Biaxially oriented polyolefin film

Abstract

To provide a biaxially oriented polyolefin film that allows for simultaneous realization of excellent release properties and handling characteristics in high-temperature environments of 160°C or more.SOLUTION: A biaxially oriented polyolefin film comprising a layer (A layer) mainly composed of a polypropylene-based resin and a layer (B layer) mainly composed of a 4-methyl-1-pentene-based resin having a melting point of 190°C or more and 280°C or less, the layers A and B being in contact with each other. The interlayer adhesion strength between the A layer and the B layer is 1.4 N / 19 mm or more and 10 N / 19 mm or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a biaxially oriented polyolefin having excellent heat resistance, releasability and handleability. [Background technology]

[0002] Polyolefin films are highly hydrophobic films because they are primarily composed of resins whose main structural units are hydrocarbon-derived units. As a result, polyolefin films have excellent releasability and low moisture absorption, making them suitable for use as release films and process films.

[0003] Widely used unstretched polyolefin films include films made of polyolefin resins such as polyethylene, polypropylene, and 4-methyl-1-pentene resins. Polyolefin films made of 4-methyl-1-pentene resins in particular have excellent heat resistance and releasability. For example, Patent Document 1 describes an unstretched film made of 4-methyl-1-pentene resin that can be suitably used as a release film for semiconductor molds. However, the film has poor mechanical strength and insufficient handleability.

[0004] On the other hand, as a stretched polyolefin film, a biaxially oriented polypropylene film, which has excellent mechanical strength and thickness accuracy, is often used. For example, Patent Document 2 describes a biaxially oriented polypropylene film that has excellent heat resistance and can be suitably used as a processing film or a release film.

[0005] However, for example, when a biaxially oriented polypropylene film with a surface layer melting point of approximately 160°C is used as a release film or process film in a high-temperature environment of 160°C or higher, more than half of the crystals in the surface layer melt, causing the film to fuse to the adherend (the mating member), making it impossible to peel. As such, biaxially oriented polypropylene films have a limited usable temperature range. In other words, it has been difficult for conventional biaxially oriented polypropylene films to achieve both good releasability and handleability in a high-temperature environment of 160°C or higher.

[0006] In consideration of the above problems, for example, Patent Documents 3 to 5 describe laminated stretched films using a polypropylene resin for the inner layer and a 4-methyl-1-pentene resin for the surface layer. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-361643 [Patent Document 2] International Publication No. 2022 / 210688 [Patent Document 3] Japanese Patent Application Publication No. 2019-1139 [Patent Document 4] International Publication No. 2018 / 097161 [Patent Document 5] Japanese Patent Application Publication No. 2018-204001 Summary of the Invention [Problem to be solved by the invention]

[0008] However, while the laminated stretched film described in Patent Document 3 is somewhat suitable for use as a process film or release film in high-temperature environments, its low stretching ratio results in insufficient thickness uniformity and mechanical strength, resulting in issues with handleability. Furthermore, the laminated polypropylene film described in Patent Document 4 has excellent releasability and transparency / smoothness, but its low stretching temperature results in insufficient stretchability of the surface layer, which is mainly composed of a poly(4-methyl-1-pentene) resin with a high melting point and poor stretchability, resulting in the problem of easy peeling at the interface between the surface layer and the inner layer during stretching. Furthermore, the polyolefin film described in Patent Document 5 has excellent productivity and designability, but its low stretching temperature results in insufficient stretchability of the surface layer, which is mainly composed of a poly(4-methyl-1-pentene) resin with a high melting point and poor stretchability, resulting in the problem of easy peeling at the interface between the surface layer and the inner layer during stretching.

[0009] Therefore, an object of the present invention is to solve the above-mentioned problems, that is, to provide a biaxially oriented polyolefin film that can achieve both good releasability and handleability in a high-temperature environment of 160°C or higher. [Means for solving the problem]

[0010] In order to solve the above-mentioned problems, the biaxially oriented polyolefin film of the present invention has the following configuration: a layer (layer A) mainly composed of a polypropylene resin is in contact with a layer (layer B) mainly composed of a 4-methyl-1-pentene resin having a melting point of 190°C or more and 280°C or less, and the interlayer adhesion strength between layer A and layer B is 1.4 N / 19 mm or more and 10 N / 19 mm or less. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a biaxially oriented polyolefin film that can achieve both good releasability and handleability in a high-temperature environment of 160°C or higher. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 2 is a schematic diagram illustrating a pressurization method used in evaluating heat resistance characteristics. DETAILED DESCRIPTION OF THE INVENTION

[0013] The biaxially oriented polyolefin film of the present invention is a biaxially oriented polyolefin film comprising a layer (layer A) mainly composed of a polypropylene resin and a layer (layer B) mainly composed of a 4-methyl-1-pentene resin having a melting point of 190°C or more and 280°C or less, the layers being in contact with each other, and the interlayer adhesion strength between layer A and layer B is 1.4 N / 19 mm or more and 10 N / 19 mm or less. The biaxially oriented polyolefin film of the present invention will be described in detail below.

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

[0015] From the viewpoint of its temperature range of use, the biaxially oriented polyolefin film of the present invention is composed of a layer (layer A) mainly composed of a polypropylene-based resin and a layer (layer B) mainly composed of a 4-methyl-1-pentene-based resin having a melting point of 190°C or higher and 280°C or lower, which are adjacent to each other.

[0016] Here, film refers to a sheet-like molded product whose main component is a thermoplastic resin. The main component refers to a component that accounts for more than 50% by mass but not more than 100% by mass, assuming that the total components of the object (film, layer, etc.) are 100% by mass (however, if two components are present at 50% by mass each, the one with the higher melting point is considered the main component). 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.

[0017] A polyolefin film refers to a film primarily composed of an olefin resin. However, when multiple olefin resins are contained, even if the content of each individual olefin resin is 50% by mass or less of the entire film, the film is considered to be a polyolefin film if the total content of all olefin resins exceeds 50% by mass. Furthermore, an olefin resin refers to a resin that contains more than 50 mol% to 100 mol% of structural units derived from olefin hydrocarbons, where the total structural units constituting the resin are taken as 100 mol%. Hereinafter, this definition can be applied to other olefin resins (e.g., polypropylene resin, 4-methyl-1-pentene resin, etc.). Furthermore, "layer A and layer B are in contact" refers to a mode in which layer B is laminated on one or both sides of layer A without any other layer in between.

[0018] In the biaxially oriented polyolefin film of the present invention, the upper limit of the melting point of the 4-methyl-1-pentene resin is substantially 280°C, preferably 250°C, more preferably 240°C, and even more preferably 235°C, from the viewpoint of its production method. On the other hand, the lower limit of the melting point of the 4-methyl-1-pentene resin is preferably 220°C, more preferably 225°C, and even more preferably 230°C. If the lower limit of the melting point of the 4-methyl-1-pentene resin is less than 190°C, the biaxially oriented polyolefin film will be more likely to melt in a high-temperature environment, limiting the temperature range in which it can be used. The melting point of the 4-methyl-1-pentene resin can be measured by differential scanning calorimetry (DSC measurement), and details of the measurement method will be described later.

[0019] To set the melting point of the 4-methyl-1-pentene resin within the above range, the type and amount of copolymer used during polymerization can be set within the ranges described below. In particular, it is effective to use a 4-methyl-1-pentene homopolymer or a 4-methyl-1-pentene homopolymer with a small amount of copolymerization units.

[0020] In order to ensure good releasability and handleability, it is important that the biaxially oriented polyolefin film of the present invention has an interlayer adhesion between layer A and layer B of 1.4 N / 19 mm or more and 10 N / 19 mm or less (hereinafter, the "interlayer adhesion between layer A and layer B" may be simply referred to as "interlayer adhesion"). Although both polypropylene-based resin and 4-methyl-1-pentene-based resin are olefin-based resins whose main structural units are structural units derived from olefin hydrocarbons, the compatibility between the two is low due to differences in their chemical structures. Furthermore, while polypropylene-based resins have excellent stretchability when formed into a film, 4-methyl-1-pentene-based resins are characterized by poor stretchability.

[0021] For these reasons, when a layer (Layer A) primarily composed of a polypropylene resin and a layer (Layer B) primarily composed of a 4-methyl-1-pentene resin are laminated by coextrusion or other methods, interlayer adhesion may be maintained to a certain extent in the unstretched state. However, due to the significant difference in the stretchability of the two layers, uniaxial or biaxial stretching reduces interlayer adhesion and makes the film prone to delamination. Meanwhile, unstretched polyolefin films with this layer structure typically lack uniformity in thickness and mechanical strength, resulting in poor handleability and making them unsuitable for use as release films or process films in high-temperature environments. Therefore, by creating a biaxially oriented polyolefin film with this laminate structure and an interlayer adhesion strength of 1.4 N / 19 mm or greater, both release properties and handleability can be achieved.

[0022] From the above viewpoints, the lower limit of the interlayer adhesion strength of the biaxially oriented polypropylene film of the present invention is preferably 1.8 N / 19 mm, more preferably 2.2 N / 19 mm, even more preferably 2.6 N / 19 mm, and most preferably 3.0 N / 19 mm. Biaxially oriented polyolefin films with an interlayer adhesion strength of less than 1.4 N / 19 mm are prone to delamination between Layer A and Layer B. Therefore, when such a biaxially oriented polyolefin film is used as a release film or processing film, delamination may occur between Layer A and Layer B when peeled from a mating member, resulting in problems such as the layer in contact with the mating member migrating toward the mating member. From the above viewpoints, the higher the interlayer adhesion strength, the better, but the upper limit is substantially 10 N / 19 mm.

[0023] In the present invention, the interlayer adhesion strength can be measured by attaching an adhesive tape to the surface of layer B of a biaxially oriented polyolefin film, forcibly peeling layers A and B using a universal testing machine, and evaluating the peel strength at that time (details will be described later).

