents, antistatic agents, antiblocking agents, fillers, viscosity modifiers, color inhibitors, etc. These components may be used alone or in combination, and may be added to any layer, provided that the object of the present invention is not impaired.

[0065] Among these, the selection of the type and amount of antioxidant is important from the viewpoint of antioxidant bleed-out. That is, such antioxidants are preferably sterically hindered phenol-based antioxidants, with at least one of them being a high molecular weight type having a molecular weight of 500 or more. Specific examples include various antioxidants, and for example, 2,6-di-t-butyl-p-cresol (BHT: molecular weight 220.4) may be used in combination with 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene (e.g., BASF's "Irganox" (registered trademark) 1330: molecular weight 775.2) or tetrakis[methylene-3(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane (e.g., BASF's "Irganox" (registered trademark) 1010: molecular weight 1177.7). The total content of these antioxidants is preferably in the range of 0.03 to 1.0 parts by mass relative to the total amount of resin constituting the biaxially oriented polyolefin film. If the amount of antioxidant is too small, the polymer may deteriorate during the extrusion process, causing the film to discolor, or the film may have poor long-term heat resistance. If the amount of antioxidant is too large, the transparency may decrease due to bleed-out of the antioxidant. A more preferred content is 0.05 to 0.9 parts by mass, and particularly preferably 0.1 to 0.8 parts by mass.

[0066] Furthermore, a crystal nucleating agent can be added to the raw materials of each layer used in the biaxially oriented polyolefin film of the present invention, provided that it does not contradict the object of the present invention. Examples of crystal nucleating agents used in the biaxially oriented polyolefin film of the present invention include polypropylene α crystal nucleating agents (dibenzylidene sorbitols, sodium benzoate, etc.), polypropylene β crystal nucleating agents (1,2-potassium stearate, magnesium benzoate, amide compounds such as N,N'-dicyclohexyl-2,6-naphthalenedicarboxamide, quinacridone compounds, etc.). However, since excessive addition of the above-mentioned other nucleating agents can cause a decrease in stretchability or a decrease in transparency and strength due to void formation, the amount added is usually 0.5 parts by mass or less, preferably 0.1 parts by mass or less, and more preferably 0.05 parts by mass or less, based on 100 parts by mass of the total raw materials.

[0067] The biaxially oriented polyolefin film of the present invention is preferably a biaxially stretched film. As the biaxial stretching method, any of inflation simultaneous biaxial stretching method, stenter simultaneous biaxial stretching method, and stenter sequential biaxial stretching method may be used, but among them, stenter sequential biaxial stretching method is preferably used from the viewpoint of film formation stability, thickness uniformity, and control of film rigidity and dimensional stability.

[0068] Next, one embodiment of the method for producing a biaxially oriented polyolefin film of the present invention will be described using a layer structure of B layer / A layer / B layer as an example, but the present invention is not limited to this.

[0069] First, 99.7% by mass of homopolypropylene resin and 0.3% by mass of branched polypropylene resin, when the total resin is taken as 100% by mass, are dry-blended and fed into a twin-screw extruder set at 240 to 280°C, melt-kneaded, and then cooled to obtain pellets for Layer A, which is Polypropylene Composition A. The pellets for Layer A obtained by the above procedure are fed into a single-screw extruder, and 96.0% by mass of 4-methyl-1-pentene polymer and 4.0% by mass of 4-methyl-1-pentene-propylene copolymer, which are resins for Layer B (here, the mass % values ​​are mass ratios when the total resin is taken as 100% by mass), are pre-blended in pellet form and fed to another single-screw extruder, where they are melt-extruded preferably at 200 to 290°C, more preferably 240 to 280°C, and even more preferably 260 to 280°C. Then, after removing foreign matter and modified polymers using a filter installed midway through the polymer pipe, the mixture is laminated in a layer B / layer A / layer B configuration using a multi-manifold composite T-die, and then discharged onto a casting drum and cooled and solidified to obtain a laminated unstretched sheet having a layer B / layer A / layer B configuration. In this case, the lamination thickness ratio is preferably such that the total thickness of the B layer is 0.5 to 30% based on the total thickness. The lower limit for the B layer is more preferably 1%, even more preferably 2%, and particularly preferably 3%. The upper limit for the B layer is more preferably 20%, even more preferably 10%, and particularly preferably 5%. The thicknesses of the B layers on both sides may be the same or different.

[0070] The surface temperature of the casting drum is preferably 20 to 100°C, more preferably 30 to 90°C, and even more preferably 40 to 80°C. Casting temperatures within the above range can suppress the formation of β-crystals, which have a low melting point among polypropylene resin crystals, and are likely to favorably increase the proportion of high-melting-point crystals in the film. The method of adhesion to the casting drum may be any of electrostatic application, adhesion methods utilizing the surface tension of water, air knife methods, press roll methods, and underwater casting methods, but the air knife method is preferred because it allows for easy control of surface roughness. When using the air knife method, the air temperature of the air knife is preferably 20 to 100°C, and the blown air speed is preferably 130 to 150 m / s. Furthermore, in order to prevent vibration of the laminated unstretched sheet, it is also preferable to appropriately adjust the position of the air knife so that air flows downstream of the film production.

[0071] The resulting laminated unstretched sheet is introduced into the longitudinal stretching process (stretching in the longitudinal direction). In the longitudinal stretching process, the laminated unstretched sheet is preheated using a metal roll heated to 150 to 160°C, preferably 152 to 159°C, and more preferably 154 to 158°C, before stretching. When the preheating temperature is within the above range, the laminated unstretched sheet proceeds to the longitudinal stretching process in a softened state, allowing stretching without applying more stress than necessary in the longitudinal stretching process, which tends to reduce heat shrinkage stress. In addition, it is possible to reduce the low-melting-point β crystals contained in the laminated unstretched sheet, which tends to favorably increase the proportion of high-melting-point crystals in the film.

