Biaxially stretched polypropylene film

Abstract

Disclosed is a biaxially stretched polypropylene film which is characterized in that: if a surface having a relatively small peak material volume Vmp within the range of 0.1 μL / m2 to 50 μL / m2 is defined as A surface, one surface of the film is the A surface; and the value F5t obtained by multiplying the stress value F5 at an elongation of 5% in a direction orthogonal to the film main orientation at 130°C by the film thickness is 50 Pa∙m to 1,000 Pa∙m.　The present invention provides a biaxially stretched polypropylene film which has excellent quality and excellent handling properties in a high-temperature environment.

Description

Biaxially oriented polypropylene film

[0001] The present invention relates to a biaxially oriented polypropylene film that has excellent quality and handleability in high-temperature environments and can be suitably used as an industrial material film.

[0002] Biaxially oriented polypropylene films have excellent surface releasability and mechanical properties, making them suitable for use as release films, process films, and support films for a variety of components, including plastic products, building materials, and optical components.

[0003] Biaxially oriented polypropylene films generally have a surface structure with steep protrusions known as crater protrusions. Therefore, for example, when an optical film is manufactured using a biaxially oriented polypropylene film as a support, the steep protrusions may be transferred, resulting in the formation of steep recesses in the optical film. The optical film is then laminated to other components in the next process. If these steep recesses remain as voids, optical defects may result. Therefore, it has been difficult to use biaxially oriented polypropylene films as supports for high-end optical films.

[0004] Furthermore, the resin composition used to obtain the optical film is coated onto the surface of the biaxially oriented polypropylene film, and then dried and cured in a high-temperature oven. Therefore, the biaxially oriented polypropylene film must maintain high rigidity even in high-temperature environments. To maintain high rigidity in high-temperature environments, it is generally necessary to use a highly crystalline polypropylene resin as the raw material for the biaxially oriented polypropylene film. However, it is known that using a highly crystalline polypropylene raw material tends to cause the aforementioned crater protrusions to form on the surface. Thus, achieving high rigidity at high temperatures and suppressing the formation of crater protrusions are in a trade-off relationship, and it has been extremely difficult to achieve both.

[0005] Thus, the support film of an optical film is required to have both suppression of a steep protrusion structure and high rigidity under high-temperature environments. For example, Patent Documents 1 and 2 describe examples in which a copolymer or a low-crystalline polypropylene raw material is blended with a propylene-based resin as a main raw material. Furthermore, Patent Document 3 describes an example in which a crystal nucleating agent is added to increase crystallization in order to improve mechanical strength at high temperatures.

[0006] JP 2015-107612 A International Publication No. 2022 / 075101 JP 2008-050393 A

[0007] However, in the methods described in Patent Documents 1 and 2, in which a copolymer or a low-crystalline polypropylene raw material is blended with the propylene-based resin as the main raw material, the crystallinity of the substrate tends to decrease. As a result, the resulting film has low rigidity in high-temperature environments, and there are concerns about the occurrence of wrinkles and deterioration of flatness in high-temperature environments, which poses problems in terms of impairing quality and handleability in high-temperature environments. Furthermore, in the method described in Patent Document 3, the film surface has high crystallinity, and a steep protrusion structure is likely to form. Therefore, when the resulting film is used as a support film, there is a concern that this may lead to defects and a problem in terms of deteriorating quality. Therefore, the present invention aims to solve the above-mentioned problems and provide a biaxially oriented polypropylene film that has excellent quality and handleability in high-temperature environments.

[0008] In order to solve the above-mentioned problems, the biaxially oriented polypropylene film of the present invention has the following constitution: That is, the biaxially oriented polypropylene film of the present invention has a relatively small peak actual volume Vmp and a peak actual volume Vm of 0.1 μL / m ２ 50μL / m or more ２ The biaxially oriented polypropylene film is characterized in that, when the side that is equal to or less than the above is defined as side A, one side is side A, and the value F5t obtained by multiplying the stress F5 value at 5% elongation in the direction perpendicular to the main orientation of the film at 130°C by the film thickness is 50 Pa m or more and 1000 Pa m or less.

[0009] According to the present invention, it is possible to provide a biaxially oriented polypropylene film having excellent quality and handleability in a high-temperature environment.

[0010] 10 is a conceptual diagram showing the peak actual volume Vmp and the valley volume Vvv. FIG.

[0011] The biaxially stretched polypropylene film of the present invention has a relatively small peak actual volume Vmp and a peak actual volume Vmp of 0.1 μL / m ２ 50μL / m or more ２ The biaxially oriented polypropylene film of the present invention is characterized in that, when the side that is equal to or less than the above is defined as side A, one side is side A, and the value F5t obtained by multiplying the stress F5 value at 5% elongation in the direction perpendicular to the main orientation of the film at 130°C by the film thickness is 50 Pa m or more and 1000 Pa m or less. The biaxially oriented polypropylene film of the present invention will be described in detail below.

[0012] The peak actual volume Vmp (hereinafter sometimes simply referred to as Vmp) is one of the surface parameters obtained using a scanning white light interference microscope. More specifically, as shown in Figure 1, the peak actual volume Vmp (reference numeral 1) can be expressed as the area surrounded by a load curve (reference numeral 2) drawn with the vertical axis representing height and the horizontal axis representing the areal load ratio, an equivalent line (reference numeral 3) at an areal load ratio of 10%, and a position line (vertical axis) at an areal load ratio of 0%.

[0013] Biaxially oriented polypropylene films are prone to forming steep protrusion structures known as crater protrusions. Therefore, for example, when an optical film is formed on the surface of a biaxially oriented polypropylene film as a support, the steep protrusions may be transferred, resulting in the formation of steep recesses in the optical film. If such recesses are transferred to the optical film, the recesses may remain as voids when the optical film is bonded to another member in the next process, potentially resulting in optical defects.

[0014] There are various parameters that represent surface roughness, but for example, the average surface roughness Sa, the maximum height Sz, the maximum projection height Sp, etc., do not show a high correlation with the height or frequency of the steep projection structure. The inventors have found that Vmp, which represents the volume of the protruding peaks, increases when the number of steep projections is large or the height of the steep projections is high, that is, Vmp is particularly highly correlated with the steep projection structure. In the biaxially stretched propylene film of the present invention, it is important to control Vmp within an appropriate range in order to solve the problems described below. Furthermore, when the difference in Vmp between both sides is 0.1 μL / m ２ If this is the case, it is considered that there is a difference in Vmp between the two surfaces, and the "surface with a relatively small peak actual volume Vmp" refers to the surface with the relatively lower Vmp of the two surfaces of the biaxially oriented polypropylene film when there is a difference in Vmp between the two surfaces.

[0015] From the viewpoint of achieving both quality and ease of handling at high temperatures, the biaxially oriented polypropylene film of the present invention has a relatively small peak actual volume Vmp and a peak actual volume Vm of 0.1 μL / m ２ 50μL / m or more ２ When the surface that is equal to or less than the surface A is defined as the surface A, it is important that one of the surfaces is the surface A. From the above viewpoint, the upper limit of Vmp on the surface A is preferably 30 μL / m ２ , more preferably 10 μL / m ２ Vmp is 50 μL / m ２ A larger value means that many steep protrusion structures are formed on the surface of the biaxially oriented polypropylene film. Therefore, when the biaxially oriented polypropylene film is used as a support film for an optical film, if an optical film is formed on such a surface, the protrusion structures may be transferred to the surface of the optical film, resulting in a deterioration in quality. From the above viewpoint, the lower limit of Vmp on side A is preferably 0.3 μL / m ２ , more preferably 0.5 μL / m ２ Vmp is 0.1 μL / m ２A smaller value means that the surface of the biaxially oriented polypropylene film is excessively smooth, and therefore, there is a concern that a biaxially oriented polypropylene film having such a surface may be wrinkled due to an increased dynamic friction coefficient when transported in a high-temperature environment or immediately after heating.

[0016] To obtain a biaxially oriented polypropylene film having an A-side, for example, a method can be used in which the raw material composition of the biaxially oriented polypropylene film is within the ranges described below, and the film-forming conditions are within the ranges described below. In particular, it is effective to use an olefin-based elastomer resin having a softening temperature within the appropriate range as the raw material for the outermost layer, which is uniformly dispersed by, for example, melt-kneading it with a polypropylene resin and forming chips in advance, or to use a polypropylene resin having an appropriate half-crystallization time and melting point as the main component of the outermost layer (details will be described later). Furthermore, in terms of the film-forming process, it is effective to lower the casting temperature to optimize the crystallization rate during casting, thereby suppressing the formation of β-crystal spherulites and forming a dense spherulite structure, and to set the preheating temperature and stretching temperature during longitudinal and transverse stretching within the appropriate ranges described below, thereby controlling the deformation of the elastomer resin domains and precisely controlling the surface structure. These methods can also be used in combination as appropriate.

[0017] In the biaxially oriented polypropylene film of the present invention, from the viewpoint of improving handleability and quality at high temperatures, F5t, the value obtained by multiplying the stress F5 value at 5% elongation in the direction perpendicular to the main orientation of the film at 130°C by the film thickness, is 50 Pa·m or more and 1000 Pa·m or less. From the above viewpoints, the lower limit of F5t is preferably 100 Pa·m, more preferably 150 Pa·m. If F5t is less than 50 Pa·m, when a resin composition serving as a raw material for an optical film or the like is applied to the biaxially oriented polypropylene film and then dried and cured in a high-temperature oven, the biaxially oriented polypropylene film will stretch in the longitudinal direction and shrink in the width direction. As a result, the flatness of the biaxially oriented polypropylene film may be impaired, potentially deteriorating its quality and handleability. From the viewpoint of feasibility, the upper limit of F5t is 1000 Pa·m, preferably 400 Pa·m, and more preferably 300 Pa·m.

[0018] To suppress deformation under high-temperature and high-tension conditions, it is useful to maintain high mechanical strength in the low-elongation region even under high-temperature conditions, and to increase the thickness of the biaxially oriented polypropylene film. It has been found that the degree of deformation under high-temperature and high-tension conditions is strongly correlated with the F5t value. However, when thickening biaxially oriented polypropylene films, the cooling efficiency decreases during the process of obtaining a cast film by melt extrusion, resulting in the formation of many β-crystal spherulites in the cast film. The presence of such β-spherulites leads to the formation of crater protrusions after biaxial stretching, so there is a trade-off between thickening biaxially oriented polypropylene films and Vmp, and it has been difficult to achieve both in the past.