[0024] To obtain a biaxially oriented polyolefin film with interlayer adhesion within the above range, it is effective to set the composition of each layer and film-forming conditions as described below. In particular, in terms of composition, the angular frequency ω of the 4-methyl-1-pentene resin, which is the main component of Layer B, 260(described later) is set in a preferred range, the ability of layer B to follow layer A during stretching is improved, and the angular frequency ω 200 It is effective to use a material that has a small value (described later) and that easily promotes a high melting point of the crystals.

[0025] In terms of process, when producing a biaxially oriented polyolefin film, it is effective to increase the stretch ratio in each direction by performing longitudinal stretching and transverse stretching while adjusting heating and stretching speed. To increase the longitudinal stretching ratio, for example, it is effective to use a radiation heater (RH) to heat the film during longitudinal stretching to increase the ability of Layer B to follow Layer A, or to extend the stretching section and stretch while keeping the stretching speed low. To increase the transverse stretching ratio, it is effective to increase the ability of Layer B to follow Layer A during stretching by heating again with a radiation heater in the preheating step for transverse stretching. Note that, from the viewpoint of increasing the ability of Layer B to follow Layer A, heating with a radiation heater is preferably performed from the B layer side when Layer B is located on one side, or from both sides when Layer B is located on both sides (the same applies to both longitudinal stretching and transverse stretching, and hereinafter, the same applies to the items that can be achieved by heating with a radiation heater). These methods can also be combined as appropriate.

[0026] From the viewpoint of releasability, it is preferable that at least one of the outermost layers of the biaxially oriented polyolefin film of the present invention is Layer B. The 4-methyl-1-pentene resin, which is the main component of Layer B, has a higher melting point than the polypropylene resin, which is the main component of Layer A, and is less likely to melt even in high-temperature environments. If Layer B is at least one of the outermost layers, when the biaxially oriented polyolefin film is used as a release film or processing film, a layer of a mating member can be formed on Layer B. Because Layer B is less likely to melt even in high-temperature environments, even if a layer of a mating member is formed on its surface, fusion of the mating member is less likely to occur, and high releasability can be achieved.

[0027] As a method for forming at least one outermost layer of a biaxially oriented polyolefin film as layer B, for example, a method can be used in which a resin composition for layer A, which is primarily composed of a polypropylene-based resin, and a resin composition for layer B, which is primarily composed of a 4-methyl-1-pentene-based resin, are melt-kneaded in separate extruders, and then laminated into a sheet and co-extruded.

[0028] When the biaxially oriented polyolefin film of the present invention is used as a release film or a processing film, the surface free energy of the B layer surface is preferably 18.0 mN / m or more and less than 29.0 mN / m in order to improve releasability from a mating member and increase productivity. From the above viewpoint, the upper limit of the surface free energy is more preferably 28.0 mN / m, even more preferably 27.0 mN / m, particularly preferably 26.0 mN / m, and most preferably 24.2 mN / m.

[0029] If the surface free energy of the biaxially oriented polyolefin film is 29.0 mN / m or less, sufficient releasability is ensured when used as a release film, making it easier to peel from the mating member and reducing film breakage during peeling. From the above perspective, the lower the surface free energy, the better, but the practical lower limit is 18.0 mN / m. Note that when Layer B is located on the outermost surface of both sides, the surface free energy of at least one of the Layer B surfaces can be interpreted as meeting the above requirement if it is 18.0 mN / m or more and less than 29.0 mN / m.

[0030] Surface free energy can be measured by evaluating the contact angle using four types of solutions: water, ethylene glycol, formamide, and diiodomethane (details will be described later).

[0031] In order to set the surface free energy within the above range, it is effective to set the composition and film-forming conditions of the B layer within the ranges described below. In particular, the angular frequency ω of the 4-methyl-1-pentene resin contained in the B layer 260It is effective to set the temperature (described below) within a suitable range, improve the ability to follow the A layer during stretching, or heat the film at RH before longitudinal or transverse stretching. It is also effective to densely roughen the surface of the B layer, due to the lotus effect. To densely roughen the surface, it is effective, for example, to heat the casting drum and grow spherulites on the casting drum. These methods can be used in combination as appropriate.

[0032] In order to maintain the quality of the mating member when processed at high temperatures, the biaxially oriented polyolefin film of the present invention preferably has a heat shrinkage rate at 160°C of 8.0% or less in both the main orientation direction and the direction perpendicular to the main orientation. From this perspective, the upper limit of the heat shrinkage rate at 160°C is more preferably 6.0%, even more preferably 5.0%, particularly preferably 4.0%, and most preferably 3.0%, in both the main orientation direction and the direction perpendicular to the main orientation. When the heat shrinkage rate at 160°C is 8.0% or less in both the main orientation direction and the direction perpendicular to the main orientation, shrinkage deformation in a high-temperature environment is suppressed. Therefore, when the biaxially oriented polyolefin film is used as a release film or a processing film, deformation of the mating member is suppressed even in a high-temperature environment. On the other hand, if the heat shrinkage rate is 0% or less, the biaxially oriented polyolefin film will expand upon heating. Therefore, if the heat shrinkage rate is too low, the flatness of the biaxially oriented polyolefin film may be deteriorated when stored in a rolled state. From the viewpoint of reducing the deterioration of flatness due to such a mechanism, the lower limit of the heat shrinkage rate is preferably −3.0% in both the main orientation direction and the direction perpendicular to the main orientation.

[0033] The heat shrinkage at 160°C in the main orientation direction and the direction perpendicular to the main orientation direction can be measured by heating a rectangular film sample with designated marks in a 160°C oven for 10 minutes and evaluating the distance between the marks before and after heating. The main orientation direction refers to the direction with the highest Young's modulus when measuring the angles of 0°, 15°, 30°, 45°, 60°, 75°, 90°, 105°, 120°, 135°, 150°, and 165° relative to the longitudinal direction in the film plane under a 30°C environment, assuming the longitudinal direction to be 0°. If it is unclear from the appearance of the biaxially oriented polyolefin film which direction is the longitudinal direction, the X direction can be identified by drawing lines at 15° intervals based on an arbitrary straight line on the film surface, sampling slit-shaped film pieces parallel to each line, and similarly measuring the Young's modulus. Here, the longitudinal direction refers to the direction in which the film runs during the manufacturing process (corresponding to the winding direction in the case of a film roll), and the direction perpendicular to this in the plane of the film is called the width direction.

[0034] 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 in the film plane. The Y direction is the direction perpendicular to the X direction specified by the above method in the film plane, and is automatically determined once the X direction is determined. The Young's modulus (E X ) and Y-direction Young's modulus (E Y The method for measuring Young's modulus including the Young's modulus will be described in detail later.

[0035] In order to set the heat shrinkage rate at 160°C in both the main orientation direction and the direction perpendicular to the main orientation within the above ranges, it is effective to set the composition and film-forming conditions within the ranges described below. In particular, in terms of composition, the polypropylene-based resin that is the main component of Layer A has an angular frequency ω 200 It is effective to use a polymer with a small crystallization time and a short half-crystallization time that easily promotes a high melting point of the crystals. In terms of processing, it is effective to perform a heat treatment / relaxation treatment after transverse stretching to remove residual strain during stretching, and also to perform a relaxation treatment in the machine direction after longitudinal stretching.

[0036] From the viewpoint of adaptability to processing under a wide range of temperature conditions, the biaxially oriented polyolefin film of the present invention preferably has a highest melting peak temperature of 172°C or higher among the melting peaks between 30°C and 190°C in the DSC melting curve. Hereinafter, the "highest melting peak among the melting peaks between 30°C and 190°C in the DSC melting curve" may be simply referred to as the "highest melting peak." From this viewpoint, the lower limit of the highest melting peak temperature is more preferably 173°C, even more preferably 174°C, particularly preferably 175°C, and most preferably 176°C. If the highest melting peak temperature is 172°C or higher, for example, when the biaxially oriented polyolefin film is used as a press release film, it may melt or rupture upon heating at high temperatures, or some of the molten components may migrate to the mating member, making peeling difficult. From the viewpoint of heat resistance, a higher highest melting peak temperature is preferable, but 190°C is essentially the upper limit.

[0037] Of the melting peaks between 30°C and 190°C, the melting peak temperature on the highest temperature side can be measured by determining the peak temperature of the endothermic peak using a differential scanning calorimeter (details will be described later).

[0038] In order to set the highest melting peak temperature in the range from 30°C to 190°C within the above range, it is effective to set the composition and film-forming conditions within the ranges described below. In particular, in terms of composition, the polypropylene resin that is the main component of Layer A has an angular frequency ω 200 It is effective to use a polymer having a small crystallization time and a short half-crystallization time that easily promotes a high melting point of the crystals. In terms of processing, heat treatment can be performed at each stretch ratio after the longitudinal stretching step and the transverse stretching step and in the Relax step, or the same method as that for suitably controlling the interlayer adhesion can be used.

[0039] From the viewpoint of handleability, the biaxially oriented polyolefin film of the present invention preferably has a sum of the maximum point strength in the main orientation direction and the maximum point strength in the direction perpendicular to the main orientation of 150 MPa or more and 400 MPa or less. From this viewpoint, the lower limit of the sum of the maximum point strength in the main orientation direction and the direction perpendicular to the main orientation is more preferably 180 MPa, even more preferably 220 MPa, particularly preferably 260 MPa, and most preferably 300 MPa. When the sum of the maximum point strength in the main orientation direction and the direction perpendicular to the main orientation is 150 MPa or more, the conveyability is improved, and when used as, for example, a release film, a process film, or a support film, the biaxially oriented polyolefin film is less likely to rupture even when subjected to strong tension in a roll-to-roll process. From this viewpoint, the higher the sum of the maximum point strength in the main orientation direction and the direction perpendicular to the main orientation, the better, but the upper limit is substantially 400 MPa.