[0072] The film is then stretched 3.8 to 6.0 times in the machine direction between rolls having different peripheral speeds immediately after preheating to obtain a longitudinally uniaxially stretched film. The stretching ratio is preferably 4.0 to 5.5 times, and more preferably 4.2 to 4.8 times. The stretching temperature is greater than 150°C and not more than 160°C, preferably 152 to 158°C, and more preferably 154 to 158°C. When the stretching temperature is within the above range, it is possible to pull out molecular chains from the softened crystals while maintaining the stretchability of the 4-methyl-1-pentene polymer contained in Layer B and suppressing residual excessive strain in the polypropylene resin contained in Layer A, and this, together with the subsequent relaxation step, promotes a high crystalline melting point.

[0073] At the end of the longitudinal stretching step, the longitudinally uniaxially stretched film is brought into contact with metal rolls having a peripheral speed difference and maintained at 140 to 160°C, whereby the film is relaxed in the longitudinal direction at a relaxation rate of greater than 0% and not more than 10%, and then cooled to room temperature. The relaxation temperature is preferably 142 to 160°C, more preferably 145 to 160°C, even more preferably 150 to 160°C, and particularly preferably 154 to 160°C. A relaxation temperature within the above range promotes rearrangement of molecular chains extracted from the crystals in the polypropylene resin contained in Layer A by longitudinal stretching, enabling the formation of crystals with a higher melting point. Furthermore, the relaxation rate during the longitudinal stretching step is preferably 0.1 to 10%, more preferably 1.0 to 10%, even more preferably 3.0 to 10%, and particularly preferably 5.0 to 10%. A relaxation rate within the above range during the longitudinal stretching step facilitates the release of tension in the molecular chains of the polypropylene resin contained in Layer A. As a result, amorphous relaxation is promoted, shrinkage stress is reduced, and rearrangement of molecular chains is also likely to be promoted.

[0074] The longitudinally uniaxially stretched film is then introduced into a tenter, where both widthwise ends are held with clips and preheated, followed by transverse stretching in the widthwise direction at a magnification of 7.0 to 13 times (transverse stretching step). The temperature in the preheating step before stretching is 170 to 190°C, preferably 173 to 185°C, more preferably 175 to 185°C, and even more preferably 177 to 185°C. When the preheating temperature is within the above range, the transverse stretching step can be carried out in a state in which the crystals formed in the longitudinal stretching step of the polypropylene resin in Layer A are softened, while promoting structural deformation of the 4-methyl-1-pentene polymer in Layer B. This allows stretching to be carried out in the transverse stretching step without applying more stress than necessary, and the heat shrinkage stress is likely to be reduced.

[0075] The stretching temperature in the transverse stretching step after the preheating step is greater than 170°C and equal to or lower than 180°C, preferably 173 to 180°C, and more preferably 175 to 180°C. When the transverse stretching temperature is within the above range, it becomes possible to draw molecular chains from the crystals softened by preheating while promoting structural deformation of the 4-methyl-1-pentene polymer contained in Layer B and suppressing residual excessive strain in the polypropylene resin contained in Layer A. Therefore, together with the subsequent relaxation step, a high crystalline melting point is promoted.

[0076] In the subsequent relaxation step after transverse stretching, the film is relaxed in the width direction while being held with moderate tension by clips, preferably at a relaxation rate of 10 to 20%, more preferably 11 to 18%, and even more preferably 12 to 15%. A relaxation rate within the above range after transverse stretching facilitates the release of tension in the molecular chains. This promotes amorphous relaxation, reduces shrinkage stress, and provides a moderate range of molecular chain mobility, facilitating molecular chain rearrangement. The heat setting temperature during this process is preferably above 170°C and 190°C or less, preferably 173 to 185°C, and more preferably 175 to 185°C. A relaxation temperature within the above range promotes rearrangement of molecular chains extracted from crystals by transverse stretching, enabling the formation of crystals with a higher melting point and a thicker lamellar thickness.

[0077] Furthermore, the biaxially oriented polyolefin film of the present invention includes a relaxation step in the longitudinal stretching step and the transverse stretching step, and the total area relaxation rate (%) calculated from the respective stretch ratios and relaxation rates in the longitudinal stretching step and the transverse stretching step is preferably 10 to 30%, more preferably 13 to 25%, and even more preferably 15 to 20%. The total area relaxation rate (%) is calculated using the following formula, and within the above range, amorphous relaxation is likely to proceed, and the shrinkage stress of the entire biaxially oriented polyolefin film is likely to be reduced. Total area relaxation rate (%) = [1 - (1 - longitudinal stretching step relaxation rate / 100) x (1 - transverse stretching step relaxation rate / 100)] x 100

[0078] The film is then cooled at 80 to 130°C while still being held taut across the width with clips, and then led to the outside of the tenter. The clips on the film ends are released, and the film edges are slit in the winding process, and the film product roll is wound up.

[0079] The biaxially oriented polyolefin film obtained as described above can be used for various purposes such as packaging films, surface protection films, processing films, battery films, sanitary products, agricultural products, construction products, and medical products. In particular, since it has excellent heat resistance, it can be preferably used as a processing film that requires high-temperature treatment such as drying coating materials or molding thermosetting resins, a release film, a current collector substrate film for secondary batteries, or a packaging film for retort pouches, and is particularly preferably used as a release film used in high-temperature regions (details of the use will be described later).

[0080] The release film and processing film of the present invention will be described below. The release film and processing film of the present invention include the biaxially oriented polyolefin film of the present invention. Here, "including the biaxially oriented polyolefin film of the present invention" refers to both an embodiment consisting solely of the biaxially oriented polyolefin film of the present invention and an embodiment in which another layer is provided on the biaxially oriented polyolefin film of the present invention. In the present invention, the release film refers to a film that has the function of being attached to an object such as a molded body or film to protect the object from scratches, contamination, etc. during processing or transportation, and can be easily peeled off and discarded when used as a final product. In addition, the processing film refers to a film used in the manufacturing process of an object such as a molded body or film. For example, it can be attached to an object during the manufacturing process to protect it from scratches, contamination, etc., or it can function as a support when the object itself is thin or fragile and difficult to form a film from.

