WO2025205289A1 - Biaxially-oriented polyolefin film - Google Patents

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

The present invention relates to a biaxially oriented polyolefin film with excellent heat resistance.

Biaxially oriented polyolefin film is obtained by biaxially stretching an unstretched polyolefin sheet. Because of its excellent productivity and film thickness accuracy, it is used in a variety of applications, including packaging and tape. Furthermore, because biaxially oriented polyolefin film also has excellent mechanical properties and releasability, it is ideally used as a release film or process film for a variety of components, including plastic products, building materials, and optical components.

The required properties of the above-mentioned release films and processing films are set appropriately depending on their intended use, but as materials become more highly functional and productivity improves, molding and processing temperatures are rising year by year, making heat resistance increasingly important. Furthermore, there are also processing films for 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, which have a relatively low moisture content, are expected to be suitable.

However, when processing components formed on a release film, or when manufacturing components using a processing film, a process of heating at temperatures exceeding 160°C for at least several minutes is often required. Under these conditions, even polypropylene, which has relatively high heat resistance among polyolefins, is prone to fusion and deformation, which can cause problems by compromising the quality of the mating component formed on the film. For this reason, it has been extremely difficult to use biaxially oriented polyolefin film as a release film or processing film in cases such as those described above.

One indicator of heat resistance is fusion resistance, and to improve this, studies are being 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 the film. However, when these resins are laminated with other polyolefins, the difference in the melting points of the two resins results in different appropriate stretching temperatures, which can easily lead to incomplete layer formation or film rupture, posing an issue. Furthermore, these resins have poor adhesion to other polyolefins, making them prone to delamination between the substrate and the surface layer, making them less suitable for use as release films or process films.

In light of the above, for example, Patent Documents 1 and 2 describe examples in which a layer made by mixing polypropylene with a 4-methyl-1-pentene polymer or its copolymer is laminated as a surface layer on a polypropylene layer, thereby improving interlayer adhesion and mold releasability at room temperature. Furthermore, 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 also adjusting the molecular weight of the inner polypropylene layer.

International Publication No. 2018/097161 JP 2014-30974 A JP 2015-44406 A

However, the films obtained by the methods described in Patent Documents 1 and 2 above are not intended for use in high-temperature environments, and fusion and deformation can be an issue in high-temperature environments. Furthermore, the films obtained by the method described in Patent Document 3 are only intended for exposure to high temperatures for extremely short periods of time, so there is a possibility that thermal shrinkage and fusion may not be sufficiently suppressed. In other words, polyolefin films obtained by these methods are difficult to use as release films or process films in high-temperature environments.

The object of the present invention is to solve the above problems. That is, the object of the present invention is to provide a biaxially oriented polyolefin film that can be used suitably even in high-temperature environments, where it was previously impossible to use it as a release film or processing film.

In order to solve the above-mentioned problems, the biaxially oriented polyolefin film of the present invention has the following configuration: Specifically, the biaxially oriented polyolefin film has a ratio (S) of the heat of fusion in the 175-190°C range to the heat of fusion in the 30-190°C range, as determined by differential scanning calorimetry, of 10-70%, has two types of layers with different contents of 4-methyl-1-pentene polymer, and, 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 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 major component.

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 could not be used as release films or process films.

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

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 as measured by differential scanning calorimetry of 10 to 70%, has two types of layers with different contents of 4-methyl-1-pentene polymer, and, 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 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 major component. The polyolefin film of the present invention is described below.

In addition, when a numerical range is expressed as "a to b" in this invention, the numerical range includes the values at both ends, a and b, and when a unit is stated only after the numerical range a to b, the unit is assumed to be the same throughout the entire numerical range.

Here, film refers to a sheet-like molded product whose main component is a thermoplastic resin. The main component refers to a component that is contained in a proportion of more than 50% by mass but not more than 100% by mass, when all components of the object (film, layer, etc.) are taken as 100% by mass. Biaxial orientation refers to molecular orientation in two perpendicular directions. Biaxially oriented films 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.

A polyolefin film is a film whose primary component is an olefin resin. However, when multiple types of olefin resins are included, even if the content of each individual olefin resin is 50% or less by mass 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, assuming that all structural units constituting the resin composition are 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" means that the B layer is laminated on one or both sides of the A layer without any intervening layer. The same applies to the description below of one side of a film being "in contact with" a membrane.

In order to suppress 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%.

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 required to maintain the structure in high-temperature environments 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 process film in high-temperature environments, the film will experience increased thermal shrinkage and reduced rigidity, leading to excessive embedding or sticking to the mating component. Such excessive embedding or sticking not only adversely affects the surface shape of the mating component, but also raises concerns about damage or deformation of the mating component when the biaxially oriented polyolefin film is peeled from it. 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 preferable from the viewpoint of compatibility with other physical properties. Details of the method for measuring the proportion of heat of fusion S in DSC measurement will be described later (the same applies to the heat of fusion H described later). Note that 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 it is used as a release film or process film.