[0040] The maximum strength can be measured by performing a tensile test using a rectangular sample tensile tester and reading the maximum load until the sample breaks (details will be described later).

[0041] In order to set the sum of the maximum intensity in the main orientation direction and the maximum intensity in the direction perpendicular to the main orientation within the above range, it is effective to set the raw materials and film-forming conditions within the ranges described below. In particular, for the polypropylene-based resin that is the main component of Layer A, the angular frequency ω 200 In terms of the process, the total area Relaxation ratio calculated from the stretching ratios and Relaxation ratios after the longitudinal stretching step and the transverse stretching step can be set to an appropriate range, as described later, or the same method as that for suitably controlling the interlayer adhesion can be used.

[0042] From the viewpoint of improving the quality of the biaxially oriented polyolefin film of the present invention, it is preferable that the thickness unevenness is 0.1% or more and less than 2.0%. If the thickness unevenness is large, when the biaxially oriented polyolefin film is wound into a roll and stored, gaps are formed between the films in the thin parts, making it easy to shrink, while there are no gaps in the thick parts, making it difficult to shrink. As a result, the flatness of the thick parts may be deteriorated, such as by sagging. From the viewpoint of suppressing deterioration of the quality caused by such a mechanism, the upper limit of the thickness unevenness is more preferably 1.8%, even more preferably 1.6%, and particularly preferably 1.5%. On the other hand, from the above viewpoint, the smaller the thickness unevenness of the biaxially oriented polyolefin film, the better, but the lower limit is substantially 0.1%.

[0043] The thickness unevenness can be determined by continuously measuring the thickness using a contact type continuous thickness meter, the details of which will be described later.

[0044] In order to keep the thickness unevenness of the biaxially oriented polyolefin film within the above range, it is effective to set the composition and film-forming conditions within the ranges described below. In particular, in terms of composition, it is effective to increase the molecular weight of the 4-methyl-1-pentene resin, which is the main component of Layer B, to improve its ability to follow Layer A during stretching. In addition, as for the polypropylene resin, which is the main component of Layer A, it is effective to set the angular frequency ω 200 It is also effective to use a material with a small crystallization time or a material with a short half-crystallization time that easily promotes a high melting point of the crystal. In terms of processing, the same method as that for suitably controlling the interlayer adhesion can be used.

[0045] 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 μm or more and 100 μm or less from the viewpoint of handleability. From the above viewpoint, the thickness of the biaxially oriented polyolefin film is more preferably 1 μm or more and 70 μm or less, and even more preferably 1 μm or more and 55 μm or less. The thickness of the biaxially oriented polyolefin film can be adjusted by the screw rotation speed of the extruder, the width of the unstretched sheet, the film production 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.

[0046] Next, raw materials that can be used in the production of the biaxially oriented polyolefin film of the present invention will be described, but the raw materials are not necessarily limited to these.

[0047] The biaxially oriented polyolefin film of the present invention is composed of a layer (layer A) mainly composed of a polypropylene-based resin and a layer (layer B) mainly composed of a 4-methyl-1-pentene-based resin having a melting point of 190°C or higher and 280°C or lower, which are in contact with each other.

[0048] The proportion of layer A in the biaxially oriented polyolefin film of the present invention is preferably 70% or more and 99.5% or less in thickness terms, from the viewpoints of ensuring stretchability, increasing the melting peak temperature and maximum point strength, reducing thickness unevenness, and realizing ease of handling and good quality during high-temperature processing. From the above viewpoints, the lower limit of the proportion of layer A (thickness basis) is more preferably 80%, even more preferably 90%, and particularly preferably 95%. On the other hand, from the above viewpoints, the upper limit of the proportion of layer A (thickness basis) is more preferably 99%, even more preferably 97%. Note that "thickness basis" means that the ratio is calculated with the total thickness of the biaxially oriented polyolefin film being 100%.

[0049] The proportion of layer B in the biaxially oriented polyolefin film of the present invention is preferably 0.5% or more and 30% or less in thickness, from the viewpoint of structural control by stretching the 4-methyl-1-pentene resin. The lower limit of layer B is preferably 0.5%, more preferably 1.0%, and more preferably 3.0%. The upper limit of layer B is more preferably 20%, even more preferably 10%, and even more preferably 5.0%. When layer B is present on both sides, the proportion of layer B is calculated by adding together the proportions of both layers B.

[0050] The biaxially oriented polyolefin film of the present invention may be combined with layers other than layer A and layer B as long as layer A and layer B are in contact at least at one location, but from the viewpoint of heat resistance, it is preferable that the film is composed of only layers A and B. That is, particularly preferred structures are a two-type two-layer structure of layer A / layer B, and a two-type three-layer structure of layer B / layer A / layer B (or a three-type three-layer structure when the compositions of the B layers are different from each other). When multiple layers B are present, the compositions and thickness ratios of the layers B may be the same or different.

[0051] From the viewpoints of increasing the melting peak temperature and maximum strength, reducing thickness unevenness, and realizing ease of handling and good quality during high-temperature processing, Layer A constituting the biaxially oriented polyolefin film of the present invention preferably contains 90% by mass or more and 100% by mass or less of polypropylene resin A described below, when all constituent components are taken as 100% by mass. The lower limit of the amount of polypropylene resin A in Layer A is more preferably 95% by mass, even more preferably 97% by mass, and particularly preferably 99% by mass.

[0052] Polypropylene resin A has an angular frequency ω where the loss tangent obtained by melt viscoelasticity measurement at 200°C is 1. 200 is preferably 10 rad / s or more and 70 rad / s or less ("angular frequency ω at which the loss tangent obtained by melt viscoelasticity measurement at 200 ° C becomes 1" 200 " to "Angular frequency ω 200 " or "ω 200 ") Angular frequency ω 200is the angular frequency at which the loss tangent of the molten polypropylene resin becomes 1, and is an index of the relaxation characteristics of the polypropylene resin. 200 When the angular frequency ω is small, the relaxation characteristics are low (relaxation is slow) due to the entanglement of molecular chains, but the entanglement of molecular chains is easily maintained even in a high-temperature environment, which tends to work advantageously to increase the melting point of the crystal. 200 When the angular frequency ω of polypropylene resin A is large, there is little entanglement between molecular chains, the relaxation characteristics are high (relaxation is fast), residual strain is efficiently reduced in the relaxation process, and it is easy to relax the amorphous portion. 200 The lower limit of the angular frequency ω is more preferably 15 rad / s, and even more preferably 20 rad / s. 200 The upper limit of the angular frequency ω is more preferably 55 rad / s, and even more preferably 40 rad / s. 200 When is within the above range, the crystalline melting point of the polypropylene resin A is promoted to be high, and the heat resistance of the obtained biaxially oriented polyolefin film can be further improved.

[0053] Angular frequency ω of polypropylene resin A 200 In order to keep the value within the above range, it is effective to select a catalyst when polymerizing the polypropylene resin A or to control the hydrogen concentration. These methods can be combined as appropriate.

[0054] From the viewpoints of increasing the melting peak temperature and maximum strength, minimizing thickness unevenness, and realizing ease of handling and good quality during high-temperature processing, the polypropylene-based resin A preferably has a crystallization half time, which indicates the crystallization rate obtained by DSC isothermal crystallization measurement described below, of 5 to 200 seconds. From these viewpoints, the upper limit of the crystallization half time of the polypropylene-based resin 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-based resin A is likely to recrystallize even during film formation, and high-melting-point crystals are likely to form. To keep the crystallization half time of the polypropylene-based resin A within the above range, it is effective to appropriately adjust the crystallinity and molecular weight distribution of the polypropylene-based resin A. The crystallinity and molecular weight distribution of the polypropylene-based resin A can be adjusted, for example, by selecting a catalyst during polymerization or controlling the hydrogen concentration during polymerization. Furthermore, instead of adjusting the crystallinity of the polypropylene-based resin A, the formation of high-melting-point crystals can also be facilitated by adding a branched-chain polypropylene, which exhibits a crystal nucleating agent effect.

[0055] The polypropylene-based resin A may be a homopolypropylene resin or a copolymer, but is preferably a homopolypropylene resin. Furthermore, other components may be added to the polypropylene-based resin A by pre-mixing or the like, as long as the properties are not deteriorated, as described below. When other components are added to the polypropylene-based resin A, the amount added is preferably 1.2% by mass or less, more preferably 1.0% by mass or less, and even more preferably 0.5% by mass or less, relative to the polypropylene-based resin A. Here, the term "homopolypropylene resin" refers to a polypropylene-based resin in which the amount of propylene units is 99.9 mol % or more and 100 mol % or less, where 100 mol % is the total amount of all structural units constituting the molecular chain of the resin.

[0056] Examples of commercially available polypropylene resins that can be used as polypropylene resin A include F-704NP and F133A, which are polypropylene resins manufactured by Prime Polymer Co., Ltd., HC310BF, which is a polypropylene resin manufactured by Borealis, and FY6H, which is a polypropylene resin manufactured by Japan Polypropylene Corporation. Examples of commercially available branched polypropylenes that can be added to polypropylene resin A include "Daploy" (trademark) WB130HMS, WB135HMS, and WB140HMS, which are manufactured by Borealis, and "WAYMAX" (registered trademark) MFX8, MFX6, and MFX3, which are manufactured by Japan Polypropylene Corporation.

[0057] Layer B in the biaxially oriented polyolefin film of the present invention preferably contains 80% by mass or more and 100% by mass or less of 4-methyl-1-pentene resin B described below, when the total components of layer B are taken as 100% by mass. The lower limit of the amount of 4-methyl-1-pentene resin B in layer B is more preferably 85% by mass, even more preferably 90% by mass, particularly preferably 95% by mass, and most preferably 98% by mass. When the amount of 4-methyl-1-pentene resin B in layer B is 80% by mass or more, the surface of layer B is less likely to melt even in a high-temperature environment, resulting in good releasability. Furthermore, a 4-methyl-1-pentene copolymer or the like may be added to layer B to improve adhesion to other layers, for example.