[0019] Here, the "main orientation direction" refers to the direction showing the highest Young's modulus when an arbitrary direction in the film plane is set as 0° and the Young's modulus is measured in each direction forming an angle of 0° to 165° in 15° increments relative to the arbitrary direction. The "direction perpendicular to the main orientation" refers to the direction perpendicular to the main orientation direction in the film plane. The Young's modulus here refers to the Young's modulus calculated in accordance with the method specified in JIS K 7161 (2014) by setting a rectangular sample 150 mm long (measurement direction) x 10 mm wide with an initial chuck distance of 50 mm, conducting a tensile test at room temperature at a tensile speed of 300 mm / min, and then measuring the Young's modulus. Details of the Young's modulus measurement method will be described later.

[0020] However, if it is not possible to measure and calculate Young's modulus by the above method because it is not possible to obtain a sample of the above size, the crystal orientation of the polypropylene α-crystal (110) plane of the biaxially stretched polyolefin film can be measured using wide-angle X-rays as follows, and the main orientation direction can be determined based on the following criteria: That is, X-rays (CuKα rays) are incident perpendicular to the film surface, and the crystal peak at 2θ = approximately 14° (α-crystal (110) plane) is scanned in the circumferential direction to obtain a diffraction intensity distribution. The direction with the highest diffraction intensity is taken as the main orientation direction, and the direction perpendicular to this in the film plane is taken as the direction perpendicular to the main orientation direction.

[0021] The F5t of a biaxially stretched polypropylene film can be measured by the following procedure. First, the biaxially stretched polypropylene film is heated in an oven heated to 130°C for 1 minute, and then a tensile test is performed at a tensile speed of 300 mm / min to measure the load applied to the film when the biaxially stretched polypropylene film is elongated by 5%. The value obtained by dividing the value by the cross-sectional area of ​​the sample before the test (film thickness x width) is calculated as the stress at an elongation of 5%, and this value is then multiplied by the film thickness to measure and calculate the F5t. Details of the measurement method will be described later.

[0022] To achieve F5t within the above range, for example, a method can be used in which the raw material composition of the biaxially stretched polypropylene film is within the range described below, and the film-forming conditions are within the range described below. In particular, optimizing the crystallinity and orientation of the film is effective by using a polypropylene resin having an appropriate melting point and molecular weight range as the main component. Furthermore, in terms of the film-forming process, by setting the preheating temperature or stretching temperature during transverse stretching to the melting point of the main component polypropylene resin or lower, or by performing a relaxation treatment of 2.0% or more but less than 10%, preferably 3.0% or more but less than 10%, after longitudinal stretching, transverse stretching is possible without significantly relaxing the orientation structure of the uniaxially stretched film after longitudinal stretching, and high mechanical strength can be maintained even at high temperatures, which is effective. These methods can also be used in combination as appropriate.

[0023] In order to improve the handling properties of the biaxially oriented polypropylene film of the present invention in the transport and winding processes after high-temperature processing and to further improve the quality, the dynamic friction coefficient μd between the front and back surfaces of the film after heating at 130°C for 10 minutes is preferably 0.30 or more and 1.40 or less. From the above viewpoints, the upper limit of the dynamic friction coefficient μd between the front and back surfaces of the film after heating at 130°C for 10 minutes is more preferably 1.20, even more preferably 1.00, particularly preferably 0.80, and most preferably 0.75. On the other hand, the lower limit of the dynamic friction coefficient μd between the front and back surfaces of the film after heating at 130°C for 10 minutes is not particularly limited, but is substantially about 0.30. Here, the "dynamic friction coefficient μd between the front and back surfaces of the film" refers to the friction coefficient measured by rubbing different surfaces together, and the dynamic friction coefficient μd can be measured in accordance with JIS K 7125 (1999) (details will be described later).

[0024] In the biaxially oriented polypropylene film of the present invention, an olefin-based elastomer resin may be uniformly dispersed to suppress the formation of crater protrusions. In such an embodiment, the olefin-based elastomer may soften upon heating at 130°C, resulting in an increase in the coefficient of friction between the front and back surfaces of the film, resulting in a deterioration in flatness after heating. In other words, there is a trade-off between smoothing the film surface and making it easy to slip after high-temperature heating, and it is difficult to achieve both.

[0025] To achieve the desired dynamic friction coefficient μd between the front and back surfaces of the film after heating at 130°C for 10 minutes, for example, the raw material composition of the biaxially stretched polypropylene film can be set within the range described below, and the film-forming conditions can be set within the range described below. In particular, it is effective to suppress softening of the elastomer resin during heating at 130°C by setting the softening temperature and additive amount of the olefin-based elastomer resin within an appropriate range. In terms of the film-forming process, it is also effective to set the preheating temperature and stretching temperature for longitudinal and transverse stretching within an appropriate range, and to subject the uniaxially stretched film after longitudinal stretching to a relaxation treatment in the longitudinal direction at a temperature below the softening temperature of the elastomer, thereby forming a fine uneven structure on the film surface and thereby reducing the contact area between the transport roll and the film surface. These methods can be used in combination as appropriate.

[0026] In the biaxially oriented polypropylene film of the present invention, when the side opposite to the side A is designated as the side B, from the viewpoint of quality, the side B is ２ The number of craters with a major axis exceeding 100 μm is 0.001 / mm ２ 30 pieces / mm or more ２ The number of craters with a major axis exceeding 100 μm is preferably 30 / mm or less. ２ If the number of craters is larger than this, when the biaxially oriented polypropylene film is used as a support for manufacturing an optical film, the steep protrusions resulting from the craters on the B side may be transferred to the optical film when the optical film is formed on the A side and wound up, resulting in the formation of steep depressions in the optical film. When such an optical film is bonded to another member in the next step, the steep depressions may remain as voids, which may cause optical defects. From the above viewpoint, the fewer the number of craters, the better, and the upper limit is more preferably 20 / mm ２ , and more preferably 10 pieces / mm ２ , and most preferably 5.0 pieces / mm ２ The lower limit is not particularly limited, but is substantially 0.001 particles / mm ２ In the present invention, the term "crater" refers to a shape having a large depression, specifically a depression with a major axis of 150 nm or more when the height threshold is set to -0.1 μm. ２ The number of craters per unit area can be measured using a scanning white light interference microscope and an associated analysis system, and the details of the measurement method will be shown in the Examples.

[0027] B side 1mm ２To keep the number of craters with major axes exceeding 100 μm per film within the above range, for example, a method can be used in which the raw material composition of the biaxially oriented polypropylene film is within the range described below, and the film-forming conditions are within the range described below. Specifically, in addition to the method for obtaining a biaxially oriented polypropylene film having an A-side, it is preferable to lower the air temperature from the air knife, optimize the blown air velocity, and optimize the film thickness, from the viewpoint of efficiently cooling the non-cooled drum surface of the sheet on the casting drum. Note that preferred conditions for the temperature and velocity of the air from the air knife will be described later.

[0028] In the biaxially stretched polypropylene film of the present invention, from the viewpoint of improving the handling property, the peak density Spd of the A-side is 1.0 peaks / mm ２ More than 100 pieces / mm ２ It is preferable that the peak density Spd is equal to or less than 1 / 2. Hereinafter, the peak density Spd may be simply referred to as Spd. The biaxially oriented polypropylene film of the present invention is prone to deterioration in slipperiness because the formation of crater protrusions is suppressed, but in order to maintain good slipperiness even on such a surface and obtain a film with good flatness, it is preferable to form a large number of fine uneven structures on the film surface.

[0029] From the above viewpoint, the lower limit of Spd of the A side is preferably 2.0 pieces / mm ２ , more preferably 5.0 pieces / mm ２ The Spd of the A side is 1.0 pieces / mm ２ By satisfying the above condition, the biaxially oriented polypropylene film maintains its slipperiness, and therefore, the occurrence of wrinkles during transport and the resulting deterioration of flatness can be reduced. From the above viewpoint, the upper limit of Spd of the A side is preferably 60 pieces / mm ２ , more preferably 40 pieces / mm ２ and particularly preferably 20 pieces / mm ２ and most preferably 17 pieces / mm ２ The Spd of the A side is 100 pieces / mm ２By satisfying the condition below, the surface roughness of the biaxially oriented polypropylene film is appropriately suppressed from becoming finer, and therefore, when the film is used as an industrial material film, deterioration of handling due to slipperiness is reduced. Note that Spd can be measured using a scanning white light interference microscope and an associated analysis system, and details of the measurement method are shown in the Examples.

[0030] To achieve the Spd of Side A within the above range, for example, a method can be used in which the raw material composition of the biaxially stretched polypropylene film is within the range described below, and the film-forming conditions are within the range described below. In particular, it is effective to melt-knead an olefin-based elastomer resin having a softening temperature within an appropriate range in advance to uniformly disperse it in the polypropylene resin. In terms of the film-forming process, it is also effective to set the preheating temperature and stretching temperature for longitudinal and transverse stretching within appropriate ranges, or to subject the longitudinally stretched film to a relaxation treatment in the longitudinal direction at a temperature below the softening temperature of the elastomer to form a fine uneven structure on the film surface, thereby reducing the contact area between the transport roll and the film surface. These methods can be used in combination as appropriate.

[0031] When the biaxially oriented polypropylene film of the present invention is used as a protective film, from the viewpoint of maintaining the flatness and quality of the adherend, the valley volume Vvv of the A side is 2.0 μL / m ２ 20μL / m or more ２ or less. The valley volume Vvv (hereinafter sometimes simply referred to as Vvv) is one of the surface parameters obtained by measurement using a scanning white light interference microscope. More specifically, as shown in FIG. 1, the valley volume Vvv (reference numeral 4) can be expressed as the area surrounded by a load curve (reference numeral 2) drawn with the vertical axis representing height and the horizontal axis representing the areal load ratio, an equivalent line (reference numeral 5) at an areal load ratio of 80%, and a position line (reference numeral 6) at an areal load ratio of 100%. Vvv represents the volume of the space in the protruding valleys, and a small Vvv on the surface means that the surface of the biaxially oriented polypropylene film tends to have fewer coarse recessed structures. Vvv can be measured using a scanning white light interference microscope and an associated analysis system, and details of the measurement method are shown in the Examples.

