Biaxially oriented polypropylene film

Abstract

To provide a biaxially oriented polypropylene film that exhibits superior releasability and processability under high-temperature conditions, and that is suitably usable as a release film subjected to processing under high temperature.SOLUTION: A biaxially oriented polypropylene film is characterized in that a surface whose minimum autocorrelation length (Sal) is from 5 μm to 45 μm inclusive is designated as surface A, a direction in which the thermal shrinkage ratio at 150°C is maximum is designated as direction X, a direction orthogonal to the direction X in the film plane is designated as direction Y, the thermal shrinkage ratio at 150°C in the direction X is designated as Xs, the thermal shrinkage ratio at 150°C in the direction Y is designated as Ys, and a temperature at which shrinkage in the direction X reaches 0.04% is designated as XsT and a temperature at which shrinkage in the direction Y reaches 0.20% is designated as YsT in a heating process of thermomechanical analysis (TMA), wherein at least one surface is the surface A, the Xs and Ys satisfy Formula 1, XsT is in the range of 130°C to 160°C inclusive, and YsT is in the range of 140°C to 170°C inclusive. Formula 1: -10%≤Ys≤Xs≤10%.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a biaxially oriented polypropylene film having excellent releasability and processability in a high-temperature environment, and a release film using the same. [Background technology]

[0002] Polypropylene films are excellent in transparency and electrical properties, and are therefore used in a variety of applications, including packaging, tapes, cable wrapping, capacitors, and other electrical applications. Furthermore, polypropylene films are also excellent in mechanical properties and releasability, and are therefore particularly suitable for use as release films for various components, such as plastic products, building materials, and optical components.

[0003] Examples of uses for release films include supports for coated materials and melt-molding, spacers for press molding, etc. The properties required for release films are determined appropriately depending on the intended use, but heat resistance in particular has become important because temperatures during heating processes and press molding have been rising year by year as materials have become more highly functional and productivity has improved.

[0004] In the past, polyethylene terephthalate (PET) films, which have excellent heat resistance, have sometimes been used as release films at temperatures above 150°C. However, achieving both high releasability and high heat resistance has been difficult, and the required properties have not always been met. On the other hand, while conventional polypropylene films have high releasability, they begin to shrink significantly at temperatures around 120–130°C and begin to melt at around 160°C, near their melting point, significantly reducing their rigidity. Therefore, when conventional polypropylene films are used as release films, the quality of mating components made of thermoplastic or thermosetting resin compositions, which require high-temperature drying or molding, can be impaired. Therefore, it has been extremely difficult to use polypropylene films as release films at temperatures above 150°C, especially above 160°C.

[0005] Important indicators of the heat resistance of polypropylene films include heat shrinkage properties and rigidity at high temperatures. To improve heat shrinkage properties, i.e., to reduce heat shrinkage stress, a common method is to reduce the molecular weight of the polypropylene raw material and relax the orientation and residual strain developed by stretching through relaxation treatment or heat setting. However, this method tends to reduce the rigidity of the polypropylene film because it reduces the structure that contributes to mechanical properties such as elastic modulus. On the other hand, to increase the rigidity of polypropylene films, a method is used in which the molecular weight of the raw polypropylene resin is increased and the degree of molecular orientation and tension in the amorphous portion is increased through low-temperature stretching or high-magnification stretching, thereby increasing the elastic modulus from the room temperature state. However, this method has the problem of increasing the structure that is easily relaxed at high temperatures, resulting in high heat shrinkage stress in the polypropylene film. As such, it has been extremely difficult to achieve both high-temperature heat shrinkage properties and rigidity in polypropylene films using conventional techniques.

[0006] Patent Documents 1 to 6 describe biaxially oriented polypropylene films that use a high-melting-point polypropylene resin and are stretched at a high ratio in a transverse stretching process to form a dense, rough surface, thereby improving heat resistance, dimensional stability, and releasability. Furthermore, Patent Document 4 describes an example in which a low-melting-point polypropylene resin is used in the surface layer to reduce the surface elastic modulus, thereby making it difficult for dents from surface irregularities to be transferred to the adhesive resin layer and improving releasability. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 2023-118131 [Patent Document 2] Japanese Patent Application Publication No. 2023-95728 [Patent Document 3] Japanese Patent Publication No. 2022-190664 [Patent Document 4] Patent Publication No. 2021-28394 [Patent Document 5] Japanese Patent Publication No. 2020-132877 [Patent Document 6] International Publication No. 2019 / 044758 Summary of the Invention [Problem to be solved by the invention]

[0008] However, the methods described in Patent Documents 1 to 6 above result in biaxially oriented polypropylene films with insufficient heat resistance and dimensional stability, and wrinkles and fusion due to shrinkage occur in a high-temperature atmosphere. Therefore, when such biaxially oriented polypropylene films are used as release films, there are problems such as wrinkles being transferred to the mating member to which they are bonded in a high-temperature environment, and the polypropylene resin fusion with the mating member deteriorating the release properties.

[0009] More specifically, the method described in Patent Document 1 uses a polypropylene resin with low stereoregularity, and therefore, when used as a release film for components that require processing at high temperatures, fusion and wrinkling occur, resulting in insufficient releasability and processability. The methods described in Patent Documents 2 and 3 have issues with poor dimensional stability during heating due to different relaxation conditions after transverse stretching, resulting in proneness to wrinkling and poor processability. The method described in Patent Document 4 has insufficient releasability because the low-melting-point polypropylene resin in the surface layer is prone to fusion with the mating component. The methods described in Patent Documents 5 and 6 have problems with processability because the film itself is thin and therefore lacks stiffness, prone to wrinkling, and prone to processability.

[0010] Therefore, an object of the present invention is to provide a biaxially oriented polypropylene film that has excellent releasability and processability in high-temperature environments and can be suitably used as a release film that involves processing at high temperatures. [Means for solving the problem]

[0011] In order to solve the above-mentioned problems, the biaxially oriented polypropylene film of the present invention has the following configuration: That is, the biaxially oriented polypropylene film of the present invention is a biaxially oriented polypropylene film in which, when the surface having a minimum autocorrelation length (Sal) of 5 μm to 45 μm is called surface A, the direction with the largest 150°C heat shrinkage rate is called X direction, the direction perpendicular to the X direction in the film plane is called Y direction, the 150°C heat shrinkage rate in the X direction is called Xs, the 150°C heat shrinkage rate in the Y direction is called Ys, and during the temperature rise process of thermomechanical analysis (TMA), XsT is the temperature at which the film shrinks by 0.04% in the X direction and YsT is the temperature at which the film shrinks by 0.20% in the Y direction, at least one surface is surface A, and Xs and Ys satisfy the following formula 1, XsT is 130°C to 160°C and YsT is 140°C to 170°C: Equation 1: -10%≦Ys≦Xs≦10%.

[0012] The biaxially oriented polypropylene film of the present invention can also be in the following embodiments and can also be suitably used as a release film. (1) A biaxially oriented polypropylene film in which the surface having a minimum autocorrelation length (Sal) of 5 μm or more and 45 μm or less is the A-side, the direction having the largest heat shrinkage at 150°C is the X-direction, the direction perpendicular to the X-direction in the film plane is the Y-direction, the 150°C heat shrinkage in the X-direction is Xs, the 150°C heat shrinkage in the Y-direction is Ys, and during the temperature rise process of thermomechanical analysis (TMA), XsT is the temperature at which the film shrinks by 0.04% in the X-direction, and YsT is the temperature at which the film shrinks by 0.20% in the Y-direction. At least one surface of the biaxially oriented polypropylene film is the A-side, and Xs and Ys satisfy the following formula 1, in which XsT is 130°C or more and 160°C or less, and YsT is 140°C or more and 170°C or less. Formula 1: -10%≦Ys≦Xs≦10% (2) The biaxially oriented polypropylene film according to (1), wherein the surface property aspect ratio (Str) of the A-side is 0.10 or more and 0.54 or less. (3) A biaxially oriented polypropylene film according to (1) or (2), in which the sum of the heats of fusion (ΔH) in the range of 175°C to 200°C measured by a differential scanning calorimeter (DSC) is 20 J / g or more and 50 J / g or less. (4) The biaxially oriented polypropylene film according to (1) or (2), wherein the tensile elongation in the X direction is 200% or more and 400% or less, and the tensile strength in the X direction divided by the tensile elongation in the X direction is 0.05 MPa / % or more and 1.00 MPa / % or less. (5) A biaxially oriented polypropylene film according to (1) or (2), in which the static friction coefficient (μs) and the dynamic friction coefficient (μd) when different surfaces are in contact with each other are both 0.55 or more and 1.00 or less. (6) A release film made using the biaxially oriented polypropylene film according to (1) or (2). [Effects of the Invention]

[0013] According to the present invention, a biaxially oriented polypropylene film can be provided which has excellent releasability and processability in a high-temperature environment and can be suitably used as a release film that involves processing at high temperatures. DETAILED DESCRIPTION OF THE INVENTION

[0014] The biaxially oriented polypropylene film of the present invention is characterized in that, when the surface having a minimum autocorrelation length (Sal) of 5 μm to 45 μm is called surface A, the direction with the greatest 150°C heat shrinkage is called X direction, the direction perpendicular to the X direction in the film plane is called Y direction, the 150°C heat shrinkage in the X direction is called Xs, the 150°C heat shrinkage in the Y direction is called Ys, and during the temperature rise process of thermomechanical analysis (TMA), XsT is the temperature at which the film shrinks by 0.04% in the X direction and YsT is the temperature at which the film shrinks by 0.20% in the Y direction, at least one surface is surface A, Xs and Ys satisfy formula 1, XsT is 130°C to 160°C, and YsT is 140°C to 170°C. The biaxially oriented polypropylene film of the present invention will be specifically described below. Equation 1: -10%≦Ys≦Xs≦10%.