[0058] From the viewpoint of improving the stretchability of the B layer, the 4-methyl-1-pentene resin in the biaxially oriented polyolefin film of the present invention is selected from the group consisting of 4-methyl-1-pentene resins having an angular frequency ω at which the loss tangent obtained by melt viscoelasticity measurement at 260°C becomes 1. 260 is preferably 1.0 rad / s or more and 500 rad / s or less ("angular frequency ω at which the loss tangent obtained by melt viscoelasticity measurement at 260 ° C becomes 1" 260 " to "Angular frequency ω 260 " or "ω 260 ") Angular frequency ω 260is the angular frequency at which the loss tangent of the molten polypropylene resin becomes 1, and is an index of the relaxation characteristics of the polypropylene resin. The angular frequency ω of 4-methyl-1-pentene resin 260 The lower limit of the angular frequency ω of the 4-methyl-1-pentene resin is more preferably 5.0 rad / s, and even more preferably 30 rad / s. 260 The upper limit of the angular frequency ω of the 4-methyl-1-pentene polymer is more preferably 200 rad / s, and even more preferably 50 rad / sg / 10 min. 260 By keeping the stretching speed at 500 rad / s or less, entanglement of molecular chains is less likely to be relaxed during stretching, stretching stress is more likely to be transmitted uniformly, the formation of a coarse uneven structure on the surface and the occurrence of voids or holes in the layer are suppressed, and deterioration of heat resistance such as fusion properties at high temperatures is mitigated.

[0059] Angular frequency ω of 4-methyl-1-pentene resin 260 In order to keep the above range, it is effective to select a catalyst or control the hydrogen concentration when polymerizing the 4-methyl-1-pentene resin. These methods can be combined as appropriate.

[0060] The resin used in each layer of the biaxially oriented polyolefin film of the present invention may also contain various additives, such as nucleating agents, antioxidants, heat stabilizers, slipping agents, antistatic agents, antiblocking agents, fillers, viscosity modifiers, and color inhibitors, as long as the object of the present invention is not impaired.

[0061] Among these, the selection of the type and amount of antioxidant is important from the viewpoint of suppressing bleed-out. It is preferable to use multiple sterically hindered phenolic antioxidants, with at least one of them being a high-molecular-weight type with a molecular weight of 500 or more. Specific examples of preferred combinations include, but are not limited to, 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"® 1330: molecular weight 775.2) or tetrakis[methylene-3(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane (e.g., BASF's "Irganox"® 1010: molecular weight 1177.7). The total content of these antioxidants is preferably in the range of 0.03 to 1.0% by mass based on the total amount of polypropylene-based resin. If the amount of antioxidant is too small, the resin may deteriorate during the extrusion process, resulting in coloration of the resulting biaxially oriented polyolefin film, or the long-term heat resistance may be poor. If the amount of antioxidant is too large, the transparency of the resulting biaxially oriented polyolefin film may decrease due to bleed-out of the antioxidant. These problems can be alleviated by setting the layer content of the antioxidant within the above range. From the above viewpoints, the content of the antioxidant is more preferably 0.05 to 0.9% by mass, and even more preferably 0.1 to 0.8% by mass.

[0062] Generally, phosphorus-based antioxidants are highly volatile and may bleed out onto the film surface when exposed to high-temperature environments for long periods of time. Therefore, by appropriately limiting the amount of phosphorus-based antioxidant in the biaxially oriented polyolefin film, contamination caused by the antioxidant adhering to the adherend or the SUS plate during press processing can be reduced. From this perspective, the content of phosphorus-based antioxidant in the biaxially oriented polyolefin film is preferably 1000 ppm or less, and more preferably 500 ppm or less.

[0063] Furthermore, a crystal nucleating agent can be added to the raw materials for the A layer used in the biaxially oriented polyolefin film of the present invention, provided that it does not interfere with the object of the present invention. Examples include α crystal nucleating agents (dibenzylidene sorbitols, sodium benzoate, etc.), 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 reduce stretchability and transparency and strength due to void formation, the amount added is generally 0.5% by mass or less, preferably 0.1% by mass or less, and more preferably 0.05% by mass or less, based on 100% by mass of the entire raw materials.

[0064] As a biaxial stretching method for obtaining the biaxially oriented polyolefin film of the present invention, known methods such as inflation simultaneous biaxial stretching method, stenter simultaneous biaxial stretching method, stenter sequential biaxial stretching method, etc. Among these, it is preferable to adopt stenter sequential biaxial stretching method in terms of film formation stability, thickness uniformity, and control of film rigidity and dimensional stability.

[0065] From the viewpoint of releasability and handling at high temperatures, the biaxially oriented polyolefin film of the present invention preferably has an angular frequency ω of 30 rad / s or more and 400 rad / s or less. Here, the angular frequency ω refers to the angular frequency at which the loss tangent becomes 1 when the biaxially oriented polyolefin film is subjected to melt viscoelasticity measurement at 260°C, and the detailed measurement method will be described later.

[0066] From the above viewpoints, the upper limit of the angular frequency ω of the biaxially oriented polyolefin film is more preferably 300 rad / s, even more preferably 250 rad / s, and particularly preferably 150 rad / s. From the above viewpoints, the lower limit of the angular frequency ω of the biaxially oriented polyolefin film is more preferably 35 rad / s, and even more preferably 60 rad / s. When the angular frequency ω is 30 rad / s or higher, the entanglement between molecular chains does not become too strong, so relaxation does not slow down and shrinkage characteristics are less likely to deteriorate. On the other hand, when the angular frequency ω is 400 rad / s or lower, the entanglement between molecular chains becomes moderately strong, so the improvement in mechanical strength due to stretching is not limited, and handleability is improved.

[0067] To make the angular frequency ω of the biaxially oriented polyolefin film fall within the above range, it is preferable to use raw materials having the above-mentioned preferred frequencies as the polypropylene resin A and the 4-methyl-1-pentene resin, for example.

[0068] From the viewpoints of increasing the melting peak temperature and maximum strength, minimizing thickness unevenness, and realizing ease of handling and good quality during high-temperature processing, the biaxially oriented polyolefin film of the present invention preferably has a half-crystallization time, which indicates the crystallization rate obtained by DSC isothermal crystallization measurement described below, of 5 seconds or more and 200 seconds or less. From the above viewpoints, the upper limit of the half-crystallization time of the biaxially oriented polyolefin film is more preferably 100 seconds, even more preferably 50 seconds, and particularly preferably 30 seconds. When the half-crystallization time is within the above range, many high-melting point crystals are formed in the film, resulting in a desirable state. To keep the half-crystallization time of the biaxially oriented polyolefin film within the above range, it is preferable to use the polypropylene-based resin A described above as a raw material with the above-mentioned preferred half-crystallization time.

[0069] Next, one embodiment of the method for producing a biaxially oriented polyolefin film of the present invention will be explained using an example having a layer structure of B layer / A layer / B layer, but the biaxially oriented polyolefin film of the present invention is not necessarily limited to this.

[0070] First, 99.0 to 99.9 parts by mass of a homopolypropylene resin (polypropylene resin A) and 0.1 to 1.0 parts by mass of a branched-chain polypropylene resin are dry-blended and fed into a twin-screw extruder set at 240 to 280°C. After melt-kneading, the mixture is cooled to obtain pellets. The pellets of the polypropylene composition obtained by the above procedure are fed into a single-screw extruder for layer A, and the 4-methyl-1-pentene resin (4-methyl-1-pentene resin B) is fed into a single-screw extruder for layer B, and melt-extruded at 260 to 300°C, more preferably 270 to 300°C. In this case, from the viewpoint of uniformly laminating layer B on layer A, the extrusion temperature for layer B is preferably higher than the extrusion temperature for layer A, more preferably at least 5°C higher than the extrusion temperature for layer A, and even more preferably at least 10°C higher than the extrusion temperature for layer A. Then, after removing foreign matter and modified polymers from the molten resin using a filter installed midway through the polymer pipe, the resin is laminated in a layer B / layer A / layer B configuration using a multi-manifold composite T-die, and then discharged onto a casting drum and cooled and solidified to obtain a laminated unstretched sheet having a layer B / layer A / layer B configuration. In this case, the lamination thickness ratio is preferably 70% or more and 99.5% or less based on thickness. The lower limit of the proportion of layer A (based on thickness) is more preferably 80%, even more preferably 90%, and particularly preferably 95%. Meanwhile, the upper limit of the proportion of layer A (based on thickness) is more preferably 99%, even more preferably 97%. This range is preferred.

[0071] The surface temperature of the casting drum is preferably 40 to 100°C, more preferably 50 to 90°C, and even more preferably 60 to 90°C. Controlling the surface temperature of the casting drum within this range allows for the formation of a large amount of β-crystals, spherulites that are specifically formed during high-temperature casting, in Layer A. By stretching a laminated unstretched sheet containing an abundance of such spherulites at a high ratio, the surface of the resulting biaxially oriented polyolefin film can be densely roughened. More specifically, when the surface temperature of the casting drum is 40°C or higher, the surface of the resulting biaxially oriented polyolefin film is not excessively smooth, and the lotus effect of the dense, rough surface improves the mold releasability. On the other hand, when the surface temperature of the casting drum is 100°C or lower, the decrease in the lotus effect due to the coarsening of β-crystals (spherulites) is suppressed, thereby improving the mold releasability of the resulting biaxially oriented polyolefin film.