[0032] In general biaxially oriented polypropylene films for industrial use, techniques for controlling convex portions are often used, but intentional control of concave portions is rare. On the other hand, during the manufacturing process of biaxially oriented polypropylene films, coarse spherulite structures such as β crystals formed during the crystallization process are stretched, which easily forms coarse concave portions like craters. Furthermore, when an optical film is formed using a biaxially oriented polypropylene film having such concave portions as a support, air trapping may occur in the coarse concave portions and deformation of the surface of the mating component may occur. Therefore, in order to solve the problems of smoothness and handleability, it is preferable that the valley volume Vvv of the biaxially oriented polypropylene film of the present invention is controlled within an appropriate range.

[0033] From the above viewpoint, the lower limit of Vvv of the A side is more preferably 2.5 μL / m ２ , more preferably 3.0 μL / m ２ Vvv of surface A is 2.0 μL / m ２ By satisfying the above conditions, the biaxially oriented polypropylene film has appropriate slipperiness and good handling properties. From the above viewpoints, the upper limit of Vvv of the A-side is more preferably 15 μL / m ２ , more preferably 10 μL / m ２ and particularly preferably 7.0 μL / m ２ and most preferably 5.0 μL / m ２ Vvv of surface A is 20 μL / m ２ By keeping Vvv of surface A at 2.0 μL / m or less, the number of large concave portions on the surface is reduced, which makes it possible to suppress an increase in air trap defects and deterioration of flatness of the optical film formed on surface A, thereby improving the quality. ２ 20μL / m or more ２ The biaxially stretched polypropylene film described below has good handling properties and smoothness, and can be suitably used as an industrial material film for producing optical films, for example.

[0034] To achieve Vvv within the above range, for example, a method can be used in which the raw material composition of the biaxially stretched polypropylene film is within the range described below, and the film-forming conditions are within the range described below. In particular, adding an elastomer resin having a softening temperature within an appropriate range to a polypropylene resin having a crystallization rate within an appropriate range to reduce the size of spherulites formed during casting, lowering the temperature of the casting drum to improve cooling efficiency during casting, and setting the preheating and stretching temperature within an appropriate range to achieve uniform stretching while suppressing the formation of large irregularities are effective. In particular, performing a relaxation treatment of 2.0% or more but less than 10%, preferably 3.0% or more but less than 10%, after longitudinal stretching, or setting the preheating and stretching temperature during transverse stretching to a temperature higher than the softening temperature of the elastomer resin added to the film and lower than the melting point of the polypropylene resin most abundant in the film, thereby stretching the uniaxially stretched film in the width direction while maintaining the dense irregularity structure formed on the film, are particularly effective. These methods can also be used in combination as appropriate.

[0035] From the viewpoint of flatness, the biaxially oriented polypropylene film of the present invention preferably has a thickness of 10 μm or more and 100 μm or less. From this viewpoint, the lower limit of the thickness is preferably 10 μm, more preferably 15 μm. When the biaxially oriented polypropylene film is used as an industrial material film, a thickness of 10 μm or more ensures mechanical strength, thereby reducing the occurrence of wrinkles and improving flatness and handleability. From this viewpoint, the upper limit of the thickness is preferably 100 μm, more preferably 50 μm, even more preferably 40 μm, and most preferably 28 μm. A thickness of 100 μm or less allows efficient cooling during casting, thereby improving flatness. The thickness of the biaxially oriented polypropylene film can be adjusted by the screw rotation speed of the extruder, the thickness and width of the unstretched sheet, the film production speed, the stretching ratio, etc. The thickness of the biaxially oriented polypropylene film can be measured using a micro thickness gauge, and the detailed measurement method will be described later.

[0036] From the viewpoint of improving blocking resistance, the biaxially oriented polypropylene film of the present invention preferably has a heat of fusion of 160°C or less of the total heat of fusion of 5% to 50% when heated from 30°C to 260°C using a differential scanning calorimeter (DSC). Hereinafter, the "heat of fusion of 160°C or less of the total heat of fusion" may be referred to as the proportion of the heat of fusion of 160°C or less. From this viewpoint, the proportion of the heat of fusion of 160°C or less is more preferably 45% or less, and even more preferably 40% or less. By keeping the proportion of the heat of fusion of 160°C or less to 50% or less, the amount of low-melting-point components that are likely to cause deformation or fusion during long-term storage in an oven or in a wound state during heating is reduced, thereby reducing the occurrence of slippage and blocking. Furthermore, the lower limit of the proportion of the heat of fusion of 160°C or less is more preferably 8%, and even more preferably 13%. When the proportion of heat of fusion at 160° C. or less is 5% or more, film rupture during transverse stretching is suppressed, and more stable stretching is possible.

[0037] In light of the above, in order to keep the proportion of heat of fusion at 160°C or less within the above range, it is effective to, for example, set the raw material composition of the biaxially stretched polypropylene film within the range described below and set the film-forming conditions within the range described below. In particular, it is effective to optimize the crystallinity and orientation of the biaxially stretched polypropylene film by using a polypropylene resin having an appropriate melting point, molecular weight, and half-crystallization time as the main component. Furthermore, in terms of the film-forming process, it is effective to set the preheating temperature and stretching temperature during transverse stretching below the melting point of the polypropylene resin as the main component. By setting these conditions, transverse stretching is possible without significantly relaxing the orientation structure of the uniaxially stretched film after longitudinal stretching, resulting in a biaxially stretched polypropylene film with a more stable crystalline structure. These methods can also be used in combination as appropriate.

[0038] Next, resins that can be used in the biaxially oriented polypropylene film of the present invention will be explained, but the resins are not necessarily limited to these.

[0039] The biaxially oriented polypropylene film of the present invention is primarily composed of polypropylene resin. In the present invention, "primary component" refers to a component that accounts for more than 50% by mass and not more than 100% by mass of all components of the film. The lower limit of the proportion of polypropylene resin in all components of the film is more preferably 70% by mass, even more preferably 80% by mass, and particularly preferably 90% by mass. The biaxially oriented polypropylene film of the present invention may contain only one type of polypropylene resin, but may also contain two or more types of polypropylene resin. When two or more components corresponding to polypropylene resin are contained in the film, the film is considered to be "based primarily on polypropylene resin" if the total content of these components is more than 50% by mass and not more than 100% by mass. Furthermore, in the present invention, polypropylene resin refers to a resin that contains more than 50 mol% and not more than 100 mol% of propylene units, where the total content of all structural units constituting the molecular chain of the resin is 100 mol%.

[0040] The biaxially oriented polypropylene film of the present invention is not particularly limited in terms of its layer structure, and can have either a single-layer or a laminate structure. However, from the viewpoint of satisfying different properties such as smoothness, rigidity, and handleability, it is preferable that the film be composed of at least two layers, and it is particularly preferable that the film have a surface layer (I) and a base layer (II).

[0041] Furthermore, when the biaxially oriented polypropylene film of the present invention has a laminated structure, from the viewpoints of handleability at high temperatures and quality, it is preferred that a layer containing a polypropylene resin having a melting point of 155° C. or more and 175° C. or less (preferably 160° C. or more and 175° C. or less) and an elastomer resin having a softening temperature of 70° C. or more and 150° C. or less be located as the outermost layer on at least one side. For example, in the case of a biaxially oriented polypropylene film having a surface layer (I) and a base layer (II) as described above, the surface layer (I) is the outermost layer.

[0042] The polypropylene resin (polypropylene resin A) that is most preferably contained in the polypropylene film of the present invention will be described below. The weight average molecular weight Mw of polypropylene resin A is 20 × 10 ４ Above 60 x 10４ From the viewpoint of crystallization behavior and the value F5t obtained by multiplying the stress F5 value at 5% elongation in the direction perpendicular to the main orientation of the film at 130°C of the obtained biaxially stretched polypropylene film by the film thickness, it is preferable that the lower limit of Mw is 25 × 10 or less. ４ , more preferably 30 × 10 ４ The upper limit of Mw is more preferably 45×10 ４ , more preferably 40 × 10 ４ In order to make the Mw of the polypropylene resin have the above value, it is preferable to adopt a method of adjusting the hydrogen gas concentration during polymerization, or to appropriately select a catalyst and / or co-catalyst and a composition.

[0043] The lower limit of the melting point of polypropylene resin A is preferably 155 ° C, more preferably 158 ° C, even more preferably 160 ° C, and particularly preferably 163 ° C. When the melting point of the polypropylene resin is 155 ° C or higher, the resulting biaxially oriented polypropylene film has excellent heat resistance, and when used as a process film for the production of, for example, a release film or an optical film, it maintains its shape even when passing through a heat-exposed process after being laminated to the adherend, and deformation of the adherend can be suppressed. The upper limit of the melting point of polypropylene resin A is preferably 170 ° C, more preferably 168 ° C, and even more preferably 165 ° C. When the melting point of the polypropylene resin of the present invention is 170 ° C or lower, the crystallization rate is appropriately suppressed, thereby suppressing the formation of a coarse uneven structure during crystallization by casting, improving the smoothness of the biaxially oriented polypropylene film, and also making it easier to control the dynamic friction coefficient μd of the front and back of the film within a suitable range after heating at 130 ° C for 10 minutes.

[0044] The polypropylene resin A is preferably a homopolypropylene resin from the viewpoints of excellent heat resistance, good mechanical properties and lubricity even in a high-temperature environment, and good handleability.

[0045] The polypropylene resin A is not particularly limited as long as it satisfies the above-mentioned preferred conditions, but examples of commercially available polypropylene resins that can be suitably used in the biaxially oriented polypropylene film of the present invention include E-203GP and F113G polypropylene resins manufactured by Prime Polymer Co., Ltd., D101, FS2011DG3, and FSX20L8 polypropylene resins manufactured by Sumitomo Chemical Co., Ltd., and FY6H and FL4 polypropylene resins manufactured by Japan Polypropylene Corporation.