[0081] Because of its excellent heat resistance and releasability, the release film of the present invention is particularly preferably used for fiber-reinforced composite curing, substrates, or molds. All of these applications correspond to release applications used in high-temperature regions. For fiber-reinforced composite materials, the film is attached to a material reinforced by mixing glass fibers or carbon fibers into a resin. For substrates, the film is attached to a plate (substrate) on which components for achieving a certain function are arranged. Examples of substrates include electronic circuit boards such as printed circuit boards and glass substrates used in displays. For molds, the film is attached to a product (molded body) processed in a molding process to protect semiconductor elements or integrated circuits (ICs) with a semiconductor package. Because fiber-reinforced composite materials, substrates, and molded bodies all involve high-temperature heating during processing, the release film of the present invention, which has excellent releasability in high-temperature environments, can be suitably used to protect them.

[0082] The laminate of the present invention will be described below. The biaxially oriented polyolefin film of the present invention has excellent heat resistance and releasability, and therefore can be preferably used when forming a laminate with a metal film or a transparent conductive film by subjecting the biaxially oriented polyolefin film of the present invention to vapor deposition or sputtering. That is, one embodiment of the laminate of the present invention is one in which a metal film is in contact with at least one surface of the biaxially oriented polyolefin film of the present invention.

[0083] In vapor deposition and sputtering processes, polyethylene terephthalate (PET) film has often been used due to its excellent heat resistance and rigidity. However, PET has high hydrophilicity due to the presence of ester bonds, and PET film contains trace amounts of moisture. Such trace amounts of moisture can have adverse effects during vapor deposition and sputtering, particularly when vapor-depositing metals belonging to Group 1 or 2 of the periodic table, or compounds containing these metals, which are susceptible to moisture. In light of this, when the laminate of the present invention has a metal film, the laminate of the present invention preferably contains a metal belonging to Group 1 or 2 of the periodic table, since this allows for a suitable laminate to be obtained even when the presence of trace amounts of moisture makes it difficult to use PET film. Here, the metal belonging to Group 1 or 2 refers to lithium, sodium, potassium, rubidium, cesium, francium, beryllium, magnesium, calcium, strontium, barium, or radium.

[0084] Furthermore, in high-temperature environments, moisture in the film evaporates as outgassing, and this effect is particularly pronounced under high-vacuum conditions such as in metal vapor deposition processes. Outgassing from the film can deteriorate the degree of vacuum in the system, potentially reducing the quality of the metal film formed by vapor deposition and the yield of the vapor deposition process. From this perspective, the biaxially oriented polyolefin film of the present invention, which has a lower moisture content than PET film, can be suitably used in applications where a metal film is to be formed, and the laminate of the present invention can maintain good metal film quality.

[0085] Furthermore, electrolyte membranes used in fuel cells, semi-solid batteries, all-solid batteries, etc. are usually produced in an environment where temperature and humidity are strictly controlled. In particular, sulfide-type electrolyte membranes react with moisture to generate hydrogen sulfide, so the processing film used in their production is also required to have an extremely low moisture content. Therefore, the biaxially oriented polyolefin film of the present invention is preferably used as the processing film in the production of such electrolyte membranes. That is, a preferred embodiment of the laminate of the present invention includes one in which an electrolyte membrane is in contact with at least one side of the biaxially oriented polyolefin film, and more particularly, an electrolyte membrane for a fuel cell, a semi-solid battery, or an all-solid battery is particularly preferred.

[0086] From the above viewpoints, the upper limit of the moisture content of the biaxially oriented polyolefin film of the present invention is preferably 2000 ppm, more preferably 1000 ppm, even more preferably 500 ppm, particularly preferably 200 ppm, and most preferably 100 ppm. The lower limit of the moisture content is not particularly limited, but is substantially 1 ppm. The moisture content of the biaxially oriented polyolefin film can be measured by the Karl Fischer method, the details of which will be described later.

[0087] In order to achieve the moisture content of the biaxially oriented polyolefin film of the present invention within the above range, it is preferable to minimize the content of hydrophilic resins in the biaxially oriented polyolefin film and to minimize the amount of additives. Specifically, of all the components of the biaxially oriented polyolefin film of the present invention, the lower limit of the content of polyolefin resins such as polypropylene resins is preferably 90% by mass, more preferably 95% by mass, and even more preferably 97% by mass. The upper limit of the content of these resins is substantially 100% by mass.

[0088] From the above perspectives, when the total resin components of the biaxially oriented polyolefin film of the present invention are taken as 100 parts by mass, the content of the antioxidant is preferably 0.05 to 0.9 parts by mass, more preferably 0.1 to 0.8 parts by mass. In particular, it is preferable to control the content of phosphorus-based antioxidants, since bleed-out phosphorus-based antioxidants to the surface may degrade the properties and quality of the metal film formed on the surface. More specifically, the content of phosphorus-based antioxidants among all the components of the biaxially oriented polyolefin film of the present invention is preferably 0.01 parts by mass or less, more preferably 0.005 parts by mass ppm or less, and even more preferably 0.001 parts by mass or less. There is no particular lower limit for the content of phosphorus-based antioxidants, and theoretically it is 0 parts by mass (i.e., no phosphorus-based antioxidants are included). It should be noted that the content of antioxidants, including phosphorus-based antioxidants, is preferably the same even when the film formed on the surface is not a metal film, from the viewpoints of properties, quality, and reducing inhibition of effects.

[0089] In order to reduce defects due to bleed-out in the biaxially oriented polyolefin film of the present invention, the total content of additives other than antioxidants (e.g., antistatic agents, viscosity modifiers, color inhibitors, slip agents, etc.) is preferably 0 to 0.05 parts by mass. A content of these additives of 0 parts by mass is synonymous with not containing these additives.