[0072] The molten resin sheet (laminated unstretched sheet) can be adhered to the casting drum using any of the following methods: electrostatic application, adhesion methods utilizing the surface tension of water, air knife, press roll, and underwater casting. 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 50 to 110°C, more preferably 60 to 100°C, and even more preferably 70 to 100°C. By setting the air knife temperature at least 5°C, preferably at least 10°C, higher than that of the casting drum, the temperature of the unstretched laminated sheet can be controlled within a suitable range even from the side not in contact with the casting drum, making it easier to control the structure of β crystals (spherulites) in the laminated unstretched sheet. The air blowing speed of the air knife 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 formation process to prevent vibration of the laminated unstretched sheet.

[0073] The resulting laminated unstretched sheet is introduced into a longitudinal stretching process (stretching in the machine direction) to become a uniaxially oriented film. In the longitudinal stretching process, it is preferable to preheat the laminated unstretched sheet with a preheating roll whose temperature has been adjusted to 150°C or higher and 165°C or lower, and to further increase the temperature of the film during longitudinal stretching using a radiation heater (RH) to improve the conformability of Layer B to Layer A. From the viewpoint of improving the conformability of Layer B to Layer A, heating with a radiation heater is preferably performed from the Layer B side when Layer B is located on one side, or from both sides when Layer B is located on both sides (the same applies to heating in the transverse stretching process described below).

[0074] The output of the radiation heater can be adjusted as appropriate, but is preferably 1.0 kW or more and 5.0 kW or less from the viewpoint of more uniformly laminating Layer B while maintaining film-forming properties. When the output of the radiation heater is 1.0 kW or more, Layer B is sufficiently heated, improving stretchability and allowing Layer B to be more uniformly laminated to Layer A. As a result, the release properties and fusion resistance in high-temperature environments of the resulting biaxially oriented polyolefin film are improved. On the other hand, when the upper limit of the output of the radiation heater is 5.0 kW or less, the temperature of the laminated unstretched sheet does not become excessively high during stretching, reducing breakage during stretching.

[0075] From the viewpoint of uniformly and efficiently heating the film, the distance between the radiation heater and the film is preferably 100 to 300 mm.

[0076] The longitudinal stretching is preferably performed between rolls with a peripheral speed difference at a ratio of 5.0 to 6.5, more preferably 5.3 to 6.0, and even more preferably 5.5 to 6.0. A longitudinal stretching ratio of 5.0 or more allows the melting peak temperature, maximum point strength, and thickness unevenness of the resulting biaxially oriented polyolefin film to be adjusted within suitable ranges, thereby improving handleability and quality during high-temperature processing. On the other hand, a longitudinal stretching ratio of 6.5 or less reduces film breakage during stretching. Furthermore, the longitudinal stretching temperature is preferably greater than 142°C and less than 155°C, more preferably 145°C to 153°C, and even more preferably 148°C to 153°C. A stretching temperature within the above range allows the 4-methyl-1-pentene resin contained in Layer B to maintain its ability to stretch to Layer A, while suppressing excessive residual strain in the polypropylene resin A contained in Layer A and allowing molecular chains to be drawn from softened crystals. This, together with the subsequent relaxation step, promotes a higher crystal melting point.

[0077] From the viewpoint of uniform stretchability, the stretching section of the longitudinal stretching is preferably 20 mm or more and 200 mm or less. From the above viewpoint, the stretching section of the longitudinal stretching is more preferably 40 mm or more and 180 mm or less, and even more preferably 60 mm or more and 150 mm or less. If the stretching section is less than 20 mm, the stretching speed becomes too fast, and Layer B cannot keep up with the stretching of Layer A, which may result in film rupture or worsening thickness unevenness. On the other hand, if the stretching section is greater than 200 mm, the neckdown of the film becomes large, which may narrow the film width and reduce productivity, or may result in film rupture during transverse stretching.

[0078] The uniaxially oriented film obtained in the longitudinal stretching step is then brought into contact with a metal roll maintained at 140 to 160°C and drawn down in the longitudinal direction at a ratio of more than 0% to 10%, thereby subjecting the film to a relaxation treatment in the longitudinal direction, and then cooled to room temperature. The temperature for the relaxation treatment after longitudinal stretching is preferably 150 to 165°C, more preferably 155 to 165°C, and even more preferably 155 to 160°C. The temperature for the relaxation treatment after longitudinal stretching is preferably at least the longitudinal stretching temperature but not more than the longitudinal stretching temperature + 20°C, more preferably at least the longitudinal stretching temperature + 5°C but not more than the longitudinal stretching temperature + 18°C, and even more preferably at least the longitudinal stretching temperature + 10°C but not more than the longitudinal stretching temperature + 15°C.

[0079] When the temperature of the Relax treatment after longitudinal stretching is within the above range, excess strain in the longitudinal direction caused by longitudinal stretching can be efficiently released, and the heat shrinkage rate in the longitudinal direction at 160°C can be reduced. Furthermore, the Relax rate in the longitudinal direction is preferably 0.1 to 10%, more preferably 1.0 to 10%, even more preferably 3.0 to 10%, and particularly preferably 5.0 to 10%. When the Relax rate after longitudinal stretching is within the above range, excess strain in the longitudinal direction caused by longitudinal stretching can be released, and the heat shrinkage rate in the longitudinal direction at 160°C can be reduced, and thickness unevenness can be improved.

[0080] Next, the uniaxially oriented film is preferably guided to a tenter, with both widthwise ends held by clips, preheated in a radiation heater and a preheating chamber, and then transversely stretched 7.0 to 13 times in the widthwise direction (transverse stretching step). The output of the radiation heater can be adjusted as needed, but from the viewpoint of more uniform lamination of Layer B, it is preferably 1.5 kW or more, more preferably 2.5 kW or more, and even more preferably 3.5 kW or more. The upper limit of the radiation heater output is preferably 5.0 kW from the viewpoint of maintaining film formability. When the radiation heater output is 1.5 kW or more, the temperature of Layer B of the uniaxially oriented film can be sufficiently raised, improving the stretchability of Layer B and allowing Layer B to be more uniformly laminated to Layer A. As a result, the resulting biaxially oriented polyolefin film has improved releasability and resistance to fusion in high-temperature environments. On the other hand, when the radiation heater output is 5.0 kW or less, the temperature of the uniaxially oriented film does not rise too much, thereby reducing breakage due to preheating and stretching during the transverse stretching step.

[0081] From the viewpoint of uniformly and efficiently heating the film, the distance between the radiation heater and the film is preferably 100 to 300 mm.

[0082] The preheating temperature in the transverse stretching step is preferably 170°C to 190°C, more preferably 173°C to 185°C, even more preferably 175°C to 185°C, and particularly preferably 177°C to 185°C. When the preheating temperature is within the above range, the 4-methyl-1-pentene resin contained in Layer B, which has been heated by a radiation heater, remains in an elevated temperature state, and Layer B is stretched in a softened state. This allows Layer B, which has low stretchability, to be laminated more uniformly on Layer A and stretched more uniformly.

[0083] The stretching temperature in the transverse stretching step is preferably greater than 170°C and not greater than 183°C, more preferably 173°C to 180°C, and even more preferably 175°C 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 resin contained in Layer B, while suppressing residual excessive strain in the polypropylene resin A contained in Layer A, and to draw molecular chains from the crystals softened by preheating. Therefore, together with the subsequent relaxation step, the melting point of the crystals is promoted. The lower limit of the transverse stretching ratio is more preferably 8.5 times, more preferably 9.0 times. The upper limit of the transverse stretching ratio is more preferably 12 times, even more preferably 11 times. When the transverse stretching ratio is 7.0 times or more, the melting peak temperature and maximum strength are maintained at appropriate levels, resulting in good thickness unevenness, good handleability during high-temperature processing, and good quality. On the other hand, if the transverse stretching ratio is 13 times or less, Layer B will easily follow Layer A during stretching, preventing partial exposure of Layer A on the film surface, improving mold releasability and fusion properties during high-temperature processing.

[0084] Next, a Relax treatment is performed after transverse stretching. In the Relax treatment, both widthwise ends of the biaxially stretched film are held with clips to maintain a moderate tension, and the distance between the opposing clips is reduced to relax the biaxially stretched film at a Relax ratio in the width direction of preferably 10.5% to 20%, more preferably 11% to 18%, and even more preferably 12% to 15%. A Relax ratio after transverse stretching within the above range facilitates the release of tension in the molecular chains of the biaxially stretched film. This promotes amorphous relaxation, reduces shrinkage stress, and also brings molecular chain mobility into a moderate range, facilitating molecular chain rearrangement.

[0085] The heat setting temperature (temperature during the Relax treatment) is preferably above 170°C and not higher than 190°C, more preferably from 173°C to 185°C, and even more preferably from 175°C to 185°C, and is preferably equal to or higher than the stretching temperature in the immediately preceding stretching step, and more preferably 2°C or higher than the stretching temperature in the immediately preceding stretching step. If the relaxation temperature is within the above range, it will promote the rearrangement of molecular chains pulled out of the crystals by transverse stretching, making it possible to form crystals with a thicker lamellar thickness and a higher melting point.

[0086] As described above, the manufacturing process for the biaxially oriented polyolefin film of the present invention preferably includes a Relax step after the longitudinal stretching step and the transverse stretching step. The total area Relax rate (%) calculated from the respective stretch ratios and Relax rates after 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 22%. The total area Relax rate (%) is calculated using the following formula, and when the total area Relax rate (%) is within the above range, amorphous relaxation is likely to proceed, and the shrinkage stress of the resulting biaxially oriented polyolefin film as a whole is likely to be reduced. Total area relaxation rate (%)=[1−(1−Relax rate after longitudinal stretching process / 100)×(1−Relax rate after transverse stretching process / 100)]×100.

[0087] Thereafter, while the widthwise ends are held taut with clips, the film is cooled at 80 to 130°C and then guided outside the tenter, and the clips are released. Furthermore, the widthwise ends of the biaxially oriented polyolefin film thus obtained are cut off in a winding process and wound into a roll to obtain a product roll of the biaxially oriented polyolefin film of the present invention.