[0046] When the biaxially oriented polypropylene film of the present invention has a laminated structure, the polypropylene resin (polypropylene resin B) that is most preferably contained in the outermost layer on the A-side side will be described below. The lower limit of the crystallization time of polypropylene resin B is preferably 50 seconds, more preferably 100 seconds, even more preferably 200 seconds, particularly preferably 300 seconds, and most preferably 400 seconds. When the crystallization time of the polypropylene resin is 50 seconds or more, crystal formation is moderately suppressed, making it difficult for coarse structures to form during crystallization by casting. This improves the smoothness of the biaxially oriented polypropylene film and makes it easier to control the dynamic friction coefficient μd between the front and back of the film after heating at 130°C for 10 minutes within a suitable range. Furthermore, the upper limit of the crystallization time is preferably 1500 seconds or less, more preferably 1000 seconds or less. A crystallization time of 1500 seconds or less ensures sufficient crystal formation, making it easier to form fine irregularities on the film surface. This makes the biaxially oriented polypropylene film smoother and easier to handle.

[0047] The crystallization half time of polypropylene resin B is an index of the crystallization rate determined by isothermal crystallization measurement at 130°C using DSC. The crystallization rate is determined by a combination of factors including stereoregularity, crystallinity, and molecular weight. Details of the method and apparatus for measuring the crystallization half time of polypropylene resin will be described in the Examples below.

[0048] The upper limit of the melting point of polypropylene resin B is preferably 175°C, more preferably 170°C, even more preferably 168°C, and particularly preferably 165°C. When the melting point of the polypropylene resin of the present invention is 175°C or less, the affinity with the elastomer resin is improved, and the spherulite structure formed during casting can be made finer, thereby suppressing the formation of steep protrusions derived from craters. The lower limit of the melting point of polypropylene resin B is preferably 155°C, more preferably 158°C, and even more preferably 160°C. When the melting point of the polypropylene resin is 155°C or more, the transparency of the film can be easily maintained even under conditions of high transverse stretching temperatures.

[0049] The polypropylene resin B is not particularly limited as long as it satisfies the above-mentioned preferred conditions. Examples of commercially available polypropylene resins that can be used in the biaxially oriented polypropylene film of the present invention include, in addition to the polypropylene resins listed above as polypropylene resin A, polypropylene resins E-200GP, E-203GP, and Y-400GP manufactured by Prime Polymer Co., Ltd., polypropylene resin WF836DG3 manufactured by Sumitomo Chemical Co., Ltd., polypropylene resins FY6H, MA3U, and SA4L manufactured by Japan Polypropylene Corporation, and polypropylene resin VS700A manufactured by SunAllomer Co., Ltd. Note that some polypropylene resins combine both the characteristics preferably possessed by the above-mentioned polypropylene resin A and the characteristics preferably possessed by polypropylene resin B. These polypropylene resins can be suitably used for either polypropylene resin A or B, and the same polypropylene resin can also be used for polypropylene resin A and B.

[0050] In addition to the polypropylene resin described above, the biaxially stretched polypropylene film of the present invention may contain a polypropylene resin (hereinafter sometimes referred to as a copolymerized polypropylene resin) containing copolymerized components such as other unsaturated hydrocarbons, provided that the effects of the invention are not impaired. Examples of monomer components constituting the copolymerized components in the copolymerized polypropylene resin include ethylene, 1-butene, 1-pentene, 3-methylpentene-1, 3-methylbutene-1, 1-hexene, 4-methylpentene-1, 5-ethylhexene-1, 1-octene, 1-decene, 1-dodecene, vinylcyclohexene, styrene, allylbenzene, cyclopentene, norbornene, and 5-methyl-2-norbornene. From the viewpoint of dimensional stability of the resulting biaxially stretched polypropylene film, the copolymerization amount of the copolymerized polypropylene resin is preferably less than 10 mol % when the total structural units constituting the resin are taken as 100 mol %.

[0051] When the copolymerized polypropylene resin contains ethylene as a copolymerization component, the ethylene component content is preferably 10 mol% or less, more preferably 5 mol% or less, and even more preferably 3 mol% or less. The higher the ethylene component content, the lower the crystallinity and the easier it is to improve transparency. However, if the ethylene component content exceeds 10 mol%, the strength of the biaxially oriented polypropylene film may decrease, or the heat resistance may decrease, resulting in a worsening heat shrinkage rate. Furthermore, the resin may be prone to degradation during the extrusion process, and fisheyes may be more likely to occur in the resulting biaxially oriented polypropylene film.

[0052] The biaxially stretched polypropylene film of the present invention preferably contains an elastomer resin, which is a polyolefin resin different from the polypropylene resin described above. Adding and dispersing the elastomer resin in the polypropylene resin described above facilitates the promotion of structural refinement during casting and stretching.

[0053] The elastomer resin is preferably an olefin-based elastomer, more preferably a propylene-based elastomer, from the viewpoint of dispersibility in the polypropylene resin and the effect of miniaturizing the structure formed by the polypropylene resin.

[0054] From the viewpoint of dispersibility in polypropylene resin, the MFR of the elastomer resin is preferably 3.0 g / 10 min or more and 50 g / 10 min or less, with the lower limit being more preferably 5.0 g / 10 min and the upper limit being more preferably 40 g / 10 min, even more preferably 10 g / 10 min, and most preferably 8.0 g / 10 min. The MFR can be measured in accordance with JIS K7210-1 (2014).

[0055] The softening temperature of the elastomer resin is preferably 70°C or higher and 150°C or lower. From the viewpoint described below, the lower limit of the softening temperature of the elastomer resin is more preferably 110°C, even more preferably 117°C. Furthermore, the upper limit of the softening temperature of the elastomer resin is more preferably 140°C, even more preferably 130°C, and particularly preferably 123°C. When the softening temperature of the elastomer resin is 70°C or higher, films may block each other when the biaxially oriented polypropylene film is stored for a long period of time, for example, in a rolled state under a high temperature environment in summer. When the softening temperature of the elastomer resin is 150°C or lower, it is easy to control the deformation of the elastomer resin during the stretching process, thereby promoting the formation of a fine uneven surface structure and suppressing the formation of a coarse protrusion structure. As a result, the flatness and transfer suppression effect of the biaxially oriented polypropylene film are improved. The softening temperature of the elastomer resin can be measured in accordance with JIS K 7196 (1991), and details thereof will be described later.

[0056] In the biaxially stretched polypropylene film of the present invention, when the total of the polypropylene resin and elastomer resin in the outermost layer on the A-side is taken as 100% by mass, the proportions of the polypropylene resin and elastomer resin in this layer are preferably as shown below. The lower limit of the proportion of the polypropylene resin in the outermost layer is preferably 50% by mass, more preferably 60% by mass, even more preferably 65% ​​by mass, and particularly preferably 70% by mass. The upper limit of the proportion of the polypropylene resin in the outermost layer is preferably 99% by mass, more preferably 90% by mass, and even more preferably 80% by mass. The lower limit of the proportion of the elastomer resin in the outermost layer is preferably 1% by mass, more preferably 10% by mass, and even more preferably 20% by mass. The upper limit of the proportion of the elastomer resin in the outermost layer is preferably 50% by mass, more preferably 40% by mass, even more preferably 35% by mass, and particularly preferably 30% by mass. When the proportion of the elastomer resin relative to the total amount of resin components in the outermost layer on the A-side is 50% by mass or less, loss of surface flexibility after heating the biaxially oriented polypropylene film in an oven, etc. is reduced, and blocking of the biaxially oriented polypropylene film when rolled is also reduced. On the other hand, when the proportion of the elastomer resin relative to the total amount of resin components in the outermost layer on the A-side is 1% by mass or more, it becomes easy to adjust the Vmp of side A and the dynamic friction coefficient μd between the front and back of the film after heating at 130°C for 10 minutes within the aforementioned preferred ranges.

[0057] Elastomer resins are commonly used as raw materials for heat-sealable layers, and heating a biaxially oriented polypropylene film is likely to cause a decrease in lubricity and blocking, so it has not been common to use elastomer resins in the outermost layer of a film used as a protective film or support film.The biaxially oriented polypropylene film of the present invention can be used even in high-temperature environments by selecting and controlling the melting point, temperature, and amount of the elastomer resin within the appropriate ranges described above.

[0058] The elastomer resin can be appropriately selected with reference to the MFR, melting point, and softening temperature described above. Commercially available elastomer resins that can be used in the biaxially oriented polypropylene film of the present invention include, but are not limited to, "Tafmer" (registered trademark) "PN2060," "PN2070," and "BL3450" manufactured by Mitsui Chemicals, Inc., "Wintec" (registered trademark) "WMG30" manufactured by Japan Polypropylene Corporation, "Dynaron" (registered trademark) "2324P" manufactured by JSR Corporation, and "Milastomer" (registered trademark) "8030NHS" manufactured by Mitsui Chemicals, Inc.

[0059] The polypropylene resin and elastomer resin used in the biaxially oriented polypropylene film of the present invention may contain various additives, such as a crystal nucleating agent, an antioxidant, a heat stabilizer, a slipping agent, an antistatic agent, an antiblocking agent, a filler, a viscosity modifier, a color inhibitor, etc., within the range that does not impair the object of the present invention. These additives may be added to any layer, and multiple types may be used in combination, as long as they do not impair the effects of the present invention.

[0060] Among these, the selection of the type and amount of antioxidant is important from the viewpoint of antioxidant bleed-out. That is, such antioxidants are preferably sterically hindered phenol-based antioxidants, with at least one of them being a high molecular weight type having a molecular weight of 500 or more. Specific combinations include various examples, but it is preferable to use 2,6-di-t-butyl-p-cresol (BHT: molecular weight 220.4), 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), either alone or in combination.