[0015] In the present invention, a polypropylene film refers to a sheet-like molded product whose main component is polypropylene resin. The main component refers to a component that accounts for more than 50% by mass but not more than 100% by mass when the total components of the object (here, the film) are taken as 100% by mass, and the same interpretation can be used hereinafter for the main component. Note that when multiple components that fall under the category of polypropylene resin are contained, if the total amount is greater than 50% by mass, the product is considered to be mainly composed of polypropylene resin.

[0016] Polypropylene resin refers to a resin whose main structural unit is propylene, and a main structural unit refers to a structural unit that is present in an amount of more than 50 mol% but not more than 100 mol% when all structural units are taken as 100 mol%. (This definition can be interpreted similarly for other olefin resins, except that propylene units are replaced with other olefin units.) Biaxial orientation refers to molecular orientation in two perpendicular directions, and this characteristic can be achieved by stretching an unstretched polypropylene sheet in two perpendicular directions (usually the longitudinal direction and the width direction) within the film plane. Whether a film has biaxial orientation can be determined, for example, by measuring the film's planar orientation coefficient.

[0017] From the viewpoint of suitable use as a release film, it is important that at least one side of the biaxially oriented polypropylene film of the present invention is side A, where side A is the side having a minimum autocorrelation length (Sal) of 5 μm or more and 45 μm or less. Herein, "at least one side is side A" means both an embodiment in which one side is side A and an embodiment in which both sides are side A. From the viewpoint of suitable use as a release film, it is sufficient that at least one side of the biaxially oriented polypropylene film of the present invention is side A, and it is not necessarily essential that both sides simultaneously satisfy this requirement. However, from the viewpoint of mass productivity of the mating member, it is preferable that both sides of the biaxially oriented polypropylene film are side A. Hereinafter, the minimum autocorrelation length (Sal) may be simply referred to as "Sal".

[0018] If both sides of the biaxially oriented polypropylene film are side A, when it is used as a release film, a counter member can be laminated on both sides of the biaxially oriented polypropylene film, improving the mass productivity of the counter member. Here, the counter member refers to a material to be protected by the release film or a material to be manufactured using the release film as a support, and it is preferable that the counter member is formed on side A of the release film.

[0019] Sal is one of the three-dimensional parameters of surface texture, and is a parameter that serves as an index of the period of the surface texture in the in-plane direction. More specifically, Sal is the horizontal distance in the direction in which the autocorrelation function defined in ISO 25178-2 (2012) decays most rapidly to a specific value s (in the present invention, s = 0.2). A small Sal value indicates that the surface has a finer (short-period) uneven shape in the in-plane direction, while a large Sal value indicates that the surface has a coarser (long-period) uneven shape in the in-plane direction.

[0020] By making the Sal of at least one surface 5 μm or more and 45 μm or less, in other words, by making at least one surface an A-side, excellent releasability and windability can be achieved when the A-side of the biaxially oriented polypropylene film is bonded to a mating member so that it is in contact with the mating member.

[0021] From the above viewpoints, Sal on side A is 5 μm or more and 45 μm or less, and preferably 15 μm or more and 35 μm or less. Having a Sal of 5 μm or more and 45 μm or less on the surface of a biaxially oriented polypropylene film means that the surface has excellent releasability when bonded to various mating members. More specifically, by setting Sal to 5 μm or more and 45 μm or less, it is possible to create an appropriate gap between the protrusions on the surface of the biaxially oriented polypropylene film and the mating member, and to reduce the bite of the protrusions into the mating member, thereby improving the releasability.

[0022] When a biaxially oriented polypropylene film without a surface having a Sal within the above range is used as a release film, various problems arise in terms of releasability. If the Sal on both sides is less than 5 μm, the protrusion size on the biaxially oriented polypropylene film surface becomes excessively small, resulting in excessively small voids when the biaxially oriented polypropylene film is laminated to a mating component. This increases heat transfer during processing in high-temperature environments. For example, during press molding at high temperatures, the biaxially oriented polypropylene film may fuse to the mating component, resulting in poor release properties. Furthermore, if the Sal on both sides is less than 5 μm, air removal may be poor when the biaxially oriented polypropylene film is wound into a roll, resulting in poor winding properties. Furthermore, it is difficult to achieve a Sal of less than 5 μm simply by adjusting the temperature conditions during film formation or the physical properties of the resin raw materials, such as polypropylene resin. To achieve a Sal of less than 5 μm, for example, fine particles must be added to the biaxially oriented polypropylene film. The lower limit of Sal is set at 5 μm because fine particles may fall off and contaminate the film-forming process. On the other hand, if the Sal on both sides exceeds 45 μm, the surface protrusions become excessively large, increasing the contact area when the sheet is bonded to a mating component, which can result in the surfaces of both sheets fusing together or the large surface protrusions digging into the mating component, resulting in poor releasability and workability.

[0023] Sal and the aspect ratio Str of the surface texture, which will be described later, can be measured using a known layer cross-sectional shape measuring device, such as the non-contact surface / layer cross-sectional shape measuring system "VertScan" (registered trademark) 2.0 manufactured by Ryoka Systems Co., Ltd. Specific methods for measuring these parameters using this device will be described later.

[0024] In the biaxially oriented polypropylene film of the present invention, the direction with the greatest heat shrinkage at 150°C is the X direction, and the direction perpendicular to the X direction in the film plane is the Y direction. When producing a biaxially oriented polypropylene film, a non-oriented polypropylene sheet is stretched in two perpendicular directions. When stretching in this manner, the direction with the greater stretching ratio is usually the X direction, and the other stretching direction is the Y direction. However, when stretching in the longitudinal direction and the width direction, either the X direction or the Y direction may be used. Here, the longitudinal direction refers to the direction in which the film runs during the film production process, and when the film is rolled, it refers to the winding direction. The direction perpendicular to the longitudinal direction in the film plane is called the width direction.

[0025] The 150°C heat shrinkage rate, including Xs and Ys described later, can be measured as the heat shrinkage rate after leaving the sample in a 150°C oven for 15 minutes under a load of 1.5 g, and the details of the measurement method will be described later.

[0026] From the viewpoint of reducing wrinkling and folding caused by heating and improving processability in high-temperature environments, it is important that the biaxially oriented polypropylene film of the present invention satisfy the following formula 1, where Xs is the heat shrinkage rate at 150°C in the X direction and Ys is the heat shrinkage rate at 150°C in the Y direction. Xs and Ys are indicators of dimensional stability during heating, and small values ​​of these indicate that the dimensions are less likely to change during heating. A biaxially oriented polypropylene film with small Xs and Ys can reduce the occurrence of wrinkling and folding caused by deformation when it is laminated to a counterpart member in a processing step involving heating. From the above viewpoint, it is more preferable that -8.5%≦Ys≦Xs≦8.5% be satisfied. Equation 1: -10%≦Ys≦Xs≦10%.

[0027] In the biaxially oriented polypropylene film of the present invention, during the temperature rise process of thermomechanical analysis (TMA), when the temperature at which the film shrinks by 0.04% in the X direction is defined as XsT and the temperature at which the film shrinks by 0.20% in the Y direction is defined as YsT, it is important that XsT is 130°C or higher and 160°C or lower, and YsT is 140°C or higher and 170°C or lower.

[0028] By using a biaxially oriented polypropylene film of this type, it is possible to reduce the occurrence of wrinkles and folds due to fusion or deformation between the two components when they are laminated together in a high-temperature environment. Therefore, a biaxially laminated polypropylene film of this type can be suitably used as a release film for manufacturing or protecting components that undergo high-temperature processing or press molding. From the above perspective, XsT is preferably 130°C or higher and 160°C or lower, and 134°C or higher and 155°C or lower. From the same perspective, YsT is preferably 140°C or higher and 170°C or lower, and 150°C or higher and 165°C or lower.

