JP7658513B2 - Biaxially oriented polypropylene film - Google Patents

Description

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

Biaxially oriented polypropylene film has excellent surface releasability and mechanical properties, making it ideal for use as release films, process films, and support films for a variety of components, including plastic products, building materials, and optical components.

Biaxially oriented polypropylene film generally forms a surface structure with steep protrusions known as crater protrusions. For this reason, for example, when an optical film is manufactured using biaxially oriented polypropylene film as a support, the steep protrusions may be transferred and steep recesses may be formed in the optical film. The optical film is then laminated to other components in the next process, but if these steep recesses remain as voids, they will cause optical defects, making it difficult to use biaxially oriented polypropylene film as a support for high-end optical films.

In addition, the resin composition for obtaining the optical film is coated on the surface of the biaxially oriented polypropylene film, and then dried and cured in a high-temperature oven. Therefore, the biaxially oriented polypropylene film needs to maintain high rigidity even in a high-temperature environment. To maintain high rigidity in a high-temperature environment, it is generally necessary to use a highly crystalline polypropylene resin as the raw material for the biaxially oriented polypropylene film, but it is known that when a highly crystalline polypropylene raw material is used, the aforementioned crater protrusions are likely to form on the surface. In this way, there is a trade-off between achieving high rigidity at high temperatures and suppressing the crater protrusion structure, and it has been very difficult to achieve both at the same time.

Thus, the support film of the optical film is required to suppress the formation of steep protrusion structures while having high rigidity in a high-temperature environment. 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, which is the main raw material. Patent Document 3 describes an example in which a crystal nucleating agent is added to improve mechanical strength at high temperatures, resulting in high crystallization.

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

However, in the method of blending a copolymer or a low-crystalline polypropylene raw material with the propylene-based resin as the main raw material described in Patent Documents 1 and 2, the crystallinity of the substrate is likely to decrease. Therefore, the obtained film has low rigidity in a high-temperature environment, and there is a concern that wrinkles will occur and flatness will deteriorate in a high-temperature environment, which is problematic in terms of impairing the quality and handleability in a high-temperature environment. In addition, in the method described in Patent Document 3, the crystallinity of the film surface is high and a steep protrusion structure is likely to be formed. Therefore, when the obtained film is used as a support film, there is a concern that defects will occur, and there is a problem in that the quality will deteriorate. Therefore, the present invention aims to solve the above problems and provide a biaxially oriented polypropylene film with excellent quality and handleability in a high-temperature environment.

In order to solve the above-mentioned problems, the biaxially stretched polypropylene film of the present invention has the following configuration: That is, the biaxially stretched polypropylene film of the present invention is a biaxially stretched polypropylene film, characterized in that, when a surface having a relatively small peak actual volume Vmp and a peak actual volume Vmp of 0.1 μL/ ^m2 or more and 50 μL/m2 or ^less is defined as surface A, one surface is the surface A, and a 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 present invention makes it possible to provide a biaxially oriented polypropylene film that has excellent quality and handling properties even in high-temperature environments.

1 is a conceptual diagram showing a peak actual volume Vmp and a valley volume Vvv.

The biaxially stretched polypropylene film of the present invention is characterized in that, when a surface having a relatively small peak actual volume Vmp and a value between 0.1 μL/ ^m2 and 50 μL/ ^m2 is defined as surface A, one surface is surface A, and a 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 stretched polypropylene film of the present invention will be described in detail below.

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

Biaxially oriented polypropylene film is prone to forming steep protrusion structures called crater protrusions. Therefore, for example, when a biaxially oriented polypropylene film is used as a support and an optical film is formed on its surface, 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 will remain as voids when the optical film is bonded to another component in the next process, which may result in optical defects.

There are various parameters that represent surface roughness, but for example, the average surface roughness Sa, maximum height Sz, 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, becomes large 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. In addition, when the difference in Vmp between both sides is 0.1 μL/m ² or more, it is considered that there is a difference in Vmp between both sides, and the "side with a relatively small peak actual volume Vmp" refers to the side with a relatively low Vmp among the two sides of the biaxially stretched polypropylene film when there is a difference in Vmp between both sides.

From the viewpoint of achieving both quality and handleability at high temperatures, it is important that the biaxially stretched polypropylene film of the present invention has a relatively small peak actual volume Vmp, and when the surface having a peak actual volume Vmp of 0.1 μL/m ² or more and 50 μL/m ² or less is defined as surface A, one surface is surface A. From the above viewpoint, the upper limit of Vmp on surface A is preferably 30 μL/m ² , more preferably 10 μL/m ^2. Vmp being greater than 50 μL/m ² means that many steep protrusion structures are formed on the surface of the biaxially stretched polypropylene film. Therefore, when the biaxially stretched polypropylene film is used as a support film for an optical film, if an optical film is formed on such a surface, the protrusion structure 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 surface A is preferably 0.3 μL/m ² , more preferably 0.5 μL/m ² . A Vmp of less than 0.1 μL/ ^m2 means that the surface of the biaxially oriented polypropylene film is excessively smooth. Therefore, 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.

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 range described below, and the film-forming conditions are within the range described below. In particular, it is effective to use an olefin-based elastomer resin having a softening temperature within an appropriate range, which is uniformly dispersed by melt-kneading it with a polypropylene resin in advance and forming chips, as the raw material for the outermost layer, and to use a polypropylene resin having a half crystallization time and melting point within an appropriate range as the main component of the outermost layer (details will be described later). In addition, in terms of the film-forming process, it is effective to set the casting temperature low to optimize the crystallization rate during casting, suppress the formation of β-crystal spherulites, and form a dense spherulite structure, and to set the preheating temperature and stretching temperature during longitudinal stretching and transverse stretching within the appropriate range described later, and precisely control the surface structure by controlling the deformation of the elastomer resin domain. These methods can also be used in combination as appropriate.

In the biaxially oriented polypropylene film of the present invention, from the viewpoint of improving the handling property and quality at high temperatures, 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. From the above viewpoint, 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 that is a raw material for an optical film or the like is applied onto the biaxially oriented polypropylene film and then dried and cured in a high-temperature oven, the biaxially oriented polypropylene film stretches in the longitudinal direction and shrinks in the width direction. As a result, there is a concern that the flatness of the biaxially oriented polypropylene film is impaired, and the quality and handling property are deteriorated. In addition, from the viewpoint of feasibility, the upper limit of F5t is 1000 Pa · m, preferably 400 Pa · m, and more preferably 300 Pa · m.

In order to suppress deformation in a high-temperature, high-tension environment, it is useful to maintain high mechanical strength in the low-elongation region even in a high-temperature environment, and to increase the thickness of the biaxially oriented polypropylene film. It was found that the degree of deformation in a high-temperature, high-tension environment is strongly correlated with the value of F5t. However, when making a biaxially oriented polypropylene film thicker, the cooling efficiency decreases during the process of obtaining a cast film by melt extrusion, and many β-crystal spherulites are formed in the cast film. The presence of such β-spherulites causes crater protrusions after biaxial stretching, so there is a trade-off between thickening the biaxially oriented polypropylene film and Vmp, and it was previously difficult to achieve both.