[0061] In particular, phosphorus-based antioxidants may be prone to bleed-out onto the surface of biaxially stretched polypropylene films. Such bleed-out can lead to various problems. For example, when a resin film is formed on the surface of a biaxially stretched polypropylene film, curing the uncured or semi-cured resin film may inhibit curing. Furthermore, when an electrolyte membrane or metal membrane is formed on the surface of a biaxially stretched polypropylene film, the properties of the electrolyte membrane and the quality of the metal layer may be reduced. From the above viewpoints, the content of the phosphorus-based antioxidant in the biaxially stretched polypropylene 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. From the above viewpoints, it is preferable that no phosphorus-based antioxidant is contained in the biaxially stretched polypropylene film, and the lower limit of the content is 0 ppm. Examples of phosphorus-based antioxidants include tris(2,4-di-t-butylphenyl)phosphite (e.g., "Irgafos" (registered trademark) 168 manufactured by BASF, molecular weight 647). In biaxially stretched polypropylene films, the total content of these antioxidants is preferably in the range of 0.03 to 1.0 parts by mass relative to the total amount of resin. If the amount of antioxidant is too small, the polymer may deteriorate during the extrusion process, resulting in coloration of the film, or long-term heat resistance may be poor. If the amount of antioxidant is too large, the transparency may be reduced due to bleed-out of the antioxidant. From the above viewpoints, the amount of antioxidant is more preferably 0.05 to 0.9 parts by mass, and particularly preferably 0.1 to 0.8 parts by mass, relative to the total amount of resin.

[0062] A crystal nucleating agent can be added to the biaxially stretched polypropylene film of the present invention, provided that the addition does not adversely affect the intended purpose. Specific examples of crystal nucleating agents 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, since excessive addition of the above-mentioned other nucleating agents may result in reduced stretchability and reduced transparency and strength due to void formation, the amount added is 0.5 parts by mass or less, preferably 0.1 parts by mass or less, and more preferably 0.05 parts by mass or less, based on the total amount of resin.

[0063] The biaxially oriented polypropylene film of the present invention preferably does not contain organic or inorganic particles. The polypropylene resin that can be preferably used as the main component of the biaxially oriented polypropylene film of the present invention has low affinity with organic or inorganic particles. Therefore, if these particles are contained, the particles may fall off during the film-forming process, contaminating the process or the product. Furthermore, if high-hardness particles form coarse protrusions on the surface of the biaxially oriented polypropylene film, when the film is used as a release film for an optical component, the irregularities may be transferred to the resin layer of the optical component. Therefore, when the film is used as a protective film or a manufacturing base film for products requiring high quality, such as display components, this may cause a decrease in quality. From the above perspective, the polypropylene film of the present invention preferably does not contain organic or inorganic particles.

[0064] The biaxially oriented polypropylene film of the present invention is preferably obtained by biaxially stretching an unstretched sheet obtained using the above-mentioned resin. The biaxial stretching method may be any of simultaneous biaxial stretching by inflation, simultaneous biaxial stretching by tenter, and sequential biaxial stretching by a roll-type stretching machine and a tenter. Among these, the sequential biaxial stretching by a roll-type stretching machine and a tenter is preferably employed in terms of film formation stability, thickness uniformity, and control of high rigidity and dimensional stability of the film.

[0065] Next, one embodiment of the method for producing a biaxially oriented polypropylene film of the present invention will be explained using a two-type, three-layer biaxially oriented polypropylene film as an example, but the biaxially oriented polypropylene film of the present invention is not necessarily limited to this.

[0066] First, in order to improve the dispersibility of the elastomer resin or another type of polypropylene resin, it is preferable to melt-mix the polypropylene resin and the elastomer resin (or another type of polypropylene resin) in any ratio as described below to form a resin composition, and then pelletize the mixture.

[0067] Pellets of the resin composition obtained above are fed into a single-screw extruder for the surface layer (I), and polypropylene resin A is fed into a single-screw extruder for the base layer (II). Then, melt extrusion is performed at 200 to 280°C, more preferably 220 to 275°C, and even more preferably 240 to 270°C, respectively. After removing foreign matter and modified polymers using a filter installed midway through the polymer tube, the mixture is laminated in a layer structure of surface layer (I) / base layer (II) / surface layer (I) 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 structure of surface layer (I) / base layer (II) / surface layer (I). In this case, the lamination thickness ratio is preferably in the range of 1 / 4 / 1 to 1 / 60 / 1 from the viewpoint of balancing surface properties, mechanical strength, and thermal properties. It is more preferably 1 / 5 / 1 to 1 / 60 / 1, even more preferably 1 / 10 / 1 to 1 / 60 / 1, and particularly preferably 1 / 30 / 1 to 1 / 60 / 1. By setting the ratio within the above range, the surface layer (I) containing the elastomer resin is formed thinly and uniformly on the film surface, which increases the uniformity of the height of the irregularities formed during stretching and makes it possible to suppress the formation of a coarse irregular structure. In this case, polypropylene resins A and B may be the same polypropylene resin.

[0068] Furthermore, to suppress the formation of β-crystals during crystallization, the surface temperature of the casting drum is preferably 10 to 40°C, more preferably 15 to 30°C, even more preferably 19 to 27°C, and particularly 20 to 25°C. By setting these conditions, it becomes easy to adjust the Vmp of side A to a suitable range. Furthermore, the layer structure may be a two-layer laminate structure of surface layer (I) / base layer (II). The molten resin sheet may be adhered to the casting drum using any of the following methods: electrostatic application, adhesion methods utilizing the surface tension of water, air knife method, press roll method, and underwater casting method. However, the air knife method is preferred because it provides good flatness and allows control of surface roughness. From the perspective of cooling the uncooled drum surface of the sheet on the casting drum, it is preferable to lower the air temperature of the air knife. The air temperature of the air knife is 10 to 35°C, preferably 10 to 25°C, and more preferably 15 to 20°C, and the blown air speed is preferably 130 to 150 m / s. By setting these conditions, it becomes easy to adjust the number of craters on side B whose major axis exceeds 100 μm to a suitable range. In addition, it is also preferable to appropriately adjust the position of the air knife so that air flows downstream of the film formation to prevent vibration of the sheet. In addition, in the case of a two-type two-layer laminate structure of surface layer (I) / base layer (II), it is preferable that the surface layer (I) side is on the casting drum side.

[0069] The obtained laminated unstretched sheet is introduced into the longitudinal stretching step. In the longitudinal stretching step, longitudinal stretching is preferably performed in two stages in a preheating step before stretching. In the first half of the preheating step, the laminated unstretched sheet is preferably heated by contacting it with multiple metal rolls maintained at a temperature below the softening temperature of the elastomer resin added to the film, and the heating temperature is preferably the softening temperature of the elastomer resin −2°C or less, more preferably the softening temperature of the elastomer resin −10°C or less, and even more preferably the softening temperature of the elastomer resin −20°C or less. In the second half of the preheating step, the sheet is preferably heated at a temperature equal to or higher than the preheating temperature of the first half of the preheating step, and the heating temperature is more preferably the preheating temperature of the first half of the preheating step +10°C or more, and even more preferably the preheating temperature of the first half of the preheating step +20°C or more.

[0070] The longitudinal stretching temperature is preferably the softening temperature of the elastomer resin + 15° C. or more, more preferably the softening temperature of the elastomer resin + 20° C. or more, and even more preferably the softening temperature of the elastomer resin + 25° C. or more. By setting the preheating temperature and stretching temperature within the above ranges, the elastomer resin can be appropriately softened in the preheating step, and deformation during stretching can be controlled, resulting in favorable control of the surface structure of the film.

[0071] Then, in the stretching process, the laminated unstretched sheet is stretched in the longitudinal direction between rolls with different peripheral speeds by 4.0 to 6.0 times, more preferably 4.5 to 5.8 times, and even more preferably 4.5 to 5.6 times or 4.8 to 5.5 times. The longitudinal direction refers to the direction in which the film runs during the production process (or the winding direction if wound into a roll). A longitudinal stretching ratio of 4.0 times or more increases the mechanical strength in the longitudinal direction, thereby suppressing film deformation even under high tension and making it easier to maintain the flatness of the biaxially stretched polypropylene film. Furthermore, a longitudinal stretching ratio of 6.0 times or less suppresses thermal shrinkage caused by excessive strain during stretching, making it easier to maintain the flatness of the biaxially stretched polypropylene film.

[0072] Next, a relaxation treatment is performed in the longitudinal direction, thereby efficiently forming a fine uneven structure on the film surface. From the viewpoint of forming a fine uneven surface structure and improving the handleability of a film with good flatness, the temperature of the relaxation treatment in the longitudinal direction is preferably equal to or lower than the softening temperature of the elastomer resin added to the film, more preferably equal to or lower than the softening temperature of the elastomer resin minus 10°C, and even more preferably equal to or lower than the softening temperature of the elastomer resin minus 20°C. From the viewpoint of controlling the uneven surface structure and good film transportability, the relaxation rate in the longitudinal relaxation treatment is preferably 2.0% or more and less than 10%, and the lower limit of the longitudinal relaxation treatment is more preferably 3.0%, and even more preferably 5.0%. The upper limit of the longitudinal relaxation treatment is more preferably 9.0%, and even more preferably 8.0%.

[0073] Next, the uniaxially stretched film is guided into a tenter with both widthwise ends held with clips, preheated, and then transversely stretched 7.0 to 12 times in the widthwise direction. The lower limit of the stretching ratio is preferably 8.0 times, more preferably 8.6 times, and even more preferably 9.0 times, from the viewpoint of uniform stretchability of the film. The upper limit of the stretching ratio is more preferably 11 times, from the viewpoint of stable stretching without film rupture. The width direction refers to the direction perpendicular to the longitudinal direction within the film plane.

[0074] From the viewpoint of uniformly stretching the film and suppressing the formation of a sharp protrusion structure, the preheating temperature is preferably equal to or higher than the softening temperature of the elastomer resin added to the film and equal to or lower than the melting point of polypropylene resin A minus 15°C. The lower limit of the preheating temperature is more preferably the softening temperature of the elastomer resin + 5°C, and even more preferably the softening temperature of the elastomer resin + 10°C. The upper limit of the preheating temperature is more preferably the melting point of polypropylene resin A minus 20°C, and even more preferably the melting point of polypropylene resin A minus 25°C.