[0029] During the temperature rise process of thermomechanical analysis (TMA), the temperature XsT at which the film shrinks by 0.04% in the X direction and the temperature YsT at which the film shrinks by 0.20% in the Y direction are indicators of the heat resistance required of a release film, more specifically, an indicator of dimensional stability in a high-temperature environment. A high XsT or YsT for a biaxially oriented polypropylene film means that it is less likely to melt or shrink at high temperatures, in other words, it is less likely to break or wrinkle due to fusion with the mating member or deformation during processing at high temperatures.

[0030] If XsT is less than 130°C and / or YsT is less than 140°C, the biaxially oriented polypropylene film will melt and shrink at low temperatures, making it difficult to use as a release film for manufacturing or protecting components that require high-temperature processing. More specifically, if XsT is less than 130°C and / or YsT is less than 140°C, the biaxially oriented polypropylene film may fuse to the mating component when processed at temperatures above 150°C, or may undergo dimensional changes that result in wrinkles or creases, causing severe dents or molding defects in the mating component. Furthermore, because the glass transition point of polypropylene resin is below room temperature, if XsT is less than 130°C and / or YsT is less than 140°C, the biaxially oriented polypropylene film will undergo significant dimensional change over time after being wound into a roll, which may result in wrinkles, sagging, blocking, etc., of the biaxially oriented polypropylene film itself.

[0031] On the other hand, there is no particular problem if XsT exceeds 160°C and / or YsT exceeds 170°C, but to increase XsT to 160°C and YsT to temperatures higher than 170°C, it is necessary to use a polypropylene resin with higher crystallinity and to increase the areal stretch ratio in the film-forming process. Therefore, from the perspective of reducing breakage and the like in the film-forming process and improving mass productivity, the upper limits of XsT and YsT are set to 160°C and 170°C, respectively.

[0032] The XsT and YsT of a biaxially oriented polypropylene film can be measured using a known thermomechanical analyzer, such as the "TMA / SS 6100" manufactured by Seiko Instruments Inc. Specific methods for measuring these parameters using this apparatus will be described later.

[0033] As described above, in the biaxially oriented polypropylene film of the present invention, the surface having a minimum autocorrelation length (Sal) of 5 μm to 45 μm is called surface A, the direction with the greatest 150°C heat shrinkage is called the X direction, the 150°C heat shrinkage in the X direction is called Xs, the 150°C heat shrinkage in the Y direction is called Ys, and during the heating process of thermomechanical analysis (TMA), XsT is the temperature at which the film shrinks 0.04% in the X direction and YsT is the temperature at which the film shrinks 0.20% in the Y direction. When at least one surface is surface A, and Xs and Ys satisfy the following formula 1, XsT is 130°C to 160°C and YsT is 140°C to 170°C, adhesion to mating members and dimensional changes under high-temperature conditions can be reduced. Therefore, the film can be suitably used as a release film. Equation 1: -10%≦Ys≦Xs≦10%.

[0034] In order to set Sal, XsT, and YsT of Side A within the above ranges and satisfy the above formula 1, it is effective to set the raw material composition and film-forming conditions of the biaxially oriented polypropylene film within the ranges described below. More specifically, a resin composition containing a polypropylene resin as the main component and a preferred amount of a branched-chain polypropylene resin described below is used to form the surface layer or both the surface layer and the base layer (the film itself in the case of a monolayer film), and the surface temperature of the casting drum is set within the preferred ranges described below. Then, the film temperature (preheating temperature), stretch ratio, and stretch relaxation rate (relaxation rate) during stretching in the longitudinal and width directions are set within the ranges described below, thereby controlling Sal, XsT, and YsT within the preferred ranges and satisfying the above formula 1.

[0035] In the biaxially oriented polypropylene film of the present invention, from the viewpoints of reducing damage to a mating member when used as a release film, and of releasability and heat resistance, the surface texture aspect ratio (Str) of side A is preferably 0.10 to 0.54, more preferably 0.15 to 0.50, even more preferably 0.20 to 0.45, and particularly preferably 0.20 to 0.37. Hereinafter, the surface texture aspect ratio (Str) may be simply referred to as Str. When both sides are side A, if the Str of at least one side is 0.10 to 0.54, then the Str of side A is considered to be 0.10 to 0.54.

[0036] Str is one of the three-dimensional parameters that represent the surface structure and is used as an index of the isotropy of the surface structure. More specifically, Str is the value obtained by dividing the horizontal distance (corresponding to the aforementioned Sal) in the direction in which the autocorrelation function defined in ISO25178-2 (2012) decays most rapidly to a specific value s (s = 0.2 in the present invention) by the horizontal distance in the direction in which it decays most slowly to the value s. That is, the Str of a biaxially oriented polypropylene film is theoretically a value between 0 and 1, and an Str close to 0 means that the biaxially oriented polypropylene film has a surface structure with a highly anisotropic, regular pattern, while an Str close to 1 means that the biaxially oriented polypropylene film has a surface structure with a highly isotropic, random pattern.

[0037] By having an Str of 0.10 or more on the A-side of the biaxially oriented polypropylene film, a surface structure with a regular pattern and moderate anisotropy can be formed on that side. Therefore, when using the biaxially oriented polypropylene film as a release film, when the A-side is laminated to a mating member, a moderate gap can be created between the surface protrusions and the mating member. As a result, heat is less likely to be transferred to the biaxially oriented polypropylene film, making it less likely to fuse with the mating member and improving releasability. On the other hand, by having an Str of 0.54 or less on the A-side, a surface structure with a uniform pattern and moderate randomness can be formed on that side. Therefore, when the A-side of the biaxially oriented polypropylene film is laminated to a mating member, the two come into more uniform contact, suppressing dimensional changes such as localized sagging and wrinkling due to heat or room temperature shrinkage and the resulting deterioration of processability.

[0038] To achieve the Str of Side A of 0.10 to 0.54 or the preferred range described above, it is effective to set the raw material composition of the film within the ranges described below and the film-forming conditions within the ranges described below. More specifically, the same methods as those for controlling Sal, XsT, and YsT within the preferred ranges can be used.

[0039] From the viewpoint of achieving both heat resistance and productivity, the biaxially oriented polypropylene film of the present invention preferably has a total heat of fusion (ΔH) in the 175°C to 200°C range measured by differential scanning calorimetry (DSC) of 20 J / g to 50 J / g, more preferably 23 J / g to 48 J / g, and even more preferably 23 J / g to 40 J / g. Hereinafter, the "total heat of fusion (ΔH) in the 175°C to 200°C range" may be simply referred to as "ΔH".

[0040] ΔH is an index that indicates the total heat of fusion of crystals that melt at temperatures between 175°C and 200°C, which are present in extremely small proportions in conventional biaxially oriented polypropylene films.A high ΔH indicates that the biaxially oriented polypropylene film has excellent heat resistance, or more specifically, that it is difficult to melt even at high temperatures.

[0041] ΔH can be measured by a known differential scanning calorimeter, and the measuring device that can be used is, for example, "EXSTAR DSC6220" manufactured by Seiko Instruments Inc. A specific method for measuring ΔH using this device will be described later.

[0042] When a biaxially oriented polypropylene film has a ΔH of 20 J / g or more, many crystals remain unmelted even in high-temperature environments of 160°C or higher, where conventional biaxially oriented polypropylene films cannot maintain their crystalline structure. Therefore, such biaxially oriented polypropylene films are likely to achieve excellent heat resistance and can be suitably used as release films even in the above-mentioned high-temperature environments. On the other hand, to achieve a ΔH of 50 J / g or more in a biaxially oriented polypropylene film, it is important to use a polypropylene resin with high crystallinity and to increase the areal stretch ratio in the film-forming process. Therefore, from the viewpoint of reducing breakage and other problems during the film-forming process and improving mass productivity, the upper limit of ΔH is preferably 50 J / g.

[0043] To achieve a ΔH of 20 J / g or more and 50 J / g or less, or a value within the above-mentioned preferred range, it is effective to set the raw material composition and film-forming conditions of the biaxially oriented polypropylene film within the ranges described below. In particular, it is effective to use a resin composition containing a preferred amount of branched polypropylene resin described below to form the surface layer or both the surface layer and the base layer (or the film itself in the case of a monolayer film), and to set the film temperature (preheating temperature), stretch ratio, and stretch relaxation rate during stretching in the longitudinal and width directions within the ranges described below.

[0044] From the viewpoints of heat resistance and transportability, the biaxially oriented polypropylene film of the present invention preferably has a tensile elongation in the X direction of 200% to 400%, more preferably 230% to 370%, and even more preferably 235% to 370%. From the same viewpoint, the value obtained by dividing the tensile strength in the X direction by the tensile elongation in the X direction is preferably 0.05 MPa / % to 1.00 MPa / %, more preferably 0.10 MPa / % to 0.70 MPa / %. Hereinafter, "tensile elongation in the X direction" may be referred to as "tensile elongation," and "the value obtained by dividing the tensile strength in the X direction by the tensile elongation in the X direction" may be referred to as "strength-elongation ratio."