[0090] As described above, the biaxially oriented polyolefin film of the present invention has an extremely low moisture content and generates very little outgassing, and is superior in heat resistance and handling to existing polyolefin films. Therefore, it is preferably used when forming transparent conductive films that require more stringent processing conditions and heat resistance. That is, a preferred embodiment of the laminate of the present invention includes a laminate in which a transparent conductive film is in contact with at least one side of the biaxially oriented polyolefin film of the present invention. Here, the transparent conductive film refers to a thin film formed from a material that is conductive and transmits visible light. Specific examples include indium-tin composite oxide (ITO), zinc oxide (ZnO), and palladium films.

[0091] Next, the current collector of the present invention will be described. The current collector of the present invention is formed using the biaxially oriented polyolefin film of the present invention. The biaxially oriented polyolefin film of the present invention is preferably used as a current collector due to its excellent heat resistance. The current collector is a foil-like laminate used in electrodes of storage batteries such as lithium-ion batteries. Metal foils are typically used as current collectors, but laminates in which a metal film is laminated on a resin film substrate are also used for the purpose of improving safety and reducing weight. This metal film is laminated by processes such as vapor deposition, sputtering, plating, and electroless plating. Furthermore, to increase the energy density of batteries, the film substrate of the current collector is required to be thin. However, as the film becomes thinner, its stiffness decreases, significantly reducing its handleability during processing. In particular, the process of laminating the metal film requires good handleability because it is exposed to high heat such as radiant heat during processing and also to tension in the conveying direction. The biaxially oriented polyolefin film of the present invention can be thinned and has good handleability, making it suitable for use as a current collector.

[0092] Next, the storage battery of the present invention will be described. The storage battery of the present invention uses the biaxially oriented polyolefin film of the present invention. The biaxially oriented polyolefin film of the present invention has excellent heat resistance and is therefore preferably used as a current collector, and is used in storage batteries in which the current collector is used as an electrode. A storage battery is a device that stores electrical energy and converts it back into electrical energy when needed. Specific examples include lead-acid batteries, nickel-metal hydride batteries, lithium-ion batteries, NAS batteries, and redox flow batteries.

[0093] The present invention will be described in detail below with reference to examples. The properties were measured and evaluated by the following methods.

[0094] (1) Film Thickness The film thickness was measured using a micro thickness meter (manufactured by Anritsu Corporation). The film was sampled in a 10 cm square area, and measurements were taken at five arbitrary points to calculate the average value. The obtained value was taken as the film thickness.

[0095] (2) The ratio S of the heat of fusion between 175 and 190°C and the heat of fusion between 190 and 260°C. Using a differential scanning calorimeter (Rigaku Corporation's "Thermo plus EV02 DSCvesta" differential scanning calorimeter), 3 mg of biaxially oriented polyolefin film was heated from 25°C to 260°C at a rate of 20°C / min in a nitrogen atmosphere to obtain a melting curve.

[0096] <Proportion S of heat of fusion between 175 and 190 ° C> For the obtained melting curve, a linear baseline was set within the range of 30 to 190 ° C, and the heat of fusion was calculated from the area enclosed by the linear baseline and the melting curve. This was converted to the heat per sample mass to calculate the heat of fusion between 30 and 190 ° C (J / g). In addition, the heat of fusion was calculated from the area enclosed by the linear baseline and the melting curve within the range of 175 to 190 ° C, and this was converted to the heat per sample mass to calculate the heat of fusion between 175 and 190 ° C (J / g). The obtained heat of fusion between 30 and 190 ° C and the heat of fusion between 175 and 190 ° C were applied to the following formula to determine the proportion S (%) of the heat of fusion between 175 and 190 ° C. Percentage of heat of fusion between 175 and 190°C S (%) = [heat of fusion between 175 and 190°C] x 100 / [heat of fusion between 30 and 190°C]

[0097] <Heat of fusion H between 190 and 260°C> For the melting curve, a linear baseline was set within the range of 190 to 260°C, and the heat of fusion was calculated from the area enclosed by the linear baseline and the melting curve. This was converted into the heat per mass of the sample to calculate the heat of fusion H (J / g) between 190 and 260°C.

[0098] (3) Developed Area Ratio (Sdr) Measurements were performed using a scanning white light interference microscope "VS1540" (manufactured by Hitachi High-Tech Science Corporation; measurement conditions and instrument configuration are described below). Measurements were performed using multiple 5 x 5 fields of view, with each field measuring 561.1 μm x 561.5 μm. All images were then stitched with a 20% overlap to obtain surface profile data of 2356.716 μm x 2358.294 μm. The captured image was interpolated (fully interpolated) using the accompanying analysis software, surface correction was performed using a polynomial fourth-order approximation, and the surface profile was determined by processing with a median filter (3 x 3 pixels). Measurements were performed starting from the intersection of the diagonals of a biaxially oriented polyolefin film cut into a 5 cm x 5 cm square. A total of three measurement positions were determined according to the following procedure, and measurements were performed at each measurement position. The developed area ratio (Sdr) at each measurement position was determined according to the above procedure, and the average value was used. Note that the measurements were performed on both the front and back surfaces of the film.

[0099] <How to determine the measurement position> Measurement 1: Position of the measurement start point Measurement 2: Position 10.0 mm to the right of the measurement start point Measurement 3: Position 10.0 mm to the left of the measurement start point

[0100] <Measurement conditions and device configuration> Objective lens: 10x Lens tube: 1x Zoom lens: 1x Wavelength filter: 530 nm white Measurement mode: Wave Measurement software: VS-Measure 10.0.4.0 Analysis software: VS-Viewer 10.0.3.0 Measurement area: 561.1 μm × 561.5 μm Number of pixels: 1,024 × 1,024

[0101] Device name: "VertScan" (registered trademark) 2.0 R5300GL-Lite-AC manufactured by Ryoka Systems Co., Ltd. Measurement conditions: CCD camera SONY HR-57 1 / 2 inch Objective lens: 10x Intermediate lens: 0.5x Wavelength filter: 530 nm white Measurement mode: Wave Measurement software: VS-Measure Version 5.5.1 Analysis software: VS-Viewer Version 5.5.1 Measurement area: 561.097 μm × 561.473 μm

[0102] (4) Diiodomethane Contact Angle I Using diiodomethane (methylene iodide) as the measurement liquid, the static contact angle on the film surface was determined using a contact angle meter CA-D manufactured by Kyowa Interface Science Co., Ltd., and this was taken as contact angle I. Measurements were made at three locations on a 5 cm x 5 cm film, and the average value was adopted. Contact angle I was measured 30 seconds after the liquid was dropped onto the film surface. The above measurement was made on both the front and back surfaces of the film.