[0088] The biaxially oriented polyolefin film obtained as described above can be used for various purposes such as release films, packaging films, surface protection films, processing films, battery films, sanitary products, agricultural products, construction products, and medical products. In particular, since it has excellent heat resistance, it can be preferably used as processing films that require high-temperature treatment such as drying coating materials and molding thermosetting resins, release films, current collector substrate films for secondary batteries, and retort packaging films, and is particularly preferably used as release films and processing films used in high-temperature regions.

[0089] The release film of the present invention will be described below. The release film of the present invention is made using the biaxially oriented polyolefin film of the present invention and is preferably used for at least one of the following purposes: a release film for fiber composite curing, a release film for substrates, and a mold release film. A release film is a film that is attached to an object such as a molded body or film to protect the object from scratches and contamination during processing or transportation, and can be easily peeled off and discarded when used as a final product. A release film for fiber-reinforced composite materials is a release film that is attached to a material reinforced by mixing glass fiber or carbon fiber into a resin. A release film for substrates is a release film that 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. A release film is a release film that is attached to a product (molded body) processed in a molding process to protect semiconductor elements or integrated circuits (ICs) with a semiconductor package. Since fiber-reinforced composite materials, substrates, and molded bodies are heated at high temperatures during processing, it is preferable to use the release film of the present invention, which has excellent releasability in high-temperature environments, for their protection.

[0090] The processing film of the present invention will be described below. The processing film of the present invention is made using the biaxially oriented polyolefin film of the present invention. In the present invention, the processing film refers to a film used in the manufacturing process of an object such as a molded body or film, and examples include a film that is attached to an object during the manufacturing process to protect it from scratches, contamination, etc., and a film that functions as a support when it is difficult to form a film from the object itself because it is thin or fragile. Preferred embodiments of the processing film of the present invention will be described below.

[0091] The casting film of the present invention is preferably used for producing a metal membrane or an electrolyte membrane because of its excellent heat resistance, releasability, and handleability. When using the casting film of the present invention, the casting film of the present invention is subjected to vapor deposition or sputtering of a metal or electrolyte, a metal membrane or an electrolyte membrane is formed on the surface, and the casting film is then peeled off, thereby producing a metal membrane or an electrolyte membrane.

[0092] In vapor deposition and sputtering processes, polyethylene terephthalate (PET) film is often used due to its excellent heat resistance and rigidity. However, PET has high hydrophilicity due to the presence of ester bonds, and PET film contains trace amounts of moisture. Such trace amounts of moisture can have adverse effects during vapor deposition and sputtering, particularly when vapor-depositing metals belonging to Group 1 or 2 of the periodic table, or compounds containing these metals, which are susceptible to moisture. In light of this, it is preferable that the metal film produced using the casting film of the present invention contains a metal belonging to Group 1 or 2 of the periodic table, since this allows for the production of a suitable laminate even when the presence of trace amounts of moisture makes it difficult to use PET film. Here, the metal belonging to Group 1 or 2 refers to lithium, sodium, potassium, rubidium, cesium, francium, beryllium, magnesium, calcium, strontium, barium, and radium.

[0093] Furthermore, in high-temperature environments, moisture in the film evaporates as outgassing, and this effect is particularly pronounced under high-vacuum conditions such as in metal vapor deposition processes. Outgassing from the film can deteriorate the degree of vacuum 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 processing film of the present invention, which contains less moisture than PET film, is suitable for use in forming metal films.

[0094] Furthermore, electrolyte membranes used in fuel cells, semi-solid batteries, all-solid batteries, and the like are usually produced in an environment where temperature and humidity are strictly controlled. In particular, sulfide-type electrolyte membranes react with moisture to generate hydrogen sulfide, so the water content of the processed film used in their production is required to be extremely low. Therefore, the biaxially oriented polyolefin film of the present invention is preferably used as the processed film in the production of such electrolyte membranes. In other words, the electrolyte membrane produced using the processed film of the present invention is preferably used for at least one of fuel cells, semi-solid batteries, and all-solid batteries.

[0095] From the above viewpoints, the upper limit of the moisture content of the biaxially oriented polyolefin film of the present invention is preferably 2000 ppm, more preferably 1000 ppm, even more preferably 500 ppm, particularly preferably 200 ppm, and most preferably 100 ppm. The lower limit of the moisture content is not particularly limited, but is 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. By setting the moisture content of the biaxially oriented polyolefin film of the present invention within the above range, it can be suitably used as the aforementioned processing film where reduced moisture content is required.

[0096] 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-based resins is preferably 90% by mass, more preferably 95% by mass, and even more preferably 97% by mass. The upper limit of the content of these resins is substantially 100%.

[0097] From the above viewpoints, the content of the antioxidant in all the components of the biaxially oriented polyolefin film of the present invention is preferably 500 to 9000 ppm, more preferably 1000 to 8000 ppm. In particular, it is preferable to control the content of phosphorus-based antioxidants, since 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 in all the components of the biaxially oriented polyolefin film of the present invention is preferably 100 ppm or less, more preferably 50 ppm or less, and even more preferably 10 ppm or less. There is no particular lower limit for the content of phosphorus-based antioxidants, and theoretically it is 0 ppm (equivalent to no phosphorus-based antioxidants). It should be noted that the content of antioxidants, including phosphorus-based antioxidants, is preferably the same even when the film formed on the surface is not a metal film, from the viewpoints of properties, quality, and reducing inhibition of effects.

[0098] In order to reduce defects caused by bleed-out in the biaxially oriented polyolefin film of the present invention, the total content of additives other than antioxidants (e.g., antistatic agents, viscosity modifiers, color inhibitors, slip agents, etc.) is preferably 0 to 500 ppm or less.

[0099] The antioxidant preferably used in the biaxially oriented polyolefin film of the present invention is a sterically hindered phenolic antioxidant, and at least one of these is preferably a high molecular weight type having a molecular weight of at least 500. Specific examples include various antioxidants, but it is preferable to use 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).

[0100] Next, the current collector of the present invention will be described. The current collector of the present invention is formed using the biaxially oriented polyolefin film of the present invention. The biaxially oriented polyolefin film of the present invention is preferably used as a current collector due to its excellent heat resistance. The current collector is a foil-like laminate used in electrodes of storage batteries such as lithium-ion batteries. Metal foils are typically used as current collectors, but laminates in which a metal film is laminated on a resin film substrate are also used for the purpose of improving safety and reducing weight. This metal film is laminated by processes such as vapor deposition, sputtering, plating, and electroless plating. Furthermore, to increase the energy density of batteries, the film substrate of the current collector is required to be thin. However, as the film becomes thinner, its stiffness decreases, significantly reducing its handleability during processing. In particular, the process of laminating the metal film requires good handleability because it is exposed to high heat such as radiant heat during processing and also to tension in the conveying direction. The biaxially oriented polyolefin film of the present invention can be thinned and has good handleability, making it suitable for use as a current collector.

[0101] Next, the storage battery of the present invention will be described. The storage battery of the present invention uses the biaxially oriented polyolefin film of the present invention. The biaxially oriented polyolefin film of the present invention has excellent heat resistance and is therefore preferably used as a current collector, and is used in storage batteries in which the current collector is used as an electrode. A storage battery is a device that stores electrical energy and converts it back into electrical energy when needed. Specific examples include lead-acid batteries, nickel-metal hydride batteries, lithium-ion batteries, NAS batteries, and redox flow batteries. [Example]

[0102] The present invention will be described in detail below with reference to examples. The properties were measured and evaluated by the following methods.

[0103] (1) Film thickness Measurements were taken 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 random points, and the average value was calculated.

[0104] (2) Interlayer adhesion The biaxially oriented polyolefin film of the present invention was cut into a 25 mm width in the main orientation direction and a 70 mm length perpendicular to the main orientation direction, and 80 mm of acrylic polyester adhesive tape (Nitto Denko Corporation, Nitto 31B tape, 19 mm wide) was laminated to it. At this time, one end of the biaxially oriented polyolefin film and the adhesive tape were aligned, and the other end of the adhesive tape was laminated so that it protruded 10 mm from the end of the biaxially oriented polyolefin film. A 2 kgf roller was rolled back and forth over the biaxially oriented polyolefin film to prepare a tape-laminated laminate film sample, which was then left to stand for 24 hours at 25°C and a humidity of 55% ± 5%. The tape-laminated laminate film sample was then laminated to a 1.5 mm thick SUS plate using double-sided tape (Nitto Denko Corporation, No. 532, tape thickness 0.08 mm) on the side opposite the adhesive tape. The adhesive tape was gripped at a 10 mm protruding portion, and layers A and B were pre-peeled over a 3 mm area from the edge of the biaxially oriented polyolefin film. The SUS plate was then secured to an air chuck on an AG-1S universal testing machine (registered trademark) manufactured by Shimadzu Corporation. The adhesive tape was then secured at a 10 mm protruding portion with an air chuck, and the peel force (N / 19 mm) was measured when peeled at a peel angle of 180° and a tensile speed of 300 mm / min. The average peel force over a 10-40 mm range was calculated from the graph of peel force (N / 19 mm) versus test length (mm) obtained by the measurement, and evaluated according to the following criteria. When layers B were present on both sides, the surface roughness of both the front and back surfaces was measured, and the surface with the lowest maximum peak height (S) was measured.