Here, the main orientation direction refers to the direction that shows the highest value when the Young's modulus is measured in each direction that forms an angle of 0° to 165° in 15° increments with respect to an arbitrary direction in the film plane, with the arbitrary direction being set as 0°. 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 with a length of 150 mm (measurement direction) and a width of 10 mm with an initial chuck distance of 50 mm and performing a tensile test at room temperature at a tensile speed of 300 mm/min. The details of the method for measuring the Young's modulus will be described later.

However, in cases where it is not possible to obtain samples of the above size and therefore to measure and calculate Young's modulus using the above method, the crystal orientation of the polypropylene α crystal (110) plane of the biaxially oriented 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 perpendicularly 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 determined as the main orientation direction, and the direction perpendicular to this in the film plane is determined as the main orientation perpendicular direction.

The F5t of a biaxially oriented polypropylene film can be measured by the following procedure. First, the biaxially oriented polypropylene film is heated for 1 minute in an oven heated to 130°C, 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 elongation of the biaxially oriented polypropylene film is 5%. The value obtained by dividing the value by the cross-sectional area (film thickness x width) of the sample before the test is calculated as the stress at an elongation of 5%, and this can be measured and calculated by multiplying this by the film thickness. Details of the measurement method will be described later.

To make 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 set to the range described below, and the film-forming conditions are set to the range described below. In particular, it is effective to optimize the crystallinity and orientation of the film by using a polypropylene resin having an appropriate melting point and molecular weight as the main component. In addition, in terms of the film-forming process, by setting the preheating temperature and 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 and less than 10%, preferably 3.0% or more and 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.

In the biaxially oriented polypropylene film of the present invention, from the viewpoint of improving the handling property in the transport and winding process after high-temperature processing and further improving the quality, it is preferable that the dynamic friction coefficient μd of the front and back of the film after heating for 10 minutes at 130 ° C is 0.30 or more and 1.40 or less. From the above viewpoint, the upper limit of the dynamic friction coefficient μd of the front and back of the film after heating for 10 minutes at 130 ° C 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 of the front and back of the film after heating for 10 minutes at 130 ° C is not particularly limited, but is substantially about 0.30. Here, the "dynamic friction coefficient μd of the front and back of the film" is the friction coefficient measured by overlapping and rubbing different surfaces against each other, and the dynamic friction coefficient μd can be measured in accordance with JIS K 7125 (1999) (details will be described later).

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 to 130°C, resulting in an increase in the coefficient of friction between the front and back of the film, leading to 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.

In order to set the dynamic friction coefficient μd of the front and back of the film in the above-mentioned preferred range after heating at 130 ° C for 10 minutes, for example, a method can be used in which the raw material composition of the biaxially oriented polypropylene film is set in the range described below, and the film-forming conditions are set in the range described below. In particular, it is effective to suppress the softening of the elastomer resin when heated at 130 ° C by setting the softening temperature and the amount of addition of the olefin-based elastomer resin in the appropriate range. In addition, in terms of the film-forming process, it is also effective to set the preheating temperature and stretching temperature of the longitudinal stretching and transverse stretching in the appropriate range, and to perform a relaxation treatment in the longitudinal direction of the uniaxially stretched film after longitudinal stretching in a temperature range 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.

From the viewpoint of quality, the biaxially stretched polypropylene film of the present invention, when the surface opposite to the A surface is the B surface, preferably has 0.001 to 30 craters per ^mm2 of the B ^surface with a major axis exceeding 100 μm. If the number of craters with a major axis exceeding 100 μm is more than ³⁰ craters per ^mm2 , when the biaxially stretched polypropylene film is used as a support for optical film production, the steep protrusions originating from the craters on the B surface may be transferred to the optical film when the optical film is formed on the A surface and wound up, and steep recesses may be formed in the optical film. When such an optical film is laminated to another member in the next process, the steep recesses 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 craters/ ^mm2 , even more preferably 10 craters/ ^mm2 , and most preferably 5.0 craters/ ^mm2 . The lower limit is not particularly limited, but is substantially 0.001 pieces/ ^mm2 . Note that, in the present invention, "crater" refers to a shape having a coarse depression, specifically, a depression whose major axis is 150 nm or more when the height threshold is set to -0.1 μm. The number of such craters per ^mm2 of surface B can be measured using a scanning white light interference microscope and an associated analysis system, and the details of the measurement method are shown in the examples.

In order to keep the number of craters with a major axis exceeding 100 μm per 1 ^mm2 of the B side 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 of obtaining a biaxially oriented polypropylene film having an A side, it is preferable to lower the air temperature of the air knife, optimize the blown air speed, and optimize the thickness of the film, from the viewpoint of efficiently cooling the non-cooled drum surface of the sheet on the casting drum. Note that the preferred conditions for the temperature and speed of the air from the air knife will be described later.

In the biaxially stretched polypropylene film of the present invention, from the viewpoint of improving the handleability, the peak density Spd of side A is preferably 1.0 peaks/ ^mm2 or more and 100 peaks/ ^mm2 or less. Hereinafter, the peak density Spd may be simply referred to as Spd. Since the biaxially stretched polypropylene film of the present invention suppresses the formation of crater protrusions, the slipperiness is easily deteriorated, but in order to maintain good slipperiness even on such a surface and obtain a film with good flatness, it is preferable to form many fine uneven structures on the film surface.

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. By having the Spd of the A side be 1.0 pieces/mm ² or more, the slipperiness of the biaxially oriented polypropylene film is maintained, so that the occurrence of wrinkles during transportation and the associated deterioration of flatness can be reduced. Also, from the above viewpoint, the upper limit of the Spd of the A side is preferably 60 pieces/mm ² , more preferably 40 pieces/mm ² , particularly preferably 20 pieces/mm ² , and most preferably 17 pieces/mm ^2. By having the Spd of the A side be 100 pieces/mm ² or less, the fineness of the uneven structure on the surface of the biaxially oriented polypropylene film is appropriately suppressed, so that when used as an industrial material film, the deterioration of handleability due to the slipperiness is reduced. In addition, Spd can be measured by a scanning white light interference microscope and an associated analysis system, and the details of the measurement method are shown in the examples.

In order to set the Spd of the A side within the above range, for example, a method can be used in which the raw material composition of the biaxially oriented polypropylene film is set within the range described below, and the film-forming conditions are set 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 addition, in terms of the film-forming process, it is also effective to set the preheating temperature and stretching temperature for longitudinal stretching and transverse stretching within an appropriate range, and to subject the film after longitudinal stretching to a relaxation treatment in the longitudinal direction at a temperature range 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.