[0045] The tensile strength and tensile elongation can be measured using a known tensile tester, and an example of a measuring device that can be used is the automatic film strength and elongation measuring device "Tensilon" (registered trademark) AMF / RTA-100 manufactured by A&D Co., Ltd. A specific method for measuring the tensile strength and elongation using this device will be described later.

[0046] A biaxially oriented polypropylene film with a tensile elongation of 200% to 400% and a strength-strain ratio of 0.05 MPa / % to 1.00 MPa / % indicates high flow characteristics in the molten state during the manufacturing process. Therefore, such biaxially oriented polypropylene films have reduced thickness unevenness and film breakage during film formation, improving windability. Furthermore, when rolled, the film is less likely to experience localized dimensional changes, maintaining high dimensional stability at room temperature.

[0047] To achieve the tensile elongation and strength-strain ratio within the above-mentioned preferred ranges, it is effective to set the raw material composition and film-forming conditions of the biaxially oriented polypropylene film within the ranges described below. In particular, it is effective to use a resin composition containing a preferred amount of branched polypropylene resin described below to form the surface layer or both the surface layer and the base layer (or the film itself in the case of a monolayer film), and to set the film temperature (preheating temperature), stretch ratio, and stretch relaxation rate during stretching in the longitudinal and transverse directions within the ranges described below.

[0048] In order to reduce defects such as meandering and wrinkles during transport, winding, and unwinding, the biaxially oriented polypropylene film of the present invention preferably has a static friction coefficient (μs) and a kinetic friction coefficient (μd) of 0.55 to 1.00 when the two opposite surfaces are in contact with each other, more preferably at least one of which is 0.65 to 0.90, and even more preferably both of which are 0.65 to 0.90. The static friction coefficient (μs) and kinetic friction coefficient (μd) herein refer to the values ​​calculated from the maximum resistance value at the moment the film begins to slide and the resistance value during the slide when the two films are stacked so that the two opposite surfaces are in contact with each other in an environment of 23±3°C and 65±5% humidity, a 200g load is applied from above, and the upper film is caused to slide, using the following formula (details of the measurement method are described below). Hereinafter, the "static friction coefficient (μs)" and "kinetic friction coefficient (μd)" may be collectively referred to as the "friction coefficient," and both can be calculated using the following formula 2: Equation 2: Friction coefficient = resistance value (g) / load (g).

[0049] A friction coefficient of 0.55 or higher for biaxially oriented polypropylene film ensures a certain degree of slipperiness, reducing problems such as wrinkles in the film during transport and winding, leading to a more stable winding appearance. On the other hand, a friction coefficient of 1.00 or lower prevents excessive friction, reducing meandering and winding slippage that occurs when the biaxially oriented polypropylene film is wound into a roll, improving winding properties.

[0050] The coefficient of friction can be adjusted to 0.55 or more and 1.00 or less, or to the preferred range described above, by, for example, adding a branched chain polypropylene resin or a resin other than polypropylene resin (such as polymethylpentene (PMP) resin) to the surface layer (for example, the biaxially oriented polypropylene film itself in the case of a single layer structure) instead of particles that tend to produce coarse protrusions, to form surface irregularities. The coefficient of friction can also be adjusted by adjusting Sal, and more specifically, the coefficient of friction can be increased by reducing Sal. The method for adjusting Sal is as described above.

[0051] Next, the layer structure and raw materials of the biaxially oriented polypropylene film of the present invention will be explained, but the layer structure and raw materials of the biaxially oriented polypropylene film of the present invention are not necessarily limited to those shown above.

[0052] In the biaxially oriented polypropylene film of the present invention, when all components constituting the film are taken as 100% by mass, the polypropylene resin content is preferably 95% by mass or more and 100% by mass or less in total, more preferably 96% by mass or more and 100% by mass or less in total, even more preferably 97% by mass or more and 100% by mass or less in total, and particularly preferably 98% by mass or more and 100% by mass or less in total.

[0053] The thickness of the biaxially oriented polypropylene film of the present invention is not particularly limited, but from the viewpoint of handleability, it is preferably from 1 μm to 100 μm, more preferably from 5 μm to 50 μm, and even more preferably from 10 μm to 40 μm. The film thickness can be measured with a known electronic micrometer (details will be described later), and can be adjusted by the screw rotation speed of the extruder, the width of the unstretched polypropylene sheet, the film production speed, the stretching ratio, etc.

[0054] The biaxially oriented polypropylene film of the present invention may have a single-layer structure or a laminated structure, but from the viewpoint of achieving both releasability and heat resistance, a laminated structure is preferred, and in the case of a laminated structure, a two-type two-layer structure of surface layer (A layer) / base layer (B layer), a two-type three-layer structure of surface layer (A layer) / base layer (B layer) / surface layer (A layer), or a three-type three-layer structure of surface layer (A layer) / base layer (B layer) / surface layer (C layer) is preferred. Note that hereinafter, the surface layer (A layer), base layer (B layer), and surface layer (C layer) may be referred to as A layer, B layer, and C layer, respectively.

[0055] Resins and other components that can be used in each layer are described below (the surface layer will be described as "layer A," but unless otherwise specified, "layer C" can be interpreted similarly). When the biaxially oriented polypropylene film of the present invention has a laminated structure, the main component of layers A and B is preferably a polypropylene resin. Hereinafter, the polypropylene resin that is the main component of layers A and B of the biaxially oriented polypropylene film may be referred to as polypropylene resin I. The content of polypropylene resin I in layers A and B is more preferably 70% by mass or more and 100% by mass or less, even more preferably 75% by mass or more and 100% by mass or less, and particularly preferably 80% by mass or more and 100% by mass or less (however, when other resins, additives, etc. are contained, the upper limit is set taking into consideration the content of these components).

[0056] In view of strength and heat resistance, the polypropylene resin I in the biaxially oriented polypropylene film of the present invention is preferably a homopolypropylene, which refers to a polypropylene resin in which 99 mol % to 100 mol % of the total structural units constituting the resin are propylene units.

[0057] The polypropylene resin I preferably has a melting point of 160°C or higher, more preferably 162°C or higher, and even more preferably 165°C or higher. Having a melting point of 160°C or higher improves the heat resistance of the resulting biaxially oriented polypropylene film. Therefore, when the biaxially oriented polypropylene film is used, for example, as a release film, softening of the biaxially oriented polypropylene film during the heating process after laminating its side A to a mating member, as well as the resulting expansion and contraction in the tension direction, can be suppressed. As a result, deformation of the mating member and deterioration of releasability due to fusion between the biaxially oriented polypropylene film and the mating member can be reduced. Furthermore, the melting point of the polypropylene resin I is preferably 200°C or lower, more preferably 190°C or lower, and even more preferably 180°C or lower. Having a melting point of 200°C or lower reduces the likelihood of equipment constraints, such as ensuring the heating capacity of the melt extruder and managing filters.

[0058] The mesopentad fraction of polypropylene resin I is preferably 0.90 or more, more preferably 0.93 or more, and even more preferably 0.94 or more. The mesopentad fraction is an index of the stereoregularity of the crystalline phase of polypropylene measured by nuclear magnetic resonance (NMR). Generally, the higher the mesopentad fraction, the higher the crystallinity and melting point. Therefore, by using a polypropylene resin with a high mesopentad fraction as polypropylene resin I, the dimensional stability at high temperatures is improved when the polypropylene resin is made into a biaxially oriented polypropylene film. The upper limit of the mesopentad fraction is not particularly specified, but is set to 0.99 from the viewpoint of use in film formation.

[0059] To obtain such a polypropylene resin with a high mesopentad fraction, it is preferable to use a method of washing the obtained resin powder with a solvent such as n-heptane, a method of appropriately selecting a catalyst and / or co-catalyst, and a method of appropriately selecting the composition, etc. Usually, a polymerization catalyst containing a metallocene compound having a cyclopentadienyl skeleton in the molecule is preferably used as the catalyst.

[0060] Furthermore, from the viewpoint of film formability and strength when made into a biaxially oriented polypropylene film, the polypropylene resin I preferably has a melt flow rate (MFR) of 1.0 g / 10 min or more and 10 g / 10 min or less, more preferably 1.0 g / 10 min or more and 8.0 g / 10 min or less, and even more preferably 2.0 g / 10 min or more and 6.0 g / 10 min or less. The MFR here refers to the MFR (JIS K7210 2014) measured under conditions of a temperature of 230°C and a load of 21.18 N, and the same applies to MFR hereinafter.

[0061] To achieve the above melt flow rate (MFR) of polypropylene resin, methods such as controlling the average molecular weight and molecular weight distribution are employed. More specifically, the MFR can be increased by reducing the number average molecular weight (Mn), weight average molecular weight (Mw), and molecular weight distribution (JIS K7210 2014).