To achieve the ratio S of heat of fusion of the biaxially oriented polyolefin film within the above range, methods can be used in which the raw material composition is within the range described below, or the film-forming conditions are within the range described below. In terms of raw material composition, in order to promote the formation of high-melting-point crystals by film-forming at high temperatures, it is particularly effective to use a polyolefin with a high melting point (e.g., polypropylene) as the raw material, and to appropriately adjust the half-crystallization time and angular frequency _ω200 of the resin constituting the biaxially oriented polyolefin film to control the relaxation characteristics. In terms of 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, treating at a temperature and relaxation rate within the ranges described below. These methods can also be used in combination as appropriate.

By adopting the above raw material composition and process conditions, stretching and relaxation treatments can be performed under high-temperature conditions, promoting the relaxation of the amorphous portions that cause thermal shrinkage stress while enabling the formation of high-melting-point crystals that were previously unimaginable 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.

The biaxially oriented polyolefin film of the present invention has two types of layers with different contents of 4-methyl-1-pentene polymer, from the viewpoint of suppressing fusion in high-temperature environments. Of the two types of layers, the layer with a relatively low content of 4-methyl-1-pentene polymer is designated Layer A, and the layer with a relatively high content is designated Layer B. Layer B is located on at least one surface, and Layer B contains 4-methyl-1-pentene polymer as its main component.

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 a 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 calculating the amount of constitutional units derived from 4-methyl-1-pentene by ^13C -NMR.

"Layer B being 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 adhesion, and from the viewpoint of achieving this effect on both surfaces, an embodiment in which Layer B is located on both surfaces is preferred.

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

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). 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). Furthermore, when Layer B is present on both surfaces, their compositions may be the same or different. An example of an embodiment in which the composition of layer B is different is when the outermost layers on both sides have the same and highest content (mass%) of 4-methyl-1-pentene polymer, but the types of 4-methyl-1-pentene polymer and other resins are different.

In order to prevent delamination that impairs the quality of the mating member when the biaxially oriented polyolefin film of the present invention is used as a release film or process film, it is preferable that the developed area ratio Sdr of at least one film surface be 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 process film, it is preferable to laminate the mating member onto Layer B. In other words, when Layer B is present on one surface, it is preferable that the Sdr of the Layer B surface be within the above range; when Layer B is present on both surfaces, the Sdr of one or both sides can be within the above range depending on the side to which the mating member is laminated.

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 dense uneven structure on the surface indicates a high value. 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 perspective of film formability, it is essentially 10, preferably 5.0. When Sdr is 0.2 or higher, the B layer 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 as a release film, the releasability of the biaxially oriented polyolefin film and the mating component, as well as the fusion resistance of the biaxially oriented polyolefin film itself, are more likely to be 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.

In order to achieve 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, from the raw material perspective, in order to form a dense uneven structure in combination with stretching, it is effective to set the angular frequency _ω260 of the 4-methyl-1-pentene polymer contained in Layer B within an appropriate range (for example, 200 rad/s or less), thereby strengthening the entanglement of molecular chains and improving compliance during stretching. Furthermore, from the process perspective, it is effective to set the preheating and stretching temperatures in the stretching step within preferred ranges. These methods can be used in combination as appropriate.

From the viewpoint of peelability 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 it is more preferable that the surface of Layer B satisfy the above requirement. 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; conversely, it tends to be low when a large amount of polyolefin other than 4-methyl-1-pentene polymer (e.g., polypropylene) is present in the outermost layer. Note that hereinafter, "static contact angle I with diiodomethane" may also be referred to as "contact angle I."

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°. The upper limit of the contact angle I is not particularly limited, but is preferably 90°, more preferably 80°, from the viewpoint of the raw material characteristics of the 4-methyl-1-pentene polymer and the film formability. When the contact angle I is 60° or greater, 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 to form a mating component on the surface of the layer, the releasability of the mating component and the fusion resistance of the biaxially oriented polyolefin film itself are more likely to be maintained even in high-temperature environments.

In order to set the contact angle I within the above range, it is effective to set the layer structure and film-forming 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, it is effective to set the angular frequency ω ₂₆₀ within a suitable range. It is also effective to adjust the thickness of layer B within a suitable range. These methods can be used in combination as appropriate.

From the viewpoint of handleability, the biaxially oriented polyolefin film of the present invention preferably has a sum E X + Y of the Young's modulus in the X direction and the Young's modulus in the Y direction, where the direction of the main orientation axis is the _{X direction and} the direction perpendicular to the main orientation is the Y direction, of 5.2 to 15 GPa. From the above viewpoint, the lower limit of E _{X + Y} is more preferably 6.0 GPa. From the viewpoint of achieving both film formability and heat resistance of the film, the upper limit of E X + _Y is preferably 10 GPa. When _{E X + Y} is 5.2 GPa or more, the stiffness of the biaxially oriented polyolefin film tends to be high, which improves handleability when used as a release film or process film, and improves workability and transportability in high-temperature environments.

In order to achieve the above-described range of E _X+Y for the biaxially oriented polyolefin film of the present invention, it is effective to set the raw materials and film-forming conditions within the ranges described below. In particular, in terms of raw materials, it is effective to set the melting point and angular frequency ω ₂₀₀ of the polyolefin resin (e.g., polypropylene resin) other than the 4-methyl-1-pentene polymer contained in Layer A within appropriate ranges. In terms of processing, it is effective to set the longitudinal stretching ratio and transverse stretching ratio within appropriate ranges. Furthermore, in terms of 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.