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

In general biaxially oriented polypropylene films for industrial use, methods for controlling convex portions are often used, but intentional control of concave portions is rare. On the other hand, in the manufacturing process of biaxially oriented polypropylene films, coarse spherulite structures such as β crystals formed during the crystallization process are stretched, and coarse concave portions such as craters are easily formed. When an optical film is formed using a biaxially oriented polypropylene film having such concave portions as a support, there is a possibility that air pockets will occur in the coarse concave portions and deformation of the surface of the mating member will occur. Therefore, in order to solve the problems of smoothness and handleability, it is preferable that the biaxially oriented polypropylene film of the present invention has a valley volume Vvv controlled within an appropriate range.

From the above viewpoint, the lower limit of Vvv of surface A is more preferably 2.5 μL/m ² , and even more preferably 3.0 μL/m ² . By having Vvv of surface A of 2.0 μL/m ² or more, the biaxially oriented polypropylene film has moderate slipperiness and good handleability. Also, from the above viewpoint, the upper limit of Vvv of surface A is more preferably 15 μL/m ² , even more preferably 10 μL/m ² , particularly preferably 7.0 μL/m ² , and most preferably 5.0 μL/m ² . By having Vvv of surface A of 20 μL/m ² or less, the number of coarse recesses on the surface is reduced, so that the increase in air pocket defects and the deterioration of flatness of the optical film formed on surface A can be suppressed, and the quality is improved. Thus, a biaxially oriented polypropylene film having a Vvv of 2.0 μL/ ^m2 or more and 20 μL/ ^m2 or less on side A has good handleability and smoothness, and can be suitably used, for example, as an industrial material film for producing optical films.

To set 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 set to the range described below, and the film-forming conditions are set to the range described below. In particular, it is effective to add an elastomer resin having a softening temperature in an appropriate range to a polypropylene resin having a crystallization rate in an appropriate range to refine the spherulites formed during casting, to lower the temperature of the casting drum to increase the cooling efficiency during casting, to set the preheating and stretching temperature in an appropriate range, and to stretch uniformly while suppressing the formation of coarse irregularities. In particular, it is effective to perform a relaxation treatment of 2.0% or more and less than 10%, preferably 3.0% or more and less than 10%, after longitudinal stretching, or to set the temperature in the preheating and stretching process 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 contained most in the film, and to stretch in the width direction while maintaining the dense irregular structure formed on the uniaxially stretched film. These methods can also be used in combination as appropriate.

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 the above 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, the mechanical strength is ensured, so that the occurrence of wrinkles is reduced, and the flatness is improved, and the handling property is improved. From the above 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. When the thickness is 100 μm or less, cooling proceeds efficiently during casting, so that the flatness is improved. 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-forming speed, the stretching ratio, and the like. The thickness of the biaxially oriented polypropylene film can be measured with a micro thickness meter, and the detailed measurement method will be described later.

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 in the total heat of fusion of 5% to 50% when heated from 30°C to 260°C using a differential scanning calorimeter DSC. In the following, the "heat of fusion of 160°C or less in the total heat of fusion" may be referred to as the ratio of the heat of fusion of 160°C or less. From the above viewpoint, the ratio of the heat of fusion of 160°C or less is more preferably 45% or less, and even more preferably 40% or less. By having the ratio of the heat of fusion of 160°C or less be 50% or less, the amount of low-melting point components that are likely to cause deformation or fusion when stored for a long period of time in an oven during heating or in a wound state is suppressed, thereby reducing the occurrence of slippage and blocking. In addition, the lower limit of the ratio of the heat of fusion of 160°C or less is more preferably 8%, and even more preferably 13%. By making the proportion of the heat of fusion at 160° C. or less 5% or more, film rupture during transverse stretching can be suppressed, making it possible to perform stretching more stably.

In consideration of the above, in order to set the ratio of the heat of fusion at 160°C or less within the above range, it is effective to set the raw material composition of the biaxially stretched polypropylene film to the range described below, and to set the film-forming conditions to 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. In addition, in terms of the film-forming process, it is effective to set the preheating temperature and stretching temperature during transverse stretching to be equal to or lower than the melting point of the polypropylene resin, which is the main component. By setting such 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 crystal structure. These methods can also be used in combination as appropriate.

Next, we will explain the resins that can be used in the biaxially oriented polypropylene film of the present invention, but they are not necessarily limited to these.

The biaxially oriented polypropylene film of the present invention is mainly composed of polypropylene resin. In the present invention, the term "main component" means a component whose proportion in the total components of the film is more than 50% by mass and not more than 100% by mass. The lower limit of the proportion of polypropylene resin in the total 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. In addition, when two or more components corresponding to polypropylene resin are contained in the film, if the total of these components is more than 50% by mass and not more than 100% by mass, it is considered that the film is "mainly composed of polypropylene resin". In addition, 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 when the total structural units constituting the molecular chain of the resin are 100 mol%.

The biaxially oriented polypropylene film of the present invention is not particularly limited in terms of its layer structure, and may 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 is composed of at least two layers, and it is particularly preferable that the film has a surface layer (I) and a base layer (II).

In addition, 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 preferable 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 is 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.

The polypropylene resin (polypropylene resin A) suitable as the component most abundant in the polypropylene film of the present invention will be described below. The weight average molecular weight Mw of polypropylene resin A is preferably 20×10 ⁴ or more and 60×10 ⁴ or less 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. by the film thickness. From the above viewpoint, the lower limit of Mw is more preferably 25×10 ⁴ , more preferably 30×10 ⁴ , and 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 the above value, a method of adjusting the hydrogen gas concentration during polymerization, the selection of a catalyst and/or a co-catalyst, and the selection of the composition are preferably adopted.

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 can maintain its shape even when passing through a heat-applied process after being laminated with an adherend, and can suppress deformation of the adherend. 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. By having the melting point of the polypropylene resin of the present invention be 170°C or lower, the crystal formation rate is appropriately suppressed, so that the formation of a coarse uneven structure during crystallization by casting is suppressed, the smoothness of the biaxially oriented polypropylene film is improved, and the dynamic friction coefficient μd of the front and back of the film after heating at 130°C for 10 minutes can be easily controlled within a suitable range.

Polypropylene resin A is preferably a homopolypropylene resin from the viewpoint of excellent heat resistance, good mechanical properties and lubricity even in high temperature environments, and good handleability.

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

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. The lower limit of the semi-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 semi-crystallization time of the polypropylene resin is 50 seconds or more, crystal formation is moderately suppressed and coarse structures are less likely to be formed during crystallization by casting, improving the smoothness of the biaxially oriented polypropylene film, and making it easier to control the dynamic friction coefficient μd of the front and back of the film after heating at 130 ° C for 10 minutes within a suitable range. In addition, the upper limit of the semi-crystallization time is preferably 1500 seconds or less, more preferably 1000 seconds or less. By having a semi-crystallization time of 1500 seconds or less, crystal formation is sufficient and fine unevenness is easily formed on the film surface. Therefore, the biaxially oriented polypropylene film becomes easy to slip and has good handleability.