[0062] Furthermore, from the viewpoint of film-forming properties of the biaxially oriented polypropylene film, Layer A and Layer B may contain a polypropylene resin (hereinafter sometimes referred to as polypropylene resin II) having an MFR within the above range and higher than that of Polypropylene Resin I. By adopting such an embodiment, it is possible to produce a biaxially oriented polypropylene film more stably. In Layer A and Layer B, the content of Polypropylene Resin II is preferably from 75% to 100% by mass in total with Polypropylene Resin I when all constituent components of each layer are taken as 100% by mass, and it is more preferable to adjust the content within this range taking into consideration the content of the branched polypropylene resin and other components described below.

[0063] Layer B of the biaxially oriented polypropylene film of the present invention may use a resin composition in which polypropylene resin I contains a branched polypropylene resin, from the viewpoint of improving heat resistance and dimensional stability. The branched polypropylene resin acts as a crystal nucleating agent, thereby contributing to improving heat resistance and dimensional stability. When layer B contains a branched polypropylene resin, its content is preferably 1% by mass or more and less than 10% by mass, and more preferably 3% by mass or more and 7% by mass or less, of all components constituting the layer.

[0064] Furthermore, it is preferable that Layer A of the biaxially oriented polypropylene film of the present invention also contain a branched polypropylene resin from the viewpoint of controlling the Sal and coefficient of friction within suitable ranges. From the above viewpoints, the content (mass%) of branched polypropylene resin in Layer A is preferably 1% to 10% by mass of all components constituting the layers, and the content (mass%) in Layer B is preferably 1% to 10% by mass, and more preferably exceeds the content (mass%) in Layer B and is 10% to 10% by mass. Incorporation of branched polypropylene in both Layer A and Layer B not only improves heat resistance but also facilitates the formation of numerous protrusions on the surface, making it easier to form a dense rough surface like that of Layer A. Furthermore, by keeping the amount of branched polypropylene resin in Layer B equal to or less than that in Layer A, it is possible to prevent the surface from becoming smooth beyond the appropriate dense rough surface due to excessive protrusion formation, thereby maintaining better releasability.

[0065] As the branched chain polypropylene resin, it is preferable to use a polypropylene resin having a branched structure in the molecular chain. Note that, "a polypropylene resin having a branched structure in the molecular chain" is a polypropylene resin having 5 or less internal tri-substituted propylenes per 10,000 carbon atoms in the molecular chain, and the presence of this internal tri-substituted propylene is 1 This can be confirmed by the proton ratio in the H-NMR spectrum. From the viewpoint of film formation stability, the MFR of the branched-chain polypropylene resin is preferably 1.0 g / 10 min or more and 10.0 g / 10 min or less. The branched-chain polypropylene resin can be used not only for the B layer but also for the A layer, and in both cases the same resin is suitable.

[0066] Examples of branched polypropylene resins that can be suitably used in each layer of the biaxially oriented polypropylene film of the present invention include "Profax" (registered trademark) (PF-814, etc.) manufactured by Lyondell Basell, "Daploy" (trademark) (WB130HMS, WB135HMS, etc.) manufactured by Borealis, and "WAYMAX" (registered trademark) (MFX3, MFX6, MFX8, EX6000, EX8000, etc.) manufactured by Japan Polypropylene Corporation.

[0067] Although the branched polypropylene resin itself has the effect of forming a crystal nucleating agent for α or β crystals, other crystal nucleating agents such as α crystal nucleating agents (e.g., dibenzylidene sorbitols, sodium benzoate, etc.) or β crystal nucleating agents (e.g., potassium 1,2-hydroxystearate, magnesium benzoate, amide compounds such as N,N'-dicyclohexyl-2,6-naphthalenedicarboxamide, quinacridone compounds, etc.) may also be added within the scope of the present invention. These crystal nucleating agents containing branched polypropylene may be added to either Layer A or Layer B, or to both.

[0068] However, excessive addition of the above-mentioned other nucleating agents may cause a decrease in the stretchability when producing the biaxially oriented polypropylene film, or a decrease in the transparency and strength of the biaxially oriented polypropylene film due to void formation, etc., so the amount added is usually 0.5 mass% or less, preferably 0.1 mass% or less, and more preferably 0.05 mass% or less, when all components constituting the biaxially oriented polypropylene film are taken as 100 mass%.

[0069] Polypropylene resin I may contain copolymerization components such as unsaturated hydrocarbons other than the main structural units, as long as the objectives of the present invention are not impaired. Examples of monomers constituting such copolymerization components 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. The copolymerization amount is preferably less than 1 mol% from the viewpoint of dimensional stability when formed into a biaxially oriented polypropylene film.

[0070] Furthermore, Layer A and Layer B may contain a polyolefin resin other than the polypropylene resin that is the main component. Here, "polyolefin resin other than polypropylene resin" refers to a polyolefin resin whose main structural unit is different from that of polypropylene resin. Examples of main structural units of polyolefin resins other than polypropylene resin include structural units derived from 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. The content of polyolefin resin other than polypropylene resin is preferably 15% by mass or less, and more preferably 10% by mass or less, when the total mass of all components constituting Layer A and Layer B is 100% by mass.

[0071] When polyethylene is contained as a polyolefin resin other than polypropylene resin, the polyethylene resin contained in Layer A and Layer B is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 3% by mass or less. The higher the polyethylene resin content in Layer B, the lower the crystallinity when made into a biaxially oriented polypropylene film, making it easier to improve transparency. On the other hand, by keeping the polyethylene resin content at 10% by mass or less, it is possible to reduce the decrease in strength and heat resistance when made into a biaxially oriented polypropylene film, and it is also possible to reduce resin degradation during the extrusion process and suppress the occurrence of fisheyes when made into a biaxially oriented polypropylene film.

[0072] The biaxially oriented polypropylene film of the present invention can contain various additives, such as antioxidants, heat stabilizers, slipping agents, antistatic agents, antiblocking agents, fillers, viscosity modifiers, and color inhibitors, as long as they do not impair the object of the present invention. Among these, the selection of the type and amount of antioxidant is important from the viewpoint of antioxidant bleed-out. As such antioxidants, phenolic antioxidants having steric hindrance are preferred, and when multiple types of antioxidants are used in combination, at least one is preferably a high molecular weight type having a molecular weight of 500 or more. Specific examples include various compounds, but it is preferable to use, for example, 2,6-di-t-butyl-p-cresol (BHT: molecular weight 220.4) in combination with 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene (e.g., BASF's "Irganox" (registered trademark) 1330: molecular weight 775.2) or tetrakis[methylene-3(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane (e.g., BASF's "Irganox" (registered trademark) 1010: molecular weight 1177.7).

[0073] The total content of these antioxidants is preferably in the range of 0.03% by mass or more and 1.0% by mass or less, based on 100% by mass of all raw materials used to obtain the biaxially oriented polypropylene film, in order to reduce discoloration due to polymer degradation and loss of transparency due to antioxidant bleed-out. By using an antioxidant content of 0.03% by mass or more, discoloration of the biaxially oriented polypropylene film and loss of long-term heat resistance due to polymer degradation during the extrusion process can be reduced. On the other hand, by using an antioxidant content of 1.0% by mass or less, loss of transparency of the biaxially oriented polypropylene film due to antioxidant bleed-out can be suppressed. From the above viewpoints, the content of the antioxidant is more preferably 0.05% by mass or more and 0.9% by mass or less, and even more preferably 0.1% by mass or more and 0.8% by mass or less. These antioxidants may be added to either Layer A or Layer B, or both.

[0074] Furthermore, the biaxially oriented polypropylene film of the present invention preferably does not contain organic or inorganic particles. For example, the polypropylene resin that can be suitably used for the biaxially oriented polypropylene film of the present invention has low affinity with organic or inorganic particles, so these particles may fall off during the production process and contaminate the production line or product, or large protrusions formed by high-hardness particles may be transferred as irregularities to the resin layer of the mating member. Therefore, when used as a release film for optical components used in products requiring high quality, such as display components, it is preferable that the biaxially oriented polypropylene film does not contain organic or inorganic particles.

[0075] As a biaxial stretching method for obtaining the biaxially oriented polypropylene film of the present invention, any of inflation simultaneous biaxial stretching method, stenter simultaneous biaxial stretching method, and stenter sequential biaxial stretching method may be used, but among them, stenter sequential biaxial stretching method is preferably used from the viewpoints of film formation stability, thickness uniformity, and controlling high rigidity and dimensional stability of the film. Here, the stenter sequential biaxial stretching method refers to a method in which stretching in the longitudinal direction and width direction is carried out in separate steps, and at least stretching in the width direction is carried out using a stenter.

[0076] Next, one embodiment of the method for producing the biaxially oriented polypropylene film of the present invention will be explained using an example of a two-type, three-layer structure of surface layer (layer A) / base layer (layer B) / surface layer (layer A), but the biaxially oriented polypropylene film of the present invention is not necessarily limited to this.