[0103] (5) Young's modulus E Ｘ＋Ｙ Five biaxially oriented polyolefin films were cut into rectangular samples measuring 150 mm in length (measurement direction) x 10 mm in width. The measurement direction was selected arbitrarily, and the selected measurement direction was defined as the 0° direction. Then, using a tensile tester (Orientec "Tensilon" (registered trademark) UCT-100) manufactured by Orientec, a tensile test was performed on the samples at a room temperature of 23°C and a relative humidity of 65%, with an initial tensile chuck distance of 50 mm and a tensile speed of 300 mm / min. The Young's modulus was calculated according to the method specified in JIS K7161 (2014). Each sample was measured five times, and the average values ​​were used as the tensile strength and Young's modulus of the sample. The Young's modulus was measured in each direction forming an angle of 0 to 175° with respect to the measurement direction at 5° increments within the film plane. The direction showing the highest value was designated the main orientation direction (X direction), and the direction perpendicular to this was designated the main orientation perpendicular direction (Y direction). The test was carried out five times, and the average value was calculated. The Young's modulus in the X direction was defined as EX, the Young's modulus in the Y direction as EY, and the sum of these was defined as E. Ｘ＋Ｙ It was decided.

[0104] (6) 160°C shrinkage stress P Ｘ＋ＹUsing a thermomechanical analyzer (manufactured by SII Nanotechnology Co., Ltd. / Model TMA / SS6100), heat shrinkage stress curves in the measurement directions (X direction and Y direction) were obtained under the following conditions. The X direction and Y direction were specified by the method described in (5). (a) Sample: Width 4 mm x Length 20 mm (b) Initial load: 0.0 mN (c) Temperature program: Heating from 30°C to 200°C at a heating rate of 10°C / min (d) Preparation of heat shrinkage stress curve: The load (N) observed at each temperature was divided by the cross-sectional area (thickness x sample width) of the biaxially oriented polyolefin film to calculate the shrinkage stress (MPa) at each temperature, and a temperature-shrinkage stress curve (heat shrinkage stress curve) was prepared. The shrinkage stress (MPa) at 160°C was read from the heat shrinkage stress curve. Measurements were performed three times in each direction, and the average was calculated. The value in the X direction was defined as PX, the value in the Y direction as PY, and the sum of PX and PY was defined as P Ｘ＋Ｙ It was decided.

[0105] (7) Melting point of raw material and Tm of biaxially oriented polyolefin film １ and Tm ２ A 5 mg sample was placed in an aluminum pan and measured under a nitrogen atmosphere using a differential scanning calorimeter (Rigaku Corporation, "Thermo plus EV02 DSCvesta" differential scanning calorimeter). The raw material was first heated from 30°C to 260°C at 20°C / min, and then held at 260°C for 5 minutes. The temperature was then lowered from 260°C to 30°C at 20°C / min, and then again heated from 30°C to 260°C at 20°C / min. The maximum peak temperature of the melting curve observed when the temperature was increased from 30°C to 260°C at 20°C / min was taken as the melting point of the raw material. Biaxially oriented polyolefin films were also measured in the same manner, and the maximum peak temperature between 30 and 190°C was taken as the melting point of the polypropylene resin in the biaxially oriented polyolefin film, Tm １ The temperature of the maximum peak between 190 and 260°C was taken as the melting point Tm of the 4-methyl-1-pentene polymer in the biaxially oriented polyolefin film. ２ In addition, when the polyolefin composition was one in which a plurality of resin components were used without pre-mixing, the resin composition obtained by melt-extrusion of only Layer A was measured, or only Layer A was sampled from an unstretched sheet or stretched film and measured.

[0106] (8) Crystallization Half Time Using a differential scanning calorimeter (Rigaku Corporation, "Thermo plus EV02 DSCvesta" differential scanning calorimeter), 3 mg of polypropylene resin or polypropylene composition was heated in a nitrogen atmosphere from 25°C to 250°C at 20°C / min and held for 5 minutes. The temperature was then lowered from 250°C to 130°C at 20°C / min and held at 130°C for 30 minutes. The time when the sample temperature reached 130°C was defined as 0 seconds, and the elapsed time from the maximum intensity peak appearing in the endothermic curve obtained during isothermal holding at 130°C was defined as the crystallization half time (seconds). When multiple resin components were used as the polyolefin composition without pre-mixing, the resin composition obtained by melt-extrusion of only Layer A, or Layer A alone was sampled from an unstretched sheet or stretched film, was used for the measurement.

[0107] (9) Angular frequency ω ２００ , angular frequency ω ２６０ Measurements were performed using a rotational rheometer (MCR302 manufactured by Anton Paar Japan) equipped with a 25 mm diameter cone plate. The polypropylene composition (or polypropylene resin and polyolefin resin) was left standing for 10 minutes under a nitrogen atmosphere on a plate heated to 200°C or 260°C. Then, while maintaining the temperature, the angular frequency was changed from low to high from 0.5 rad / s to 500 rad / s at 5% strain, and viscoelasticity measurements were performed. From the obtained curve of angular frequency vs. loss tangent, the angular frequency obtained in the measurement at 200°C was calculated as ω for the angular frequency at which the loss tangent was 1. ２００ (rad / s), and the angular frequency obtained in the measurement at 260°C is ω ２６０ When a plurality of resin components were used as the polyolefin composition without pre-mixing, the resin composition obtained by melt-extrusion of only the layer A, or the layer A alone was sampled from an unstretched sheet or a stretched film, and the measurement was performed.