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. Detailed measurement methods and devices for the crystallization half time of polypropylene resin are described in the examples below.

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. By having the melting point of the polypropylene resin of the present invention be 175°C or less, the affinity with the elastomer resin is improved, and the spherulite structure formed during casting can be refined, and as a result, the formation of steep protrusions derived from craters can be suppressed. 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. By having the melting point of the polypropylene resin be 155°C or more, the transparency of the film can be easily maintained even under conditions of high transverse stretching temperature.

Polypropylene resin B is not particularly limited as long as it satisfies the above-mentioned preferred conditions, but examples of polypropylene resins that can be used in the biaxially oriented polypropylene film of the present invention and are commercially available 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 resins WF836DG3 manufactured by Sumitomo Chemical Co., Ltd., polypropylene resins FY6H, MA3U, and SA4L manufactured by Japan Polypropylene Co., Ltd., and polypropylene resin VS700A manufactured by Sun Allomer Co., Ltd., etc. In addition, there are polypropylene resins that have both the characteristics that are preferably possessed by the above-mentioned polypropylene resin A and the characteristics that are preferably possessed by polypropylene resin B. These can be suitably used for both polypropylene resins A and B, and it is not prohibited to use the same polypropylene resin as polypropylene resins A and B.

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 with other unsaturated hydrocarbons within a range that does not impair the effects of the invention. 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. In terms of the dimensional stability of the resulting biaxially stretched polypropylene film, it is preferable that the copolymerization amount of the copolymerized polypropylene resin is less than 10 mol% when the total constituent units constituting the resin are 100 mol%.

When the copolymerized polypropylene resin has ethylene as a copolymerization component, the content of the ethylene component is preferably 10 mol% or less. More preferably, it is 5 mol% or less, and even more preferably, it is 3 mol% or less. The higher the content of the ethylene component, the lower the crystallinity and the easier it is to improve transparency. However, if the content of the ethylene component exceeds 10 mol%, the strength of the biaxially oriented polypropylene film may decrease, or the heat resistance may decrease, resulting in a worsening of the heat shrinkage rate. In addition, the resin may be easily deteriorated during the extrusion process, and fish eyes may easily occur in the obtained biaxially oriented polypropylene film.

The biaxially stretched polypropylene film of the present invention preferably contains an elastomer resin, which is a polyolefin resin different from the above-mentioned polypropylene resin. By adding and dispersing the elastomer resin in the above-mentioned polypropylene resin, it becomes easier to promote structural refinement during casting and stretching.

From the viewpoints of dispersibility in polypropylene resin and the effect of refining the structure formed by the polypropylene resin, olefin-based elastomers are preferred as elastomer resins, and propylene-based elastomers are more preferred.

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).

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, and even more preferably 117°C. 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, the films may block each other when the biaxially oriented polypropylene film is stored for a long period of time 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, so that the formation of a fine uneven surface structure is promoted and the formation of a coarse protrusion structure can be suppressed, resulting in improved flatness and transfer suppression effect of the biaxially oriented polypropylene film. The softening temperature of the elastomer resin can be measured in accordance with JIS K 7196 (1991), and the details will be described later.

In the biaxially stretched polypropylene film of the present invention, when the total of the polypropylene resin and the elastomer resin in the outermost layer on the A-side side is taken as 100% by mass, the proportions of the polypropylene resin and the elastomer resin in the 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 the biaxially oriented polypropylene film is heated in an oven, etc. is reduced, and blocking between the biaxially oriented polypropylene films 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 the A-side and the dynamic friction coefficient μd of the front and back of the film after heating at 130° C. for 10 minutes, etc., to the preferred ranges described above.

Elastomer resins are commonly used as raw materials for heat seal layers, and there is a high possibility that heating a biaxially oriented polypropylene film will cause a decrease in slipperiness and blocking, so it has not been common to apply elastomer resins to 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 to be added within the appropriate ranges described above.

The elastomer resin can be appropriately selected with reference to the above MFR, melting point, and softening temperature. Examples of commercially available elastomer resins that can be used in the biaxially oriented polypropylene film of the present invention include, but are not limited to, Mitsui Chemicals' "Toughmer" (registered trademark) "PN2060", "PN2070", and "BL3450", Japan Polypropylene's "Wintec" (registered trademark) "WMG30", JSR's "Dynaron" (registered trademark) "2324P", and Mitsui Chemicals' "Milastomer" (registered trademark) "8030NHS".

The polypropylene resin and elastomer resin used in the biaxially oriented polypropylene film of the present invention may contain various additives, such as crystal nucleating agents, antioxidants, heat stabilizers, slipping agents, antistatic agents, antiblocking agents, fillers, viscosity modifiers, and color inhibitors, to the extent that the object of the present invention is not impaired. These additives may be added to any layer, and multiple types may be used in combination, as long as they do not impede the effects of the present invention.

Among these, the selection of the type and amount of antioxidant is important from the viewpoint of antioxidant bleed-out. In other words, such antioxidants are sterically hindered phenol-based, and at least one of them is preferably a high molecular weight type with a molecular weight of 500 or more. Specific combinations include various ones, but for example, 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), and tetrakis[methylene-3(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane (e.g., BASF's "Irganox" (registered trademark) 1010: molecular weight 1177.7), etc., are preferably used alone or in combination.

In particular, phosphorus-based antioxidants may easily bleed out onto the surface of the biaxially oriented polypropylene film. Such bleed-out may induce various problems. For example, when a resin film is formed on the surface of the biaxially oriented polypropylene film, curing may be inhibited if the uncured or semi-cured resin film is subjected to a curing treatment. In addition, when an electrolyte membrane or a metal membrane is formed on the surface of the biaxially oriented polypropylene film, the properties of the electrolyte membrane and the quality of the metal layer may be deteriorated. From the above viewpoint, the content of the phosphorus-based antioxidant in the biaxially oriented 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 viewpoint, it is preferable that the phosphorus-based antioxidant is not contained in the biaxially oriented 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., BASF's "Irgafos" (registered trademark) 168: molecular weight 647). In the biaxially oriented polypropylene film, 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, causing the film to become discolored, or the film may have poor long-term heat resistance. If the amount of antioxidant is too large, the transparency may decrease due to the bleed-out of these antioxidants. From the above viewpoints, the content of the 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.

A crystal nucleating agent can be added to the biaxially stretched polypropylene film of the present invention to the extent that it does not interfere with the 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 types of nucleating agents may cause a decrease in stretchability or a decrease in transparency and strength due to void formation, etc., 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.