[0075] From the viewpoints of uniformly stretching the film and controlling the surface uneven structure, the stretching temperature is preferably at least the softening temperature of the elastomer resin + 20°C and not more than the melting point of polypropylene resin A. From the viewpoints of stably stretching the film without melting and rupturing the film, the lower limit of the stretching temperature is more preferably the softening temperature of the elastomer resin + 25°C, and even more preferably the softening temperature of the elastomer resin + 30°C. The upper limit of the stretching temperature is more preferably the melting point of polypropylene resin A - 5°C, and even more preferably the melting point of polypropylene resin A - 10°C. By setting the preheating and stretching temperatures within the above ranges, the uniaxially stretched film can be uniformly stretched laterally, and a dense uneven structure can be formed on the surface of the biaxially stretched film.

[0076] In the subsequent heat treatment and relaxation treatment steps, the biaxially stretched film is relaxed and shrunk uniformly, and from the viewpoint of suppressing the formation of steep protrusion structures, both widthwise ends of the film are tension-held with clips, and the film is heat-set at 140°C to 180°C, more preferably 150°C to 173°C, and even more preferably 150°C to 170°C, while being relaxed at a relaxation rate of 5.0 to 20%, more preferably 7.0 to 15%, and even more preferably 9.0 to 12% in the width direction. Furthermore, while both widthwise ends are tension-held with clips, the film is cooled at 80 to 100°C, and then guided to the outside of the tenter, and the clips at both widthwise ends are released. Thereafter, in the winding step, the film edges are slit, and a biaxially stretched polypropylene film product roll is wound up.

[0077] The biaxially oriented polypropylene film obtained as described above can be used for various industrial applications such as packaging films, cover films, process films, sanitary products, agricultural products, construction products, and medical products. However, since it has particularly excellent surface smoothness, handleability, and blocking resistance, it can be preferably used for surface protection films, support films, and release films.

[0078] The biaxially oriented polypropylene film of the present invention has excellent transparency, and therefore, when focusing on the use of the biaxially oriented polypropylene film as a cover film, it can be particularly preferably used as a cover film for photocurable resins and dry photoresists. Here, the cover film refers to a film that is attached to an object such as a molded body or film and has the function of protecting the object from scratches, contamination, etc. that occur during processing or transportation.

[0079] The support film refers to a film used to form a film by laminating an object on the film when it is difficult to form a film from the object alone due to reasons such as the object itself being thin or fragile.

[0080] A release film is a film that has high releasability and can be attached to objects such as molded bodies and films to protect them from scratches and contamination that occur during processing and transportation, and can be easily peeled off and discarded when used as a final product.

[0081] The biaxially oriented polypropylene film of the present invention is excellent in quality and in handleability in a high-temperature environment, and therefore can be preferably used when forming a laminate with a metal film by subjecting the biaxially oriented polypropylene film of the present invention to sputtering or vapor deposition. That is, the biaxially oriented polypropylene film of the present invention can be made into a laminate by forming a metal film on at least one side thereof.

[0082] In sputtering and vapor deposition processes, polyethylene terephthalate (PET) films are often used due to their excellent surface smoothness, mechanical strength, and heat resistance. However, PET is highly hydrophilic due to its ester bonds, and PET films contain trace amounts of moisture. Such trace amounts of moisture can have adverse effects during sputtering and vapor deposition processes, particularly when vapor-depositing metals belonging to Group 1 or 2 of the periodic table, or compounds containing these metals, which are susceptible to moisture. Given this background, the biaxially oriented polypropylene film of the present invention is particularly suitable for forming metal films containing metals belonging to Group 1 or 2 of the periodic table or compounds thereof, since it can produce laminates even in cases where the presence of trace amounts of moisture makes it difficult to use PET films. Here, metals belonging to Group 1 or 2 refer to lithium, sodium, potassium, rubidium, cesium, francium, beryllium, magnesium, calcium, strontium, barium, and radium.

[0083] Furthermore, under high-temperature conditions, moisture in the film evaporates as outgassing, and this effect is particularly pronounced under high-vacuum conditions such as in metal sputtering processes. Outgassing from the film reduces the degree of vacuum in the system, which can lead to deterioration in the quality of the metal film formed by sputtering and a decrease in the sputtering yield. From this perspective, the biaxially oriented polypropylene film of the present invention, which has a low moisture content, is more suitable for use in forming metal films than PET films with a high moisture content, and can also maintain good quality of the formed metal film. From the above perspectives, the upper limit of the moisture content of the biaxially oriented polypropylene film of the present invention is preferably 2000 ppm, more preferably 1000 ppm, and even more preferably 500 ppm. The lower limit of the moisture content is not particularly limited, but is essentially 1 ppm. The moisture content of the biaxially oriented polypropylene film can be measured by the Karl Fischer method, the details of which will be described later.

[0084] In order to set the moisture content of the biaxially oriented polypropylene film of the present invention within the above range, it is preferable to reduce the amount of hydrophilic resin or additive in the biaxially oriented polypropylene film. Specifically, of all the components of the biaxially oriented polypropylene film, the lower limit of the content of polyolefin resin such as polypropylene resin or propylene-based elastomer resin is preferably 90 mass%, more preferably 95 mass%, and even more preferably 97 mass%. The upper limit of the content of these resins is substantially 100 mass%. Polyolefin resins such as polypropylene resin and propylene-based elastomer resin are relatively hydrophobic, and increasing their proportions can reduce the moisture content of the biaxially oriented polypropylene film.

[0085] In order to reduce defects due to bleed-out, the biaxially stretched polypropylene film of the present invention preferably contains 0 to 500 ppm in total of additives other than antioxidants (e.g., antistatic agents, viscosity modifiers, color inhibitors, slip agents, etc.). It is preferable that the content of antioxidants such as phosphorus-based antioxidants is adjusted as described above.

[0086] As described above, the biaxially oriented polypropylene film of the present invention has an extremely low moisture content and generates very little outgassing, and is superior in smoothness and handling to existing olefin-based films, and is therefore preferably used when forming transparent conductive films that require stricter vacuum conditions or high quality. Here, the term "transparent conductive film" refers to a thin film formed from a material that is conductive and yet transmits visible light, and specific examples include an indium-tin composite oxide (ITO) film, a zinc oxide (ZnO) film, and a palladium film.

[0087] Furthermore, electrolyte membranes used in fuel cells, semi-solid batteries, all-solid batteries, etc. are usually produced in environments where temperature and humidity are strictly controlled. In particular, sulfide-type electrolyte membranes generate hydrogen sulfide when they react with moisture, so the process film used in producing the electrolyte membrane is required to have an extremely low moisture content. Therefore, the biaxially oriented polypropylene film of the present invention is preferably used for such applications.

[0088] The biaxially oriented polypropylene film of the present invention is preferably used as a current collector due to its excellent handleability. A current collector is a foil-like laminate used in electrodes of storage batteries such as lithium-ion batteries, and is typically formed by laminating a metal film on a base resin film. This metal film is laminated by processes such as vapor deposition, sputtering, plating, and electroless plating. To increase the energy density of batteries, the film serving as the base material of the current collector must be thin. However, as the film becomes thinner, its stiffness decreases, significantly reducing its handleability during processing. In particular, the process of laminating the metal film requires the film to be highly heat-sensitive, such as radiant heat, during processing, and also to be tensioned in the conveying direction, so the film must have good handleability. The biaxially oriented polypropylene film of the present invention is preferably used as a current collector because it can be thinned and has good handleability.

[0089] The biaxially oriented polypropylene film of the present invention is preferably used as a current collector due to its excellent handling properties, and the current collector is generally used as an electrode for a storage battery. A storage battery is a device that stores electrical energy and converts it back into electrical energy when needed, and specific examples include lead-acid batteries, nickel-metal hydride batteries, lithium-ion batteries, NAS batteries, and redox flow batteries.

[0090] The present invention will be described in detail below with reference to examples. The properties were measured and evaluated by the following methods.

[0091] (1) Film Thickness: Measured using a micro thickness meter (manufactured by MITSUTOYO Corp.) A biaxially stretched polypropylene film was sampled in a 10 cm square area, and measurements were taken at five random points, and the average value was taken as the film thickness.

[0092] (2) Peak volume (Vmp), valley volume (Vvv), and peak density (Spd) Measurements were performed using a scanning white light interference microscope "VS1540" (manufactured by Hitachi High-Tech Science Corporation; measurement conditions and instrument configuration are described below). Measurements were performed using a 5 x 5 field of view, with 561.1 μm x 561.5 μm per field. All images were then stitched with a 20% overlap to obtain surface profile data of 2356.716 μm x 2358.294 μm. The captured image was interpolated (fully interpolated) using the accompanying analysis software, and surface correction was performed using a polynomial fourth-order approximation. The surface profile was then determined using a median filter (3 x 3 pixels). The measurement surfaces were both sides of a biaxially oriented polypropylene film, and a total of nine measurement positions were determined according to the following procedure, starting from the intersection of the diagonals of a 5 cm x 5 cm square cut of biaxially oriented polypropylene film. Measurements were taken at each measurement position, and Vmp, Vvv, and Spd were calculated at each measurement position according to the procedure described above, and the average values ​​were used as Vmp, Vvv, and Spd. ２ When there was a difference between the above, the side with the relatively smaller Vmp was designated as side A, and the opposite side as side B.

[0093] <How to determine the measurement position> Measurement 1: Position of the starting point Measurement 2: Position 5.0 mm to the right of the starting point Measurement 3: Position 10.0 mm to the right of the starting point Measurement 4: Position 5.0 mm below the starting point Measurement 5: Position 5.0 mm below and 5.0 mm to the right of the starting point Measurement 6: Position 5.0 mm below and 10.0 mm to the right of the starting point Measurement 7: Position 10.0 mm below the starting point Measurement 8: Position 10.0 mm below and 5.0 mm to the right of the starting point Measurement 9: Position 10.0 mm below and 10.0 mm to the right of the starting point <Measurement conditions and device configuration> Objective lens: 10x Lens tube: 1x Zoom lens: 1x Wavelength filter: 530 nm white Measurement mode: Wave Measurement software: VS-Measure 10.0.4.0 Analysis software: VS-Viewer 10.0.3.0 Measurement area: 561.1 μm × 561.5 μm Number of pixels: 1,024 × 1,024.