[0108] (10) Proportion of Components with Logarithm of Molecular Weight M, Log M, of 5.0 or Less and Proportion of Components with Log M, of 6.0 or More A biaxially oriented polyolefin film was dissolved in 1,2,4-trichlorobenzene as a solvent by stirring at 165°C for 30 minutes. The solution was then filtered using a 0.5 µm filter, and the molecular weight distribution of the filtrate was measured by gel permeation chromatography (GPC). The proportions of components with Logarithm of Molecular Weight M, Log M, of 5.0 or less and components with Log M, of 6.0 or more were determined from the integral curve of the obtained molecular weight distribution. The measurement by gel permeation chromatography (GPC) was performed using the following equipment and conditions.

[0109] <Apparatus and measurement conditions> Apparatus: Agilent high-temperature GPC apparatus PL-GPC220 Detector: Agilent differential refractive index detector (RI detector) Column: Agilent PL1110-6200 (20 μm MIXED-A) × 2 Flow rate: 1.0 mL / min Column temperature: 145°C Injection volume: 0.500 mL Sample concentration: 0.1 wt% Standard sample: Tosoh monodisperse polystyrene, Tokyo Chemical Industry dibenzyl.

[0110] (11) Moisture Content A biaxially oriented polyolefin film or PET film sample was left for 4 hours or more in a room conditioned at 23°C and a relative humidity of 20%, and then immersed for 24 hours in distilled water at 23°C. Thereafter, the moisture on the surface of the sample was wiped off, and the moisture in the sample was dried and evaporated at a temperature of 150°C using a trace moisture meter (manufactured by Mitsubishi Chemical Corporation, CA-20 model), and the moisture content was then determined by the Karl Fischer method to calculate the moisture content.

[0111] (12) Evaluation of Heat Resistance Properties Cardboard (product number C-55, manufactured by Daio Paper Co., Ltd.) was cut into 10 cm squares, and biaxially oriented polyolefin film or PET film cut into 15 cm squares was placed on both sides of the cardboard along with a 20 cm square SUS plate as shown in Figure 1. Using a heating press, the biaxially oriented polyolefin film or PET film was heated and pressed at a pressure of 1.0 MPa and temperatures of 175°C, 180°C, and 185°C for 5 minutes, respectively, and then removed from the press and cooled to room temperature. The biaxially oriented polyolefin film or PET film protruding from the cardboard was then peeled away from the biaxially oriented polyolefin film or PET film, and the cardboard was then peeled away from the biaxially oriented polyolefin film or PET film. The symbols 1 to 4 in Figure 1 represent the SUS plate, the film to be measured, the cardboard, and the pressure direction, respectively. The state of the cardboard and biaxially oriented polyolefin film or PET film after treatment at each temperature or after peeling was visually observed, and shape stability and high-temperature peel resistance were evaluated according to the following criteria. Evaluation was initially carried out at a heating temperature of 175°C for each evaluation item, and only evaluation items that passed the evaluation proceeded to evaluation at a heating temperature of 180°C. Furthermore, only items that passed the evaluation at 180°C proceeded to evaluation at a heating temperature of 185°C.

[0112] (Shape Stability) Shape stability was evaluated according to the following criteria: Pass if no folds or wrinkles were observed on the cardboard; Fail if at least either folds or wrinkles were observed.

[0113] (High-temperature peel resistance) After pressure treatment, if the biaxially oriented polyolefin film or PET film protruding from the cardboard could be peeled off at the interface between the films and no part of the film peeled off from the cardboard remained on the cardboard, the film was judged to be pass; if it was not peelable or if even part of the film remained on the cardboard even if it was peelable, the film was judged to be fail, and the evaluation was based on the following criteria.

[0114] <Evaluation criteria (common to shape stability and high-temperature peel resistance)> Shape stability and high-temperature peel resistance were evaluated according to the following criteria, and if each characteristic was A to C, it was determined to have heat resistance. A: Passed at 175°C, 180°C, and 185°C. B: Passed at 175°C and 180°C, but failed at 185°C. C: Passed at 175°C, but failed at 180°C. D: Failed at 175°C.

[0115] (13) Interlayer Adhesion The biaxially oriented polyolefin film of the present invention was cut into a width of 25 mm in the main orientation (X) direction and a length of 70 mm in the direction perpendicular to the main orientation (Y), and 80 mm of acrylic polyester adhesive tape (Nitto Denko Corporation, Nitto 31B tape, 19 mm wide) was attached to both ends of the longitudinal direction. A 2 kgf roller was run over the film to prepare a tape-attached laminated film sample, which was then left to stand for 24 hours under an environment of 25 ° C and humidity 55% ± 5%. Then, the tape-attached laminated film sample was attached to a 1.5 mm thick SUS plate with double-sided tape (Nitto Denko No. 532, tape thickness 0.08 mm) on the side opposite the adhesive tape-attached surface, and the SUS plate was fixed to a universal testing machine "Autograph" (registered trademark) AG-1S manufactured by Shimadzu Corporation with an air chuck. Furthermore, a portion of the adhesive tape peeled 5 mm from the bonded edge was fixed with an air chuck, and the peel force (N / 19 mm) was measured when peeled at a peel angle of 180° and a tensile speed of 300 mm / min. From the graph of peel force (N / 19 mm) vs. test length (mm) obtained by the measurement, the average peel force from 10 to 40 mm was calculated and evaluated according to the following criteria. A: The average peel strength was 2.0 N / 19 mm or more, and the standard deviation of the peel strength was less than 10% of the average value. B: The average peel strength was 2.0 N / 19 mm or more, and the standard deviation of the peel strength was 10% or more but less than 20% of the average value. C: The average peel strength was 2.0 N / 19 mm or more, and the standard deviation of the peel strength was 20% or more of the average value. D: The peel strength was less than 2.0 N / 19 mm, or the standard deviation of the peel strength was 20% or more of the average value.