It is preferable that the biaxially oriented polypropylene film of the present invention does not contain organic particles and 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 particles and inorganic particles, so if these particles are contained, the particles may fall off during the film-forming process and contaminate the process or product. In addition, if coarse protrusions are formed on the surface of the biaxially oriented polypropylene film by particles with high hardness, when the film is used as a release film for optical components, the unevenness may be transferred to the resin layer of the optical components. Therefore, when the film is used as a protective film or a manufacturing base film for products that require high quality, such as display components, it may cause a decrease in quality. From the above viewpoint, it is preferable that the polypropylene film of the present invention does not contain organic particles, inorganic particles, etc.

The biaxially oriented polypropylene film of the present invention is preferably obtained by biaxially stretching an unstretched sheet obtained using the above-mentioned resin. As a method of biaxial stretching, any of the inflation simultaneous biaxial stretching method, the tenter simultaneous biaxial stretching method, and the sequential biaxial stretching method using a roll-type stretching machine and a tenter can be used. Among these, it is preferable to adopt the sequential biaxial stretching method using a roll-type stretching machine and a tenter in terms of controlling the film formation stability, thickness uniformity, high rigidity and dimensional stability of the film.

Next, one embodiment of the manufacturing method for the 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.

First, in order to improve the dispersibility of the elastomer resin or other polypropylene resin, it is preferable to melt mix the polypropylene resin and the elastomer resin (or other polypropylene resin) in any ratio as described below, and pelletize the resin composition before use.

The pellets of the resin composition obtained above are fed to a single-screw extruder for the surface layer (I), and polypropylene resin A is fed to 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. Then, after removing foreign matter and modified polymers with a filter installed in the middle of the polymer tube, the layers are laminated in a layer structure of surface layer (I) / base layer (II) / surface layer (I) with a multi-manifold type composite T-die, and 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). At this time, the lamination thickness ratio is preferably in the range of 1/4/1 to 1/60/1 from the viewpoint of the balance between surface properties, mechanical strength, and thermal properties. More preferably, it is 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 it within the above range, the surface layer (I) containing the elastomer resin is formed thinly and uniformly on the film surface, so that the uniformity of the height of the unevenness formed during stretching is increased, and the formation of a coarse uneven structure can be suppressed. In this case, the polypropylene resins A and B may be the same polypropylene resin.

In order 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 such conditions, it becomes easy to adjust the Vmp of the A side to a suitable range. In addition, the layer structure may be a two-layer laminate structure of surface layer (I)/base layer (II). As a method for adhering the molten resin sheet to the casting drum, any method may be used, such as an electrostatic application method, an adhesion method using the surface tension of water, an air knife method, a press roll method, or an underwater casting method, but the air knife method, which has good flatness and allows control of surface roughness, is preferred. From the viewpoint of cooling the non-cooled 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 even more preferably 15 to 20°C, and the blowing air speed is preferably 130 to 150 m/s. By setting such conditions, it becomes easy to adjust the number of craters on the B side 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 in order 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.

The obtained laminated unstretched sheet is introduced into the longitudinal stretching process. In the longitudinal stretching process, it is preferable to perform longitudinal stretching in two stages in the preheating process before stretching. In the first half of the preheating process, it is preferable to heat the laminated unstretched sheet 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 minus 2°C or less, more preferably the softening temperature of the elastomer resin minus 10°C or less, and even more preferably the softening temperature of the elastomer resin minus 20°C or less. In the second half of the preheating process, it is preferable to heat at a temperature equal to or higher than the preheating temperature of the first half of the preheating process, and the heating temperature is more preferably the preheating temperature of the first half of the process + 10°C or more, and even more preferably the preheating temperature of the first half of the process + 20°C or more.

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 process and deformation can be controlled during stretching, resulting in favorable control of the surface structure of the film.

Then, in the stretching process, the laminated unstretched sheet is stretched in the longitudinal direction between rolls with a difference in peripheral speed 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 manufacturing process (the winding direction when wound into a roll). By stretching the film in the longitudinal direction by 4.0 times or more, the mechanical strength in the longitudinal direction is increased, so that deformation of the film is suppressed even under high tension, and the flatness of the biaxially stretched polypropylene film is easily maintained. In addition, by stretching the film in the longitudinal direction by 6.0 times or less, heat shrinkage caused by excessive distortion during stretching is suppressed, and the flatness of the biaxially stretched polypropylene film is easily maintained.

Next, a relaxation treatment is performed in the longitudinal direction, so that a fine uneven structure can be efficiently formed on the film surface. The temperature of the longitudinal relaxation treatment 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 forming a fine uneven surface structure and improving the handleability of the film as a film with good flatness. The relaxation rate in the longitudinal relaxation treatment is preferably 2.0% or more and less than 10%, from the viewpoint of controlling the uneven surface structure and good film transportability, 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%.

The uniaxially stretched film is then guided into a tenter with both widthwise ends held by clips, preheated, and 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 plane of the film.

From the viewpoint of uniformly stretching the film and suppressing the formation of steep protrusion structures, 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.

From the viewpoint of uniform stretching of the film and control of the uneven surface structure, the stretching temperature is preferably at least the softening temperature of the elastomer resin +20°C and at most the melting point of polypropylene resin A. From the viewpoint of stable stretching 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.

In the subsequent heat treatment and relaxation treatment process, the film is uniformly relaxed and shrunk, and from the viewpoint of suppressing the formation of a steep protrusion structure, the film is relaxed in the width direction at a relaxation rate of 5.0 to 20%, more preferably 7.0 to 15%, and even more preferably 9.0 to 12%, while both ends of the film after biaxial stretching are tensely held with clips, and heat set at 140°C to 180°C, more preferably 150°C to 173°C, and even more preferably 150°C to 170°C. Furthermore, while both ends of the film are tensely held with clips, the film is guided to the outside of the tenter through a cooling process at 80 to 100°C, and the clips at both ends of the film width are released. After that, the film edge is slit in the winder process, and the biaxially stretched polypropylene film product roll is wound up.

The biaxially oriented polypropylene film obtained in the above manner can be used for various industrial applications such as packaging films, cover films, process films, sanitary products, agricultural products, construction products, and medical products, but since it has particularly excellent surface smoothness, handling properties, and blocking resistance, it can be preferably used as a surface protection film, support film, and release film.

The biaxially oriented polypropylene film of the present invention has excellent transparency, and therefore, when focusing on the use as a cover film, it can be particularly preferably used as a cover film for photocurable resins and cover films for 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.

A support film is a film used to laminate an object onto a film when it is difficult to form a film from the object alone due to reasons such as the object being thin or fragile.

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 it is time to use the film as a final product.

The biaxially oriented polypropylene film of the present invention has excellent quality and 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. In other words, 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 of the film.