[0077] First, polypropylene resin I or a mixture of polypropylene resin I and a branched polypropylene resin is fed into a single-screw extruder for layer B, and then the mixture of polypropylene resin I and a branched polypropylene resin is fed into a single-screw extruder for layer A, and each is melt-extruded at 200°C to 280°C, preferably 215°C to 270°C, and even more preferably 230°C to 260°C. After removing foreign matter and modified polymers using a filter installed midway through the polymer tube, the mixture is laminated in a multi-manifold composite T-die so as to have a layer A / layer B / layer A structure, and then extruded onto a casting drum and cooled to solidify, yielding an unstretched polypropylene sheet. Preferably, the amount of branched polypropylene resin (mass%) in the mixture for layer A is relatively higher than that for layer B.

[0078] The surface temperature of the casting drum is preferably 21° C. or higher and 100° C. or lower, more preferably 30° C. or higher and 100° C. or lower, even more preferably 50° C. or higher and 100° C. or lower, and particularly preferably 50° C. or higher and 85° C. However, within the above range, cooling of the casting drum alone, or cooling by a water bath or air blowing may be used, or these methods may be combined as appropriate.

[0079] Alternatively, a casting drum may be provided further downstream, and the unstretched polypropylene sheet may be cooled by bringing the surface opposite to the surface in contact with the upstream casting drum into close contact with this casting drum. The surface temperature of the downstream casting drum is preferably at least 50°C lower than the surface temperature of the upstream casting drum and lower than the surface temperature of the upstream casting drum, more preferably at least 40°C lower than the surface temperature of the upstream casting drum and lower than the surface temperature of the upstream casting drum by 15°C.

[0080] Regardless of the number of casting drums, the temperature of the unstretched polypropylene sheet after cooling is preferably 20°C or higher and 83°C or lower, more preferably 20°C or higher and 80°C or lower. By keeping the surface temperature of the casting drum and the temperature of the unstretched polypropylene sheet within the above ranges, the unstretched polypropylene sheet can be slowly cooled and moderately crystallized. The crystallized portions not only increase heat resistance but also contribute to the formation of appropriate surface protrusions (side A). Therefore, by obtaining an unstretched polypropylene sheet under these conditions, a biaxially oriented polypropylene film with excellent releasability and workability in high-temperature environments can be obtained.

[0081] The unstretched polypropylene sheet may be adhered to the casting drum using any of the following methods: electrostatic application, adhesion using the surface tension of water, air knife method, press roll method, underwater casting method, etc. However, the air knife method is preferred from the viewpoint of improving the flatness of the resulting biaxially oriented polypropylene film and facilitating control of surface roughness (formation of side A).

[0082] Next, the obtained unstretched polypropylene sheet is biaxially stretched to biaxially orient it. Specific stretching and preheating conditions are preferably as follows. Preheating methods may include a method using a temperature-controlled rotating roll, a method using a hot air oven, or the like, either alone or in combination. Taking into consideration the fact that stretching at a high temperature controls the protrusions on the film surface to a preferred range, the preheating temperature on both sides of the unstretched polypropylene sheet is preferably 140°C or higher and 165°C or lower, more preferably 145°C or higher and 160°C or lower, and even more preferably 150°C or higher and 154°C or lower.

[0083] From the viewpoint of controlling the surface roughness, heat resistance, and dimensional stability within preferred ranges by high-temperature stretching after preheating under the above conditions, the stretching temperature when stretching in the longitudinal direction is preferably 140°C or higher and the preheating temperature or lower, more preferably 142°C or higher and the preheating temperature or lower, and even more preferably 145°C or higher and the preheating temperature or lower.

[0084] The stretching ratio is preferably 2.0 to 8.0 times, more preferably 3.0 to 6.0 times, and even more preferably 3.5 to 5.5 times. A stretching ratio of 2.0 times or more allows for more uniform stretching, thereby suppressing thickness unevenness. On the other hand, a stretching ratio of 8.0 times or less reduces film breakage during the longitudinal stretching step and the subsequent transverse stretching step. Furthermore, by performing stretch relaxation after stretching in the machine direction, dimensional stability can be controlled within a preferred range. From this perspective, the relaxation rate after stretching in the machine direction is preferably 4.0% to 8.0% and more preferably 4.6% to 7.5%.

[0085] The uniaxially oriented polypropylene film obtained by longitudinal stretching is then introduced into a tenter, where both widthwise ends are held with clips and preheated, and then transversely stretched at a ratio of preferably 4.0 to 12.0, more preferably 5.0 to 11.0, even more preferably 6.0 to 10.0, and particularly preferably 6.5 to 10.0. From the viewpoint of controlling dimensional stability within a preferred range by high-temperature stretching, the preheating and stretching temperatures are 155°C to 190°C, more preferably 162°C to 172°C, and even more preferably 166°C to 172°C.

[0086] Furthermore, it is preferable to heat-treat the polypropylene film after transverse stretching directly in the tenter. From the viewpoint of controlling dimensional stability in the width direction, the heat-treatment temperature is preferably 140°C or higher and 175°C or lower, more preferably 145°C or higher and 170°C or lower, and even more preferably 150°C or higher and 170°C or higher. Furthermore, from the viewpoint of slowing the shrinkage rate after transverse stretching and removing residual shrinkage stress to reduce the thermal shrinkage rate, it is also preferable to heat-treat the film after transverse stretching by increasing the temperature by 2 to 3 steps within the above temperature range. Here, "within the above temperature range" means that both the lowest and highest temperatures are within the above temperature range.

[0087] From the same viewpoint, it is preferable to carry out the heat treatment while relaxing the polypropylene film after transverse stretching in the width direction. From the above viewpoint, the relaxation rate after stretching in the width direction is preferably 5.0% to 20.0%, more preferably 7.0% to 15.0%. Thereafter, the obtained biaxially oriented polypropylene film is released from the clips and cut parallel to the longitudinal direction using a known slitter or the like, thereby cutting and removing the portion held by the clips, and winding it up into a roll to obtain a roll of biaxially oriented polypropylene film. Note that the width of the biaxially oriented polypropylene film roll may be adjusted by unwinding the biaxially oriented polypropylene film of the present invention from the biaxially oriented polypropylene film roll, slitting it to the desired width, and rewinding it.

[0088] The biaxially oriented polypropylene film of the present invention can be used for various purposes such as packaging films, release films, sanitary products, agricultural products, construction products, and medical products, but is preferably used as a release film because it has excellent releasability and processability in high-temperature environments.

[0089] The release film of the present invention is made using the biaxially oriented polypropylene film of the present invention. A release film refers to a film that is peeled off from a mating member before use as a final product, such as a substrate used to protect the surface of the mating member or to manufacture the mating member, and is also called a surface protection film, process film, etc. The biaxially oriented polypropylene film of the present invention has excellent releasability and processability in high-temperature environments, making it suitable for use as a release film, and is particularly advantageous when used as a release film for a mating member that requires processing at high temperatures. [Example]

[0090] The biaxially oriented polypropylene film of the present invention will be described in more detail below with reference to examples, but the biaxially oriented polypropylene film of the present invention is not limited to these examples. The properties were measured and evaluated by the following methods.

[0091] [Measurement and evaluation methods for each characteristic] (1) Film thickness Five biaxially oriented polypropylene films were stacked to prepare a measurement sample, and the thickness of the measurement sample was measured using an electronic micrometer (TT80 manufactured by TESA). The obtained value was then divided by 5 to calculate the thickness per biaxially oriented polypropylene film. The same measurement was then performed at a position shifted 50 mm in the width direction as the measurement point, and this measurement was repeated a total of five times. From the obtained measurement results, the average thickness per biaxially oriented laminated polypropylene film was calculated and used as the film thickness (μm).

[0092] (2) Minimum autocorrelation length (Sal), surface aspect ratio (Str) Measurements were carried out using the non-contact surface / layer cross-sectional shape measurement system "VertScan" (registered trademark) 2.0 (model: R3300GL-Lite-AC) manufactured by Ryoka Systems Co., Ltd., under the following procedures and conditions. First, the biaxially oriented polypropylene film was unwound from the film roll, and measurement samples were taken at 10 measurement points randomly determined on a line passing through the center in the width direction and parallel to the longitudinal direction, and Sal and Str were measured at the 10 points. The average values ​​of the obtained measurements were calculated and used as Sal and Str of the biaxially oriented polypropylene film. Note that in one measurement, one field of view (field area: vertical 939 μm × horizontal 1,252 μm = 1,175,628 μm) was used. 2 ) was measured, and the measurements were carried out on both sides in the same manner.

[0093] A. Measurement conditions CCD camera: "SONY" (registered trademark) HR-57 1 / 2 manufactured by Sony Corporation Objective lens: 10X Lens tube: 0.5X BODY Wavelength filter: 530 white Measurement mode: Wave Field of view size: 640 x 480 Scan range: (Start) 5μm, (Stop) -5μm.

[0094] B. How to fix the measurement sample A dedicated sample holder was used to secure the measurement sample. The sample holder consists of two detachable metal plates with a circular hole in the center, and the measurement sample was sandwiched and fixed between them without any wrinkles, and measurements were taken of the sample positioned in the circular hole.