[0105] (3) Surface free energy First, a biaxially oriented polyolefin film was left in an atmosphere at room temperature (23°C) and a relative humidity of 65% for 24 hours. Then, under the same atmosphere, the contact angles of four solutions (pure water, ethylene glycol, formamide, and diiodomethane) on the surface of Layer B of the biaxially oriented polyolefin film were measured at five points using a DropMaster DM-501 contact angle analyzer (Kyowa Interface Science Co., Ltd.). The average of the three measurements, excluding the maximum and minimum values, was used as the contact angle for each solution. Next, using the contact angles obtained for the four solutions, the surface free energy of the biaxially oriented polyolefin film, which is the dispersion force, polar force, hydrogen bonding force, and the sum of the dispersion force and polar force, was calculated using the geometric mean method proposed by Hata et al., which separates the surface free energy (γ) of a solid into three components: the dispersion force component (γSd), the polar force component (γSp), and the hydrogen bonding force component (γSh). The following is a specific calculation method. The meaning of each symbol is explained below. When γSL is the tension at the interface between a solid and a liquid, formula (a) holds. If layers B exist on both sides, the surface roughness of both the front and back surfaces was measured, and the surface with the lowest maximum peak height Sp was measured. γSL: Surface free energy of the resin layer and the known solution listed in Table 1 γS: Surface free energy of the resin layer γL: Surface free energy of known solutions listed in Table 1 γSd: Dispersion force component of the surface free energy of the resin layer γSp: Polar component of the surface free energy of the resin layer γSh: Hydrogen bonding force component of the surface free energy of the resin layer γLd: Dispersion force component of the surface free energy of the known solutions listed in Table 1 γLp: Polar component of the surface free energy of known solutions listed in Table 1 γLh: Hydrogen bonding force component of the surface free energy of the known solutions listed in Table 1 γSL=γS+γL-2(γSd·γLd)1 / 2-2(γSp·γLp)1 / 2-2(γSh·γLh)1 / 2 ··· Formula (a).

[0106] Furthermore, when a droplet is in contact with a smooth solid surface at a contact angle (θ), the state is expressed by the following equation (Young's equation): γS=γSL+γLcosθ... Formula (b).

[0107] Combining these formulas (a) and (b), we obtain the following formula: (γSd·γLd)1 / 2+(γSp·γLp)1 / 2+(γSh·γLh)1 / 2=γL(1+cosθ) / 2 ··· Formula (c).

[0108] In practice, the contact angles (θ) of four types of solutions (water, ethylene glycol, formamide, and diiodomethane) and the surface tension components (γLd, γLp, γLh) of known solutions listed in Table 1 are substituted into equation (c) to solve four simultaneous equations. As a result, the surface free energy (γ), dispersion force component (γSd), polar force component (γSp), and hydrogen bonding force component (γSh) of the solid are calculated.

[0109] (4) Heat shrinkage rate at 160°C Five samples of 10 mm width and 150 mm length (measurement direction) were cut out from the biaxially oriented polyolefin film in the direction of the main orientation axis, and five samples of 10 mm width and 150 mm length (measurement direction) were similarly cut out in the direction perpendicular to the main orientation axis, and marks were made 5 mm from each end to obtain a test length of 100 mm (d). Next, the test pieces were sandwiched between paper and held horizontally in an oven kept at 160 ° C for 10 minutes, then removed and cooled in an environment of 23 ° C. After that, the dimension (e) was measured and calculated using the following formula, and the average of the five samples was used as the thermal shrinkage in each direction. Heat shrinkage rate = {(d) - (e) / (d)} x 100 (%).

[0110] (5) The highest melting peak temperature (melting point) among the melting peaks between 30°C and 190°C Using a differential scanning calorimeter (Rigaku Thermo plus EV02 DSCvesta), a 3 mg biaxially oriented polyolefin film sample was heated in a nitrogen atmosphere from 30°C to 300°C at a rate of 20°C / min. The highest peak temperature in the temperature range of 30 to 190°C of the endothermic curve obtained during this temperature increase was taken as the melting peak temperature of the biaxially oriented polyolefin film. The melting point of the resin was also measured using the same method, and the highest melting peak temperature was taken as the melting point of the resin.

[0111] (6) The sum of the maximum point strength in the main orientation direction and the maximum point strength in the direction perpendicular to the main orientation Biaxially oriented polyolefin films were prepared, and samples with the main orientation direction and the transverse direction as the long side directions were cut into 50 mm long x 10 mm wide rectangles. The rectangular sample tensile tester (Orientec "Tensilon" (registered trademark) UCT-100) was set with an initial chuck distance of 20 mm and subjected to a tensile test at a tensile speed of 300 mm / min. The maximum load until the sample broke was measured and divided by the cross-sectional area of ​​the sample before the test (film thickness x width (10 mm)). The maximum point strength stress was calculated. Measurements were performed five times for each sample in the main orientation direction and the transverse direction, and the maximum point strength in each direction was calculated and averaged. The film thickness used to calculate the maximum point strength was the value measured in (1) above.

[0112] (7) Uneven thickness A 30 mm (main orientation direction) x 1 m (orthogonal to the main orientation direction) sample of biaxially oriented polyolefin film was then taken. Using a contact-type continuous thickness meter (KG601B manufactured by Anritsu Corporation), the thickness of the biaxially oriented polyolefin film was continuously measured while it was being transported parallel to the orthogonal to the main orientation direction. The transport speed of the biaxially oriented polyolefin film was 0.5 m / min, the measurement speed was 0.1 s, and the number of measurement points was 2,056. From the thickness data obtained by the measurement, the maximum value Tmax (μm), the minimum value Tmin (μm), and the average value Tave (μm) were calculated, and the thickness unevenness was calculated using the following formula (f). Similar measurements were performed 10 times, and the obtained values ​​were used as the thickness unevenness. Thickness variation (%) = (Tmax - Tmin) x 100 / Tave.

[0113] (8) Crystallization half time Using a differential scanning calorimeter (Rigaku Thermo plus EV02 DSCvesta), 3 mg of raw material or film was heated in a nitrogen atmosphere from 25°C to 250°C at a rate of 20°C / min and held there for 5 minutes. The temperature was then lowered from 250°C to 130°C at a rate of 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 first peak in the endothermic curve obtained during isothermal holding at 130°C was defined as the half-crystallization time (seconds).

[0114] (9) Angular frequency ω 200 , angular frequency ω 260 , angular frequency ω Measurements were performed using a rotational rheometer (MCR302 manufactured by Anton Paar Japan) equipped with a 25 mm diameter cone plate. The raw material was left to stand 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 curve of angular frequency vs. loss tangent obtained, the angular frequency at which the loss tangent was 1 was calculated by taking the angular frequency obtained in the measurement of the raw material or film at 200°C as ω. 200 (rad / s), and the angular frequency obtained in the measurement at 260°C is ω260 The angular frequency ω was measured in rad / s. For biaxially oriented polyolefin films, the same measurement was carried out at 260°C, and the obtained value was taken as the angular frequency ω. In this case, the biaxially oriented polyolefin films were stacked so that the total thickness was 500 μm.

[0115] (10) Moisture content A sample of biaxially oriented polyolefin film or PET film was left in a room conditioned at 23°C and a relative humidity of 20% for at least 4 hours, and then immersed in distilled water at 23°C for 24 hours. After that, 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 (Mitsubishi Chemical Corporation, CA-20 model), after which the amount of moisture was quantified by the Karl Fischer method to calculate the moisture content.

[0116] (11) Maximum mountain height (Sp) Measurements were performed using a scanning white light interference microscope "VS1540" (Hitachi High-Tech Science Corporation; measurement conditions and instrument configuration are described below). Measurements were performed using multiple 5 × 5 fields of view, with each field measuring 561.1 μm × 561.5 μm. All images were then stitched with a 20% overlap to obtain surface profile data of 2356.716 μm × 2358.294 μm. The images were interpolated (fully interpolated) using the accompanying analysis software, surface correction was performed using a polynomial fourth-order approximation, and the surface profile was calculated using a median filter (3 × 3 pixels). The measurement surfaces were both sides of the biaxially oriented polyolefin film. A total of nine measurement positions were determined according to the following procedure, starting from the intersection of the diagonals of a 5 cm × 5 cm square cut of biaxially oriented polypropylene film. Measurements were performed at each measurement position, and the SP value was calculated according to the procedure described above. The average SP value was used.

[0117] <How to determine the measurement position> Measurement 1: Starting point position Measurement 2: 5.0 mm to the right of the starting point Measurement 3: 10.0 mm to the right of the starting point Measurement 4: 5.0 mm below the starting point Measurement 5: 5.0 mm below and 5.0 mm to the right of the starting point Measurement 6: 5.0 mm below and 10.0 mm to the right of the starting point Measurement 7: 10.0 mm below the starting point Measurement 8: 10.0 mm below and 5.0 mm to the right of the starting point Measurement 9: 10.0 mm below and 10.0 mm to the right of the starting point <Measurement conditions and equipment configuration> Objective lens: 10x Telescope tube: 1x Zoom lens: 1x Wavelength filter: 530nm white Measurement mode: Wave Measurement software: VS-Measure 10.0.4.0 Analysis software: VS-Viewer 10.0.3.0 Measurement area: 561.1μm×561.5μm Number of pixels: 1,024 x 1,024.

[0118] (12) High-temperature press evaluation Cardboard (product number C-55, manufactured by Daio Paper Co., Ltd.) was cut into 10 cm squares, and biaxially oriented polyolefin film cut into 15 cm squares was placed on both sides of the cardboard along with 20 cm square SUS plates as shown in Figure 1. The plates were heated and pressed at 1.5 MPa and 180°C for 5 minutes using a heating press, then removed from the press and cooled to room temperature. The biaxially oriented polyolefin film protruding from the cardboard was then peeled away from the biaxially oriented polyolefin film, and the cardboard was then peeled away from the biaxially oriented polyolefin film. The symbols 1 to 4 in Figure 1 represent the SUS plate, biaxially oriented polyolefin film, cardboard, and pressure direction, respectively. The condition of the cardboard and biaxially oriented polyolefin film after treatment was visually evaluated based on the following criteria. A: The biaxially oriented polyolefin film could be peeled off from the cardboard and SUS plate, and no defects in flatness such as wrinkles were observed on the biaxially oriented polyolefin film. B: The biaxially oriented polyolefin film could be peeled off from the cardboard and SUS plate, but poor flatness such as wrinkles was observed on the biaxially oriented polyolefin film. C: At least a part of the biaxially oriented polyolefin film adhered to the cardboard or SUS plate.