In sputtering and deposition processing, polyethylene terephthalate (PET) films, which have excellent surface smoothness, mechanical strength, heat resistance, and other properties, are often used. However, PET has high hydrophilicity due to its ester bonds, and PET films contain a small amount of moisture. Such a small amount of moisture can have a negative effect on sputtering and deposition processing, and the negative effect is particularly noticeable when depositing metals belonging to Group 1 or Group 2 of the periodic table, which are easily reactive with moisture, or compounds containing these metals. In this context, the biaxially oriented polypropylene film of the present invention can be particularly suitably used when forming a metal film containing a metal belonging to Group 1 or Group 2 of the periodic table or a compound thereof, since a laminate can be obtained suitably even in cases where it is difficult to use a PET film due to the presence of a small amount of moisture. Here, the metals belonging to Group 1 or Group 2 refer to lithium, sodium, potassium, rubidium, cesium, francium, beryllium, magnesium, calcium, strontium, barium, and radium.

In addition, under high temperature conditions, the moisture in the film evaporates as outgassing, and the effect is particularly noticeable under high vacuum conditions such as metal sputtering processes. When outgassing occurs from the film, the degree of vacuum in the system decreases, which may induce deterioration of the film quality of the metal film formed by sputtering or reduce the yield of sputtering. From this perspective, the biaxially oriented polypropylene film of the present invention, which has a low moisture content, can be used more preferably for forming a metal film than a PET film, which has a high moisture content, and can also maintain the quality of the formed metal film in good condition. From the above perspective, 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 substantially 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.

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 resins and additives in the biaxially oriented polypropylene film. Specifically, it is preferable to set the lower limit of the content of polyolefin resins such as polypropylene resins and propylene-based elastomer resins in the total components of the biaxially oriented polypropylene film to 90% by mass, more preferably 95% by mass, and even more preferably 97% by mass. The upper limit of the content of these resins is substantially 100% by mass. Polyolefin resins such as polypropylene resins and propylene-based elastomer resins are relatively hydrophobic, and by increasing the ratio of these, the moisture content of the biaxially oriented polypropylene film can be reduced.

In order to reduce defects caused by bleed-out, the biaxially stretched polypropylene film of the present invention preferably contains additives other than antioxidants (e.g., antistatic agents, viscosity modifiers, color inhibitors, slip agents, etc.) in a total amount of 0 to 500 ppm or less. It is preferable to adjust the amount of antioxidants such as phosphorus-based antioxidants as described above.

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 smoother and easier to handle than existing olefin-based films, so it is also preferably used when forming transparent conductive films that require stricter vacuum conditions and high quality. Here, the transparent conductive film refers to a thin film formed from a material that is conductive and transmits visible light, and specific examples include an indium-tin oxide (ITO) film, a zinc oxide (ZnO) film, and a palladium film.

Furthermore, electrolyte membranes used in fuel cells, semi-solid batteries, all-solid batteries, etc. are usually manufactured 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 manufacturing the electrolyte membrane is required to have an extremely low moisture content. For this reason, the biaxially oriented polypropylene film of the present invention is preferably used for such applications.

The biaxially oriented polypropylene film of the present invention is preferably used as a current collector because of its excellent handling properties. A current collector is a foil-shaped laminate used in electrodes of storage batteries such as lithium ion batteries, and is usually formed by laminating a metal film on a resin film as a base material. This metal film is laminated by processing such as vapor deposition, sputtering, plating, and electroless plating. In addition, from the viewpoint of increasing the energy density of a battery, the film as the base material of the current collector is required to have a small thickness, but as the film becomes thinner, its stiffness decreases, and the handling properties during processing are greatly reduced. In particular, in the process of laminating the metal film, high heat such as radiant heat is applied during processing, and tension is also applied in the conveying direction, so the film is required to have good handling properties. The biaxially oriented polypropylene film of the present invention is preferably used as a current collector because it can be made thin and has good handling properties.

The biaxially oriented polypropylene film of the present invention is preferably used as a current collector because of 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 to electrical energy when needed. Specific examples include lead-acid batteries, nickel-metal hydride batteries, lithium-ion batteries, NAS batteries, and redox flow batteries.

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

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

(2) Peak actual volume (Vmp), valley volume (Vvv), peak density (Spd)
The measurement was performed using a scanning white light interference microscope "VS1540" (manufactured by Hitachi High-Tech Science Corporation; measurement conditions and device configuration will be described later), and measurements of 561.1 μm × 561.5 μm per field were performed by measuring multiple fields of 5 × 5 fields. Then, all images were stitched with a 20% overlap to obtain surface shape data of 2356.716 μm × 2358.294 μm. The captured screen was complemented (completely complemented) using the attached analysis software, surface correction was performed using a polynomial 4th order approximation, and then the surface shape was obtained by processing with a median filter (3 × 3 pixels). The measurement surfaces were both sides of the biaxially stretched polypropylene film, and the measurement positions were determined in a total of nine positions according to the following procedure, starting from the intersection of the diagonals of the biaxially stretched polypropylene film cut into a 5 cm × 5 cm square shape. Measurements were performed at each measurement position, and Vmp, Vvv, and Spd at each measurement position were obtained according to the above procedure, and the average values were adopted as Vmp, Vvv, and Spd. When there was a difference of 0.1 μL/m2 or ^more in Vmp between the two surfaces, the surface with the relatively smaller Vmp was designated as surface A, and the opposite surface as surface B.

<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
Optical tube: 1x
Zoom lens: 1x
Wavelength filter: 530 nm white
Measurement mode: Wave
Measurement software: VS-Measure 10.0.4.0
Analysis software: VS-Viewer 10.0.3.0
Measurement area: 561.1μm x 561.5μm
Number of pixels: 1,024 x 1,024.

(3) Identification of the main orientation direction and the main orientation perpendicular direction When an arbitrary direction in the film plane is set to 0°, the Young's modulus is measured in each direction that forms an angle of 0° to 175° in 5° increments with respect to the arbitrary direction. The direction showing the highest value was the main orientation direction of the film, and the direction perpendicular to this in the film plane was the main orientation perpendicular direction of the film. The Young's modulus was measured according to the following procedure in accordance with JIS K 7161 (2014). First, a rectangular sample with a length (measurement direction) of 150 mm x width of 10 mm was cut out. A tensile test was performed using a tensile tester (Orientec "Tensilon" (registered trademark) UCT-100) 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 chuck distance became 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 straight line passing through the origin and the point used for measuring the F2 value was determined as Young's modulus (MPa).

(4) F5t value Five samples with a width of 10 mm and a length of 50 mm (measurement direction) were cut out from the biaxially stretched polypropylene film so that the measurement direction was the orthogonal direction to the main orientation, and marks were made at positions 15 mm from both ends to make a test length of 20 mm. Next, the rectangular sample was set in a tensile tester (Orientec's "Tensilon" (registered trademark) UCT-100) with an initial chuck distance of 20 mm, heated for 1 minute in an oven heated to 130°C, and then a tensile test of the film was performed at a tensile speed of 300 mm/min. The load applied to the film when the sample was elongated by 5% was read, and the value divided by the cross-sectional area of the sample before the test (film thickness x width (10 mm)) was calculated as the stress (unit: MPa) at an elongation of 5%, and multiplied by the film thickness obtained in (1). The measurement was performed five times for each sample, and the average value was calculated to be the F5t value (unit: Pa·m).