[0094] (3) Identification of the Main Orientation Direction and the Direction Perpendicular to the Main Orientation Direction When an arbitrary direction in the film plane was defined as 0°, the Young's modulus was measured in each direction forming an angle of 0° to 175° in 5° increments relative to the arbitrary direction. The direction showing the highest value was defined as the main orientation direction of the film, and the direction perpendicular to this in the film plane was defined as the direction perpendicu- lar to the main orientation direction of the film. The Young's modulus was measured in accordance with JIS K 7161 (2014) using the following procedure. First, a rectangular sample measuring 150 mm in length (measurement direction) and 10 mm in width was cut out. Using a tensile tester (Orientec "Tensilon" (registered trademark) UCT-100), a tensile test was performed in an atmosphere of room temperature of 23°C and relative humidity of 65%, with an initial tensile chuck distance of 50 mm and a tensile speed of 300 mm / min. The load applied to the film when the sample was elongated by 2% (when the distance between the chucks reached 51 mm) was read and divided by the cross-sectional area of ​​the sample before the test (film thickness x 10 mm) to obtain the stress F2 value at 2% elongation. Then, a stress-strain curve was drawn with the elongation on the horizontal axis and the stress on the vertical axis, and the slope of the line passing through the origin and the point used to measure the F2 value was defined as Young's modulus (MPa).

[0095] (4) F5t Value Five samples measuring 10 mm wide and 50 mm long (measurement direction) were cut from biaxially stretched polypropylene film so that the measurement direction was the direction perpendicular to the main orientation. Marks were placed 15 mm from each end to obtain a test length of 20 mm. Next, the rectangular samples were placed in a tensile tester (Orientec "Tensilon" (registered trademark) UCT-100) with an initial chuck distance of 20 mm. After heating for 1 minute in an oven heated to 130°C, a tensile test was performed on the film at a tensile speed of 300 mm / min. The load applied to the film when the sample was elongated by 5% was read, and this value was divided by the cross-sectional area of ​​the sample before the test (film thickness x width (10 mm)) to calculate the stress (unit: MPa) at 5% elongation. This value was then multiplied by the film thickness determined in (1). Measurements were performed five times for each sample, and the average value was calculated to obtain the F5t value (unit: Pa m).

[0096] (5) μd of the front and back of the film after heating at 130 ° C for 10 minutes A biaxially oriented polypropylene film was cut into a width of 6.5 cm and a length of 12 cm, and the test piece was sandwiched between 20 cm square pieces of paper and stacked. The test piece was heated for 10 minutes in an oven maintained at 130 ° C under zero load, and then removed. After cooling at room temperature, the test piece was measured at room temperature of 25 ° C and a relative humidity of 65% using a slip tester manufactured by Toyo Seiki Co., Ltd. in accordance with JIS K 7125 (1999). The measurement was performed with the biaxially oriented polypropylene film in the directions perpendicular to the main orientation and with different surfaces overlapping, i.e., with the main orientation perpendicular directions overlapping and the surface of one film in contact with the back of the other film. The same measurement was performed five times for each sample, and the average of the obtained values ​​was calculated and used as the dynamic friction coefficient (μd) of the sample.

[0097] (6) Number of craters with a major axis of 150 μm or greater Measurements were performed using a scanning white light interference microscope "VS1540" (manufactured by Hitachi High-Tech Science Corporation; measurement conditions and instrument configuration are described below). Measurements were performed using multiple 5 × 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 shape data of 2356.716 μm × 2358.294 μm. In the pore analysis of the particle analysis mode of the accompanying analysis software "VS-Viewer 10.0.3.0," the height threshold was set to -0.1 μm, and the number of depressions with a major axis of 150 nm or greater was counted. Measurements were performed on side B of the biaxially oriented polypropylene 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 taken at each measurement position, and the number of particles at each measurement position was determined according to the procedure described above, and the average value was adopted. Note that the "method of determining the measurement position" and the "measurement conditions and device configuration" were the same as in the previous section (2).

[0098] (7) Heat of fusion below 160°C: 5 mg of biaxially oriented polypropylene film was placed in an aluminum pan as a sample and measured using a differential scanning calorimeter (Rigaku Thermoplus EVO2 DSCvesta). For the melting curve observed when the temperature was raised from 23°C to 260°C at 20°C / min (first run) under a nitrogen atmosphere, the heat flow at each temperature was corrected using a straight line that served as the baseline from 60°C to 250°C. The heat of fusion from 60°C to 250°C was then divided by the heat of fusion from 60°C to 160°C to calculate the ratio of the heat of fusion below 160°C.

[0099] (8) Crystallization Half Time Using a differential scanning calorimeter (Rigaku Thermoplus EVO2 DSCvesta), 3 mg of polypropylene resin was heated in a nitrogen atmosphere from 25°C to 250°C at 20°C / min and held for 5 minutes. The temperature was then lowered from 250°C to 130°C at 20°C / min and held at 130°C for 30 minutes. The time when the sample temperature reached 130°C was defined as 0 seconds, and the elapsed time from the peak appearing 60 seconds or later in the endothermic curve obtained during isothermal holding at 130°C was defined as the crystallization half time (seconds).

[0100] (9) Weight-average molecular weight Mw Polypropylene resin was dissolved in 1,2,4-trichlorobenzene as a solvent by stirring at 165°C for 30 minutes. The solution was then filtered using a 0.5 μm filter, and the molecular weight distribution of the filtrate was measured. The weight-average molecular weight Mw of the sample was determined using a molecular weight calibration curve prepared using the following standard samples. Apparatus: Agilent PL-GPC220 high-temperature GPC apparatus Detector: Agilent differential refractive index detector (RI detector) Column: Agilent PL1110-6200 (20 μm MIXED-A) x 2 Flow rate: 1.0 mL / min Column temperature: 145°C Injection volume: 0.500 mL Sample concentration: 0.1 wt% Standard samples: Tosoh monodisperse polystyrene, Tokyo Chemical Industry dibenzyl.

[0101] (10) Softening Temperature The softening temperature was determined in accordance with JIS K 7196 (1991) using the following equipment and conditions. Using a hydraulic hot press molding machine set at 190°C, the elastomer resin was preheated for 5 minutes, press-molded for 2 minutes under a pressure of 10 MPa, and then cooled at 20°C for 4 minutes under a pressure of 10 MPa to produce a sheet of a predetermined thickness, thereby obtaining a sheet sample. After 72 hours at room temperature after molding, the softening temperature was measured using a thermomechanical analyzer TMASS6100 (manufactured by Hitachi High-Tech Science Corporation) at a pressure of 2 kg / cm using a 1 mmφ probe. ２ The displacement (penetration depth) was measured when the sample was heated at a temperature increase rate of 5° C. / min while applying a pressure of 100 μm, and the temperature at which the displacement reached 500 μm was used as the softening temperature.

[0102] (11) Melting Point Using a differential scanning calorimeter (Rigaku Thermoplus EVO2 DSCvesta), 5 mg of polypropylene resin or elastomer resin was heated in a nitrogen atmosphere from -70°C to 250°C at a rate of 20°C / min and held for 5 minutes. The temperature was then lowered from 250°C to -70°C at a rate of 20°C / min and held for 5 minutes. The temperature was then raised from -70°C to 250°C at a rate of 20°C / min. The peak temperature of the curve obtained during this temperature rise was taken as the melting point (Tm, °C).

[0103] (12) Evaluation of Transfer to Adherend Biaxially oriented polypropylene film and 20 μm thick "ZEONORFILM" (registered trademark) manufactured by Zeon Corporation were sampled into a square shape measuring 100 mm wide and 100 mm long. The biaxially oriented polypropylene film was placed on top of the "ZEONORFILM" (registered trademark) so that the surface A of the biaxially oriented polypropylene film was in contact with the "ZEONORFILM" (registered trademark). The sample was sandwiched between two acrylic plates (100 mm wide and 100 mm long), a load of 5 kg was applied, and the sample was left to stand for 24 hours in an atmosphere at 40°C. After 24 hours, the surface of the "ZEONORFILM" (registered trademark) (the surface that was in contact with the biaxially oriented polypropylene film) was visually observed and evaluated according to the following criteria: A: Clean and the same as before the load was applied. B: Weak irregularities were observed. C: Strong irregularities were observed.

[0104] (13) Flatness during Film Heating A 500 mm wide polypropylene film was subjected to corona treatment on the side with a low Vmp value, and then coated with an acrylic emulsion adhesive (EX243, manufactured by E-Tech Co., Ltd.). The film was then introduced into a drying oven at 130 °C and conveyed for 60 seconds at a conveying tension of 200 N to remove the solvent from the coating, resulting in an adhesive film having an adhesive layer with a thickness of 1 μm. This adhesive film was then wound into a roll with a length of 200 m to produce an adhesive film roll. Next, 1 m of the 500 mm wide adhesive film was unwound and subjected to free tension (a state in which the film was hanging vertically due to its own weight) and tensions of 1 kg / m and 3 kg / m were applied uniformly and evenly across the entire film width. The presence or absence of flatness defects such as wrinkles and dents was visually confirmed using the obtained values. Flatness was evaluated according to the following criteria: S: No areas of flatness defects were observed under free tension. A: Poor flatness was observed with free tension, but no poor flatness was observed with a tension of 1 kg / m width. B: Poor flatness was observed with a tension of 1 kg / m width, but no poor flatness was observed with a tension of 3 kg / m width. C: Poor flatness was observed even with a tension of 3 kg / m width.

[0105] (14) Blocking Resistance Biaxially oriented polypropylene films were cut to a size of 60 mm x 210 mm, and five sheets were stacked on an iron plate so that side A and side B of the film were in contact. Then, an iron plate was placed on top of the five films so that a load of 1 kgf was applied, and the films were heated in an oven set to 120°C for 15 minutes. After being removed from the oven, the films were allowed to stand and slowly cooled to room temperature. Then, the laminated biaxially oriented polypropylene films were peeled off and evaluated according to the following criteria. A: The films did not adhere to each other. B: The films adhered to each other, but could be peeled off without rupturing. C: Peeling was impossible, or the films ruptured during peeling.