[0116] (14) High-Temperature Conveyance A 500 mm wide biaxially oriented polyolefin film was introduced into a drying oven at 140°C, conveyed for 20 seconds under a conveying tension of 1.0 MPa, and wound into a 200 m long roll at a take-up tension of 200 N / m to obtain a film roll. Next, 1 m of the 500 mm wide biaxially oriented polyolefin film was unwound under free tension (a state in which the biaxially oriented polyolefin film was hanging vertically under its own weight), and tensions of 1 kg / m and 3 kg / m were applied uniformly and evenly across the entire width of the biaxially oriented polyolefin film, and the presence or absence of defects in flatness such as wrinkles or dents was visually confirmed. Evaluation was performed according to the following criteria. A: No defects in flatness were observed under free tension. B: Defects in flatness were observed under free tension, but the defects disappeared under a tension of 1 kg / m width. C: Poor flatness was observed at a tension of 1 kg / m width, but the poor flatness disappeared at a tension of 3 kg / m width. D: Poor flatness was observed even at a tension of 3 kg / m width.

[0117] (15) Yield evaluation during metal film formation: Using a vacuum deposition device, the pressure was reduced from atmospheric pressure to 1 × 10 －５ The pressure was reduced to 100 kJ, and a magnesium vapor deposition film with a thickness of 200 angstroms was formed on one side of the biaxially oriented polyolefin film or PET film by vacuum deposition using magnesium as a vapor deposition source. －５ The time required for the pressure to be reduced to a full pressure was measured, and the unevenness of the metal film surface was visually observed to evaluate the yield according to the following criteria. The evaluation criteria were A as pass and B as fail. A: 1×10 －５ B: The time required for reducing the pressure to a full pressure was 50 minutes or less, and no irregularities were observed on the surface of the metal film. －５ The time required to reduce the pressure to the full pressure was longer than 50 minutes, or irregularities were observed on the surface of the metal film.

[0118] (Polypropylene Resin, etc.) The raw materials used in the biaxially oriented polyolefin films of the Examples and Comparative Examples and their properties are shown in Tables 1 and 2 below. Note that these property values ​​are values ​​evaluated in the form of resin pellets. Two types of polypropylene resin pellets were prepared by pre-kneading (kneading was performed by dry blending, feeding into a twin-screw extruder, kneading at 260°C, and cooling). Homopolypropylene resin 1 (PP1): manufactured by Prime Polymer Co., Ltd. Homopolypropylene resin 2 (PP2): manufactured by Prime Polymer Co., Ltd. Homopolypropylene resin 3 (PP3): manufactured by Prime Polymer Co., Ltd. Homopolypropylene resin 4 (PP4): manufactured by Prime Polymer Co., Ltd. Homopolypropylene resin 5 (PP5): manufactured by Prime Polymer Co., Ltd. ("Prime Polypro" (registered trademark) F113G) Homopolypropylene resin 6 (PP6): manufactured by Sumitomo Chemical Co., Ltd. (Sumitomo "Noblen" (registered trademark) FLX80E4) Branched polypropylene resin 7 (PP7): manufactured by Boreales Co., Ltd. Polypropylene resin 8 (PP8): PP1 / PP7 = 99.7 / 0.3 was charged into a twin-screw extruder, kneaded at 260 ° C., and cooled to obtain pellets. Polypropylene resin 9 (PP9): PP1 / PP2 was charged into a twin-screw extruder at a mass ratio of 20 / 80, kneaded at 260 ° C., and cooled to obtain pellets. Polyolefin resin 1 (PO1): Propylene-1-butene copolymer manufactured by Mitsui Chemicals, Inc.

[0119]

[0120] 4-Methyl-1-pentene polymer 1 (PMP1): manufactured by Mitsui Chemicals, Inc. 4-Methyl-1-pentene polymer 2 (PMP2): manufactured by Mitsui Chemicals, Inc. 4-Methyl-1-pentene polymer 3 (PMP3): manufactured by Mitsui Chemicals, Inc. 4-Methyl-1-pentene polymer 4 (PMP4): manufactured by Mitsui Chemicals, Inc. 4-Methyl-1-pentene polymer 5 (PMP5) 4-Methyl-1-pentene polymer 6 (PMP6): manufactured by Mitsui Chemicals, Inc. 4-Methyl-1-pentene-propylene copolymer 1 (MP-P1): manufactured by Mitsui Chemicals, Inc. 4-Methyl-1-pentene-propylene copolymer 2 (MP-P2)

[0121]

[0122] Example 1 Polypropylene resin 8 (PP8) was fed into a single-screw extruder for the base layer (A layer), and 4-methyl-1-pentene polymer 1 (PMP1) was fed alone into a single-screw extruder for the surface layer (B layer). The resin mixture for each layer was melt-extruded at 260°C, and after removing foreign matter with a 20 μm cut-off sintered filter, the layers were laminated in a feedblock B / A / B composite T-die so that the thickness ratio of surface layer (B layer) / base layer (A layer) / surface layer (B layer) was 1 / 48 / 1. The resulting molten laminate was molded into a sheet using a T-die. The molten sheet was then ejected from the T-die onto a casting drum whose surface temperature was controlled to 60°C, and compressed air at 25°C was sprayed using an air knife at an air speed of 140 m / s to adhere the molten sheet to the casting drum, resulting in an unstretched sheet. The unstretched sheet was then preheated to 156°C using a ceramic roll and stretched 4.3 times in the longitudinal direction between 155°C rolls with a peripheral speed difference, followed by a 5.2% longitudinal relaxation between 155°C rolls with a peripheral speed difference to obtain a uniaxially stretched film. The uniaxially stretched film was then introduced into a tenter-type stretching machine with both widthwise ends held by clips, preheated at 180°C for 10 seconds, stretched 9.8 times in the widthwise direction at 177°C, and heat-set at 178°C while providing 13% relaxation in the widthwise direction. After a cooling step at 100°C, the film was introduced to the outside of the tenter-type stretching machine, the clips at both widthwise ends were released, and the film was wound around a core to obtain a 50 μm thick biaxially oriented polyolefin film. The physical properties and evaluation results of the resulting biaxially oriented polyolefin film are shown in Table 3.