(5) μd of the front and back of the film after heating at 130° C. for 10 minutes
A biaxially stretched 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 kept at 130°C under zero load, and then removed. After cooling the test piece at room temperature, it was measured at room temperature of 25°C and 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 in the directions perpendicular to the main orientation of the biaxially stretched polypropylene film and with different surfaces overlapping each other, that is, with the main orientation perpendicular directions overlapping and the surface of one film contacting the back surface of the other film. The same measurement was performed five times for one sample, and the average value of the obtained values was calculated to be the dynamic friction coefficient (μd) of the sample.

(6) Number of craters with major axis of 150 μm or more Measurement was performed using a scanning white light interference microscope "VS1540" (manufactured by Hitachi High-Tech Science Corporation, measurement conditions and device configuration will be described later), and measurements of 561.1 μm x 561.5 μm per field were performed by multiple field measurements of 5 x 5 fields. Then, all images were stitched with 20% overlap to obtain surface shape data of 2356.716 μm x 2358.294 μm. In the hole analysis of the particle analysis mode of the attached analysis software "VS-Viewer 10.0.3.0", the height threshold was set to -0.1 μm, and the number of dents with a major axis of 150 nm or more was measured. The measurement was performed on the B side of the biaxially stretched polypropylene film, and the measurement position was determined in a total of 9 positions according to the following procedure, starting from the intersection of the diagonal lines of the biaxially stretched polypropylene film cut into a 5 cm x 5 cm square shape. Measurements were performed at each measurement position, and the number of particles at each measurement position was calculated according to the above procedure, 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).

(7) Heat of fusion below 160°C 5 mg of biaxially oriented polypropylene film was collected as a sample in an aluminum pan and measured using a differential scanning calorimeter (Thermoplus EVO2 DSCvesta manufactured by Rigaku). 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 rate at each temperature was corrected with a straight line that was the baseline from 60°C to 250°C, and the heat of fusion ratio below 160°C was calculated by subtracting the heat of fusion from 60°C to 250°C from the heat of fusion from 60°C to 160°C.

(8) Half-crystallization time Using a differential scanning calorimeter (Thermoplus EVO2 DSCvesta manufactured by Rigaku), 3 mg of polypropylene resin was heated from 25°C to 250°C at 20°C/min in a nitrogen atmosphere and held for 5 minutes. Then, the temperature was 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 after 60 seconds in the endothermic curve obtained during isothermal holding at 130°C was defined as the half-crystallization time (seconds).

(9) Weight average molecular weight Mw
The polypropylene resin was dissolved in 1,2,4-trichlorobenzene 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 calculated using a molecular weight calibration curve prepared using the following standard sample.
Apparatus: Agilent high temperature GPC device PL-GPC220
Detector: Agilent refractive index detector (RI detector)
Column: Agilent PL1110-6200 (20 μm MIXED-A) × 2 Flow rate: 1.0 mL / min
Column temperature: 145° C.
・Injection volume: 0.500mL
Sample concentration: 0.1 wt%
-Standard samples: Monodisperse polystyrene manufactured by Tosoh, dibenzyl manufactured by Tokyo Kasei.

(10) Softening temperature The softening temperature was determined according to 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 under a pressure of 10 MPa for 4 minutes to produce a sheet of a predetermined thickness, thereby obtaining a sheet sample. After 72 hours at room temperature after molding, the displacement (penetration depth) was measured when heating at a heating rate of 5°C/min while applying a pressure of 2 kg/ ^cm2 to a 1 mmφ probe using a thermomechanical analyzer TMASS6100 (manufactured by Hitachi High-Tech Science Corporation), and the temperature at which this reached 500 μm was used as the softening temperature.

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

(12) Evaluation of transfer onto adherend A biaxially oriented polypropylene film and 20 μm thick "ZEONORFILM" (registered trademark) manufactured by Zeon Corporation were sampled into a square shape measuring 100 mm in width and 100 mm in length, and the sample was overlapped so that the A side of the biaxially oriented polypropylene film and the "ZEONORFILM" (registered trademark) were in contact with each other. The sample was sandwiched between two acrylic plates (100 mm in width, 100 mm in length), a load of 5 kg was applied, and the sample was allowed 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.

(13) Planarity during film heating A 500 mm wide polypropylene film was subjected to corona treatment on the side with a low Vmp value, and the treated surface was 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 200N to remove the solvent in the coating agent, thereby obtaining an adhesive film having an adhesive layer with a thickness of 1 μm. This adhesive film was then wound into a roll with a winding length of 200 m to obtain an adhesive film roll. Next, the 500 mm wide adhesive film was unwound by 1 m, and tensions of 1 kg/m and 3 kg/m were applied uniformly and evenly across the entire film width to visually check for the presence or absence of flatness defects such as wrinkles and dents. Using the obtained values, flatness evaluation was performed according to the following criteria.
S: There was no poor flatness with 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 at a tension of 1 kg/m width, but no poor flatness was observed at a tension of 3 kg/m width.
C: Even at a tension of 3 kg/m width, some areas were found to be poorly flat.

(14) Blocking Resistance The biaxially oriented polypropylene film was cut to a size of 60 mm x 210 mm, and five sheets were stacked on an iron plate so that the A side and the B side of the film were in contact with each other. 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 at 120°C for 15 minutes. After being removed from the oven, the films were left to stand and gradually cooled to room temperature. Then, the stacked 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 were in close contact with each other, but could be peeled off without rupturing the films.
C: Peeling was impossible, or the film broke during peeling.

(15) Yield evaluation during metal film formation A metal film made of magnesium oxide was formed on one surface of a biaxially oriented polypropylene film or a PET film by sputtering. A magnetron sputtering device was used for sputtering, and after reducing the pressure from atmospheric pressure to 1×10 ⁻⁵ Torr, argon gas was introduced and sputtering was performed at 2×10 ⁻³ Torr to form a magnesium oxide film on the surface of the biaxially oriented polypropylene film or the PET film. At that time, the time it took to reduce the pressure from atmospheric pressure to 1×10 ⁻⁵ Torr was measured, and the unevenness of the metal film surface was visually observed to evaluate the yield according to the following criteria. The evaluation criteria were A as pass and B as fail.
A: The time required for reducing the pressure from atmospheric pressure to 1×10 ⁻⁵ Torr was 50 minutes or less, and no irregularities were observed on the surface of the metal film.
B: The time required to reduce the pressure from atmospheric pressure to 1×10 ⁻⁵ Torr was longer than 50 minutes, or irregularities were observed on the surface of the metal film.