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

Furthermore, the "direction perpendicular 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 within the film plane. The Y direction is the direction perpendicular to the X direction specified by the above method within the film plane, and is automatically determined once the X direction is determined. Details of the measurement method for Young's modulus, including the Young's modulus in the X direction (E _X ) and the Young's modulus in the Y direction (E _Y ), will be described later.

In order to be used as a release film or process film in high-temperature environments, 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 temperature range of 190°C to 260°C, as determined by differential scanning calorimetry. From the above perspective, 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 perspective, 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 process film, it maintains 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 high-temperature environments.

In order to achieve the heat of fusion H of the biaxially oriented polyolefin film of the present invention 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 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 extrusion 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.

From the viewpoint of heat shrinkage properties in a high-temperature environment, the biaxially oriented polyolefin film of the present invention preferably has a sum of the 160°C shrinkage stress in the X direction and the 160°C shrinkage stress in the Y direction, P _{X + Y} , of -1.0 to 3.0 MPa. From the above viewpoints, the upper limit of P X _{+ Y} is more preferably 2.0, even more preferably 1.5, and particularly preferably 1.0. By having _{P X + Y} be 3.0 or less, heat shrinkage of the biaxially oriented polyolefin film is suppressed when a mating member is overlaid or attached to the biaxially oriented polyolefin film and processed in a molding press or heating oven in a high-temperature environment. This reduces the occurrence of deformations such as wrinkles and curls in the biaxially oriented polyolefin film, thereby reducing the transfer of these deformations to the mating member. On the other hand, from the viewpoint of film-forming properties of the biaxially oriented polyolefin film, the lower limit of P _{X + Y} is preferably -1.0, and from the viewpoint of achieving both high-temperature rigidity, 0.0 is preferred. Details of the method for measuring the 160°C shrinkage stress in TMA measurement will be described later.

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

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%.

Furthermore, it is preferable that the content of components whose logarithm of molecular weight M, LogM, is 6.0 or greater is 3.0 to 10.0% by mass. The upper limit of the content of components whose LogM is 6.0 or greater 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 LogM is 6.0 or greater is more preferably 3.5% by mass, and even more preferably 4.0% by mass.

When the content ratio of the component with Log M (logarithm of molecular weight M) of 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. 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, it is effective to set the angular frequency ω ₂₀₀ (details described below) of the polyolefin resin other than the 4-methyl-1-pentene polymer used in layer A within the preferred range described below and to adjust it by the pre-mixing and the mixing temperature during film formation.

From the viewpoint of heat resistance, etc., the biaxially oriented polyolefin film of the present invention preferably has a melting point Tm ₁ , which is the melting point with the highest peak intensity obtained in the second run of DSC measurement at a heating rate of 20°C/min, of 165.0 to 170.0°C. From the above viewpoint, the lower limit of Tm ₁ is more preferably 166.0°C, even more preferably 166.5°C, and particularly preferably 167.0°C. Furthermore, the upper limit of Tm ₁ is 170.0°C, taking into account the properties of the polypropylene resin as described below. The method for measuring Tm ₁ will be described in detail below.

_Tm1 is an index showing the raw material melting point of the polyolefin resin other than the 4-methyl-1-pentene polymer contained in the biaxially oriented polyolefin film of the present invention. If this index is within the above range, film formation under higher temperature conditions becomes possible. This facilitates improvement of heat shrinkage properties and is advantageous in terms of the formation of high-melting-point crystals, resulting in improved heat resistance of the resulting biaxially oriented polyolefin film. To achieve _Tm1 within the above range, it is effective to set the raw material composition of the biaxially oriented polyolefin film within the range described below, particularly by using a high-melting-point raw material and minimizing mixtures other than the high-melting-point raw material, or by increasing the proportion of layer A in the entire biaxially oriented polyolefin film. These methods may be used in combination as appropriate.

From the viewpoint of suppressing fusion at high temperatures, the biaxially oriented polyolefin film of the present invention preferably has a melting point _Tm2 observed at 190°C or higher in the second run of DSC measurement at a temperature rise rate of 20°C/min of 200 to 250°C. From the above viewpoint, the lower limit of _Tm2 is more preferably 220°C, and even more preferably 225°C. The upper limit of _Tm2 is more preferably 240°C. Details of the method for measuring _Tm2 will be described later.

_Tm2 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 if it is within the above range, it is possible to achieve both film formability and resistance to fusion at high temperatures. To achieve _Tm2 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 in particular to use a 4-methyl-1-pentene polymer having a melting point within an appropriate range and to add it in an appropriate proportion. These methods may be used in combination as appropriate.

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.

Next, we will explain the raw materials that can be used to produce the biaxially oriented polyolefin film of the present invention, but they are not necessarily limited to these.

The biaxially oriented polyolefin film of the present invention has two types of layers that differ from each other in the 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 is referred to as Layer B. Note that the 4-methyl-1-pentene polymer content here refers to the content (mass %) of 4-methyl-1-pentene polymer when the entire layer is taken as 100 mass %.

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% by thickness. 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 ensuring that the proportion of layer A is 70% or more by thickness, it is possible to maintain favorable thermal shrinkage properties. On the other hand, by ensuring that the proportion of layer A is 99.5% or less by thickness, it is possible to ensure that layer B is thick enough to exhibit releasability.