[0119] (13) High-temperature transport evaluation A 500mm wide biaxially oriented polyolefin film was introduced into a drying oven at 120°C, conveyed for 30 seconds under a conveying tension of 1.0 MPa, and wound up into a 200m long roll at a take-up tension of 200 N / m to produce a film roll. Next, 1m of the 500mm wide biaxially oriented polyolefin film was unwound and subjected to free tension (a state in which the biaxially oriented polyolefin film was hanging vertically due to its own weight), and tensions of 1kg / m and 3kg / 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. S: There were no areas with poor flatness under free tension. A: With free tension, areas of poor flatness were observed, but with a tension of 1 kg / m width, the areas of poor flatness disappeared. B: When the tension was 1 kg / m width, some areas of poor flatness were observed, but when the tension was 3 kg / m width, the areas of poor flatness disappeared. C: Even with a tension of 3 kg / m width, areas of poor flatness were observed.

[0120] (14) Yield evaluation during metal film formation Using a vacuum deposition device, atmospheric pressure was reduced to 1×10 -5 The pressure was reduced to 1000 kJ / cm², and a magnesium vapor deposition film with a thickness of 200 Å 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. -5 The time required for the pressure to be reduced to a full pressure was measured, and the unevenness of the metal film surface was visually observed to evaluate the yield according to the following criteria: A was considered pass, and B was considered fail. A: 1×10 above atmospheric pressure -5 The time to reduce the pressure to a full pressure was less than 50 minutes, and the metal film surface did not appear smooth. B: 1×10 above atmospheric pressure-5 The time to reduce the pressure to the full pressure was longer than 50 minutes, or irregularities were observed on the metal film surface.

[0121] (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): Prime Polymer Homopolypropylene resin 2 (PP2): Prime Polymer Homopolypropylene resin 3 (PP3): Prime Polymer Homopolypropylene resin 4 (PP4): Prime Polymer Homopolypropylene resin 5 (PP5): Prime Polymer Branched polypropylene resin 6 (PP6): manufactured by Boreales Polypropylene resin 7 (PP7): PP1 / PP6 = 99.7 / 0.3 was charged into a twin-screw extruder, kneaded at 260°C, and cooled to obtain pellets. Copolymer polypropylene resin 8 (PP8): Sumitomo Chemical Co., Ltd. Polyolefin resin 1 (PO1): Propylene-1-butene copolymer. Manufactured by Mitsui Chemicals.

[0122] [Table 1]

[0123] 4-Methyl-1-pentene resin 1 (PMP1): Mitsui Chemicals 4-Methyl-1-pentene resin 2 (PMP2): Mitsui Chemicals 4-Methyl-1-pentene resin 3 (PMP3): Mitsui Chemicals 4-Methyl-1-pentene resin 4 (PMP4): Mitsui Chemicals 4-Methyl-1-pentene-propylene copolymer 1 (MP-P1): Mitsui Chemicals 4-Methyl-1-pentene resin 5 (PMP5): PMP1 / MP-P1 = 90 / 10 were charged into a twin-screw extruder, kneaded at 280°C, and cooled to obtain pellets. 4-Methyl-1-pentene resin 6 (PMP6): PMP2 / PP1 were charged into a twin-screw extruder in a ratio of 78 / 22, kneaded at 260°C, and cooled to obtain pellets.

[0124] [Table 2]

[0125] Example 1 Homopolypropylene resin 7 was fed into a twin-screw extruder set at 255°C, melt-kneaded, and cooled to obtain pellets. The polypropylene composition pellets obtained by the above procedure were fed into a single-screw extruder set at 270°C for Layer A, and 4-methyl-1-pentene resin 1 was fed into a single-screw extruder set at 285°C for Layer B, and melt-extruded. After removing foreign matter and modified polymers from the molten resin using a filter installed midway through the polymer tube, the resulting mixture was laminated in a multi-manifold composite T-die with a layer structure of Layer B / Layer A / Layer B at a thickness ratio of 1 / 40 / 1. The mixture was then extruded onto a casting drum set at 80°C and cooled and solidified by blowing 90°C air at a speed of 140 m / s to obtain a laminated unstretched sheet. The unstretched sheet was then preheated to 155°C using a ceramic roll and stretched 5.8 times in the longitudinal direction between 150°C rolls with different peripheral speeds. The distance between the rolls was 100 mm, and the film was heated from both the top and bottom sides using a radiation heater set to 4 kW between the rolls (the distance between the radiation heater and the film was 200 mm). The uniaxially oriented film obtained in the longitudinal stretching process was then brought into contact with a metal roll maintained at 158°C and drawn down 8% in the longitudinal direction, thereby relaxing the film in the longitudinal direction, and then cooled to room temperature. Next, the uniaxially stretched film was introduced into a tenter-type stretching machine, with both ends in the width direction held with clips, and heated from above and below using a radiation heater with an output of 4.0 kW. After preheating at 180°C for 10 seconds, the film was stretched 10.5 times in the width direction at 178°C and heat-set at 179°C while relaxing 14% in the width direction (the distance between the radiation heater and the film was 200 mm). After that, after a cooling step at 110°C, the film was introduced to the outside of the tenter-type stretching machine, the clips were released, and the film was taken up around a core to obtain a biaxially oriented polyolefin film with a thickness of 45 μm. The physical properties and evaluation results of the obtained biaxially oriented polyolefin film are shown in Table 4.

[0126] (Examples 2 to 8, Comparative Examples 1 to 3) A biaxially oriented polyolefin film was obtained in the same manner as in Example 1, except that the raw material composition and film-forming conditions for each layer were as shown in Table 3. At this time, 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 4.

[0127] Comparative Example 4 Instead of the biaxially oriented polyolefin film, a PET film "Lumirror" (registered trademark) S10 (manufactured by Toray Industries, Inc.) was used to evaluate the yield during metal film formation, and the thickness unevenness and moisture content were measured. The evaluation results are shown in Table 3.

[0128] The moisture content and the yield in producing the metal film were evaluated only for Example 1 and Comparative Example 4.

[0129] [Table 3] [Industrial Applicability]

[0130] As described above, the biaxially oriented polyolefin film of the present invention can be used in various applications such as packaging films, release films, processing films, sanitary products, agricultural products, construction products, medical products, etc. In particular, because of its excellent heat resistance, it can be preferably used as a release film or processing film to be used at high temperatures, which is difficult for biaxially oriented polyolefin films to be used at. [Explanation of symbols]

[0131] 1:SUS board 2: Film to be measured 3: Cardboard 4: Pressure direction

Claims

1. A biaxially oriented polyolefin film comprising a layer (layer A) mainly composed of a polypropylene resin and a layer (layer B) mainly composed of a 4-methyl-1-pentene resin having a melting point of 190°C or more and 280°C or less, the layer being in contact with the layer A, and the interlayer adhesion strength between the layer A and the layer B is 1.4 N / 19 mm or more and 10 N / 19 mm or less.

2. 2. The biaxially oriented polyolefin film according to claim 1, wherein at least one outermost layer is the layer B.

3. 3. The biaxially oriented polyolefin film according to claim 1, wherein the surface free energy of the surface of the layer B is 18.0 mN / m or more and less than 29.0 mN / m.

4. 4. The biaxially oriented polyolefin film according to claim 1, wherein the heat shrinkage at 160° C. is 8.0% or less in both the main orientation direction and the direction perpendicular to the main orientation.

5. 5. The biaxially oriented polyolefin film according to claim 1, wherein the highest melting peak temperature among the melting peaks from 30°C to 190°C in a DSC melting curve is 172°C or higher.

6. 6. The biaxially oriented polyolefin film according to claim 1, wherein the sum of the maximum point strength in the main orientation direction and the maximum point strength in the direction perpendicular to the main orientation is 150 MPa or more and 400 MPa or less.

7. The biaxially oriented polyolefin film according to any one of claims 1 to 6, wherein the thickness variation is 0.1% or more and less than 2.0%.

8. 8. The biaxially oriented polyolefin film according to claim 1, wherein the half-crystallization time is from 5 seconds to 400 seconds, and the angular frequency ω is from 30 rad / s to 400 rad / s.

9. The biaxially oriented polyolefin film according to any one of claims 1 to 8, wherein the layer A comprises a 4-methyl-1-pentene resin.

10. 10. The biaxially oriented polyolefin film according to claim 1, comprising a resin obtained by copolymerizing 4-methyl-1-pentene and an α-olefin.

11. A release film comprising the biaxially oriented polyolefin film according to any one of claims 1 to 10.

12. The release film according to claim 11, which is used for at least one of a release film for a fiber-reinforced composite material, a release film for a substrate, and a release film for a mold.

13. A processing film comprising the biaxially oriented polyolefin film according to any one of claims 1 to 10.

14. 14. The process film of claim 13 for use in the manufacture of metal films.

15. The process film of claim 14 , wherein the metal film comprises a metal from Group 1 or Group 2 of the periodic table.

16. 16. The casting film of claim 14 or 15, wherein the metal film is a transparent conductive film.

17. 14. The process film of claim 13 for use in the manufacture of electrolyte membranes.

18. 20. The process film of claim 17, wherein the electrolyte membrane is used in at least one of a fuel cell, a semi-solid battery, and an all-solid battery.

19. A current collector comprising the biaxially oriented polyolefin film according to any one of claims 1 to 10.

20. An electric storage element comprising the biaxially oriented polyolefin film according to any one of claims 1 to 10.

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