(16) Moisture Content A biaxially oriented polypropylene film or PET film sample was left for 4 hours or more in a room conditioned at 23° C. and 20% relative humidity, and then immersed for 24 hours in distilled water at 23° C. Thereafter, 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 quantified by the Karl Fischer method to calculate the moisture content.

(Polypropylene resin, etc.)
The resins used in the manufacture of the biaxially stretched polypropylene films of the Examples and Comparative Examples are shown in Tables 1 and 2 below. These characteristic values are values 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, a resin composition for the surface layer (I) was used that was obtained by melt-kneading a polypropylene resin and an elastomer resin in advance so as to have the mass ratio shown in Table 3. All of PP1 to PP7 contain tetrakis[methylene-3(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane as an antioxidant in the range of 1000 ppm to 5000 ppm, and only PP7 further contains 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 Co., Ltd. Polypropylene resin 3 (PP3): manufactured by Prime Polymer Co., Ltd. Polypropylene resin 4 (PP4): manufactured by Japan Polypropylene Co., Ltd. Polypropylene resin 5 (PP5): manufactured by Idemitsu Kosan Co., Ltd. Polypropylene resin 6 (PP6): manufactured by Japan Polypropylene Co., Ltd., "WAYMAX" (registered trademark) MFX6
Polypropylene resin 7 (PP7): "WINTEC" (registered trademark) WFW4M, manufactured by Japan Polypropylene Corporation.

Elastomer resin 1 (ELA1): "Toughmer" (registered trademark) PN2060, manufactured by Mitsui Chemicals, Inc.
Elastomer resin 2 (ELA2): "Toughmer" (registered trademark) BL3450, manufactured by Mitsui Chemicals, Inc.
Elastomer resin 3 (ELA3): "Toughmer" (registered trademark) PN2070, manufactured by Mitsui Chemicals, Inc.
Elastomer resin 4 (ELA4): "Wintec" (registered trademark) WMG30, manufactured by Japan Polypropylene Corporation
Elastomer resin 5 (ELA5): "Vistamaxx" (registered trademark) 3020FL, manufactured by Exxon Mobil
Elastomer resin 6 (ELA6): "Welnex" (registered trademark) RFX4V, manufactured by Japan Polypropylene Corporation.

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

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 discharged from a die in the form of a strand. The discharged resin composition was cooled and solidified in a water tank 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 sintered filter with a 20 μm cut, the mixture 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 obtained molten sheet-like material was discharged onto a casting drum whose surface temperature was controlled at 20 ° C., and was adhered to the casting drum by an air knife. Thereafter, the sheet on the casting drum was cooled by injecting compressed air at 15 ° C. with an air knife at an air speed of 140 m / s onto the non-cooled drum surface of the sheet on the casting drum, to obtain an unstretched sheet. Subsequently, the unstretched sheet was contacted with a metal roll at 95 ° C. in the first half of the preheating process, and was 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 with a metal roll heated to 95 ° C., to obtain a uniaxially stretched film. Next, the obtained uniaxially stretched film was introduced into a tenter-type stretching machine with both ends in the width direction held by clips, preheated at 137°C for 2 seconds, stretched 9.5 times in the width direction at 152°C, and heat-treated at 155°C while giving 10% relaxation in the width direction. After a cooling process at 100°C, the film was guided to the outside of the tenter, the clips at both ends in the width direction of the film were released, and the film was wound up on a core to obtain a biaxially stretched polypropylene film with a thickness of 25 μm. The physical properties and evaluation results of the obtained film are shown in Table 3.

(Examples 2 to 5, Comparative Examples 1 to 4)
A biaxially stretched 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. In Examples 2 and 3, a resin composition obtained by melt-kneading a PP resin and an elastomer resin in advance in the same manner as in Example 1 was used for the resin composition for the surface layer (I) so that the mass ratio was as shown in Table 3. The thickness was adjusted by adjusting the discharge amount during extrusion and the speed of the casting drum. The physical properties and evaluation results of the obtained biaxially stretched polypropylene film are shown in Table 3.

(Comparative Example 5)
The raw material composition and film-forming conditions of each layer are as shown in Table 3. 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 rolls at 90°C with a difference in peripheral speed. The film after the initial stretching was then preheated to 140°C, and the second stage of longitudinal stretching was performed at a stretching ratio of 3.5 times. Except for this, a biaxially stretched polypropylene film was obtained in the same manner as in Example 1. At this time, the thickness was adjusted by adjusting the discharge amount during extrusion and the speed of the casting drum. The physical properties and evaluation results of the obtained biaxially stretched polypropylene film are shown in Table 3.

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

In the table, in Comparative Examples 1, 2, and 4, even the surface with a relatively small Vmp did not reach 50 μL/ ^m2 or less, so there is no A-side, but the value of the surface with a relatively small Vmp is recorded as the A-side Vmp. Note that the Vmp of both sides was different in all examples. In addition, the moisture content and the yield during metal film production were evaluated only for Example 1, Comparative Examples 1, and 6.

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 and a release film or support film for dry film resists.

1: Peak actual volume Vmp
2: Load curve with the vertical axis being height and the horizontal axis being 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 (9)

A biaxially oriented polypropylene film, characterized in that ^one side is side A, and a layer containing a polypropylene resin having a melting point of 160°C or more and 175° ^C or less as a main component and containing an elastomer resin is located as an outermost layer on at least one side, when a side A is defined as a side having a relatively small peak actual volume Vmp and being 0.1 μL/m2 or more and 50 μL/m2 or less, 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. The biaxially oriented polypropylene film according to claim 1, in which the dynamic friction coefficient μd between the front and back of the film after heating at 130°C for 10 minutes is 0.30 or more and 1.40 or less. The biaxially oriented polypropylene film according to any one of claims 1 to ² , wherein the number of craters having a major axis exceeding 100 μm per 1 mm2 of the B side is 0.001 pieces/ ^mm2 or more and 30 pieces/ ^mm2 or less when the surface opposite the A side is the B side. The biaxially oriented polypropylene film according to any one of claims 1 to 2, wherein the peak density Spd of the A-side is 1.0 peaks/ ^mm2 or more and 100 peaks/ ^mm2 or less. The biaxially oriented polypropylene film according to any one of claims 1 to 2, wherein the valley volume Vvv of the A side is 2.0 μL/m ² or more and 20 μL/m ² or less. The biaxially stretched polypropylene film according to any one of claims 1 to 2, in which the heat of fusion at 160°C or less accounts for 5% to 50% of the total heat of fusion when heated from 30°C to 260°C using a differential scanning calorimeter (DSC). The biaxially oriented polypropylene film according to any one of claims 1 to 2 , wherein the softening temperature of the elastomer resin is 70°C or higher and 150°C or lower. The biaxially oriented polypropylene film according to any one of claims 1 to 2, used for release film applications. The biaxially oriented polypropylene film according to any one of claims 1 to 2, used for a support film.

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