[0106] (15) Evaluation of yield when forming metal film A metal film made of magnesium oxide was formed on one side of a biaxially stretched polypropylene film or a PET film by sputtering. A magnetron sputtering device was used for sputtering, and the pressure was adjusted from atmospheric pressure to 1 × 10 －５ After reducing the pressure in the tank, argon gas was introduced and 2×10 －３ A magnesium oxide film was formed on the surface of a biaxially oriented polypropylene film or a PET film by sputtering at a pressure of 1×10 －５ The time required for the pressure to be reduced to a full pressure was measured, and the unevenness of the metal film surface was visually observed to evaluate the yield according to the following criteria. The evaluation criteria were A as pass and B as fail. A: 1×10 －５ B: The time required for reducing the pressure to a full pressure was 50 minutes or less, and no irregularities were observed on the surface of the metal film. －５ The time required to reduce the pressure to the full pressure was longer than 50 minutes, or irregularities were observed on the surface of the metal film.

[0107] (16) Moisture Content A biaxially stretched polypropylene film or PET film sample was left for 4 hours or more in a room conditioned at 23° C. and a relative humidity of 20%, and then immersed for 24 hours in distilled water at 23° C. Thereafter, the moisture on the surface of the sample was wiped off, and the moisture in the sample was dried and evaporated at a temperature of 150° C. using a trace moisture meter (manufactured by Mitsubishi Chemical Corporation, CA-20 model), and the moisture content was then determined by the Karl Fischer method to calculate the moisture content.

[0108] (Polypropylene Resin, etc.) The resins used in the production of biaxially oriented polypropylene films in the Examples and Comparative Examples are shown in Tables 1 and 2 below. These property values ​​were evaluated in the form of resin pellets. Seven types of polypropylene resins (PP1 to PP7) and six types of elastomer resins (ELA1 to ELA6) were prepared. Furthermore, in Examples 1 to 3, the resin composition for the surface layer (I) was prepared by pre-melting and kneading polypropylene resin and elastomer resin in the mass ratios shown in Table 3. Note that all of PP1 to PP7 contained tetrakis[methylene-3(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane as an antioxidant in the range of 1000 ppm to 5000 ppm, while PP7 additionally contained 3000 ppm of tris(2,4-di-t-butylphenyl)phosphite. Polypropylene resin 1 (PP1): manufactured by Prime Polymer Co., Ltd. Polypropylene resin 2 (PP2): manufactured by Borealis Polypropylene resin 3 (PP3): manufactured by Prime Polymer Co., Ltd. Polypropylene resin 4 (PP4): manufactured by Japan Polypropylene Corporation Polypropylene resin 5 (PP5): manufactured by Idemitsu Kosan Co., Ltd. Polypropylene resin 6 (PP6): manufactured by Japan Polypropylene Corporation, "WAYMAX" (registered trademark) MFX6 Polypropylene resin 7 (PP7): manufactured by Japan Polypropylene Corporation, "WINTEC" (registered trademark) WFW4M.

[0109]

[0110] Elastomer resin 1 (ELA1): Mitsui Chemicals, Inc., "Tafmer" (registered trademark) PN2060 Elastomer resin 2 (ELA2): Mitsui Chemicals, Inc., "Tafmer" (registered trademark) BL3450 Elastomer resin 3 (ELA3): Mitsui Chemicals, Inc., "Tafmer" (registered trademark) PN2070 Elastomer resin 4 (ELA4): Japan Polypropylene Corporation, "Wintec" (registered trademark) WMG30 Elastomer resin 5 (ELA5): ExxonMobil Corporation, "Vistamaxx" (registered trademark) 3020FL Elastomer resin 6 (ELA6): Japan Polypropylene Corporation, "Welnex" (registered trademark) RFX4V.

[0111]

[0112] In the table, MFR is a value measured in accordance with JIS K7210-1 (2014).

[0113] (Example 1) Polypropylene resin 1 (PP1) and elastomer resin 1 (ELA1) were fed from a lightweight hopper to a twin-screw extruder so that the ratio was 70:30 (mass ratio), melt-kneaded at 250 ° C, and extruded from a die in the form of a strand. The extruded resin composition was cooled and solidified in a water bath at 25 ° C and cut into chips to obtain a resin composition for the surface layer (I). Thereafter, the resin composition for the surface layer (I) was fed to a single-screw extruder, and PP1 was fed to a single-screw extruder for the base layer (II) as a raw material for the base layer (II). Each resin mixture was melt-extruded at 250 ° C, and after removing foreign matter with a 20 μm cut sintered filter, it was laminated in a feed block type A / B / A composite T die so that the thickness ratio of the surface layer (I) / base layer (II) / surface layer (I) was 1 / 50 / 1. The resulting molten sheet was discharged onto a casting drum with a surface temperature controlled at 20 ° C. and adhered to the casting drum using an air knife. Then, compressed air at 15 ° C. was sprayed onto the uncooled drum surface of the sheet on the casting drum using an air knife at an air speed of 140 m / s to cool the sheet on the casting drum, obtaining an unstretched sheet. Subsequently, the unstretched sheet was brought into contact with a metal roll at 95 ° C. in the first half of the preheating process, and heated to 120 ° C. in the second half of the preheating process, and stretched 4.9 times in the longitudinal direction between rolls at 147 ° C. with a peripheral speed difference. Next, the sheet was subjected to a 5.5% relaxation treatment in the longitudinal direction using a metal roll heated to 95 ° C. to obtain a uniaxially stretched film. The resulting uniaxially stretched film was then clamped at both widthwise ends with clips and introduced into a tenter-type stretching machine. After preheating at 137°C for 2 seconds, it was stretched 9.5 times in the widthwise direction at 152°C and then heat-treated at 155°C while providing 10% relaxation in the widthwise direction. The film was then cooled to 100°C and guided outside the tenter. The clips at both widthwise ends of the film were released and the film was wound around a core to obtain a biaxially stretched polypropylene film with a thickness of 25 μm. The physical properties and evaluation results of the resulting film are shown in Table 3.

[0114] (Examples 2 to 5, Comparative Examples 1 to 4) Biaxially stretched polypropylene films were 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. In Examples 2 and 3, a resin composition was used for the surface layer (I) by previously melt-kneading a PP resin and an elastomer resin in the same manner as in Example 1, so that the resin composition had the mass ratio shown in Table 3. 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 stretched polypropylene films are shown in Table 3.

[0115] Comparative Example 5 A biaxially oriented polypropylene film was obtained in the same manner as in Example 1, except that the raw material composition of each layer and the film-forming conditions were as shown in Table 3, and that in the longitudinal stretching step, the unstretched sheet was preheated to 90°C with a ceramic roll and initially stretched 1.3 times in the longitudinal direction between 90°C rolls with a peripheral speed difference, and then the film after the initial stretching was preheated to 140°C and a second-stage longitudinal stretching was performed at a stretching ratio of 3.5 times. 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 polypropylene film are shown in Table 3.

[0116] Comparative Example 6 Instead of the biaxially stretched polypropylene film, a PET film "Lumirror" (registered trademark) S10 (manufactured by Toray Industries, Inc.) was used to evaluate various properties, and the moisture content and thickness were measured. The evaluation results are shown in Table 3.

[0117]

[0118] In the table, in Comparative Examples 1, 2, and 4, even on the surface where Vmp is relatively small, 50 μL / m ２ Since the Vmp value was not less than 0.01, there was no A-side, but the value of the side with a relatively small Vmp was recorded as the A-side Vmp. Note that the Vmp values ​​of both sides were different in all examples. Furthermore, the moisture content and the yield during metal film production were evaluated only for Example 1, Comparative Examples 1, and 6.

[0119] As described above, the biaxially oriented polypropylene of the present invention can be used for various industrial applications such as packaging films, surface protection films, release films, support films, sanitary products, agricultural products, construction products, and medical products. However, since it has particularly excellent quality and handleability in high-temperature environments, it can be preferably used as a release film or support film for photocurable resins, or a release film or support film for dry film resists.

[0120] 1: Peak actual volume Vmp 2: Load curve drawn with the vertical axis as height and the horizontal axis as areal load ratio 3: Equivalent line at areal load ratio of 10% 4: Valley volume Vvv 5: Equivalent line at areal load ratio of 80% 6: Position line at areal load ratio of 100%

Claims

1. The peak actual volume Vmp is relatively small and is 0.1 μL / m ２ More than 50μL / m ２ A biaxially oriented polypropylene film, characterized in that one side is the A-side, and a value F5t obtained by multiplying the stress F5 value at 5% elongation in a direction perpendicular to the main orientation of the film at 130°C by the film thickness is 50 Pa・m or more and 1000 Pa・m or less, when the side which is equal to or less than the A-side is defined as the A-side.

2. 2. The biaxially oriented polypropylene film according to claim 1, wherein the dynamic friction coefficient μd between the front and back surfaces of the film after heating at 130° C. for 10 minutes is 0.30 or more and 1.40 or less.

3. When the surface opposite to the surface A is surface B, the surface B is 1 mm ２ The number of craters with a major axis exceeding 100 μm is 0.001 / mm ２ 30 pieces / mm or more ２ The biaxially oriented polypropylene film according to any one of claims 1 to 2, wherein:

4. The peak density Spd of the A side is 1.0 peaks / mm ２ More than 100 pieces / mm ２ The biaxially oriented polypropylene film according to any one of claims 1 to 2, wherein:

5. The valley volume Vvv of the surface A is 2.0 μL / m ２ 20μL / m or more ２ The biaxially oriented polypropylene film according to any one of claims 1 to 2, wherein:

6. The biaxially stretched polypropylene film according to any one of claims 1 to 2, wherein when the film is heated from 30 ° C. to 260 ° C. using a differential scanning calorimeter DSC, the heat of fusion at 160 ° C. or less accounts for 5% to 50% of the total heat of fusion.

7. 3. The biaxially oriented polypropylene film according to claim 1, wherein a layer containing a polypropylene resin having a melting point of 160° C. or more and 175° C. or less and an elastomer resin is located as an outermost layer on at least one side.

8. The biaxially oriented polypropylene film according to claim 7, wherein the softening temperature of the elastomer resin is 70°C or higher and 150°C or lower.

9. The biaxially oriented polypropylene film according to any one of claims 1 to 2, which is used for a release film application.

10. The biaxially oriented polypropylene film according to any one of claims 1 to 2, which is used for a support film application.