[0095] C. Analysis method The data obtained by the above measurements was analyzed using VS-Viewer, an image analysis software for "VertScan" (registered trademark) 2.0. First, noise was removed using a median filter (5 × 5), and waviness components were removed using a Gaussian filter with a cutoff value of 250 μm. Next, Sal and Str, as defined in ISO25178 (2012), were measured using the "ISOPara" function. In the "ISOPara" function, the S-Filter was set to 6.0 μm.

[0096] (3) 150℃ heat shrinkage rate Xs, Ys Samples were prepared on the surface of polypropylene film, with a width of 10 mm and a measurement length of approximately 100 mm, with the measurement length aligned with the longitudinal and transverse stretching directions. (These measurement directions were determined because, in principle, if the stretching direction is known, the 150°C heat shrinkage is predicted to be greatest in one of the stretching directions.) Two lines were then drawn on each sample, and the distance between the two lines was accurately measured at 23°C and designated as L0. These samples were then placed in a 150°C oven for 15 minutes under a 1.5 g load. The distance between the two lines was then measured again at 23°C and designated as L1. The heat shrinkage at 150°C was calculated using the following equation (3). As mentioned above, the larger 150°C heat shrinkage was designated as the X-direction heat shrinkage, Xs, and the other as the Y-direction heat shrinkage, Ys. If the 150°C heat shrinkages in two orthogonal directions were the same, one of them was designated as the X-direction and the other as the Y-direction. Equation 3: Dimensional change rate (%) = [(L0-L1) / L0] × 100.

[0097] (4) XsT and YsT during the temperature rise process of thermomechanical analysis (TMA) Using the TMA (Thermo Mechanical Analysis) method, the dimensional change rate of the sample in the X and Y directions at each temperature was measured, and a curve (TMA curve) was drawn by plotting the temperature on the horizontal axis and the rate of change in the length of the sample on the vertical axis. During the temperature rise process in the X direction TMA curve, the temperature that first reached 0.04% was read as XsT. Furthermore, YsT was measured in the same way as XsT, except that the TMA curve in the Y direction was used. The measurement equipment and conditions were as follows: <Measurement equipment and conditions> Stress loading device: "TMA / SS 6100" manufactured by Seiko Instruments Inc. Data processing device: "EXSTAR 6000" manufactured by Seiko Instruments Inc. Measurement mode: Constant temperature rise of 10°C / min Device installation atmosphere: Room temperature air Sample: 15mm x 2mm rectangle (measurement direction is 15mm).

[0098] (5) Sum of heat of fusion between 175°C and 200°C (ΔH) Using a differential scanning calorimeter (EXSTAR DSC6220 manufactured by Seiko Instruments Inc.), 3 mg of biaxially oriented polypropylene film was heated from 25°C to 250°C at a rate of 20°C / min in a nitrogen atmosphere to obtain a melting curve. In the obtained melting curve, a linear baseline was set in the range of 60°C to 200°C, and the total heat quantity was calculated from the area enclosed by the linear baseline and the melting curve at 175°C or higher. This was converted to the total heat quantity of fusion (ΔH) from 175°C to 200°C per sample mass.

[0099] (6) Friction coefficient Test pieces (measurement standard length: width 75 mm × length 100 mm) that had been temperature- and humidity-controlled under specified conditions (temperature: 23±3°C, humidity: 65±5%) were stacked so that the different surfaces were in contact and set in a measuring device (slip tester). A 200 g load was placed on the two stacked test pieces, and the upper test piece was slid using the measuring device to measure the resistance value, and the friction coefficient was calculated using the following equation 2. Note that both the static friction coefficient (μs) and the kinetic friction coefficient (μd) were calculated using the following equation 2, but for the resistance value (g), the maximum resistance value at the moment the sliding began was used to calculate the former, and the resistance value during sliding was used to calculate the latter. Equation 2: Coefficient of friction = resistance (g) / load (g).

[0100] (7) Melting point of resin Five mg of the raw material was placed in an aluminum pan as a sample and measured using a differential scanning calorimeter (Seiko Instruments EXSTAR DSC6220) under a nitrogen atmosphere according to the following procedure. First, the temperature was increased from 30°C to 260°C at 20°C / min, and then held at 260°C for 5 minutes. Then, the temperature was decreased from 260°C to 30°C at 20°C / min, and then increased again from 30°C to 260°C at 20°C / min. The maximum peak temperature of the melting curve observed when the temperature was increased again was taken as the melting point of the resin.

[0101] (8) Mesopentad fraction Mesopentad fraction (mmmm) A polypropylene resin sample is dissolved in a solvent. 13 The mesopentad fraction (mmmm) was determined using C-NMR under the following conditions (Reference: New Edition Polymer Analysis Handbook, edited by the Japan Society for Analytical Chemistry and the Polymer Analysis Research Forum, 1995, pp. 609-611).

[0102] A. Measurement conditions Equipment: Bruker DRX-500 Measurement nucleus: 13C nucleus (resonance frequency: 125.8MHz) Measured concentration: 10% by mass Solvent: Benzene / d-o-dichlorobenzene = 1:3 mass ratio mixed solution Measurement temperature: 130℃ Spin speed: 12Hz NMR sample tube: 5mm tube Pulse width: 45° (4.5 μs) Pulse repetition time: 10 seconds Data points: 64K Conversion count: 10,000 times Measurement mode: complete decoupling.

[0103] B.Analysis conditions Fourier transform was performed with LB (line broadening factor) set to 1.0, and the mmmm peak was set to 21.86 ppm. Peak splitting was performed using WINFIT software (Bruker). Peak splitting was performed as follows, starting from the peak on the high magnetic field side, and automatic fitting was then performed using the accompanying software. After optimizing the peak splitting, the sum of the mmmm peak fractions was calculated. The above measurement was performed five times, and the average value was used as the mesopentad fraction (mmmm) of this sample. (peak splitting) (a)mrrm (b)(c) rrrm (split into two peaks) (d)rrrr (e)mrmr (f)mrmm+rmrr (g)mmrr (h)rmmr (i) mmmr (j)mmmm.

[0104] (9) Melt flow rate (MFR) (unit: g / 10 min) Measurements were performed at 230°C and 2.16 kg in accordance with JIS K 7210-1 (2014).

[0105] (10) Mold releasability A SUS plate (10 cm x 10 cm, SUS304), biaxially oriented polypropylene film, and the SUS plate were laminated in this order, with the inner surface of the biaxially oriented polypropylene film in contact with the lower SUS plate. The resulting laminate was pressed at 150°C under a constant pressure of 1.5 MPa for 15 minutes. After pressing, the laminate was removed from the press, and the upper SUS plate was removed, leaving the SUS plate and biaxially oriented polypropylene film stacked in that order. The biaxially oriented polypropylene film was peeled from the laminate, and the releasability of the inner surface of the biaxially oriented polypropylene film was evaluated according to the following criteria: ○, △, and ×. The outer surface of the biaxially oriented polypropylene film was evaluated in the same manner, and the better result of the inner surface or outer surface was used as the evaluation result. ◯: No resistance was felt when peeling off the biaxially oriented polypropylene film, and neither damage such as tearing of the biaxially oriented polypropylene film nor adhesion to the SUS plate was observed. △: There was resistance when peeling off the biaxially oriented polypropylene film, but neither damage such as tearing of the biaxially oriented polypropylene film nor fusion to the SUS plate was observed. ×: At least one of damage such as tearing of the biaxially oriented polypropylene film and fusion to the SUS plate was observed.

[0106] (11) Heat wrinkle detection The laminate obtained by pressing in (10), in which the SUS plate and the biaxially oriented polypropylene film were laminated in this order from the bottom, was visually observed for deformation, including folds and wrinkles, in the biaxially oriented polypropylene film, and evaluated according to the following criteria (○ and △ were considered pass, and × was considered fail). ◯: There were no folds or wrinkles in the biaxially oriented polypropylene film. △: The biaxially oriented polypropylene film was free from folds and wrinkles, but had slight deformation. ×: The biaxially oriented polypropylene film was folded or wrinkled.

[0107] (12) Windability When the biaxially oriented polypropylene film was wound into a roll, the film was visually inspected for deterioration of the roll shape that would impair its function as a release film, such as uneven creases that would cause dents on the adherend, blocking that would cause destruction of the film surface or film tearing, and marks of destruction such as holes, and judged according to the following criteria (○ and △ were considered pass, and × was considered fail). ◯: No irregular creases, blocking, holes or other damage marks were observed from the core to the surface layer of the biaxially oriented polypropylene film roll. △: Minor uneven creases were observed from the core to the surface of the biaxially oriented polypropylene film roll, but no signs of destruction such as blocking or holes were observed. ×: At least one of the following marks of destruction, such as non-minor uneven creases, blocking, holes, etc., was observed from the core to the surface layer of the biaxially oriented polypropylene film roll.