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.

The biaxially oriented polyolefin film of the present invention may be composed of layers other than layer A and layer B, but from the standpoint of heat resistance, it is preferable that it be composed of only layer A and layer B.

From the viewpoints of biaxial stretchability and heat shrinkage properties, it is preferable to use polypropylene composition A for Layer A in the biaxially oriented polyolefin film of the present invention. In the present invention, polypropylene composition A refers to a composition that contains 90 to 100% by mass of polypropylene resin, when all constituent components are taken as 100% by mass, and has a melting point of 165.0 to 170.0°C, an angular frequency _ω200 of 10 to 70 rad/s, and a crystallization half time (indicating the crystallization rate) 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. It is most preferable that the polypropylene composition A is 100% by mass, i.e., a simple polypropylene resin is the only constituent component. When a composition contains multiple components corresponding to polypropylene resin, the composition is considered to correspond to polypropylene composition A as long as the total amount of these components exceeds 95% by mass of the entire resin and the melting point, crystallization half time, and angular frequency _ω200 are within the above-mentioned ranges. Furthermore, 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 %.

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 more likely to form 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 included, the melting point of polypropylene composition A is determined to be the temperature with the highest peak intensity in the DSC melting curve.

From the viewpoint of the heat resistance of the resulting biaxially oriented polyolefin film, polypropylene composition A has an angular frequency _ω200 of 10 to 70 rad/s, at which the loss tangent obtained by melt viscoelasticity measurement at 200°C is 1. The angular frequency _ω200 is an index of the relaxation characteristics of the polypropylene composition. When this angular frequency _ω200 is low, the relaxation characteristics are low (i.e., relaxation is slow) due to entanglement between molecular chains, etc., but the entanglement between molecular chains is easily maintained even at high temperatures, which tends to work advantageously for increasing the melting point of the crystal. On the other hand, when the angular frequency _ω200 is high, the entanglement between molecular chains is low, so the relaxation characteristics are high (i.e., relaxation is fast), and it is easy to efficiently reduce residual strain and relax the amorphous portion in the Relax step. From the above viewpoints, the lower limit of the angular frequency _ω200 of polypropylene composition A is more preferably 15 rad/s, and even more preferably 20 rad/s. From the above viewpoints, the upper limit of the angular frequency _ω200 is more preferably 50 rad/s, and even more preferably 40 rad/s or less. When the angular frequency _ω200 is within the above range, relaxation of the amorphous portion of the polypropylene composition A and an increase in the crystalline melting point are promoted, making it possible to further improve the heat resistance of the obtained biaxially oriented polyolefin film. The angular frequency _ω200 can be measured using a known rotational rheometer, and details will be described later (the same applies to the angular frequency _ω260 described later).

In order to set the angular frequency _ω200 of polypropylene composition A within 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 within 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.

The crystallization half time of polypropylene composition A is preferably 5 to 200 seconds. The upper limit of the crystallization half time of 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 keep the crystallization half time of polypropylene composition A within the above range, it is effective to appropriately adjust the crystallinity and molecular weight distribution of the polypropylene resin constituting polypropylene composition A, or to add 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 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 polypropylene composition A.

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 branched polypropylene resins, which are homopolypropylene resins useful for controlling crystallization rate, and block copolymer polypropylene resins and random copolymer polypropylene resins other than homopolypropylene resins, may be mixed in by pre-kneading or other methods, provided that the properties are not impaired. When components other than branched polypropylene resins or homopolypropylene resins are added to polypropylene composition A, the amount added is preferably 1.2 mass% or less, more preferably 1.0 mass% or less, and even more preferably 0.5 mass% or less, of the total components of polypropylene composition A. Here, homopolypropylene resin refers to a polypropylene resin in which the amount of propylene units is 99.9 to 100 mol%, where the total number of structural units constituting the molecular chain of the resin is 100 mol%.

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

Layer B in the biaxially oriented polyolefin film of the present invention preferably contains the aforementioned 4-methyl-1-pentene polymer in a proportion of 80 to 100%. The lower limit of the proportion of 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 4-methyl-1-pentene polymer in Layer B is less than 80%, the high-temperature fusion resistance of the resulting biaxially oriented polyolefin film will be affected. Furthermore, a 4-methyl-1-pentene copolymer or the like may be added to Layer B for purposes such as improving adhesion to other layers.

From the viewpoint of structure formation by stretching, the 4-methyl-1-pentene polymer in the biaxially oriented polyolefin film of the present invention preferably has an angular frequency ω ₂₆₀ of 1 to 500 rad/s, at which the loss tangent obtained by melt viscoelasticity measurement at 260°C becomes 1. The angular frequency ω ₂₆₀ is the angular frequency at which the loss tangent of the molten polypropylene resin becomes 1, and serves as an index of the relaxation characteristics of the polypropylene resin. The lower limit of the angular frequency ω ₂₆₀ of the 4-methyl-1-pentene polymer is more preferably 5.0 rad/s, and even more preferably 30 rad/s. On the other hand, the upper limit of the angular frequency ω ₂₆₀ of the 4-methyl-1-pentene polymer is more preferably 200 rad/s, and even more preferably 50 rad/s.