[0123] (Examples 2 to 5, Comparative Examples 1 to 5) Biaxially oriented polyolefin films were obtained in the same manner as in Example 1, except that the raw material composition of each layer and the film-forming conditions were as shown in Table 3. In Example 5, 60 parts by mass of PP1, 34 parts by mass of PP2, and 6.0 parts by mass of PMP-1 were mixed in the form of pellets and fed into an extruder. The thickness was adjusted by adjusting the discharge rate during extrusion and the speed of the casting drum. The physical properties and evaluation results of the obtained biaxially oriented polyolefin films are shown in Table 3. The thickness was adjusted by adjusting the discharge rate of the extruder and the speed of the casting drum.

[0124] Comparative Example 6: Instead of the biaxially oriented polyolefin film, a PET film "Lumirror" (registered trademark) S10 (manufactured by Toray Industries, Inc.) was used to evaluate heat resistance and metal film formation yield, and the moisture content was measured. The evaluation results are shown in Table 3.

[0125] The moisture content and the yield in producing the metal film were evaluated only for Example 1 and Comparative Examples 4 and 6.

[0126]

[0127] In the table, the diiodomethane contact angles I and Sdr were measured on the front surface (the surface in contact with the casting drum) and the back surface (the surface opposite to the front surface), and the respective measured values ​​are shown.

[0128] As described above, the biaxially oriented polyolefin film of the present invention can be used in various applications such as packaging films, release films, processing films, sanitary products, agricultural products, construction products, medical products, etc. In particular, the biaxially oriented polyolefin film of the present invention has excellent heat resistance and can therefore be preferably used as a release film or processing film to be used at high temperatures, which is generally considered difficult to use with biaxially oriented polyolefin films.

[0129] 1: SUS plate 2: Film to be measured 3: Cardboard 4: Pressure direction

Claims

1. a biaxially oriented polyolefin film, characterized in that a ratio S of the heat of fusion in the range of 175 to 190°C to the heat of fusion in the range of 30 to 190°C, as determined by differential scanning calorimetry, is 10 to 70%, the film has two types of layers each having a different content of 4-methyl-1-pentene polymer, and when the layer with a relatively lower content of 4-methyl-1-pentene polymer is designated as layer A and the layer with a relatively higher content of 4-methyl-1-pentene polymer is designated as layer B, layer B is located on at least one of the film surfaces, and layer B contains the 4-methyl-1-pentene polymer as a main component.

2. 2. The biaxially oriented polyolefin film according to claim 1, wherein the developed area ratio Sdr of at least one of the film surfaces is 0.2 to 10.

0.

3. 3. The biaxially oriented polyolefin film according to claim 1, wherein the static contact angle I with diiodomethane on at least one of the film surfaces is 60 to 90°.

4. When the main orientation axis direction is the X direction and the direction perpendicular to the main orientation is the Y direction, the sum E of the Young's modulus in the X direction and the Young's modulus in the Y direction is Ｘ＋Ｙ The biaxially oriented polyolefin film according to claim 1 or 2, wherein the modulus is 5.2 to 15 GPa.

5. 3. The biaxially oriented polyolefin film according to claim 1, wherein the heat of fusion H in the range of 190 to 260°C obtained by differential scanning calorimetry is 0.1 to 20 J / g.

6. The sum of the 160°C shrinkage stress in the X direction and the 160°C shrinkage stress in the Y direction measured by thermomechanical analysis, P Ｘ＋Ｙ The biaxially oriented polyolefin film according to claim 1 or 2, wherein the elongation stress is -1.0 to 3.0 MPa.

7. 3. The biaxially oriented polyolefin film according to claim 1, wherein the polyolefin resin contained therein satisfies all of the following characteristics 1 to 3. Feature 1: The melting point Tm with the largest peak intensity obtained in the second run of differential scanning calorimetry performed at a heating rate of 20°C / min １ is 165.0 to 170.0°C. Feature 2: The content of components having a logarithm LogM of molecular weight M of 5.0 or less is 30.0 to 39.0% by mass. Feature 3: The content of components having a logarithm LogM of molecular weight M of 6.0 or more is 3.0 to 10% by mass.

8. 3. The biaxially oriented polyolefin film according to claim 1, wherein the A layer and the B layer are in contact with each other, and the 4-methyl-1-pentene polymer has the following characteristics 4 and 5: Feature 4: A melting point (Tm) observed at 190°C or higher in the second run of differential scanning calorimetry performed at a heating rate of 20°C / min ２ is 200 to 250°C. Feature 5: Angular frequency ω at which the loss tangent obtained by melt viscoelasticity measurement performed at 260 ° C. becomes 1 ２６０ is 1 to 500 rad / s.

9. A release film comprising the biaxially oriented polyolefin film of claim 1 or 2.

10. The release film according to claim 9 , which is used for a fiber-reinforced composite material, a substrate, or a mold.

11. A processing film comprising the biaxially oriented polyolefin film of claim 1 or 2.

12. A laminate comprising the biaxially oriented polyolefin film according to claim 1 or 2, and a metal film in contact with at least one surface of the biaxially oriented polyolefin film.

13. 13. The laminate of claim 12, wherein the metal film comprises a metal belonging to Group 1 or Group 2 of the periodic table.

14. A laminate comprising the biaxially oriented polyolefin film according to claim 1 or 2 and a transparent conductive film in contact with at least one surface of the biaxially oriented polyolefin film.

15. 3. A laminate comprising the biaxially oriented polyolefin film according to claim 1 or 2 and an electrolyte membrane in contact with at least one surface of the film.

16. The laminate according to claim 15, wherein the electrolyte membrane is used in a fuel cell, a semi-solid battery, or an all-solid battery.

17. A current collector comprising the biaxially oriented polyolefin film according to claim 1 or 2.

18. A storage battery comprising the biaxially oriented polyolefin film according to claim 1 or 2.