[0108] (13) Tensile test Five rectangular samples, each 10 mm wide and 50 mm long (measurement direction), were cut from the biaxially oriented polypropylene film, with the measurement direction aligned with the X axis. Marks were placed 15 mm from each end to set the sample length at 20 mm. The rectangular samples were then placed in a tensile tester (A&D's "Tensilon"® AMF / RTA-100) with an initial chuck distance of 20 mm. A tensile test was then conducted at a tensile speed of 300 mm / min, yielding the elongation (%) and strength (MPa) at break. The measurement was performed five times, and the average values ​​of the elongation and strength at break were calculated. These were used as the elongation at break (%) and strength / elongation ratio (MPa / %) of the biaxially oriented polypropylene film, respectively.

[0109] (14) Film Formation Evaluation The film formability of the biaxially oriented polypropylene film was evaluated according to the following criteria. Note that the time from when film production was stopped due to film breakage during the process until film production was restarted was excluded from the observation time. ◯: No film breakage occurred for over 48 hours. △: The film broke 1 to 3 times in 48 hours. ×: The film broke four or more times within 48 hours.

[0110] [Raw materials] The following raw materials were used to produce the biaxially oriented laminated polypropylene films of the Examples and Comparative Examples. The antioxidants listed below were already mixed with the polypropylene resin I and the branched polypropylene resin, so the content was not finely adjusted in the Examples and Comparative Examples. Furthermore, the antioxidants contained in each resin were in trace amounts (less than 1.0% by mass in total), and the amount would be even less if the volatile content in the film was taken into account. Therefore, they are not listed in the production methods of the Examples and Comparative Examples described below or in Table 1.

[0111] (1) Resin Polypropylene resin I (homopolypropylene resin): manufactured by Prime Polymer Co., Ltd., a highly stereoregular homopolypropylene resin having an MFR of 2.5 g / 10 min, a melting point of 166°C, and a mesopentad fraction of 0.98. Polypropylene resin II (homopolypropylene resin): manufactured by Prime Polymer Co., Ltd., a highly stereoregular homopolypropylene resin with an MFR of 4.1 g / 10 min, a melting point of 166°C, and a mesopentad fraction of 0.98. Polypropylene resin III (homopolypropylene resin mixture): Manufactured by Sumitomo Chemical Co., Ltd., this polypropylene resin has an MFR of 4.0 g / 10 min and a melting point of 158°C (homopolypropylene resin mixed with ethylene-propylene copolymer so that the ethylene unit content is 0.5% by mass), and is mixed with 0.3% by mass of acrylic beads with an average particle size of 2 μm measured by the electrical detection zone method. Note that the mesopentad fraction cannot be measured. Branched chain polypropylene resin: (Branched chain polypropylene resin / Branched PP1) "WAYMAX" (registered trademark) (MFX3) manufactured by Japan Polypropylene Corporation. A branched chain polypropylene resin having an MFR of 9.0 g / 10 min. Polymethylpentene (PMP) resin: "TPX" (registered trademark) MX004 manufactured by Mitsui Chemicals, Inc.

[0112] (2) Antioxidants Antioxidant 1: "Irganox" (registered trademark) 1010 manufactured by BASF Japan Ltd. Antioxidant 2: 2,6-di-t-butyl-p-cresol (BHT) manufactured by ADEKA Corporation.

[0113] Example 1 As the raw material for the base layer (B layer), a mixture containing 80% by mass of polypropylene resin I, 15% by mass of polypropylene resin II, and 5% by mass of branched polypropylene resin was fed into a single-screw extruder. As the raw material for the surface layer (A layer), a mixture containing 80% by mass of polypropylene resin I, 13% by mass of polypropylene resin II, and 7% by mass of branched polypropylene resin was fed into another single-screw extruder, and each was melt-extruded at 260 ° C. Next, after removing foreign matter with a 30 μm cut sintered filter, the raw materials were laminated in a feedblock-type composite T-die so that the thickness ratio of A layer / B layer / A layer was 1 / 28 / 1, and then formed into a sheet and extruded. The extruded molten sheet was adhered to an upstream casting drum with a surface temperature adjusted to 70 ° C. using an air knife, cooled, and solidified. The opposite surface was further adhered to a downstream casting drum with a surface temperature of 35 ° C. to obtain an unstretched polypropylene sheet. At this time, compressed air was sprayed at 40 m / s onto the non-drum contact side of the unstretched polypropylene sheet (molten sheet-like material) on the upstream casting drum, adhering the unstretched polypropylene sheet to the casting drum. After passing through the downstream casting drum, the unstretched polypropylene sheet was cooled to 34 ° C. Next, the unstretched polypropylene sheet was preheated to 154 ° C using a conveying roll, and then stretched 4.3 times in the longitudinal direction between a 150 ° C stretching roll with a peripheral speed difference from the preheating roll. A uniaxially stretched film was obtained by applying 6.1% relaxation in the longitudinal direction using a conveying roll with a peripheral speed difference from the stretching roll. Next, the uniaxially stretched film was introduced into a tenter-type stretching machine, with both widthwise ends held with clips, stretched 7.9 times at 167 ° C., and then heat-treated by air blowing at three temperatures set from upstream to downstream: 162 ° C., 160 ° C., and 155 ° C., while applying 11.3% relaxation in the widthwise direction. The clips at the widthwise ends were then released, and both widthwise ends (portions held by the clips) were cut off with a slitter. The film was then wound up with a winder to obtain a biaxially oriented polypropylene film with a thickness of 30 μm. The physical properties and evaluation results of the obtained biaxially oriented polypropylene film are shown in Table 1.In Table 1, the inner surface of the roll refers to the surface located on the core side of the film roll when the film is wound into a roll, and the outer surface of the roll refers to the surface opposite the inner surface of the roll (the surface that was in contact with the upstream casting drum is the inner surface of the roll).

[0114] (Examples 2 to 8, Comparative Examples 1 to 6) In Example 1, a biaxially oriented polypropylene film having the thickness shown in Table 1 was obtained in the same manner as in Example 1, except that the film configuration and film-forming conditions were as shown in Table 1. The physical properties and evaluation results of the obtained biaxially oriented polypropylene film are shown in Table 1. The film thickness was adjusted by adjusting the screw rotation speed of the extruder, the width of the unstretched polypropylene sheet, the film-forming speed, the stretching ratio, etc.

[0115] [Table 1]

[0116] In the table, the antioxidant is not taken into consideration in calculating the composition because it is an extremely small amount relative to the resin in the film composition. Also, the heat treatment temperature was adjusted from right to left. [Industrial Applicability]

[0117] As described above, the biaxially oriented polypropylene film of the present invention has excellent releasability and processability in high-temperature environments, making it particularly suitable as a release film for use at high temperatures or as a cover film for an adhesive resin layer. Furthermore, the biaxially oriented polypropylene film of the present invention can also be made to have excellent surface smoothness, making it suitable for use as a release film or processing film for applications requiring product surface smoothness. Furthermore, the biaxially oriented polypropylene film of the present invention, which has the above-mentioned properties, can also be used in a variety of applications, such as packaging films, processing films, sanitary products, agricultural products, construction products, and medical products.

Claims

1. a biaxially oriented polypropylene film, wherein the surface having a minimum autocorrelation length (Sal) of 5 μm or more and 45 μm or less is the A-side, the direction having the largest heat shrinkage rate at 150°C is the X-direction, the direction perpendicular to the X-direction in the film plane is the Y-direction, the 150°C heat shrinkage rate in the X-direction is Xs, the 150°C heat shrinkage rate in the Y-direction is Ys, and during a temperature rise process in thermomechanical analysis (TMA), XsT is the temperature at which the film shrinks by 0.04% in the X-direction, and YsT is the temperature at which the film shrinks by 0.20% in the Y-direction, and at least one surface is the A-side, and Xs and Ys satisfy the following formula 1, wherein XsT is 130°C or more and 160°C or less, and YsT is 140°C or more and 170°C or less: Formula 1: -10%≦Ys≦Xs≦10%

2. The biaxially oriented polypropylene film according to claim 1, wherein the surface property aspect ratio (Str) of the A-side is 0.10 or more and 0.54 or less.

3. 3. The biaxially oriented polypropylene film according to claim 1, wherein the sum of the heats of fusion (ΔH) in the range of 175°C to 200°C measured by a differential scanning calorimeter (DSC) is 20 J / g or more and 50 J / g or less.

4. 3. The biaxially oriented polypropylene film according to claim 1, wherein the tensile elongation in the X direction is 200% or more and 400% or less, and the tensile strength in the X direction divided by the tensile elongation in the X direction is 0.05 MPa / % or more and 1.00 MPa / % or less.

5. 3. The biaxially oriented polypropylene film according to claim 1, wherein the static friction coefficient (μs) and the dynamic friction coefficient (μd) when different surfaces are in contact with each other are both 0.55 or more and 1.00 or less.

6. A release film comprising the biaxially oriented polypropylene film according to claim 1 or 2.

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