When the angular frequency _ω260 of the 4-methyl-1-pentene polymer is 1.0 to 500 rad/s, entanglement of molecular chains is not relaxed during stretching, and 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.

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 other layers primarily composed of polyolefins. 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.

The entire resin used in the biaxially oriented polyolefin film of the present invention can also contain various additives, such as inorganic and organic particles, crystal nucleating agents, antioxidants, heat stabilizers, slip agents, antistatic agents, antiblocking agents, fillers, viscosity modifiers, and color inhibitors, as long as the purpose of the present invention is not impaired. These components may be used alone or in combination, and may be added to any layer.

Among these, the selection of the type and amount of antioxidant is important from the perspective of antioxidant bleed-out. In other words, such antioxidants are preferably sterically hindered phenol-based, with at least one of them being a high molecular weight type with a molecular weight of 500 or more. Specific examples include various antioxidants, such as 2,6-di-t-butyl-p-cresol (BHT: molecular weight 220.4) 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 based on the total amount of resin that makes up 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 antioxidant may bleed out, resulting in a decrease in transparency. A more preferred content is 0.05 to 0.9 parts by mass, and especially preferred is 0.1 to 0.8 parts by mass.

In addition, a crystal nucleating agent can be added to the raw materials for each layer used in the biaxially oriented polyolefin film of the present invention, provided that it does not conflict with the objectives 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 (potassium 1,2-hydroxystearate, magnesium benzoate, amide compounds such as N,N'-dicyclohexyl-2,6-naphthalenedicarboxamide, quinacridone compounds, etc.). However, because 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 typically 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.

The biaxially oriented polyolefin film of the present invention is preferably a biaxially stretched film. The biaxial stretching method may be any of the inflation simultaneous biaxial stretching method, stenter simultaneous biaxial stretching method, and stenter sequential biaxial stretching method. Among these, it is preferable to use the stenter sequential biaxial stretching method in terms of film formation stability, thickness uniformity, and control of film rigidity and dimensional stability.

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

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 240-280°C. After melt-kneading, the mixture is 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, the 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 into another single-screw extruder, where they are melt-extruded preferably at 200-290°C, more preferably 240-280°C, and even more preferably 260-280°C. After removing foreign matter and modified polymers using a filter installed midway through the polymer pipe, the layers are laminated in a layer B/layer A/layer B configuration using a multi-manifold composite T-die, and then extruded 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.

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. A casting temperature within this range can suppress the formation of β-crystals, which have a low melting point among polypropylene resin crystals, and is likely to favorably increase the proportion of high-melting-point crystals in the film. The method of adhesion to the casting drum can be any of the following: electrostatic application, adhesion methods utilizing the surface tension of water, air knife method, press roll method, and underwater casting method. However, 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. It is also preferable to appropriately adjust the position of the air knife so that air flows downstream of the film production process to prevent vibration of the laminated unstretched sheet.

The resulting laminated unstretched sheet is then introduced into the longitudinal stretching process (stretching in the lengthwise direction). In the longitudinal stretching process, the laminated unstretched sheet is preheated before stretching using a metal roll heated to 150-160°C, preferably 152-159°C, and more preferably 154-158°C. When the preheating temperature is within the above range, the laminated unstretched sheet proceeds to the longitudinal stretching process in a softened state, allowing it to be stretched without applying more stress than necessary in the longitudinal stretching process, which tends to reduce thermal shrinkage stress. It is also 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.

Then, immediately after preheating, the film is stretched 3.8 to 6.0 times in the machine direction between rolls with a difference in peripheral speed 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 above 150°C and 160°C or less, 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 maintain the stretchability of the 4-methyl-1-pentene polymer contained in Layer B, suppress residual excessive strain in the polypropylene resin contained in Layer A, and pull out molecular chains from the softened crystals, which, combined with the subsequent relaxation process, promotes a high crystalline melting point.

At the end of the longitudinal stretching process, the longitudinally uniaxially stretched film is brought into contact with metal rolls with a peripheral speed difference and maintained at 140-160°C, where it is relaxed in the longitudinal direction at a relaxation rate greater than 0% and less than 10%, and then cooled to room temperature. The relaxation temperature is preferably 142-160°C, more preferably 145-160°C, even more preferably 150-160°C, and particularly preferably 154-160°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, making it possible to form crystals with a higher melting point. Furthermore, the relaxation rate during the longitudinal stretching process is preferably 0.1-10%, more preferably 1.0-10%, even more preferably 3.0-10%, and particularly preferably 5.0-10%. A relaxation rate within the above range during the longitudinal stretching process 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 molecular chain rearrangement is also facilitated.

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 structural deformation of the 4-methyl-1-pentene polymer contained in Layer B is promoted, while the crystals formed in the longitudinal stretching step of the polypropylene resin contained in Layer A can be softened before proceeding to the transverse stretching step. This allows stretching to be performed without applying more stress than necessary in the transverse stretching step, making it easier to reduce thermal shrinkage stress.

The stretching temperature in the transverse stretching step after the preheating step is above 170°C and not higher 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 is possible to promote structural deformation of the 4-methyl-1-pentene polymer contained in Layer B, while suppressing residual excessive strain in the polypropylene resin contained in Layer A, and to pull out molecular chains from the crystals softened by preheating. This, combined with the subsequent relaxation step, promotes a high crystalline melting point.

In the subsequent relaxation process 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 10-20%, more preferably 11-18%, and even more preferably 12-15%. A relaxation rate after transverse stretching within the above range facilitates the release of tension in the molecular chains. This promotes amorphous relaxation, reduces shrinkage stress, and ensures that molecular chain mobility is within a moderate range, facilitating molecular chain rearrangement. The heat setting temperature during this process is preferably above 170°C and 190°C or less, preferably 173-185°C, and more preferably 175-185°C. A relaxation temperature within the above range promotes rearrangement of molecular chains extracted from crystals by transverse stretching, making it possible to form higher-melting-point crystals with thicker lamellae.

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 when it is within the above range, amorphous relaxation is likely to proceed, and the shrinkage stress of the biaxially oriented polyolefin film as a whole is likely to be reduced.
Total area relaxation rate (%) = [1 - (1 - longitudinal stretching process relaxation rate / 100) x (1 - transverse stretching process relaxation rate / 100)] x 100

Then, while still tensely gripped across the width with clips, the film is cooled at 80-130°C and then led to the outside of the tenter. The clips on the film edges are released, and the film edges are slit in the winding process, and the film product roll is wound up.

The biaxially oriented polyolefin film obtained in this manner can be used for a variety of purposes, including packaging film, surface protection film, processing film, battery film, sanitary products, agricultural products, construction products, and medical products. Because of its particularly excellent heat resistance, it is preferably used as processing film that requires high-temperature processing, such as drying coatings and molding thermosetting resins, as well as release film, base film for current collectors in secondary batteries, and packaging film for retort pouches, and is particularly preferably used as release film for use in high-temperature regions (details of uses will be described later).

The release film and processing film of the present invention are 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, a 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. Furthermore, a 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 manufacturing to protect it from scratches, contamination, etc., or it can function as a support when the object itself is thin or fragile and therefore difficult to form a film from.

Because the release film of the present invention has excellent heat resistance and releasability, it is particularly preferred for use in fiber-reinforced composite curing, substrates, or molds. All of these applications correspond to release applications used in high-temperature environments. For fiber-reinforced composite materials, the film is attached to a material reinforced by mixing glass fiber or carbon fiber into resin. For substrates, the film is attached to a plate (substrate) on which components that achieve a certain function are placed. 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 used advantageously to protect them.

The laminate of the present invention will now be described. Because the biaxially oriented polyolefin film of the present invention has excellent heat resistance and release properties, it can also 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. In other words, 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.

In vapor deposition and sputtering processes, polyethylene terephthalate (PET) film has often been used due to its excellent heat resistance and rigidity. However, PET is highly hydrophilic due to its ester bonds, and PET film contains trace amounts of moisture. This trace amount 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. Given this background, when the laminate of the present invention has a metal film, it is preferable that the metal film contains a metal belonging to Group 1 or 2 of the periodic table, since this allows for a suitable laminate to be obtained even in cases where the presence of trace amounts of moisture makes it difficult to use PET film. Here, metals belonging to Group 1 or 2 refer to lithium, sodium, potassium, rubidium, cesium, francium, beryllium, magnesium, calcium, strontium, barium, and radium.

Furthermore, in high-temperature environments, moisture in the film evaporates as outgassing, with the effects becoming particularly pronounced under high-vacuum conditions such as in metal vapor deposition processes. Outgassing from the film can worsen the degree of vacuum within 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 ideally used in applications involving the formation of metal films, and the laminate of the present invention can maintain good metal film quality.

Furthermore, electrolyte membranes used in fuel cells, semi-solid batteries, all-solid batteries, etc. are typically manufactured in environments 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. For this reason, the biaxially oriented polyolefin film of the present invention is preferably used as the processing film in the production of such electrolyte membranes. In other words, a preferred embodiment of the laminate of the present invention is one in which an electrolyte membrane is in contact with at least one side of the biaxially oriented polyolefin film, and it is particularly preferred that the electrolyte membrane is for use in fuel cells, semi-solid batteries, or all-solid batteries.

From the above perspective, 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 essentially 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.

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 limit the amount of additives to the minimum necessary. 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 essentially 100% by mass.

From the above perspective, when the total resin components of the biaxially oriented polyolefin film of the present invention are taken as 100 parts by mass, the antioxidant content is preferably 0.05 to 0.9 parts by mass, and more preferably 0.1 to 0.8 parts by mass. In particular, it is preferable to control the content of phosphorus-based antioxidants, as they may bleed out onto the surface and 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 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). From the perspectives of properties, quality, and reducing inhibition of effects, it is preferable that the content of antioxidants, including phosphorus-based antioxidants, be the same even when the film formed on the surface is not a metal film.

In order to reduce defects caused by bleed-out in the biaxially oriented polyolefin film of the present invention, it is preferable that the total content of additives other than antioxidants (e.g., antistatic agents, viscosity modifiers, color inhibitors, slip agents, etc.) be 0 to 0.05 parts by mass. A content of these additives of 0 parts by mass is the same as not including these additives.

As described above, the biaxially oriented polyolefin film of the present invention has an extremely low moisture content and generates very little outgassing. In addition, it has superior heat resistance and handling properties compared to existing polyolefin films, making it suitable for use in forming transparent conductive films that require more stringent processing conditions and heat resistance. In other words, a preferred embodiment of the laminate of the present invention is one 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, a transparent conductive film refers to a thin film formed from a material that is conductive yet transmits visible light; specific examples include indium-tin composite oxide (ITO), zinc oxide (ZnO), and palladium films.

Next, we will explain the current collector of the present invention. The current collector of the present invention is made 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. A current collector is a foil-like laminate used in electrodes of storage batteries such as lithium-ion batteries. Metal foil is typically used as a current collector, but laminates in which a metal film is laminated onto a base resin film 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 base of the current collector must 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 involves high heat, such as radiant heat, during processing, and tension in the transport direction, so the film must have good handleability. The biaxially oriented polyolefin film of the present invention can be made thin and has good handleability, making it preferable for use as a current collector.

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 generally used in storage batteries that use the current collector 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.

The present invention will be explained in detail below using examples. The properties were measured and evaluated using the following methods.

(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.

(2) The ratio S of the heat of fusion between 175 and 190°C and H 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.

<Proportion S of heat of fusion at 175 to 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 (J / g) from 30 to 190 ° C. 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 (J / g) from 175 to 190 ° C. The obtained heat of fusion from 30 to 190 ° C. and the heat of fusion from 175 to 190 ° C. were applied to the following formula to determine the ratio S (%) of the heat of fusion from 175 to 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]

<Heat of fusion H at 190 to 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 sample mass to calculate the heat of fusion H (J/g) from 190 to 260°C.

(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 quartic 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 5 cm x 5 cm square cut biaxially oriented polyolefin film. 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. The measurements were performed on both the front and back surfaces of the film.

<How to determine the measurement position>
Measurement 1: Position of 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

<Measurement conditions and device configuration>
Objective lens: 10x
Telescope 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: 561.1μm x 561.5μm
Number of pixels: 1,024 x 1,024

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: 530nm white
Measurement mode: Wave
Measurement software: VS-Measure Version 5.5.1
Analysis software: VS-Viewer Version 5.5.1
Measurement area: 561.097μm x 561.473μm

(4) Diiodomethane contact angle I
Using diiodomethane (methylene iodide) as the measurement liquid, a contact angle meter CA-D manufactured by Kyowa Interface Science Co., Ltd. was used to determine the static contact angle on the film surface, which was designated 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.

(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 values were calculated. The Young's modulus in the X direction was defined as EX, the Young's modulus in the Y direction was defined as EY, and the sum of these was defined as _EX+Y .

(6) 160°C shrinkage stress P _{X + Y}
Using a thermomechanical analyzer (Model TMA/SS6100 manufactured by SII Nanotechnology Co., Ltd.), thermal 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 identified by the method described in (5).
(a) Sample: Width 4 mm x Length 20 mm
(b) Initial load: 0.0 mN
(c) Temperature program: Heat 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 designated PX, the value in the Y direction was designated PY, and the sum of PX and PY was designated PX _+Y .

(7) Melting point of raw material and Tm ₁ and Tm ₂ of biaxially oriented polyolefin film
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, 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 heated again from 30°C to 260°C at 20°C/min. The maximum peak temperature of the melting curve observed when the temperature was increased again 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 Tm ₁ (°C), the melting point of the polypropylene resin in the biaxially oriented polyolefin film. The temperature of the maximum peak between 190 and 260°C was taken as _Tm2 , which is the melting point of the 4-methyl-1-pentene polymer in the biaxially oriented polyolefin film. 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 a stretched film and measured.

(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.

(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 placed on a plate heated to 200°C or 260°C under a nitrogen atmosphere for 10 minutes. 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 resulting curve of angular frequency vs. loss tangent, the angular frequency at which the loss tangent was 1 was determined. The angular frequency obtained in the measurement at 200°C was designated ω ₂₀₀ (rad/s), and the angular frequency obtained in the measurement at 260°C was designated ω ₂₆₀ (rad/s). When multiple resin components were used as the polyolefin composition without pre-mixing, measurements were performed on a resin composition obtained by melt-extrusion of only Layer A, or on Layer A alone sampled from an unstretched sheet or stretched film.

(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.

<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: 0.500mL
Sample concentration: 0.1 wt%
Standard samples: monodisperse polystyrene manufactured by Tosoh, dibenzyl manufactured by Tokyo Chemical Industry.

(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.

(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.

(shape stability)
The cardboard was evaluated as pass if no folds or wrinkles were observed, and as fail if at least one of folds or wrinkles was observed, and was evaluated according to the following criteria.

(High temperature peel resistance)
After the 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.

<Evaluation criteria (shape stability and high-temperature peel resistance)>
The shape stability and high-temperature peel resistance were evaluated according to the following criteria, and if each characteristic was A to C, it was judged 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.

(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 bonding 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 pulling rate of 300 mm/min. From the graph of peel force (N/19 mm) vs. test length (mm) obtained by the measurement, the average value of the 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 value of the peel strength was 2.0 N/19 mm or more, and the standard deviation of the peel strength was 10% or more and 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.

(14) High-temperature transportability A 500 mm wide biaxially oriented polyolefin film was introduced into a drying oven at 140 ° C., transported for 20 seconds under a transport tension of 1.0 MPa, and wound up into a roll with a winding length of 200 m at a winding tension of 200 N / m to obtain a film roll. Next, 1 m of the 500 mm wide biaxially oriented polyolefin film was unwound, 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 poor flatness such as wrinkles or dents was visually confirmed. Evaluation was performed according to the following criteria.
A: There were no areas with poor flatness under free tension.
B: Poor flatness was observed in some areas with free tension, but the poor flatness disappeared with 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: Even at a tension of 3 kg/m width, some areas of poor flatness were observed.

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

(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® F113G)
Homopolypropylene resin 6 (PP6): manufactured by Sumitomo Chemical Co., Ltd. (Sumitomo "Noblen" (registered trademark) FLX80E4)
Branched chain polypropylene resin 7 (PP7): manufactured by Boreales Polypropylene resin 8 (PP8): PP1/PP7 = 99.7/0.3 were charged into a twin-screw extruder, kneaded at 260°C, and cooled to obtain pellets.
Polypropylene resin 9 (PP9): PP1/PP2 were charged into a twin-screw extruder in 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.

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)

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 the 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.

(Examples 2 to 5, Comparative Examples 1 to 5)
A biaxially oriented polyolefin film was 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 film are shown in Table 3. The thickness was adjusted by adjusting the discharge rate of the extruder and the speed of the casting drum.

(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 the heat resistance and the yield during metal film formation, and the moisture content was measured. The evaluation results are shown in Table 3.

Note that moisture content and yield during metal film production were evaluated only for Example 1 and Comparative Examples 4 and 6.

In the table, the diiodomethane contact angles I and Sdr were measured on the front surface (the surface that came into contact with the casting drum) and the back surface (the surface opposite the front surface), and the measured values for each are listed.

As mentioned above, the biaxially oriented polyolefin film of the present invention can be used in a variety of applications, including packaging films, release films, processing films, sanitary products, agricultural products, construction products, and medical products. In particular, the biaxially oriented polyolefin film of the present invention has excellent heat resistance, making it suitable for use as release films and processing films for use at high temperatures, which are generally considered difficult to use with biaxially oriented polyolefin films.

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

Claims (18)

A biaxially oriented polyolefin film in which the proportion S of the heat of fusion in the 175-190°C range to the heat of fusion in the 30-190°C range obtained by differential scanning calorimetry is 10-70%, the film has two types of layers with different contents of 4-methyl-1-pentene polymer, the layer with a relatively low content of 4-methyl-1-pentene polymer being designated as layer A and the layer with a relatively high 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 major component. 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. The biaxially oriented polyolefin film according to claim 1 or 2, wherein the static contact angle I with diiodomethane on at least one of the film surfaces is 60 to 90°. 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 _{X + Y} of the Young's modulus in the X direction and the Young's modulus in the Y direction is 5.2 to 15 GPa. The biaxially oriented polyolefin film according to any one of claims 1 to 3. The biaxially oriented polyolefin film according to any one of claims 1 to 4, 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. The biaxially oriented polyolefin film according to any one of claims 1 to 5, wherein the sum P _{X + Y} of the 160 ° C shrinkage stress in the X direction and the 160 ° C shrinkage stress in the Y direction measured by thermomechanical analysis is -1.0 to 3.0 MPa. The biaxially oriented polyolefin film according to any one of claims 1 to 6, wherein the polyolefin resin contained therein satisfies all of the following characteristics 1 to 3.
Feature 1: The melting point Tm _1, which is the melting point with the largest peak intensity obtained in the second run of differential scanning calorimetry performed at a heating rate of 20°C/min, 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. The biaxially oriented polyolefin film according to any one of claims 1 to 7, 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: The melting point _Tm2 observed at 190°C or higher in the second run of differential scanning calorimetry performed at a heating rate of 20°C/min is 200 to 250°C.
Feature 5: The angular frequency ω ₂₆₀ at which the loss tangent obtained by melt viscoelasticity measurement carried out at 260° C. becomes 1 is 1 to 500 rad/s. A release film comprising the biaxially oriented polyolefin film according to any one of claims 1 to 8. The release film according to claim 9, which is used for fiber-reinforced composite materials, substrates, or molds. A processing film comprising the biaxially oriented polyolefin film described in any one of claims 1 to 8. A laminate comprising a biaxially oriented polyolefin film according to any one of claims 1 to 8, with a metal film in contact with at least one surface. The laminate described in claim 12, wherein the metal film comprises a metal belonging to Group 1 or Group 2 of the periodic table. A laminate comprising a biaxially oriented polyolefin film according to any one of claims 1 to 8, with a transparent conductive film in contact with at least one surface of the biaxially oriented polyolefin film. A laminate comprising an electrolyte membrane in contact with at least one surface of the biaxially oriented polyolefin film described in any one of claims 1 to 8. The laminate described in claim 15, wherein the electrolyte membrane is used in a fuel cell, a semi-solid battery, or an all-solid battery. A current collector made using the biaxially oriented polyolefin film described in any one of claims 1 to 8. A storage battery comprising the biaxially oriented polyolefin film according to any one of claims 1 to 8.